Sea Surface Temperature Biases under the Stratus Cloud Deck in the Southeast Pacific Ocean in 19 IPCC AR4 Coupled General Circulation Models

This study examines systematic biases in sea surface temperature (SST) under the stratus cloud deck in the southeast Pacific and upper ocean processes relevant to the SST biases in 19 coupled general circulation models (CGCMs) participating in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). The twenty years of simulations from each model are analyzed. Pronounced warm SST biases in a large portion of the southeast Pacific stratus region are found in all models. Processes that could contribute to the SST biases are examined in detail based on the computation of major terms in the upper ocean heat budget. Negative biases in net surface heat fluxes are evident in most of the models, suggesting that the error of net surface heat flux is not a primary source of the warm SST biases. Biases in heat transport by Ekman currents largely contribute to the warm SST biases both near the coast and the open ocean. In the coastal area, southwestward Ekman currents and upwelling in most models are much weaker than observed due to weaker alongshore winds, resulting in insufficient advection of cold water from the coast. In the open ocean, warm advection due to the Ekman currents is overestimated in models because of the larger meridional temperature gradient.