Satellite Derived Sea Surface Temperature Variability Off California During the Upwelling Season

Abstract

An empirical orthogonal function analysis of an advanced very high resolution radiometer sea surface temperature data set encompassing the spring–summer period during 1991–1993 was used to examine patterns of temperature variability off northern and central California. This analysis decomposed the spatial variability of remotely sensed sea surface temperature in 124 images that covered an offshore distance of 200 km and an alongshore distance of 500 km. The first mode empirical orthogonal function described 65% of the spatial variability in the data set. The spatial amplitude pattern of the first mode, which closely resembled the mean of all the images, revealed a strong cross-shore gradient with much cooler water nearshore and warmer water offshore. These areas of cool waters were associated with known upwelling locales, and the large-scale temperature structure compared favorably to that derived from historical California Cooperative Oceanographic Fisheries Investigation (CalCOFI) climatology. The mode 1 temporal amplitude function, representing the temporal modulation of this spatial pattern, was coupled with changes in the alongshore component of the coastal wind field. Interannual variations in the timing of the spring transition were reflected in this temporal amplitude time series. The second mode empirical orthogonal function described 11% of the variability in the data set. The spatial pattern revealed a north-south structure that appeared to be related to regional spatial variations in the strength of upwelling-favorable wind. The third empirical orthogonal function described only 3% of the data variability and the spatial structure suggested a meandering of the coastal surface flow. The results of this investigation indicate that the empirical orthogonal function method was effective at characterizing from the satellite data the dominant nearshore surface properties of the California Current System related to upwelling and wind, and by extension suggest that it is a suitable approach for remotely investigating other less well known eastern boundary current systems.