High-resolution satellite-derived sea surface temperature
variability over the Gulf of Maine and Georges Bank region
J. J. Bisagni, K. W. Seemann and T. P. Mavor
Empirical
orthogonal function (EOF) analyses were completed using over four
years (October 1993 through December 1997) of 1.4-km resolution,
satellite-derived sea surface temperature (SST) imagery from a large
domain which 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 291 cloud-free, five-day averaged SST fields, produced at
five-day intervals for the study period. After removal of the
four-year temporal mean SST at each gridpoint, a covariance EOF
analysis of OI SST shows three significant modes, accounting for a
total of 95.8 % of observed SST variance. Covariance EOF modes 1 and
3 represent 92.1% and 1.2% 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 2.5% 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 1993-1996 study period. Additional Gulf Stream
warm-core ring data is being obtained for 1997.