AIR-SEA INTERACTIONS OF RELEVANCE TO THE ITCZ - ANALYSIS OF COUPLED INSTABILITIES AND EXPERIMENTS IN A HYBRID COUPLED GCM

The eastern Pacific and Atlantic have a curious climatic asymmetry rel ative to the equator. Whereas the intertropical convergence zone (ITCZ ) characterized by persistent and heavy rainfall and the warmest surfa ce waters reside north of the equator, a cold tongue in sea surface te mperature (SST) occurs at and south of the equator even though the tim e-mean solar radiation is approximately symmetric about the equator. I n this paper the author investigates the relative role of three types of coupled ocean-atmosphere interaction processes-the meridional wind- SST feedback, the evaporation-wind feedback, and the low-level stratus cloud-SST feedback-in determining the climatic asymmetry relative to the equator. This study has two components. First, a simple analytical model is constructed in which the aforementioned three positive-feedb ack mechanisms are all included in a unified dynamic framework. The au thor's stability analysis indicates that in a reasonable parameter reg ime the growth rates associated with the three coupled instabilities a re of the same order of magnitude, suggesting that they are all import ant in contributing to the climatic asymmetry. Because of the dependen ce of the three feedback mechanisms on the existence of a shallow ocea nic mixed layer that, in turn, is a result of equatorial easterlies, t he existence of the equatorial easterlies is essential for the amplifi cation of the climatic asymmetry. Next, a hybrid coupled general circu lation model is used in which a realistic continental and coastal geom etry is presented. The model starts from an ideal symmetric condition forced only by the annual-mean insolation at the top of the atmosphere which is approximately symmetric about the equator. In the presence o f the three air-sea interaction mechanisms, the coupled model is capab le of reproducing a realistic asymmetric time-mean state in the easter n Pacific and Atlantic. The fundamental cause of the asymmetry in the eastern Pacific is the tilt of the western coast of the Americas, whic h perturbs SST in the vicinity of the coastal region through a so-call ed coastal wind-upwelling mechanism. The asymmetry in the Atlantic, on the other hand, results from the land-ocean thermal contrast between the bulge of northwestern Africa and the ocean to the south. The ocean -atmosphere interactions act as an amplifier to amplify the asymmetry set up by the continental or coastal asymmetry. Numerical experiments presented here demonstrate the importance of the geographic asymmetrie s and the ocean-atmosphere interactions in determining the preferred c limatic position for the ITCZ.