Observed Trends of Coastal Upwelling in Eastern Boundary Upwelling Systems

Alyssa Walker

University of Colorado Boulder

A thesis submitted to the University of Colorado at Boulder In partial fulfillment Of the requirements to receive Honors designation in Environmental Studies May 2020

Defense Date: April 8th, 2020

Thesis Advisors:

Kristopher Karnauskas, Department of Atmospheric and Oceanic Sciences Dale Miller, Department of Environmental Studies Nicole Lovenduski, Department of Atmospheric and Oceanic Sciences

Observed Trends of Coastal Upwelling in Eastern Boundary Upwelling Systems ii

Abstract

Eastern boundary upwelling systems (EBUS) create unique ecosystems that serve as habitats for cold water species of marine life. In addition, these upwelling systems may serve as a potential buffer zone to the ongoing and future global oceanic warming. Prior research conducted on EBUS has brought forth two conflicting hypotheses: will EBUS intensify due to enhanced land-sea temperature gradients or weaken due to enhanced vertical stratification? Furthermore, shifting atmospheric circulations may cause EBUS to migrate poleward. While enhanced ocean stratification is a viable constraint on the future health of these marine ecosystems, my research focuses on using observations to detect and understand mechanisms responsible for EBUS intensification and poleward migration. Specifically, I am to examine whether EBUS are latitudinally intensifying following the poleward migration of the Hadley Cell (HC). I show that natural climate variability, such as El Niño-Southern Oscillation (ENSO), can be used to identify the mechanisms responsible for current and future EBUS intensification. Through linear regression, I expose ENSO’s effect on EBUS and subsequently connect this effect to the structure of the HC. As proposed by others, my results suggest that EBUS are dependent on the HC for their strength and structure, and as a result of this interdependence, future alterations of the HC will lead to corresponding changes in EBUS, specifically in their latitudinal extent.

Observed Trends of Coastal Upwelling in Eastern Boundary Upwelling Systems iii

Preface

I have spent my college career learning about the physical and social consequences accompanying climate change. Although I have spent four years learning about these repercussions, I feel as though these consequences have been expressed through blanket statements, such as Earth’s oceans are warming, and as a result of this warming, hundreds of species will perish. I decided to research Eastern Boundary Upwelling Systems because I wanted to explore and understand the true complexity behind climate change. I wanted to learn more about oceanic and atmospheric processes that might respond to climate change in a manner that’s potentially adaptable for humans and the aquatic species dependent on these current systems. I would like to thank my thesis committee who repeatedly encouraged me throught this lengthy process. I would like to thank Dr. Karnauskas for teaching me everything I know about coding, preventing me from diving too deep into an off-topic rabbit hole, and reminding me that I need to give myself more credit. I would like to thank Dr. Miller for consoling me during my weekly anxious rants and acting as the little voice in the back of my head reminding me of deadlines and thesis format. I would like to thank Dr. Lovenduski for providing guidance on my paper. Lastly, I would like to thank Riley Brady for meeting with me several times to discuss my qualms about the direction of my research and for repeatedly sending me helpful literature related to my topic.

Observed Trends of Coastal Upwelling in Eastern Boundary Upwelling Systems iv

Table of Contents Abstract ...... iii Preface ...... iv Abbreviations ...... vi List of Figures ...... vii 1.0 Introduction ...... 1 2.0 Background...... 3 2.1 The Importance of Coastal Upwelling ...... 3 2.2 Background on Coastal Upwelling and Pressure Systems ...... 4 2.3 The Bakun Hypothesis Explained ...... 6 2.4 Complementary Hypothesis for Changes within Coastal Upwelling ...... 7 2.5 Summary of the Outstanding Literature on the Potential Mechanisms Responsible for EBUS Alterations ...... 9 2.6 Outstanding Literature on the Past, Present, and Future State of Upwelling Systems ...... 11 2.6.1 The Humboldt Current System ...... 11 2.6.2 The California Current System ...... 12 2.6.3 The Benguela and Canary Current Systems ...... 13 2.7 Natural Climate Variability and its Implication on Coastal Upwelling ...... 14 3.0 Methodology...... 15 3.1 Sources of Observational Data ...... 15 3.2 Latitudinal Sub-sectioning ...... 16 3.3 Calculation of Trends and Creation of Time Series ...... 17 3.4 Creation of Regression Maps ...... 20 4.0 Observational Results and Discussion ...... 22 4.1 Time Series Results for Individual EBUS ...... 22 4.1.1 CCS Trends ...... 22 4.1.2 HCS Trends ...... 25 4.1.3 BCS Trends ...... 28 4.1.4 CnCS Trends ...... 30 4.2 Regression Map Results for Individual EBUS ...... 31 4.2.1 CCS Regression Analysis...... 32 4.2.2 HCS Regression Analysis ...... 35 4.2.3 BCS Regression Analysis...... 37 4.2.4 CnCS Regression Analysis ...... 38 5.0 Summary and Conclusion ...... 42 6.0 Limitations and Recommendations for Future Research ...... 44 7.0 Bibliography...... 46

Observed Trends of Coastal Upwelling in Eastern Boundary Upwelling Systems v

Abbreviations

BH Bakun Hypothesis

SST Sea Surface Temperature SLP Sea Level Pressure EBUS Eastern Boundary Upwelling Systems CCS California Current System BCS Benguela Current System HCS Humboldt Current System CnCS Canary Current System HC Hadley Cell CMIP (5 &6) Coupled Model Intercomparison Project AOGCM Atmospheric/Ocean General Circulation Model PP Primary Productivity NOAA National Oceanic and Atmospheric Administration JJA June, July, August (boreal summer) DJF December, January, February (boreal winter) ENSO El Nino-Southern Oscillation

Observed Trends of Coastal Upwelling in Eastern Boundary Upwelling Systems vi

List of Figures

Figure 1 Visualization of the Bakun Hypothesis...... 7 Figure 2 Trend Map of Insignificant SST and SLP Overlaid with Significant Surface Winds...... 18 Figure 3 CCS Poleward SST vs Equatorward SST ...... 19 Figure 4 CCS Poleward SST Minus Equatorward SST...... 20 Figure 5 CCS SST from 1982–2018 ...... 23 Figure 6 HCS Difference Plot of SST ...... 25 Figure 7 BCS SST from 1982–2018 ...... 28 Figure 8 CnCS SST from 1982–2018 ...... 30 Figure 9 CCS Regressed SST (Poleward-Equatorward) Overlaid with Regressed SLP (Poleward-Equatorward) ...... 32 Figure 10 HCS Regressed SST (Poleward-Equatorward) Overlaid with Regressed SLP (Poleward-Equatorward) ...... 35 Figure 11 BCS Regressed SST (Poleward-Equatorward) Overlaid with Regressed SLP (Poleward-Equatorward) ...... 37 Figure 12 CnCS Regressed SST (Poleward-Equatorward) Overlaid with Regressed SLP (Poleward-Equatorward) ..... 38 Figure 13 Displays the Wind-Evaporation-SST Feedback ...... 40

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Observed Trends of Coastal Upwelling in Eastern Boundary Upwelling Systems viii

1.0 Introduction

This paper seeks to examine two proposed mechanisms behind upwelling intensification in Eastern Boundary Upwelling Systems (EBUS), as well as examine their interannual variability contributing to anomalous values in the current system’s strength and structure. EBUS, defined as narrow regions of cold, nutrient-rich upwelled water along the eastern side of their respective ocean basins, play a vital role in the fixation of atmospheric carbon and the creation of unique ecosystems suited for cold-affinity species (Ducklow, Steinberg, & Buesseler, 2001; IPCC, 1990). In this paper, I examine observational data for the four main EBUS: the California Current System (CCS), the Humboldt Current System (HCS), the Benguela Current System (BCS), and the Canary Current System (CnCS). While the importance of these current systems is widely recognized, their response to climate change remains an open and actively debated question. As argued by many researchers, these upwelling zones are most likely to respond to increases in atmospheric and oceanic temperatures through one of three means: first, because Earth’s oceans warm from solar insolation reaching its surface, the pervasive warming of the ocean will result in increased vertical stratification that inhibits upwelling and diminishes the vigor of EBUS; second, an intensification of upwelling-favorable winds by means of land-sea surface temperature contrasts will result in an overall amplification of upwelling systems; third, EBUS may migrate northward or southward, depending on the migration of the Hadley Cell (HC). A poleward shift, for example, would appear as a change along the present-day boundaries of EBUS (Bakun et al., 2015; Rykaczewski et al., 2015; Seabra, Wethey, Santos, Gomes, & Lima, 2016). Although there is rational behind all three of these responses, I center my study on examining the latter two of these hypotheses. The second climatic response, proposed by Andrew Bakun in 1990, is coined as the Bakun hypothesis (BH), and for the sake of this paper, the third climatic response, proposed by Ryan Rykaczewski et al. in 2015, will hereafter be referred to as the Hadley Cell hypothesis (HCH). Throughout the past few decades, the scientific community has attempted to refute or prove the validity of the BH and the HCH through the inspection of observational data and the employment of atmospheric and oceanic general circulation models (AOGCMs). Although there is already an extensive amount of literature on EBUS, few observational studies explicitly examine how the current system’s interannual variability can used to infer the mechanisms

Observed Trends of Coastal Upwelling in Eastern Boundary Upwelling Systems 1 responsible for upwelling modifications. In an effort to explore this relationship between natural climate variability and the strength and structure of EBUS, I ask research questions such as: • What specific modes of natural climate variability contribute to anomalies in the strength and structure of EBUS? • Can natural climate variability be used as a means of proving the supposed mechanisms associated with EBUS intensification and migration? • Can we understand the changes in SST in EBUS through couple ocean atmosphere processes? • How can various modes of interannual variability be used to predict the future state of EBUS? First, I set up my paper with a background section that covers basic principles needed to understand my results. Second, I discuss the methods used for answering my research questions. Lastly, I analyze my results and discuss the implications of my findings.

Observed Trends of Coastal Upwelling in Eastern Boundary Upwelling Systems 2

2.0 Background Before diving into the methodology and results of my research, a basic knowledge of the physical processes at play is essential to the development of my paper. This section is dedicated to explaining the physical processes behind coastal upwelling, as well as the natural climate variability that modulates its strength.

