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Investigating the influence of atmospheric changes on the variability of the North Pacific using paleoproxy data and a fully coupled GCM

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Investigating the influence of atmospheric changes on the variability of the North Pacific using paleoproxy data and a fully coupled GCM

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Content INVESTIGATING THE INFLUENCE OF ATMOSPHERIC CHANGES ON THE
VARIABILITY OF THE NORTH PACIFIC USING PALEOPROXY DATA AND
A FULLY COUPLED GCM
by
Paola Gomez
A Thesis is Presented to the
FACULTY OF THE GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment o f the
Requirements for the Degree
MASTER OF SCIENCE
EARTH SCIENCES
December 2005
Copyright 2005 Paola Gomez
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UMI Number: 1435080
Copyright 2005 by
Gomez, Paola
All rights reserved.
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DEDICATION
To my nephew, Joseph James Gingeleskie.
Hope and opportunity are most easily recognized when it comes in the small package
o f a child. Every day I am amazed by the possibilities within you and it makes me
strive for the best within myself.
“To know even one life has breathed easier because you have lived. This is to have
succeeded.” -Ralph Waldo Emerson
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ACKNOWLEDGEMENTS
There are a number o f people without whom this thesis may never have been written,
and to whom I am forever grateful and greatly indebted.
To my parents, Jaime and Luz Marina Gomez, who have always loved and supported
me in all that I have ever done, in spite o f the fact that I have often walked to the
offbeat rhythm o f my own drummer.
To my sister, Catherine Gomez Gingeleskie, who has always been my biggest fan. I
will always be amazed at the unending confidence that I will find my way and be
great at whatever I do.
To Lowell D. Stott, who has supported me through the years and given me the
opportunity to explore and experience science in a way that is only possible by
throwing yourself into it head first.
To Christopher J. Poulsen, for allowing me to take a chance and reach past the limits
I thought I could not surpass and only to find that I could.
To Robert G. Douglas, William Berelson, Douglas E. Hammond, Steve Lund, and
Donn S. Gorsline, who have all shared with me their knowledge and love o f science
but have also taught me to learn for myself.
iii
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To David Wedgwood Carlson Farris, who, second only to my family, has been my
greatest supporter in this endeavor. I will forever be thankful for the faith you have
shown in me throughout my graduate career.
To Miguel Rincon, without whom I would have not had any data to present in a
thesis. You are a great friend and teacher and colleague, always patient with me even
as I was flooding the lab or some other small disaster graduate students tend to
cause.
To the Crews o f the RV Sproul, RV New Horizon, and the RV Yellowfin, I loved
every moment I was out at sea and am thankful for having the opportunity to go out
on the ocean as much as I did.
Lastly, to all o f my fellow graduate students, both within the laboratory and the
department, you have been a great support system and each in your own way have
taught me something about myself. I will appreciate that long after we part ways.
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TABLE OF CONTENTS
Dedication ii
Acknowledgements iii
List o f Tables vi
List o f Figures vii
Abstract ix
I. Introduction 1
II. North Pacific High 3
a. Local Influences 3
b. Remote Influences 8
III. Decadal Variability 16
IV. NPH Influence 19
V. Coastal California 21
VI. Santa Barbara Basin- Geologic Setting 25
VII. Mg/Ca Paleothermometry 29
a. Globigerina bulloides 29
b. Mg/Ca 31
VIII. Methodology 35
a. Santa Barbara Basin Proxy Record 35
b. Model Description (FOAM 1.5) 37
IX. Results and Discussion 39
a. Instrumental Records 39
b. Proxy Records 40
c. Model Results 49
X. Conclusions 68
References 71
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LIST OF TABLES
Table: Page:
1. Summary o f Pacific and North American climate anomalies associated 12
with extreme phases o f the PDO
2. Atmospheric Gas Concentrations 38
vi
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LIST OF FIGURES
Figure: Page:
1. April/May/June Averages 4
2. North Pacific High Pressure Center 5
3. Hadley Cell Circulation 7
4. The Atmospheric Bridge 9
5. Diatom Abundances 11
6 . Pacific Decadal Oscillation 12
7. Major Surface Currents 22
8 . Monthly Temperatures for the Surface Ocean 23
9. Santa Barbara Basin Bathymetry 25
10. Varved Sediments from Santa Barbara Basin 27
11. Globigerina bulloides 30
12. Globigerina bulloides 32
13. Varved Sediments from Study Area 35
14. Age Model for NH01-3-MC7 37
15. Instrumental Records in California Borderlands 40
16. Instrumental vs. Proxy Records 41
17. Proxy Data vs. PDO Index 43
18. Proxy Data vs. Upwelling Index 44
19. Comparison o f Independent Proxy Temperature Records 46
20. Mg/Ca Derived Temperatures (NH01-3-MC7) 47
21. Interannual Variability in the Mg/Ca Temperature Record 48
vii
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LIST OF FIGURES
Figure: Page:
22. Model Validation using PDO Variability in the Instrumental and 51
Modeled Data
23. Spectral Analysis- Model Results o f PDO 52
24. Simulated Surface Wind Patterns in the North Pacific 53
25. Atmospheric Temperature Difference Between Present Day and 54
Preindustrial Model Runs
26. Present Day - Preindustrial Spring SST and SSS 55
27. North Pacific High Strength - 10 year running average 56
28. Spectral Analysis AMJ Preindustrial NPH 58
29. Spectral Analysis AMJ Present Day NPH 59
30. Coastal California AMJ SSTs 60
31. Detrended Coastal California AMJ SSTs 61
32. Coastal California AMJ SSTs - 10 Year Running Average 62
33. Coastal California AMJ SSTs - 60 Year Running Average 63
34. Spectral Analysis AMJ Coastal CA SST 65
35. Preindustrial Model PDO and SST Results 66
36. Present Day Model PDO and SST Results 67
v iii
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ABSTRACT
We investigate whether changes in atmospheric concentrations in ozone and
greenhouse gases influence decadal-scaled oceanic and atmospheric dynamics in the
Northeast Pacific. Using a coupled ocean-atmosphere GCM we simulate
preindustrial and present day climatic conditions, focusing on the North Pacific.
Using modeled and paleoproxy data, we explore how the changed composition o f the
atmosphere influences the PDO and North Pacific High, two dominant modes of
regional variability. We examine spatial and temporal patterns o f regional sea-
surface temperatures on interannual to interdecadal timescales. Within these
simulations the influence o f the prescribed atmospheric perturbations is significant
on longer time scales at the smaller spatial scales examined. The significance o f the
variability evident in both the proxy and modeled results is a matter o f the temporal
and spatial scale examined.
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I. Introduction
During the latter part o f the 20th century the increase in greenhouse gas
(GHG) concentrations in the atmosphere has raised questions regarding the
influences an altered atmosphere can have on climate on various temporal and spatial
scales. For example, in the last decades o f the previous century, the ocean o f the
North Pacific sustained El Nino-like conditions even during times in which the
tropical Pacific was not experiencing El Nino events (Mantua and Hare, 2002). We
are also interested in which conditions are being preserved via proxy records in the
sediments o f the North Pacific. How are larger shifts in climate within the North
Pacific being translated into viable proxy records within the Santa Barbara Basin? If
we consider the increase in the global average temperature during the latter part of
the 20th century, would the increased concentrations o f GHGs and decreased
concentrations o f ozone, create a significant change in the mean climate state o f the
North Pacific? To what extent do both natural and anthropogenic variability
influence the resulting climate conditions we are currently experiencing in the
Northeast Pacific? Specifically, does the change in the atmospheric composition
cause a shift in the strength and position of the North Pacific High on decadal and
longer timescales? Lastly, how does this change in atmospheric composition
ultimately influence the sedimentary proxy record off o f the coast o f California?
The shorter scaled variability in atmospheric and oceanic conditions has
become essential to those who use an understanding o f present day physics and
dynamics in an effort better predict future climate interactions (Schneider and Miller,
1
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2001; Schneider et al., 2002). Considering the complexity o f ocean and atmosphere
dynamics, it is still unclear what particular effect specific perturbations to the steady
state o f this system can have on our society on these short time scales. Variability in
the mean climate state over the North Pacific ocean can influence terrestrial systems
extending from Siberia to the Southeastern United States (Horel and Wallace, 1981).
This region is subject to variability in atmospheric and oceanic conditions on decadal
and interdecadal timescales that strongly influence the weather o f North America
(Biondi et al., 2001) as well as shorter time-scaled variability such as that associated
with the El Nino/Southern Oscillation (ENSO). Utilizing sedimentary proxy records
together with global climate modeling can be useful to better understand the natural
variability that affects the coastal California region via perturbations in the mean
North Pacific climate. This combination o f resources can also be useful in learning
how the climatic system could potentially change with the introduction o f an
increase in GHG concentrations.
Our approach to understanding whether changes in atmospheric
concentrations in ozone and GHGs have an influence on decadal-scaled oceanic and
atmospheric dynamics in the Northeast Pacific is two-fold. We use Mg/Ca
paleothermometry applied to Globigerina bulloides (a surface dwelling planktonic
foraminifera) to reconstruct the pattern o f SST variability during the past two
centuries. We use this data to test whether the sediments in this basin record climatic
changes that originated as part o f the tropical-extratropical atmospheric bridge. In an
effort to place these findings in a larger temporal and spatial context we use a
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coupled ocean-atmosphere GCM (FOAM1.5) to simulate climatic conditions for the
preindustrial and the present day while focusing on the North Pacific. We explore
how the expanding ozone hole over the Southern Hemisphere and increased
concentrations in GHGs, particularly CO2, N 2O, and CH4, observed in the present
day influence the Pacific (Inter)Decadal Oscillation (PDO) and the North Pacific
High (NPH). In each model simulation we examine the spatial and temporal patterns
of the NPH, sea-surface temperatures and salinities (SSTs, SSSs) as well as wind and
ocean currents on the order o f interannual to interdecadal time scales. Proxy data
results together with climate modeling can place our simulated results within a
realistic and tangible context.
II. The North Pacific High
Ila. Local Influences
In this study, we focus on the strength and position o f the North Pacific High
(NPH) and its influence on the coastal Californian region. O f particular importance is
the seasonal pressure gradient between the NPH and the low pressure system over
the North American continent (Harms and Winant, 1998; Thunell et al., 1995). This
gradient creates and drives the regional winds, which in turn force much o f the
climate in the coastal regions o f the Northeast Pacific. The sea level pressure (SLP)
gradients play an important role in shaping the ocean’s climate and are linked to
changes in sea surface temperatures (SSTs) and mixed layer depth through shifts in
surface winds (Schwing et al., 2002). The strength o f the pressure center varies
3
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seasonally with a stronger NPH during the spring and a weaker center in the winter
(Emery and Hamilton, 1985; Harms and Winant, 1998). The stronger winds
generated during the spring are the basis for the wind-driven upwelling which is
recorded by
proxy in the
sea surface
temperatures
(SSTs) o f the
California
Borderlands.
