A New Look at Stratospheric Sudden Warmings. Part III: Polar Vortex Evolution and Vertical Structure

Abstract

The evolution of the Arctic polar vortex during observed major midwinter stratospheric sudden warmings (SSWs) is investigated for the period 1957–2002, using 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) Ertel’s potential vorticity (PV) and temperature fields. Time-lag composites of vertically weighted PV, calculated relative to the SSW onset time, are derived for both vortex-displacement SSWs and vortex-splitting SSWs, by averaging over the 15 recorded displacement and 13 splitting events. The evolving vertical structure of the polar vortex during a typical SSW of each type is clearly illustrated by plotting an isosurface of the composite PV field, and is shown to be very close to that observed during representative individual events. Results are verified by comparison with an elliptical diagnostic vortex moment technique. For both types of SSW, little variation is found between individual events in the orientation of the developing vortex relative to the underlying topography; that is, the location of the vortex during SSWs of each type is largely fixed in relation to the earth’s surface. During each type of SSW, the vortex is found to have a distinctive vertical structure. Vortex-splitting events are typically barotropic, with the vortex split occurring near simultaneously over a large altitude range (20–40 km). In the majority of cases, of the two daughter vortices formed, it is the “Siberian” vortex that dominates over its “Canadian” counterpart. In contrast, displacement events are characterized by a very clear baroclinic structure; the vortex tilts significantly westward with height, so that the top and bottom of the vortex are separated by nearly 180° longitude before the upper vortex is sheared away and destroyed.

Examining the Predictability of the Stratospheric Sudden Warming of January 2013 Using Multiple NWP Systems

“…The elliptical diagnostics are based on the spatial moments of the PV field, and the diagnostics are used to calculate various geometric parameters of the vortex structure such as the area, the ellipticity, the aspect ratio, and the centroid of the vortex equivalent ellipse. Several previous studies have used elliptical diagnostics to examine stratospheric vortex structure and evolution during deformed stratospheric vortex events (e.g., Waugh 1997;Waugh and Randel 1999;Matthewman et al 2009;Mitchell et al 2011). Tracking elliptical diagnostics in time provides information related to the state of the vortex; these diagnostics can be used to track the spatial evolution of the vortex during the deformed vortex events.…”

Winter 2015/16: A Turning Point in ENSO-Based Seasonal Forecasts

“…When the polar vortex is strong, the jet stream is displaced farther northward. Furthermore, during weak and strong polar vortex events, the atmosphere tends to be barotropic, with the jets in both the stratosphere and troposphere aligned (Matthewman et al, 2009). A strong polar vortex is a plausible explanation for the poleward-displaced jet stream during winter 2015/16, despite a record strong El Niño that favors just the opposite response.…”

The nature of Arctic polar vortices in chemistry–climate models

“…The validity of these data was found to be sound in a companion study undertaken by Hannachi et al (2011). For a full description of these vortex diagnostics, the reader is referred in the first case to M11, but also to Matthewman et al (2009). However, in brief, the objective area gives a measure of the vortex's area and strength, the centroid latitude gives a measure of the displacement of the centroid of the vortex, the aspect ratio is a ratio between the major and minor axes of the vortex and the kurtosis gives a measure of vortex variability, where negative kurtosis values indicate a vortex split, values between 0 and 0.5 indicate a stable vortex, and high positive values indicate strong filamentation.…”

“…Andrews et al, 1987;O'Neill, 2003;Charlton and Polvani, 2007), with the geometric-based moments. A brief summary of the moment diagnostics is presented here, but for technical details readers are referred to Matthewman et al (2009) and M11.…”

“…An alternative approach to this problem is to use the vortex benchmark developed in Mitchell et al (2011) (hereafter M11), which itself is an extension of previous studies (Butchart and Remsberg, 1986;Waugh, 1997;Waugh and Rendel, 1999;Matthewman et al, 2009). M11 used the application of 2D moments to measure the area, centroid, aspect ratio and kurtosis of the vortex throughout the ERA-40 dataset over the winter (DJFM) period.…”