THE ROLE OF TRANSIENT DISTURBANCES IN THE DYNAMICS OF THE PACIFIC-NORTH AMERICAN PATTERN

A 2.5-yr dataset is used to investigate the role of transient eddies i n the dynamics of the Pacific-North American (PNA) pattern. Monthly me an vorticity and sensible heat flux divergences associated with submon thly transients are computed over the Northern Hemisphere for each win ter month. These fields are composited over months with strong PNA pat terns, and the average over all winter months is subtracted to obtain anomaly fields. The vorticity flux divergence anomaly is found to be w ell correlated with the PNA height field, particularly in the upper tr oposphere, where an eddy vorticity flux convergence (divergence) is fo und in the low thigh) height regions of the PNA anomaly. The sensible heat flux divergence, on the other hand, is negatively correlated with the PNA temperature anomaly, so that the transient eddies produce a s ensible heat flux out of the warm regions of the PNA and into the cold regions, thus tending to destroy the temperature anomaly. A linear qu asi-nondivergent global steady-state model is constructed using the ob served climatology. The eddy vorticity and sensible heat Bur divergenc e anomalies are treated as empirical forcing functions to simulate the response of the atmosphere. The model response to the transient-eddy forcing is found to be qualitatively similar to the PNA pattern. The a mplitude of the response is weaker than observed over the North Pacifi c but nearly as observed over North America. The wave-activity flux co mputed from the model response is in reasonable agreement with that ob tained from the observed PNA, except that the model shows a weaker wav e activity and a spurious flux southward from the main cell of the PNA in the North Pacific. A possible explanation for this deficiency, as for the underestimation of the response in the North Pacific, is the a bsence of tropical forcing in the model. The results clearly show the crucial role of the transient eddies in the dynamics of the PNA, parti cularly over North America.