Satellite observations of sea surface temperature variability on southern Georges Bank

Abstract

Sea surface temperature (SST) residual time series, derived from declouded Advanced Very High Resolution Radiometer (AVHRR) satellite imagery for southern Georges Bank, were examined for the period April–October 1987. Significant negative correlations were computed between the low-pass filtered SST residuals and daily-averaged tidal current magnitude, suggesting that for periods of ∼15 and ∼28 days, SST variability was related to the spring-neap tidal cycle in both the stratified and unstratified portions of southern Georges Bank during the stratified season. Maximum negative correlations occurred at a lag of ∼3 days, indicating that negative (positive) SST residuals lagged the maximum spring (minimum neap) tidal current by this amount. This lag and the measured 3σ SST residual of ∼3.0°C are in agreement with the summer hydrography on southern Georges Bank and a one-dimensional model in which the depth-independent vertical eddy diffusivity varies with time. Because meteorological forcing at the sea surface occurs on time scales of ∼1–7days, the data suggest that the observed, negative SST residuals are caused by periodic, enhanced vertical mixing of colder, sub-thermocline water into surface waters as a result of spring tidal currents during the stratified season. Given the inverse temperature-nitrate correlation for the region during the stratified season, the data suggest the occurrence of increased nitrate flux into the stratified and well-mixed regions of southern Georges Bank during spring tides. Periods of spring tides may result in the transport of a large fraction of the new nitrogen needed to sustain high primary production observed in the chlorophyll maximum within the stratified and well-mixed regions of southern Georges Bank during the stratified season.