2.1 The Importance of Coastal Upwelling Coastal upwelling is a physical oceanic process where cold, nutrient-dense upwelled waters are uplifted by a means of surface wind direction and strength. This cool, nutrient-rich water impacts local climates and ecosystems by lowering nearby atmospheric surface temperatures (which can influence winds) and feeding aquatic, cold-affiliated species. EBUS are narrow bands of water that are comprised of upwelled water located on the Eastern side of their respective ocean basin. Upwelling bands, which tend to have a width of 10–30 km, serve as a host of nutrients for phytoplankton and phytophagous organisms (Send, Beardsley, & Winant, 1987). Small pelagic fish, such as sardines and anchovies, rely on these boundaries for survival and prosperity (Carr, 2001). Although EBUS make up less than 1% of Earth’s oceans, they account for 5% of global marine primary productivity (PP) and 17% of global fish catch (Carr, 2001; Pauly & Christensen, 1995). While the width of these upwelling bands only extends 10–30 km, the ‘productive’ bands they create span over a width of 100 km. This productive band is essential to fisheries operating along their boundaries. Since EBUS are responsible for over 17% of global fish catch, their response to climate change has become of particular interest to oceanographers and climatologists investigating both the boundaries rate of PP and the underlying mechanisms responsible for their creation (Carr, 2001). While EBUS have long been on the minds of researchers, their reaction to climate change was not fully questioned until 1990 after the publication of the controversial BH. Andrew Bakun, professor of marine biology and fisheries, published his seemingly simple, yet polemic hypothesis, which predicted that anthropogenic climate change will generate an acceleration in coastal upwelling along Eastern boundaries (Bakun et al., 2015; Sydeman et al., 2014). Although in 1990 Bakun did not explicitly conclude that the primary productivity (PP) of these regions will increase with climate change, it is well understood that the livelihood of pelagic fish is linked to upwelling trends; therefore, in addition to predicting an increase in the rate of

Observed Trends of Coastal Upwelling in Eastern Boundary Upwelling Systems 3 upwelling, the BH has been extended to postulations that envisage direct benefits for EBUS fisheries (Bakun, 1990; Bakun, 2015).

2.2 Background on Coastal Upwelling and Pressure Systems Because the potential implications of the BH are widespread, any investigation of it must be accompanied by an in-depth understanding of the physical processes responsible for the formation of EBUS. To begin, EBUS are part of wind-driven ocean circulation, and their existence is entirely dependent on wind direction and strength (Capet, Marchesiello, & McWilliams, 2004). The winds responsible for the creation of coastal upwelling are dependent on preexisting pressure gradients which follow two rules: first, matter and energy flow from areas of high density (pressure) to areas of low density (pressure); second, warm parcels have a lower density therefore they tend to rise, where as cold parcels have higher densities and sink (Mote & Mantua, 2002). These two rules impact the strength and direction of coastal surface winds. The initiation of these surface winds begins with pressure contrasts between the land and sea. Due to the land’s low heat capacity––compared to Earth’s oceans, less energy is required to raise the temperature of continental surfaces by 1°C––Earth’s terrestrial crust is typically characterized as having a low surface pressure. In contrast, since Earth’s oceans have a higher heat capacity, meaning more energy is required to raise the temperature of water by 1°C, they emit lower amounts of thermal energy leading to a cooler ocean surface with heavier residing air. As a result of this temperature contrast, air flow adheres to the pressure gradient force (PGF) by flowing from the Earth’s oceanic highs to Earth’s terrestrial lows. For example, in California, the land has a relatively low surface pressure compared to the pressure of the neighboring Pacific Ocean; therefore, air flows towards Los Angeles. If Earth’s atmospheric circulation was only dictated by the PGF, one would expect surface winds to flow towards the coastal land; however, in reality, surface winds adhere to the direction of geostrophic flow and move along the coast (equatorward). Geostrophic flow is the processes whereby the PGF is balanced by the Coriolis Force (CF) resulting in the net movement of air or water perpendicular to the PGF. The CF can be defined as “an effect whereby a mass moving in a rotating system experiences a force acting perpendicular to the direction of motion and to the axis of rotation” (Britannica, 2019). In the case of atmospheric circulation, the spherical nature of Earth causes winds to be deflected to the right in the Northern Hemisphere

(NH) and the left in the Southern Hemisphere (SH). Using California as an example again, airflow moving from the Pacific Ocean to Los Angeles will be deflected to the right and result in an equatorward geostrophic flow heading towards Baja. The CF is the underlying mechanism responsible for directing all EBUS winds in a net equatorward direction; however, the CF does not stop exerting its force once the winds are directed equatorward; rather, the CF continues to impact winds so that they flow in a circular motion. The circular direction of the wind flow is determined by the system’s central pressure. Systems with a central low pressure are classified as a cyclone. Cyclone’s rotate counterclockwise in the NH and clockwise in the SH. In contrast, systems with a central high pressure are classified as anticyclones. Anticyclones rotate clockwise in the NH and counterclockwise in the SH (Allen, 1973; Yasuhiro, 2002). Using California as an example again, the cold water of the Pacific Ocean creates a pressure system with a central high. This anticyclone rotates clockwise since it’s in the NH; therefore, winds in this system will be pushed equatorward near California’s coast, but it will once again adhere to geostrophic flow and be deflected towards the right, rotating around the central high in a clockwise manner. In addition to having an impact on the direction of winds, the CF also influences the path of ocean surface currents, leading to a substantial fraction of the ocean circulation being in geostrophic balance. As mentioned earlier, surface winds help form oceanic surface currents; therefore, one would expect currents to follow surface winds and flow parallel to coastal shorelines. However, identical to the impact it has on surface winds, the CF continues to exert its force and deflect current flow to the right in the NH and the left in the SH. Specifically, when the phenomenon occurs in the oceans, it is defined as Ekman Transport. Ekman Transport is a key process responsible for the formation of coastal upwelling. Ekman Transport, which is a combination of the effects of the CF and friction from the wind above, deflects water flow originally headed equatorward away from shorelines along eastern boundaries of oceans. In the case of California, surface currents that are dragged equatorward by wind stress are simultaneously deflected to the right (i.e., westward) and huge volumes of water are transported away from California’s coastline. The effects of Ekman Transport extend to a depth of tens to hundreds of meters, creating current flows that move in the motion of a spiral; this corkscrew movement is defined as the Ekman spiral (NOAA, 2020). The Ekman spiral directs huge volumes of surface water away from shorelines, and in order to adhere to the physics of mass continuity, the mass of water leaving the coastal shore

Observed Trends of Coastal Upwelling in Eastern Boundary Upwelling Systems 5 must be replaced by some other water. Cold, nutrient-rich water residing below the surface upwells and fills in the space of the displaced surface water, thereby completing the process of coastal upwelling.

2.3 The Bakun Hypothesis Explained As demonstrated above, the atmospheric and oceanic mechanics responsible for coastal upwelling are interdependent, and changes in one variable, such as wind speeds, can have a domino effect and impact seemingly distant variables. This domino effect plays a central role in the rational proposed by the BH. There’s no debate that Earth’s surfaces are rapidly warming in response to the accumulation of anthropogenic greenhouse gasses (GHG). The understanding of this anthropogenic GHG effect paired with the knowledge of coastal upwelling basics inspired Bakun to publish his logical theory on EBUS (Di Lorenzo, 2015). As covered earlier, miniscule changes in one variable involved in development of coastal upwelling can have a rippling effect on the whole process. Since the 1800s, the Earth’s atmospheric temperature has warmed by about 1.2°C (Marchitto, 2020). Over the past 40 years, Earth’s oceans have warmed by approximately 0.6°C, and from the ocean’s surface to a depth of 500m, the water is expected to warm at a rate of 0.05°C per decade (Marchitto, 2020). Although both Earth’s terrestrial and oceanic surfaces are expected to experience increases in temperature, lands lower heat capacity will cause it to warm more effectively than Earth’s oceans. (Karnauskas, 2019; Huyer, 1983). As a result of these unequal heat capacities, surface air residing over the continents will rise at swifter rate. In other words, the differing heat capacities have led to an overall increase in the PGF occurring between the land and sea. Although an intensified PGF may seem trivial, amplifications in the PGF directly increase the equatorward windspeeds. For example, as Los Angeles warms more rapidly than the neighboring Pacific Ocean, the coastal winds flowing towards the equator follows amplifications in the PGF and also increase their speed. As windspeeds accelerate, the rate of Ekman Transport follows and the amount of surface waters transferred offshore increases as well (Huyer, 1983). If coastal surface water is displaced at a swifter rate, nations should expect to see rates of coastal upwelling increase correspondingly (Bakun et al., 2015). Figure 1, taken from Bakun’s 2015 study, visually describes the hypothesis explained in this section. Figure 1A identifies the four EBUS in question and exposes their high levels of chlorophyll concentration; chlorophyll

concentration exposes the PP of each upwelling system. Figure 1B demonstrates the general mechanisms responsible for upwelling explained in section 2.2. As shown above, coastal California is marked with a permanent low- pressure system and the neighboring Pacific Ocean is marked with a permanent high-pressure system. This pressure gradient forms the resultant equatorward wind, which in return fuels coastal upwelling signified as the blue upward arrows. Lastly, figure 1C demonstrates the proposed mechanisms of upwelling intensification laid out by the BH. The enlarged H, L, and arrows are meant to signify substantial increases in the PGF, coastal windspeeds, and coastal upwelling.

2.4 Complementary Hypothesis for Changes within Coastal Upwelling While the BH coincides with the known physical processes accredited for the creation of these current systems, the BH remains actively debated throughout the scientific community. Few researchers deny the logic

Figure 1 Visualization of the Bakun Hypothesis. behind the BH: eastern boundary coastlines warm more

A.) Depiction of the four-primary focused EBUS. quickly than their neighboring oceans which in theory Global chlorophyll concentrations average annually. B.) Displays the current state of the EBUS. Uses the intensifies preexisting land-sea pressure gradients; California current system as an example, but mechanisms are identical for all four current systems. however, some researchers question whether the impact C.) Shows the predicted changes within EBUS. Demonstrates a poleward migration of the Oceanic differential heating has on the PGF ultimately drives High, but shows the strengthening of the continental thermal low as the true source of the proposed intensifications in coastal upwelling. Rather, a upwelling intensification. Obtained from: (Bakun, et al., 2015) complementary hypothesis proposes that evidence of an

intensifying PGF is limited to the poleward migration of the HC (Brady, Alexander, Lovenduski, & Rykaczewski, 2017; Rykaczewski et al., 2015). I identify this hypothesis as the HCH. As explained in section 2.2, upwelling is fundamentally a part of wind driven circulation. While land-sea temperature contrasts are essential to the formation of surface winds via pressure gradients, the different amounts of solar radiation received at Earth’s surfaces and the subsequent

