On the triggering of Benguela Niños – equatorial vs. local forcing

Several studies have highlighted the importance of equatorial wind forcing on the development of Benguela Niños, suggesting that downwelling Kelvin waves propagate the signal along the equator and southwest African coast in a time span of about 1-2 months. The influence of local along shore winds, on the other hand, has received less attention. The present study compares the relative importance of remote and local effects in the development of Benguela Niños using both observations and a 120-year simulation of the coupled GCM for the Earth Simulator (CFES). Compared to many current GCMs, CFES achieves a rather realistic simulation of the tropical Atlantic. In addition to capturing the mean seasonal cycle the model also simulates Atlantic and Benguela Niños with realistic amplitude and phase locking. Both observations and CFES output suggest that anomalous along-shore winds in the Angola Benguela Area (ABA) are an important component in the development of Benguela Niños. These wind variations induce SST anomalies through both increased latent heat flux and reduced upwelling. They form part of a large-scale weakening of the South Atlantic subtropical anticyclone that appears to originate from the midlatitudes. Forcing by anomalous winds in the equatorial region also plays a role by preconditioning the ABA, but it is found that the local forcing is dominant. The large spatial extent of the wind anomalies may explain the high correlation between Benguela and Atlantic Niños, which exists in both CFES and observations. The possible influence of Benguela Niños on the formation of Atlantic Niños will also be discussed.