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The Equatorially-Asymmetric Mode of the Hadley Circulation

Paper Reviewed
Feng, J., Li, J., Zhu, J., Li, F. and Sun, C. 2015. Simulation of the equatorially asymmetric mode of the Hadley Circulation in CMIP5 models. Advances in Atmospheric Sciences 32: 1129-1142.

Noting that "the tropical Hadley circulation (HC) plays an important role in influencing the climate in the tropics and extra-tropics," Feng et al. (2015) describe how they examined "the realism of the climatological characteristics, spatial structure and temporal evolution of the long-term variation of the principal mode of the annual mean HC (i.e., the equatorially asymmetric mode, EAM)," working with models employed in the Coupled Model Intercomparison Project Phase 5 (CMIP5). And what did they thereby learn?

The five Chinese scientists report that (1) "none of the models can successfully capture the differences in the warming rate between the tropical Southern Hemisphere (SH) and Northern Hemisphere (NH)," that (2) "most of the models produce a faster warming in the NH than in the SH, which is the reverse of the observed trend," that this fact (3) "contributes to the poor simulation of EAM variability," that (4) "limited skill in simulating the underlying thermal variation within the tropics may contribute to the poor ability of the models to reproduce the long-term variability of the EAM," while on the other hand, they report that (5) "the magnitude of the gradient within the tropical hemispheres is much smaller than observed," which (6) "may also contribute to the poor simulation of the EAM."

And so it was that in light of their several findings, Feng et al. were forced to admit that their study (7) "did not identify why the models cannot reproduce the warming differences between the tropical SH and NH," although they did note that (8) "the variation of the underlying sea surface temperature is a complex issue that is not only linked to atmospheric processes, but is also affected by processes in the interior ocean, as well as air-sea interactions," which linked relationships obviously have yet to be adequately integrated into the EAM modelling process.

Posted 4 September 2015