On the seasonal onset of polar mesospheric clouds and the breakdown of the stratospheric polar vortex in the Southern Hemisphere

Abstract Statistical models of climate generally regard climate variability as anomalies about a climatological seasonal cycle, which are treated as a stationary stochastic process plus a long-term seasonally dependent trend. However, the climate system has deterministic aspects apart from the climatological seasonal cycle and long-term trends, and the assumption of stationary statistics is only an approximation. The variability of the Southern Hemisphere zonal-mean circulation in the period encompassing late spring and summer is an important climate phenomenon and has been the subject of numerous studies. It is shown here, using reanalysis data, that this variability is rendered highly nonstationary by the organizing influence of the seasonal breakdown of the stratospheric polar vortex, which breaks time symmetry. It is argued that the zonal-mean tropospheric circulation variability during this period is best viewed as interannual variability in the transition between the springtime and summertime regimes induced by variability in the vortex breakdown. In particular, the apparent long-term poleward jet shift during the early-summer season can be more simply understood as a delay in the equatorward shift associated with this regime transition. The implications of such a perspective for various open questions are discussed.

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A meridional structure of static stability and ozone vertical gradient around the tropopause in the Southern Hemisphere extratropics

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-10-19175-2010 ◽

2010 ◽

Vol 10 (8) ◽

pp. 19175-19194 ◽

Cited By ~ 1

Author(s):

Y. Tomikawa ◽

T. Yamanouchi

Keyword(s):

Southern Hemisphere ◽

Seasonal Variations ◽

Inversion Layer ◽

Polar Vortex ◽

Vertical Gradient ◽

Static Stability ◽

Radiative Heating ◽

Stratospheric Polar Vortex ◽

Close Relationship ◽

The Antarctic

Abstract. An analysis of the static stability and ozone vertical gradient in the ozone tropopause based (OTB) coordinate is applied to the ozonesonde data at 10 stations in the Southern Hemisphere (SH) extratropics. The tropopause inversion layer (TIL) with a static stability maximum just above the tropopause shows similar seasonal variations at two Antarctic stations, which are latitudinally far from each other. Since the sunshine hour varies with time in a quite different way between these two stations, it implies that the radiative heating due to solar ultraviolet absorption of ozone does not contribute to the seasonal variation of the TIL. A meridional section of the static stability in the OTB coordinate shows that the static stability just above the tropopause has a large latitudinal gradient between 60° S and 70° S in austral winter because of the absence of the TIL over the Antarctic. It is accompanied by an increase of westerly shear with height above the tropopause, so that the polar-night jet is formed above this latitude region. This result suggests a close relationship between the absence of the TIL and the stratospheric polar vortex in the Antarctic winter. A vertical gradient of ozone mixing ratio, referred to as ozone vertical gradient, around the tropopause shows similar latitudinal and seasonal variations with the static stability in the SH extratropics. In a height region above the TIL, a small ozone vertical gradient in the midlatitudes associated with the Antarctic ozone hole is observed in a height region of the subvortex but not around the polar vortex. This is a clear evidence of active latitudinal mixing between the midlatitudes and subvortex.

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Bifurcation of potential vorticity gradients across the Southern Hemisphere stratospheric polar vortex

Atmospheric Chemistry and Physics ◽

10.5194/acp-18-8065-2018 ◽

2018 ◽

Vol 18 (11) ◽

pp. 8065-8077 ◽

Cited By ~ 2

Author(s):

Jonathan Conway ◽

Greg Bodeker ◽

Chris Cameron

Keyword(s):

