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

On the relationship between Atlantic Niño variability and ocean dynamics

“…Based on observations, Marin et al (2009) argue that weak variability in WAtl zonal surface wind fails to precondition the basin-wide thermocline slope for the subsequent summer-the initial cooling of the cold tongue is hence weakened or delayed. In agreement with Richter et al (2014a), a similar process could be at work in CGCMs: Spring zonal winds that are systematically too weak in the western equatorial Atlantic could inhibit seasonal thermocline shoaling in the eastern ocean basin and hence intense surface cooling during early boreal summer. Here, we demonstrate that the KCM, too, develops a zonal wind bias in boreal spring that is, however, largely independent of the SST bias in the eastern ocean basin.…”

Mechanisms rectifying the annual mean response of tropical Atlantic rainfall to precessional forcing

“…The overestimation of Atlantic precipitation seasonality south of the equator is a result of Atlantic mean state biases common amongst CMIP5 GCMs, which simulate a reversal of the annual mean SST gradient and a boreal spring westerly wind bias (e.g. Richter et al 2014;Siongco et al 2014). Richter et al (2012) attribute this wind bias to precipitation deficits over the Amazon basin and excess precipitation over the Congo basin, also seen for CESM in Fig.…”

Growth and decay of the equatorial Atlantic SST mode by means of closed heat budget in a coupled general circulation model

“…Regarding the simulation of the TAV in the current GCMs, all of the models participating in the Coupled Model Intercomparison Project (CMIP3, CMIP5) fail to reproduces both, the seasonal cycle and the inter-annual variability over the equatorial Atlantic (Breugem et al, 2006(Breugem et al, , 2007Large and Danabasoglu, 2006;Chang et al, 2007;Richter and Xie, 2008;Wahl et al, 2011;Richter et al, 2012a; see also Tokinaga and Xie, 2011 additional material;Richter et al, 2014). The strong bias over the tropical Atlantic is a feature of the coupled models and it has been related to several processes: for instance, the complex land-sea interaction in the Atlantic basin, the oceanic coarse resolution for reproducing enough upwelling and the problems in simulating winds and clouds by the atmospheric model component (Large and Danabasoglu, 2006;Richter and Xie, 2008;Richter et al, 2012a;Toniazzo and Woolnough, 2014).…”

“…As a result of an inadequate seasonal cycle over the equatorial Atlantic, coupled models also fail in reproducing the proper leading mode of the inter-annual variability (i.e., Atlantic Niño, Breugem et al, 2006Breugem et al, , 2007Richter et al, 2014). Nevertheless, most of the CMIP models show variability over the equatorial Atlantic associated with a Bjerknes feedback (Breugem et al, 2007;Richter et al, 2014), suggesting that they are able to produce some thermocline feedbacks even when the mean thermocline is much more deeper in the eastern equatorial Atlantic compared with the observations (i.e., see variability of HadCM3 Figure 2 in Polo et al, 2013).…”

“…Nevertheless, most of the CMIP models show variability over the equatorial Atlantic associated with a Bjerknes feedback (Breugem et al, 2007;Richter et al, 2014), suggesting that they are able to produce some thermocline feedbacks even when the mean thermocline is much more deeper in the eastern equatorial Atlantic compared with the observations (i.e., see variability of HadCM3 Figure 2 in Polo et al, 2013).…”