Atmosphere-ocean thermal coupling in the North Atlantic: A positive feedback

In this article, it is examined whether a positive atmosphere-ocean feedback is found over the North Atlantic through a series of general circulation model (GCM) experiments. A 60-year integration of an atmospheric GCM (AGCM) coupled with a 50 m deep motionless ocean was conducted. The coupled run was compared with two uncoupled runs, one with prescribed mid-latitude climatological sea surface temperatures (SSTs) and the other with daily SSTs derived from the coupled experiment. The uncoupled atmosphere forces the slab ocean, to obtain SST responses in these runs. Consistent with previous studies, the coupled atmosphere-ocean fields show interannual variances larger than those in the uncoupled runs, due to reduction in the local thermal damping. Patterns of maximum atmosphere-ocean covariability show the Noah Atlantic Oscillation (NAO) and tripole SST anomalies both in the coupled and uncoupled fields, indicating a dominant role of the atmosphere in generating the SST anomalies. On the other hand, analyses of the temporal variability in the three runs suggest an active role of SST anomalies in determining the polarity of the air-sea coupled variability longer than the interannual time-scale. A combined analysis of forcing SST, upper-air height, and the response SST anomalies in the uncoupled run identified that a patch of positive SST anomalies in the mid-latitude band around 40 degreesN effectively excite the positive phase of the NAG, which in turn reinforces the tripole SST anomalies. This relationship was further confirmed by a nine-member ensemble AGCM experiment forced by the relevant SST anomalies. Since the forcing and response SST anomaly patterns bear some resemblance, these results manifest positive feedback at work in the coupled atmosphere-ocean patterns. The processes responsible for this positive feedback were elaborated by a series of linear model experiments. The local thermal adjustment of the atmosphere to the SST anomalies results in increased precipitation over the Gulf Stream region. Associated diabatic heating applied to a linear baroclinic model yields a positive height response to the east, which highly influences the southern part of the NAG. This stationary response in turn induces a northward deflection in the storm track activity, leading to an eddy vorticity feedback that tends to force the positive phase of the NAO.