A Possible Influence of Equatorial Winds on the September 2002 Southern Hemisphere Sudden Warming Event
Open Access
- 1 March 2005
- journal article
- Published by American Meteorological Society in Journal of the Atmospheric Sciences
- Vol. 62 (3), 651-667
- https://doi.org/10.1175/jas-3339.1
Abstract
The stratospheric sudden warming in the Southern Hemisphere (SH) in September 2002 was unexpected for two reasons. First, planetary wave activity in the Southern Hemisphere is very weak, and midwinter warmings have never been observed, at least not since observations of the upper stratosphere became regularly available. Second, the warming occurred in a west phase of the quasi-biennial oscillation (QBO) in the lower stratosphere. This is unexpected because warmings are usually considered to be more likely in the east phase of the QBO, when a zero wind line is present in the winter subtropics and hence confines planetary wave propagation to higher latitudes closer to the polar vortex. At first, this evidence suggests that the sudden warming must therefore be simply a result of anomalously strong planetary wave forcing from the troposphere. However, recent model studies have suggested that the midwinter polar vortex may also be sensitive to the equatorial winds in the upper stratosphere, the region dominated by the semiannual oscillation. In this paper, the time series of equatorial zonal winds from two different data sources, the 40-yr ECMWF Re-Analysis (ERA) and the Met Office assimilated dataset, are reviewed. Both suggest that the equatorial winds in the upper stratosphere above 10 hPa were anomalously easterly in 2002. Idealized model experiments are described in which the modeled equatorial winds were relaxed toward these observations for various years to examine whether the anomalous easterlies in 2002 could influence the timing of a warming event. It is found that the 2002 equatorial winds speed up the evolution of a warming event in the model. Therefore, this study suggests that the anomalous easterlies in the 1–10-hPa region may have been a contributory factor in the development of the observed SH warming. However, it is concluded that it is unlikely that the anomalous equatorial winds alone can explain the 2002 warming event.Keywords
This publication has 18 references indexed in Scilit:
- Flow regimes in the winter stratosphere of the northern hemisphereQuarterly Journal of the Royal Meteorological Society, 2003
- A data study of the influence of the equatorial upper stratosphere on northern‐hemisphere stratospheric sudden warmingsQuarterly Journal of the Royal Meteorological Society, 2001
- Model studies of the interannual variability of the northern‐hemisphere stratospheric winter circulation: The role of the quasi‐biennial oscillationQuarterly Journal of the Royal Meteorological Society, 2001
- Seasonal and interannual variability of the stratosphere diagnosed from UKMO TOVS analysesQuarterly Journal of the Royal Meteorological Society, 2000
- Interaction of the quasi‐biennial oscillation and stratopause semiannual oscillationJournal of Geophysical Research: Atmospheres, 1997
- Climatology of the semiannual oscillation of the tropical middle atmosphereJournal of Geophysical Research: Atmospheres, 1997
- The Meteorological Office analysis correction data assimilation schemeQuarterly Journal of the Royal Meteorological Society, 1991
- Simulations of linear and nonlinear disturbances in the stratosphereQuarterly Journal of the Royal Meteorological Society, 1988
- Associations between the 11-year solar cycle, the QBO and the atmosphere. Part I: the troposphere and stratosphere in the northern hemisphere in winterJournal of Atmospheric and Terrestrial Physics, 1988
- The Middle Atmosphere In the Absence of Dynamical Heat FluxesQuarterly Journal of the Royal Meteorological Society, 1987