Observed Trends of Coastal Upwelling in Eastern Boundary Upwelling Systems 7 formation of the HC may play a stronger role in the creation of semi-permanent high- and low- pressure systems that create the initial PGF that instigates coastal upwelling. The HC is a climatic phenomenon created by Earth’s spherical nature and controlled by consequent varying surface temperatures. That being said, since the Sun occupies an overhead position at the equator and a more angular one at the subtropics, the differences in total amount of solar insolation reaching Earth’s tropics and subtropics lead to a permanent low surface pressure paired with steady rising air at Earth’s equator and a high surface pressure paired with subsiding air at Earth’s subtropics (30º N&S).The subsiding air at 30° N&S completes the circulation of the HC by moving winds back towards the equator. In other words, the equatorward winds blowing along eastern boundary coastlines are part of the HC; therefore, if the structure and strength of the HC changes, the equatorward surface winds of EBUS also change. Outstanding literature proposes that over the past 40 years, the HC has demonstrated observational characteristics that suggests it is slowly expanding poleward (Grise et al., 2019; Hudson, 2012; Lu, Vecchi, & Reichler, 2007; Tao, Hu, & Liu, 2016). While there is some disagreement in regards to the methods of measuring the HC’s expansion, it is widely recognized that the HC has expanded by 0.25°–0.5° latitude per decade (Grise et al., 2019; Staten et al., 2019). Since the HC is observable via zonally averaged pressure zones, researchers question whether shifts within the location of the descending branch of the HC––which namely creates subtropical anticyclones that fuel coastal upwelling––will have outstanding impacts on distinct climatic processes dependent on these pressure systems. Specifically, EBUS and their relationship to the observed poleward migration of the HC have come into question. Through the use of 21 AOGCMs operating under RCP 8.5 (a high radiative forcing scenario with GHG emissions continuing as business as usual) conditions, Rykaczewski, et al. (2015) proposes that any future observations of intensified coastal upwelling are expected to occur at the poleward boundaries of these upwelling systems. Furthermore, upon observing the models output for likely changes within land-sea surface temperature contrasts, Rykaczewski et al. (2015) finds that although all 21 AOGCMs reveal summertime intensifications between these temperature gradients, there appears to be no resultant modifications within continental SLP. As put differently, the study did not detect Bakun’s supposed connection between land-sea temperature contrasts and escalations in cross-shore SLP gradients. Therefore, unlike the BH, Rykaczewski et al. (2015) explains that the mechanisms

Observed Trends of Coastal Upwelling in Eastern Boundary Upwelling Systems 8 responsible for the intensification of the upwelling-dependent PGF is not driven by differential warming between the land and sea; rather, because models illustrate intensifications within EBUS are constrained to the poleward boundaries of the system and the upwelling seasons (summertime for the observed hemisphere), the study concludes that land-sea surface temperature contrasts cannot be the lone mechanism contributing to changes within EBUS. In conclusion, they find that Bakun’s logic is sensible, but fails to encompass peripheral mechanisms potentially having a greater impact on coastal upwelling.

2.5 Summary of the Outstanding Literature on the Potential Mechanisms Responsible for EBUS Alterations Here, I summarize the findings of studies examining the observed and future mechanisms responsible for alterations within the strength and structure of EBUS. Furthermore, this section examines literature that both explicitly or indirectly claims to support either the BH or HCH. In addition to the Rykaczewski et al. (2015) study, several studies investigating the validity of the BH have found results that seem to back the HCH (Aguirre, Rojas, Garreaud, & Rahn, 2019; Baumann & Doherty, 2013; Di Lorenzo, 2015; Sydeman et al., 2014; Wang, Gouhier, Menge, & Ganguly, 2015). Efforts of using coupled climate models as a method of examining the BH began in 1992. In 1992 Hsieh and Boer doubled atmospheric CO2 concentrations and found surface temperatures over land increased, but there were no corresponding changes associated with SLP or upwelling trends. Furthermore, Mote and Mantua (2002) used two AOGCMs and discovered no notable changes in coastal upwelling. In an experiment where CO2 concentrations were quadrupled, Belmadani, et al. (2013) reported significant changes in surface temperatures over land, but the study concluded that such changes were not significant enough to increase pressure gradients to the point where upwelling would be impacted. The studies mentioned above serve as the basis of evidence contradicting the mechanisms proposed by the BH, and further evidence arguing against the BH extends into the specific location of upwelling intensification as well as the designated season of the current systems supposed enhancements. While majority of the literature shows that intensifications within EBUS are expected to occur at high latitudes, not all studies find that system intensifications correspond to their appropriate upwelling season (June, July, August (JJA) for the NH and

December, January, February (DJF) for the SH). In their original paper, Rykaczewski et al. (2015) determines that intensifications of the CCS are not only limited to the Northern extent of the system, but also limited to April and May. Following the BH, one would expect the CCS to intensify during its upwelling season—JJA—when land-sea temperature contrasts are at their greatest extent; however, because models show intensifications occurring in seasons outside of the typical NH upwelling season, conclusions can be made that differential heating is not the sole source of alterations to the CCS. In fact, based on this seasonal distinction, recent CMIP6 studies indirectly support the mechanism of upwelling intensification proposed by the HCH. In their study comparing the findings of CMIP5 to CMIP6, Grise and Davis (2020) note that HC expansion of the North Pacific high is expected to shift poleward during March, April, May (MAM) and equatorward during JJA. While their report does not mention the CCS or its relationship to the HC, I conclude that the corresponding relationship between HC expansion and the predicted months of CCS’s poleward migration serve as evidence of the HCH in the future. Although literature dominantly shows a poleward migration of EBUS, several studies still associate the mechanisms responsible for upwelling intensification with Bakun’s hypothesis: the greenhouse-warming effect on the thermal sea-land difference directly increases offshore Ekman Transport (Seabra et al., 2016; Sydeman et al., 2014; Wang et al., 2015). Sydeman et al. (2014) found wind intensifications for the Benguela Current (poleward of 20ºS), California Current (poleward of 32.5ºN), and the Humboldt Current (poleward of 14ºS). The study attributes the varying latitudinal wind intensities to the consistent warming patterns associated with climate change: stronger warming trends observed at the poles compared to the equator. Furthermore, through confirmation of the interdependence between coastal upwelling and land-sea temperature contrasts, the study explicitly claims it supports the mechanisms proposed in the BH. In addition to Sydeman et al. (2014), Wang et al. (2015) conducted a study investigating greenhouse-warming impacts on the intensification and spatial homogenization of coastal upwelling systems. While the study shows the intensification of upwelling and expansion of its season at higher latitudes, their land-sea thermal tests show a “robust relationship” between temperature differences and upwelling intensity; therefore, overall, the study encompasses a trend that supports the BH (Wang et al., 2015). Rykaczewski et al. (2015) warns against the conclusions found by Wang et al. (2015) because the latitudes used within the analysis of the

Observed Trends of Coastal Upwelling in Eastern Boundary Upwelling Systems 10 relationship between upwelling and land-sea surface temperature contrasts are constrained to regions that reflected positive trends in coastal upwelling. Although Wang et al. (2015) discovered the mechanisms responsible for upwelling intensification through a potentially biased method, the study is still frequently cited. As literature shows, it is commonly suggested that intensifications within EBUS are likely to take place at higher latitudes; however, an explanation for the observed latitudinal differences within EBUS intensification is still needed and efforts to account for these discrepancies are often made through the consideration of varying regional processes. For example, efforts to explain weakening upwelling trends at the lowest latitudes of the CnCS evoke questions centered on the roll the southwesterly monsoon circulation plays in the vigor of subtropical downwelling-favorable winds (Wang et al., 2015). Furthermore, although a recent study utilizing methods similar to this report (analysis of observational SST trends) found the weakening of upwelling near equatorial ridges of Atlantic EBUS to be indicative of the mechanisms proposed by Rykaczewski et al. (2015), it also suggests the possibility of regional oceanic currents accounting for the equatorial warming (Seabra et al., 2019). Due to the close proximity between the observed warming regions and neutral/negative net warming regions, the study suggests that upwelling is either relocating geographically like Rykaczewski proposed or impacted by changes in nearby regional oceanic currents. For example, in the case of Benguela’s equatorial warming, changes could be caused by the migration of the Angola Current, and its indication of warming at the polar edge of the current could be due to changes in the intensity of the Agulnas current (Seabra et al., 2019).

2.6 Outstanding Literature on the Past, Present, and Future State of Upwelling Systems This section examines and summarizes the outstanding literature that quantifies observed and future alterations in all four EBUS. 2.6.1 The Humboldt Current System Although the mechanisms responsible for observed and predicted changes within EBUS are contested, there seems to see some degree of consensus surrounding one upwelling system that has and is expected to intensify: the HCS. Observational studies investigating variable changes within the Humboldt system have found it to be the only EBUS whose SST trends show Observed Trends of Coastal Upwelling in Eastern Boundary Upwelling Systems 11 a consistent negative linear trend over the data time period: 36 years (Baumann & Doherty, 2013; Seabra et al., 2019). Specifically, the latitudes of 6ºS–14ºS have experienced the most consistent cooling (Baumann & Doherty, 2013). Furthermore, Sydeman et al. (2014), who did a statistical analysis of observational studies vs. model-based ones, found the winds responsible for the creation of the HCS show signs of intensification. Furthermore, CMIP5 models, operating on a timescale from 1950 to 2099, reveal that the HCS is the only EBUS to exhibit positive trends of upwelling intensification at all latitudes, with the most vigorous augmentations occurring at the three southern-most latitudes (Wang et al., 2015). Out of the 21 AOGCMs analyzed by Rykaczewski et al. (2015), 62% expected the HCS’s regional upwelling to increase by 10% (± 12% standard deviation) compared to the 1861–1890 mean. Aguirre et al. (2019) concurs with Rykaczewski et al. (2015) through their analysis declaring that upwelling-favorable winds are expected to intensify in the poleward regions of the HCS.

2.6.2 The California Current System As explained above, the HCS appears to be the strongest, most consistent EBUS. In contrast to the harmony surrounding the present and future fate of the HCS, the CCS seems to be the most puzzling EBUS. Observational analysis of the CCS suggests that warm seasons are significantly more likely to show wind intensification, and SST patterns mainly show a negative linear trend (Seabra et al., 2016; Sydeman et al., 2014). However, observational data does show a tapering of this negative linear trend between 1982–2012 which is attributed to the robust 2014– 2016 Northeast Pacific marine heat wave. Furthermore, in their observational study on California’s SST, Baumann and Doherty (2013) compare the latitudinal SST patterns to that of the HCS. Although the HCS demonstrates stronger patterns of SST cooling, the study still makes note of the North American Pacific gradient and its cooling trend which is latitudinally dependent. In contrast to the observed cooling and wind intensifying trends found within the CCS, studies that employ the use of AOGCMs show trends for California that suggest a weakening of future upwelling. Rykaczewski et al. (2015) finds that 71% of the 21 AOGCMs project a significant decrease in California’s summertime upwelling. Weakening of upwelling-favorable winds is expected to be observable at the current system’s core (30º–40ºN), and any slight future intensification of the system are anticipated to take place in Northern California during April and

May (Aguirre et al., 2019; Rykaczewski et al., 2015). It is hypothesized that lack of uncertainty for the future of the CCS is attributed to natural climate variability, such as ENSO, Pacific Decadal Oscillation, and North Pacific Gyre Oscillation (Wang et al., 2015).

