The Stratospheric Pathway of La Niña

Abstract

A Northern Hemisphere (NH) polar stratospheric pathway for La Nina events is established during win tertime 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 oscil lation, 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.

Characterizing Stratospheric Polar Vortex Variability With Computer Vision Techniques

“…Our results highlight the wide spectrum of behavior exhibited by the Arctic polar vortex in both time and altitude, which includes small localized weak/strong events, as well as prolonged and vertically deep ones. Distinguishing between such “major” and “minor” events has recently been highlighted by many as important for more completely describing the full variability of the stratosphere and its importance for numerous research topics including definitions of sudden warming events (e.g., Butler et al, ; Maury et al, ), polar chemical processing and ozone loss (e.g., Manney & Lawrence, ; Strahan et al, ), coupling with the troposphere (e.g., Dunn‐Sigouin & Shaw, ; Runde et al, ), understanding climate connections to, for example, the quasi‐biennial oscillation and El Niño/La Niña (e.g., Díaz‐Durán et al, ; Garfinkel et al, ; Iza et al, ), forecasting and prediction (e.g., Butler et al, ; Tripathi, Charlton‐Perez, et al, ), and defining and attributing events in climate models (e.g., Ayarzagüena et al, ; Kim et al, ).…”

The Impact of Strong El Niño and La Niña Events on the North Atlantic

“…The present study considers the above pathways and the response of North Atlantic MSLP in January–February following La Niña events in addition to El Niño events. The response to observed La Niña events is an anomalously positive NAO, opposite (in sign) to that for moderate El Niño events (Iza et al, ). No strong La Niña events have yet been observed reflecting the asymmetry between the amplitude of El Niño and La Niña (An & Jin, ), although future strong La Niña events are possible.…”

The Role of the Stratosphere in Subseasonal to Seasonal Prediction: 2. Predictability Arising From Stratosphere‐Troposphere Coupling

“…El Niño conditions in the tropical Pacific have been shown to lead to a weakened stratospheric vortex (Domeisen, Garfinkel, & Butler, ; García‐Herrera et al, ; Garfinkel & Hartmann, ; Manzini et al, ), while La Niña tends to be associated with a strengthening, though this connection is less robust (Iza et al, ; Polvani et al, ). The second column of Figure shows the PDF of zonal wind at 10 hPa and 60°N for week 6 (days 36 to 42) after initialization for November and December hindcasts initialized during El Niño and La Niña.…”