The Life Cycle and Variability of Antarctic Weak Polar Vortex Events

Temperature And Precipitation ◽

Scale Temperature ◽

Precipitation Anomalies ◽

Polar Stratosphere ◽

Abstract Motivated by the strong Antarctic sudden stratospheric warming (SSW) in 2019, a survey on the similar Antarctic weak polar events (WPV) is presented, including their life cycle, dynamics, seasonality, and climatic impacts. The Antarctic WPVs have a frequency of about four events per decade, with the 2002 event being the only major SSW. They show a similar life cycle to the SSWs in the Northern Hemisphere but have a longer duration. They are primarily driven by enhanced upward-propagating wavenumber 1 in the presence of a preconditioned polar stratosphere, i.e., a weaker and more contracted Antarctic stratospheric polar vortex. Antarctic WPVs occur mainly in the austral spring. Their early occurrence is preceded by an easterly anomaly in the middle and upper equatorial stratosphere besides the preconditioned polar stratosphere. The Antarctic WPVs increase the ozone concentration in the polar region and are associated with an advanced seasonal transition of the stratospheric polar vortex by about one week. Their frequency doubles after 2000 and is closely related to the advanced Antarctic stratospheric final warming in recent decades. The WPV-resultant negative phase of the southern annular mode descends to the troposphere and persists for about three months, leading to persistent hemispheric scale temperature and precipitation anomalies.

Skillful Seasonal Prediction of the Southern Annular Mode and Antarctic Ozone

Journal of Climate ◽

10.1175/jcli-d-14-00264.1 ◽

2014 ◽

Vol 27 (19) ◽

pp. 7462-7474 ◽

Cited By ~ 35

Author(s):

William J. M. Seviour ◽

Steven C. Hardiman ◽

Lesley J. Gray ◽

Neal Butchart ◽

Craig MacLachlan ◽

...

Keyword(s):

Polar Vortex ◽

Seasonal Prediction ◽

Southern Annular Mode ◽

Significant Advance ◽

Seasonal Forecasts ◽

Austral Spring ◽

Annular Mode ◽

The North ◽

Abstract Using a set of seasonal hindcast simulations produced by the Met Office Global Seasonal Forecast System, version 5 (GloSea5), significant predictability of the southern annular mode (SAM) is demonstrated during the austral spring. The correlation of the September–November mean SAM with observed values is 0.64, which is statistically significant at the 95% confidence level [confidence interval: (0.18, 0.92)], and is similar to that found recently for the North Atlantic Oscillation in the same system. Significant skill is also found in the prediction of the strength of the Antarctic stratospheric polar vortex at 1 month average lead times. Because of the observed strong correlation between interannual variability in the strength of the Antarctic stratospheric circulation and ozone concentrations, it is possible to make skillful predictions of Antarctic column ozone amounts. By studying the variation of forecast skill with time and height, it is shown that skillful predictions of the SAM are significantly influenced by stratospheric anomalies that descend with time and are coupled with the troposphere. This effect allows skillful statistical forecasts of the October mean SAM to be produced based only on midstratosphere anomalies on 1 August. Together, these results both demonstrate a significant advance in the skill of seasonal forecasts of the Southern Hemisphere and highlight the importance of accurate modeling and observation of the stratosphere in producing long-range forecasts.

The influence of large amplitude planetary waves on the Antarctic ozone hole of austral spring 2017

Journal of Southern Hemisphere Earth System Science ◽

10.1071/es19022 ◽

2019 ◽

Vol 69 (1) ◽

pp. 57

Author(s):

Oleksandr Evtushevsky ◽

Andrew R. Klekociuk ◽

Volodymyr Kravchenko ◽

Gennadi Milinevsky ◽

Asen Grytsai

Keyword(s):