2.6.3 The Benguela and Canary Current Systems To continue with observational trends of EBUS, both the BCS and CnCS demonstrate reduced coastal warming; however, unlike the HCS, this attenuation of oceanic warming is restricted to several nearshore pockets (Seabra et al., 2019). Seabra et al. (2019) found the observational trends show high warming rates at the equatorial regions of Atlantic EBUS. When looking at the BCS, Baumann and Doherty (2013) found the South African Atlantic latitudinal temperature gradient to be warming along 12–23ºS, whereas significant cooling trends were observed at approximately 31–34ºS. Santos et al. (2011), through observations, concludes that overall the BCS has strengthened during a time period from 1970 to 2009. When looking at the CnCS, observational studies typically report slight positive warming trends or no significant trends for changes in SST(Baumann & Doherty, 2013; Santos, Gomez-Gesteira, deCastro, & Alvarez, 2012; Seabra et al., 2019). Although slight warming trends are occasionally detected, the CnCS is said to have expressed nearshore warming rates lower than those of the entire global ocean; therefore, researchers believe that it’s resistance to adhere to the global nearshore rate of warming qualifies the current system as a “buffer” to oceanic warming (Baumann & Doherty, 2013; Santos, Gomez-Gesteira, deCastro, & Alvarez, 2012; Seabra et al., 2019). To continue with the state of Atlantic EBUS, trends pulled from AOGCMs tend to be slightly more ambiguous. The Aguirre et al. (2019) reanalysis of an ensemble of GCMs found increases in summertime upwelling-favorable winds for both the BCS and CnCS. For the CnCS, Rykaczewski et al. (2015) found that majority of the models project significant increases in summertime intensity of upwelling; however, both Wang et al. (2015) and Rykaczewski et al. (2015) detected weakening in upwelling for the lowest latitudes of the CnCS. For the BCS, out of 21 AOGCMs, 29% of the models concluded the system would experience significant increases in future upwelling, whereas 24% of the models suggests the system would experience significant decreases in future upwelling (Rykaczewski et al., 2015).

2.7 Natural Climate Variability and its Implication on Coastal Upwelling The existing literature effectively portrays the observed and anticipated trends in EBUS, and more often than not, natural climate variability is referenced as a potential influencer of significant trends within these current systems. Natural climate variability can be identified as climatic phenomenon’s that reoccur at differing temporal periods. These differing phenomena are typically accompanied by reoccurring climatic conditions but exert their climatic conditions with a different magnitude for each individual occurrence. The most commonly used example of natural climate variability is ENSO. During ENSO, Walker Cell Circulation is weakened or reversed, resulting in a multitude of teleconnections that have global impacts. Walker Cell Circulation serves as the climatic median for wind direction, SLP, and SST distribution. Under normal conditions, Easterly Trade winds along both sides of the equator create divergent Ekman Transport that instigates upwelling in the central Pacific Ocean, creating a tongue of cold SST along the equator. During ENSO, the Trade winds are weakened or reversed, and as a result, central Pacific upwelling is turned into downwelling, creating a warm tongue. Furthermore, the high SLP normally accompanying the Western coast of South America is replaced with a relatively low pressure, whereas the low SLP typically occupying Australia is replaced with a high SLP. In addition to the discouragement of coastal upwelling along Western South America, South America also experiences an abnormal increase in precipitation accompanying the increased coastal SST. Although ENSO seems to be confined to the Pacific Ocean, its extent reaches beyond Australia and South America having global teleconnections. Specifically, for the purpose of this paper, I focus on its impact on the HC. As a result of the reversal of the normal surface pressures and the subsequent standstill of equatorial upwelling, SSTs in the central and Eastern Pacific Ocean increase. This subsequent warm SST enhances tropical convection. Since tropical convection occurring at the equator is the main contributor to the formation of the HC, any changes in the rate of convection can have detrimental impacts on the location and strength of the HC. In the case of ENSO, as tropical convection enhances along the equatorial central Pacific, the HC responds in the form of a latitudinal contraction and an overall intensification. Although ENSO’s impact on the HC is widely understood, it has not been connected and studied alongside EBUS migration.

3.0 Methodology Although AOGCMs are an effective method for predicting the future of these current systems and determining the specific mechanisms responsible for changes within EBUS, this paper centers itself on the use of observational data. This chapter is dedicated to discussing the origin of the data used within my report and the process of compiling trends and creating regression maps. 3.1 Sources of Observational Data I gathered the observational data for each variable—SST, SLP, and surface winds—from three sources. First, the observational data for SST was retrieved from NOAA Optimum Interpolation (OI) Sea Surface Temperature (SST) V2 (Reynolds et al., 2002). The data time period reaches from 1982 to 2018 (36 years) and has a spatial grid resolution of 1.0° latitude x 1.0° longitude. The temporal coverage looks at both weekly and monthly means. The OI analysis is conducted over all oceans and the Great Lakes; however, it does not include terrestrial surface temperatures. Second, the data for SLP was pulled from the NOAA/NCEP-DOE Reanalysis 2 (Kanamitsu et al., 2002). Reanalysis data is an objective combination of real observations and numerical models. The blended outcome produces an approximation of the physical state of the examined system. Although the SLP data used in this report is a combination of observations and numerical models, I refer to it as yielding solely observational data because it is exceptionally close to actual observations. This source provided SLP data with a 2.5° resolution and a timeline from 1979 to present. Lastly, surface wind data was pulled from the Cross–Calibrated Multi– Platform (CCMP) Ocean Surface Winds with a resolution of 0.25° and a timeline of 1988–2017. The surface wind data comes from a blend of satellite estimates (over a dozen satellites merged) and in situ (anemometers on buoys) observations of near-surface (10 m) winds over the ocean (Atlas et al., 2011). Although the observational data has a relatively short time record, trend maps are still a valuable tool for investigating whether equatorward winds have been increasing or decreasing over the past several decades; however, the short time record is certainly one limitation within my study. Since my study relies on relatively short time periods, isolating significant trends not attributed to natural climate variability is somewhat challenging. Processes like ENSO, Atlantic Multi-decadal Oscillation, and the West Pacific monsoon all play a role in the creation of these

Observed Trends of Coastal Upwelling in Eastern Boundary Upwelling Systems 15 observational trends. Although a longer time record would be beneficial, there is still value in working with the current data.

3.2 Latitudinal Sub-sectioning After obtaining the data, I focused on the latitudinal sub-sectioning of each EBUS as my primary method of ascertaining whether observational trends follow the mechanisms proposed by the BH or those of the HCH. I use two variables essential to the overall formation of coastal upwelling as the primary approach for examining the potential mechanisms responsible for alterations with EBUS: SST and SLP. Although I obtained observational data for surface winds, I do not analyze the surface wind results. Due to geostrophic balance, SLP gradients effectively diagnose the surface winds. Like other papers on EBUS, I exclude Australia and narrow my scope to the four main EBUS: The Canary, Benguela, Humboldt, and California current systems. Although Australia’s western coast is considered an EBUS, it is not typically included in scientific studies on coastal upwelling because it does not contribute much to net global fish catch. The latitudes for each corresponding upwelling system can be found on the web; however, in order to ensure I encompassed the latitudinal boundaries targeted by Rykaczewski et al. (2015), I obtained the latitudes for this study from their 2015 report and only slightly extended them by one- or two-degrees latitude. I slightly extended the latitudes because I wanted to ensure I encompassed all cooling and warming trends that potentially reach farther than the original boundaries. The latitudes for each EBUS are as follows: CCS (26–49°N), HCS (2.5–44°S), BCS (16–36°S), and CnCS (21–44°N). Although EBUS are relatively narrow, their productive regions extend over 100km offshore. Literature investigating EBUS trends typically compares the current system’s trends with those of the open ocean; therefore, longitudes often extend approximately 500–600km offshore (Rykaczewski et al., 2015). I intended for the longitudes used in this study to be no greater than 500km offshore, but due to time constraints, the longitudes used were only estimates of where upwelling trends should be expected. The lack of consistency in total offshore distance used to observe each system is another limitation of my study. As mentioned earlier, I use latitudinal sub-sectioning as my primary method of testing the results proposed by Rykaczewski et al. (2015); therefore, each observed EBUS was divided into three sections: poleward, mid-latitude, and equatorward. The three subdivisions serve as a

way of examining whether historical intensifications within the upwelling systems agree with the mechanisms proposed in Rykaczewski et al. (2015) or support different mechanisms, such as those proposed by Bakun (1990). If these upwelling systems possess intensification characteristics—such as decreasing SST or increasing PGF—but only demonstrate these characteristics in their poleward latitudes, then there is observational evidence that corresponds to the mechanisms for upwelling intensification proposed by the HCH. If trends were to follow the mechanisms proposed by the BH, the differing land-sea pressure gradients should be strong enough to intensify coastal upwelling at all latitudes, not just poleward ones. Latitudinal sub-sectioning is essential to uncovering which hypothesis is more likely to have played a role in the alteration of these upwelling systems. To determine the three subsections, I divide the current system’s total latitudinal range by three and then assign a corresponding category (poleward, midlatitude, and equatorward). EBUS Poleward Midlatitude Equatorward Table 1 displays the sectioned Latitude Latitude latitudes for each EBUS based on the HCS 44–30.2°S 30.2–16.4°S 16.4–2.5°S use of this method. Figure two on the following page shows an image of CCS 49–40°N 40–33°N 33–26°N insignificant SST, insignificant SLP, BCS 36–29.4°S 29.4–22.8°S 22.8–16°S and significant surface wind trends CnCS 44–36.4°N 36.4–28.8°N 28.8–21°N overlaid with draw boxes indicating

Table 1 Subdivided EBUS Latitudes the observed EBUS and their

Displays the subdivided latitudes for each EBUS. Calculated by dividing the corresponding sectional divisions. total latitudinal range of each system by three.

