The exceptionally strong and persistent Arctic stratospheric polar vortex in the winter of 2019–2020 2019-2020

Yuli Zhang, Zhaonan Cai, Yi Liu

Key Laboratory of Middle Atmosphere and Global Environment Observation, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China

Keywords:Arctic stratospheric polar vortex Warm winter Ozone depletion

ABSTRACT The Arctic stratospheric polar vortex was exceptional strong, cold and persistent in the winter and spring of 2019–2020.Based on reanalysis data from the National Centers for Environmental Prediction/National Center for Atmospheric Research and ozone observations from the Ozone Monitoring Instrument, the authors investigated the dynamical variation of the stratospheric polar vortex during winter 2019–2020 and its influence on surface weather and ozone depletion.This strong stratospheric polar vortex was affected by the less active upward propagation of planetary waves.The seasonal transition of the stratosphere during the stratospheric final warming event in spring 2020 occurred late due to the persistence of the polar vortex.A positive Northern Annular Mode index propagated from the stratosphere to the surface, where it was consistent with the Arctic Oscillation and North Atlantic Oscillation indices.As a result, the surface temperature in Eurasia and North America was generally warmer than the climatology.In some places of Eurasia, the surface temperature was about 10 K warmer during the period from January to February 2020.The most serious Arctic ozone depletion since 2004 has been observed since February 2020.The mean total column ozone within 60°–90°N from March to 15 April was about 80 DU less than the climatology.

1.Introduction

The stratosphere is dominated by a low-pressure cold vortex in the polar region in winter.This stratospheric polar vortex is surrounded by strong westerly winds at high latitudes.It is often disturbed and weakened by upward planetary waves during the whole winter.Such strong disturbance sometimes leads to a sudden stratospheric warming event during which the stratospheric polar vortex weakens rapidly( Andrews et al., 1987 ; Matsuno, 1971 ).On the contrary, less propagation of planetary waves into the stratosphere results in a strong and cold stratospheric polar vortex ( Sabutis and Manney, 2000 ; Xie et al., 2012 ;Zhang et al., 2016 ).

Studies have shown that a strong (weak) stratospheric polar vortex is followed by a positive (negative) Northern Annular Mode (NAM)index, which propagates from the upper stratosphere to the surface( Baldwin and Dunkerton, 2001 ; Charlton and Polvani, 2007 ; Xie et al.,2016 ).The downward propagation of the stratospheric anomaly is closely related to surface weather ( Kidston et al., 2015 ).The NAM near the surface is highly correlated with the Arctic Oscillation (AO)and the regional North Atlantic Oscillation (NAO).The positive phase during a strong stratospheric polar vortex event is characterized by a larger pressure gradient and stronger westerly winds, leading to less frequent blocking events and occurrence of surface cold extremes( Thompson and Wallace, 2001 ; Trigo et al., 2004 ; Croci-Maspoli et al.,2007 ; Cheung et al., 2012 ).

The occurrence of a strong and cold stratospheric polar vortex is worthy of investigation because of the possibility of consequent ozone depletion in spring ( WMO, 2014 ).Low temperature is required to form the polar stratospheric clouds that drive the heterogeneous reactions and lead to ozone loss.The consistency between colder temperatures and ozone depletion has been observed in the Northern Hemispheric polar vortex ( Proffitt et al., 1993 ; Manney et al., 1996 ; Kuttippurath et al.,2011 ).Furthermore, the ozone depletion in turn affects the stratospheric polar vortex, with some studies ( Coy et al., 1997 ; Newman et al., 2001 ;Hu and Tung, 2003 ) having indicated that ozone depletion can result in a decrease in upward planetary waves, a decrease in polar downwelling,and a strengthened polar vortex, but others ( Albers and Nathan, 2013 ;Manzini et al., 2003 ) arguing that it will cause an increased downwelling and a weak polar vortex.

Fig.1.The 50-hPa variations of (a) minimum geopotential height in the northern polar region, (b) minimum temperature in the northern polar region, (c) zonal-mean zonal wind at 60°N, and (d) 100-hPa mean eddy heat flux within 45°–75°N.The red lines are the values of winter 2019–2020.The black lines are the climatological values.The gray lines are the winters from 1958–1959 to 2018–2019.

The winter of 2019–2020 witnessed an exceptionally strong, cold,and persistent Arctic stratospheric polar vortex.In this study, we focus on the variation of the stratospheric polar vortex during winter 2019–2020 and its influence on surface weather and ozone depletion.

