Organization of Extratropical Transients during El Niño

Abstract

Four observed El Niño-Southern Oscillation (ENSO) events are studied to determine the mechanisms responsible for the anomalous extratropical atmospheric circulation during northern winter. A parallel analysis of a GCM's response to El Niño is performed in order to assess if similar mechanisms are operative in the model atmosphere. The observed stationary wave anomalies over the Pacific/North American (PNA) region are found to he similar during the four winters despite appreciable differences in sea surface temperatures. The anomalous transient vorticity fluxes are remarkably robust over the North Pacific during each event, with an eastward extension of the climatological storm track leading to strong cyclonic forcing near 40°N, 150°W. This forcing is in phase with the seasonal mean Aleutian trough anomaly suggesting the importance of eddy-mean flow interactions. By comparison, the intersample variability of the GCM response over the PNA region is found to exceed the observed inter-El Niño variability. This stems primarily from a large variability in the model's anomalous transients over the North Pacific. Further analysis using a linear stationary wave model reveals that the extratropical vorticity transients are the primary mechanism maintaining the stationary wave anomalies over the PNA region during all four observed ENSO winters. In the case of the GCM, the organization of transient eddies is ill defined over the North Pacific, a behavior that appears more indicative of model error than the unpredictable component of seasonal mean storm track anomalies. A physical model is proposed to explain the robustness of the tropical controlling influence of the extratropical transients in nature. A simple equatorial Pacific heat source directly forces a tropical anticyclone whose phase relative to the climatological tropical anticyclone leads to an eastward extension of the subtropical jet stream. This mechanism appears to be equally effective for a beat source located either in the central or eastern Pacific basin.