Links between annual variations of Peruvian stratocumulus clouds and of SST in the eastern equatorial Pacific

The hypothesis that Peruvian stratocumulus play an important role on both t he annual mean and annual variations of sea surface temperature (SST) in th e eastern equatorial Pacific is examined. The problem is addressed by perfo rming sensitivity experiments using the University of California, Los Angel es, coupled atmosphere-ocean GCM with different idealized temporal variatio ns of stratocumulus in a region along the coast of Peru. The results obtained are consistent with the notion that Peruvian stratocum ulus are a key component of the interhemispherically asymmetric features th at characterize the annual mean climate of the eastern equatorial Pacific, including the cold SSTs off Peru and the absence of a southern ITCZ. The pr incipal new finding of this study is that the annual variations (i.e, devia tions from the annual mean) of Peruvian stratocumulus are linked to the dif ferences between the amplitude, duration, and westward propagation of the w arm and cold phases of the equatorial cold tongue. In the model's context, only if the prescribed annual variations of Peruvian stratocumulus have the same phase as the observed variations are those differences successfully c aptured. The impact of Peruvian stratocumulus on equatorial SST involves "dynamical" and "thermal" effects. The former develop through an enhancement of the no rtherly component of the surface wind from the Peruvian coast to the equato r. The thermal effects develop through the special relationships between SS T and surface evaporation over the equatorial cold tongue, which contribute s to extend the cold phase until the end of the year. A successful portraya l of this behavior requires a realistic simulation of the annual variations of surface wind over the equatorial cold tongue.