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Modeling the Indian Ocean Dipole: A Progress Report (of Sorts)

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.
The Indian Ocean Dipole (IOD) is an irregular oscillation of sea-surface temperatures in which the western Indian Ocean becomes alternately warmer and then colder than the eastern part of the ocean; and, hence, it is important that it is correctly incorporated into modern-day climate models. However, Cai and Cowan (2013) note that "in most models, IOD peak-season amplitudes are systematically larger than that of the observed," and they state that "understanding the cause of this bias is therefore essential for alleviating model errors and reducing uncertainty in climate projections."

Hoping to provide some of that needed "understanding," the two Australian researchers analyzed sea surface temperatures (SSTs), thermocline characteristics (20°C isotherm depth), zonal wind and precipitation outputs from 23 CMIP3 models and 33 CMIP5 models that attempted to simulate these climatic features over the last half of the 20th century, after which they compared the model simulations with real-world observations.

Cai and Cowan report that "most models generate an overly deep western Indian Ocean thermocline that results in an unrealistic upward slope toward the eastern tropical Indian Ocean," while noting that "the unrealistic thermocline structure is associated with too strong a mean easterly wind over the equatorial Indian Ocean, which is in turn supported by a slightly stronger mean west minus east SST gradient, reinforced by the unrealistic thermocline slope." And they conclude by stating that "these biases/errors have persisted in several generations of models," such that "there is no clear improvement from CMIP3 to CMIP5."

Archived 3 September 2013