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Lamenting the Double ITCZ Bias in CMIP5 Climate Models

Paper Reviewed
Oueslati, B. and Bellon, G. 2015. The double ITCZ bias in CMIP5 models: interaction between SST, large-scale circulation and precipitation. Climate Dynamics 44: 585-607.

In what must be a difficult acknowledgement to make, Oueslati and Bellon (2015) write that "the double intertropical convergence zone (ITCZ) bias still affects all the models that participate in CMIP5," - i.e. the Coupled Model Intercomparison Project, phase 5 -- while further noting that "as an ensemble, general circulation models have improved little between CMIP3 and CMIP5 as far as the double ITCZ is concerned." And, therefore, they proceed to discuss some of the many other problems that have ultimately led to this problem.

The two French researchers begin by noting that (1) "the double ITCZ bias affecting the central Pacific can be connected to the simulation of a too-zonally elongated South Pacific convergence zone," as well as (2) "a too-zonally elongated SPCZ and a spurious ITCZ in the Eastern Pacific, that (3) the spatial distribution of sea surface temperature is "poorly simulated in coupled ocean-atmosphere GCMs (OAGCMs)," with (4) "a positive SST bias over the southeastern Pacific," as well as (5) "an excessive equatorial cold tongue extending too far west in the Pacific," which latter biases are attributed to coupled ocean-atmosphere feeedbacks such as (i) "the SST-wind-induced surface fluxes feedback," (ii) "the SST-stratus feedback," and (iii) "the SST gradient-trade wind feedback associated with vertical upwelling."

After further studying this sad situation, Oueslati and Bellon additionally, and quite rationally, come to the conclusion that "overestimated ascending regimes suggest that processes inhibiting deep convection (e.g. convective entrainment, downdrafts and large-scale subsidence) are still poorly represented in CMIP5 models," all of which makes one wonder if it will ever be possible to correctly represent these several interacting phenomena in a fail-safe climate model.

Posted 16 June 2015