East-west connectivity in the subpolar gyre: impact on Labrador Sea Water properties

We use the adjoint of the MIT general circulation model to explore how Labrador Sea Water (LSW) – defined in terms of instantaneous potential vorticity and potential density – responds to surface buoyancy fluxes. We focus on key properties of this important water mass: its time-evolving volume, salinity, and temperature, as represented in the ECCO state estimate. In our adjoint analysis, we consider the response of LSW properties to net surface heat and freshwater fluxes such that air-sea feedback mechanisms are assumed to be included in the fluxes rather than the sensitivity. We furthermore decompose the sensitivities to surface heat fluxes into a dynamical component due to density and a separate component due to density-compensated changes. Our analysis highlights an important dynamical pathway that facilitates the response of LSW properties to surface buoyancy anomalies over the Eastern North Atlantic at lead times of approximately three years. Our results suggest that the mechanism of east-west connectivity involves more than just a passive advection of surface temperature and salinity anomalies downstream from the Eastern North Atlantic into the Labrador Sea. In addition to passive advection, we identify a dynamical response of the density-space overturning circulation in the Eastern Subpolar Gyre to local surface buoyancy fluxes. The resulting circulation anomaly impacts transport into the Labrador Sea and ultimately, the properties of LSW as a distinct water mass.