A Physical Explanation for Ocean Air–Water Warming Differences under CO2-Forced Warming

Modeled global warming is often quantified using global near-surface air temperature ( T as ). Meanwhile, long-term temperature datasets combine observations of T as over land with sea surface temperature (SST) over ocean. Modeled ocean T as warms more than SST, which can bias model–observation comparisons. Skin temperature ( T s ), which is typically warmer than T as , follows SST changes so the ocean surface temperature discontinuity δT s = T s − T as decreases with warming. Here I show that under CO 2 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 a LongRunMIP climate simulation, radiative kernels, and standard methods for diagnosing forcing and feedbacks from the CMIP5 ensemble. CO 2 forcing establishes longwave heating of the lower atmosphere and subsequent adjustments that result in a small T as increase, and therefore a δT s decrease. During the subsequent warming in response to CO 2 forcing, the model-mean surface evaporation feedback is 3.6 W m −2 °C −1 over oceans, which reduces T s warming relative to T as 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.