Air–Sea Interactions of Relevance to the ITCZ: Analysis of Coupled Instabilities and Experiments in a Hybrid Coupled GCM

Abstract

The eastern Pacific and Atlantic have a curious climatic asymmetry relative to the equator. Whereas the intertropical convergence zone (ITCZ) characterized by persistent and heavy rainfall and the warmest surface waters reside north of the equator, a cold tongue in sea surface temperature (SST) occurs at and south of the equator even though the time-mean solar radiation is approximately symmetric about the equator. In 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-feedback mechanisms are all included in a unified dynamic framework. The author?s stability analysis indicates that in a reasonable parameter regime the growth rates associated with the three coupled instabilities are of the same order of magnitude, suggesting that they are all important in contributing to the climatic asymmetry. Because of the dependence of the three feedback mechanisms on the existence of a shallow oceanic mixed layer that, in turn, is a result of equatorial easterlies, the existence of the equatorial easterlies is essential for the amplification of the climatic asymmetry. Next, a hybrid coupled general circulation model is used in which a realistic continental and coastal geometry 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 of the three air?sea interaction mechanisms, the coupled model is capable of reproducing a realistic asymmetric time-mean state in the eastern Pacific and Atlantic. The fundamental cause of the asymmetry in the eastern Pacific is the tilt of the western coast of the Americas, which perturbs SST in the vicinity of the coastal region through a so-called 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 asymmetries and the ocean?atmosphere interactions in determining the preferred climatic position for the ITCZ.