Small scale processes influencing the deep western boundary current in the Labrador Sea

Abstract:

The aim of this study is to address the impact of small scale processes on large scale circulation in the North Atlantic with a focus on the Labrador Sea where strong mesoscale and submesoscale activity occurs.
The Atlantic Meridional Overturning Circulation (AMOC) plays a key role in the regulation of the earth climate. This large circulation represents a synthetic view of the basin-wide transport with a northward warm and salty flow at the surface and an equatorward cold and fresh flow at depth. The latter is dominated by the Deep Western Boundary Current (DWBC) which exports to the south dense water masses such as the Labrador Sea Water (LSW) formed during winter convective events in the interior Labrador Sea.

In a simplified, laminar view of the AMOC, a direct link could exist between dense water formation in the Labrador Sea and the DWBC transport intensity. Nevertheless, recent studies suggest that this link may be more complex. Indeed, it seems that the LSW, once formed, may be exported to the surrounding DWBC by lateral density turbulent fluxes. It is also put forward that dense water formation signal may be lagged by several years when reaching the DWBC due to eddies. So the small scale processes may be key in the LSW export process. The exploration of these dynamics requires numerical models of very high resolution due to the small Rossby radius deformation in the Labrador Sea (about 7km).

We develop a high resolution primitive equation configuration which relies on a global config- uration at 1/2◦ horizontal resolution including two embedded grids covering respectively the North Atlantic (1/8◦) and the subpolar gyre (1/32◦). At the highest resolution, the mesoscale and submesoscale processes are explicitely resolved by the model and lead for instance to a strong seasonality in the dynamics due to convection. Our objective here is to evaluate their impact on the DWBC by reconstructing the potential vorticity budget in the Labrador Sea.