Department of Earth System Science

Sea surface temperatures (SSTs) in the eastern equatorial Pacific have a pronounced annual variation. The SSTs are warmer than the annual mean in the first half of the year (warm phase) and colder in the latter half (cold phase). The warm phase is not identical to the cold phase with opposite sign. For example, SST anomalies in the warm phase have stronger westward propagation and larger amplitude than those in the cold phase. Also, the strongest SST anomalies in the cold phase last longer than those in the warm phase. The reasons for this "temporal asymmetry" are investigated by performing sensitivity experiments with the UCLA coupled ocean-atmosphere general circulation model (GCM), in which an idealized, time-independent distribution of Peruvian stratus clouds is prescribed at different seasons of the year.

The results suggest that the annual evolution of SSTs over the eastern equatorial Pacific is highly sensitive to the annual variation of Peruvian stratus. It is found that Peruvian cloudiness affects the surface wind speed in the equator through local meridional circulation. The strength of surface wind determines the correlationship between SST and surface evaporation over the eastern equatorial Pacific. The observed negative correlation, which is needed to produce the observed temporal asymmetry, can be simulated by the coupled GCM only when the observed annual variation of Peruvian stratus cloudiness is prescribed in the model.

A simulation with the revised version of the UCLA coupled GCM, in which the parameterization of marine stratus clouds is improved, shows significant improvement in the simulated annual cycle and interannual variability. Results from this simulation confirm the findings obtained from the sensitivity study.