An assessment of equatorial Atlantic interannual variability in Ocean Model Intercomparison Project (OMIP) simulations

Abstract

The eastern equatorial Atlantic (EEA) seasonal cycle and interannual variability strongly influence the climate of the surrounding continents. It is thus crucial that models used in both climate predictions and future climate projections are able to simulate them accurately. In that context, the EEA monthly climatology and interannual variability are evaluated over the period 1985–2004 for models participating in the Ocean Model Intercomparison Project Phases 1 and 2 (OMIP1 and OMIP2). The main difference between OMIP1 and OMIP2 simulations is their atmospheric forcing: CORE-II and JRA55-do, respectively. Monthly climatologies of the equatorial Atlantic zonal wind, sea level anomaly, and sea surface temperature in OMIP1 and OMIP2 are comparable to reanalysis products. Yet, some discrepancies exist in both OMIP ensembles: the thermocline is too diffusive, and there is a lack of cooling during the development of the Atlantic cold tongue. The EEA interannual sea surface temperature variability during May–June–July in the OMIP1 ensemble mean is found to be 51 % larger (0.62 ± 0.04 °C) than that in the OMIP2 ensemble mean (0.41 ± 0.03 °C). Likewise, the May–June–July interannual sea surface height variability in the EEA is 33 % larger in the OMIP1 ensemble mean (0.02 ± 0.002 m) than in the OMIP2 ensemble mean (0.015 ± 0.002 m). Sensitivity experiments demonstrate that the discrepancies in interannual sea surface temperatures and sea surface height variabilities between OMIP1 and OMIP2 are mainly attributable to their wind forcings and, specifically, to their variability. While the April–May–June zonal wind variability in the western equatorial Atlantic is similar in both forcings, the zonal wind variability peaks in April for JRA55-do and in May for CORE-II.