Southern Hemisphere ◽

Potential Vorticity ◽

Polar Vortex ◽

Trace Gas ◽

Stratospheric Polar Vortex ◽

Distinct Structure ◽

Barrier Region ◽

Westerly Winds ◽

Antarctic Continent ◽

The Antarctic

Abstract. The wintertime stratospheric westerly winds circling the Antarctic continent, also known as the Southern Hemisphere polar vortex, create a barrier to mixing of air between middle and high latitudes. This dynamical isolation has important consequences for export of ozone-depleted air from the Antarctic stratosphere to lower latitudes. The prevailing view of this dynamical barrier has been an annulus compromising steep gradients of potential vorticity (PV) that create a single semi-permeable barrier to mixing. Analyses presented here show that this barrier often displays a bifurcated structure where a double-walled barrier exists. The bifurcated structure manifests as enhanced gradients of PV at two distinct latitudes – usually on the inside and outside flanks of the region of highest wind speed. Metrics that quantify the bifurcated nature of the vortex have been developed and their variation in space and time has been analysed. At most isentropic levels between 395 and 850 K, bifurcation is strongest in mid-winter and decreases dramatically during spring. From August onwards a distinct structure emerges, where elevated bifurcation remains between 475 and 600 K, and a mostly single-walled barrier occurs at other levels. While bifurcation at a given level evolves from month to month, and does not always persist through a season, interannual variations in the strength of bifurcation display coherence across multiple levels in any given month. Accounting for bifurcation allows the region of reduced mixing to be better characterised. These results suggest that improved understanding of cross-vortex mixing requires consideration of the polar vortex not as a single mixing barrier but as a barrier with internal structure that is likely to manifest as more complex gradients in trace gas concentrations across the vortex barrier region.

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Bifurcation of potential vorticity gradients across the Southern Hemisphere stratospheric polar vortex

10.5194/acp-2017-1016 ◽

2017 ◽

Author(s):

Jonathan Conway ◽

Greg Bodeker ◽

Chris Cameron

Keyword(s):

Southern Hemisphere ◽

Potential Vorticity ◽

Polar Vortex ◽

Stratospheric Polar Vortex ◽

Annual Variations ◽

Distinct Structure ◽

Barrier Region ◽

Antarctic Continent ◽

Winter Time ◽

The Antarctic

Abstract. The winter-time stratospheric westerly winds circling the Antarctic continent, also known as the Southern Hemisphere polar vortex, create a barrier to mixing of air between middle and high latitudes. This dynamical isolation has important consequences for export of ozone-depleted air from the Antarctic stratosphere to lower latitudes. The prevailing view of this dynamical barrier has been an annulus compromising steep gradients of potential vorticity (PV) that create a single semi-permeable barrier to mixing. Analyses presented here show that this barrier often displays a bifurcated structure where a doubled-walled barrier exists. The bifurcated structure manifests as enhanced gradients of PV at two distinct latitudes – usually on the inside and outside flanks of the region of highest wind speed. Metrics that quantify the bifurcated nature of the vortex have been developed and their variation in space and time has been analysed. At most isentropic levels between 370 K and 850 K, bifurcation is strongest in winter and reduces dramatically during spring. From August onwards a distinct structure emerges, where elevated bifurcation remains between 475 K and 600 K, and a mostly single walled barrier occurs at other levels. While bifurcation at a given level evolves from month to month, and does not always persist through a season, inter-annual variations in the strength of bifurcation display coherence across multiple levels in any given month. Accounting for bifurcation allows the region of reduced mixing to be better characterized. These results suggest that improved understanding of cross-vortex mixing requires consideration of the polar vortex not as a single mixing barrier, but as a barrier with internal structure that is likely to manifest as more complex gradients in trace gas concentrations across the vortex barrier region.

