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Simulating the South Asian Monsoon: A Revealing Status Update
Volume 17, Number 6: 5 February 2014

In a study published a few months back, Islam et al. (2013) wrote that "an intensive research effort has been made to improve simulation of monsoon systems by climate models," while also noting, in this regard, that "significant progress has been made in recent years." And, therefore, they said it was "of interest and importance to evaluate the ability of these new model versions," which is what they therefore did for Community Atmosphere Models (CAM4 and CAM5) and the most recent Community Climate System Model (CCSM4).

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." And what did they learn? The following is what 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 that [6] "CCSM4 underestimated the precipitation over the equatorial area in the Pacific Ocean," and that [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 indicate that [8] "CCSM4 showed a systematic cold bias in the simulation of SSTs over the tropical Pacific Ocean" and that it [9] "showed problems in simulating the observed SST-precipitation relationship." And they write, last of all, that [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. were forced to acknowledge - in spite of some significant improvements over earlier versions of the three models they had studied - that [11] "many biases are still present" and that [12] the latest versions of the models "still have simulation errors that need further consideration," which facts could well be considered to transform the models' top ten embarrassments into their dirty dozen failures.

Sherwood, Keith and Craig Idso

Islam, S., Tang, Y. and Jackson P.L. 2013. Asian monsoon simulations by Community Climate Models CAM4 and CCSM4. Climate Dynamics 41: 2617-2642.