High-resolution satellite-derived sea-surface temperature variability over the Gulf of Maine and Georges Bank region, 1993–1996

Abstract

Empirical orthogonal function (EOF) analyses were completed using 3 yr (fall 1993 through late-summer 1996) of 1.4-km resolution, satellite-derived sea-surface temperature (SST) imagery from a large domain that includes the Gulf of Maine, Georges Bank, southern New England and western Scotian shelves, and the adjacent slope water region located seaward of the 200-m isobath. Prior to analysis, raw satellite-derived SST images were carefully cloud masked using automated and manual cloud detection techniques and then interpolated to a 7-km resolution grid using optimal interpolation (OI). OI resulted in 218 cloud-free, five-day-averaged SST fields, produced at five-day intervals for the study period. After removal of the 3 yr temporal mean SST at each gridpoint, a covariance EOF analysis of OI SST shows three significant modes, accounting for a total of 96.5% of observed SST variance. A gradient EOF analysis also shows three significant modes, accounting for a total of 92.7% of observed SST variance after removal of the spatial mean SST from each OI SST field. Covariance EOF modes 1 and 3 represent 92.2 and 1.3% of observed SST variance, and depict rich spatial structure in the seasonal heating cycle and additional spatial variation in the seasonal heating cycle over the study domain, respectively. SST variability noted for covariance EOF mode 1 is largely related to local differences in the seasonal mixed-layer depth, with deeper mixed layers resulting from strong tidal mixing over offshore banks and shoals contrasting with more- shallow-mixed layers and seasonal density stratification caused by weaker tidal mixing over regions of greater bottom depth. Additional spatial differences in seasonal SST variability and noted in covariance EOF mode 3 are related to regional differences in seasonal thermohaline and density characteristics of surface waters across the Gulf of Maine. Regional surface water differences across the Gulf of Maine are caused by different phasing between annual cycles of vertical heat flux and inputs of low-salinity Scotian Shelf water in eastern Gulf of Maine during late winter and local freshwater runoff from rivers in western Gulf of Maine during late spring. Covariance EOF mode 2 represents 3.0% of observed SST variance and describes interannual SST variability manifested by a cooling trend within the slope water region caused by a decrease in Gulf Stream warm-core ring activity during the study period. The gradient EOF analysis shows results that support and supplement the covariance EOF analysis by depicting the year-round nature of the SST gradient associated with the shelf break front (mode 1, 86.4% of SST variance) and SST gradients arising from spatial variation in the seasonal heating cycle (modes 2 and 3, 4.3% and 2.0% of SST variance, respectively) and some degree of interannual variability as described for covariance EOF mode 2 (mode 3).