The Asian Monsoon and the TBO in the NCAR CCSM

The NCAR Community Climate System Model (CCSM) 300-year baseline run is analyzed for for its ability to reproduce the south Asian Monsoon. The climatological seasonal evolution of precipitaion, SST and surface winds during the period spanning the boreal summer and winter monsoons is illustrated, and the ability of the model to reproduce the coupled interactions and the transitions from a strong monsoon years to weak monsoon years is explored. Composites of these transitional periods are displayed, and their relation to the Tropospheric Biennial Oscillation (TBO), as well as the Indian Ocean Dipole and its possible role in the biennial cycle, is illustrated. Comparisons to observations are made in terms of the indices that have been developed for the description of the Indian summer monsoon. These indices often disagree on their assessment of monsoon strength, and Meehl (2000) has shown that this could be the result of several different phenomena involved in the dynamics of the monsoon circulation competing with one another on various temporal and spatial scales. Instead of using area-averaged indices, it is possible, using SVD analysis, to relate the actual patters of rainfall over the Indian monsoon region to two mechanisms involved with the TBO to account for interannual variations of Indian summer rainfall. Two dominant mechanisms appear to be involved in the development of the TBO's influence on the south Asian monsoon: Large-scale forcing from the tropical Pacific, and regional forcing associated with the meridional temperature gradient between the Asian continent and the Indian Ocean. The impacts of these mechanisms will be explored in the CCSM and compared to observations.