3.3 Calculation of Trends and Creation of Time Series After subdividing the current systems, I employed several different methods for investigating the supposed trends between the poleward and equatorward latitudes of these upwelling systems. First, through the use of a function called sigtrendmap, I created significant and insignificant trends for each observational variable. The insignificant trends were then displayed on a single global map, serving as a general reference for viewing the location of trends that suggest a potential intensification of coastal upwelling. Outside of figure two, this paper only considers trends with a significance level of 0.05. With a significance level of 0.05,

Observed Trends of Coastal Upwelling in Eastern Boundary Upwelling Systems 17 there is only a 5% risk of concluding that a difference in my results exists when there is no actual difference. Figure two is a compiled map showing the insignificant global trends for SST and SLP. The total range for each EBUS is outlined in green; the poleward regions are outlined in magenta, the midlatitude regions are in blue, and the equatorward regions are in black. Although I only consider the significant trends for data analysis, this insignificant SST trend map helped me identify which latitudes were worth investigating.

Figure 2 Trend Map of Insignificant SST and SLP Overlaid with Significant Surface Winds

This preliminary figure was mainly used to help identify and visualize the latitudes for each EBUS, as well as display their three sectional boxes.

Although figure two provides a good reference of where SST, SLP, and surface winds have been intensifying over their respective time periods, individual time series became my primary method of understanding changes within these upwelling systems. I used time series as a way of investigating significant trends because the results show me the seasonality of the variable and whether or not the variable is stationary. Seasonality refers to the periodic fluctuations in the variable; in this case, seasonality may show ENSO. If the time series is found to be stationary, the examined variable has a constant mean and variance, meaning its properties do not change over time. Through the use of a function called anom_ts, all monthly anomalies–– data points that deviate from the normal––were excluded from the timeseries. Overall, I relied on

twenty time series for determining any modifications occurring within the upwelling systems. Each EBUS had five time series associated with it. The time series can be separated into two categories: total latitudinal range and poleward vs. equatorward. First, I made a time series for each variable––SST, SLP, and surface winds––relying on the total latitudinal range of each upwelling system. For example, when observing SST data for the CCS, I made a time series that used data points from the full latitudinal length of the upwelling system (26–49°N). Second, I repeated the same process and made time series for each variable of each EBUS, however, under this method I used the Figure 3 CCS Poleward SST vs Equatorward SST latitudinal sections defined earlier to create Displays SST time series for the CCS. Both poleward SST and poleward and equatorward ones. For equatorward SST are shown, as well as their corresponding running means. example, looking at SST in the CCS again, I made two timeseries: one constrained to the poleward latitudes (49–40°N) and one constrained to the equatorward latitudes (26–33°N). I repeated this process for each EBUS and each variable until I had twenty time series figures.

After I created the timeseries, I began the analysis section of the report. First, using a window length of 25, I calculated the moving average of each timeseries. A window length of 25 corresponds to 2 years. The moving average smooths the time series and highlights different trends. The moving average clearly displayed seasonality influence, so in addition to calculating the moving average, MATLAB’s built in statistical analysis functions helped find linear trendlines for each timeseries. After plotting each time series and their moving averages, I focused on investigating the poleward intensification discussed in Rykaczewski et al. (2015); therefore, I concentrated mainly on the poleward and equatorward time series. To examine observational changes within the upwelling systems, I created two plots. One plot simply compared the poleward trends to the equatorward ones by overlaying the two series on the same plot. For example, figure three shows SST time series for the CCS constrained to both the

poleward latitudes (49–40°N) of the system and the equatorward ones (33–26°N). Since the poleward portions of the upwelling systems are expected to intensify seasonally and the equatorward portions are expected to weaken, I did not focus on analyzing the midlatitude portions of the systems. Although my study does not look deeply into the midlatitudes, it is still an important part of the upwelling system as a whole and is worth investigating in the future. In addition to plots that compare the time series of poleward and equatorward latitudes, I created plots that displayed the Figure 4 CCS Poleward SST Minus Equatorward SST. differences between the poleward and Displays SST time series for the CCS. Poleward SST minus equatorward SST. equatorward timeseries. To do so, I

created new values that expressed SST or SLP of the poleward region of the current system minus its equatorward region. For example, figure 4 again shows the SST time series for the CCS, but displays one plot line expressing (49–40°N) – (33–26°N).

3.4 Creation of Regression Maps While analyzing my time series plots, I observed anomalous SST and SLP values that clearly represented some form of natural climate variability. In order to observe the interannual climate variability within each EBUS, I created regression maps. The regression maps serve as a way to explore spatial relationships between the poleward and equatorward portions of the current system and determine outlying factors that may be responsible for the formation of these spatial patterns. I created four regression maps displaying regressed SST (poleward minus equatorward for the respective latitudes of the EBUS) overlaid with regressed SLP (poleward minus equatorward for the respective latitudes of the EBUS). I designed the regression map to display values high on the respective index in the poleward portion of the current system. In other words, on the CCS regression map, SST anomalies are represented by unusually warm SST in the poleward portion of the current system. Had I done equatorward minus poleward, the values of SST would be reversed: poleward portions of the current system would express

Observed Trends of Coastal Upwelling in Eastern Boundary Upwelling Systems 20 relatively cool or less warm anomalies and equatorward portions of the system would express warm anomalies. The same can be said for the SLP values. The regressed low SLP over the poleward portion of the CCS would be reversed had I done equatorward minus poleward. I chose not to look at regressed surface winds because the wind speed and direction are largely determined by the SLP gradients via geostrophic balance (plus the influence of friction). Overall, there are a total of four regression maps. Each regression map has the potential to highlight mechanisms and remote connections—such as ENSO or an Indian monsoon—that may be a key factor in the formation of anomalous values within current systems.

4.0 Observational Results and Discussion This chapter is dedicated to reporting the results for both the time series and regression maps. First, I examine and discuss the significant trends from my time series. Significant trends were found using 95% confidence intervals based on a student’s t-test. Second, I examine and discuss my regression maps results and their connection to the mechanisms associated with alterations to EBUS strength and structure. 4.1 Time Series Results for Individual EBUS Before discussing the interannual variability highlighted by my regression maps, the significant trends I found from the varying time series are worth noting. In this section, I report the four current system’s associated trends for SST and SLP. I do not include the analysis for surface winds because changes in SLP can be used to directly infer changes in surface winds. Although the significant trends highlighted in this section are informative, overall, my trends reported by these time series are limited due to the nature of their short time record; therefore, I advise that these results be interpreted cautiously and that readers keep in mind the possibility of interannual variability interfering with my proposed trends. A time series dating back hundreds of years ago would certainly be more telling than the series reported here. 4.1.1 CCS Trends This section examines the observed SST and SLP trends for the CCS. It then reviews whether these trends can be used to support either the BH or the HCH. 4.1.1.1 SST Trends For the overall total latitudinal range of the current system (49°–26°N), the time series depicts a positive SST trend, meaning since 1982, the SST of the entire CCS has slowly increased. Specifically, I found the significant positive trend to have a value of .064 ± .054 °C/decade. Overall, this is equivalent to approximately .23°C over the total time period (1982– 2018). My findings expressing the current system’s positive trend are congruent with those of Seabra et al. (2019) who found warming rates to be approximately 0.06 °C/decade. Figure 5 on the page below displays the CCS’s SST index, overlaid with its running average and line of linear regression. Upon the separation of both the poleward and equatorward latitudinal ranges of this current system, both regions possessed similar findings. Both latitudinal ranges showed significant positive SST trends; however, the difference plot of poleward SST minus

Observed Trends of Coastal Upwelling in Eastern Boundary Upwelling Systems 22 equatorward SST showed no significant trend, meaning the difference between the two regions warming rate was not distinctive enough to be statistically significant at the 95% confidence level.

Figure 5 CCS SST from 1982–2018

Displays the CCS’s observed SST based on its full latitudinal range of 49°–26°N.

4.1.1.2 SLP Trends In addition to the results laid out by the CCS’s SST trends, the current system’s SLP results possessed similar findings. I only found one significant trend within the CCS’s observational SLP data. Out of all four SLP plots, only the plot displaying equatorward SLP possesses a significant trend. SLP in the equatorward portion of the CCS has a positive linear trendline with a value of .12 hPa ± .083 hPa/decade. This slightly positive trendline suggests that over this time record, the pressure of the equatorward portion of the CCS has slightly increased. 4.1.1.3 Potential Hypothesis Support The CCS displayed observational trends that work against hypotheses that predict its intensification. With the time record used in this study, I propose that the SST time series results for the CCS do not directly support either of the two hypotheses examined here: the BH and the HCH. First, in response to the BH, the overall positive trend line of the current system’s SST

Observed Trends of Coastal Upwelling in Eastern Boundary Upwelling Systems 23 suggests that the current system has not been intensifying over the past 36 years as predicted by Bakun (1990). Second, since the HCH relies on the mechanism of HC expansion for a consequential poleward migration of the upwelling system, time series trends that support this mechanism would reveal SST trends that suggest the poleward portions of the CCS are warming at a slower rate than the equatorward portions of the system. Since my difference plots expressing poleward SST values minus equatorward ones show no statistically significant negative trend line depicting equatorward values higher than poleward ones, I conclude that observational SST difference plots also show no signs of supporting the mechanism proposed by Rykaczewski et al. (2015). In fact, the trend line depicting poleward SST minus equatorward SST is exactly 0, meaning there is no insignificant or significant difference between the rate of warming within the two varying portions of the CCS. In regards to the positive SLP trend in the equatorward portion of the CCS, this observed trend suggests the mechanisms proposed by the HCH are not evident in the CCS. If the observational data did show trends similar to the ones presented by the HCH, then my results should display a significant positive trendline in the poleward portion of the system; however, since my results show a positive SLP trendline within the equatorward portion of the CCS, the observational data for this time period shows that the upwelling system has not yet started to shift poleward in correspondence to HC migration as predicted by the HCH. In correspondence to the minimal trends exposed by the SST and SLP timeseries, all plots and trends for alterations within the CCS’s surface winds are also insignificant. This suggests that significant SST and SLP changes within the upwelling system are not strong enough to instigate significant changes in the CCS’s surface winds. As a result of this lack of influence SST and SLP have on the current system’s wind speed, I conclude that over this time record, the rate of upwelling within the CCS has neither intensified or weakened. Furthermore, as shown by the CCS’s results, the short time record for each variable (SST, SLP, and surface winds) is a clear limitation of attempts to truly determine the mechanisms responsible alterations within the CCS.