The rest of this paper is structured as follows: Section 2 describes the data and methods.Section 3 presents the features of the strong and persistent polar vortex.Section 4 evidences the influence on tropospheric and surface anomalies, followed in Section 5 by details of the ozone depletion during that winter.Finally, Section 6 presents the conclusions and some further discussion related to the study’s findings.

2.Data and methods

2.1.Data

National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis data ( Kalnay et al.,1996 ) were used to investigate the dynamics of the polar vortex in the winter of 2019–2020.The daily mean temperature, wind velocity, and geopotential height at pressure levels, as well as the temperature at the surface, were used.The climatology was calculated during the period 1958–2020.

The Ozone Monitoring Instrument (OMI) is a nadir-viewing near-UV/Visible Charge-Coupled Device spectrometer aboard NASA’s Earth Observing System’s Aura satellite.The Level-3 Aura/OMI daily global total column ozone (TCO) dataset, which is a daily product on a 0.25°×0.25°longitude–latitude grid ( Veefkind, 2012 ), was used to evaluate the ozone depletion in the winter of 2019–2020.In this paper, we use the data covering the period from October 2004 to April 2020.

2.2.Methods

The NAM index ( Baldwin and Dunkerton, 1999,2001 ) is defined as the leading principal component of geopotential height at pressure levels.Similarly, the AO index is defined as the leading principal component of the Northern Hemisphere sea level pressure( Thompson and Wallace, 1998 ).The NAO index ( Hurrell, 1995 ) is defined as the standardized sea level pressure anomaly at Stykkisholmur, Iceland, subtracted from the standardized sea level pressure anomaly at Lisbon, Portugal.The NAM index is calculated based on NCEP/NCAR reanalysis data, and the indices of AO and NAO are updated by the National Oceanic and Atmospheric Administration( https://www.cpc.ncep.noaa.gov/products/precip/CWlink/daily_ao_index/ao.shtml ).The NAM index at 1000 hPa is highly correlated with the AO and NAO indices.

Fig.2.Monthly mean geopotential height anomalies (color shades) and horizontal circulation (red arrows) at 10 hPa.The anomalies are calculated by removing the climatologies from the values.

3.Strong and persistent polar vortex

The wintertime stratospheric polar vortex is represented by a center with low temperature and small geopotential height surrounded by strong westerly winds.The variations of 50-hPa minimum geopotential height, minimum temperature, and zonal-mean zonal wind at 60°N are shown in Fig.1 (a–c), representing the strength of the stratospheric polar vortex.Compared to other winters (gray lines in Fig.1 (a)), there were relatively small values of minimum geopotential height from the beginning of 2020, indicating a strong polar vortex in the winter and spring of 2019–2020.Usually, the polar vortex starts to weaken after it reaches its strongest point in January (black line in Fig.1 (a)).However, in 2019–2020, it remained strong from late February to the end of March (red line in Fig.1 (a)).The minimum temperature of the polar vortex during winter 2019–2020 was lower than in other winters, especially in the spring of 2020 ( Fig.1 (b)).The most significant anomalous polar vortex was observed during the period from February to April.This anomalously strong polar vortex was surrounded by strong westerly circulation at 60°N (red line in Fig.1 (c)), which was significantly stronger than the climatology (black line in Fig.2 (c)).

The wintertime polar vortex is continuously disturbed by upward propagation of planetary wave activity.When there are strong upward planetary wave activities, the polar vortex can be weakened rapidly, even resulting in a sudden stratospheric warming (SSW) event( Matsuno, 1971 ).The upward planetary wave activity in 2019–2020 winter was examined according to the 100-hPa mean eddy heat flux within 45°–75°N ( Fig.1 (d)).Compared to other winters, the eddy heat flux during winter 2019–2020 winter was relatively smaller.Most of the time, it was smaller than the climatology.The less active upward planetary wave activity is the main reason for the strong and persistent stratospheric polar vortex in the winter and spring of 2019–2020.

The geopotential height anomaly at 10 hPa ( Fig.2 ) shows the relative strength of the polar vortex in winter 2019–2020 compared with the climatology of 1958–2019.The anomalously strong polar vortex in 2019–2020 gradually enhanced from November 2019 to February 2020( Fig.2 (a–d)).The locations of polar vortices (the areas surrounded by the cyclones) were slightly eastward over Eurasia during this period.The strongest geopotential height anomaly was observed over the north of the Pacific and North America in March 2020 ( Fig.2 (e)).Finally,it weakened in April 2020, but was still stronger than the climatology( Fig.2 (f)).