Large Amplitude ◽

Polar Vortex ◽

Maximum Size ◽

Planetary Waves ◽

Ozone Hole ◽

Late Winter ◽

Austral Spring ◽

Key Factor ◽

Quasi-stationary planetary wave activity in the lower Antarctic stratosphere in the late austral winter was an important contributor to the preconditioning of the ozone hole in spring 2017. Observations show that the ozone hole area (OHA) in spring 2017 was at the level of 1980s, that is, almost half the maximum size in 2000s. The observed OHA was close to that forecasted based on a least-squares linear regression between wave amplitude in August and OHA in September–November. We show that the key factor which contributed to the preconditioning of the Antarctic stratosphere for a relatively small ozone hole in the spring of 2017 was the development of large-amplitude stratospheric planetary waves of zonal wave numbers 1 and 2 in late winter. The waves likely originated from tropospheric wave trains and promoted the development of strong mid-latitude anticyclones in the lower stratosphere which interacted with the stratospheric polar vortex and strongly eroded the vortex in August and September, mitigating the overall level of ozone loss.

The 2019 Southern Hemisphere stratospheric polar vortex weakening and its impacts

Bulletin of the American Meteorological Society ◽

10.1175/bams-d-20-0112.1 ◽

2021 ◽

pp. 1-50

Author(s):

Eun-Pa Lim ◽

Harry H. Hendon ◽

Amy H. Butler ◽

David W. J. Thompson ◽

Zachary Lawrence ◽

...

Keyword(s):

Southern Hemisphere ◽

Polar Vortex ◽

Austral Spring ◽

Climate Conditions ◽

Extreme Climate ◽

Capsule SummaryDuring austral spring 2019 the Antarctic stratosphere experienced record-breaking warming and a near-record polar vortex weakening, resulting in predictable extreme climate conditions throughout the Southern Hemisphere through December 2019.

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 ◽

Close Relationship ◽

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.

Anomalous quasi-stationary planetary waves over the Antarctic region in 1988 and 2002

Annales Geophysicae ◽

10.5194/angeo-26-1101-2008 ◽

2008 ◽

Vol 26 (5) ◽

pp. 1101-1108 ◽

Cited By ~ 11

Author(s):

A. V. Grytsai ◽

O. M. Evtushevsky ◽

G. P. Milinevsky

Keyword(s):

Lower Stratosphere ◽

Polar Vortex ◽

Amplitude Distribution ◽

Stationary Wave ◽

Planetary Waves ◽

Late Winter ◽

Latitudinal Position ◽

The Antarctic ◽

Peak Value

Abstract. Anomalies in the Antarctic total ozone and amplitudes of the quasi-stationary planetary waves in the lower stratosphere temperature during the winter and spring of 1988 and 2002 have been compared. Westward displacement of the quasi-stationary wave (QSW) extremes by 50°–70° relative to the preceding years of the strong stratospheric polar vortex in 1987 and 2001, respectively, was observed. A dependence of the quasi-stationary wave ridge and trough positions on the strength of the westerly zonal wind in the lower stratosphere is shown. Comparison of the QSW amplitude in the lower stratosphere temperature in July and August shows that the amplitude distribution with latitude in August could be considered as a possible indication of the future anomalous warming in Antarctic spring. In August 2002, the QSW amplitude of 10 K at the edge region of the polar vortex (60° S–65° S) preceded the major warming in September, whereas in August 1988, the highest 7 K amplitude at 55° S preceded the large warming in the next months. These results suggest that the peak value of the lower stratosphere temperature QSW amplitude and the peak latitudinal position in late winter can influence the southern polar vortex strength in spring.

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 ◽

Distinct Structure ◽

Barrier Region ◽

Westerly Winds ◽

Antarctic Continent ◽

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.

