Tuesday, 15 May 2001: 2:45 PM
The NCAR Community Climate System Model (CCSM) 300-year baseline run
is analyzed for its ability to reproduce the south Asian Monsoon and the
tropospheric biennial oscillation (TBO). 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 illustrated, and their relationship to the Indian
Ocean Dipole (Webster et al. 1999) and the TBO is analyzed.
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 and Arblaster (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 impact of these mechanisms will be
explored in the CCSM and compared to observations.
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