Tropical-extratropical connection in the Atlantic atmosphere-ocean variability

A question how sea surface temperature (SST) fluctuations in the tropical Atlantic affect the North Atlantic atmosphere-ocean system is addressed using an atmospheric general circulation model coupled to a 50-m-deep slab ocean (MGCM), and a linear baroclinic model. A comparison of a 60-yr integration, in which the tropical SSTs are prescribed to observations (TOGA-ML), with the control MGCM run shows that the tropical SST variations selectively enhance variance of the North Atlantic Oscillation (NAO) by about 80%. This is partly due to extratropical height anomalies associated with two dominant modes, monopole and dipole, of tropical Atlantic SST variability because they both have a projection on to the NAO in the TOGA-ML run. The simulated midlatitude height anomalies accompany characteristic SST anomaly patterns beneath, and they are consistent with observations. Analyses of steady linear responses to the heating in the tropical Atlantic and transient eddy feedback in midlatitude reveal that the substantial part of height anomalies is generated by the latter rather than the former. The anomalous eddy forcing is, however, organized not only by the heating in the tropical Atlantic but also by the tropical Pacific SST variations through the teleconnection over North America. It is suggested that the tropical Atlantic SST variability does affect the North Atlantic atmosphere and ocean, but the dynamical connection is tenuous in comparison with the Pacific.