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CMIP5 Model Simulations of the Earth's Western Pacific Monsoon
Brown, J.R., Colman, R.A., Moise, A.F. and Smith, I.N. 2013. The western Pacific monsoon in CMIP5 models: Model evaluation and projections. Journal of Geophysical Research: Atmospheres 118: 12,458-12,475.

The authors write that "the ability of coupled climate models to simulate the characteristics of the monsoon in present day climate is an important condition for the use of such models to make future climate projections." But they say that in the study of Smith et al. (2012), "the relationship between seasonal winds and rainfall was not always well represented," while adding that "the observed negative correlation between Indian rainfall and El Niņo-Southern Oscillation (ENSO) events was ... too weak in CMIP5 models," citing Sperber et al. (2013).

What was done
In the words of Brown et al., "the ability of 35 models from the Coupled Model Intercomparison Project Phase 5 (CMIP5) to simulate the western Pacific (WP) monsoon [was] evaluated over four representative regions around Timor, New Guinea, the Solomon Islands and Palau," where "coupled model simulations [were] compared with atmosphere-only model simulations (with observed sea surface temperatures, SSTs) to determine the impact of SST biases on model performance."

What was learned
The four Australian researchers report that they (1) "identified a number of biases including westerly summer monsoon winds that do not extend far enough east in many models," that they found that (2) "the AMIP [Atmospheric Model Intercomparison Project] models do not simulate the seasonal cycle of rainfall substantially better than CMIP5 [by] failing to capture the correct seasonal cycle over the Palau domain," that (3) "both CMIP5 and AMIP models appear to share problems in simulating the processes producing tropical rainfall, such as deep convection," that (4) CMIP5 models tend to simulate the boundary between positive and negative anomalies too far west," that this leads to (5) "incorrect sign anomalies over the Solomon Islands," and that (6) the models "substantially underestimate negative rainfall anomalies over the Maritime Continent region (including New Guinea and Timor)."

What it means
In the final sentence of their paper, Brown et al. conclude that "further investigation of the mechanisms determining the response of the monsoon in individual models is required to understand the reasons for model disagreement and to determine which aspects of projected monsoon change are most robust."

Smith, I., Moise, A.F. and Colman, R.A. 2012. Large-scale circulation features in the tropical western Pacific and their representation in climate models. Journal of Geophysical Research 117: 10.1029/2011JD016667.

Sperber, K.R., Annamalai, H., Kang, I.-S., Kitoh, A., Moise, A., Turner, A., Wang, B. and Zhou, T. 2013. The Asian summer monsoon: An intercomparison of CMIP5 vs. CMIP3 simulations of the late 20th century. Climate Dynamics 41: 2711-2744.

Reviewed 16 April 2014