Driving of the SAO by gravity waves as observed from satellite

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 ERAInterim 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 westwardpropagating 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.

Semi-annual oscillation (SAO) of the nighttime ionospheric D region as detected through ground-based VLF receivers

“…Thus, as we reach higher altitudes within the D region, which coincides with these altitude levels, the role of local dynamics (vertical exchange) becomes more significant (Solomon et al, 1982a). In addition, the amplitudes of gravity and planetary waves penetrating into the MLT (as a function of season; Lindzen, 1981) grow exponentially with altitude, as the ambient density drops (Ern et al, 2015;Smith, 2012). Therefore, downward transport of NO molecules, which are created mainly in the lower thermosphere (Solomon et al, 1982a, b), can increase the D region NO + concentrations, depending on the vertical wind patterns (e.g., Clilverd et al, 2006).…”

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

“…See also Preusse et al (2006), Eq. (10) in Ern et al (2008), and the discussion in Ern et al (2015).…”

“…Therefore it can be assumed that at the top of strong wind jets gravity wave potential drag can be used as a proxy for net gravity wave drag, and the direction of the drag is opposite to the vertical gradient of the wind at the top of the jet. In spite of the large uncertainty of gravity wave potential drag, relative variations of this drag have already led to meaningful results in several cases: for the mesospheric zonal wind jets in the summer hemisphere , and for both the QBO and the semiannual oscillation (SAO; Ern et al, 2015) of the zonal wind in the tropics. Therefore, it can be expected that meaningful results can also be obtained in our current study for the polar jets in boreal winter.…”

Intercomparison of AIRS and HIRDLS stratospheric gravity wave observations

“…An additional effect is that the vertical wavelength of these gravity waves is Dopplershifted towards longer vertical wavelengths, which are better visible in particular for AIRS. A more detailed discussion of this effect can be found, for example, in Ern et al (2015) and Hoffmann et al (2016). This also means that long-verticalwavelength gravity waves are preferentially found in regions of strong background winds.…”

“…In particular, absolute gravity wave momentum fluxes are derived from information about gravity wave vertical and horizontal wavelengths (Alexander et al, 2008;Wright et al, 2010;Ern et al, 2011). Based on these momentum fluxes, the intermittency in gravity wave global distributions was studied (e.g., Hertzog et al, 2012;Wright et al, 2013), as well as the interaction of gravity waves with the background circulation (e.g., Ern et al, 2014Ern et al, , 2015. In addition Geller et al (2013) used HIRDLS data to compare gravity wave momentum fluxes in models and those derived from observations.…”