Tropical-Midlatitude Interactions in the Indian and Pacific Sectors of the Southern Hemisphere

Abstract

Observations and three General Circulation Model (GCW) simulations are analyzed to study interannual processes and interactions involved in the tropical Indian and Pacific sectors and higher southern latitudes. Major features involving the observed eastward progression of the tropical convective maximum from the Indian monsoon in northern summer to the Australian monsoon and Pacific in southern summer are represented in all three model simulations. This suggests that the distribution of land and sea accounts for the observed seasonal position of the convective maximum in these regions. However, the South Pacific Convergence zone (SPCZ) is not present at any time of year in the slab-ocean models due to the anomalously warm sea-surface temperatures simulated in the equatorial Pacific. Analyses of observed zonal mean 500-mb temperatures and sea-level pressure from 10° to 80°S for Warm-Event composites minus Cold-Event composites (representing two extremes of interannual variation) show the period of anomalies at high southern latitudes to be about half a year out of phase with the tropics. Poleward migration or observed anomalies is most apparent in the Pacific sector, suggesting a delayed communication between the tropics and high latitudes. A tentative physical explanation is postulated that involves the SPCZ as a conduit for communicating tropical anomalies to higher southern latitudes by changes in the dynamically coupled ocean-atmosphere system operating in the tropics and midlatitudes or the Pacific. If such processes are taking place in the real climate system, the tropics and high southern latitudes are not in equilibrium and immediate transmission of tropical anomalies to high southern latitudes may not occur. The GCM simulations under consideration here, which are run to near-equilibrium, therefore are not capable of portraying such features of observed interannual variability. These conclusions point to the necessity of using a coupled ocean-atmosphere GCM capable of simulating realistic air-sea interactions to study such interannual and latitudinal linkages in the Indian and Pacific sectors of the Southern Hemisphere.