Figure 1
shows the
relationship
between the
upwelling
index as
calculated by
S
o
m
e
X
a
2P
e
13
£
*
April/May/June Averages
1020
1019
1018
450
400 1017
350
1016
300
- 1015
250
200
150
100
00
r
*0
o
00
o
3
cr
1940 1950 1960 1970 1980 1990 2000
Figure 1. AMJ Average values for the Upwelling Index and Oregon SLP.
(NOAA, 2004).
Pacific Fisheries Environmental Laboratory for 33°N, the approximate location o f
our study site, and the SLP off o f the coast o f Oregon, as a measure o f average SLPs
in the Northeast Pacific (NOAA, 2004). For the length o f data overlap, higher
pressures off o f the North American coast generally coincide with increased strength
in the upwelling index. In the last half o f the 20th century, the spring position of the
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NPH has also varied temporally. The spring peak occurred in April in the 1990s as
opposed to February-March as in the 1950s (Bograd et al., 2002). Seasonally, the
NPH also moves over a distance of approximately 2000km with a more eastward
position during the winter, and westward during the fall (Figure 2) (Kenyon, 1999).
NORTH PACIFIC
HIGH PRESSURE CENTER
40° N
1947 -1975 MEAN
NAM I AS
I I I 1 ___ 1 ___ 1 ___ I ___ I ___ l.......I ____1
150° 140° I30°W
LONGITUDE
Figure 2. Seasonal movement of the NPH (January = 1, December = 12) (Kenyon,
1999).
Occasionally, the spring peak occurs as a “double high” in which two centers o f high
pressure occur in the North Pacific (Kenyon, 1999). These double highs are
potentially important because o f their frequency but also the influence on the
strength and proximity to the North American coast o f the NPH. In order to
comprehend the influence of this variability on the coastal climate o f the Northeast
Pacific at these timescales it is necessary to understand the various sources o f NPH
variability.
5
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There are numerous influences on the NPH including those resulting from
local atmospheric and oceanic conditions. Although the center o f the NPH moves
seasonally, the pressure maximum tends to occur over the ocean as opposed to the
land (Kenyon, 1999). This makes direct observation o f the NPH more difficult
because there have been less frequent observations o f atmospheric and oceanic
conditions over the North Pacific Ocean gyre. A recent hypothesis with respect to
the formation and sustenance o f the NPH indicates that density o f the air within the
NPH as a result o f humidity o f the atmosphere is more important than temperature in
the formation o f the pressure center and its variability (Kenyon, 1999). Kenyon
suggests that in the mid-latitudes o f the North Pacific, the oceans act as a heat source
to the atmosphere above; in the areas surrounding this heat source the ocean cools
the atmosphere from below (1999). Small scale (~100m) vertical convection is
believed to replace the warmer air above the heat source with cooler but more
importantly, drier and denser air creating and maintaining a relatively high pressure
area (Kenyon, 1999). More evidence is necessary to substantiate such a hypothesis
but it illustrates a need to better understand the dynamics over the ocean in the NPH.
The climatic conditions that form and sustain the high pressure in the North Pacific
are the foundation for understanding why this pressure system fluctuates on various
timescales.
The NPH significantly influences the climate in the North Pacific as a
component o f the receding limb of Hadley cell circulation, the “atmospheric bridge”
(Figure 3) (Schwing et al., 2002). Through Hadley circulation, the Northeast Pacific
6
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is linked to the climate changes outside o f the North Pacific, particularly the tropics
(Emery and Hamilton, 1985; Schwing et al., 2002). This atmospheric connection
Hadley Cell Circulation
48” N
Figure 3. The Atmospheric Bridge in the Pacific Ocean (Schwing et al., 2002).
allows for anomalies within the tropical Pacific to influence the oceans o f the
extratropics from a distance (Alexander et al., in press). The NPH, in particular,
plays an important role in propagating the influence o f the atmospheric bridge
throughout the Northeast Pacific region (Schwing et al., 2002). The teleconnection
between the tropics and extratropics is a “wind-driven meridional circulation”
resulting in shifts in ocean circulation in the Northeast Pacific (Gu and Philander,
1997). Anomalous position or strength o f the NPH is transferred onto regional winds
and via those winds to ocean currents and conditions. This provides a link between
anomalous patterns in the climate within the Northeast Pacific back to the western
tropical Pacific (Schwing et al., 2002). Perturbations experienced within the tropics
have a mechanism via the atmospheric bridge to influence remote systems such as
7
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the North Pacific and, in particular, the Santa Barbara Basin. This influence of
remote regions on our area o f interest necessitates the use o f a global climate model
for our simulations.
II. b. Remote Influences
As discussed, a significant influence on the North Pacific High originates
from the variability o f SSTs in the tropical Pacific and is transferred to the higher
latitudes via the atmospheric bridge. The atmospheric bridge exists as a result of the
pressure gradient between these two regions allowing each region to influence the
other (Lau and Nath, 1996, 2001) The tropics influence the extratropics but are in
turn influenced by the extratropics. Conditions in the tropics clearly impact the
extratropics but recent studies indicate that the influence o f the extratropics on the
tropics may be just as strong (Liu and Yang, 2003). Approximately a fourth to one-
half o f the SST variability that occurs in the North Pacific can be attributed to the
perturbations in surface pressure associated with the atmospheric bridge (Alexander
et al., 2002; Latif and Barnett, 1994). Wind perturbations that form as a result of
anomalous pressure zones allow Rossby waves to propagate over the mid-latitudes
(Figure 4) (Alexander et al., 2002; Alexander et al., in press). This creates a
“wavelike” atmospheric response throughout the North Pacific and North America
(Lau and Nath, 1996). The atmosphere and its winds spread the influence o f the
anomalous pressures regulated by the atmospheric bridge through the region. These
winds influence local surface ocean circulation via Ekman transport that alter SSTs.
8
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The atmosphere systematically responds to the perturbations in mid-latitude SSTs,
propagating the influence of the atmospheric bridge throughout the North Pacific
(Latif and Barnett,
1996; Lau and Nath,
2001). Anomalies in
the tropics are
translated into the
extratropics as
changes in wind,
temperature, cloud
cover and
Sea-Air
Feedback
“The Atmospheric Bridge"
Atmosphere
Winds, temperature,
moisture, clouds
Qnet' W „ V *
ENSO
45° N Equator
SST
SSS
MLD
Pacific Ocean
Figure 4. The influence of the atmospheric bridge on higher
latitudes (Alexander et al., 2002).
atmospheric moisture (Alexander et al., in press; Schwing et al., 2002). The
temperature gradient between the tropics and extratropics is important to the overall
strength o f Hadley Cell circulation. Increased extratropical SSTs, potentially
resulting from global warming, will reduce this gradient and weaken the atmospheric
bridge (Liu and Yang, 2003). The balance o f atmospheric and oceanic feedbacks
between these two regions is important to the maintenance o f the climate patterns
presently seen in the Northeast Pacific. Changes in the temperature gradient can lead
to anomalous conditions in both regions via their ocean and atmosphere
teleconnections.
9
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II. b.l. El Nino/La Nina
ENSO events, which significantly perturb the climate state o f the tropical
Pacific, are translated into perturbations in the extratropical Pacific. These
perturbations are generally expressed as shifts in regional winds and, ultimately,
coastal SSTs in the Northeast Pacific. The atmospheric bridge provides a mechanism
that allows tropical events such as the El Nino to affect the extratropics (Emery and
Hamilton, 1985; Lau and Nath, 2001). Many warm/cool SST events within the
California Current System can be linked to El Nino/La Nina events in the tropics
(Mendelssohn et al., 2003). The tropical atmosphere responds to changes in the SSTs
of the Tropical Warm Pool as controlled by variability in heat flux via equatorial
winds, moisture, and cloud cover (Alexander et al., in press; Liu and Yang, 2003).
The shift in location and intensity o f the SSTs in the tropics in turn influences the
strength and position o f the NPH via the atmospheric bridge. Surface fluxes are also
important to the distinct signal created in the North Pacific during an El Nino event
(Lau and Nath, 2001). The SSTs o f the North Pacific are directly affected by changes
in net heat flux and indirectly via changes in local winds created by anomalous
pressure gradients which influence fresh water fluxes contributing to ocean currents
and subsequent coastal upwelling (Alexander et al., in press; Hayward et al., 1994;
Schneider et al., 2002). Warmer SSTs during El Nino events have been clearly seen
and recorded in Santa Barbara Basin (Lange et al., 1999). The reduced southward
flow o f the California Current contributes to this warmed signal during these events
(Bemis et al., 2002). Resulting SSTs within the extratropics are relatively sensitive to
10
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the conditions in the
tropics (Lau and
Nath, 2001) and can
be recorded in the
coastal sediment
proxy record. In
particular how
ENSO events are
recorded within the
laminations in Santa
Barbara Basin is of
much interest (Figure 5) (Schimmelmann et al., 1990).
II.b.2. The Pacific (Inter)Decadal Oscillation
The Pacific (Inter)Decadal Oscillation (PDO) is also an important influence
on the mean climate state o f the North Pacific. The PDO is describes as the leading
pattern of variability o f SSTs in the North Pacific from 20°N to 60°N (Biondi et al.,
2001; Mantua et al., 1997). The mechanisms causes the patterns in climate that we
identify as the PDO are still unclear (Mantua, 2003; Mantua and Hare, 2002). The
PDO index is characterized by positive (warm) and negative (cool) phases as
indicated in Figure 6 and Table 1 (Mantua, 2003; Mantua and Hare, 2002; Mantua et
al., 1997). The positive (warm) phase o f the PDO is characterized by warmer SSTs
11
Diatom Abundances
1 0 0
so
III
1860
Figure 5. Relative abundances of diatom assemblages. "A"
indicates a warm water assemblage which peaks during El
Nino years (Schimmelmann et al., 1990).
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Pacific Dcjc& dsil Oscillation
Figure 6. Anomalous conditions associated with the warm phase of the PDO
(Mantua and Hare, 2002).
Summary of Pacific and North American dim.
associated with extreme phases of the
ate anomalies
PDO.
Climate Anomalies Warm Phase PDO Cool Phase PDO
Ocean surface temperatures in the
northeastern and tropical Pacific
Above average Below average
October-March northwestern North
American air temperatures
Above average Below average
October-March Southeastern US air
temperatures
Below average Above average
October-March southern US/Northern
Mexico precipitation
Above average Below average
October-March Northwestern North
America and Great Lakes precipitation
Below average Above average
Northwestern North American spring
time snow pack and water year
(October-September) stream flow
Below average Above average
Winter and spring time flood risk in the
Pacific Northwest
Below average Above average
Table 1. Climate Conditions associated with the warm and cool phases of the PDO (Mantua,
2003).
12
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in the Northeast Pacific, associated with reduced upwelling. Cooler SSTs in the
Northeast Pacific generally occur during the negative (cool) phase o f the PDO. The
shift in North Pacific climate recorded in the mid-1970s influenced changes in ocean
mixing, heat flux, and Ekman transport (Mantua et al., 1997; Schneider et al., 2002).
