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Observed Trends of Coastal Upwelling in Eastern Boundary Upwelling Systems

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Observed Trends of Coastal Upwelling in Eastern Boundary Upwelling Systems Observed Trends of Coastal Upwelling in Eastern Boundary Upwelling Systems By 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
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