Temporal evolution of chlorine and minor species related to ozone depletion observed with ground-based FTIR at Syowa Station, Antarctica and satellites during austral fall to spring in 2007 and 2011

Abstract. To understand and project future ozone recovery, understanding of mechanisms related to polar ozone destruction is crucial. For polar stratospheric ozone destruction, chlorine species play an important role, but detailed temporal evolution of chlorine species in the Antarctic winter is not well understood. We retrieved lower stratospheric vertical profiles of O3, HNO3, and HCl from solar spectra taken with a ground-based Fourier-Transform infrared spectrometer (FTIR) installed at Syowa Station, Antarctica (69.0º S, 39.6º E) from March to December 2007 and September to November 2011. We analyzed temporal variation of these species combined with ClO, HCl, and HNO3 data taken with the Aura/MLS (Microwave Limb Sounder) satellite sensor, and ClONO2 data taken with the Envisat/MIPAS (The Michelson Interferometer for Passive Atmospheric Sounding) satellite sensor at 18 and 22 km over Syowa Station. When the stratospheric temperature over Syowa Station fell below polar stratospheric cloud (PSC) saturation temperature in early winter, PSCs started to form and heterogeneous reaction on PSCs convert chlorine reservoirs into reactive chemical species. HCl and ClONO2 decrease occurred at both 18 and 22 km, and soon ClONO2 was almost depleted in early winter. When the sun returned to Antarctica in spring, enhancement of ClO and gradual O3 destruction were observed. During the ClO enhanced period, negative correlation between ClO and ClONO2 was observed in the time-series of the data at Syowa Station. This negative correlation was associated with the distance between Syowa Station and the inner edge of the polar vortex. Temporal variation of chlorine species over Syowa Station was affected by both heterogeneous chemistry related to PSC occurrence deep inside the polar vortex, and transport of an NONOx-rich airmass from lower latitudinal polar vortex boundary region which can produce additional ClONO2 by reaction between ClO and NO2. We used MIROC3.2 Chemistry-Climate Model (CCM) results to see the comprehensive behavior of chlorine and related species inside the polar vortex and the edge region in more detail. Rapid conversion of chlorine reservoir species (HCl and ClONO2) into Cl2, gradual conversion of Cl2 into Cl2O2, increase of ClO when sunlight became available, and conversion of ClO into HCl, was successfully reproduced by the CCM. HCl decrease in the winter polar vortex core continued to occur due to the transport of ClONO2 from the subpolar region (55–65º S) to higher latitudes (65–75º S), providing a flux of ClONO2 from more sunlit latitudes into the polar vortex. The deactivation pathways from active ClO into reservoir species (HCl and/or ClONO2) were found to be highly dependent on the availability of ambient O3 and NOx. At an altitude where most ozone was depleted in Antarctica, most ClO was converted to HCl. However, when there were some O3 and NOx available, super-recovery of ClONO2 can occur, similar to the case in the Arctic.

Download Full-text

Chlorine partitioning near the polar vortex boundary observed with ground-based FTIR and satellites at Syowa Station, Antarctica in 2007 and 2011

10.5194/acp-2019-443 ◽

2019 ◽

Author(s):

Hideaki Nakajima ◽

Isao Murata ◽

Yoshihiro Nagahama ◽

Hideharu Akiyoshi ◽

Kosuke Saeki ◽

...

Keyword(s):

