Heat Budget of the Equatorial Cold Tongue in GFDL FLOR Global Coupled GCM

To shed light on the dynamics and thermodynamics of the equatorial Pacific cold tongue, we examine the upper-ocean heat budget in GFDL’s FLOR global coupled GCM, which performs well in this region and is used routinely for ENSO forecasts. Using an exact mixed layer heat budget diagnosed directly from the FLOR model at hourly time scales, we systematically approximate the heat budget to facilitate comparison with the monthly-mean fields typically diagnosed from ocean reanalyses and CMIP simulations. Although mixed layer depth varies on a wide range of timescales, from sub-diurnal to decadal, useful insights into the cold tongue climatology can be gained by examining the heat budgets of two nested, temporally-fixed layers -- one sufficiently thin to be closely linked to SST,  and one sufficiently thick to subsume most of the subweekly vertical diffusion.  For the thin layer, the climatological heat budget is mainly a balance between the net surface flux (particularly shortwave heating during the day) and parameterized vertical diffusion across the layer base (particularly at night).  For the thick layer, the dominant terms are the surface fluxes and resolved advection, with parameterized diffusion playing a redistributive role that transfers heat between the thin and thick layers.  A crucial term in the heat budget is advection from submonthly phenomena -- particularly Tropical Instability Waves, which transfer heat meridionally and vertically within both the thin and thick layers, especially during La Niña and boreal summer.  The heat budget framework is applied to evaluate FLOR against observations, with an eye toward improving simulations, reanalyses, observations, and understanding of tropical Pacific climate.