DYNAMICS OF THE EARLY AND MIDDLE WINTER ATMOSPHERIC RESPONSES TO THE NORTHWEST ATLANTIC SST ANOMALIES

The differences between early and middle winter atmospheric responses to the sea surface temperature anomalies (SSTA) in the northwest Atlan tic are examined using a linear baroclinic model. Using a global spect ral model, Peng et al. found a positive height anomaly in the perpetua l November and a negative height anomaly in the Perpetual January expe riments in response to a warm SSTA over the northwest Atlantic. These height anomalies are found to be associated with the reduced Atlantic jet stream in November and enhanced jet in January. Linear model diagn ostics suggest that the difference in jet stream response may induce a nomalous storm track eddy vorticity fluxes, which in turn maintain the different atmospheric responses under the early and middle winter con ditions. The different jet stream responses in November and January ar e further traced to the initial atmospheric response to a local heat s ource accompanying the warm SSTA. Under both the January and November conditions, the atmospheric response is dominated by an anticyclone do wnstream from the heat source at the jet stream level. The anticyclone is shifted northward in November, however, from its position in Janua ry. Combined with a northeast-southwest tilted jet stream in January a nd an east-west oriented, southward shifted November jet stream in the Atlantic, the above difference in the atmospheric responses to the in itial heat source may lead to a reduced jet in November and an enhance d jet in January. The feedback between the anomalous storm track eddy vorticity fluxes and the anomaly flow induced by the heat source may f urther enhance the different equilibrium responses in-the global spect ral model.