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Beijing Climate Center's Climate System Model BCC_CSM1.1(m)

Paper Reviewed
Liu, X., Wu, T., Yang, S., Jie, W., Nie, S., Li, Q., Cheng, Y. and Liang, X. 2015. Performance of the seasonal forecasting of the Asian Summer Monsoon by BCC_CSM1.1(m). Advances in Atmospheric Sciences 32: 1156-1172.

The recently published study of Liu et al. (2015), as they describe it, "provides a comprehensive assessment of Asian Summer Monsoon prediction skill as a function of lead time and its relationship to sea surface temperature prediction using the seasonal hindcasts of the Beijing Climate Center's Climate System Model, BCC.CSM1.1(m)," which assessment focuses on "deficiencies such as significant forecast errors over the tropical western North Pacific and the eastern equatorial Indian Ocean." And what were their most important findings?

The eight Chinese researchers report finding (1,2) "significant forecast errors over the tropical western North Pacific and the eastern equatorial Indian Ocean," that (3,4) "overestimation of the connections of some dynamical monsoon indices with large-scale circulation and precipitation patterns exists in most ensemble mean forecasts, even for short lead-time forecasts," and that (5,6) "predictions show apparent biases for winds and precipitation over the monsoon region," as well as (7) "an apparent overestimation of the inter-annual variance over the tropical western Pacific." And they say that forecasts often (8) "show gradually decreasing skill with increasing lead time," and that they (9) "become unskillful over many regions when the lead time is longer than four months."

Liu et al. additionally report that (10) "the prediction of wind often suffers apparent initial errors over the tropical western North Pacific," that (11-13) "the rainfall forecast is affected by obvious initial errors over the south of the Indian subcontinent and the western Indo-China Peninsula, as well as the tropical western Pacific." They also note that (14-16) "relationships between the first principal component of Pacific SST and the winds over the sub-tropics, and between the first principal component of Indian Ocean SST [sea surface temperature] and the winds over the tropical Indian Ocean and central-western Pacific, are always exaggerated by most ensemble mean predictions."

Continuing with their listing of problems associated with the BCC.CSM1.1(m) climate model, Liu et al. note that (17,18) "relationships between the first principal component of Pacific SST and the winds over the tropical Indian Ocean and central-western Pacific, are always exaggerated by most ensemble mean predictions," that (19) "the observed connections of the Webster-Yang (WY) index and SASM index with the principal component of the Pacific SST are clearly overestimated in most forecasts," while (20) "their links with the tropical Indian Ocean SST are often underestimated."

And with this they conclude that (21) "unrealistic connections among monsoons, atmospheric circulation, and tropical Pacific SST in the atmosphere-only model are similarly reproduced and further exaggerated by the coupled ensemble forecasts," thereby suggesting (22) "the existence of an unreasonable atmospheric response to tropical SST in both the simulation and predictions," which leads them to conclude that "the model is incapable of reproducing the appropriate interaction between monsoon and SST."

Posted 30 September 2015