An empirical orthogonal function analysis of spring-summer sea surface temperature variability off northern and central California from AVHRR satellite imagery

Abstract

An empirical orthogonal function (EOF) analysis of an AVHRR sea surface temperature (SST) data set encompassing the spring-summer period during 1991-1993 was used to examine patterns of SST variability off northern and central California. This EOF analysis decomposed the spatial variability of SST in 124 images that covered an offshore distance of 200 km and an alongshore distance of 500 km. The first EOF mode 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 offshore gradient with much cooler water nearshore and warmer water offshore. These areas of cool waters were associated with known upwelling locales and the structure of the mode was closely coupled with the alongshore component of the coastal wind field. The temporal modulation of this spatial pattern was given by the eigenvector time series of temporal amplitudes. The second EOF mode described 11% of the remaining variability in the data set. Its spatial pattern revealed a north-south structure that appeared to be related to regional spatial variations in the strength of upwelling favorable wind. These variations apparently resulted in regions of reduced upwelling, increased solar surface heating, onshore transport of offshore waters, and perhaps poleward surface flow that were evident in the spatial pattern as areas of relatively warmer water. Interannual variation in the mode 1 spatial structure was investigated by performing a separate EOF analysis of each yearly data set. The combined results of this analysis indicate that it may be possible to use remotely sensed SST to study and infer circulation of other eastern boundary currents, such as the Peru Current, that have dynamics similar to the California Current but where in situ hydrographic and wind observations are much less frequent.