Wind, Circulation, and Topographic Effects on Alongshore Phytoplankton Variability in the California Current

A physical‐biogeochemical model is used to produce a retrospective analysis at 3‐km resolution of alongshore phytoplankton variability in the California Current during 1988–2010. The simulation benefits from downscaling a regional circulation reanalysis, which provides improved physical ocean state estimates in the high‐resolution domain. The emerging pattern is one of local upwelling intensification in response to increased alongshore wind stress in the lee of capes, modulated by alongshore meanders in the geostrophic circulation. While stronger upwelling occurs near most major topographic features, substantial increases in phytoplankton biomass only ensue where local circulation patterns are conducive to on‐shelf retention of upwelled nutrients. Locations of peak nutrient delivery and chlorophyll accumulation also exhibit interannual variability and trends noticeably larger than the surrounding shelf regions, thereby suggesting that long‐term planktonic ecosystem response in the California Current exhibits a significant local scale (O(100 km)) alongshore component. Plain Language Summary The California Current region off the west coast of North America is one of the four main upwelling systems of the World Ocean, and it supports a productive marine ecosystem including economically important fisheries. In the present study, a numerical model of the ocean circulation and planktonic food web is used to explain how changes in coastline features (such as capes), regional wind patterns, and transport by ocean currents combine to create local regions of enhanced production at the base of the food web along the west coast of the United States. The main finding from this study is that even though most of the California Current region is subjected to wind patterns that favor the growth of phytoplankton by bringing deep nutrients near the ocean's surface, only a few localized areas of the coastline see significant increases in phytoplankton biomass because of how ocean currents tend to redistribute available nutrients. Key Points Long‐term phytoplankton variability in the California Current is analyzed using a high‐resolution historical simulation Alongshore patterns of phytoplankton biomass result from increased upwelling intensity near capes and onshore retention of nutrients Solution benefits from downscaling of ocean circulation reanalysis to improve regional‐ and basin‐scale influences in high‐resolution domain