and Yanai, 1996) drive relatively weak direct atmospheric circulations. These circulations are then substantially amplified by the diabatic heating during precipitation caused by convergence of moisture supplied by the surrounding ocean. These monsoonal diabatic heat sources, which drive global-scale atmospheric circulations, migrate seasonally across the equator. They are also subject to interannual variations in their intensity and location, dominated by the quasibiennial oscillation (QBO) and ENSO.
The largest and most intense mean heat source is found over the western Pacific warm pool. The warm pool is a thermal reservoir characterized by ocean temperatures warmer than 29℃ in a layer about 80 m deep, covering an area about the size of Australia. This warm pool is usually centered near the equator northeast of Papua New Guinea; it owes its existence to the convergence, into the western boundary, of ocean currents driven by the convergent atmospheric circulation set up by the diabatic heat sources of the Asian-Australian monsoons (cf. Chervin and Druyan, 1984). The poleward edges of the warm pool migrate northward and southward with the seasonal cycle of solar heating, and with the seasonally-varying surface currents.
The warm pool and its associated precipitation maximum is subject to eastward displacement during ENSO events (Fig. 1). This effectively moves the ascending branch of the zonal Walker circulation into the central Pacific (cf. Barnett, 1983), affecting the subsequent monsoon season (Soman and Slingo, 1996). Cold events appear to be associated with more intense monsoons, and a warm pool pushed farther west than usual. Soman and Slingo (1996) showed that the intensity of the monsoonal convection is directly influenced by these warm SST anomalies.
Figure 1. Time series of the longitude of the eastern edge of the western Pacific warm pool as indicated by the 4°S-4°N average location of the 28.5℃ isotherm. Years of ENSO warm events are indicated. Extreme eastward and westward displacements are labeled with month and year. Long-term mean longitude is indicated by the horizontal line.
Ocean dynamics (currents and baroclinic waves), forced by the atmosphere, are largely responsible for the mobility of the warns pool. Near. equatorial westerly winds associated with the Asian winter (northwest) monsoon are known to drive the warm pool eastward during the onset of ENSO events; concurrent upwelling and vertical mixing in the far western Pacific cause cooling there, helping to suppress convection and rainfall. Also, ocean dynamics in the off-equatorial regions appear to be responsible for interannual variations of the poleward edges of the warm pool; these variations of sea surface temperature near the Asian continent may influence the interannual variability of the monsoons (Kawamura et al., 1994; Joseph et al., 1994; Li and Yanai, 1996; Soman and Slingo, 1996). The shape of the warm pool along the western boundary is influenced by the surface western boundary currents. In particular, the bifurcation of the North Equatorial Current near 14。N into the Mindanao Current and the Kuroshio appears to be subject to considerable interannual variability related to the QBO and ENSO (Lukas, 1988; Qiu and Lukas, 1996).
Because of the interannual migrations of the warm pool, interactions between the monsoon diabatic heat sources and the warm pool heat source vary from year-to-year. These variable interactions are responsible for the QBO and the extremes of the monsoons associated with ENSO events (Soman and Slingo, 1996). Monitoring these interactions is important for improved understanding of their physics, and for initializing prediction models. Capturing the physics of these interactions in numerical models of the coupled ocean-atmosphere-land system is critical for improved ENSO forecasts, as well as improved monsoon forecasts.
The Climate Variability and Predictability Program (CLIVAR) of the World Climate Research Programme brings together and builds upon the objectives of TOGA and WOCE in articulating a research agenda for the next 15 years (WCRP, 1995). Because of the differences in objectives and strategy for research on the predictability of shorter and longer time scale climate variability, two components of CLIVAR have been developed. The Global Ocean-Atmosphere-Land System (GOALS) component is pursuing research on the predictability and prediction of seasonal-to-interannual climate variability. The decadal-to-centennial (Dec-Cen) component is pursuing understanding and assessment of decadal-to- centennial time scale variability, anthropogenic changes, and sudden changes.
An important research objective of GOALS is the determination of the predictability of the Asian- Australian monsoon system, and the relationship of its interannual variability to the ENSO phenomenon.