On longer timescales such “shifts” have been related to atmospheric forcing (Lau
and Nath, 1996), in particular to changes in the strength and position of the NPH in
the region. The final two decades o f the 20th century were dominated by El Nino-like
conditions, included muted coastal upwelling and warmer SSTs, despite a lack o f
concurrent El Nino events in the tropics (Mantua and Hare, 2002). Both ENSO
events and a decadal-scaled PDO have a major climate influence within the North
Pacific (Dean and Kemp, 2004). The conditions that the PDO produces in the North
Pacific are very similar to those o f ENSO (Mantua, 2003; Mantua and Hare, 2002;
Seager et al., 2001) and has been characterized as “ENSO-like interdecadal climate
variability” (Mantua et al., 1997).
Despite various similarities, these two climate oscillations are distinct in their
expressions and timescales. The PDO oscillates on a much longer, decadal to
interdecadal, timescale than ENSO (Liu et al., 2002; Mantua and Hare, 2002).
Although the effects o f ENSO events are transmitted to both hemispheres via the
atmospheric bridge, the anomalies produced by the PDO are generally restricted to
the North Pacific (Table 1, Figure 6) (Mantua, 2003). There are no strong signals
associated with the PDO in SST data from either the Atlantic or Indian oceans
(Mantua et al., 1997). The PDO signal is strongest in the extratropical North Pacific
13
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where as ENSO is more strongly located within the tropics but these indices are not
completely independent as a result o f overlap in the regions they describe (Lluch-
Cota et al., 2001). Significant anomalies characterized by a shift in the PDO are
generally restricted to the Pacific ocean between 20°-60°N (Biondi et al., 2001;
Mantua, 2003; Mantua et al., 1997). The coastal region north o f approximately 33°N
is more greatly impacted by the PDO than ENSO, which is more important to the
south (Lluch-Cota et al., 2001). The anomalies experienced for the most part are
associated with changes created by the changes in the position o f the Aleutian Low
(Dean and Kemp, 2004). The strength and position of the Aleutian Low has the
potential to influence the strength and position o f the NPH. The surface anomalies
generated during shifts in the PDO are consistent with changes in circulation on a
hemisphere scale (Mantua et al., 1997).
Although these climate regimes act on different timescales with varying areas
of prominent influence, there is a link between them. It is understood that the PDO
modulates the influence o f ENSO in the extratropical Pacific. Stronger responses in
the extratropics are observed when the PDO and ENSO are in phase, and weaker
when they are out o f phase (Biondi et al., 2001). A time period characterized by an
El Nino event will generally experience warmer SSTs off o f Southern California’s
coast if the region is experiencing the warm, as opposed to cool, phase o f the PDO.
The particular influence o f the PDO on the climate of the North Pacific was clearly
demonstrated during the 1976 shift that intensified the warming o f the coastal
surface waters (Mendelssohn et al., 2003). Since the middle o f the 20th century there
14
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has been an increased significance in the PDO and its effect on the North Pacific
climate (Moore et al., 2002). A more “energetic” PDO regime has coincided with the
increase in ENSO variability in the past two centuries (Biondi et al., 2001). The
cause for this is still unclear but important for understanding the relationship
between natural and anthropogenic influences. O f particular importance is the
interaction between ENSO and the PDO within the North Pacific. The influence of
these two climate systems occurs over a similar area in the North Pacific and
interaction would result in a regional signature o f anomalies (Lluch-Cota et al.,
2001). Atmospheric forcing associated with the PDO has been associated with the
variability o f SSTs in the North Pacific in both the western and eastern coastal
regions (Schneider and Miller, 2001). The magnitude o f variability also differs, for
the PDO the magnitude o f variability is similar in both the tropics and extratropics
but the magnitude o f tropical variability dominates ENSO events (Seager et al.,
2001). Also, the mechanisms which produce and control the PDO are still uncertain
although work continues (Mantua and Hare, 2002) in an attempt to better understand
this climate system.
ll.b.3. Global Warming
Questions have arisen with respect to what effect climate warming in the
latter part o f the 20th century has had on the position and strength o f the NPH. The
importance o f the temperature gradient between the tropics and the extratropics in
determining the strength o f Hadley cell circulation would indicate a warmer climate
15
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could weaken and shift the NPH southward. The 1920s also was a time of rapid
global warming. The high-pressure systems in both the North Atlantic and North
Pacific weakened during the 1920s concurrent with the strengthening of the Aleutian
Low (Fu et al., 1999). In the 1930s and 1940s, the Aleutian Low returned to a more
northerly and westerly position (Fu et ah, 1999). The warming during the 20t h
century has been marked by a “more energetic” PDO and an increased ENSO
variability (Biondi et ah, 2001) consistent with a weaker NPH in the North Pacific.
The warming associated with the latter part o f the 20th century may have influenced
the significance o f the influence o f the PDO on regional ocean conditions such as
SSTs (Moore et ah, 2002). The PDO regime shift experienced in the mid-1970s
seems to have been intensified by a warming that had begun a few years prior
(Mendelssohn et ah, 2003). Considering the numerous feedbacks both positive and
t h
negative in the Northeast Pacific, it is difficult to isolate the influence the 20
century would impose on the NPH but it is clear that there is an influence.
III. Decadal Variability
Although variability on shorter, interannual timescales has a significant
influence on the North Pacific climate, longer-term variability at decadal and
interdecadal timescales, can also have a major impact on the coastal regions of the
Northeast Pacific. Decadal and interdecadal variability in the North Pacific oscillates
on 20- to 30-year and 50- to 70-year periodicities (Latif and Barnett, 1996; Liu et ah,
2002). The variations that occur on these timescales can manifest themselves as a
16
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gradual drift, an oscillation, or a step-like shift (Miller and Schneider, 2000). The
actual mechanisms that maintain a decadal-scaled variability is still unclear. Local
processes, stochastic forcing and the interaction between ocean and atmosphere have
all been credited with influencing the timescales o f the climate recorded in the North
Pacific (Gu and Philander, 1997; Latif and Barnett, 1996; Liu et al., 2002; Miller and
Schneider, 2000). The simplest o f these theories indicates that the length of
variability in the North Pacific is regulated by stochastic forcing (Miller and
Schneider, 2000). Stochastic climate models have shown that it is possible to achieve
decadal-scaled variability in the North Pacific without an ocean feedback system
(Miller and Schneider, 2000). Stochastic forcing within the extratropics causes the
transition o f climate oscillations from one phase to another to be more gradual (Gu
and Philander, 1997), lengthening the timescale o f that variability.
Different modeling studies have shown that much o f the decadal variability
o f the North Pacific is produced within the region via local processes (Liu et al.,
2002). For example, the effect o f surface wind anomalies on SSTs may not
immediately be observed and may lag upto five years (Seager et al., 2001). Climate
modeling studies have concluded that the ocean-atmosphere interactions are in some
way contributing to the longer scaled variability in this region (Latif and Barnett,
1996). The transfer o f energy from the atmosphere to the ocean can be inefficient,
creating a longer lag between a perturbation and its response. In the mid-latitudes
ocean anomalies can lag changes in wind from approximately 5 to 10 years (Miller
and Schneider, 2000; Schneider et al., 2002). This delay between the atmosphere and
17
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the ocean forcing is related to “internal atmospheric variability” and the high heat
storage capabilities o f the mixed layer in the surface ocean (Schneider et al., 2002).
The ocean’s capacity to store a vast amount o f energy decreases the ability o f the
atmosphere to efficiently transfer energy to the ocean. Feedbacks between the ocean
and the atmosphere act to lengthen the timescales o f variability within the North
Pacific. In particular, the interactions associated with Ekman transport and wind
stress curl can excite Rossby waves that take years to cross from the western to
eastern boundaries o f the North Pacific (Alexander et al., 2002; Miller and
Schneider, 2000; Schneider and Miller, 2001).
Energy that is transferred from the atmosphere to the ocean introduces a
delay that is decadal, which is the timescale associated with the dynamics o f energy
transfer within the ocean (Schneider et al., 2002). The majority o f the “memory” of
the ocean-atmosphere coupled system is within the ocean; it reacts and adjusts
slowly to perturbations (Latif and Barnett, 1996). Processes within the ocean
involving the ocean’s heat budget function on a decadal timescale (Schneider et al.,
2002). This relationship resembles ENSO variability but on a longer timescale. SST
anomalies outside o f the tropics occur approximately 2 to 6 months after a tropical
shift (Alexander et al., 2002). The atmosphere requires a few weeks to respond to the
original SST anomaly in the tropics and the ocean in the extratropics incorporates the
forcing from the atmospheric bridge over the next months (Alexander et al., 2002).
Ocean teleconnections also work towards lengthening variability across the North
Pacific, such as coastally trapped waves that move poleward along the Eastern
18
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Pacific boundary (Schwing et al., 2002). The slow movement o f the water has the
potential to increase the time needed to influence a significant area o f ocean to
decadal and interdecadal timescales.
IV NPH influence
The influence o f the NPH on the climate o f the Northeast Pacific is the focus
o f this study. It is important, therefore, to understand exactly how the strength and
the position o f the high-pressure system affects coastal California winds, currents,
and sea surface conditions. Shifts in the strength or position o f the NPH would
primarily lead to changes in the direction and strength o f the local winds. During an
El Nino a relatively stronger Aleutian Low forces strong cyclonic wind flow in the
Northeast Pacific which, as a result o f Ekman transport, warms the surface waters
along the Californian coast (Alexander et al., in press; Miller and Schneider, 2000).
Concurrent with the 1976 shift in the PDO, the Aleutian low again shifted into a
southward position causing anomalous cyclonic wind flow (Seager et al., 2001). At
the same time, the NPH weakened and spring winds also weakened resulting from
the decreased pressure gradient between the ocean and continent. Ekman transport,
as a result o f the altered winds, contributed to a shift in the surface currents and
upwelling along the coast. Alongshore coastal stress, produced by wind stress,
induces up- or downwelling depending on whether the surface currents are being
moved off- or onshore (Murphree et al., 2003). The upwelling that influences the
SSTs also regulates sea surface salinity (Mendelssohn et al., 2003) and determines
19
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the depth o f the mixed layer due to the vertical transfer o f deep, cold ocean water.
The coastal warming occurred with the 1976 PDO shift (Mendelssohn et al., 2003)
can be explained by cyclonic winds the created conditions unfavorable to upwelling
along the Californian coast giving rise to a strong link between the NPH and the
resulting coastal SSTs in the Northeast Pacific.