Stratospheric influence on idealised baroclinic life cycles

10.5194/egusphere-egu2020-13680 ◽

2020 ◽

Author(s):

Philip Rupp ◽

Thomas Birner

Keyword(s):

Life Cycle ◽

Initial Conditions ◽

Polar Vortex ◽

Life Cycles ◽

Shift Response ◽

Tropospheric Circulation ◽

Wave Development ◽

Final Steady State ◽

Numerical Model Experiments

The importance of understanding the dynamical coupling of troposphere and stratosphere to make accurate weather and climate predictions is well-known. Over the past years and decades various signatures of such a coupling have been discovered. A very robust result, for example, seems to be an equatorward shift of the tropospheric eddy driven jet following sudden stratospheric warming events, where the westerly winds of the stratospheric polar vortex weaken or even reverse. However, many aspects of this fundamental coupling are still not fully understood and research on how the state of the stratosphere can influence the tropospheric circulation and what dynamical processes are involved is still ongoing. An important such process arises due to the interaction of a sharp, localised maximum in potential vorticity gradient near the tropopause with baroclinic eddies in the troposphere. Here, we analyse the sensitivity of baroclinic wave development and evolution to changes of various basic state characteristics, by performing a series of idealised baroclinic eddy life cycle experiments. Special attention is paid to sensitivities associated with the dynamical state of the stratosphere. We find that the final (steady) state of the life cycle simulations corresponds to an equatorward shift of the tropospheric jet in cases where the initial conditions do not include a stratospheric polar vortex (such as following sudden warming events) compared to those that do. These results further support the idea that the stratospheric state can strongly influence tropospheric dynamics and, in particular, highlight the robustness of the jet shift response following sudden warmings, that can be seen in a range of observations and numerical model experiments.

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 ◽

Annual Variations ◽

Distinct Structure ◽

Barrier Region ◽

Antarctic Continent ◽

Winter Time ◽

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.

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 ◽

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.

Teleconnection patterns in the Southern Hemisphere in subseasonal to seasonal models hindcasts and influences on South America

10.5194/egusphere-egu2020-2755 ◽

2020 ◽

Author(s):

Iracema Cavalcanti ◽

Naurinete Barreto

Keyword(s):

South America ◽

Atmospheric Circulation ◽

Southern Hemisphere ◽

Austral Summer ◽

Polar Vortex ◽

Southern Annular Mode ◽

Atmospheric Teleconnection ◽

Teleconnection Patterns ◽

South Pacific Ocean ◽

Precipitation Anomalies

The main atmospheric teleconnection patterns in the Southern Hemisphere are the Southern Annular Mode (SAM) and the Pacific South American (PSA). The SAM has opposite atmospheric anomalies between high and middle latitudes and it is linked with the polar vortex intensity and jet streams. PSA shows a wavetrain pattern from tropical to the extratropical atmosphere over the South Pacific Ocean triggered by convection in the tropical Indian, Maritime Continent and tropical Pacific. These modes modulate the atmospheric circulation variability and have an influence on the precipitation over Southern Hemisphere continents, mainly in South America (SA). Global models are able to represent these modes in climate simulations of seasonal timescale. The objective of this study is to analyse these teleconnections in hindcasts of subseasonal timescale and the relations to precipitation anomalies over South America. Predictions in the subseasonal time scale of austral summer are very important for several sectors of Southeastern and Southern regions of SA, as these are very populated regions and have agriculture and the largest hydropower,&#160; which are very much affected by precipitation extremes, both excess and lack of rain. Two models of the S2S project (ECMWF and NCEP) are used for the summer seasons of 1999 to 2011 and the patterns are compared to ERA5 reanalyses and GPCP data. EOF analyses of geopotential at 200 hPa and regression analyses against precipitation show the patterns and the influences over South America. The SAM pattern is represented in predictions of 1 to 4 weeks in advance, and PSA pattern, from 1 to 3 weeks in advance. Then, the atmospheric circulation and meteorological variables composites of extreme positive and negative amplitudes of SAM and PSA are analysed to interpret precipitation anomalies during these specific periods for predictions of weeks 2 and 3.