The Stratospheric Pathway of La Niña

Abstract

A Northern Hemisphere (NH) polar stratospheric pathway for La Nina events is established during wintertime based on reanalysis data for the 1958-2012 period. A robust polar stratospheric response is observed in the NH during strong La Nina events, characterized by a significantly stronger and cooler polar vortex. Significant wind anomalies reach the surface, and a robust impact on the North Atlantic-European (NAE) region is observed. A dynamical analysis reveals that the stronger polar stratospheric winds during La Nina winters are due to reduced upward planetary wave activity into the stratosphere. This finding is the result of destructive interference between the climatological and the anomalous La Nina tropospheric stationary eddies over the Pacific-North American region. In addition, the lack of a robust stratospheric signature during La Nina winters reported in previous studies is investigated. It is found that this is related to the lower threshold used to detect the events, which signature is consequently more prone to be obscured by the influence of other sources of variability. In particular, the occurrence of stratospheric sudden warmings (SSWs), partly linked to the phase of the quasi-biennial oscillation, modulates the observed stratospheric signal. In the case of La Nina winters defined by a lower threshold, a robust stratospheric cooling is found only in the absence of SSWs. Therefore, these results highlight the importance of using a relatively restrictive threshold to define La Nina events in order to obtain a robust surface response in the NAE region through the stratosphere.