The fluctuations in the NPH can also influence SSTs in the Central North
Pacific. The Aleutian Low generates temperature anomalies in both the central and
eastern Pacific where the SST patterns tend to be o f opposite sign (Miller and
Schneider, 2000). If the low-pressure system strengthens, the westerlies in the
Central North Pacific increase and local SSTs cool (Miller and Schneider, 2000). A
large area o f cool surface water has been situated over the Central North Pacific
during the 20 years since the 1976 climate shift (Mantua et al., 1997). This is in
contrast with the warmer waters that have characterized the Northeast Pacific during
this same interval (Mendelssohn et al., 2003). Ultimately, heat is being transferred
throughout the Pacific via the perturbations that are creating the anomalies both in
the ocean and in the atmosphere. Surface winds and temperature are important to the
sensible and latent heat exchanges occurring in both the extratropics and tropics (Lau
and Nath, 1996) and to the formation o f the SST anomalies that feedback onto the
surface fluxes (Alexander et al., 2002; Lau and Nath, 2001). Ultimately, in
examining how the climate o f the North Pacific adjusting to various perturbations,
we are coming to understand how the climate is equilibrating and redistributing heat
throughout the North Pacific.
20
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V . Coastal California
The pressure gradient between the NPH and the low-pressure system situated
over the western United States forces, during the spring, the coastal conditions by
influencing the velocity and direction o f regional and local winds. During the
summer months the gradient between these two pressure systems is at a maximum
(Harms and Winant, 1998). The increased pressure gradient sustains a stronger anti-
cyclonic flow over the Northeast Pacific, bringing strong southward winds through
the California Borderlands in the spring. These local wind patterns are translated into
surface ocean currents via Ekman transport. The ocean responds to atmospheric
anomalies, including sea level pressure (Emery and Hamilton, 1985). The
relationship between the local winds and surface ocean currents influence Ekman
pumping and upwelling (Murphree et al., 2003). The coastal currents are well
correlated with the winds during the spring months (Harms and Winant, 1998; Lau
and Nath, 2001).
Coastal ocean conditions that are o f particular interest in our study include
SSTs and sea surface salinities (SSSs). The surface waters o f the California
Borderlands, including Santa Barbara Basin, are a composite o f distinct waters
masses originating from difference regions with particular signature properties.
Warm, high-salinity, pole-ward moving waters flow north and mix with cold, low-
salinity California Current System (CCS) waters (Figure 7) (Harms and Winant,
1998; Hendy and Kenneth, 2000; Kincaid et al., 2000; Lynn et al., 2003). In
particular, the surface conditions within Santa Barbara Basin are forced by the
21
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upwelling off o f Point Conception and saltier waters o f the California Countercurrent
(Pak et al., 2004; Shipe et al., 2002). The characteristics o f the surface mixed layer
depend on the seasonal wind patterns and the interaction between the cold California
Current and the warm
California Countercurrent
(Friddell et al., 2003). The
variable strength o f these two
currents and their influence on
the Santa Barbara Basin
depends on the regional winds.
During the spring the northerly
winds strengthen the southward
flow o f the California Current
and upwelling increases (Bemis
et al., 2002; Kincaid et al.,
2000; Lynn et al., 2003), visible
in the coastal SST records once
the cold water has filled most o f the coastal region (Capet et al., 2004). The seasonal
changes in the regional wind direction and strength controls, in turn, the strength o f
the California Current (Friddell et al., 2003; Thunell, 1998). Examining Santa
Barbara Basin’s temperature and primary productivity, it is clear that the upwelling
season extends from April to June (Figure 8) (Thunell et al., 1995). During the spring
22
Major Surface Currents
i w elling1
P93A
Figure 7. Surface ocean circulation through the
Santa Barbara Basin (Hendy and Kenneth, 2000).
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months the currents are well correlated with the strength o f the local winds but in
later seasons the relationship is not as distinct (Harms and Winant, 1998). In the
Monthly Temperatures for the Surface Ocean
S
€
o .
a
s
I S
I
75 -
1905 1993 19%
Figure 8. Subsurface temperatures for Santa Barbara Basin (Thunell, 1998).
spring upwelling season the current becomes a mixture o f the low salinity waters
from the north and the upwelled, higher salinity water from the deep ocean (Lynn et
al., 2003). Wind stress is an important factor for surface flow in the Santa Barbara
region that is exposed to the wind throughout the year (Harms and Winant, 1998).
The SST anomalies within the basin are an indication o f the local changes in coastal
upwelling as well as the influence o f the larger scaled atmospheric and ocean forcing
(Schwing et al., 2000). Seasonal variations in the wind stress curl strongly influence
the temperatures just below the thermocline (Miller and Schneider, 2000; Murphree
et al., 2003). Upwelling o f nutrient-rich water directly influences the amount of
primary productivity, which in turn, influences the particulate flux to the ocean
23
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bottom. In Santa Barbara Basin the particulate flux is seasonal with the largest
biogenic flux occurring during the spring and summer (Shipe et al., 2002). The
seasonal fluxes strongly influence the sediment archive. Hence, samples of
sediments will be primarily recording the upwelling season.
The presence o f La Nina- or El Nino-forced conditions disrupts the seasonal
cycle by affecting the initial regional winds. The SSTs o f the North Pacific are
significantly sensitive to ENSO forcing in the tropics (Lau and Nath, 2001). Most of
the significant warm or cool subsurface temperatures in this region can be linked to
El Nino or La Nina events with longer decadal variability evident as well
(Mendelssohn et al., 2003). During the 1999 La Nina, the stronger NPH created
faster than normal anti-cyclonic wind in the region leading to increased upwelling-
favorable conditions along the coast (Schwing et al., 2000). El Nino events also have
the potential to influence large-scale atmospheric pressure patterns and via these
changes can influence the strength o f upwelling, sea level, SSTs and the depth o f the
thermocline (Hayward et al., 1994). The strong cyclonic flow in the North Pacific
during an El Nino cools the central North Pacific while warming the Northeast
Pacific SSTs (Alexander et al., in press). The polarity between the central and
eastern North Pacific is common with respect to seasonal and anomalous variability
(Miller and Schneider, 2000). The flow o f the California currents weakens during El
Nino events (Bemis et al., 2002; Thunell, 1998). During the 1997-98 El Nino,
upwelling was considerably depressed when the SSTs recorded a sharp increase in
the coastal regions along with a decrease in primary productivity and an increase in
24
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rain (Friddell et al., 2003; Schwing et al., 2002). ENSO influences the coastal
regions southward o f Southern California whereas the PDO has a greater influence
northward o f this region (Lluch-Cota et al., 2001). The influence that the PDO has on
the California Borderlands is consistent with hemisphere-scaled circulation
anomalies (Mantua et al., 1997) and is similar to the changes seen in relation to
ENSO variability including the warming in the Northeast Pacific (Schneider et al.,
2002). By initially perturbing the regional surface pressure, both ENSO and the PDO
have a significant influence on the local SSTs during these climate shifts.
VI. Santa Barbara Basin - Geologic Setting
Santa Barbara Basin Bathymetry
Basin Bathymetry in meters
_ —
20 km
Santa Barbara
300’
-M C I
.ir t r i
u r n
Figure 9. Santa Barbara Basin bathymetry with multicore locations indicated.
The Santa Barbara Basin is one o f several silled basins within the California
Borderlands (Friddell et al., 2003). The basin is located in a region in which cold
upwelled waters from the north mix with warmer subtropical waters from the south
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(Harms and Winant, 1998). The center o f the basin is approximately 600m deep with
sills at 475m and 230m depths at the western and eastern boundaries, respectively
(Figure 9) (Lange et al., 1999; Shipe et al., 2002). The sills prevent the direct
ventilation o f the basin bottom waters below its sill depth (Lange et al., 1999). This
results in suboxic conditions that precluded the formation and preservation of
laminated sediments within the basin. The restricted ventilation is important to the
preservation o f the sediment and proxy record within the basin but is not the sole
factor influencing the bottom water concentration o f oxygen. Although the intensity
o f the OMZ (Oxygen Minimum Zone) has been linked to changes in the ventilation
o f the basin (Lange et al., 1999; vanGeen et al., 2003) the changes seen within the
California Borderlands indicates that another process may be a factor. The oxygen
concentrations in the bottom waters o f Santa Barbara Basin are dependant on the
amount o f oxygen entering the basin along with the residence time and consumption
rates via carbon oxidation within the basin (Stott et al., 2000b). Changes in export o f
organic carbon to the bottom waters resulting from changes in surface production
can affect the concentration o f oxygen over time suggesting that the supply of
organic material to the basin bottom plays an important role in both the formation
and preservation o f the laminated sediment in Santa Barbara Basin (Stott et al.,
2000a; Stott et al., 2000b; vanGeen et al., 2003).
This preservation along with high sedimentation rates characterizes the Santa
Barbara Basin as a useful site for high-resolution records o f hydrological conditions
in this area. High-resolution records necessary for annual to century scaled ocean
26
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studies are best recorded in coastal margins which produce and preserve varved
formations (Thunell et al., 1995). The most detailed records are formed in varved
sediments maintain in an anaerobic environment (Schimmelmann et al., 1990). Near
annual resolution records occur in the basin as a result o f the high sedimentation
rates, averaging approximately 4mm/year (Lange et al., 1999; Schimmelmann et al.,
1990). Preserving the sediment record is as important as initially creating the record.
Beggiatoa mats at the sediment-water interface serve to trap and protect material
traveling through the water column to the basin floor (Lange et al., 1999; Soutar and
Crill, 1977).
Varved sediments are couplets that represent the seasonal to annual cycle
within a basin and are generally preserved in
an oxygen-depleted environment (Figure 10)
(Lange et al., 1999; Reimers et al., 1990).
The sediment within the basin is primarily
composed o f olivine-rich sediments that are
laminated (Lange et al., 1999). The
composition and concentration o f material
sinking to the Santa Barbara Basin’s floor is
highly seasonal. The sediment accumulation
rates within the basin tend to be highly
correlated to the seasonal distribution o f rainfall (Soutar and Crill, 1977). A rainy
winter season results in greater terrigenous flux to the basin whereas during the
27
Varved Sediments from Santa
Barbara Basin
Figure 10. X-radiograph of varved
sediments from NH01-3-MC13.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
spring months there is a greater biogenic contribution. The different colors o f the
laminae indicate differences in density and composition o f the sediments deposited
seasonally (Reimers et al., 1990; Thunell, 1998). The dark laminae are composed of
terrigenous material transported by rivers; the light colored laminae are associated
with the spring and early summer upwelling season flux o f the biogenic material
(Thunell, 1998; Thunell et al., 1995). There is a strong relationship between the local
rainfall and the varve thickness; during the winter, higher rainfall introduces more
terrestrial material into the basin via rivers (Lange et al., 1999; Thunell, 1998).
In Santa Barbara Basin, the best preserved varved records are found at the
very center o f the basin where the water is calmest and deepest, the surface is flattest
and where the oxygen concentrations are at a minimum (Lange et al., 1999). The
cycle o f oxygen depletion and replenishment is a significant influence on the
formation and preservation o f these laminated sediments (Reimers et al., 1990).
