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Modelling the Recent History of Changes in the Walker Circulation

Paper Reviewed
Kociuba, G. and Power, S.B. 2015. Inability of CMIP5 models to simulate recent strengthening of the Walker Circulation: Implications for projections. Journal of Climate 28: 20-35.

Introducing their paper, Kociuba and Power (2015) write that Earth's Walker Circulation (WC) "is one of the world's most prominent and important atmospheric systems," in that "it extends across the entire tropical Pacific Ocean, encompassing (1) the trade winds blowing from east to west; (2) air forced to rise over the western Pacific, Southeast Asia, and northern Australia through enhanced convection; (3) winds blowing counter to the trades aloft; and (4) air descending over the eastern Pacific Ocean." And in light of these several significant aspects of the WC, they go on to evaluate how well they are replicated by CMIP5 climate models between 1980 and 2012. And what did they learn by so doing?

The two Australian researchers report that "on the one hand, 78% of the models show a weakening of the WC over the twentieth century, consistent with the observations." But on the other hand, they say "the observations also exhibit a strengthening in the last three decades (i.e., from 1980 to 2012) that is statistically significant at the 95% level," whereas the models "show no consensus on the sign of change, and none of the models shows a statistically significant strengthening over the same period."

Therefore, and in light of the fact that "the reasons for the inconsistency between modeled and observed trends in the last three decades are not fully understood," Kociuba and Power rightly note that "confidence in the model projections is reduced." In fact, it is pretty much destroyed.

Posted 27 April 2015