Abstract
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.
Highlights
The unperturbed arctic winter stratosphere is characterized by a strong eastward-directed zonal wind jet
This effect is clearly seen in Fig. 3: during the mentioned periods of strong unperturbed polar jets gravity wave squared amplitudes are quite high with values of about 5 K2 around 30 km altitude, and of about 20 K2 around 50 km altitude
This is the case for all polar-night jet oscillation (PJO) events in the time period considered – even for the PJO event in winter 2011/2012 that is associated with only a minor stratospheric warmings (SSWs), the zonal wind in the mid and lower stratosphere only weakens after the SSW, and does not reverse on zonal average in the latitude range considered
Summary
The unperturbed arctic winter stratosphere is characterized by a strong eastward-directed zonal wind jet (polar vortex). Forcing by upward propagating planetary Rossby waves can lead to strong deceleration and even reversals of this polar jet. These events are associated with strong warming of the polar stratosphere, and they are called sudden stratospheric warmings (SSWs). Much of the dynamics during SSWs can be understood by upward propagation of planetary waves from the troposphere, amplification of their amplitudes, followed by wave dissipation and heat flux convergence. During this process considerable wave drag is exerted on the zonal mean background flow. As was shown by Holton (1983), accurate representation of major SSWs requires the inclusion of gravity wave drag in models
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