Multi-model assessment of the late-winter stratospheric response to El Niño and La Niña

Abstract

The impact of El Niño-Southern Oscillation (ENSO) on the late-winter extra-tropical stratosphere (January–March) is assessed in a multi-model framework. Three state-of-the-art atmospheric models are run with prescribed SST anomalies representative of a strong ENSO event, with symmetric patterns for El Niño and La Niña. The well-known temperature perturbation in the lower stratosphere during El Niño is captured by two models, in which the anomalous warming at polar latitudes is accompanied by a positive geopotential height anomaly that extends over the polar cap. In the third model, which shows a lack of temperature anomalies over the pole, the anomalous anticyclone is confined over Canada and does not expand to the polar cap. This anomalous center of action emerges from the large-scale tropospheric Rossby wave train forced by ENSO, and shrinking/stretching around the polar vortex is invoked to link it to the temperature response. No disagreement across models is found in the lower stratosphere for La Niña, whose teleconnection is opposite in sign but weaker. In the middle-upper stratosphere (above 50 hPa) the geopotential height anomalies project on a wavenumber-1 (WN1) pattern for both El Niño and, more weakly, La Niña, and show a westward tilt with height up to the stratopause. It is suggested that this WN1 pattern arises from the high-latitude lower-stratospheric anomalies, and that the ENSO teleconnection to the polar stratosphere can be interpreted in terms of upward propagation of the stationary Rossby wave train and quasi-geostrophic balance, instead of wave breaking.

Tropospheric pathways of the late-winter ENSO teleconnection to Europe

“…Domeisen et al 2019;Mezzina et al 2020) anomalies in the 50-hPa geopotential height-although less significant in LR-that can be interpreted as a weakening and displacement of the polar vortex towards the North Atlantic sector (e.g. Mezzina et al 2021b). Note also that the response in the middle stratosphere (10 hPa) is less significant in MR (Fig.…”

“…Cagnazzo and Manzini 2009). While we acknowledge a possible role of the stratosphere (see Mezzina et al 2021b), in this work we pursue a comprehensive understanding of the ENSO-NAE teleconnection in terms of tropospheric dynamics, which appear to dominate in the western North Atlantic.…”

Interannual Bottom-Intensified Current Thickening Observed on the Continental Slope Off the Southeastern Coast of Hokkaido, Japan

“…The North Pacific Index (NPI), which is defined as the area-weighted sea-level pressure in the region of 30 • N-65 • N, 160 • E-140 • W, is an indicator of the strength of the Aleutian Low [33]. During El Niño (La Niña) winters, the Aleutian Low becomes stronger (weaker) via atmospheric teleconnection, e.g., [34]. The NPI anomaly was calculated from the seasonal mean.…”

“…The time coefficient of the first rotary EOF mode exhibits interannual variation, with significant positive peaks in the winters of 2002/2003, 2007/2008, 2009/2010, 2011/2012, and 2015/2016 (Figure 6a). Most of these positive peaks lag behind the positive peaks of the inverted NPI anomaly (gray line), which represent the strengthening of the Aleutian Low, principally due to the occurrence of El Niño events, e.g., [34]. With a lag of 3 months, the correlation coefficient between the first rotary EOF mode and inverted NPI anomaly reaches a maximum of 0.52, which is higher than the 99% confidence limit (0.47) and, therefore, statistically significant.…”

Driving mechanisms for the El Niño–Southern Oscillation impact on stratospheric ozone

“…As a consequence, the propagation of Rossby waves into the stratosphere is enhanced through the intensification of stationary wave number 1 (Manzini et al, 2006). Increased upward propagation of planetary waves during El Niño into the stratosphere results in a weakened polar vortex and a strengthening of the residual circulation of the Brewer-Dobson circulation (BDC), which leads to tropical stratospheric cooling and stratospheric polar-cap warming (e.g., Calvo et al, 2010;Mezzina et al, 2021). In contrast, during La Niña, a weakening of the Aleutian low and destructive linear interference with the climatological wave pattern occur, resulting in a stronger and colder NH polar vortex and a weakening of the residual circulation (Iza et al, 2016).…”