Fig.3.(a) Onset time of SFW.The blue dashed line is the mean time of SFW during 1957–2020; the gray dashed lines are the mean time ± standard deviation of SFW.(b) The 10-hPa zonal-mean zonal wind at 60°N.Day 0 is the date of SFW.The black line is the climatology during 1957–2020.The red line is the value in spring 2020.The yellow lines are the variations during late SFW events (the dates of SFW later than mean time + standard deviation).The gray solid lines are values in other spring seasons.

Fig.4.(a) Time–pressure distribution of NAM index.(b) Variations of NAM index at 1000 hPa, along with the AO and NAO indices.

Weakening of the stratospheric polar vortex in spring is known as stratospheric final warming (SFW).SFW is characterized by an increase in polar stratospheric temperature and the transition of circumpolar zonal winds from westerly to easterly ( Andrews et al., 1987 ).The onset date of SFW is defined as the last time in spring when the 10-hPa zonalmean zonal wind at 60°N drops below zero and never exceeds 5 m s1 until autumn ( Black et al., 2007 ).Fig.3 (a) shows the variation of the SFW date from 1957 to 2020.The mean date of SFW is around 7 April.We define an SFW as a late SFW when the onset date is later than the mean date plus the standard deviation of all SFWs.The SFW in 2020 spring occurred on 29 April, which was a late SFW event owing to the persistence of the stratospheric polar vortex in winter 2019–2020.The circumpolar zonal winds before a late SFW are smaller than the climatology ( Fig.3 (b)).Like other late SFW events, the circumpolar westerly before the 2020 SFW was relatively weaker than in other winters.

4.Influence on tropospheric and surface anomalies

Stratospheric anomalies can propagate downward into the troposphere to influence tropospheric circulation and even surface weather( Baldwin and Dunkerton, 2001 ).Such downward propagation can be represented by the NAM index, which is positive during a strong polar vortex event.A positive NAM index extended from 10 to 1000 hPa from January to late March 2020 ( Fig.4 (a)), indicating a stronger and more stable polar vortex throughout the whole stratosphere and troposphere.

The response of surface weather to the polar vortex variability is traditionally discerned by the AO and more regional NAO indices.The AO and NAO indices are associated with extratropical surface temperature and precipitation anomalies.Generally, these indices are negative following a weak polar vortex, especially SSW events.There is colder air at the surface under this condition.On the contrary, a strong stratospheric polar vortex is followed by positive indices, indicating warmer air at the surface.Fig.4 (b) compares the NAM index at 1000 hPa with the AO and NAO indices.The three indices show similar variations during the whole of winter 2019–2020.They were almost all positive from January to March 2020.

Due to the strong and stable low (negative geopotential height anomaly) in the polar region at 500 hPa ( Fig.5 (c–e)), most of the middle latitudes was dominated by higher centers of geopotential height/pressure.In this case, the monthly mean surface temperature anomaly during winter 2019–2020 was positive in the middle latitudes of the Northern Hemisphere ( Fig.5 ).The most significant warmer surface temperature occurred in the north of Eurasia in January and February 2020.The largest surface temperature anomaly was more than 10 K in some places of Eurasia.Surface temperature over North America was also slightly warmer than the climatology during this period.

5.Ozone depletion

The strong and persistent polar vortex resulted in cold temperatures in the polar stratosphere during winter 2019–2020 ( Fig.1 (b)).Low temperature is usually associated with ozone depletion in the polar vortex of the Northern Hemisphere ( Proffitt et al., 1993 ; Manney et al., 1996 ;Kuttippurath et al., 2011 ).Fig.6 shows the monthly mean OMI TCO anomalies during winter 2019–2020.The TCO anomalies from December 2019 to January 2020 were not significant ( Fig.6 (a, b)).From February 2020, there was a strong negative TCO anomaly in the polar region( Fig.6 (c)).The negative TCO anomaly was clearer in March 2020, with a minimum TCO 150 DU smaller than the climatology ( Fig.6 (d)).This negative TCO anomaly weakened slightly and shifted to the northern edge of Eurasia in April 2020 ( Fig.6 (e)).Finally, in May 2020, the negative TCO anomaly was not as remarkable as in earlier months, but the TCO in the polar region and the north of Eurasia was still lower than the climatology ( Fig.6 (f)).

