The North American Spring Coldness Response to the Persistent Weak Stratospheric Vortex Induced by Extreme El Niño Events

Xin Zhou,Jingtao Zhou,Yong Zhao,Yawei Yang,Shaobo Zhang,Quanliang Chen,Yunliang Qi ,Yang Li

doi:10.3389/feart.2021.626244

Xin Zhou, Jingtao Zhou + Show 6 more

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https://doi.org/10.3389/feart.2021.626244

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Abstract

The stratospheric pathway is a major driver of El Niño–Southern Oscillation (ENSO) impacts on mid-latitude tropospheric circulation and winter weather. The weak vortex induced by El Niño conditions has been shown to increase the risk of cold spells, especially over Eurasia, but its role for North American winters is less clear. This study involved idealized experiments with the Whole Atmosphere Community Climate Model to examine how the weak winter vortex induced by extreme El Niño events is linked to North American coldness in spring. Contrary to the expected mid-latitude cooling associated with a weak vortex, extreme El Niño events do not lead to North American cooling overall, with daily cold extremes actually decreasing, especially in Canada. The expected cooling is absent in most of North America because of the advection of warmer air masses guided by an enhanced ridge over Canada and a trough over the Aleutian Peninsula. This pattern persists in spring as a result of the trapping of stationary waves from the polar stratosphere and troposphere, implying that the stratospheric influence on North America is sensitive to regional downward wave activities.

Highlights

The El Niño-Southern Oscillation (ENSO) has far-reaching impacts globally and provides the most reliable source of seasonal to interannual climate prediction for North America (e.g., Tippett et al, 2012; L’Heureux et al, 2015)
This study involved idealized experiments with WACCM4 to explore how spring coldness is reduced by the persistent weak vortex induced by extreme El Niño events
The isolated stratospheric signal persisting in spring was shown to reduce the probability of daily cold extremes in North America

Summary

Introduction

The El Niño-Southern Oscillation (ENSO) has far-reaching impacts globally and provides the most reliable source of seasonal to interannual climate prediction for North America (e.g., Tippett et al, 2012; L’Heureux et al, 2015). Stratospheric Pathway of El Niño intensifies the upward propagation of planetary Rossby waves to the stratosphere in the Northern Hemisphere during winter, resulting in a warmer and weaker polar vortex (Manzini et al, 2006; Camp and Tung, 2007; Free and Seidel, 2009; Fletcher and Kushner, 2011; Lan et al, 2012; Ren et al, 2012; Xie et al, 2012; Garfinkel et al, 2013a; Garfinkel et al, 2013b; Fletcher and Cassou, 2015) These EN-related stratospheric anomalies subsequently propagate downward into the troposphere, where they project a negative North Atlantic Oscillation (NAO) phase and an equatorward-shifted jet (Perlwitz and Graf, 1995; Thompson et al, 2002; Brönnimann et al, 2007; Kolstad et al, 2010). For North America, the influence of the stratospheric EN pathway is less pronounced

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