Monthly to Decadal Variability of Mesoscale Stirring in the California Current System: Links to Upwelling, Climate Forcing, and Chlorophyll Transport

The California Current System (CCS) is a highly productive eastern boundary upwelling system. Cross-shore transport is driven by horizontal stirring from mesoscale (and submesoscale) eddies, fronts, and filaments, which shift surface productivity away from the narrow upwelling zone inshore. Using an unprecedented 25-year daily record of finite size Lyapunov exponents (FSLEs), we characterize the spatial and temporal patterns of mesoscale stirring in the entire CCS and quantify correlations of FSLEs with satellite-measured chlorophyll a and climate indices. The annual cycle of mesoscale stirring is highly correlated with seasonal upwelling in the CCS, with bands of intense FSLE propagating to the west during the upwelling season. Annual cycles and interannual variability in FSLE intensity were most prominent to the north and inshore of the CCS, and weaker to the south and offshore. 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 is positively correlated with FSLE intensity in a meridional band 200-600 km offshore, consistent with the hypothesis that increased horizontal stirring increases the export of chlorophyll a to offshore waters. When mesoscale stirring intensifies during negative phases of the Pacific Decadal Oscillation and Ocean Nino index, we predict greater offshore transport of organic carbon. Our analyses underscore the predictability of annual and interannual cycles of mesoscale stirring and chlorophyll a fluxes in the CCS and their links with climate indices.