Seasonal upper-layer T–S structure in the Gulf of St. Lawrence during the ice-free months

A 75-year hydrographic data set was analyzed to determine the monthly mean vertical fields of temperature (T) and salinity (S) for 15 sub-regions of the Gulf of St. Lawrence (GSL). Objective fields of sea-surface temperature (SST), salinity and chlorophyll-a (Chl a) also were computed and compared with similar climatologies. Use of recent satellite-derived data enhanced observational coverage of SST and Chl a. These interpolated fields reproduced several known physical surface characteristics of the GSL with a very small relative interpolation error. The surface heat budget was computed over the entire GSL for the 7 ice-free months (May–November incl.) by using SST maps in combination with atmospheric fields and satellite-derived monthly averaged cloud cover data. Monthly means of mixed-layer depth, upper-layer heat content and upper-layer stratification were obtained for 15 sub-regions of the GSL. Simple 1D calculations (heat transfer, mixed-layer deepening rates, stratification change rates, heat storage rates) were used to study the seasonal evolution of the upper-layer thermohaline structure. It was observed that the upper-layer could be relatively well modeled in most areas by considering only the vertical processes.