Separating the Tropospheric and Stratospheric Pathways of ENSO

Understanding the linkage between El Niño Southern Oscillation (ENSO) and Northern Hemisphere climate has found to be one of the key mechanisms to improve the predictability skill in extratropical regions. The El Niño signal propagates poleward via Rossby wave trains in the upper troposphere. In the North Pacific, this contributes to a deepening of the winter Aleutian low pressure system (AL), and hence to a strengthening of the Pacific North America (PNA) pattern. As a consequence, the strengthened wave anomaly interacts constructively with the climatological planetary wave and thereby enhances upward propagation of planetary waves into the stratosphere. This can lead to Sudden Stratospheric Warming (SSW) events, which can then have a lasting impact on the North Atlantic and Europe by projecting onto the North Atlantic Oscillation (NAO) pattern, thereby influencing weather over Europe. Pressure anomalies in the North Pacific can also propagate to the North Atlantic and Eurasia through the mid- and upper troposphere as a Rossby wave train, but the relative importance of the tropospheric and stratospheric pathways is still not resolved. In this study, we evaluate the relative importance and the role of both pathways. We use a 3D wave activity flux to identify the propagating Rossby waves in the troposphere and in the stratosphere. On the one hand, that the stratospheric pathway can be due to planetary wave reflection or absorption, exhibiting different surface responses and different time scales. On the other hand, when the stratosphere is less active, in neutral Vortex events, we show that the tropospheric pathway is dominant leading to a negative(positive) NAO during El Niño(La Niña) years. We also investigate the role of the synoptic eddies, which play a significant role in maintaining the southward(northward) shift of the storm track in the Atlantic during El Niño(La Niña) conditions.