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Correctly Modelling the Indian Summer Monsoon
Prodhomme, C., Terray, P., Masson, S., Izumo, T., Tozuka, T. and Yamagata, T. 2014. Impacts of Indian Ocean SST biases on the Indian Monsoon: as simulated in a global coupled model. Climate Dynamics 42: 271-290.

The authors write that "the Asian Summer Monsoon is one of the most dominant tropical atmospheric circulations, and the economies and livelihoods of the populations of India and Southeast Asia depend heavily on its rainfall (Wang, 2006)." However, they note that "important SST [Sea Surface Temperature] biases in coupled models drastically limit our understanding of the physical processes involved in the climate fluctuations, especially those associated with the ISM [Indian Summer Monsoon] and Indian Ocean Dipole," as reported by Bollasina and Nigam (2009), Fischer et al. (2005), Terray et al. (2012) and Levine and Turner (2012).

What was done
"In this study," according to Prodhomme et al., "the impact of the ocean-atmosphere coupling on the atmospheric mean state over the Indian Ocean and the ISM is examined in the framework of the SINTEX-F2 coupled model through forced and coupled control simulations and several sensitivity coupled experiments."

What was learned
In the words of the six scientists, they determined that (1) "during boreal winter and spring, most of the Indian Ocean biases are common in forced and coupled simulations, suggesting that the errors originate from the atmospheric model, especially a dry islands bias in the Maritime Continent," that (2) "during boreal summer, the air-sea coupling decreases the ISM rainfall over South India and the monsoon strength to realistic amplitude, but at the expense of important degradations of the rainfall and SST mean states in the Indian Ocean," that (3) "strong SST biases of opposite sign are observed over the western (WIO) and eastern (EIO) tropical Indian Ocean," that (4) "rainfall amounts over the ocean (land) are systematically higher (lower) in the Northern Hemisphere," that (5) "the south equatorial Indian Ocean rainfall band is missing in the control coupled simulation," and that (6) "during boreal fall, positive dipole-like errors emerge in the mean state of the coupled model, with warm and wet (cold and dry) biases in the WIO (EIO), suggesting again a significant impact of the SST errors."

What it means
So are we "there" yet? Unfortunately not, for in spite of all that has been accomplished in the quest to correctly model the Indian Summer Monsoon, the world's climate modelers still have a significant way to go before they arrive at their designated destination.

Bollasina, M.A. and Nigam, S. 2009. Indian Ocean SST, evaporation, and precipitation during the South Asian summer monsoon in IPCC AR4 coupled simulations. Climate Dynamics 33: 1017-1032.

Fischer, A., Terray, P., Guilyardi, E. and Delecluse, P. 2005. Two independent triggers for the Indian Ocean Dipole/zonal mode in a coupled GCM. Journal of Climate 18: 3428-3449.

Levine, R.C. and Turner, A.G. 2012. Dependence of Indian Monsoon rainfall on moisture fluxes across the Arabian Sea and the impact of coupled model sea surface temperature biases. Climate Dynamics 38: 2167-2190.

Terray, P., Kamala, K., Masson, S., Madec, G., Sahai, A.K., Luo, J.-J. and Yamagata, T. 2012. The role of the intra-daily SST variability in the Indian Monsoon variability and monsoon-ENSO-IOD relationships in a global coupled model. Climate Dynamics 39: 729-754.

Wang, B. 2006. The Asian Monsoon. Springer/Praxis Publishing, New York, New York, USA, p. 787.

Reviewed 26 March 2014