Driving of the SAO by gravity waves as observed from satellite

M Ern,M Riese,P Preusse

doi:10.5194/angeo-33-483-2015

Abstract

Abstract. It is known that atmospheric dynamics in the tropical stratosphere have an influence on higher altitudes and latitudes as well as on surface weather and climate. In the tropics, the dynamics are governed by an interplay of the quasi-biennial oscillation (QBO) and semiannual oscillation (SAO) of the zonal wind. The QBO is dominant in the lower and middle stratosphere, and the SAO in the upper stratosphere/lower mesosphere. For both QBO and SAO the driving by atmospheric waves plays an important role. In particular, the role of gravity waves is still not well understood. In our study we use observations of the High Resolution Dynamics Limb Sounder (HIRDLS) satellite instrument to derive gravity wave momentum fluxes and gravity wave drag in order to investigate the interaction of gravity waves with the SAO. These observations are compared with the ERA-Interim reanalysis. Usually, QBO westward winds are much stronger than QBO eastward winds. Therefore, mainly gravity waves with westward-directed phase speeds are filtered out through critical-level filtering already below the stratopause region. Accordingly, HIRDLS observations show that gravity waves contribute to the SAO momentum budget mainly during eastward wind shear, and not much during westward wind shear. These findings confirm theoretical expectations and are qualitatively in good agreement with ERA-Interim and other modeling studies. In ERA-Interim most of the westward SAO driving is due to planetary waves, likely of extratropical origin. Still, we find in both observations and ERA-Interim that sometimes westward-propagating gravity waves may contribute to the westward driving of the SAO. Four characteristic cases of atmospheric background conditions are identified. The forcings of the SAO in these cases are discussed in detail, supported by gravity wave spectra observed by HIRDLS. In particular, we find that the gravity wave forcing of the SAO cannot be explained by critical-level filtering alone; gravity wave saturation without critical levels being reached is also important.

Highlights

In the tropical stratosphere and lower mesosphere, the zonal wind is dominated by an interplay of the quasi-biennial oscillation (QBO) in the lower and middle stratosphere and the semiannual oscillation (SAO) in the upper stratosphere/lower mesosphere
In our study we have investigated the momentum budget of the semiannual oscillation of the zonal wind (SAO) in the tropical latitude band 10◦ S–10◦ N for the period January 2005 until mid-2008
Temperature observations of the High Resolution Dynamics Limb Sounder (HIRDLS) satellite instrument were used to derive absolute values of gravity wave momentum flux, as well as total gravity wave drag from momentum flux vertical gradients. These values of gravity wave drag were compared to the different terms in the momentum budget of the ERA-Interim reanalysis of the European Centre for Medium-Range Weather Forecasts (ECMWF), in particular to the zonal wind tendency and the missing drag

Summary

Introduction

In the tropical stratosphere and lower mesosphere, the zonal wind is dominated by an interplay of the quasi-biennial oscillation (QBO) in the lower and middle stratosphere and the semiannual oscillation (SAO) in the upper stratosphere/lower mesosphere. The QBO has an average period of 28 months. The SAO has a period of 6 months, and both eastward and westward winds can be quite strong: about −60 to −20 m s−1 for westward wind, and about 20 to 40 m s−1 for eastward wind (e.g., Hirota, 1980; Delisi and Dunkerton, 1988; Dee et al, 2011, and references therein). The QBO and the SAO are important processes in atmospheric dynamics. Both QBO and SAO have an effect on the tracer transport in the stratosphere (e.g., Huang et al, 2008; Punge et al, 2009; Khosrawi et al, 2013; Shu et al, 2013). It has been found that the QBO has an effect on the weather and climate in the lower atmosphere and even at the surface (e.g., Ebdon, 1975; Boer and Hamilton, 2008; Marshall and Scaife, 2009)

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Discussion

Conclusion