4.1.2 HCS Trends This section examines the observed SST and SLP trends for the HCS. It then reviews whether these trends can be used to support either the BH or the HCH. 4.1.2.1 SST Trends To continue the examination of the observational data used within my study, this section looks at the results obtained from the time series for the HCS. To begin, over the total latitudinal range of the HCS, the SST expressed a negative linear trend, meaning over this time period, the HCS’s SST has slowly been decreasing; however, I found this negative linear trend to be insignificant at the 95% confidence level. Although I found this negative trend to be insignificant, my results still coincide with the results of Seabra et al. (2019) where the HCS is classified as the only EBUS with a consistently negative trend in SST. Upon the separation of the

Figure 6 HCS Difference Plot of SST

Displays the HCS’s poleward SST minus its equatorward SST. system’s poleward and equatorward regions, only the equatorward portion of the system possessed a significant negative trendline. With a trendline of -.103 ± .094 °C/decade, between 1982 and 2018, the equatorward portion of the HCS decreased by roughly 0.33°C. Although only the equatorward portion of the current system possessed a significant trend, the HCS difference

Observed Trends of Coastal Upwelling in Eastern Boundary Upwelling Systems 25 plots of poleward SST minus equatorward SST possessed a significant positive linear trendline of approximately .12 ± 0.9 °C/decade). Above, figure 6 displays a plot of this trend which indicates that over this time record, the poleward SST possessed a larger rate of warming than the equatorward portion of the system. 4.1.2.2 SLP Trends To continue with the observational trends for the HCS, in this paragraph I analyze the trends for SLP and infer its effect on the strength of surface winds. To begin, my plots for the total latitudinal range of the HCS produced a significant positive linear trendline, indicating that over this time record, the SLP of the HCS has been slowly increasing by .13 ± .066 hPa/decade. This positive trendline indicates that over this time record, the subtropical anticyclone associated with the HCS has slowly been increasing. When looking at the separated latitudinal subsections of the HCS, only the poleward section of the current system contained a significant trend. The poleward SLP of the HCS has a positive trendline of .26 ± 0.14 hPa/decade. 4.1.2.3 Potential Hypothesis Support Upon analysis of the HCS’s timeseries, several conjectures can be made from the few significant trends they contain. In regards to the SST analysis, while the entire latitudinal range of the current system expresses a decreasing SST over this time record, the insignificance of the linear trendline prevents this specific trend from being used as evidence in support of total intensification of the current system; however, in regards to SST, the significant negative trend found in the equatorward portion of the system paired with the significant positive trend displayed in the current system’s difference plots can be used to infer potential mechanisms outside of the HCH responsible for HCS’s intensification. In a way, both plots display data that challenges the mechanisms proposed by Rykaczewski et al. (2015). If the observational data for this time record did show evidence of the HCH in action, I would expect my results to show a significant negative SST trend in the poleward portion of the system, corresponding to the poleward migration of the descending high-pressure branch of the HC. Furthermore, a difference plot that supports the mechanisms proposed by the HCH would display a negative trendline expressing that the equatorward portion of the HCS possesses a smaller rate of SST cooling than the system’s poleward portion. Although the observational SST data shows no signs of poleward intensification of the upwelling system, the current system’s SLP trend for its poleward extent suggests otherwise.

Since the positive SLP trend occurs in poleward portion of the HCS, it is possible that this is a sign of the HCH in action. Although the HCS’s equatorward SLP is insignificant, it still displays a negative SLP trendline. Since the SLP only seems to be increasing over the poleward portion of the current system, the SLP of the HCS seems to be following a trend similar to what is proposed by the HCH. As demonstrated in this section, the trends held by the HCS are inconsistent and are therefore inconclusive in regards to satisfying the two examined hypotheses. If the trends were harmonious, a negative SST trend and positive SLP trend would occur within the same latitudinal region of the system. Therefore, because my results show evidence of opposing trends occurring in opposing regions, I conclude these results incapable of supporting both the BH and the HCH.

4.1.3 BCS Trends This section examines the observed SST and SLP trends for the BCS. It then reviews whether these trends can be used to support either the BH or the HCH. 4.1.3.1 SST Trends The full latitudinal range of the BCS displayed SST trends similar to that of the CCS. Overall, I found the BCS to possess a significant positive SST trend throughout the entire time record. Increasing by approximately 0.09 ± 0.03 °C/decade, the BCS rose by roughly .32°C over the full time period. Figure 7 displays this warming trend.

Figure 7 BCS SST from 1982–2018

Displays the BCS’s SST from its full latitudinal range of 36–16°S.

The overall warming of the BCS matches the findings of Seabra et al. (2019); however, the Seabra et al. (2019) study found different rates of warming amounting to 0.17 ± 0.11°C/decade. I hypothesize our numerical rates of warming are different because their study uses SST data operating on a ¼° resolution, whereas my SST data operates on a scale of 1° resolution. Although the full latitudinal range of the BCS expresses a significant increase in SST, upon examination of the divided latitudinal ranges, the poleward portion of the system appears to be warming at a slower rate than the equatorward portion. For instance, the poleward extent of the BCS has a warming rate of approximately 0.06 ± 0.03°C/decade, whereas the equatorward portion of the

Observed Trends of Coastal Upwelling in Eastern Boundary Upwelling Systems 28 current systems has a warming value of 0.16 ± 0.05°C/decade. Furthermore, difference plots of poleward SST minus equatorward SST distinguish this difference in the current systems latitudinal warming rates through a negative trend of -0.096 ± 0.05°C/decade. This negative trend in the system’s difference plot paired with the substantial difference in the current system’s sectional rate of monthly warming indicates that the poleward portion of the system is acting as a larger buffer to global oceanic warming than its equatorward portion. 4.1.3.2 SLP Trends Although both latitudinal portions of the BCS expressed significant positive trends for the SST data, only the equatorward portion of the current system possessed a significant SLP trend. The equatorward portion of the BCS portrays a significant negative SLP trend, indicating that over the time period, SLP at this latitudinal region has been decreasing by approximately -0.08 ± 0.06 hPa/decade. As covered in section 2.2, the strength of the PGF dictates the strengths of the surface winds, which in end determines the strength of coastal upwelling. The negative SLP trend occurring in the equatorward portion of the current system coincides with its higher decadal SST warming rate; that is to say that the decreasing strength of the subtropical anticyclone extending into the equatorward portion of the system weakens wind strength, resulting in the slight deterrent of coastal upwelling. 4.1.3.3 Potential Hypothesis Support As demonstrated by section 4.1.3.1, over this time record, the BCS has warmed overall; therefore, in contrast to the general predictions made by the BH, I conclude the BCS has not intensified correspondingly to the degree of average global warming over the past 36 years. However, I do hypothesize that the differing rates of oceanic warming occurring in the two regional boundaries of the current system suggest a connection between HC and EBUS migration. As explained by the HCH, if EBUS were to follow the migration of the HC’s descending branch, intensifications in upwelling should occur in the poleward portion of current systems. Although the BCS displays no observational evidence of its intensification, I conclude that the slower warming rate of its poleward latitudes more closely follows the mechanism proposed by the HCH as opposed to the BH.

4.1.4 CnCS Trends This section examines the observed SST and SLP trends for the CCS. It then reviews whether these trends can be used to support either the BH or the HCH. 4.1.4.1 SST Trends For the CnCS, I found a significant SST warming trend for the current system’s full latitudinal extent. With a trend of approximately 0.14 ± 0.04°C/decade, I found the entire current system to have increased by approximately .51°C over the whole-time record. Figure 8 below displays this significant warming trend in the CnCS. In contrast to the results found for the BCS

Figure 8 CnCS SST from 1982–2018

Displays the CnCS’s SST from its full latitudinal range of 21–44°N. that suggest poleward SST buffering, both the equatorward and poleward portions of the CnCS possess a warming trend of 0.12 ± 0.05°C/decade. From 1982–2018, both regions of the current system warmed by roughly .44°C. Seabra et al. (2019) also found the CnCS to have experienced overall warming during this time period; however, their study differs from mine in that they found “remarkably high warming rates” in the equatorial portion of the CnCS (0.60 °C/decade off Mauritania, southern Canary). Although I found uniform warming rates throughout the current system, it is possible my warming rates differ from those of the Seabra et al. (2019) study

Observed Trends of Coastal Upwelling in Eastern Boundary Upwelling Systems 30 because the spatial resolution of their observational SST data has a greater ability to distinguish specific latitudinal warming trends.

4.1.4.2 SLP Trends To continue with the CnCS’s time series results, I only found one significant SLP trend for the entire system. Only the current systems equatorial range possessed a significant negative trend amounting to -0.22 ± 0.95 hPa/decade. This negative trend indicates that the CnCS’s equatorial SLP has decreased over the studies time record. 4.1.4.3 Potential Hypothesis Support Although the CnCS did yield some significant trends, I conclude that the trends produced for this current system fail to support both the BH and the HCH. The overall warming in SST suggests the current system is not intensifying, but rather it is weakening. This warming contradicts the mechanisms proposed by the BH which proposes that terrestrial warming will enhance the processes behind the formation of EBUS. Furthermore, the identical rate of warming experienced by both portions of the CnCS suggests that based on this data, there is no evidence of the current system’s poleward migration.

4.2 Regression Map Results for Individual EBUS

Due to the short nature of the time record used to create trends within my timeseries, it is likely that natural climate variability impacted the trends discussed in section 4.1. For example, the occurrence of ENSO at the end of the time record could result in anomalously warm SSTs in EBUS and potentially impact the current system’s entire trend; therefore, while working with the observational time series used in this paper, I began to question what specific modes of interannual variability cause anomalies within the strength and structure of EBUS. Because these anomalous values impact the state and shape of EBUS, they directly influence the productivity of fisheries located along these current systems; therefore, the more investigation done on EBUS’s responses to interannual variability, the likelihood of researchers and fisheries ability to predict the future state of these current systems increases. In order to observe the interannual variability impacting EBUS, I created regression maps that display outlying climatic processes that occur during the presence of anomalous values in EBUS’s SST and SLP. These regression maps provide a number of conclusions, but mainly they expose the potential connection between ENSO and the physical state of EBUS. Since my project seeks to examine whether the migration Observed Trends of Coastal Upwelling in Eastern Boundary Upwelling Systems 31 of the HC impacts the location and strength of EBUS as proposed by Rykaczewski et al. (2015), each EBUS regression map displays the current system’s difference of its poleward latitudinal values and equatorward latitudinal values. Throughout this section, I will examine the climatic processes, such as ENSO, that occur during the current system’s high SST and SLP indices and discuss the implications of the relationship between the current system and its interannual variability.

