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Abstract:
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According to current knowledge, convection over the tropical oceans increases with sea surface temperature
(SST) from 26 to 29 °C, and at SSTs above 29 °C, it sharply decreases. Our research shows that it is only over the summer
warm pool areas of Indian and west Pacific Oceans (monsoon areas) where the zone of maximum SST is away from the
equator that this kind of SST-convection relationship exists. In these areas (1) convection is related to the SST gradient
that generates low-level moisture convergence and upward vertical motion in the atmosphere. This has modelling support.
Regions of SST maxima have low SST gradients and therefore feeble convection. (2) Convection initiated by SST gradient
produces strong wind fields particularly cross-equatorial low-level jetstreams (LLJs) on the equator-ward side of the warm
pool and both the convection and LLJ grow through a positive feedback process. Thus, large values of convection are
associated with the cyclonic vorticity of the LLJ in the atmospheric boundary layer. In the inter-tropical convergence zone
(ITCZ) over the east Pacific Ocean and the south Pacific convergence zone (SPCZ) over the west Pacific Ocean, low-level
winds from north and south hemisphere converge in the zone of maximum SST, which lies close to the equator producing
there elongated bands of deep convection, where we find that convection increases with SST for the full range of SSTs
unlike in the warm pool regions. The low-level wind divergence computed using QuikSCAT winds has large and significant
linear correlation with convection in both the warm pool and ITCZ/SPCZ areas. But the linear correlation between SST
and convection is large only for the ITCZ/SPCZ. These findings have important implications for the modelling of largescale
atmospheric circulations and the associated convective rainfall over the tropical oceans |