Spectral analysis of the AVHRR sea surface temperature variability off the west coast of Vancouver Island

Abstract

From 16 AVHRR infrared satellite images of the west coast of Vancouver Island, British Columbia, collected during the five summers of 1984–1988, 4 alongshore temperature transects were sampled. Upon Fourier transforming the transect data, we found that the energy spectra of the temperature variance in alongshore wavenumber space in general followed a –2.1 power law, which agreed with previous observations from other parts of the world.

Summer images may be divided into 2 types: upwelling dominated and non‐upwelling dominated. When a strong upwelling‐induced alongshore cold front was observed, the regimes shoreward and seaward of the front had distinctly different spectra. Cross‐spectral analysis of transect data between images taken a day apart in the presence of strong upwelling events revealed significant coherence at the low wavenumber regime (wavelength 300 km and above, corresponding to the large eddies) and often at the high wavenumber regime (wavelength 30 km or below, corresponding to the fine structures of the eddies). The coherence dropped for images taken 2 or more days apart, suggesting a decorrelation time‐scale of about 2 days. In the absence of strong upwelling and associated eddies, summer transect temperature data from different years often showed a similar alongshore linear trend in addition to possible large differences in the mean temperature.