Together with high sedimentation rates, low oxygen concentrations lead to the
preservation o f these detailed marine records (Bemis et al., 2002). The
concentrations o f oxygen in the bottom water is less than 0.2ml/l (Schimmelmann et
al., 1990). Seasonal flushing o f the basin does not seem to disrupt the laminated
sediments in the basin (Shipe et al., 2002). The limited concentrations of oxygen are
further depleted by limited amounts o f biodegradation o f organic material that can
occur in such a stressed environment (Lange et al., 1999; Stott et al., 2000a; Stott et
al., 2000b; vanGeen et al., 2003). The demand for oxygen at the sediment-water
interface coupled with degradation of organic material leads to minimal
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concentrations o f oxygen, which in turn suppressed bioturbation (Thunell, 1998).
This depressed bioturbation acts to aid in the preservation o f the laminated
sediments, protected from benthic organisms that may flourish in a more oxygenated
environment. The lack o f burrowing organisms is necessary for the preservation of
laminated features in the center o f the basin where large bioturbators are rare (Lange
et al., 1999; Soutar and Crill, 1977).
The Santa Barbara Basin is sensitive to shifts in climate centered in the North
Pacific (Bemis et al., 2002) primarily as a result o f its position along the California
coast. It has been recognized that the sediments deposited in anaerobic environments
are a important record o f “broader climate events” that impact the atmospheric and
oceanic conditions throughout the Pacific (Soutar and Crill, 1977). The strong El
Nino events are markedly recorded within the basin’s sediment record (Shipe et al.,
2002). Changes in diatom assemblages have been used down-core in response to El
Nino events have been used as a tool to aid in the dating o f core horizons
(Schimmelmann et al., 1990). The influence o f the regional coastal winds connects
the basin to the larger-scaled North Pacific variability.
VII. Mg/Ca Paleothermometry
Vila. Globigerina bulloides
Globigerina bulloides is a predaceous, non-symbiont bearing, planktonic
foraminifera that is frequently characterized as an upwelling indicator (Kincaid et al.,
2000) (Figure 11). The species dominates sediment trap assemblages throughout the
29
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year in Santa Barbara Basin but generally has its highest fluxes in association with
periods o f upwelling and diatom blooms and
accounts for approximately 42% o f the total
foraminiferal flux from the surface ocean
(Kincaid et al., 2000; Pak et al., 2004). This
strong seasonal pattern is well recorded by this
species in sediment traps (Pak et al., 2004). G.
bulloides is also useful for proxy records because
o f its extensive distribution, both spatially and within various environments
(Puechmaille, 1994) It is important to note that only those organisms with hard shells
or other parts resistant to dissolution will be preserved in the sediment record and
therefore the sediment may not record a true indication o f what the living assemblage
o f the basin is (Lange et al., 1999). Sediment trap data completed within the Santa
Barbara Basin indicates that the maximum primary production occurs between April
and June although a lag time o f a month can occur before the material reaches the
basin floor (Thunell et al., 1995). The foraminifera are recording the seasonal and
interannual variability within the environment in which their calcite shells are
precipitating (Kincaid et al., 2000). Some sediment trap studies indicate that
although the greatest flux o f G. bulloides reaches the sea floor during the spring
upwelling, the core tops analyzed indicated that the foraminifera may be recording
annual instead o f seasonal SSTs (Pak et al., 2004). Comparison o f this study’s proxy
record to instrumental records is necessary to determine which season or time period
30
Globigerina bulloides
Figure II. Globigerina bulloides
from the University of
Southampton SOES Fossil
Collection.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
is being recorded downcore. Ensuring the preservation o f the calcite shells, G.
bulloides within the Santa Barbara Basin has the potential to record a high-resolution
history o f the upwelling season.
VII. b. Mg/Ca
Magnesium (Mg) is a conservative element with an approximately fixed ratio
to calcium (Ca) within the ocean with no fractionation between the ocean basins
(Lea, 1999; Niirnberg et al., 1996). These elements are precipitated into the calcite
shells during precipitation (Lea, 1999). Variability o f the ratio o f trace elements to
calcium within foraminiferal shells is related to the environmental changes in the
water in which the calcite is precipitated (Lea et al., 1999; Puechmaille, 1994).
Plankton tow studies have shown that once the calcite is precipitated as shell there
are no more biological influences while the material travels from the surface to the
sea floor (Puechmaille, 1994). In particular, Mg/Ca ratios as a paleothermometer
have gained increasing attention because o f the various limitations o f other proxies
for temperature currently available (Mashiotta et al., 1999).
The potential o f the proxy lays in the reproducibility o f Mg/Ca ratios along
with the exponential relationship between Mg/Ca in the calcite and temperature (Lea
et al., 1999). Temperature is the primary influence o f Mg/Ca in foraminiferal shells
with salinity and pH having a minimal effect (Lea, 1999; Lea et al., 1999; Mashiotta
et al., 1999; Niirnberg et al., 1996). The response of Mg/Ca rations within the calcite
shells is approximately 10% per 1°C change in ambient temperature (Pak et al.,
31
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2004). Species-specific relationships also allow for a more accurate
paleothermometer (Bemis et al., 2002; Lea, 1999) with a precision o f 1.1 °C (Lea et
al., 1999). The relationship used in this study was empirically derived from culturing
and core-top studies.
Mg/Ca = 0.474e0 107 x tem Perature (Figure 12) (Mashiotta et al., 1999).
Difficulties with culturing G. bulloides in the laboratory made necessary the use of
core tops samples to
calibrate the relationship
below 16°C (Pak et al.,
2004). N ew studies
utilizing sediment traps
have verified that the
above relationship is
valid down to 10°C (Pak
et al., 2004). As a result
o f the exponential
relationship, foraminifera
show greater shifts o f concentrations in warmer waters (Puechmaille, 1994). As an
independent record o f temperature, Mg/Ca can be utilized together with stable
isotope geochemistry to generate a more complete record o f hydrology within a
location. These two proxies can be measured on the foraminiferal shells from the
same sediment horizon allowing for a direct comparison o f results (Mashiotta et al.,
32
Globigerina bulloides
Mg/Ca
(mmol/mol)
Mg/Ca = 0.474 e°
R2 = 0.98
8 tO 12 14 16 18 20 22 24 26
Temperature (°C)
Figure 12. Relationship between G. bulloides' shells and
ambient water temperature (Mashiotta et al., 1999).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
1999; Pak et al., 2004). This adds to the usefulness o f this paleoproxy for more
encompassing studies o f marine environments.
There are various complications to the use o f Mg/Ca paleothermometry
including the other influences on the ratios recorded in the calcite shells and
preservation o f the material analyzed. Shell formation can be influenced by external
forces that cause the calcite to be precipitated out o f equilibrium with the ocean
water (Lea, 1999). If these influences can be excluded then Mg/Ca ratios can be
utilized to reconstruct ocean temperatures (Niirnberg et al., 1996). Although extreme
variations in salinity can influence the proxy’s signal, small and natural shifts do not
significantly affect the Mg/Ca ratio (Niirnberg et al., 1996). The incorporation o f pH
and salinity effects on the Mg/Ca ratios increases the uncertainty associated with this
proxy from 1.1 °C to 1.3°C (Lea et al., 1999). Alternatively, it is important to
consider how the foraminiferal shell is formed, a chamber added to the end o f the
shell one at a time. Trace elements are not incorporated homogeneously throughout
the shell (Lea, 1999). Small differences can be measured from the inner and outer
layers of shells analyzed o f G. bulloides (Puechmaille, 1994). The Mg concentrations
can be very heterogeneous within the shell o f this species (Elderfield and Ganssen,
2000), particularly the final chamber, generated during gamteogenesis which tends to
be enriched in Mg (Niirnberg et al., 1996). Another possible source o f complications
with the use o f this proxy is the influence o f the foraminiferal life cycle in which the
individuals move throughout the surface waters potentially recording temperatures of
depths up to 20-40 m (Pak et al., 2004). This can be important when considering an
33
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offset that may occur between our proxy record and a comparable instrumental
record as a result o f a difference in depth o f record. Apart from these concerns, the
use o f multiple adult specimens in the analysis will give a better representative result
with respect to the temperatures recorded throughout the upwelling season.
Dissolution o f the calcite after it has been deposited is also an important
consideration in the use o f Mg/Ca ratios for paleothermometry. Dissolution can
significantly modify the concentrations o f Mg within the foraminiferal shells
(Niirnberg et al., 1996). Mg is preferentially removed when the calcite is dissolved
(Brown and Elderfield, 1996; Lea, 1999). The bottom water chemistry along with the
rain rate o f the calcite and the break down o f the organic carbon in the sediment all
contribute to the dissolution o f the shells (Brown and Elderfield, 1996). The calcium
compensation depth (CCD) within the basin is an important indicator o f how well
preserved samples can be expected to be. Mg rich shells tend to occur in shallower
sites because o f the greater dissolution at depth (Elderfield and Ganssen, 2000). For
the most part G. bulloides is assumed to be resistant to changes in Mg concentrations
as a result o f selective dissolution (Mashiotta et al., 1999). Examination o f the
specimens to ensure whole individuals, preferably with the delicate spines intact,
gives some assurance that dissolution is not influencing our results.
34
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VIII. M ethodology
V illa. Santa Barbara Basin Proxy Record.
In November 2001, as a part o f the CalMex cruise aboard the RV New
Horizon, laminated sediment cores were retrieved from the center o f Santa Barbara
Basin using a multi-corer (Figure 13). The use o f multi-cores preserves the sediment-
water interface and the bacterial mats that protect the upper layers o f the sediment
core as w ell as multiple samples from the same location. A core retrieved from
NH01-3-MC7 was dried out and then sliced longitudinally into slabs before being x-
radiographed to ensure that the material was
indeed laminated (Figure 13). A neighboring
core from the same deployment is the primary
source o f material for this study and was
sectioned onboard using a foot pump extruder.
The top 10 cm o f the core was sampled at an
approximate 1.1mm resolution and the lower 50
cm at a resolution approximately 3.3mm. The
sample depth assigned to each sample has an
estimate error o f approximately 1mm as
calculated by comparing the number o f samples
to the total length o f the multi-core.
Samples retrieved were washed using de-ionized water over a >63pm sieve.
Varve counting in Santa Barbara Basin is difficult because seasonal layers are
35
Varved Sediments
from Study Area
Figure 13. X-radiograph of
NH01-3-MC7.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
sometimes misidentified as annual layers (Schimmelmann et al., 1990) and so the
<63 pm sample portion was dried, weighed and stored for 210Pb analysis to date the
layers o f the core. G. bulloides were picked from the remaining sample using a >180
pm sieve ensuring the individuals being analyzed were adults. Excluding juveniles
from the sample gives some assurance that magnesium and calcium concentrations
within the shells are indicative o f the temperature o f the water in which the
community was living. Approximately 100-250 pg o f G. bulloides ’ shells (40-60
individuals) were mechanically cleaned with an added leaching process to ensure
that the shells were without contamination to influence the results. Samples from the
top o f the core were not originally cleaned with the additional leaching but samples
were re-analyzed using the leaching technique to ensure that the results were not
influenced by the hot PECh-NaOH bath. Once cleaned the shells were dissolved in a
weak acid solution and run on an Inductively Coupled Plasma Mass Spectrometer
(ICP-MS) to determine the concentrations o f magnesium and calcium in the solution.
