Linkage between overturning and density anomaly over the subpolar gyre

The surface forced water mass transformation (SFWMT) is known to be the main contributor of the Atlantic Meridional Overturning Circulation (AMOC) over the subpolar gyre. Over the eastern part of the subpolar gyre, a recent study revealed the dominant role of surface density changes in driving the SFWMT as opposed to the direct influence of air-sea fluxes. Indeed, the distribution at surface of the isopycnal associated with the maximum overturning streamfunction, Smoc, modulates the area of dense water formation induced by the air-sea fluxes.

The Overturning in the Subpolar North Atlantic Program (OSNAP) showed that the density of Smoc is highly variable in time along each section of the array. However, the drivers of Smoc remain unclear. In our work, we use a combination of atmospheric reanalysis and coupled simulations of HadGEM3-GC3.1 to evaluate the Smoc variability over the subpolar gyre as well as its connection with the overturning strength. At interannual timescale, the variability of Smoc at OSNAP East is strongly related to those at OSNAP West and at 45°N. However, its connection with the overturning strength is more complex. Although Smoc is not well related to the overturning at OSNAP, it is associated with a shift in density of the overturning stream function. The Irminger Sea is identified as being the centre of action driving this variability.