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Satellite Observations of Middle Atmosphere Gravity Wave Absolute Momentum flux and of Its Vertical Gradient During Recent Stratospheric Warmings

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Satellite Observations of Middle Atmosphere Gravity Wave Absolute Momentum flux and of Its Vertical Gradient During Recent Stratospheric Warmings Atmos. Chem. Phys., 16, 9983–10019, 2016 www.atmos-chem-phys.net/16/9983/2016/ doi:10.5194/acp-16-9983-2016 © Author(s) 2016. CC Attribution 3.0 License. Satellite observations of middle atmosphere gravity wave absolute momentum flux and of its vertical gradient during recent stratospheric warmings Manfred Ern1, Quang Thai Trinh1, Martin Kaufmann1, Isabell Krisch1, Peter Preusse1, Jörn Ungermann1, Yajun Zhu1, John C. Gille2,3, Martin G. Mlynczak4, James M. Russell III5, Michael J. Schwartz6, and Martin Riese1 1Institut für Energie- und Klimaforschung, Stratosphäre (IEK–7), Forschungszentrum Jülich GmbH, 52425 Jülich, Germany 2Center for Limb Atmospheric Sounding, University of Colorado at Boulder, Boulder, Colorado, USA 3National Center for Atmospheric Research, Boulder, Colorado, USA 4NASA Langley Research Center, Hampton, Virginia, USA 5Center for Atmospheric Sciences, Hampton University, Hampton, Virginia, USA 6Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA Correspondence to: Manfred Ern ([email protected]) Received: 30 March 2016 – Published in Atmos. Chem. Phys. Discuss.: 25 April 2016 Revised: 18 July 2016 – Accepted: 20 July 2016 – Published: 9 August 2016 Abstract. Sudden stratospheric warmings (SSWs) are cir- potential drag at the top of the jet likely contributes to the culation anomalies in the polar region during winter. They downward propagation of both the jet and the new elevated mostly occur in the Northern Hemisphere and affect also sur- stratopause. During PJO events, we also find some indication face weather and climate. Both planetary waves and gravity for poleward propagation of gravity waves. Another strik- waves contribute to the onset and evolution of SSWs. While ing finding is that obviously localized gravity wave sources, the role of planetary waves for SSW evolution has been rec- likely mountain waves and jet-generated gravity waves, play ognized, the effect of gravity waves is still not fully under- an important role during the evolution of SSWs and poten- stood, and has not been comprehensively analyzed based on tially contribute to the triggering of SSWs by precondition- global observations. In particular, information on the gravity ing the shape of the polar vortex. The distribution of these wave driving of the background winds during SSWs is still hot spots is highly variable and strongly depends on the zonal missing. and meridional shape of the background wind field, indicat- We investigate the boreal winters from 2001/2002 un- ing that a pure zonal average view sometimes is a too strong til 2013/2014. Absolute gravity wave momentum fluxes simplification for the strongly perturbed conditions during and gravity wave dissipation (potential drag) are estimated the evolution of SSWs. from temperature observations of the satellite instruments HIRDLS and SABER. In agreement with previous work, we find that sometimes gravity wave activity is enhanced be- fore or around the central date of major SSWs, particularly 1 Introduction during vortex-split events. Often, SSWs are associated with polar-night jet oscillation (PJO) events. For these events, we The unperturbed arctic winter stratosphere is characterized find that gravity wave activity is strongly suppressed when by a strong eastward-directed zonal wind jet (polar vortex). the wind has reversed from eastward to westward (usually Occasionally, however, forcing by upward propagating plan- after the central date of a major SSW). In addition, grav- etary Rossby waves can lead to strong deceleration and even ity wave potential drag at the bottom of the newly forming reversals of this polar jet. These events are associated with eastward-directed jet is remarkably weak, while considerable strong warming of the polar stratosphere, and they are there- fore called sudden stratospheric warmings (SSWs). Such Published by Copernicus Publications on behalf of the European Geosciences Union. 9984 M. Ern et al.: Gravity wave effects during SSWs events were first reported by Scherhag(1952), and the im- lating the tropical ascent of trace species into the strato- portance of upward propagating planetary waves for the driv- sphere (Tao et al., 2015a,b). These teleconnections be- ing of SSWs was first pointed out by Matsuno(1971). A tween tropics and high latitudes are still not fully cap- climatology and characterization of SSW events into differ- tured by GCMs/CCMs (e.g., Scaife et al., 2014). ent categories can be found, for example, in Charlton and Polvani(2007). Often, the following classification is used: 3. The forecast skill of weather prediction is related to during a “minor warming”, the temperature gradient in the the occurrence of SSWs (e.g., Sigmond et al., 2013; stratosphere between 60◦ latitude and pole, on zonal aver- Domeisen et al., 2015), and this can be utilized for an age, becomes positive over a certain altitude