THE DIFFERENCES BETWEEN EARLY AND MIDWINTER ATMOSPHERIC RESPONSES TO SEA-SURFACE TEMPERATURE ANOMALIES IN THE NORTHWEST ATLANTIC

Using an atmospheric global spectral model, it is shown that the winte r atmosphere in the midlatitudes is capable of reacting to prescribed sea surface temperature (SST) anomalies in the northwest Atlantic with two very different responses. The nature of the response is determine d by the climatological conditions of the winter regime. Experiments a re performed using either the perpetual November or January conditions with or without the prescribed SST anomalies. Warm SST anomalies in N ovember result in a highly significant anomalous ridge downstream over the Atlantic with a nearly equivalent barotropic structure; in Januar y the response is a statistically less significant trough. The presenc e of the SST anomalies also causes a northward (southward) shift of th e Atlantic storm track in the November (January) cases. A diagnostic a nalysis of the anomalous heat advection in the simulations reveals tha t in the January cases, the surface heating is offset primarily by the strong horizontal cold advection in the lower troposphere. In the Nov ember cases, there is a vitally important vertical heat advection thro ugh which a potential positive ocean-atmosphere feedback was found. Th e positive air temperature anomalies exhibit a deep vertical penetrati on in the November eases but not in the January cases. The simulated a tmospheric responses to the warm SST anomalies in the November and Jan uary cases are found to be in qualitative agreement with the observati onal results using 50-yr (1930-1979) records. The atmospheric response s to the cold SST anomalies in the simulations are found to be insigni ficant.