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Simulating the Trigger of the Onset of the Indian Summer Monsoon
Sabeerali, C.T., Dandi, A.R., Dhakate, A., Salunke, K., Mahapatra, S. and Rao, S.A. 2013. Simulation of boreal summer intraseasonal oscillations in the latest CMIP5 coupled GCMs. Journal of Geophysical Research: Atmospheres 118: 4401-4420.

The authors write that "in almost all years, the onset of the Indian summer monsoon is triggered by a northward propagation of the wet phase of the BSISO [boreal summer intraseasonal oscillation] with considerable inter-annual variations," citing Goswami (2005) and Ajayamohan et al. (2008). And they state that considering the crucial role that the BSISO plays in the ASM [Asian summer monsoon], "the proper simulation of this variability in the state-of-the-art coupled GCMs is important for the short-term and long-term prediction of the ASM."

What was done
Sabeerali et al. evaluated the abilities of 32 CMIP5 models to simulate the BSISO by comparing their projections against observations covering the last twenty years.

What was learned
This work revealed, in the words of the six scientists, that (1) "most of the models still have great difficulty to simulate the large-scale mean feature of precipitation over the ASM region," that (2) "the magnitude of the annual cycle varies from model to model," that (3) "most of the models underestimate (overestimate) the rainfall during the early (late) stages of the boreal summer monsoon season," that (4) "many models showed a remarkable dry bias over the eastern equatorial Indian Ocean and wet bias over the southwestern equatorial Indian Ocean," that (5) "the majority of the models are unable to simulate the spatial pattern of BSISO variance over the ASM region," that (6) these biases are "mostly due to the erroneous representation of the seasonal mean precipitation," that (7) "many models failed to simulate realistic equatorial eastward propagation beyond Maritime continent," that (8) "the simulated amplitude of the northward propagating band is weak in most of the models," that (9) "most of the models failed to simulate a realistic southward propagating mode as seen in observations," and that (10) "a fundamental error noted in many models is the incorrect evolution of the BSISO."

What it means
In one of the chief understatements of the climatological year, Sabeerali et al. concluded that "many models still face difficulties" to simulate what they are attempting to simulate in the case of the BSISO.

Ajayamohan, R.S. and Goswami, B.N. 2007. Dependence of simulation of boreal summer tropical intraseasonal oscillations on the simulation of seasonal mean. Journal of the Atmospheric Sciences 64: 460-478.

Goswami, B.N. 2005. South Asian monsoon. In: Lau, W.K.M. and Waliser, D.E. (Eds.). Intraseasonal Variability of the Atmosphere-Ocean Climate System. Springer, Berlin, Germany, pp. 19-61.

Reviewed 22 January 2014