Relative roles of different key forcing and preconditioning factors for recurrent deep convection in the Labrador Sea from observations and ocean models

The Labrador Sea in the subpolar North Atlantic is one of the few special regions, where strong wintertime buoyancy loss, consecutive substantially reduced vertical stratification, and the prevailing circulation facilitate the transfer of water mass properties from the surface to depths exceeding 1500 m through deep convective mixing. Hence, impacting the characteristics of the intermediate and deep waters in the entire Atlantic basin. Despite ever-growing evidence of the freshwater and atmospheric gas contents of these waters being directly affected by the strength of wintertime mixing in the Labrador Sea, the relative importance of the Labrador Sea convection for the strength of the overall Atlantic meridional overturning is still under debate, often leading to contradicting conclusions. This ongoing debate highlights the need for an in-depth all-inclusive investigation of the processes responsible for both occurrence and persistence of deep convective mixing events. Here, we make a first step in this direction by aligning multiplatform observations with model runs and quantifying the roles of the local atmospheric forcing (e.g., cumulative wintertime air-sea flux), the remote oceanic forcing (e.g., horizontal advection) and the ocean’s own memory of the past convective events (e.g., weak stratification resulting from convective preconditioning).

These three key factors, fully responsible for initiation and undergoing of winter convection, and both seasonal and interannual heat content changes in the Labrador Sea, are analyzed based on long time series. These are comprised from all available thoroughly quality-controlled ship, profiling float and mooring measurements in the central Labrador Sea and state-of the-art ocean models. The resulting variables compared between the observations and models include time series of the characteristic ocean state variables, such as temperature, salinity and density over the entire water column. Additionally, the variables quantifying specific outcomes of each winter convection, such as depth, density and volume of the newly mixed intermediate-depth water in the Labrador Sea are considered. 

We show that the seasonal evolution of the deep winter convective mixed layer is a result of the sum of the surface cooling and the overall multiyear inertia in density changes and variations in the heat, freshwater and salt imports from the neighboring North Atlantic and Arctic regions. This, in turn means that not forcibly the strongest surface cooling induces the deepest convection with maximum density water, but rather a combination of the three factors. Through the combined analyses of observations and model-based time series we are able to properly assess the relative contribution of these three factors to the development of deep convective mixing in the Labrador Sea.