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Biases Driving Biases in CMIP5 Models of Earth's Major Monsoons

Paper Reviewed
Sandeep, S. and Ajayamohan, R.S. 2014. Origin of cold bias over the Arabian Sea in climate models. Scientific Reports 4: 10.1038/srep06403.

In prefacing their work, Sandeep and Ajayamohan (2014) write that "sea surface temperature (SST) biases are arguably the most prominent error in Coupled General Circulation Model (CGCM) simulations," citing Large and Danabasoglu (2006), while further noting that these biases "can result in amplification of model error due to the feedback between different components of the climate system," citing Cai et al. (2011). More specifically, they report that "tropics-wide bias in SSTs in the fifth phase of the Coupled Model Inter-comparison Project (CMIP5) models has been traced to biases in the simulations of clouds and thermocline depth by the coupled models," citing Li and Xie (2012). And they additionally indicate that both "local and large-scale oceanic and atmospheric processes are dominant elements for the SST biases," citing the findings of Toniazzo and Woolnough (2013), Xu et al. (2013) and Vanniere et al. (2014).

To further explore this important issue, Sandeep and Ajayamohan focus on "the pre-monsoon SST bias over the Arabian Sea in CMIP5 historical simulations," specifically investigating "the possible presence of an equator-ward bias in STJs [Sub-Tropical Jet Streams]," in view of "the wake of known biases in ITCZ and eddy driven jets," citing Ceppi et al. (2013) and Hwang and Frierson (2013). And what rewards did these efforts yield them?

The two researchers state that the biases in the location and strength of the STJs "are explained by the location of northern hemispheric Hadley Cell subsidence," while "biases in the strength and location of Hadley Cell subsidence may be linked to the biases in the radiative forcing," once again citing Hwang and Frierson (2013). In addition, they write that "the equator-ward shift coupled with enhanced strength of the subtropical jet produce a stronger upper tropospheric convergence, leading to a subsidence and divergence at lower levels over the Arabian deserts," such that "the low entropy air flowing from the Arabian land mass cools the northern Arabian Sea." And they say that "the weaker meridional temperature gradients in the colder models substantially weaken Indian Summer Monsoon precipitation," all of which leads one to wonder when we will ever be able to say "bye, bye to biases" in ever-evolving - but never quite getting there - climate models.

Cai, W., Sullivan, A., Cowan, T., Ribbe, J. and Shi, G. 2011. Simulation of the Indian Ocean Dipole: A relevant criterion for selecting models for climate projections. Geophysical Research Letters 38: 10.1029/2010gl046242.

Ceppi, P., Hwang, Y.-T., Liu, X., Frierson, D.M.W. and Hartmann, D.L. 2013. The relationship between the ITCZ and the Southern Hemispheric eddy-driven jet. Journal of Geophysical Research 118: 5136-5146.

Hwang, Y.T. and Frierson, D.M.W. 2013. Link between the double-Intertropical Convergence Zone problem and cloud biases over the Southern Ocean. Proceedings of the National Academy of Sciences USA 110: 4935-4940.

Large, W.G. and Danabasoglu, G. 2006. Attribution and impacts of upper-ocean biases in CCSM3. Journal of Climate 19: 2325-2346.

Li, G. and Xie, S.-P. 2012. Origins of tropical-wide SST biases in CMIP multi-model ensembles. Geophysical Research Letters 39: 10.1029/2012GL053777.

Toniazzo, T. and Woolnough, S. 2013. Development of warm SST errors in the southern tropical Atlantic in CMIP5 decadal hindcasts. Climate Dynamics: 10.1007/s00382-013-1691-2.

Vanniere, B., Guilyardi, E., Toniazzo, T., Madec, G and Woolnough, S.A. 2014. A systematic approach to identify the sources of tropical SST errors in coupled models using the adjustment of initialized experiments. Climate Dynamics: 10.1007/s00381-014-2051-6.

Xu, Z., Li, M., Particols, C. and Chang, P. 2013. Oceanic origin of southeast tropical Atlantic biases. Climate Dynamics: 10.1007/s00382-013-1901-y.

Posted 14 January 2015