Abstract
Abstract. The Planetary Waves (PWs) are believed to have significant role in generating the wintertime warming over the polar stratosphere, known as Stratospheric Sudden Warming (SSW). However, the origin, characteristics and evolution of these waves are still speculative. The possibility that the PWs over the polar stratosphere, which play an important role in the generation of SSW, could also have contribution from the tropics has been indicated through many numerical simulations in the past, but due to the paucity of global measurements it could not be established unequivocally. The earlier numerical studies also indicated the presence of a zero-wind line (more general the critical layer, where the zonal wind amplitude becomes zero) whose real counterparts were not observed in the atmosphere. The present study based on the NCEP/NCAR reanalysis of stratospheric wind and temperatures of recent years clearly shows that (i) the zero-wind line appears over the tropics ~60 days prior to the major SSWs and progresses towards the Pole and (ii) an enhanced PW activity of quasi periodicity 16-days, which is also seen almost simultaneously with the zero-wind line, shows a propagation from equator to the Pole. This result is significant as it presents for the first time the connection between the tropics during the SSW events and the pole, through the quasi 16-day wave.
Highlights
Planetary Waves (PWs) are large-scale perturbations of the atmospheric dynamical structure that extends coherently around a full longitude circle
In this paper we present comparative case studies from one database, primarily to investigate (i) whether a real counterpart of the critical layer, as inferred through the simulations, exists in the atmosphere during the Stratospheric Sudden Warming (SSW) (ii) whether there is any systematic trend in the PW activity over different latitudes, which could be indicative of a possible coupling between the tropics and the Pole and (iii) has the phase of the Quasi Biennial Oscillation (QBO) play any role in such coupling
In a recent study, using the National Centers for Environmental Prediction (NCEP)/National Center for Atmospheric Research (NCAR) data and the TIMEGCM, the dynamical changes in the mesosphere and the stratosphere were analyzed for the Southern Hemisphere SSW event of 2002 and a strong feedback taking place between the PWs and the mean dynamics of the mesosphere and stratosphere was shown (Liu and Roble, 2005)
Summary
Planetary Waves (PWs) are large-scale perturbations of the atmospheric dynamical structure that extends coherently around a full longitude circle. Some of the recent studies discussing the dynamical changes in the lower atmosphere over the northern high latitude regions indicate the presence of a zero wind line and discuss about significance in context of the propagation of PWs (Baldwin and Dunkerton, 1999; Walterscheid et al, 2000; Coughlin and Tung, 2005; Liu and Roble, 2005) In this context, the global reanalysis data of recent years, provided by the National Centers for Environmental Prediction (NCEP) and National Center for Atmospheric Research (NCAR) have been found to be useful to investigate processes, which are global and large-scale in nature due to a better coverage of actual observations and modeling database (Kalnay et al, 1996).
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