A continuum of sudden stratospheric warmings

Coughlin, K. and Gray, L. J. (2009) A continuum of sudden stratospheric warmings. Journal of the Atmospheric Sciences, 66 (2). pp. 531-540. ISSN 1520-0469 doi: 10.1175/2008JAS2792.1

Abstract/Summary

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.

Item Type	Article
URI	https://reading-clone.eprints-hosting.org/id/eprint/4331
Item Type	Article
Refereed	Yes
Divisions	Science > School of Mathematical, Physical and Computational Sciences > NCAS Science > School of Mathematical, Physical and Computational Sciences > Department of Meteorology
Uncontrolled Keywords	elliptic diagnostics; northern-hemisphere; planetary-waves; polar vortices; propagation; troposphere; climatology; cycle
Publisher	American Meteorological Society
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