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The Combined Influence of the Stratospheric Polar Vortex and ENSO on Zonal Asymmetries in the Southern Hemisphere Upper Tropospheric Circulation during Austral Spring and Summer

10.5194/egusphere-egu21-5311 ◽

2021 ◽

Author(s):

Marisol Osman ◽

Theodore Shepherd ◽

Carolina Vera

Keyword(s):

Southern Hemisphere ◽

Southern Oscillation ◽

Polar Vortex ◽

Austral Spring ◽

Stratospheric Polar Vortex ◽

Wave Source ◽

Weather Forecasts ◽

Rossby Wave Source ◽

Enso Teleconnections ◽

Tropospheric Circulation

The influence of El Nin&#771;o Southern Oscillation (ENSO) and the Stratospheric Polar Vortex (SPV) on the zonal asymmetries in the Southern Hemisphere atmospheric circulation during spring and summer is examined. The main objective is to explore if the SPV can modulate the ENSO teleconnections in the extratropics. We use a large ensemble of seasonal hindcasts from the European Centre for Medium-Range Weather Forecasts Integrated Forecast System to provide a much larger sample size than is possible from the observations alone.We find a small but statistically significant relationship between ENSO and the SPV, with El Nin&#771;o events occurring with weak SPV and La Nin&#771;a events occurring with strong SPV more often than expected by chance, in agreement with previous works. We show that the zonally asymmetric response to ENSO and SPV can be mainly explained by a linear combination of the response to both forcings, and that they can combine constructively or destructively. From this perspective, we find that the tropospheric asymmetries in response to ENSO are more intense when El Nin&#771;o events occur with weak SPV and La Nin&#771;a events occur with strong SPV, at least from September through December. In the stratosphere, the ENSO teleconnections are mostly confounded by the SPV signal. The analysis of Rossby Wave Source and of wave activity shows that both are stronger when El Nin&#771;o events occur together with weak SPV, and when La Nin&#771;a events occur together with strong SPV.

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How Do Weakening of the Stratospheric Polar Vortex in the Southern Hemisphere Affect Regional Antarctic Sea Ice Extent?

Geophysical Research Letters ◽

10.1029/2021gl092582 ◽

2021 ◽

Vol 48 (11) ◽

Author(s):

Shaoyin Wang ◽

Jiping Liu ◽

Xiao Cheng ◽

Tobias Kerzenmacher ◽

Yongyun Hu ◽

...

Keyword(s):

Sea Ice ◽

Southern Hemisphere ◽

Polar Vortex ◽

Sea Ice Extent ◽

Stratospheric Polar Vortex ◽

Antarctic Sea Ice

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Emergence of Southern Hemisphere circulation changes in response to ozone recovery

10.5194/egusphere-egu2020-22211 ◽

2020 ◽

Author(s):

Brian Zambri ◽

Susan Solomon ◽

David Thompson ◽

Qiang Fu

Keyword(s):

Southern Hemisphere ◽

Ozone Depletion ◽

Stratospheric Ozone ◽

Polar Vortex ◽

Montreal Protocol ◽

Stratospheric Polar Vortex ◽

Surface Climate ◽

Antarctic Ozone ◽

Ozone Depleting Substances ◽

Circulation Changes

Ozone depletion in the Southern Hemisphere (SH) stratosphere in the late 20th century cooled the air there, strengthening the SH stratospheric westerly winds near 60&#186;S and altering SH surface climate. Since ~1999, trends in Antarctic ozone have begun to recover, exhibiting a flattening followed by a sign reversal in response to decreases in stratospheric chlorine concentration due to the Montreal Protocol, an international treaty banning the production and consumption of ozone-depleting substances. Here we show that the post&#8211;1999 increase in ozone has resulted in thermal and circulation changes of opposite sign to those that resulted from stratospheric ozone losses, including a warming of the SH polar lower stratosphere and a weakening of the SH stratospheric polar vortex.&#160; Further, these altered trends extend to the upper troposphere, albeit of smaller magnitudes. &#160;Observed post&#8211;1999 trends of temperature and circulation in the stratosphere are about 20&#8211;25% the magnitude of those of the ozone depletion era, and are broadly consistent with expectations based on modeled depletion-era trends and variability of both ozone and reactive chlorine, thereby indicating the emergence of healing of dynamical impacts of the Antarctic ozone hole.

