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Model Biases Leading to a Double Intertropical Convergence Zone

Paper Reviewed
Wang, C.-C., Lee, W.-L., Chen, Y.-L. and Hsu, H.-H. 2015. Processes leading to Double Intertropical Convergence Zone bias in CESM1/CAM5. Journal of Climate 28: 2900-2915.

As background for their most interesting study, Wang et al. (2015) write that "although the double intertropical convergence zone (ITCZ) is an observed phenomenon in the tropical central-eastern Pacific during boreal spring, it is often overemphasized in general circulation models (GCMs) and is a common bias that has plagued GCMs for a few decades." And, therefore, they set about to revisit the issue by analyzing the double ITCZ bias in a set of simulations produced by the Community Atmosphere Model version five (CAM5) and the Community Earth System Model version one (CESM1). And what did they discover by so doing in terms of relevant problems?

The four researchers from Taiwan report that in CAM5 alone they found a (1) "weaker-than-observed equatorial easterly in the tropical eastern South Pacific" that leads to (2) "weaker evaporation" and (3) "an increase in local sea surface temperature," that (4) "the shallow meridional circulation overly converges in the same region in the CAM 5 stand-alone simulation," that (5,6) "the planetary boundary layer and middle troposphere are too humid," that (7) "the large-scale subsidence is too weak at the middle levels," and that these problems (8) "may result from excessive shallow convection behavior in CAM5."

In the ocean model (CESM1), they found that (9) "the South Equatorial Current is underestimated," that (10) "the North Equatorial Counter-current is located too close to the equator," which causes (11) "a warm SST bias in the southeastern Pacific," and (12) "a cold bias in the northeastern Pacific." In addition, Wang et al. note that (13) "these sea surface temperature biases feed back to the atmosphere and further influence convection and the surface wind biases in the coupled simulation," with the ultimate end result of these many model errors and deficiencies being the erroneously-predicted Double Intertropical Convergence Zone.

Posted 17 July 2015