Peruvian Stratus Clouds and the Tropical Pacific Circulation: A Coupled Ocean-Atmosphere GCM Study

Ma, Chung-Chun; Mechoso, Carlos R.; Robertson, Andrew W.; Arakawa, Akio

Extensive and persistent stratus cloud decks are prominent climatic features off the Peruvian coast. They are believed to play a key role in the coupled atmosphere-ocean processes that determine the sea surface temperature (SST) throughout the eastern tropical Pacific. This notion is examined and further developed using a coupled ocean-atmosphere general circulation model (GCM): a control simulation, in which the simulated amount of Peruvian stratus clouds is unrealistically low, is compared with an experiment in which a stratus cloud deck is prescribed to persistently cover the ocean off the Peruvian coast. Beneath the prescribed cloud deck SSTs are reduced by up to 5 K, as expected from decreased solar radiation reaching the surface. In addition, there is significant cooling over much of the eastern tropical Pacific south of the equator, and even along the equator well into the central Pacific. The prescribed stratus deck largely alleviates the coupled GCM's warm bias in SST in the southeastern Pacific, which is common to most contemporary coupled GCMS, and produces a distribution of SST with more realistic interhemispheric asymmetries. Examination of differences between SST evolutions in the enhanced stratus experiment and the control circulation reveals that the remote ocean cooling is not due to a single mechanism. The cooling immediately to the west and north of the region with the prescribed stratus deck is primarily associated with increased evaporation as the southeast trades strengthen. The cooling along the equator in the central Pacific is mainly due to increased oceanic cold advection. The results of this study suggest that the Peruvian stratus clouds are important in modulating the circulation of the tropical Pacific. The "double ITCZ" syndrome of the coupled GCM, however, does not appear to be solely due to underpredicted stratus cloud cover and requires consideration of other processes in the coupled GCM.

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International Research Institute for Climate and Society
Published Here
August 21, 2012