Isopycnal eddy stirring dominates thermohaline mixing in the upper
subpolar North Atlantic
Abstract
The Atlantic Meridional Overturning Circulation (AMOC) entails vigorous
thermohaline transformations in the subpolar North Atlantic (SPNA).
There, warm and saline waters originating in the subtropics are
converted into cooler and fresher waters by a combination of surface
fluxes and sub-surface thermohaline mixing. Using microstructure
measurements and a small-scale variance conservation framework, we
quantify the diapycnal and isopycnal contributions to thermohaline
mixing within the eastern SPNA. Isopycnal stirring is found to account
for 65% of thermal and 84% of haline
variance dissipation in the upper 400 m of the eastern SPNA, suggesting
an important role of isopycnal stirring in regional water-mass
transformations. By applying the tracer variance method to two tracers,
we underscore the special significance of isopycnal stirring for tracers
weakly coupled to density, such as biologically-active tracers. Our
findings thus highlight the central role of isopycnal stirring in both
the AMOC and biogeochemical dynamics within the SPNA.