Linear and nonlinear responses of annual mean sea surface temperature to orbital forcing

The response of sea surface temperature (SST) to orbital-induced insolation is not well understood. Here, using transient climate simulation of the last 200,000 years, we investigate whether mean annual SST (MASST) responds linearly or nonlinearly to orbital forcing. Our results show that the response of SST to insolation is regionally variable. It is nearly linear across most of the tropics and high latitudes, with the MASST dominated by obliquity variability. In the mid-latitude and eastern equatorial Pacific, the response of SST is, however, nonlinear with MASST dominated by precession variability. The nonlinear MASST response can be biased towards winter or summer SST, caused by the rectification of the seasonal insolation forcing by cloud albedo or mixed layer depth depending on the region. Specifically, the MASST in the northwest Pacific is biased towards its winter season SST, which could mainly be attributed to the seasonal cycle of cloud cover. The summer season biased MASST in the mid-latitude south Pacific might be related to the seasonal variation of the oceanic mixed layer. The summer season biased MASST in the northeast Pacific might be a combined effect of atmospheric cloud and oceanic mixed layer. The peculiar seasonal bias of MASST in the eastern equatorial Pacific, however, could be related to the remote transmission signal from the mid-latitude south Pacific. Our study has implications for better interpreting proxy SST records through gaining further insights on the physical mechanisms controlling SST responses to orbital forcing. • MASST is dominated by a linear response to insolation over most of the tropical oceans with dominant obliquity variability. • MASST exhibits nonlinear response over mid-latitude and eastern equatorial Pacific with dominant precession variability. • Nonlinear response in mid-latitude can be rectified by cloud or oceanic MLD, leading MASST biased towards winter/summer SST.