Modeling results suggest that the extensive marine stratus deck off the west coast of South America plays a crucial role in the dynamics of the coupled ocean-atmosphere system of the eastern Pacific (Philander, et al., 1996; Ma et al., 1996). This is because stratus reflects a large fraction of the incident solar radiation, which would otherwise be absorbed by the ocean. At the same time it only slightly decreases the net energy loss by sea-air latent and sensible heat fluxes and outgoing longwave radiation. Thus, the effect of low stratus clouds is to decrease the net radiative energy input into the underlying ocean. This causes an oceanic cooling tendency which reinforces the cross-equatorial contrast in SST, and thus intensifies the cross-equatorial Hadley cell. The subsidence in the downward branch of the Hadley cell is in turn at least partly responsible for the existence of the stratus layer, so the net result is a hypothesized instability which simultaneously strengthens the cross-equatorial SST contrast while intensifying the associated atmospheric circulation.
Nilsson and Emanuel (1999) found that an instability of the above type develops in a two column numerical model even in the absence of stratus formation in the downward branch. This instability occurred in their model because the subsiding column became drier and therefore more transparent to outgoing longwave radiation than the ascending column. However, Philander et al. (1996) found that the instability was not manifested in the east Pacific in a coupled ocean-atmosphere global circulation model unless the additional cooling effects of stratus clouds were included. Thus, a proper understanding of the southern hemisphere marine stratus in the eastern Pacific appears to be important for modeling the coupling of the atmospheric and oceanic circulations there.
A great deal of work has recently been done on the development and evolution of subtropical stratus and stratocumulus clouds. The ASTEX (Atlantic Stratocumulus Transition Experiment; Randall, 1995, and associated papers) studied the transition of stratus and stratocumulus clouds into tradewind clouds as air moved over higher SSTs. Much of the knowledge gained from this and similar work can possibly be applied to the eastern Pacific. However, to use this knowledge with confidence, it is important to determine whether the clouds in this region have characteristics similar to those observed in the ASTEX region.