Abstract
<jats:p>Stratospheric polar vortex (SPV) stretching events are typically of 1-2 weeks in duration when the SPV is elongated and locally extends anomalously south into lower latitudes. Such events are often associated with extreme cold over the Great Plains to southern and eastern states of the US. Here we present an observational study of SPV stretching events. First, empirical orthogonal function (EOF) analyses were performed for daily geopotential height fields (in the Northern Hemisphere domain) at 500 and 100 hPa levels, respectively. The second modes (EOF2) are identified to be associated with SPV stretching events. Second, the evolution and morphology of geopotential height anomalies are described based on composite averaging over all individual events. The retrograding 500 hPa EOF2 positive circulation pattern may lead to SPV stretching (i.e., 100 hPa EOF2) and a cold wave in North America. Long-duration EOF2+ events (principal component amplitude &gt; 1 for longer than 5 days) are associated with a Siberian/East Asian trough, while short events are not. The frequency of long-duration 500 hPa EOF2+ occurrence is found to be 1.1 and 2.6 per season (November to March) following Octobers of low and high snow cover extent (SCE) over Eurasia, respectively, or high and low autumn sea ice concentration (SIC) from the Greenland Sea to the Kara Sea, respectively. The occurrences of long events are more likely initiated when the zonal-mean zonal wind (60-70˚N) is reduced in the lower stratosphere, with the Siberian trough deepened and the Alaskan ridge enhanced (i.e., amplified stationary eddies). The anomalies grow rapidly after initial perturbations and move westward, leading to a deep trough anomaly and a “wavier” jet stream over North America. Meanwhile in the stratosphere, an enhanced ridge over the Bering Strait and Alaska is accompanied by a downstream trough, which extends to the surface over Canada. It is suggested that the frequency of North American cold waves may be modulated by the interannual variations of snow cover and sea ice through perturbations to the zonal mean zonal wind and stationary eddies.</jats:p>