Abstract 042

Abstract ID: 042

Causality between Extreme Stratospheric Wave Activity and Cold Extremes over North America

Lead Author: Xiuyuan Ding
University of California, Los Angeles, United States of America

Keywords: Cold extremes, Stratosphere

Abstract: Recent studies have suggested that stretching of the stratospheric polar vortex, featuring planetary wave reflection, is linked to North American cold extremes such as the February 2021 Texas cold wave. However, it is not well resolved on which timescales the stratosphere contributes to the surface anomalies, and the causality between them is still under debate. With reanalysis and CMIP6 historical simulations, we show that weaker-than-normal stratospheric planetary wave activity is associated with surface cold anomalies over North America, similar to wave reflection events. But these cold anomalies occur before and near the onset of the weak stratospheric wave activity, suggesting that this surface anomaly is likely a precursor rather than an effect of the stratosphere. Strong-than-normal stratospheric wave activity, on the other hand, is followed by robust North American cold anomalies in about 10 days. The risk of cold extremes also increases significantly over North America 5–25 days later.
We further examine the causality between stratospheric wave activity and cold extremes using idealized nudging simulations in an AGCM with a well-resolved stratosphere. The stratosphere in the nudging run is linearly damped to that in a free-running control simulation. With nearly identical stratospheric evolution, the nudging run largely reproduces the surface temperature fingerprints following strong stratospheric wave events as the control run. But the surface signals before these events nearly vanish in the nudging simulation. Thus similar surface fingerprints follow strong wave events in the two simulations in spite of very different tropospheric conditions. This indicates that the North American cold anomalies following strong wave events are plausibly driven by the stratosphere through vertical wave coupling. These findings have important implications for our understanding of the role of the stratosphere in surface cold extremes and their predictability on subseasonal timescales.

Gang Chen (University of California, Los Angeles)