UC San Diego

The California Current System (CCS) is a highly productive Eastern Boundary Upwelling System. Cross-shore transport of recently-upwelled water is driven by horizontal stirring from mesoscale eddies, fronts, and filaments, that shift surface productivity away from the narrow upwelling zone inshore. In addition to driving the cross-shore transport of upwelled water, mesoscale features form pelagic habitats through biophysical coupling, which create regions where the physical convergences and enhanced growth of plankton attract higher trophic levels. Mesoscale features are therefore important foraging habitats for many organisms. Climate variability, such as the El Niño Southern Oscillation, can drive large changes in ecosystems through physical processes such as changes in upwelling that can reverberate through the ecosystem. This dissertation combines a 25-year daily record of finite-size Lyapunov exponents (FSLEs), a measure of mesoscale features and horizontal stirring, with 21 years of satellite-derived chlorophyll a measurements and 18 years of quarterly marine mammal visual surveys in order to investigate the links between mesoscale features, chlorophyll, climate variability, and cetaceans in the CCS. The annual cycle of mesoscale stirring was found to be highly correlated with seasonal upwelling in the CCS. Interannual fluctuations in mesoscale stirring showed 3-12 month lagged responses to climate indices, with the CCS oscillating out of phase with waters west and north. Chlorophyll a was positively correlated with FSLE intensity in a meridional band 200-600 km offshore. Annual cetacean sightings (scaled for effort) were stable for most species but scaled sightings of Dall’s porpoise and Pacific white-sided dolphin decreased over the study period. During the 2014-2016 marine heatwave, scaled sightings of common and Risso’s dolphins increased, an effect driven by large groups in the winter and spring of 2015. Scaled sightings of several dolphin species were higher during El Niño events, whereas blue and fin whale summertime scaled sightings were higher during La Niñas. Blue, fin, and humpback whales showed habitat partitioning through a combination of geographic separation and species-specific preferences for certain mesoscale features. Dall’s porpoises had a strong association with rare, intense FSLE ridges.