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Past studies of sea surface temperature (SST) in relation to tropical ocean rainfall have arrived at somewhat divergent conclusions with respect to SST and gradients thereof (e.g. Neelin and Held,1987;Lindzen and Nigam1991). Unlike most studies, Li and Carbone (2012) examined this issue from a mesoscale perspective, nominally at 100km scale, in an attempt to clarify the circumstances under which rainfall is triggered in the tropical western Pacific. They examined a 49 month timeseries of SST and rainfall as estimated from satellite observations. The seminar will begin with a brief summary of results, which shows a high frequency of mesoscale SST gradients and a propensity for onset of rainfall near the mode of the regional SST spectrum. While the statistical inference of dependence on SST is weaker than one might expect, coincidence with the convergent Laplacian of SST is strong.
The current work mainly emphasizes anomalies of and correlations between SST structure and rainfall in the MJO pass-band across the eastern hemisphere. The statistical inferences are, at once, broadly consistent with conventional wisdom on the role of SST in rainfall production and the pivotal role played by the convergent Laplacian of SST. We conclude that the main role of SST in tropical oceanic rainfall is in production of moist static energy. That is to say, SST bears a decidedly conditional and somewhat indirect relationship to the occurrence of rainfall: i.e. if rain occurs, then more rain is likely over warm anomalies of SST. The SST Laplacian mainly triggers rainfall events through induced vertical air motion with sufficient kinetic energy to overcome convective inhibition in a conditionally unstable troposphere. Its role is directly associated with the frequency of rainfall events; however, it bears little direct relationship to cumulative rainfall beyond its capacity to influence approximately where, when, and how many events occur.
Seminars are live webcast: http://www.fin.ucar.edu/it/mms/ml-live.htm