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Evolution of the stratospheric polar vortex edge intensity and duration in the Southern hemisphere over the 1979–2020 period

10.5194/acp-2021-676 ◽

2021 ◽

Author(s):

Audrey Lecouffe ◽

Sophie Godin-Beekmann ◽

Andrea Pazmiño ◽

Alain Hauchecorne

Keyword(s):

Solar Cycle ◽

Southern Hemisphere ◽

Southern Oscillation ◽

Polar Vortex ◽

Onset Date ◽

Late Winter ◽

Stratospheric Polar Vortex ◽

The Difference ◽

Lower Variability

Abstract. The intensity and position of the Southern Hemisphere stratospheric polar vortex edge is evaluated as a function of equivalent latitude over the 1979–2020 period on three isentropic levels (475 K, 550 K and 675 K) from ECMWF ERA-Interim reanalysis. The study also includes an analysis of the onset and breakup dates of the polar vortex, which are determined from wind thresholds (e.g. 15.2 m.s−1, 20 m.s−1and 25 m.s−1) along the vortex edge. The vortex edge is stronger in late winter, over September–October – November with the period of strongest intensity occurring later at the lowermost level. A lower variability of the edge position is observed during the same period. Long-term increase of the vortex edge intensity and break-up date is observed over the 1979–1999 period, linked to the increase of the ozone hole. Long-term decrease of the vortex onset date related to the 25 m.s−1wind threshold is also observed at 475 K during this period. The solar cycle and to a lower extent the quasi-biennal oscillation (QBO) and El Niño Southern Oscillation (ENSO) modulate the inter-annual evolution of the strength of the vortex edge and the vortex breakup dates. Stronger vortex edge and longer vortex duration is observed in solar minimum (minSC) years, with the QBO and ENSO further modulating the solar cycle influence, especially at 475 K and 550 K: during West QBO (wQBO) phases, the difference between vortex edge intensity for minSC and maxSC years is smaller than during East QBO (eQBO) phases. The polar vortex edge is stronger and lasts longer for maxSC/wQBO years than for maxSC/eQBO years. ENSO has a weaker impact but the vortex edge is somewhat stronger during cold ENSO phases for both minSC and maxSC years.

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Evolution of the stratospheric polar vortex in the Southern and Northern Hemispheres over the period 1979 – 2020

10.5194/egusphere-egu21-9523 ◽

2021 ◽

Author(s):

Audrey Lecouffe ◽

Sophie Godin-Beekmann ◽

Andrea Pazmiño ◽

Alain Hauchecorne

Keyword(s):