Fig.7 (a) shows the time series of mean TCO within 60°–90°N from November to April.The 2019–2020 TCO from November to January was close to the climatology.From early February, the TCO gradually decreased.The most significant ozone depletion was observed from March to the middle of April 2020, during which time the mean TCO within 60°–90°N was only about 330 DU (80 DU smaller than the climatology).The TCO then gradually recovered up until the end of May 2020.There were only two ozone depletion events in Arctic winters during the period of 2004–2020.The other one occurred in winter 2010–2011,during which the mean TCO in the polar region decreased as significantly as in February and March 2020.However, the ozone depletion in winter and spring 2019–2020 lasted much longer (until May 2020).Generally, Arctic TCO is much higher than Antarctic TCO ( Fig.7 (b)),and the frequency of Arctic ozone depletion is much lower.However, it is interesting that a significant Antarctic ozone recovery (anomalously higher TCO) occurred in the austral spring of 2019, half a year ahead of the ozone depletion in the boreal spring of 2020.It is still unknown if there is any relationship between these two anomalous events.

Fig.5.Surface temperature anomalies (color shades) and geopotential height anomalies at 500 hPa (purple contours).The anomalies are calculated by removing the climatologies from the values.

6.Conclusion and discussion

The Arctic stratospheric polar vortex was exceptionally strong, cold,and persistent in winter and spring of 2019–2020.Based on NCEP/NCAR reanalysis data, we investigated the dynamical characteristics and variations of the stratospheric polar vortex during winter 2019–2020 and its influence on tropospheric circulation and surface weather.Affected by the less-active upward propagation of planetary waves, the geopotential height and temperature in the polar vortex were lower and more persistent than most other winters.Meanwhile, the westerly circulation around the polar vortex was stronger than the climatology.The date of SFW was late in spring 2020 owing to the persistence of the stratospheric polar vortex, indicating a late transition of the stratosphere from winter to summer.

The strong stratospheric polar vortex manifested as a positive NAM index that propagated from the stratosphere to the surface.The NAM index was consistent with the AO and NAO indices when it propagated to the surface.These positive indices indicated a stronger westerly wind in the middle latitudes and pressure gradient between the middle latitudes and high latitudes, leading to less occurrence of surface cold extremes.The surface temperatures in Eurasia and North America were generally warmer than the climatology, especially in the period from January to February 2020, during which the temperature was about 10 K warmer in some places of Eurasia.

The exceptionally strong, cold, and persistent stratospheric polar vortex also led to the most serious Arctic ozone depletion since 2004.The ozone depletion was observed by the OMI from February 2020.The minimum TCO anomaly was less than 150 DU.The mean TCO within 60°–90°N decreased to about 330 DU (about 80 DU smaller than the climatology) from March to 15 April.

The Arctic ozone depletion in spring 2020 followed the Antarctic ozone recovery in the austral spring of 2019.However, another Arctic ozone depletion, in spring 2011, did not follow any austral anomaly.Thus, it whether there is any relationship between the 2020 Arctic ozone depletion and the 2019 Antarctic ozone recovery is worthy of investigation.Additionally, the causes and consequences of the remarkable stratospheric polar vortex event in 2019–2020 require further in-depth studies.For example, was this event related to quasi-biennial oscillation( Zhang et al., 2019 ), sea-ice variability ( Kim and Kim, 2020 ), or sea surface temperature anomalies ( Garfinkel and Hartmann, 2007,2008 )?These possibilities certainly merit further examination.

Fig.6.Monthly mean TCO anomalies (units: DU) from OMI.The blank areas are missing values.

Funding

This research was supported by the Key Laboratory of Middle Atmosphere and Global Environment Observation [grant number LAGEO-2019-01] .

Acknowledgments

The NCEP/NCAR reanalysis data are available at the website of the NOAA-CIRES Climate Diagnostics Center( http://www.cdc.noaa.go ).We thank the OMI science team for providing the total column ozone data, which are freely available at https://cmr.earthdata.nasa.gov/search/concepts/C1266136037-GES_DISC.html .

Fig.7.Variations of mean OMI TCO in the polar region of the (a) Northern Hemisphere and (b) Southern Hemisphere.The red and blue lines in (a) are the values of winter 2019–2020 and 2010–2011 in the Northern Hemisphere,respectively.The red line in (b) is the variation of winter 2019 in the Southern Hemisphere.The gray lines are the winters from 2004–2005 to 2018–2019, with their average been marked as black lines.