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Simulating the South Asian Monsoon: A Revealing Status Update

Reference
Islam, S., Tang, Y. and Jackson P.L. 2013. Asian monsoon simulations by Community Climate Models CAM4 and CCSM4. Climate Dynamics 41: 2617-2642.
In a study published a few months back, Islam et al. (2013) write "an intensive research effort has been made to improve simulation of monsoon systems by climate models," while also noting, in this regard, "significant progress has been made in recent years." And, therefore, they note it is "of interest and importance to evaluate the ability of these new model versions," which is what they did for Community Atmosphere Models (CAM4 and CAM5) and the most recent Community Climate System Model (CCSM4). Specifically focusing on the South Asian Monsoon (SAM), Islam et al. explored in detail "the strengths and limitations of CAM4, CAM5 and CCSM4 in simulating SAM precipitation with an emphasis on the mean climate, seasonal and inter-annual variability and the relationship between SAM and SST (sea surface temperature, local and remote) in the simulations." As for what they learned, the following could well be called the models' top ten embarrassments.

In the words of the three Canadian researchers, but with italics added, "both [1] CAM4 and [2] CAM5 poorly simulate the ENSO-monsoon teleconnection," while "over the SAM region their simulations show significant large-scale biases such as [3] excessive precipitation over the Arabian bay and [4] over the Western Ghats of India, and [5] reduced precipitation over the eastern Indian Ocean extending into the Bay of Bengal." In addition, they say [6] "CCSM4 underestimated the precipitation over the equatorial area in the Pacific Ocean," and [7] it "still has the double ITCZ [Intertropical Convergence Zone] problem that was also present in the previous versions of the CCSM model." Furthermore, they write [8] "CCSM4 showed a systematic cold bias in the simulation of SSTs over the tropical Pacific Ocean" and it [9] "showed problems in simulating the observed SST-precipitation relationship." And they state, last of all, [10] "significant cold biases over the equatorial Pacific Ocean are found in CCSM4, particularly in winter and early summer."

When all was said and done, therefore, Islam et al. acknowledge, in spite of some significant improvements over earlier versions of the three models they had studied, "many biases are still present" and the latest versions of the models "still have simulation errors that need further consideration," which facts reveal the models' still have much to improve on.

Archived 11 February 2014