The Role of Transient Disturbances in the Dynamics of thePacific–North American Pattern

Abstract

A 25-yr dataset is used to investigate the role of transient eddies in the dynamics of the Pacific?North American (PNA) pattern. Monthly mean vorticity and sensible heat flux divergences associated with submonthly transients are computed over the Northern Hemisphere for each winter month. These fields are composited over months with strong PNA patterns, and the average over all winter months is subtracted to obtain anomaly fields. The vorticity flux divergence anomaly is found to be well correlated with the PNA height field, particularly in the upper troposphere, where an eddy vorticity flux convergence (divergence) is found in the low (high) 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 sensible heat flux out of the warm regions of the PNA and into the cold regions, thus tending to destroy the temperature anomaly. A linear quasi-nondivergent global steady-state model is constructed using the observed climatology. The eddy vorticity and sensible heat flux divergence 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 amplitude of the response is weaker than observed over the North Pacific but nearly as observed over North America. The wave-activity flux computed from the model response is in reasonable agreement with that obtained from the observed PNA, except that the model shows a weaker wave 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 absence of tropical forcing in the model. The results clearly show the crucial role of the transient eddies in the dynamics of the PNA, particularly over North America.