LAUSR

The Atlantic Meridional Overturning Circulation (AMOC) plays a central role in the decadal variability of global and regional climate through changing poleward transport of heat. However, realistic simulation of the AMOC, i.e., its strength and spatial structure, remains a challenge for ocean general circulation models (OGCMs) and coupled climate models. Here, we investigate how the simulated AMOC could be affected by improved accuracy of the seawater equation of state (EOS) with an OGCM. Two EOSs used in this study: the UNESCO EOS80, and the “stiffened” EOS derived from the compressibility of sea water and the UNESCO EOS80. Compared to the model using the UNESCO EOS80, the model using the “stiffened” EOS yields stronger deep convection in the Labrador Sea, the Irminger-Iceland-Scotland Basin, and the Greenland-Iceland-Norwegian (GIN) seas, which leads to an improvement in the simulation of the AMOC: Along 26.5°N, the maximum transport is increased from 14.9 to 17.4 Sv and the interface between the upper clockwise cell and lower counterclockwise cell is deepened from 2.8 to 3.3 km, both matching the observations better. Taken the Labrador Sea as an example, the processes, including both direct and indirect causes, that in part responsible for the improved AMOC are as follows. The use of “stiffened” EOS increases the density throughout the water column and weakens the stability of sea water. Moreover, the enhanced cabbeling and thermobaric effect strengthen the vertical advection, intensifying the deep convection and increasing formation of deep water, which eventually improves the simulation of the AMOC. The intensified AMOC, in turn, speeds up the surface return flow, transporting more warm and saline water to the high latitudes in the North Atlantic, which contributes to the densification of surface water. Similar analyses can be applied to the Iceland-Scotland Basin and GIN seas. Thus, the enhanced deep convection and formation of deep water in the Labrador Sea, as well as in the Iceland–Scotland Basin and GIN seas, improve the simulated AMOC.