Date of Award

Spring 2020

Access Restriction

Thesis

Degree Name

Master of Science

School or College

Seaver College of Science and Engineering

First Advisor

Jeremy S. Pal

Second Advisor

John Dorsey

Third Advisor

Donald R. Kendall

Abstract

The California Current system (CCS) is home to a vast and diverse marine coastal ecosystem. Upwelling is an oceanic phenomenon wind-driven displacement of surface water brings cold nutrient-rich water from the ocean bottom to surface waters. Along the coast in the CCS, upwelling occurs via the advection of water perpendicular to the shoreline northerly winds, a phenomenon called “Ekman Transport”. In the open ocean. Wind stress curl causes disruption of normal currents, resulting in small pockets of upwelled or downwelled water (downwelling is the movement of water downward in the ocean, the opposite of upwelling). This process is called “Ekman Pumping”, and over large swaths of ocean, it can result in a notable increase/decrease in net upwelling. Upwelling is one of the main driving forces behind the diversity and strength of the ecosystems within the CCS. The Bakun hypothesis (Bakun, 1990) suggests that with the future increase of atmospheric greenhouse gases (GHGs), Eastern Boundary Upwelling Systems (EBUSs) will experience an increase in upwelling intensity and season duration. The Bakun hypothesis has been proven to be an accurate description of the mechanisms of change expected in all major EBUSs in the world, except for the CCS, where only weak correlations have been made (Sydeman, García-Reyes, Schoeman, D. S. Rykaczewski, R. R. . Thompson, Black, & Bograd, 2014). A recent study by Wang et al. 2015 found weak correlations of decreases in upwelling intensity in the CCS, which would suggest that the Bakun Hypothesis does not accurately depict the future of the CCS. Previous climate change CCS studies have relied on atmosphere-ocean global climate models (AOGCMs), which are typically performed on a horizontal grid too coarse to accurately depict the physical changes expected in the CCS.

In this study, a 10-member ensemble of high resolution regional climate model (RCM) climate change experiments driven by 10 AOGCMs is used to project changes in the timing and intensity of the upwelling season within the CCS and test the Bakun Hypothesis. We also consider Surface Temperature to extrapolate how changes, if any, compound or counteract the accuracy of the Bakun Hypothesis. We find a significant decrease of 6 m^3/s/100m in mean Ekman Transport and an increase of 1.3 ⁰C in mean Surface Temperature. A 1.1×10^(-06) m/s decrease in mean Ekman Pumping is also observed, but these findings are not robust. We also find no change to the length or timing of the upwelling season. The decrease in Ekman Transport and increase in Surface Temperature could compound upon each other and cause damage to vulnerable ecosystems within the CCS, most notably in the latitudinal range of 34°-40° along the coast, as future GHGs concentrate in the atmosphere. In the shallow ocean, decreased upwelling intensity and increased stratification will inhibit the movement of nutrients that primary producers rely on, thus stifling phytoplankton and zooplankton growth. Increases in surface temperature could also create new avenues of environmental stress that local communities are not prepared to adapt to, resulting in decreased physiological performance and potentially geographical shifts in species residence. These changes to shallow ocean communities could have cascading effects on the availability of prey up the trophic web. In the deeper ocean, decreased upwelling intensity and increased stratification will inhibit ocean mixing, and thus decrease availability of dissolved oxygen in the deeper ocean. This could result in hypoxic and anoxic events and create major species die-off in local communities.

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