Abstract
Dramatic meteorological phenomena in the winter polar stratosphere known as Sudden Stratospheric Warming (SSW) events are well recognized for their impacts felt across the whole atmosphere. Apart from the influence of tropospheric forcing and stratospheric control, many studies have addressed the possible role of external factors on the occurrence of SSW events. Here, with the help of reanalysis datasets, we present a hitherto unexplored connection between the tropical upper stratosphere and the polar vortex. We identify enhanced planetary wave driving around the tropical stratopause and poleward progression of the zero-wind line as early indicators for the occurrence of SSW events. We demonstrate that the poleward progression of the zero wind line results in efficient focusing of planetary waves into the polar vortex which culminates in its disruption. Statistically, nearly 70% of the SSW events that took place so far have been preceded by enhanced tropical stratopause wave driving which points towards identifying this as a potential precursor for the occurrence of SSW events. After the year 2000, significantly a greater number of SSW events have been found to be preceded by enhanced tropical stratopause wave driving.
Highlights
Dramatic meteorological phenomena in the winter polar stratosphere known as Sudden Stratospheric Warming (SSW) events are well recognized for their impacts felt across the whole atmosphere
The Holton-Tan[27] mechanism has been widely invoked to explain the control of the polar vortex by the tropical stratospheric QBO28,29. As per this classical mechanism, weak vortex states during the easterly phase of the Quasi Biennial Oscillation (QBO) are attributed to the presence of the zero-wind line in the subtropics of the winter hemisphere, which facilitates the focusing of planetary waves into the polar vortex[19,24,30]
Our results show that the poleward propagation of the zero-wind line helps in better focusing of planetary waves into the polar mesosphere, eventually culminating in the breakdown of the vortex through wave-mean flow interaction. (Readers are referred to Supplementary Fig. S2 for Eliassen Palm Flux (EP Flux) vectors corresponding to Fig. 2)
Summary
Dramatic meteorological phenomena in the winter polar stratosphere known as Sudden Stratospheric Warming (SSW) events are well recognized for their impacts felt across the whole atmosphere. The poleward excursion of the zero-wind line is driven by the deposition of easterly momentum by the interaction of planetary waves with the mean flow. Our results show that the poleward propagation of the zero-wind line helps in better focusing of planetary waves into the polar mesosphere, eventually culminating in the breakdown of the vortex through wave-mean flow interaction.
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