Standards were run in between each sample to ensure a maximum certainty of
accuracy in our results.
Sediment samples from the upper 30cm o f NH01-3-MC7 were analyzed for
210Pb to reconstruct an age model for the core. The age o f a sample is determined by
■ j i a Tin
analyzing the activity o f Pb relative to its grand-daughter Po (Christensen et al.,
1994). The sediment horizons that provided samples to generate the age model are
indicated with open triangles in Figure 14. Counting errors were calculated to be
approximately 5%. This results in an uncertainty o f 0-2.5 years for the upper 10cm
36
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
o f the core and approximately 10 years down to 25cm. Below 25cm, the age model
was adjusted downcore to account for the compression that the sediment potentially
experiences at depth
1 within the core. The
o ./jsr
100 .
Age Model for NH01-3-MC7
200
?
e, 300
J g
C L
E
$ 400
500
600
700
2000 1950 1900 1850 1800
Figure 14. Age model reconstructed for NH01-3-MC7 using 2 1 0 Pb
and sample weights to account for compression downcore.
increase in average
sample density
deeper in the core is
used as a measure of
the compaction of
the deeper sediment
layers relative to the
surface sample
horizons. It is
calculated utilizing the total dry sample weights recorded when the samples were
initially washed. Incorporating core compaction in the age model allows for a more
realistic analysis o f the time represented by the multicore.
VIII. b. Model Description ( FOAM 1.5)
Previous studies on the climate dynamics o f the extratropics typically have
been conducted either in an atmospheric or oceanic model without fully coupled and
dynamic interaction between the two systems (Liu and Yang, 2003). In this study we
use a fully coupled, general circulation model, the Fast Ocean and Atmosphere
37
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Model (FOAM) Version 1.5. The atmospheric and oceanic components o f the model
were developed separately and a surface model couples the two components,
allowing communication between the atmosphere and ocean (Jacob, 1997). The
atmospheric model contains 18 vertical levels and resolution o f approximately 4.5°
latitude by 7.5° longitude, a parallel version o f the NCAR-CCM2 (Community
Climate Model 2) (Liu and Yang, 2003). The ocean model is comprised o f 24
vertical levels with a 1.4° by 2.8°, latitude and longitude, resolution developed from
the GFDL MOM (Liu and Yang, 2003). The model includes a thermodynamic sea-
ice model but does not include sea-ice dynamics. The control run simulation models
present day climate using current vegetation distributions, ice sheet cover,
topography, and bathymetry. Modern GHG concentrations were set to the values
indicated in Table 2 as present day levels. Ozone concentrations in the control run
reflect the seasonal
hole that appears
over Antarctica in
the winter months.
The Preindustrial
run maintains
modern vegetation
distributions, ice sheet cover, topography, and bathymetry but lowers GHG
concentrations to those prior to the industrial revolution in the late 1800s. Ozone
concentrations are increased, in the preindustrial run, throughout the troposphere and
38
Atmospheric Gas Concentrations
Preindustrial Present Day/Control
C 0 2 (ppm) 280 360
CH4 (ppb) 700 1720
N 2O (ppb) 275 310
Ozone
Distribution from
(Martinerie et al., 1995)
Distribution from
(Martinerie etal., 1995)
Table 2. Parameters for two model simulations
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
stratosphere, particularly over the Southern Hemisphere indicating more complete
atmospheric cover even during the winter. To test how an increase in greenhouse
gas concentrations influence the climate in the Northeast Pacific, two model runs
were initiated and simultaneously rim. Each model was run for 2100 model years
with the initial 500 years allocated for “spin up” time. This ensures that both the
atmosphere and the ocean are circulating and interacting realistically.
IX. Results and Discussion
IX. a. Instrumental Records
Sea surface temperature record at the Scripps and Santa Barbara Piers
comprise the instrumental records used in this study. Both records are limited in
length, approximately 100 and 50 years long, respectively. Despite the relative
brevity o f the records, both are still useful for calibrating what the Mg/Ca
temperature proxy is recording in the region. Figure 15 demonstrates that both piers
are experiencing and recording similar shorter and possibly longer-scaled variability
in spite o f an average 1.35°C offset between the two sites. The offset in temperature
experienced between the two sites is consistent with their relative positions in the
California Current system. The southerly situated Scripps Pier is further removed
from the site o f most intense seasonal upwelling as well as the source o f the colder
surface waters originating in the higher latitudes o f the Pacific Ocean and so tends to
record warmer temperatures. Despite this offset Scripps Pier is nevertheless
recording the pattern o f SST variability experienced in Santa Barbara Basin for the
39
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period o f record
overlap. Although
it would be ideal
to have a longer
instrumental
/— s
O
record to compare j
20
19
1 8
17
Instrumental Records in the California Borderlands
: ' : | |
Scripps Pier
£
16
15
14
M M .
: i > > J 1
« 1 1 ‘
if
1 j I
1 i
£
M
I
■ 1 i
I If!
i I
1 M
V
if
I
i 1
.a Barbara Pier
to our results from g *
the Santa Barbara
Basin multicore
site, we use the
Scripps Pier
record because o f
its length and its
consistent similarity to the Santa Barbara Pier. The pier’s instrumental record is
utilized as a tool to validate the ability o f our paleotemperature proxy to record SSTs.
The instrumental record is also useful in the determination o f exactly which seasons
the proxy is recording. Calibrating our proxy record to the present day records will
lend validity to our use o f this method further down core.
1910 1920 1930 1940 1950 1960 1970 1980 1990 2000
Figure 15. Instrumental SST records from Scripps Pier in San Diego,
CA and Santa Barbara Pier.
IX. b. Proxy Records
To determine which time period, seasonal or annual, is being recorded by the
foraminifera we compare our proxy record to various components o f the Scripps Pier
40
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instrumental record. Figure 16 shows that the proxy record reflects the variability in
the instrumental
record for both the
April/May/June
(AMJ) upwelling
season as well as
the annually
averaged record.
Although the
Mg/Ca ratios o f G.
bulloides do not
perfectly
correspond to the
Scripps Pier
records, there is
marked similarities
between the
records. The
Scripps Pier’s
instruments record
the temperature at
Instrumental vs. Proxy Records
Scripps
AMJ
‘ Mg/Ca
Derived
p 18
Scripps _
ANN "
1920 1940 1960 1980 2000 1900
Figure 16. Mg/Ca derived SSTs compared with the instrumental
SST records from Scripps Pier.
an average water depth o f > 10m (Scripps Institute o f Oceanography, 2001) where as
41
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Temperature (°C)
the work done by Pak et al. indicates the G. bulloides is most likely recording
temperatures between 20-40m (2004). During the upwelling season when the water
column is more homogenous and the differing depths o f the records should not be as
major a concern. The proxy record generally agrees with the overall trend o f the
annual instrumental record that varies with a similar magnitude as seen in the AMJ
SST record. All three records show a distinctive increase in temperature trend
following the mid-1970s. The timing o f this change in temperature corresponds to
the change in regime o f the PDO from a predominantly negative phase to a more
positive phase (Figure 17). The pattern o f variability seems better related between
the PDO and the proxy record after this regime shift in the mid-1970s. The beginning
o f the PDO record does not seem as well matched with the corresponding proxy
record. The reduced compatibility between the June and May PDO index and our
proxy record may be attributed to the weaker PDO in the early part o f the 20th
century. A longer PDO record could potentially give a better understanding o f how
well these records correspond back through time.
Examining the instrumental SST record it is clear that the AMJ spring
temperatures are similar to the annual average temperatures. The correlation between
the two data sets is high with and R factor o f 0.85 for the length o f the record. A
potential interpretation o f the similarities is that the strength o f the spring upwelling
is the dominant signal within the annual variability. The general agreement between
our proxy temperature record and the instrumental record also indicates that G.
bulloides Mg/Ca ratios are a reasonable recorder o f the spring seasonal upwelling
42
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Proxy Data vs. PDO Index
y is
i
June PDO
1900 1920 1940 1960 1980 2000
Figure 17. The May and June PDO index compared to the derived SSTs
from Santa Barbara Basin.
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P D O Index P D O Index
SSTs in the Santa Barbara Basin. As a potential measure o f the strength o f upwelling
through time, we
Proxy Data vs. Upwelling Index
compare our
proxy record
with the
upwelling index
(Figure 18). An
increase in
upwelling
generally
corresponds to a
decrease in SSTs
in the proxy
record.
Unfortunately,
the upwelling
100
150
AMJ Average
S 200
400 18H
450
Mg/Ca Derived
1940 1950 1960 1970 1980 1990 2000
Figure 18. The calculated upwelling index compared to the
corresponding portion of our proxy record.
index only extends approximately 50 years and so it becomes difficult to determine
how well correlated these two records are in the long term. Down-core the
temperatures recorded by the foraminifera should give an indication o f how the
strength o f upwelling has changed through time
Another potential comparison to validate our proxy record is to the alkenone
records that have been generated in Santa Barbara Basin. The advantage o f
44
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comparing our record to the alkenone timeseries is the length o f that record as well
as its location within the basin. Unfortunately, the alkenone record does not extend to
present day and so a comparison with our entire record is not possible. Alkenones are
lipid biomarkers o f a small selection o f micro-algae which can be used in the
creation o f the index, a proxy for the reconstruction o f SSTs (Zhao et al., 2000).
The advantage to comparing our record to the alkenone-derived SSTs is the ability of
both methods to reconstruct high-resolution records back through time in Santa
Barbara Basin. For the time period o f overlap between the two records there are
noticeable similarities with respect to variability recorded. There is also an offset of
approximately 1°C between the two records (Figure 19). If we consider the Mg/Ca
derived temperatures as an indicator o f the strength o f upwelling, the offset between
the two proxy records may not be unreasonable. There also seems to be an offset
temporally but the uncertainty o f the age model for our proxy record at that depth
may account for this offset. We do not have specific dates for that section o f the
multicore. The alkenone record can potentially be a useful tool for solidifying the
o i n
age model for our proxy record outside o f the limits o f Pb. For this study we have
not adjusted the age model to better coincide with the alkenone record but this can
potentially be done for future studies. For the most part the alkenone records
supports that this Mg/Ca proxy is a reasonable measure o f the SSTs back through
time in Santa Barbara Basin. Considering our interest in the upwelling season and
the compatibility o f the proxy record to the AMJ SSTs, we focus on the spring
season for the remainder o f our study.
45
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Comparison of Independent Proxy Temperature Records
C O
5 Z >
x>
& 14
1860 1880 1900 1920 1940 1820 1840 1800
20
19
18
17
16
15
Figure 19. Alkenone derived-temperatures (Zhao et al., 2000) compared to Mg/Ca derived-
temperatures in Santa Barbara Basin.
