Filaments in the California Current

Upwelling systems on the Eastern boundaries of oceans are areas of high oceanic productivity. The winds drive nutrient-rich water stirring from depth into the euphotic zone, close to surface, thus stimulating primary production. Those areas are usual fisheries grounds, and biological hotspots. The California Current System, off the Pacific coasts of Canada, United States, and Mexico, is one of such systems. It shows seasonal variations, in phase with wind shifts.

To better understand the dynamic of the upwelling and its variability including in stirring, satellite Chlorophyll-A data can be used, but they are highly sensitive to clouds. Altimetry is not, and FSLEs provide with a possibility of reaching the same kind of resolution with an information in correlation with it. FSLEs show areas of high horizontal convergence characteristic of fronts, filaments, and the edges of eddies. Thus, FSLEs serve as both a measure of stirring in the ocean and as a measure of mesoscale activity.

Comparing 25 years of altimetry-derived FSLEs with chlorophyll-A, and also with climate indices (ENSO, PDO, etc.) in the California Current System area show that the annual cycle of mesoscale stirring is highly correlated with seasonal upwelling in a meridional band 200–600 km offshore. This is consistent with the hypothesis that increased horizontal stirring increases the export of chlorophyll-A to offshore waters.. Interannual variations show lags of varying length with the climate indices; the inflow from the North Pacific Current into the California Current System might be the source of this.