What determines the spatial pattern in summer upwelling trends on the U.S. West Coast?

Analysis of sea surface temperature (SST) from coastal buoys suggests that the summertime over-shelf water temperature off the U.S. West Coast has been declining during the past 30 years at an average rate of -0.19 °C per decade. This cooling trend manifests itself more strongly off south-central California than off Oregon and northern California. The variability and trend in the upwelling north of off San Francisco are positively correlated with those of the equatorward wind, indicating a role of offshore Ekman transport in the north. In contrast, Ekman pumping associated with wind stress curls better explains the stronger and statistically more significant cooling trend in the south. While the coast-wide variability and trend in SST are strongly correlated with those of large-scale modes of climate variability, they in general fail to explain the southward intensification of the trend in SST and wind stress curl. This result suggests that the local wind stress curl, often topographically forced, may have played a role in the upwelling trend pattern.