From CO2 Science, something that Willis Eschebach holds dear as a regulating mechanism: The Inter-Tropical Convergence Zone
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.
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. http://link.springer.com/article/10.1007%2Fs00382-015-2468-6
The double intertropical convergence zone (ITCZ) bias still affects all the models that participate to CMIP5 (Coupled Model Intercomparison Project, phase 5). As an ensemble, general circulation models have improved little between CMIP3 and CMIP5 as far as the double ITCZ is concerned. The present study proposes a new process-oriented metrics that provides a robust statistical relationship between atmospheric processes and the double ITCZ bias, additionally to the existing relationship between the sea surface temperature (SST) and the double ITCZ bias. The SST contribution is examined using the THR-MLT index (Bellucci et al. in J Clim 5:1127–1145, 2010), which combines biases on the representation of local SSTs and the SST threshold leading to the onset of ascent in the double ITCZ region. As a metrics of a model’s bias in simulating the interaction between circulation and precipitation, we propose to use the Combined Precipitation Circulation Error (CPCE). It is computed as the quadratic error on the contribution of each vertical regime to the total precipitation over the tropical oceans. CPCE is a global measure of the circulation-precipitation coupling that characterizes the model physical parameterizations rather than the regional characteristics of the eastern Pacific. A linear regression analysis shows that most of the double ITCZ spread among CMIP5 coupled ocean–atmosphere models is attributed to SST biases, and that the precipitation large-scale dynamics relationship explains a significant fraction of the bias in these models, as well as in the atmosphere-only models.
Full paper (draft) http://www.cnrm-game.fr/IMG/pdf/ditcz_cmip5.pdf