46
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M g/Ca Derived Temperature (°C)
o
S
«
1 “
(2
Mg/Ca Derived Temperatures (NH01-3-MC7)
20
19
18
17
16
15
14
1840 1860 1880 1900 1920 1940 1960 1980 2000
Figure 20. The entire Mg/Ca-derived paleotemperature record from Santa Barbara
Basin.
Examining the proxy record alone, it becomes evident that there are both longer as
well as shorter scaled variability. The record indicates that noticeable changes in the
strength o f upwelling occurred throughout the timeseries (Figure 20) The brevity of
the record makes a meaningful spectral analysis o f the timeseries difficult. A
potential shorter time-scaled influence on the strength o f upwelling included the
presence and strength o f the El Nino events in the tropical Pacific, as previously
discussed. A moderate to strong El Nino event should be expressed in the Santa
Barbara Basin as a muted upwelling season with warmer SSTs recorded by both
47
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instrumental and proxy records. Figure 21 compares our proxy record to the
occurrences o f El Nino events and their relative strengths. The presence of an El
Nino event in the tropics does generally correlate well with warmer SSTs in Santa
Barbara Basin. Considering the interactions that produce the relationship between the
tropics and SSTs in the Northeast Pacific, it could be expected that the strength o f a
particular El Nino events may be correlated to the strength o f regional upwelling in
the months following. NOAA has characterized the El Nino o f the past two centuries
Interarm ual Variability in the Mg/Ca Temperature Record
19 -
Strong El Nino
Moderate El Nino
W eak El Nifio
1880 1900 1920 1940 1960 1980 2000
Figure 21. The strength of El Nino events in the tropical Pacific as compared to the Mg/Ca-
derived paleotemperatures for Santa Barbara Basin.
as strong (circles), moderate (squares), or weak (triangles) (NOAA, 2005) (Figure
21). Although the occurrence o f the El Nino event is recorded by the basin, the
strength o f the event does not seem to be correlated to the degree o f warmth
48
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recorded. The majority o f the events concurrent with our proxy record are associated
with a strong peak in SST, but there are still some El Nino events not associated with
any distinctive variations in temperature. There are also discrete warm events that
cannot be linked to an El Nino event, even a weak event. Further down core, sharp
warm events can be interpreted as the likelihood o f and El Nino event, although it
may not always be the case. A longer scaled influence potentially visible in our
record is the PDO. The length o f our record does not allow the study o f numerous
regime changes, but the one significant shift o f the late 1970s does appear but a
longer proxy record is necessary to fully understand how well defined the PDO is
within the Santa Barbara Basin SSTs.
IX. c. Model Results
IX.c.l. Model Validation
The climate model used for this study, FOAM1.5, as with all GCMs,
recreates a global simulation o f climate on various scales, both temporally and
spatially. It is important to validate the model, to ensure that it is recreating
interactions between various components o f the climate system realistically and
within the bounds o f modem climate, particularly because this is the time period we
are interested in studying. In order to assess the ability o f the model to reproduce the
decadal and multidecadal variability we are examining, we determine whether or not
the model recreates the variability and then if it resembles what we experience in
nature. Focusing on the PDO, in particular, allows us to determine how climate in the
49
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North Pacific is recreated. It is important for the model to accurately model the PDO
because o f its overall influence on the Northeast Pacific. Figure 22 demonstrates that
the model does recreate variability that represents the PDO as described by Mantua
et al. (1997). In both the preindustrial and present day simulations, FOAM
successfully reproduces many aspects o f the PDO. It also accounts for the greatest
component o f variability in SSTs between 20°N and 60°N further validating it as the
PDO as described in the model. As well as producing the correct manifestations of
the PDO (Figure 22B,C) the spectral analysis o f the PDO demonstrates that the index
oscillates within the multi-decadal periodicity see in nature (Figure 23). If the model
can accurately recreate the regional variability in the ocean and atmosphere that
produce the PDO, results on within this region can be interpreted as being realistic.
Also o f importance is how and where the model simulates the NPH and its
variability. Figure 24 shows the prominent location o f the NPH during the spring
season with its corresponding regional winds. The location o f the NPH corresponds
in the model to that experienced naturally creating wind patterns that promote
upwelling in the coastal regions both in the model and in reality. The model also
recreates within this region what Kenyon refers to as a “double high” which are most
likely to occur in the spring or fall (1999). The model simulates on a regular basis
two high-pressure centers within the Northeast Pacific as is experienced in nature.
Although the “double high” is not extensively examined in this study, its presence
lends promise o f legitimacy of the region’s climate as simulated by the model runs.
Together with the reproduction o f the PDO the model reasonably recreates the
50
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Model Validation using PDO varibility in the Instrumental
and Modelled Data
Preindustrial PDO
- f I i t I i I 1
-0 .0 7 -0 .0 6 -0 .0 3 -0 .0 1 0.01 0.03 0.06 0.07
Present Day PDO
EOF 1 o f SSTs
-0 .0 7 -0 .0 5 -0 .0 3 -0 .0 1 0.01 0.03 0.05 0.07
Figure 22. PDO as calculated by Mantua et al. (1997) using instrumental records (A)
as compared with the Model's simulations of the PDO in the Preindustrial (B) and
Present Day (C) runs.
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Spectral Analysis- Model Results o f PDO
Frequency (cycles/year)
0.00 0.01 0.02 0.03 0.04 0.04 0.06
* ■ ■ « ■ ■ ■ ‘ ■ ■ -i ■ * ■ ■ t » * * 1 i ■ ■ <-■
Preindustrial PDO
o 60
Present Day PDO
i r
100 50 25 20
Frequency (years/cycle)
Figure 23. Spectral Analysis of the Model Simulated PDO for both the
Preindustrial and Present Day runs.
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Simulated Surface Wind Patterns in the North Pacific
Sea level pressure
i ____ L
Preindustrial Average AMJ
6 0 1 *
40N
SON
150W
103400
102200
102000
101800
101800
101400
101200
101000
100800
1202
1 0
Sea level pressure
-i— i „» i
Present Day Average AMJ
_ | , , |_
eon -
40N
S O N
150V 120V
102400
102200
102000
101800
101800
101400
101200
101000
100800
150E
10
—►
Figure 24. SLP in the North Pacific as simulated within the Preindustrial and Present Day
runs. The wind vectors are shown relative to 10 m/s.
climate on both shorter, as in the NPH’s variability, and longer, as in the PDO,
timescales within the North Pacific.
IX.c.ii. Large-Scaled Results
Examining overall global trends resulting from the increase in GHGs in the
model simulations, Figure 25 indicates the locations o f warming and cooling in the
Present Day relative to the Preindustrial run. Cooling is experienced within the
53
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model at higher latitudes, particularly n the North Atlantic and the Southern Ocean in
the southern portions o f the Pacific and Indian Oceans. Warming at the lower
latitudes and the North Pacific also characterize the changes in air temperature
between the two model simulations. As experienced in the natural environment,
larger changes in temperature tend to occur at the higher latitudes as opposed to the
Atmospheric Temperature Difference Between Present Day and
Preindustrial Model Runs
90N
60N
30N
30S
60S
90S
180 150W 120W 90W 60W 30W 0 30E 60E 90E 120E 150E 180
[CONTOUR FROM - 4 TO 3 BY ,2l
Figure 25. Difference of surface temperatures in the atmosphere (Present Day- Preindustrial
values). Dashed contours are negative values which indicates cooling in the Present Day run.
Solid contours are positive, warmer Present Day values.
relatively small changes occurring in the tropical regions. The temperature changes
in Figure 25 are comparable to surface atmospheric temperature shifts experienced
since the preindustrial era. O f importance is the question o f whether or not these
global changes attributed to the increase in GHGs since the late 1800s will influence
the coastal Northeast Pacific atmospheric and oceanic interactions and resulting
54
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climates. O f interest is how these global changes in temperature are translated into
regional shifts that can potentially be recorded by a proxy record.
Maintaining a regional view o f climate in the Northeast Pacific, the change in
atmospheric
Present Day - Preindustrial
Spring SST and SSS
60N
30N
r,.
Temp (” C )
composition from the
Preindustrial run to the
Present Day does not
seem to significantly
180 135W
Contour From -0.5 to 0.5 by 0.02 PSU
Figure 26. Difference in SST and SSS in the Northeast Pacific.
Colored contours are temperature differences. All temperatures
are warmer (positive) in the Present Day simulation. Lined
contours indicate differences in surface salinities. Dashed
contours indicate freshening and solid contours, an increase
in salinity in the Present Day Model.
** alter SSTs and sea
surface salinities
14S
(SSSs) in the region
0.2
(Figure 26). The
surface waters, as
defined as the upper
100m o f the model’s
ocean, in the
Northeast Pacific are
warmer and generally
fresher in the Present Day simulation but the variation between the two model runs is
not significant. The overall sea surface temperatures and salinities on a regional scale
do not appear to be influenced by the changes in atmospheric composition between
the model simulations. Although the increase in GHGs do not seem to have a
55
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significant influence on the climate on this larger scale, it is important to examine the
smaller, basin-scaled results to understand how the change in atmospheric
composition may be recorded in the basin’s sediments.
IX. c. iii. Regional/Basin Results
Focusing on the variability o f the North Pacific High as a precursor to
changes in coastal upwelling, there are distinctions between the records generated by
the model for the preindustrial and present day simulations. O f importance is
determining whether these differences are significant, if the changes in atmospheric
North Pacific High Strength
10 year running average
i I 1 i i I i i i i i I i L
102600
Present Day
£ 102400
f i
Preindustrial
— I ---- 1 ---- ,---- j ---- 1 ---- 1 ---- 1 ---- ,---- j ---- j ---- ,---- ,---- 1 ---- [ ----- j ------
600 900 1200 1500 1800 2100
Model Year
Figure 27. Strength of the April/May/June seasonally averaged NPH as simulated by the two
model runs.
composition can significantly alter the variability o f the NPH. Figure 27 shows a 10-
year running average o f the spring seasonal strength o f the NPH. The center o f the
high-pressure zone does occur further east in the present day simulation but the
56
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strength o f the pressure within that region does not significantly change between the
model runs. The closer proximity o f the NPH to the continental low-pressure system
in the present day simulation has the potential to alter the strength o f the coastal
upwelling by intensifying the pressure gradient between the ocean and land that
drives the regional winds. The position as well as the strength o f the NPH influences
the regional upwelling we are interested in and we will discuss how the resulting
regional SSTs are affected by this spatial shift in the high pressure center.
