The stratopause evolution during different types of sudden stratospheric warming event

Etienne Vignon,Daniel M Mitchell

doi:10.1007/s00382-014-2292-4

Etienne Vignon, Daniel M Mitchell

Open Access

https://doi.org/10.1007/s00382-014-2292-4

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Abstract

Recent work has shown that the vertical structure of the Arctic polar vortex during different types of sudden stratospheric warming (SSW) events can be very distinctive. Specifically, SSWs can be classified into polar vortex displacement events or polar vortex splitting events. This paper aims to study the Arctic stratosphere during such events, with a focus on the stratopause using the Modern Era-Restrospective analysis for Research and Applications reanalysis data set. The reanalysis dataset is compared against two independent satellite reconstructions for validation purposes. During vortex displacement events, the stratopause temperature and pressure exhibit a wave-1 structure and are in quadrature whereas during vortex splitting events they exhibit a wave-2 structure. For both types of SSW the temperature anomalies at the stratopause are shown to be generated by ageostrophic vertical motions. Transformed Eulerian mean diagnostics are used to show differences in the planetary wave activity between displacement and splitting events. The convergence of Eliassen-Palm flux, which leads to SSWs is longer for displacement events and a persistent mesospheric Eliassen-Palm flux divergence can be observed about 20 days after displacement events. Finally, although this work focuses on the stratopause at high latitudes, associated observations of the equatorial middle atmosphere are also examined to explore links between the equator and polar evolution during SSWs.

Highlights

The stratopause is characterised by a reversal of the atmospheric lapse rate at around 50 km (~1 hPa)
A comparison between Microwave limb sounder (MLS), HIRDLS and Modern Era-Restrospective analysis for Research and Application (MERRA) data has been performed to evaluate the quality of the stratopause identification in the MERRA dataset
An underestimation of the drop during strong stratospheric warming (SSW) and the absence of the 2006 and 2009 ES events are noticeable in the MERRA data comparing with the satellite data

Summary

Introduction

The stratopause is characterised by a reversal of the atmospheric lapse rate at around 50 km (~1 hPa). The westward and stationary gravity wave (GW) breaking induces a mesospheric meridional flow toward the winter pole with a confined descending branch over the vortex It leads to a warming around 60 km and so to a separation between the polar stratopause and the lower latitude stratopause, with the climatological zonally-averaged summer hemisphere and lower latitude stratopause generally positioned at a lower altitude than the winter stratopause. Their study focused on the link between the stratopause, the polar vortex and the middle atmosphere winter anticyclones They show that in October, as the polar sun heating has decreased, the northern polar stratopause inside the vortex is not due to ozone heating anymore but is maintained by westward GW driven descents at higher altitude. In November, the vortex becomes stronger and allows a higher critical level for westward GWs and a higher stratopause

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