Southern Hemisphere ◽

Potential Vorticity ◽

Southern Oscillation ◽

Stratospheric Ozone ◽

Polar Vortex ◽

Geophysical Research ◽

Quasi Biennial Oscillation ◽

Stratospheric Polar Vortex ◽

Antarctic Polar Vortex ◽

The Antarctic

The stratospheric polar vortex in the Southern Hemisphere plays an important role in the intensity of the stratospheric ozone destruction during austral spring, which started in the late 1970s. The so-called ozone hole has in turn influenced the evolution of weather patterns in the Southern Hemisphere in the last decades (WMO, 2018). The Northern Hemisphere polar vortex is less stable because of larger dynamical activity in winter. It is thus less cold and polar arctic ozone losses are less important. The seasonal and interannual evolution of the polar vortex in both hemispheres has been analyzed using meteorological fields from the European Center for Meteorology Weather Forecasts ERA-Interim reanalyses and the MIMOSA model (Mod&#233;lisation Isentrope du transport M&#233;so-&#233;chelle de l&#8217;Ozone Stratosph&#233;rique par Advection, Hauchecorne et al., 2002). This model provides high spatial resolution potential vorticity (PV) and equivalent latitude fields at several isentropic levels (675K, 550K and 475K) that are used to evaluate the temporal evolution of the polar vortex edge. The edge of the vortex is computed on isentropic surfaces from the wind and gradient of PV as a function of equivalent latitude (e.g. Nash et al, 1996; Godin et al., 2001). On an interannual scale, the signature of some typical forcings driving stratospheric natural variability such as the 11-year solar cycle, the quasi-biennial oscillation (QBO), and El Ni&#241;o Southern Oscillation (ENSO) is evaluated. The study includes analysis of the onset and breakup dates of the polar vortex, which are determined from the wind field along the vortex edge. Several threshold values, such as 15.2m/s, 20m/s and 25m/s following Akiyoshi et al. (2009) are used. Results on the seasonal and interannual evolution of the intensity and position of the vortex edge, as well as the onset and breakup dates of the Southern and Northern polar vortex edge over the 1979 &#8211; 2020 period will be shown.References:<ul><li>Akiyoshi, H., Zhou, L., Yamashita, Y., Sakamoto, K., Yoshiki, M., Nagashima, T., Takahashi, M., Kurokawa, J., Takigawa, M., and Imamura, T. A CCM simulation of the breakup of the Antarctic polar vortex in the years 1980&#8211;2004 under the CCMVal scenarios, Journal ofGeophysical Research: Atmospheres, 114, 2009.</li> <li>Godin S., V. Bergeret, S. Bekki, C. David, G. Me&#769;gie, Study of the interannual ozone loss and the permeability of the Antarctic Polar Vortex from long-term aerosol and ozone lidar measurements in Dumont d&#8217;Urville (66.4&#9702;S, 140&#9702;E), J. Geophys. Res., 106, 1311-1330, 2001.</li> <li>Hauchecorne, A., S. Godin, M. Marchand, B. Hesse, and C. Souprayen, Quantification of the transport of chemical constituents from the polar vortex to midlatitudes in the lower stratosphere using the high-resolution advection model MIMOSA and effective diffusivity, J. Geophys. Res., 107 (D20), 8289, doi:10.1029/2001JD000491, 2002.</li> <li>Nash, E. R., Newman, P. A., Rosenfield, J. E., and Schoeberl, M. R. (1996), An objective determination of the polar vortex using Ertel&#8217;s potential vorticity, Journal of geophysical research, VOL.101(D5), 9471- 9478</li> <li>World Meteorological Organization, Global Ozone Research and Monitoring Project &#8211; Report No. 58, 2018.</li> </ul>

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The Splitting of the Stratospheric Polar Vortex in the Southern Hemisphere, September 2002: Dynamical Evolution

Journal of the Atmospheric Sciences ◽

10.1175/jas-3318.1 ◽

2005 ◽

Vol 62 (3) ◽

pp. 590-602 ◽

Cited By ~ 55

Author(s):

Andrew J. Charlton ◽

Alan O’Neill ◽

William A. Lahoz ◽

Paul Berrisford

Keyword(s):

High Resolution ◽

Southern Hemisphere ◽

Large Scale ◽

Dynamical Evolution ◽

Polar Vortex ◽

Dynamical Analysis ◽

Stratospheric Polar Vortex ◽

Upper Troposphere ◽

Dynamical Features ◽

Stratospheric Circulation

Abstract The polar vortex of the Southern Hemisphere (SH) split dramatically during September 2002. The large-scale dynamical effects were manifest throughout the stratosphere and upper troposphere, corresponding to two distinct cyclonic centers in the upper troposphere–stratosphere system. High-resolution (T511) ECMWF analyses, supplemented by analyses from the Met Office, are used to present a detailed dynamical analysis of the event. First, the anomalous evolution of the SH polar vortex is placed in the context of the evolution that is usually witnessed during spring. Then high-resolution fields of potential vorticity (PV) from ECMWF are used to reveal several dynamical features of the split. Vortex fragments are rapidly sheared out into sheets of high (modulus) PV, which subsequently roll up into distinct synoptic-scale vortices. It is proposed that the stratospheric circulation becomes hydrodynamically unstable through a significant depth of the troposphere–stratosphere system as the polar vortex elongates.

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