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Errors in Modelling the South Indian Ocean Thermocline Dome

Paper Reviewed
Li, G., Xie, S.-P. and Du, Y. 2015. Climate model errors over the South Indian Ocean thermocline dome and their effect on the basin mode of interannual variability. Journal of Climate 28: 3093-3098.

Writing in the Journal of Climate, Li et al. (2015) describe how a common equatorial easterly wind bias forces a westward-propagating down-welling Rossby wave in the southern Indian Ocean (IO) that "induces too deep a thermocline dome over the southwestern IO (SWIO) in state-of-the-art climate models." And they indicate that "such a deep SWIO thermocline weakens the influence of subsurface variability on sea surface temperature (SST), reducing the Indian Ocean Basin (IOB) amplitude and possibly limiting the models' skill of regional climate predictions."

In further studying this knotty problem, they also note that (1) "the westerly wind over the equatorial IO is too weak during SON [Sept, Oct, Nov] in most CMIP5 CGCMs [coupled general circulation models]," that (2) "the deep thermocline dome bias over the SWIO in CMIP5 CGCMs could significantly reduce the effect of subsurface thermocline variability on SST [sea surface temperature] there," that (3) "too weak a thermocline-SST feedback over the SWIO in CMIP5 CGCMs results in a deficiency in the simulated amplitude for the IOB mode of inter-annual variability," that this phenomenon (4) "also lowers their skill in predicting the IOB warming following El Niño," that (5) "the easterly wind error in CGCMs can result in a too steep eastward shoaling of [the] thermocline in the equatorial IO," that (6) "the unrealistically steep thermocline slope generates too strong a thermocline feedback on SST," and thus (7) "develops an excessively large IOD amplitude of inter-annual variability in CGCMs," which (8) "exerts profound social and economic consequences for the IO rim countries such as Indonesia and Kenya," that (9) the "too weak cross-equatorial easterly wind bias can be traced back to errors in the South Asian summer monsoon," that (10) the "too weak cross-equatorial monsoon over the western basin in JJA [Jun, Jul, Aug] causes a sustained warm SST bias in the western equatorial IO," that (11) "in SON, Bjerknes feedback helps amplify this SST error into an IOD-like pattern," with (12) "a strong equatorial easterly bias accompanied by a physically consistent bias in the precipitation dipole."

These results, in the words of the three Chinese researchers, "imply that reducing the monsoon errors in CGCMs will improve climate simulation and prediction for the IO and rim countries, and increase our confidence in their application for regional climate projection." But in light of the multiple problems that remain to be resolved -- and if past is prologue to the future -- that confidence may well be weakened, or possibly even shattered, before we're half-way there.

Posted 29 July 2015