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Abstract:
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The SST convection relation over tropical ocean and its impact on the South Asian monsoon
is the first part of this thesis. Understanding the complicated relation between SST
and convection is important for better prediction of the variability of the Indian monsoon
in subseasonal, seasonal, interannual, and longer time scales. Improved global
data sets from satellite scatterometer observations of SST, precipitation and refined reanalysis
of global wind fields have made it possible to do a comprehensive study of
the SST convection relation. Interaction of the monsoon and Indian ocean has been
discussed. A coupled feedback process between SST and the Active-Break cycle of the
Asian summer monsoon is a central theme of the thesis. The relation between SST and
convection is very important in the field of numerical modeling of tropical rainfall. It is
well known that models generally do very well simulating rainfall in areas of tropical
convergence zones but are found unable to do satisfactory simulation in the monsoon
areas. Thus in this study we critically examined the different mechanisms of generation
of deep convection over these two distinct regions.The study reported in chapter 3 has shown that SST - convection relation over the
warm pool regions of Indian and west Pacific oceans (monsoon areas) is in such a way
that convection increases with SST in the SST range 26-29 C and for SST higher than
29-30 C convection decreases with increase of SST (it is called Waliser type). It is found that convection is induced in areas with SST gradients in the warm pool areas of
Indian and west Pacific oceans. Once deep convection is initiated in the south of the
warmest region of warm pool, the deep tropospheric heating by the latent heat released
in the convective clouds produces strong low level wind fields (Low level Jet - LLJ)
on the equatorward side of the warm pool and both the convection and wind are found
to grow through a positive feedback process. Thus SST through its gradient acts only
as an initiator of convection. The central region of the warm pool has very small SST
gradients and large values of convection are associated with the cyclonic vorticity of the
LLJ in the atmospheric boundary layer. The conditionally unstable atmosphere in the
tropics is favorable for the production of deep convective clouds. |