Air–sea turbulent heat fluxes in climate models and observational analyses: What drives their variability?

A traditional view is that the ocean outside of the tropics responds passively to atmosphere forcing, which implies that air-sea heat fluxes are mainly driven by atmosphere variability. This paper tests this viewpoint using state-of-the-art air-sea turbulent heat flux observational analyses and a climate model run at different resolutions. It is found that in midlatitude ocean frontal zones the variability of air-sea heat fluxes is not predominantly driven by the atmosphere variations but instead is forced by sea surface temperature (SST) variations arising from intrinsic oceanic variability. Meanwhile in most of the tropics and subtropics wind is the dominant driver of heat flux variability, and atmosphere humidity is mainly important in higher latitudes. The predominance of ocean forcing of heat fluxes found in frontal regions occurs on scales of around 700 km or less. Spatially smoothing the data to larger scales results in the traditional atmosphere-driving case, while filtering to retain only small scales of 5 degrees or less leads to ocean forcing of heat fluxes over most of the globe. All observational analyses examined (1 degrees OAFlux; 0.25 degrees J-OFURO3; 0.25 degrees SeaFlux) show this general behavior. A standard resolution (1 degrees) climate model fails to reproduce the midlatitude, small-scale ocean forcing of heat flux: refining the ocean grid to resolve eddies (0.1 degrees) gives a more realistic representation of ocean forcing but the variability of both SST and of heat flux is too high compared to observational analyses.