Mechanisms behind the temporary shutdown of deep convection in the Labrador Sea: Lessons from the Great Salinity Anomaly years 1968-1971

From 1969 to 1971 convection shut down in the Labrador Sea thus interrupting the formation of the intermediate/dense watermasses. The shutdown has been
attributed to the surface freshening induced by the Great Salinity Anomaly (GSA), a fresh water anomaly in the subpolar North Atlantic. The abrupt resumption of convection in
1972, in contrast, is attributed to the extreme atmospheric forcing of that winter. Here we use oceanic and atmospheric data collected in the Labrador Sea at Ocean Weather
Station Bravo and a one-dimensional mixed layer model to examine the causes of the shutdown and resumption of convection in detail. Our analysis shows that the shutdown started
as a result of the GSA-induced freshening as well as the mild 1968-1969 winter. After the shutdown began, however, two positive feedbacks (both associated with the sea-surface temperature (SST) decrease due to lack of convective mixing with the warmer subsurface water) further inhibited convection. First, the SST decrease reduced the heat flux to the atmosphere by reducing the air-sea temperature gradient. Second, it further reduced the surface buoyancy loss by reducing the thermal expansion coefficient of the surface water.
Convection resumed in 1972 both because of the extreme atmospheric forcing as well as advection of saltier waters into the convection region. These results highlight the tight coupling of the ocean/atmosphere in convection regions and the need to resolve both components to correctly represent convective processes in the ocean. They are also relevant to present-day conditions given the increased ice melt in the Arctic Ocean and from the Greenland Ice Sheet.