Extratropical Forcing of Tropical Atlantic Variability during Boreal Summer and Fall

The connection between midlatitude Atlantic sea surface temperature (SST) anomalies and tropical SST variations during boreal summer and fall are investigated using a coupled general circulation model (GCM). This research follows on an observational study that finds that, using linear inverse modeling (LIM), predictions of boreal summer tropical Atlantic Meridional Mode (AMM) variations can be made with skill exceeding persistence with lead times of about one year. The LIM framework identified extratropical Atlantic SST anomalies as important precursors to the AMM variations. The authors have corroborated this finding using a general circulation model coupled to a slab ocean, which represents a completely different physical basis from the LIM. Initializing the GCM with the LIM-derived “optimal” SST anomaly in November results in a steady equatorward propagation of SST anomalies into the subtropics during the following boreal spring. Thereafter, the GCM suggests that two possible feedbacks propagate the SST anomalies farther equatorward and westward with minimal loss of amplitude: the dominant wind–evaporation–SST (WES) thermodynamic feedback and a secondary low-cloud–SST radiative feedback. This study shows that this result has strong seasonal dependence and consists of nonlinear interactions when considering warm and cold “optimal” conditions separately. One main finding is that oceanic dynamics are not essential to understanding extratropical–tropical interaction in the Atlantic basin. The authors also discuss the results of the study in context with previous studies investigating the extratropical forcing of tropical air–sea variability.