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Simulating the Indian Ocean Subtropical Dipole

Kataoka, T., Tozuka, T., Masumoto, Y. and Yamagata, T. 2012. The Indian Ocean subtropical dipole mode simulated in the CMIP3 models. Climate Dynamics 39: 1385-1399.
Writing as background for their work, authors Kataoka et al. (2012) note that the Indian Ocean Subtropical Dipole (IOSD; Behera et al., 2000; Behera and Yamagata, 2001) is "one of the climate modes that generate climate variations in the Southern Hemisphere," having "a great impact on the surrounding countries through its influence on the rainfall (Behera and Yamagata, 2001; Reason, 2001; Washington and Preston, 2006)." This mode is characterized by "a dipole pattern in the sea surface temperature anomaly in the southern Indian Ocean with a warm (cold) southwestern pole and cold (warm) northeastern pole during austral summer." And they say that "since southern Africa is one of the most vulnerable regions to abnormal weather events, an accurate prediction of the IOSD together with its influence on rainfall is necessary to mitigate the impacts."

Against this backdrop and using real-world observational data and mathematical outputs from 22 state-of-the-art coupled general circulation models (CGCMs) submitted to the World Climate Research Programme's Coupled Model Intercomparison Project phase 3 (CMIP3), Kataoka et al. proceeded to assess each model's ability to simulate the IOSD and its influence on rainfall anomalies over southern Africa.

In discussing their findings, the four Japanese researchers report that the location and orientation of sea surface temperature anomaly poles "differ considerably" from one model to another, owing primarily to model biases in sea level pressure anomalies, which finding, as they describe it, supports "the earlier results of Morioka et al. (2010) based on an ocean general circulation model." And this problem, in their words, "may partly explain the poor skills of CGCMs in simulating the influence of the IOSD on the rainfall anomalies." In addition, they state that "some models fail to simulate the statistical relation between the positive (negative) rainfall anomaly and La Niña (El Niño)."

The authors' parting words, as expressed in the final sentence of their paper, is that their study suggests that "more accurate simulation of the IOSD as well as the influence of the ENSO is necessary to improve the seasonal prediction of southern African rainfall," which feat, it might be added, is but one small aspect of purported global climate change, about which so many people believe we so thoroughly understand and can therefore so accurately model ... when we obviously don't and can't, respectively, at least at the present time.

Additional References
Behera, S.K. and Yamagata, T. 2001. Subtropical SST dipole events in the southern Indian Ocean. Geophysical Research Letters 28: 327-330.

Behera, S.K., Salvekar, P.S. and Yamagata, T. 2000. Simulation of interannual SST variability in the tropical Indian Ocean. Journal of Climate 13: 3487-3499.

Morioka, Y., Tozuka, T. and Yamagata, T. 2010. Climate variability in the southern Indian Ocean as revealed by self-organizing maps. Climate Dynamics 35: 1059-1072.

Reason, C.J.C. 2001. Subtropical Indian Ocean SST dipole events and southern African rainfall. Geophysical Research Letters 28: 2225-2227.

Washington, R. and Preston, A. 2006. Extreme wet years over southern Africa: Role of Indian Ocean sea surface temperatures. Journal of Geophysical Research 111: 10.1029/2005JD006724.

Archived 5 March 2013