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A Multiplicity of Model Biases in NCEP's Climate Forecast System: How Do They Propagate in Modeling the Indian Summer Monsoon?

Reference
Chaudhari, H.S., Pokhrel, S., Saha, S.K., Dhakate, A., Yadav, R.K., Salunke, K., Mahapatra, S., Sabeerali, C.T. and Rao, S.A. 2013. Model biases in long coupled runs of NCEP CFS in the context of Indian summer monsoon. International Journal of Climatology 33: 1057-1069.
In the words of Chaudhari et al. (2013), "despite the potential for tropical climate predictability, and the advances made in the development of climate models, the seasonal dynamical forecast of [the] Indian summer monsoon remains a challenging problem," which they thus proceed to explore via a study of model biases and how they create further biases as they wend their way through multiple stages of both simultaneous and sequential processes. Specifically, Chaudhari et al. examined the performance of the National Centers for Environmental Prediction (NCEP) Climate Forecast System (CFS) over the Indian monsoon region in a 100-year-long coupled run, which was framed "in terms of biases of sea surface temperature (SST), rainfall and circulation," while also exploring "the role of feedback processes in maintaining these biases." And what did that examination reveal?

According to the nine researchers, the model [1] shows dry (wet) rainfall bias concomitant with cold (warm) SST bias over the east (west) equatorial Indian Ocean; and they say that these biases of SST and rainfall [2] affect both lower- and upper-level circulations in a feedback process, which in turn [3] regulates the SST and rainfall biases by maintaining a coupled feedback process. Subsequently, a dry (wet) rainfall bias over the east (west) Indian Ocean [4] induces anomalous low level easterlies over the tropical Indian Ocean and [5] causes cold SST bias over the east Indian Ocean by triggering evaporation and warm SST bias over the west Indian Ocean through advection of warm waters. The persistent SST bias then [6] retains the zonal asymmetric heating and meridional temperature gradient resulting in a circum-global subtropical westerly jet core, which in turn [7] magnifies the mid-latitude disturbances and [8] decreases the Mascarene high, which in its turn [9] diminishes the strength of monsoon cross-equatorial flow and [10] results in less upwelling as compared to that in the observations, which latter phenomenon [11] increases the SST bias over the West Indian Ocean.

And thus it is, in the words of Chaudhari et al., that "the coupled interaction among SST, rainfall and circulation works in tandem through a closed feedback loop to maintain the model biases over the tropical Indian Ocean." And once significant biases worm their way into the innards of today's complex climate models, they are extremely difficult to expel.

Archived 25 September 2013