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Westerly Biases Over the Equatorial Atlantic

Zermeño-Diaz, D.M. and Zhang, C. 2013. Possible root causes of surface westerly biases over the equatorial Atlantic in global climate models. Journal of Climate 26: 8154-8168.
According to authors Zermeño-Diaz and Zhang (2013), "most global climate models (GCMs) suffer from biases of a reversed zonal gradient in sea surface temperature (SST) and weak surface easterlies (the westerly bias) in the equatorial Atlantic during boreal spring." And they say that "these biases exist in the atmospheric GCMs (AGCMs) and are amplified by air-sea interactions in atmospheric-oceanic GCMs," noting that "this problem has persisted despite considerable model improvements in other aspects."

As their contribution to the subject, Zermeño-Diaz and Zhang, as they describe it, "used a simple model for a well-mixed boundary layer over the tropical oceans and simulations from eight AGCMs to diagnose possible root causes of the surface westerly bias over the Atlantic during boreal spring," examining "the possible roles of the vertical structure of diabatic heating and zonal momentum entrainment across the top of the boundary layer."

When all was said and done, little was to be learned from their analysis, as the two researchers report their work merely lays the foundation for a mere hypothesis in diagnosing the cause of the bias. Neverthless, they add that "the implication of our results is that there might be no simple or single remedy for the westerly bias in GCMs." And they say that "this may be why this problem has been so stubborn and persistent up to the new generation of CMIP5 models." And, it might be added, why the problem still remains ... unresolved!

Archived 15 January 2014