range at or be- improvement of weather forecasts. low the 10 hPa level (about 32 km altitude). During a “major warming”, additionally, the stratospheric zonal wind at 60◦ 4. SSWs have effect on the average temperature of the latitude reverses from eastward to westward at or below the stratosphere at high northern latitudes. Changes in the 10 hPa level (e.g., Chandran et al., 2014). frequency of SSWs will therefore result in temperature SSWs are dynamical processes that involve strong vertical trends (Angot et al., 2012). In addition, the warming coupling through atmospheric waves. Much of the dynamics of the lower stratosphere induced by SSWs can lead to during SSWs can be understood by upward propagation of temperatures too warm for polar stratospheric clouds to planetary waves from the troposphere, amplification of their form, which has a strong influence on ozone depletion amplitudes, followed by wave dissipation and heat flux con- in the Arctic. vergence. During this process considerable wave drag is ex- On the other hand, changes in the frequency of SSWs erted on the zonal mean background flow. Particularly during will also affect the meridional circulation and tracer major warmings, stationary planetary waves will encounter transport from lower latitudes, possibly resulting in critical levels when the zonal jet reverses from eastward to changes in ozone depletion of opposite sign (e.g., westward. The waves dissipate and can no longer propagate Konopka et al., 2007). to higher altitudes. However, as was shown by Holton(1983), accurate representation of major SSWs also requires the in- Particularly the timing of SSWs is of importance, with clusion of gravity wave drag in models. later SSWs favoring stronger ozone depletion (e.g., Kut- tipurath and Nikulin, 2012; von Hobe et al., 2013). The 1.1 Relevance of SSWs for global modeling timing of SSWs, however, may change in response to climate change. If SSWs are shifted to later in win- Simulating realistic SSWs in general circulation models and ter, as expected by Naoe and Shibata(2012), stronger chemistry climate models (GCMs/CCMs) and including the ozone depletion would be expected in the Arctic. There- physical processes that are involved in the onset and evolu- fore, realistic representation of SSWs and their evo- tion of SSWs is important for several reasons: lution could be important for obtaining more realistic ozone projections for a changing climate. 1. Effects of SSWs are not limited to the polar region. SSWs influence the global meridional residual circu- 5. It has been found that the circulation anomalies of lation, and meridional coupling between different lati- SSWs have influence on surface weather and cli- tudes is observed. For example, SSWs have influence mate (e.g., Baldwin and Dunkerton, 2001; Kodera on mesospheric temperatures in the tropics (e.g., Shep- et al., 2016), as well as on sea surface temperature herd et al., 2007), and they likely also have an effect on (O’Callaghan et al., 2014). Near-surface effects of the opposite hemisphere (e.g., Becker and Fritts, 2006; SSWs are not limited to the polar region, but can also de Wit et al., 2015; Cullens et al., 2015). extend to low latitudes (e.g., Hitchcock and Simpson, 2014). 2. There are teleconnections between high latitudes and the tropics. For example, it is known that the fre- A recent review on the mechanism of downward cou- quency of SSWs is influenced by the direction of the pling during SSWs is given, for example, by Kidston et stratospheric quasi-biennial oscillation (QBO) of the al.(2015). Because of this downward influence, repre- zonal wind in the tropics (e.g., Holton and Tan, 1980): sentation of SSWs is required for accurate simulation SSWs are more frequently observed if the zonal wind of Northern Hemisphere climate both on global and re- in the tropics at 50 hPa is westward. However, there gional scales (e.g., Gerber et al., 2012, and references are also indications for influences in the opposite direc- therein). tion: SSWs can influence the tropospheric temperature and convection in the tropics (e.g., Kodera, 2006), and All these effects and interlinks show the importance of thereby also tropical clouds and moisture (Eguchi and understanding all processes that are relevant for the for- Kodera, 2010). In particular, SSWs have strong influ- mation and evolution of SSWs, and of including them in ence on the composition of the stratosphere by modu- GCMs/CCMs. As mentioned before, one important process Atmos. Chem. Phys., 16, 9983–10019, 2016 www.atmos-chem-phys.net/16/9983/2016/ M. Ern et al.: Gravity wave effects during SSWs 9985 for the onset and evolution of SSWs is driving by atmo- westward propagating gravity waves is higher after the SSW spheric waves, particularly by planetary waves. The effect than before. Consequently, the residual circulation induced of gravity waves is still not well known. However, there are by those gravity waves that is responsible for the formation indications that they significantly contribute. of the stratopause is also raised, which explains the forma- tion of the new stratopause at an elevated level (Tomikawa et 1.2 Effects of gravity waves during SSWs al., 2012).
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