Large-scale anomalies in sea-surface temperature and air-sea fluxes during wind relaxation events off the United States West Coast in summer

In summertime along the U.S. West Coast, the winds exhibit a three-stage cycle spanning ∼12 days. The prevailing upwelling-favorable winds weaken (relax) or reverse off the Pacific Northwest, then reintensify, then weaken off central California. We study the sea-surface temperature (SST) response to these “northern” and “southern” wind relaxations. (1) Satellite data indicate northern wind relaxations result in SST anomalies O(+1°C) extending ∼2000 km offshore. Surface heat flux reanalyses indicate the warm anomaly is mainly from decreased latent cooling. (2) After the winds reintensify, SST becomes anomalously cold along central and southern California. (3) During the southern wind relaxations, the cold SST anomaly persists but the SST warms with time. This warming is not driven by surface heat flux. The latent cooling is reduced, yet unlike during the northern relaxation, this change is canceled by a decrease in solar radiation due to increased cloudiness. In the region south of Point Conception, reduced southward advection of cold water and increased northward advection of warm water by the coastal countercurrent could explain the warming. Reduced Ekman pumping likely contributes to the warming trend during the southern relaxations, and reduced wind-driven entrainment at the base of the mixed layer likely contributes to the warming during both relaxations. Whether the net surface heat flux is the main driver of SST anomalies during wind relaxation depends on the regional response of clouds. Southern wind relaxations follow episodes of enhanced surface cooling, which may contribute to greater cloudiness during southern than northern wind relaxations.