Circulation and mixing in the subpolar North Atlantic diagnosed from climatology using a Regional Thermohaline Inverse Method (RTHIM)

The Overturning in the Subpolar North Atlantic Program (OSNAP) aims to quantify the subpolar Atlantic Meridional Overturning Circulation (AMOC), including associated advective and diffusive transport of heat and freshwater. The OSNAP observational array will provide a continuous subpolar record of the AMOC from Labrador-Greenland-Scotland during 2014-2018. To understand the significance of high- and low- frequency changes measured by the array, including changes to AMOC metrics, water mass transformation and transports, Argo observations provide a useful complementary constraint for an inverse method, with the aim of resolving intra-seasonal timescales.

A novel inverse method in thermohaline coordinates has recently been demonstrated as being able to diagnose aspects of the global overturning circulation and mixing from model data. Here we have further developed a Regional Thermohaline Inverse Method, (RTHIM) and have validated it with the NEMO model in the OSNAP region, before applying it to a seasonal Argo climatology.

In an ocean basin there exists a balance between surface heat and freshwater fluxes, advective fluxes at an open boundary and interior diffusive mixing. RTHIM makes use of this balance to determine unknown velocities at the open boundary and diffusive fluxes of heat and salt within the domain volume. We identify key transport and mixing regions and events, relevant to the subpolar AMOC, and discuss the robustness of the inverse solutions. RTHIM is also able to identify the particular contributions to AMOC volume transport changes from temperature and salinity components.