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The South Asian Monsoon: How Difficult It Has Been to Model

Bollasina, M.A. and Ming, Y. 2013. The general circulation model precipitation bias over the southwestern equatorial Indian Ocean and its implications for simulating the South Asian monsoon. Climate Dynamics 40: 823-838.
In the words of Bollasina and Ming (2013), most current general circulation models (GCMs) "show a remarkable positive precipitation bias over the southwestern equatorial Indian Ocean (SWEIO), which can be thought of as a westward expansion of the simulated IO convergence zone toward the coast of Africa." And they note, in this regard, that "the bias is common to both coupled and uncoupled models, suggesting that its origin does not stem from the way boundary conditions are specified." To further explore this issue, as briefly described by Bollasina and Ming, "the spatio-temporal evolution of the precipitation and associated three-dimensional atmospheric circulation biases [were] comprehensively characterized by comparing the GFDL [Geophysical Fluid Dynamics Laboratory] AM3 atmospheric model to observations." So what did they learn?

In the words of the two researchers, "the oceanic bias, which develops in spring and reduces during the monsoon season, is associated [with] a consistent precipitation and circulation anomalous pattern over the whole Indian region," where "in the vertical, the areas are linked by an anomalous Hadley-type meridional circulation, whose northern branch subsides over northeastern India significantly affecting the monsoon evolution (e.g., delaying its onset)." And they indicate that "the ability of local anomalies over the SWEIO to force a large-scale remote response to the north is further supported by numerical experiments with the GFDL spectral dry dynamical core model."

Given such findings, Bollasina and Ming say their study "makes the case that the precipitation bias over the SWEIO is forced by the model excess response to the local meridional sea surface temperature gradient through enhanced near-surface meridional wind convergence," and they thus conclude that "a detailed investigation into the model physics to identify possible parameters which may alleviate the model bias would be the natural extension of this work."

Archived 10 July 2013