Also o f interest is the timescale o f variability as recreated in the simulations
(Figures 28 and 29). Both model runs generate a peak in the spectral analysis equal
to or greater than the total length o f the runs that are analyzed. Although the potential
for millennial-scaled variability could be o f interest, the runs are currently too short
in length to properly investigate this scaled variability. The interdecadal and
interannual timescales indicative o f the influence of the PDO and ENSO,
respectively, do appear significant in both spectral analyses. There does seem to be
clear similarities between the scale o f variability experienced by the NPH both in
strength and period that the increase in green house gases do not appear to
significantly alter the major influence on the coastal region that we are interested in
observing
Ultimately, we are interested in a better understanding o f the conditions that
can be potentially recorded in Santa Barbara Basin by our proxy methods. The
resolution o f the model does not allow us to examine the basin specifically but we
57
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Spectral Analysis
AMJ Preindustrial NPH
210000
180000
150000
a
0
1 120000
>
90000
60000
30000
0
0.00 0.10 0.20 0.30 0.40 0.50
Frequency (cycles/year)
Figure 28. Spectral Analysis of the April/May/June seasonal strength of the NPH as
simulated in the Preindustrial Model Run.
Frequency (years/cycle)
10( 0 25 Ip
2000
(1600 yr data set)
95%
90%
" r e d "
2.1
7.3
36.4
13.3
5-6 3. 1 - 3.5 3
2 .6 - 2.1
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Spectral Analysis
AMJ Present Day NPH
Frequency (years/cycle)
300000
0
1
&
>
100 25 10
J L m m m m J L m m m m m m m m m m t J L m m
250000 ”
200000
150000
100000
50000
1600
(length o f run)
95%
90%
’ ’ red”
66
3:6
4,9
2.72.5
12,81.0
Q — i— i t i | i— i i i— j— i— i— i— i | T i— i— i— |— r —i— r— r
0.00 0.10 0.20 0.30 0.40 0.50
Frequency (cycles/year)
Figure 29. Spectral Analysis of the April/May/June seasonal strength of the NPH as
simulated in the Present Day Model Run.
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Coastal California AMJ SSTs
are able to examine the conditions o f the coastal Californian region referred to as the
California Borderlands. Figure 30 shows the spring SSTs o f this region for the final
1100 years o f the
model data. The
present day
simulation
experiences
temperatures o f an
average 1.2°C
warmer than the
preindustrial model
run. There are
numerous methods
available to
interpret this data
set. In an effort to
Present Day
Preindustrial
Avg: 14.901°C ± 1.358
— i— i— i— |— i— i— i— |— i— i— i— |— i— i— i— |— i— i— i— |— i
1000 1200 1400 1600 1800 2000
Model Year
Figure 30. SSTs as simulated by the model runs in the Preindustrial
and Present Day. The average temperatures are indicated for the
final 1600 years of the model runs as well as the value of 1 standard
deviation for each average.
remove the influence o f time on the data, to examine only its variability, we detrend
both data sets, removing the long-term influence on the data set (Figure 31). The
detrended data sets are very similar indicating that the GHGs in the present day
model run do not influence the spring SSTs on this scale. To determine if the altered
atmospheric composition has an influence on a longer timescale we examine the 10-
year and 60-year running averages o f the data sets (Figures 32 and 33, respectively).
60
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Detrended Coastal California AMJ SSTs
0,5
V
1 0
0
1
o
p
-0.5
-1
-1.5
Present Day
Pretndustnal
1000 1200 1400 1600 1800 2000
Model Year
Figure 31. SSTs from Figure 30 are detrended to remove the temporal influence on the
temperature variability.
61
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Temperature (°C)
Temperature (°C)
Coastal California AMJ SSTs
10 Year Running Average
17.5
17.0
16.5
16.0
15.5
15.0
14.5
14.0
600 900 1200 1500 1800 2100
Model Year
Figure 3 2 .10-year running average of the SSTs as simulated by the model runs in the
Preindustrial and Present Day.
J 1 ___ 1 ___ I ___ I ___ 1 ___ 1 ___ 1 ___ I ___ I ___ 1 ___ I ___ I ___ I ___ L
Present Day
Preindustrial
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Coastal California AMJ SSTs
60 Year Running Average
17.0
Present Day
16.5
e i6.o
< D
15.0
Preindustrial
14.5
14.0
600 900 1200 1500 1800 2100
Model Year
Figure 33. 60-year running average of the SSTs as simulated by the model runs in the
Preindustrial and Present Day.
63
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The difference between the present day and preindustrial simulations are more
pronounced at these longer timescales indicating that the GHGs do influence longer
termed trends o f the model results. The frequency of the variability experienced in
both model runs also generates some interesting results. The interannual periodicity
that we have associated with ENSO variability does appear in the spectral analysis of
both simulations (Figure 34). The preindustrial model run also experienced
interdecadal, and potentially PDO-related, variability although the present day
simulation does not demonstrate any significant peaks within this time period. This
raises the question o f how important the PDO is to the variability o f SSTs in this
region within the model. The proxy data indicates that there is a link between the
PDO and SSTs in the Santa Barbara Basin but the modeled data does not indicate a
strong relationship. There is an absence o f an interdecadal periodicity in the present
day data as well as a low correlation coefficient between the PDO and the
corresponding regional SSTs, with an R-value o f 0.215 and 0.252 for the present day
and preindustrial simulations, respectively. Increasing the lag time between the two
variables does not increase the correlation coefficient as may be expected. This
indicates that the model does not depict a significant relationship between these
components o f Northeast Pacific climate, as indicated by the paleoproxy record.
Figures 35 and 36 show a comparison o f the PDO and coastal SSTs in a 500-year
window within the model runs. Peaks in the temperature records do seem to occur
during positive (warm) phases o f the PDO and valleys during the negative (cooler)
phases but this is not always the case.
64
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Spectral Analysis AMJ Coastal CA SST
Frequency (years/cycle)
t
1<|0^5 L O
Preindustrial
266.67
3 .0 -
5,0-
4 .0 -
4 )
S 3.0-
■1600
Present Day
• 95%
- 90%
■ "red"
A 34 3 ^ 2.6 2-2.5
-1— 1 — 1 — 1 — j— i— 1 — 1 — 1 — j— 1— 1 — 1 — 1 — |— 1 — 1 — 1 — 1 — p
0.00 0.10 0.20 0.30 0.40 0.50
Frequency (cycles/year)
Figure 34. Spectral Analysis of the April/May/June seasonal SST as simulated in the
Preindustrial and Present Day Model Runs. It is interesting to note that no decadal
or multidecadal scaled variability is calculated as significant in this simulation.
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P D O Index
Preindustrial Model PDO and SST Results
3 |iii[iii|iin im n f]itip n [iiq iii|iii|H n m |iii|iin iii|M i|n i) in |iii|m jiii[ iiin iijiii|[ii[n i|iii]ifi|iin m [n i|iii|iu p n |T iqn ip i; j T i r ) (
AMJ Average PDO Index
AMJ Coastal Cahforma SSTs
16.5
16
15.5
15
14.5
14
1500 1600 1700 1800
Model Year
1900 2000
Figure 35. A comparison of the PDO index and Coastal California SSTs for the AMJ
spring upwelling season in the Preindustrial model run.
66
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Tem perature fC )
Present Day Model PDO and SST Results
1
O
Q1
a*
AMJ Average PDO Index
-2
-3
AMJ Coastal California SSTs
17.5
17
16.5
16
15.5
1500 1600 1700 1800 1900 2000
Model Year
Figure 36. A comparison of the PDO index and Coastal California SSTs for the AMJ spring
upwelling season in the Present Day model run.
67
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Tem perature (°C)
Longer periods characterized by a positive PDO in the model are not necessarily
characterized by equally long and strong warming in the Southern California coastal
region. This absence o f a stronger correlation is a potential avenue for further
research with respect to the usefulness o f the comparisons between modeled and
proxy data.
X. Conclusions
It is important to note that although the proxy and model data support each
other there are instances in which there is not complete agreement between the
results o f these two methods o f climate analysis. The model does not support a
strong link between the PDO and coastal surface temperatures although a
relationship as observed in the Mg/Ca derived SSTs. This disagreement is important
as an indication that much work remains before the regional climate, both in the
ocean and atmosphere, can be well constrained back through time. Nevertheless, the
work done within this study has made important strides to placing the numerical
simulations o f climate modeling within a context tangible in nature via our proxy
record.
With respect to our paleoproxy work, in this study we are primarily interested
in determining what our proxy is recording down core and how we can extract
information about regional conditions when instrumental records are not available.
Reconstructions o f Mg/Ca-derived SSTs in the basin indicate that record left in the
sediment is more complex than simply SST. The planktonic foraminifera, G.
68
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bulloides, is principally recording the strength o f upwelling via its influence on SSTs
in Santa Barbara Basin back through time. The proxy record indicates that
interannual variability akin to ENSO and interdecadal variability associated with the
PDO is chronicled in the calcareous shells o f the planktonic foraminifera. Utilizing
Mg/Ca concentration ratios these shells in Santa Barbara Basin is a reasonable
method for determining how local oceanic conditions change through time. The
shifts in the strength o f upwelling can potentially be applied to speculate how
atmospheric conditions, primarily regional winds and sea level pressure, have varied
through time if the relationship between the atmosphere and ocean can be well
defined. Together with the high-resolution records that Santa Barbara Basin is
capable o f preserving, there is potential in the application o f Mg/Ca ratios to
constrain how climate in the Northeast Pacific has fluctuated through time.
The FOAM 1.5 results indicate that the model is able to accurately recreate
the PDO and the climate influences on the Northeast Pacific as experienced in the
present day as well as the preindustrial era. The variability in Californian coastal
SSTs and the NPH is comparable to what is expected based on modern relationships
between the atmosphere and ocean. As in nature, the PDO is the dominant pattern of
variability within the North Pacific regardless o f the change in atmospheric
composition between the two simulations and does not appear to change either in
character or expression as a result o f the atmospheric perturbations. This difference
in composition, including the increase in GHGs and the ozone hole over the
Antarctic in the present day run relative to the preindustrial, is sufficient to produce a
69
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significant change in the climate, on a local scale over longer periods of time. We
find that within these simulations the influence o f the prescribed atmospheric
perturbations is dependant on the temporal and spatial scale examined. On larger
spatial scales, we observe that the present-day SST and SSS fields o f the Northeast
Pacific are similar in both model runs as well as sea level pressure indicated a weak
influence o f GHGs in our study area. Alternately, on the smaller, basin-scaled
variability, GHGs appear to have a much larger influence, particularly over longer
periods o f time. The increase in GHGs experienced since the preindustrial period
does appear to have some impact on the coastal regions that has been a concern in
our society. What becomes important is a question of perspective and context. The
significance o f the variability examined in the model simulation is a matter of
temporal and spatial scale.
70
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Asset Metadata

Creator Gomez, Paola (author)

Core Title Investigating the influence of atmospheric changes on the variability of the North Pacific using paleoproxy data and a fully coupled GCM

School Graduate School

Degree Master of Science

Degree Program Earth Sciences

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag geochemistry,OAI-PMH Harvest,physical oceanography,Physics, Atmospheric Science

Language English

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Permanent Link (DOI) https://doi.org/10.25549/usctheses-c16-48395

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