Characteristics of the Sinking Branch of the Atlantic Meridional Overturning Circulation in a Global Ocean Model - the Impacts of Model Resolution

The Atlantic Meridional Overturning Circulation (AMOC) plays an important role in climate. The classical view of an ocean conveyor belt with northward surface currents and southward return currents transporting convectively-formed waters from the subpolar North Atlantic Ocean to other ocean basins suggests a tight relation between convection and sinking. However, convection regions feature a large vertical heat and salt transport, but very little vertical mass transport. Instead, it has been argued that the net sinking of dense waters that constitute the sinking branch of the AMOC must take place near boundaries and steep topography rather than in the ocean interior.

So far, this theoretical result has been confirmed in highly idealized regional model studies and in laboratory experiments. It is, however, unclear how well the sinking of dense waters is represented in the current generation of ocean models and climate models, and whether the factors driving and controlling the sinking are in accordance with the developed theory. The latter is of crucial importance for the reliability of the projected future behavior of the AMOC and its impacts on climate.

In this study, we address this issue by analyzing the outcomes of two global ocean models that differ only in their horizontal resolution (an eddy-permitting version, and a climate-model like version). We analyze the characteristics of the sinking, like the amount and spatial pattern. Moreover, the partition between sinking in the boundary current region and sinking in the interior is quantified. It appears that model resolution (and applied numerical diffusion) have a strong impact on the characteristics of the modeled sinking. These impacts are discussed in light of differences in the characteristics of the subpolar gyre circulation.