Global analysis for periodic variations in gravity wave squared amplitudes and momentum fluxes in the middle atmosphere

Abstract

Abstract. Atmospheric gravity waves (GWs) are an important coupling mechanism in the middle atmosphere. For instance, they provide a large part of the driving of long-period atmospheric oscillations such as the Quasi-Biennial Oscillation (QBO) and the semiannual oscillation (SAO) and are in turn modulated. They also induce the wind reversal in the mesosphere–lower thermosphere region (MLT) and the residual mean circulation at these altitudes. In this study, the variations in monthly zonal mean gravity wave square temperature amplitudes (GWSTAs) and, for the first time, absolute gravity wave momentum flux (GWMF) on different timescales such as the annual, semiannual, terannual and quasi-biennial variations are investigated by spectrally analyzing SABER observations from 2002 to 2015. Latitude–altitude cross sections of spectral amplitudes and phases of GWSTA and absolute GWMF in the stratosphere and mesosphere are presented and physically interpreted. It is shown that the time series of GWSTA and GWMF at a certain altitude and latitude results from the complex interplay of GW sources, propagation through and filtering in lower altitudes, oblique propagation superposing GWs from different source locations, and, finally, the modulation of the GW spectrum by the winds at a considered altitude and latitude. The strongest component is the annual variation, dominated in the summer hemisphere by subtropical convective sources and in the winter hemisphere by polar vortex dynamics. At heights of the wind reversal, a 180∘ phase shift also occurs, which is at different altitudes for GWSTA and GWMF. In the intermediate latitudes a semiannual variation (SAV) is found. Dedicated GW modeling is used to investigate the nature of this SAV, which is a different phenomenon from the tropical SAO also seen in the data. In the tropics a stratospheric and a mesospheric QBO are found, which are, as expected, in antiphase. Indication for a QBO influence is also found at higher latitudes. In previous studies a terannual variation (TAV) was identified. In the current study we explain its origin. In particular the observed patterns for the shorter periods, SAV and TAV, can only be explained by poleward propagation of GWs from the lower-stratosphere subtropics into the midlatitude and high-latitude mesosphere. In this way, critical wind filtering in the lowermost stratosphere is avoided and this oblique propagation is hence likely an important factor for MLT dynamics.

Propagation Paths and Source Distributions of Resolved Gravity Waves in ECMWF-IFS analysis fields around the Southern Polar Night Jet

“…Using ray-tracers with a global launch distribution, Sato et al (2009); Preusse et al (2009a) and Kalisch et al (2014) investigated the importance of oblique propagation for the gravity wave distributions in the upper stratosphere and mesosphere. Such modelling results are strongly supported by the patterns revealed from sub-annual cycle variations in a long time series of GWMF inferred from temperature measurements of the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument (Chen et al, 2019). All these investigations point to a continuous mostly pole-ward propagation into the jet core and focusing of gravity waves in the mid stratosphere and higher up.…”

Gravity Waves induced Wind Shears Derived from SABER Temperature Observations

“…Comparing with the AO and SAO in the GW square temperature amplitude (GWSTA) and absolute momentum flux (GWMF) presented by Chen et al 2019, we find that the AO and SAO of GW-induced shears agree with GWSTA and GWMF on the aspects of phase shifts and hemispheric asymmetry ( Fig. 2 and 3 of Chen et al, 2019). However, the heights at which phase shifts occur are different due to the AO and SAO in the background temperature and static stabilities (Liu et al, 2020).…”

“…A key step to derive the GW-induced shears is the extraction of GWs from the SABER temperature profile. The extraction methods of GWs from satellite data have been developed by Fetzer and Gille (1994) and improved greatly since (e.g., Preusse et al, 2002;Ern et al, 2004Ern et al, , 2011Ern et al, , 2018Chen et al, 2019;Alexander et al, 2008, 2018, Wang & Alexander, 2010Alexander, 2015). We have developed a similar method in our previous studies (Liu et al, 2014b(Liu et al, , 2017(Liu et al, , 2019, which is summarized here.…”

The semiannual oscillation (SAO) in the tropical middle atmosphere and its gravity wave driving in reanalyses and satellite observations

“…For the stratopause SAO, this asymmetry was confirmed by High Resolution Dynamics Limb Sounder (HIRDLS) satellite observations of gravity waves (Ern et al, 2015). Semiannual modulations of the global distribution of gravity waves are indeed observed over a large altitude range in the tropical mesosphere (e.g., Kovalam et al, 2006;Krebsbach and Preusse, 2007;Sridharan and Sathishkumar, 2008;Venkateswara Rao et al, 2012;Matsumoto et al, 2016;Chen et al, 2019). However, there is large uncertainty in which way those gravity waves contribute to the driving of the SAO, and how far the aforementioned asymmetry of gravity wave driving extends upward into the mesosphere.…”