The observation-based application of a Regional Thermohaline Inverse Method to diagnose the formation and transformation of Labrador Sea water from 2013-2015

The strength of the Meridional Overturning Circulation (MOC) in the North Atlantic is dependent upon the formation of dense waters that occurs at high northern latitudes. Wintertime deep convection in the Labrador and Irminger Seas forms the intermediate water mass known as Labrador Sea Water (LSW). Changes in the rate of formation and subsequent export of LSW are thought to play a role in MOC variability, but formation rates are uncertain and the link between formation and export is complex. We present the first observation-based application of a recently developed Regional Thermohaline Inverse Method (RTHIM) to a region encompassing the Arctic and North Atlantic subpolar gyre for the years 2013, 2014 and 2015. RTHIM is a novel method which can diagnose the formation and export rates of water masses such as the LSW identified by their temperature and salinity, apportioning the formation rates into contributions from surface fluxes and interior mixing. We find LSW formation rates of up to 12 Sv during 2014-15, a period of strong wintertime convection, and around half that value during 2013 when convection was weak. We also show that the newly convected water is not exported directly, but instead is mixed isopycnally with warm, salty waters that have been advected into the region, before the products are then exported. RTHIM solutions for 2015 volume, heat and freshwater transports are compared with observations from a mooring array deployed for the Overturning in the Subpolar North Atlantic Program and show good agreement, lending validity to our results.