Exploring atmosphere-ocean interaction through a hierarchy of simple ocean models

Recent studies have found that under the scenario of projected Arctic sea-ice loss, atmospheric responses in coupled ocean models reach beyond the equator with high equatorial symmetry: shift of intertropical convergence zone, changes in hydrological strength and tropospheric warming. Contrarily, responses of atmospheric-only model are confined to the Northern hemisphere extratropics. Thus, both dynamical and thermodynamic adjustment in the ocean are crucial.

Currently, it is difficult to address the uncertainties caused by different physical processes. It is because atmospheric modelers rely on either ocean general circulation model (OGCM) coupled simulations which impede our ability to fully understand the coupled dynamics, or a highly simplified slab ocean model (SOM) with a prescribed heat flux adjustment which is known to overestimate meridional sea surface temperature gradient under projected Arctic sea-ice loss. There is a need for a hierarchy of progressively realistic ocean models so that the insight can be gained more easily.

Here the hierarchy we develop adapts SOM to capture two additional ocean processes: Entrainment caused by mixed-layer variation and Ekman flow (horizontal transport, upwelling, downwelling) driven by surface wind stress. We test this hierarchy with pre-industrial climate conditions and compare the resulting climates to that of a full OGCM. Quantifications of how each process captures climatological means and variabilities are presented.