A physical explanation for ocean air-water warming differences under CO2-forced warming

Modelled global warming is often quantified using global near-surface air temperature (T_as). Meanwhile, long-term temperature datasets combine observations of Tas over land with sea-surface temperature (SST) over ocean. Modelled ocean Tas warms more than SST, which can bias model-observation comparisons. Skin temperature (Ts), which is typically warmer than Tas, follows SST changes so the ocean surface temperature discontinuity δT_s=T_s-T_as decreases with warming. Here I show that under CO2 forcing, decreased δT_s is consistently simulated for nonpolar ocean within ±60 °S/N, but not for other regions. I investigate the causes of oceanic δT_s decrease using aLongRunMIP climate simulation, radiative kernels, and standard methods for diagnosing forcing and feedbacks from the CMIP5 ensemble. CO2 forcing establishes longwave heating of the lower atmosphere and subsequent adjustments that result in a small Tas increase, and therefore δT_s decrease. During the subsequent warming in response to CO2 forcing, the model-mean surface evaporation feedback is 3.6 W m-2 °C-1 ¬over oceans, which reduces Ts warming relative to Tas and further shrinks δT_s. Present day forcing and feedback contributions are of similar magnitude, and both contribute to small differences in model-observation comparisons of global warming rates when these differences are not accounted for.