4.2.1 CCS Regression Analysis

Figure 9 CCS Regressed SST (Poleward-Equatorward) Overlaid with Regressed SLP (Poleward-Equatorward) To begin, I examine the CCS’s SST (poleward minus equatorward) regression map overlaid with regressed SLP (poleward minus equatorward). Figure 9 shows that the poleward latitudes of the upwelling system reflect SST values that are unusually high on the overall SST index, whereas the equatorward portion of the current system reflects SST values that are unusually low on the SST index. As explained earlier, this map displays climatic processes that are occurring outside of the examined EBUS; therefore, I examine that when CCS has an unusually high SST index value, there is also a strong Aleutian low and a weak central Pacific ENSO. The strong Aleutian low can be identified by the dashed circular system occurring at

Observed Trends of Coastal Upwelling in Eastern Boundary Upwelling Systems 32 approximately 60°N, off the southern coast of Alaska, and the weak central Pacific ENSO can be identified as the unusually warm SST occurring along the equator at 0°. Two conclusions can be made from this regression map: first, the current system’s differing SST values can be explained by the differing pressure systems off the coast of California; second, the presence of a central Pacific El Niño suggests the accompanied contraction of the HC impacts the latitudinal location of the CCS. The first conclusion made from this regression map is fairly predictable. The high SST indices in the poleward portion of the CCS are explained by the strong Aleutian low neighboring it. The Aleutian low is classified as a cyclone since its center consists of relatively low pressure. As explained by section 2.2, air following geostrophic flow in a cyclonic system flows towards the central low and is deflected to the right in the NH, resulting in a net counterclockwise flow. In the case of the CCS, surface winds moving counterclockwise are pushed poleward, resulting in an onshore Ekman Transport that moves water towards the coast and encourages coastal downwelling. As a result of this downwelling, cool upwelled water is unable to rise and the local region reflects warmer SSTs. Similar to the explanation the Aleutian low provides, the low SST indices in the equatorward portion of the CCS are explained by the neighboring North Pacific High. Again, section 2.2 explains how anticyclones, which have centers of high pressure, adhere to geostrophic flow, direct airflow away from the center, deflect this air to the right in the NH, and revolve in a net clockwise rotation around the central high. As a result of this clockwise rotation, surface wind flows towards the equator, resulting in an offshore Ekman Transport that encourages coastal upwelling. This coastal upwelling brings cool, subsurface water to the surface and results in the formation of the anomalously low SST indices in the CCS. This first conclusion made from the CCS regression map expressing the connection between these pressure systems and their corresponding SST values is unsurprising; however, the second conclusion provided by the map is slightly more surprising because it invites speculation about the potential connection between ENSO and EBUS location. In other words, the presence of a central Pacific El Niño begs the question ‘what teleconnections are possibly responsible for the formation of the CCS’s high poleward SST indices?’ As explained by section 2.7, ENSO represents a reversal or weakening of the Walker Cell Circulation. During normal Walker Cell conditions, there is consistent upwelling at the equator. This upwelling contributes to the cool SST located along the equator. Since the figure above shows relatively warm SST

Observed Trends of Coastal Upwelling in Eastern Boundary Upwelling Systems 33 values along the equator, I conclude there has been a weakening or reversal of the Trade winds and a resultant El Niño is present in this regression map. This regression map suggests that when the poleward SST indices are anomalously high in the CCS, there is an ongoing central Pacific El Niño. Although determining the presence of ENSO is relatively simple, it’s connection to the CCS is more complex and only a projection. I hypothesis that because ENSO directly impacts the strength and location of the HC, the anomalously warm SST indices in the poleward portion of the CCS are created by ENSO and the subsequent equatorward contraction it inflicts on the HC. As explained by section 2.7, ENSO thwarts equatorial upwelling, which increases equatorial SST. This increase in equatorial SST enhances tropical convection, which intensifies the HC and causes it to contract. More clearly put, in the presence of ENSO, the HC narrows its latitudinal extent and strengthens. In the CCS’s regression map, evidence of HC contraction is identifiable through two means: first, through the equatorward migration of the CCS; second, through the migration of the HC’s descending branch. To begin, the equatorward migration of the CCS is evident by the opposing SST values in the poleward and equatorward latitudes of the current system. As proposed by Rykaczewski et al. (2015), if the CCS is linked to the HC, the poleward expansion of the HC would cause the current system to also shift poleward, resulting in cooler SST values at higher latitudes. In contrast, based on Rykaczewski’s hypothesis, it can be assumed that if the HC contracts equatorward, the CCS will also contract equatorward and demonstrate cooler SST indices in its equatorward extent. Since the regression map above shows an equatorward shift of the CCS at a time when the HC is also contracting equatorward, I hypothesis that the migration of the CCS corresponds to the migration of the HC. The second piece of evidence suggesting a contracting HC is the migration of the HC’s descending branch. As mentioned in section 2.4, the ascending equatorial branch of the HC typically travels poleward until about 30°N&S where it cools and descends towards the Earth creating a zonally average high surface pressure. As shown by the image above, specifically in the Northern Pacific Ocean, the high-pressure system associated with the descending branch of the HC contracts equatorward and assumes a location of about 20°N. Furthermore, the subsequent ascending branch of the Ferrell Cell––typically located at 60°N&S––appears to have shifted equatorward in the SH and the high-pressure system associated with the Polar Cell takes its place at 60°S. These equatorward shifts in the descending branches of the HC also serve as evidence of the HC contraction that accompanies ENSO.

ENSO’s impact on oceanic and atmospheric processes is ambiguous and fluctuates with every individual occurrence, but as demonstrated by this figure it is connected to the SST and SLP values associated with the CCS. Because I hypothesize the connection between ENSO and the state of the CCS is likely a result of the migration of the HC, my findings on interannual variability within the CCS follow the mechanisms for alterations within upwelling systems proposed by Rykaczewski et al. (2015).

4.2.2 HCS Regression Analysis

Figure 10 HCS Regressed SST (Poleward-Equatorward) Overlaid with Regressed SLP (Poleward-Equatorward)

As demonstrated by the CCS, regression maps help display outlying climatic processes that impact the state of EBUS. The image above shows the HCS and its anomalous SST and SLP values. Specifically, the figure above reveals when the HCS has anomalously high poleward SST indices, there is also an ongoing La Niña directly in the equatorward region of the current system. As covered in section 2.7, ENSO sends out atmospheric and oceanic Rossby waves that literally have rippling on climatic processes. In addition to the rippling effects ENSO has on the climate, its counterpart, La Niña also produces teleconnections that alter average climatic states. In figure 10, the low SSTs dominating the equatorward portion of the HCS are a clear

Observed Trends of Coastal Upwelling in Eastern Boundary Upwelling Systems 35 representation of La Niña. During La Niña, Walker Cell circulation is enhanced, meaning preexisting trade winds, pressure gradients, and SSTs all strengthen. The low SSTs created by the presence of La Niña is rightfully accompanied by an anticyclone. This anticyclone rotates counterclockwise in the SH; therefore, its winds travel equatorward and deflect surface water away from the shoreline, resulting in a net Ekman Spiral that encourages coastal upwelling and further lowers SSTs. In contrast to the impact ENSO has on the HC, La Niña effects the HC in a completely opposing manner. Unlike ENSO, La Niña––which initiates decreased tropical convection–– causes the HC to expand poleward and weaken. I hypothesize that this poleward expansion is evident in my regression map for the HCS. As shown above, an anomalously strong anticyclone dominates the eastern portion of the Pacific Ocean. This zonally averaged high-pressure system reaches both hemispheres and stretches from a latitude higher than 60°N to one of about 57°S. Rather than demonstrating the latitudes typically associated with the HC’s zonally averaged pressure systems, the figure above shows that the descending branch of the HC has migrated past 30°N&S and assumed a poleward position. This poleward migration is specifically evident in the North Pacific where the high-pressure system occupies latitudes well above 60°N. Once again, due to the impact it has on the HC, I hypothesize that the presence of La Niña serves as evidence of the relationship between the HCS and the HC. When the HC migrates poleward, the HCS follows and continues to display evidence of anomalously cool SSTs all along the coast of South America. Although there is a clear warm tongue that jets out at about 40°S in the Pacific Ocean, this anomalously warm SST does not reach the coast, meaning that coastal upwelling is still ongoing at all latitudes of the system. Here I conclude that the HCS serves again as evidence of the validity of the mechanisms proposed by the HCH; however, in regards to the HCS, I advise these results be interpreted with caution. Because of the current system’s close proximity to the equator, it’s difficult to directly distinguish the interannual variability associated with the current system from the local manifestation of ENSO itself. Due to the time restraints of my study and my limited experience with coding, it was impossible for me to design a latitudinal box close enough to the coast that only represents Ekman Transport specific to the HCS. As a result of this limitation, it is possible that La Niña potentially contaminated the HCS regression map.

4.2.3 BCS Regression Analysis

Figure 11 BCS Regressed SST (Poleward-Equatorward) Overlaid with Regressed SLP (Poleward-Equatorward) As shown above, the BCS reflects a regression map similar to those of the CCS and the HCS. In all three maps, the equatorward portion of the system expresses low SST indices, whereas the poleward portion of the system reflects high SST indices. Furthermore, like the other two systems, these SST indices can be explained by anomalously high or anomalously low SLP. In the BCS, the low equatorward SST can be explained by the presence of an anticyclone. As air moves counterclockwise around a high-pressure system in the SH, alongshore winds move equatorward resulting in an offshore Ekman Transport that facilitates upwelling and lowers SSTs. In contrast, the poleward SST can be explained by the cyclone located at 40°S. This cyclone forces air to move in a clockwise rotation in the SH; therefore, at the coast of South Africa, winds moving towards the coast facilitate downwelling and contribute to the region’s unusually high SST. The connection between the CCS, HCS, and BCS extends beyond the relationships between cyclonic systems and their resultant SST values; specifically, all three current system demonstrate some influence of the ENSO cycle in the creation of their high poleward SST

Observed Trends of Coastal Upwelling in Eastern Boundary Upwelling Systems 37 indices. This potential connection suggests that the atmospheric and oceanic Rossby waves created by El Niño and La Niña not only impact EBUS located in the Pacific Ocean, but also impact Atlantic ones. Similar to the results found for the CCS, when the BCS has low equatorial SST indices and high poleward ones, there is a strong El Niño occurring in the Pacific Ocean. Once again, I hypothesize that the proposed mechanism behind the equatorward shift of the BCS is the contraction of the HC in the presence of ENSO. As the HC contracts equatorward, it migrates its descending branch equatorward establishing a latitude of 20°S as opposed to the typical 30°S. The migration of this high-pressure system shifts the location of upwelling in the BCS, causing it to also migrate to a more equatorial position. My findings for the BCS also support the mechanisms proposed by Rykaczewski et al. (2015).

