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Indian Ocean Dipole and ENSO Teleconnections in CMIP5 Models

Weller, E. and Cai, W. 2013. Asymmetry in the IOD and ENSO teleconnections in a CMIP5 model ensemble and its relevance to regional rainfall. Journal of Climate 26: 5139-5149.
According to Weller and Cai (2013), "recent studies have shown that the impact of the Indian Ocean dipole (IOD) on southern Australia occurs via equivalent barotropic Rossby wave trains triggered by convective heating in the tropical Indian Ocean," and they say these phenomena have played "a significant role in the region's rainfall reduction in recent decades," adding it is essential "that climate models used for future projections simulate these features." Thus the pair of researchers, as they describe it, set out to "assess and benchmark how well the CMIP5 generation of climate models simulates the IOD and ENSO-induced teleconnections/asymmetries that propagate to the extratropics in SON [September-November]," which is the time of the Southern Hemisphere's austral spring. And what did their assessment reveal?

Although the models got some things right, they got other things wrong. The two Australian researchers report, for example, that (1) "the asymmetry in the impact of the IOD is distorted by two factors," which are that (2) "the tropical and extratropical response to La Niña is situated unrealistically too far westward," and that (3) it is thus "too close to Australia," leading to (4) "an overly strong impact on southeast Australia." They also note that (5) "the majority of models simulate a positive SST [sea surface temperature] skewness in the eastern Pacific that is too weak," which leads to their (6) "overestimating the impact of La Niña relative to that of El Niño," and that (7) "low model resolution may help generate a pan-Australia rainfall effect exacerbating the tropical bias."

In the closing words of the authors, "most models simulate a slower warming rate in the eastern tropical Indian Ocean than in the western tropical Indian Ocean, inducing a positive IOD-like mean state change, in terms of zonal SST gradients." They also say, "given that the mean state change will influence mean rainfall trends, rainfall projections are likely to be distorted by the biases in the Pacific as well, making it harder for the rainfall trends to emerge." And they thus conclude by noting their results highlight "the importance of reducing the distortion in future projections of rainfall changes in IOD-affected regions such as Australia."

Archived 15 April 2014