| Summary: | The winter watermass properties and circulation of the Gulf of St. Lawrence and St. Lawrence Estuary are examined using a combination of new wintertime observations and numerical modelling. Ten moorings were deployed over the winter 2002/3, providing the first continuous observations of the upper 200m of the water column in winter. A harmonic analysis of current data in the estuary confirms the existence of a semi-diurnal internal Poincare wave. The analysis also provides strong evidence of a neap-tide intensification of the estuarine circulation. The cold intermediate layer (CIL) in the estuary is seen to be renewed abruptly in late March, with the advection of a wedge of near-freezing water from the gulf. An eddy-permitting sea ice-ocean model is used with the observations to quantify the formation and circulation of the CIL. As a first step, a two-equation second moment turbulence closure parameterization is implemented to better account for vertical mixing processes. The model sensitivity to this parameterization is tested and differences of several salinity units are found in the surface layer throughout the southern half of the gulf. The model reproduces the strong spring renewal event of the CIL into the estuary, and helps to explain the role of local dynamics, including the coupled circulation in the northwest gulf, in controlling the exchange processes at depth. The model results suggest the existence of a separated Gaspe Current that persists for more than a month. A significant correlation is found between the inflow through the Strait of Belle Isle and outflow through Cabot Strait. In order to isolate the effects of this inflow on the CIL heat content, we examine a sensitivity experiment in which the Strait of Belle Isle is closed. This simulation shows that the inflow has a relatively small effect on the total CIL heat budget. With the Strait of Belle Isle closed, the outflow of CIL through Cabot Strait is reduced by 61%. It is estimated that only 64% of CIL inflow through the Strait of Belle Isle in fall and winter contributes directly to the heat content of CIL present in the gulf.
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