A Continuum of Sudden Stratospheric Warmings

Abstract

The k-means cluster technique is used to examine 43 yr of daily winter Northern Hemisphere (NH) polar stratospheric data from the 40-yr ECMWF Re-Analysis (ERA-40). The results show that the NH winter stratosphere exists in two natural well-separated states. In total, 10% of the analyzed days exhibit a warm disturbed state that is typical of sudden stratospheric warming events. The remaining 90% of the days are in a state typical of a colder undisturbed vortex. These states are determined objectively, with no preconceived notion of the groups. The two stratospheric states are described and compared with alternative indicators of the polar winter flow, such as the northern annular mode. It is shown that the zonally averaged zonal winds in the polar upper stratosphere at ∼7 hPa can best distinguish between the two states, using a threshold value of ∼4 m s−1, which is remarkably close to the standard WMO criterion for major warming events. The analysis also determines that there are no further divisions within the warm state, indicating that there is no well-designated threshold between major and minor warmings, nor between split and displaced vortex events. These different manifestations are simply members of a continuum of warming events.

Seasonal Variability of the Polar Stratospheric Vortex in an Idealized AGCM with Varying Tropospheric Wave Forcing

“…Strangely, we find no major SSWs for even stronger forcing h 0 $ 6000 m; in such cases, the stratospheric state at 10 hPa exhibits a very high degree of interannual variability-in some winters the vortex never becomes strong-with violent midwinter disturbances that, however, fail to meet the major SSW criterion of a wind reversal at 608N. This fact may just be an indication of the arbitrariness of this criterion (Coughlin and Gray 2009), rather than having any more fundamental implications. FIG.…”

Characterizing the Variability and Extremes of the Stratospheric Polar Vortices Using 2D Moment Analysis

“…However, in reality they are often very similar to major warming events. Coughlin and Gray (2009) used the k-means clustering technique as a method of distinguishing between disturbed and undisturbed vortex states. However, using the traditional diagnostics of zonal mean winds and temperatures they found that the stratosphere was naturally split into only two states using this technique (i.e., they could not further subdivide the disturbed vortex state into splits and displacements).…”

“…During these extreme events the NH polar vortex has been shown to have a direct influence on tropospheric variability (e.g., Baldwin and Dunkerton 2001;Thompson et al 2002;Bell et al 2009). Furthermore, studies have shown that the winter stratosphere is in a highly disturbed state for around 10% of the time (e.g., Coughlin and Gray 2009). However, due to the lack of planetary waves in the SH, SSWs are almost nonexistent (e.g., Charlton et al 2004;Esler et al 2006) and most of the variability is observed in the upper stratosphere.…”

Understanding and forecasting polar stratospheric variability with statistical models

“…Lu et al (2009) used a nonlinear neural network to receive an alternative representation of the QBO. Coughlin and Gray (2009) use the K-Means cluster algorithm to determine two distinct states in the polar stratosphere, one representing normal, the other weak vortex conditions. Nao et al (2006) presents a technique based on support vector machines to estimate the surface area of polar stratospheric clouds.…”