Equatorial Atlantic variability and its relation to mean state biases in CMIP5

TitleEquatorial Atlantic variability and its relation to mean state biases in CMIP5
Publication TypeJournal Article
Year of Publication2014
AuthorsRichter I., Xie SP, Behera S.K, Doi T., Masumoto Y.
JournalClimate Dynamics
Date Published2014/01
Type of ArticleArticle
ISBN Number0930-7575
Accession NumberWOS:000329238300011
KeywordsBjerknes feedback; climatic variability; CMIP5; coupled ocean; Double ITCZ; eastern pacific; Equatorial Atlantic; Equatorial Kelvin waves; GCM biases; general-circulation models; in-situ; interannual variability; model; ocean-atmosphere; sea-surface temperature; seasonal cycle; Surface winds; tropical atlantic

Coupled general circulation model (GCM) simulations participating in the Coupled Model Intercomparison Project Phase 5 (CMIP5) are analyzed with respect to their performance in the equatorial Atlantic. In terms of the mean state, 29 out of 33 models examined continue to suffer from serious biases including an annual mean zonal equatorial SST gradient whose sign is opposite to observations. Westerly surface wind biases in boreal spring play an important role in the reversed SST gradient by deepening the thermocline in the eastern equatorial Atlantic and thus reducing upwelling efficiency and SST cooling in the following months. Both magnitude and seasonal evolution of the biases are very similar to what was found previously for CMIP3 models, indicating that improvements have only been modest. The weaker than observed equatorial easterlies are also simulated by atmospheric GCMs forced with observed SST. They are related to both continental convection and the latitudinal position of the intertropical convergence zone (ITCZ). Particularly the latter has a strong influence on equatorial zonal winds in both the seasonal cycle and interannual variability. The dependence of equatorial easterlies on ITCZ latitude shows a marked asymmetry. From the equator to 15A degrees N, the equatorial easterlies intensify approximately linearly with ITCZ latitude. When the ITCZ is south of the equator, on the other hand, the equatorial easterlies are uniformly weak. Despite serious mean state biases, several models are able to capture some aspects of the equatorial mode of interannual SST variability, including amplitude, pattern, phase locking to boreal summer, and duration of events. The latitudinal position of the boreal spring ITCZ, through its influence on equatorial surface winds, appears to play an important role in initiating warm events.

Short TitleClim. Dyn.
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