A modeling study of upper ocean winter processes in the Labrador Sea

Abstract

A coupled ice‐mixed layer model is developed to study winter processes in the Labrador Sea. The seasonal change of mixed‐layer properties, air‐sea fluxes, and ice coverage is examined. The effects of severe atmospheric conditions and salinity change on the upper ocean are investigated. The model is integrated from November to June to cover the entire winter period. Objectively analyzed temperature and salinity fields for November are used as the initial conditions of the model ocean. Meteorological parameters derived from the NCEP reanalysis are used as forcings. The results show that heat loss of the ocean reaches a maximum at the end of January. High heat loss, 250 to 400 W m−2, occurs in the northern Labrador Sea between 56°N and 63°N in ice‐free waters near the ice edge. Pack ice significantly reduces the heat loss with a typical value of 50 W m−2. In the northern Labrador Sea, surface cooling causes the mixed layer to deepen continuously through winter and reaches a maximum of 150–450 m in late March. A decrease of surface salinity by 1% (for example, from 34.35 to 34) results in a decrease of mixed‐layer depth by 17–55%. The high stratification associated with the low surface salinity has little effect on ice coverage, since the reduced level of vertical mixing cannot lower the surface temperature sufficiently to increase the ice area. This suggests that freshening of the Labrador Sea during the Great Salinity Anomaly in the late 1960s to early 1970s did not promote ice production. An increase of wind speed by 40% and a decrease of air temperature up to 4°C, representing the conditions of cold winters, doubles the mixed‐layer depth to a maximum of 900 m and extends the southern ice limit by 200 km on the northern Grand Banks. This suggests that interannual variation of convection and ice extent are mainly controlled by local meteorological conditions.