Interactions Between the Asian Monsoons, the Tropical Pacific, and the Southern Hemisphere Extratropics.

Relatively strong and weak years of Indian monsoon rainfall are selected and used as a starting point to follow the evolution of the annual cycle of precipitation in the tropical Indian and Pacific sectors. Analysis of observations shows that the dynamically coupled ocean-atmosphere system in the Indian-Pacific region produces Southern Oscillation -type signals in atmosphere and ocean in many years, with extremes being manifested as Warm and Cold Events. Four different GCM simulations are analyzed. One is run with the specified annual cycle of observed sea surface temperatures (SPEC SST), and two are coupled to a simple 50 m deep slab ocean model with present (1 times CO_ 2) and doubled (2 times CO_ 2) amounts of atmospheric carbon dioxide. The 50 m slab ocean model acts as a simple heat reservoir with no provision for currents or upwelling. Due to these inherent limitations, the SPEC SST simulation most closely represents observed mean conditions, while the 1 times CO_ 2 and 2 times CO_ 2 integrations produce a progressively more Warm Event type of response in the tropics. The high southern latitudes in these model runs show internally consistent circulation anomalies in the 2 times CO_ 2 compared with the 1 times CO _ 2 and SPEC SST. This involves a sensitivity to the annual cycle of SSTs at mid-latitudes and sea ice extent at high latitudes, both of which have an effect on the semiannual oscillation at those latitudes. A fourth GCM experiment with an altered convective scheme but with SSTs and sea ice held constant produces a warmer tropical troposphere, an intensified equator-to-pole temperature gradient, and a deepened circumpolar trough year round. However, the observations show a seasonal evolution of anomalies in the Southern Hemisphere such that the circumpolar trough is deeper during southern winter the year of a Warm Event and shallower the following summer. The observed sequence of Warm minus Cold Event composites shows a poleward migration of anomalies with the tropics leading the high latitudes by about half a year. This poleward migration is most evident in the Pacific, and appears to be related to the annual cycle of the entire coupled ocean-atmosphere system in the Indian and Pacific sectors.