4.2.4 CnCS Regression Analysis

Figure 12 CnCS Regressed SST (Poleward-Equatorward) Overlaid with Regressed SLP (Poleward-Equatorward) In addition to the impact ENSO has on the three other EBUS, evidence of its impression on the strength and structure of current systems extends to include the CnCS as well. Like the CCS, HCS, and BCS, the CnCS’s regression map displays relatively cool SST indices in the equatorward portion of the current system and relatively warm SST indices in the poleward Observed Trends of Coastal Upwelling in Eastern Boundary Upwelling Systems 38 portion of the current system; however, unlike the three other EBUS, the CnCS’s differing SST values cannot be explained by opposing pressure systems. If the CnCS followed the other current systems, its high poleward SST indices would be accompanied by a cyclone which moves counterclockwise in the NH and encourages downwelling. Instead of a cyclone residing over the Iberian coast, an anticyclone dominates all latitudes of the CnCS. At first the lack of the opposing pressure systems is troubling, but upon examination of the weak central La Niña occurring in the Pacific Ocean, the differing structure of the CnCS’s regression map makes more sense. As explained earlier, La Niña causes the HC to expand and weaken. Evidence of HC expansion is visible through the poleward migration of its descending branch positioned well above 30°N. I hypothesize that the HC expansion is the responsible for the strong anticyclone dominating the entire CnCS, and it is this anticyclone that influences the anomalous SSTs in the CnCS by creating climatic conditions supporting enhanced solar radiation and evaporation. First, I theorize that the warm SST anomalies in the poleward portion of the system owe their existence to enhanced solar radiation. As figure 12 shows, the highest SST values are located directly under the middle of the anticyclone. High-pressure systems are typically characterized as generating a climate with low cloud coverage. Low cloud coverage allows greater amounts of solar radiation to be received at the surface; therefore, the clear sky centered over Spain warms localized SSTs. The low SST indices located in the equatorial portion of the current system owe their existence to the increased amount of nearby evaporation. Figure 12 displays that the anomalously low SST indices are located in the current system’s equatorial region directly under tightly packed isobars. Isobars are “lines on a map that connect points with the same atmospheric pressure at a given time” (Britannica, 2019). These tightly packed isobars represent a steep horizontal pressure gradient, which directly indicates strong winds. In the case of the CnCS, the tightly packed isobars suggest strong winds with relatively dry air coming off of the West coast of Africa. Strong winds comprised of dry air creates climatic conditions that encourage evaporation. Evaporation requires heat, and this heat is supplied by the sea surface. Since evaporation takes heat away from the sea surface, it has an overall cooling effect. For the CnCS, the enhanced evaporation caused by the strong offshore winds cool the current system’s equatorial SSTs. As explained above, La Niña’s impact on the structure of the HC explains the creation of the opposing SST values in the equatorward and poleward portions of the CnCS; however, the

Observed Trends of Coastal Upwelling in Eastern Boundary Upwelling Systems 39 strength of the current system’s opposing SST values are controlled by a positive feedback operating as one of ENSO’s teleconnections. I propose the positive feedback responsible for the persistence and strength of the CnCS’s anomalous SST values is a meridional mode known as the Wind-Evaporation-SST (WES) feedback. Unlike ENSO––which relies on the weakening or reversal east-west climatic properties––meridional modes rely on the weakening or reversal of north-south climatic properties (Karnauskas, 2020). This paragraph is dedicated to describing how the WES feedback strengthens opposing SST values within the CnCS. The impact of the WES feedback begins with recognition of the preexisting surfaces winds occurring between the ascending equatorial branch of the HC and the

Figure 13 Displays the Wind-Evaporation-SST Feedback

Schematic illustration of WES feedback obtained from Karnauskas (2020). descending midlatitude branch of the HC. Between these two zonally averaged pressure systems, the Northeasterly Trade winds permanently exist. The Northeasterly Trade winds (also known as the Easterlies) originate from the East and flow towards the West. As mentioned in section 2.2, winds form from PGFs which direct air flow from areas of high pressure to areas of low pressure, and pressure gradients are generally created from differing surface temperatures. Cool surfaces are normally associated with high surface pressure, whereas warm surfaces are associated with low surface pressure. For the CnCS, an area of high pressure resides over the system’s cool equatorward SST. In contrast, low pressure resides over the current system’s warm poleward SST. Part A & B of figure 13 displays these opposing SST and SLP values. Part C shows the resultant PGF that pushes air flow in a South to North direction. As demonstrated by part D, this South to North pressure gradient acts as an opposing force on the Northeasterly Trade winds. Overall, the geostrophic wind created by the South to North pressure gradient is flowing in a manner that slows down the Northeasterly Trade winds, but its ability to slowdown

Observed Trends of Coastal Upwelling in Eastern Boundary Upwelling Systems 40 the Easterlies varies with different latitudes. Section 2.2 discusses the CF and its effect on wind and current direction; however, section 2.2 fails to mention that the strength of the CF increases with increasing latitude. In other words, the CF has a force of 0 at the equator; therefore, winds and currents in the NH (SH) closer to the equator will undergo less of a rightward (leftward) direction shift. In the case of the CnCS, its high SLP formed by the cool SST is centered at a latitude of about 20°N and its low SLP formed by the warm SST is centered at a latitude of about 40°N; therefore, as demonstrated by part D, the airflow following the PGF moving South to North is only slightly deflected to the right at 20°N and more dramatically deflected to the right at 40°N. Because the CF causes airflow at 40°N to assume a greater rightward direction than the airflow at 20°N, winds at 40°N work to slowdown windspeeds because they strongly oppose the direction of the Easterlies. In contrast, the CF’s minimal effect on the airflow originating in the equatorward portion of the current system causes the winds here to have less of an opposing effect on the Northeasterly Trade winds. As a result, the winds in the equatorward portion of the current system move quicker than the poleward ones. Wind speed and evaporation rate are directly proportional; in other words, increases in windspeed will result in increases in the rate of evaporation. As shown by part E, the faster wind speeds in the equatorial portion of the CnCS cause rates of evaporation to increase over the current system’s equatorward latitudes. In contrast, the slower wind speeds in the poleward portion of the system result in less ongoing evaporation. Evaporation involves the transfer of heat from the ocean to the atmosphere; therefore, it cools the ocean surface. With a greater evaporation rate in the equatorward portion of the current system, anomalously cool SST values are strengthened. In contrast, the lower rate of evaporation occurring in the poleward portion of the system warms the surface and strengthens the current system’s anomalously warm SST values. As these opposing SST values continue to strengthen, the opposing pressure gradients will also strengthen and the WES feedback will continue to intensify these opposing SST and SLP values. It is this reinforcement in the preexisting opposing SST values that classifies the WES feedback as a positive one.

5.0 Summary and Conclusion Although the significant trends highlighted by my time series provide insight on which EBUS have intensified or weakened over the past 36 years, due to the short nature of the time record, the significant trends cannot be used to infer the mechanisms responsible for changes within the current systems structures. Instead, I rely on my regression maps as a means of exposing the mechanisms associated with structural modifications in EBUS. In summary, based on the interannual variability associated with each current system, my results point to a connection between the structure of the HC and the structure of EBUS. The regression maps of all four current systems are influenced by some phase of the ENSO cycle. Whether it be the contraction of the HC in the presence of El Niño, or the expansion of the HC in the presence of La Niña, each EBUS responded to the structural changes of the HC by migrating coastal upwelling correspondingly to the position of the HC’s descending branch. Due to this harmonious migration, I suggest that the mechanisms responsible for substantial alterations to EBUS strength and structure more closely follow those proposed by the HCH as opposed to those of the BH. That being said, it is possible that the BH will still have some future impact on the localized strength of EBUS, but I hypothesize that future changes within the strength and structure of the HC will be the true determinant for the future of EBUS. As climate change progresses at an unprecedented rate, the continual investigation of its impact on EBUS is essential to understanding the future of productivity and climatic state of fisheries and nations residing in their geographic boundaries. Understanding the historic trend of these current systems is essential to calculating the degree of internal change accompanying a warming planet; however, natural climate variability may be a more important factor in determining the atmospheric and oceanic conditions nearby nations can expect to see on a yearly basis. For example, during the presence of an El Niño, fisheries along the coasts of Peru, Oregon, South Africa, and Spain can all expect a poleward decrease in upwelling and prepare for a potential decrease in their net fish catch. My study can be used by fisheries and oceanographers as a way to forecast alterations within EBUS upon the occurrence of ENSO and La Niña. Although it is widely known that the teleconnections of ENSO extend beyond the Pacific Ocean, ENSO’s explicit correspondence to EBUS is rarely studied and has been neglected to be used as a method of determining the hypothesized relationship between the HC and EBUS. Since all four of my regression maps display the presence of ENSO and the migration of the HC, I conclude

Observed Trends of Coastal Upwelling in Eastern Boundary Upwelling Systems 42 my results support the mechanisms proposed by Rykaczewski et al. (2015), and I hypothesize if the HC expands poleward as predicted, the CCS, HCS, BCS, CnCS will follow this poleward migration and intensify at higher latitudes.

6.0 Limitations and Recommendations for Future Research

My study serves its purpose in examining the proposed mechanisms behind EBUS intensification as well as identifying the important relationship between interannual variability and the geographic location of EBUS; however, there are a number of limitations to my study that function as an opportunity for further research. As mentioned in section 4.1, the short time record used for my time series is a clear limitation of exposing significant trends for SST and SLP. With a longer time record, observational time series will likely yield more conclusive trends that serve as a better mechanism of identifying the future state of EBUS. Furthermore, both methods used in my study––observational time series and regression maps––are constrained to an annual analysis. With more time, I would have created seasonal trends and seasonal regression maps, separating augmented upwelling seasons from depressed upwelling seasons. By doing so, I could examine additional modes of interannual variability besides ENSO that also impact the strength and structure of EBUS. Furthermore, seasonal regression maps would reveal whether the mechanism and hypotheses I tested for are limited to specific seasons. In addition to the need for seasonal maps, further research should be conducted on the cyclical phase between anomalous SST values in the opposing latitudinal extent of the current systems. For example, although my regression maps display regressed SST (poleward minus equatorward), it is not necessarily certain that the maps would display opposing SST values within the current system. The regression maps could display stronger warm anomalies occurring at the poleward boundaries of the system with weaker warm anomalies at the equatorward portion of the system. Since my regression maps display cool anomalies in the equatorward portion, there seems to be some type of relationship within the system that causes the latitudinal boundaries to phase in between differing SST values. Further research should be performed on the causation of this phasing relationship. Lastly, a large limitation of my study is that it fails to determine a precise offshore distance for the longitudes used in the construction of the current systems geographic locations. For example, I would have liked to create longitudinal boundaries that only included data extending 500km or less off the coast of each EBUS; however, due to my lack of time and coding knowledge, I estimated the longitudes used in my study and tried to create each current system’s box as close to the shore as possible without excluding any possible trends. If I were to redo this study, I would begin by creating a function that limits trend calculation and variable

Observed Trends of Coastal Upwelling in Eastern Boundary Upwelling Systems 44 regression to a specific offshore distance. I recommend that any further investigations of EBUS should implement such a practice before deep analysis.

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