Organization of extratropical transients during El Nino

Abstract

Four observed El Nino-Southern Oscillation (ENSO) events are studied to determine the mechanisms responsible for the anomalous extratropical atmospheric circulation during northern winter. The observed stationary wave anomalies over the Pacific/North American (PNA) region are found to be 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 Nino variability. This stems primarily from a large variability in the model's anomalous transients over the North Pacific. -from Authors