A Physically-Motivated Method for Attributing Heat Transport in the Subtropical North Atlantic

Sea surface temperatures in the subpolar North Atlantic saw a trend reversal from warming between 1994-2004 to cooling between 2005-2015. To evaluate the reversal, we present a new method for partitioning ocean heat transport, which we apply at 46N. Past decompositions of heat transport have separated the flow into a zonal-mean overturning component and a horizontal circulation component, with the latter presumed to correspond to the gyre. Instead, through the use of isopycnal coordinates, we identify physical boundaries between upper AMOC, deep western boundary current, and gyre, which produce a new division of the heat transport (while still respecting mass conservation). This physically motivated evaluation of relative heat contributions from the currents enable us to more accurately attribute changes in the North Atlantic heat content to wind-driven vs buoyancy-driven circulation. Results have important implications for the behavior of the AMOC, which plays a key role in regulating surface temperature in Western Europe and North America.