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Uncorrected Climate Model Biases Over the Tropical Indian Ocean

Paper Reviewed
Li, G., Xie, S.-P. and Du, Y. 2015. Monsoon-induced biases of climate models over the tropical Indian Ocean. Journal of Climate 28: 3058-3072.

Writing in the Journal of Climate, Li et al. (2015) say that "long-standing biases of climate models limit the skills of climate prediction and projection," while noting that persistently overlooked in this regard are tropical Indian Ocean (IO) errors. And, therefore, they go on to identify some of the common problems that plague models that hold prominent positions in phase 5 of the Coupled Model Intercomparison Project (CMIP5) that are "accompanied by physically consistent biases in precipitation, sea surface temperature (SST) and sub-surface ocean temperature."

The three Chinese researchers begin by reporting that (1) "many coupled general circulation models (CGCMs) exhibit large biases in the South Asian summer monsoon," with the upper-tropospheric temperature maximum over Tibet being (2) "too weak" and (3) "displaced southeast of its observed location," along with (4) "insufficient summer precipitation over the South Asian monsoon region." Next, they write that (5) "CGCMs suffer from serious errors in simulating tropical Pacific and Atlantic climate," including (6) "an excessive equatorial cold tongue in the Pacific," (7) "a zonal sea surface temperature gradient that is too weak along the equatorial Atlantic," (8) "SSTs over the tropical southeast Pacific and Atlantic that are too warm," and (9) "a spurious double intertropical convergence zone."

In addition, Li et al. note that (10) "Lee et al. (2013) identified a common annual-mean easterly wind bias along the equatorial IO in state-of-the-art CGCMs participating in both CMIP3 and CMIP5," while "Cai and Cowan (2013) further found that the annual-mean equatorial easterly wind bias in CGCMs is accompanied by [11] an overly strong west-minus-east SST gradient and [12] the unrealistic climatological-mean thermocline slope tilting upward toward the eastern equatorial IO."

In their own study, Li et al. determined that (13) "the southwest summer monsoon is too weak over the Arabian Sea in most CMIP5 models," that (14) "this monsoon bias creates a warm SST bias over the western equatorial IO," and that in September, October and November, "Bjerknes feedback kicks in to amplify biases into an IO dipole-like pattern, with [15,16] "easterly wind errors and a too-steep eastward shoaling of the thermocline in the equatorial IO." And, therefore, they go on to conclude that although the IPCC's Fifth Assessment Report "characterizes this future IO dipole-like projection in the mean state as robust based on consistency among models," their own findings (as well as those of many other researchers they site), "cast doubts on this conclusion."

Cai, W. and Cowan, T. 2013. Why is the amplitude of the Indian Ocean dipole overly large in CMIP3 and CMIP5 climate models? Geophysical Research Letters 40: 1200-1205.

Lee, T., Waliser, D.E., Li, J.-L. F., Landerer, F.W. and Gierach, M.M. 2013. Evaluation of CMIP3 and CMIP5 wind stress climatology using satellite measurements and atmospheric reanalysis products. Journal of Climate 26: 5810-5826.

Posted 21 September 2015