Articles | Volume 16, issue 15
Research article
09 Aug 2016
Research article |  | 09 Aug 2016

Satellite observations of middle atmosphere gravity wave absolute momentum flux and of its vertical gradient during recent stratospheric warmings

Manfred Ern, Quang Thai Trinh, Martin Kaufmann, Isabell Krisch, Peter Preusse, Jörn Ungermann, Yajun Zhu, John C. Gille, Martin G. Mlynczak, James M. Russell III, Michael J. Schwartz, and Martin Riese

Abstract. Sudden stratospheric warmings (SSWs) are circulation anomalies in the polar region during winter. They mostly occur in the Northern Hemisphere and affect also surface weather and climate. Both planetary waves and gravity waves contribute to the onset and evolution of SSWs. While the role of planetary waves for SSW evolution has been recognized, the effect of gravity waves is still not fully understood, and has not been comprehensively analyzed based on global observations. In particular, information on the gravity wave driving of the background winds during SSWs is still missing.

We investigate the boreal winters from 2001/2002 until 2013/2014. Absolute gravity wave momentum fluxes and gravity wave dissipation (potential drag) are estimated from temperature observations of the satellite instruments HIRDLS and SABER. In agreement with previous work, we find that sometimes gravity wave activity is enhanced before or around the central date of major SSWs, particularly during vortex-split events. Often, SSWs are associated with polar-night jet oscillation (PJO) events. For these events, we find that gravity wave activity is strongly suppressed when the wind has reversed from eastward to westward (usually after the central date of a major SSW). In addition, gravity wave potential drag at the bottom of the newly forming eastward-directed jet is remarkably weak, while considerable potential drag at the top of the jet likely contributes to the downward propagation of both the jet and the new elevated stratopause. During PJO events, we also find some indication for poleward propagation of gravity waves. Another striking finding is that obviously localized gravity wave sources, likely mountain waves and jet-generated gravity waves, play an important role during the evolution of SSWs and potentially contribute to the triggering of SSWs by preconditioning the shape of the polar vortex. The distribution of these hot spots is highly variable and strongly depends on the zonal and meridional shape of the background wind field, indicating that a pure zonal average view sometimes is a too strong simplification for the strongly perturbed conditions during the evolution of SSWs.

Short summary
Sudden stratospheric warmings (SSWs) influence the atmospheric circulation over a large range of altitudes and latitudes. We investigate the global distribution of small-scale gravity waves (GWs) during SSWs as derived from 13 years of satellite observations. We find that GWs may play an important role for triggering SSWs by preconditioning the polar vortex, as well as during long-lasting vortex recovery phases after SSWs. The GW distribution during SSWs displays strong day-to-day variability.
Final-revised paper