Temporal Variation ◽

Negative Correlation ◽

Climate Model ◽

Michelson Interferometer ◽

Polar Vortex ◽

The Arctic ◽

Additional Increase ◽

Syowa Station ◽

Reservoir Species ◽

Satellite Sensor

Abstract. We retrieved lower stratospheric vertical profiles of O3, HNO3, and HCl from solar spectra taken with a ground-based Fourier-Transform infrared spectrometer (FTIR) installed at Syowa Station, Antarctica (69.0° S, 39.6° E) from March to December 2007 and September to November 2011. This was the first continuous measurements of chlorine species throughout the ozone hole period from the ground in Antarctica. We analyzed temporal variation of these species combined with ClO, HCl, and HNO3 data taken with the Aura/MLS (Microwave Limb Sounder) satellite sensor, and ClONO2 data taken with the Envisat/MIPAS (The Michelson Interferometer for Passive Atmospheric Sounding) satellite sensor at 18 and 22 km over Syowa Station. HCl and ClONO2 decrease occurred at both 18 and 22 km, and soon ClONO2 was almost depleted in early winter. When the sun returned to Antarctica in spring, enhancement of ClO and gradual O3 destruction were observed. During the ClO enhanced period, negative correlation between ClO and ClONO2 was observed in the time-series of the data at Syowa Station. This negative correlation was associated with the distance between Syowa Station and the inner edge of the polar vortex. We used MIROC3.2 Chemistry-Climate Model (CCM) results to see the comprehensive behavior of chlorine and related species inside the polar vortex and the edge region in more detail. From CCM model results, rapid conversion of chlorine reservoir species (HCl and ClONO2) into Cl2, gradual conversion of Cl2 into Cl2O2, increase of ClO when sunlight became available, and conversion of ClO into HCl, was successfully reproduced. HCl decrease in the winter polar vortex core continued to occur due to the transport of ClONO2 from the subpolar region to higher latitudes, providing a flux of ClONO2 from more sunlit latitudes into the polar vortex. Temporal variation of chlorine species over Syowa Station was affected by both heterogeneous chemistry related to Polar Stratospheric Cloud (PSC) occurrence deep inside the polar vortex, and transport of an NOx-rich airmass from lower latitudinal polar vortex boundary region which can produce additional ClONO2 by reaction of ClO with NO2. The deactivation pathways from active chlorine into reservoir species (HCl and/or ClONO2) were found to be highly dependent on the availability of ambient O3. At an altitude where most ozone was depleted in Antarctica (18 km), most ClO was converted to HCl. However, at an altitude where there were some O3 available (22 km), additional increase of ClONO2 from initial value can occur, similar to the case in the Arctic.

Download Full-text

Chlorine partitioning near the polar vortex edge observed with ground-based FTIR and satellites at Syowa Station, Antarctica, in 2007 and 2011

Atmospheric Chemistry and Physics ◽

10.5194/acp-20-1043-2020 ◽

2020 ◽

Vol 20 (2) ◽

pp. 1043-1074

Author(s):

Hideaki Nakajima ◽

Isao Murata ◽

Yoshihiro Nagahama ◽

Hideharu Akiyoshi ◽

Kosuke Saeki ◽

...

Keyword(s):

Temporal Variation ◽

Negative Correlation ◽

Michelson Interferometer ◽

Polar Vortex ◽

Boundary Region ◽

The Arctic ◽

Winter Period ◽

Syowa Station ◽

Reservoir Species ◽

Satellite Sensor

Abstract. We retrieved lower stratospheric vertical profiles of O3, HNO3, and HCl from solar spectra taken with a ground-based Fourier transform infrared spectrometer (FTIR) installed at Syowa Station, Antarctica (69.0∘ S, 39.6∘ E), from March to December 2007 and September to November 2011. This was the first continuous measurement of chlorine species throughout the ozone hole period from the ground in Antarctica. We analyzed temporal variation of these species combined with ClO, HCl, and HNO3 data taken with the Aura MLS (Microwave Limb Sounder) satellite sensor and ClONO2 data taken with the Envisat MIPAS (the Michelson Interferometer for Passive Atmospheric Sounding) satellite sensor at 18 and 22 km over Syowa Station. An HCl and ClONO2 decrease occurred from the end of May at both 18 and 22 km, and eventually, in early winter, both HCl and ClONO2 were almost depleted. When the sun returned to Antarctica in spring, enhancement of ClO and gradual O3 destruction were observed. During the ClO-enhanced period, a negative correlation between ClO and ClONO2 was observed in the time series of the data at Syowa Station. This negative correlation was associated with the relative distance between Syowa Station and the edge of the polar vortex. We used MIROC3.2 chemistry–climate model (CCM) results to investigate the behavior of whole chlorine and related species inside the polar vortex and the boundary region in more detail. From CCM model results, the rapid conversion of chlorine reservoir species (HCl and ClONO2) into Cl2, gradual conversion of Cl2 into Cl2O2, increase in HOCl in the winter period, increase in ClO when sunlight became available, and conversion of ClO into HCl were successfully reproduced. The HCl decrease in the winter polar vortex core continued to occur due to both transport of ClONO2 from the subpolar region to higher latitudes, providing a flux of ClONO2 from more sunlit latitudes into the polar vortex, and the heterogeneous reaction of HCl with HOCl. The temporal variation of chlorine species over Syowa Station was affected by both heterogeneous chemistries related to polar stratospheric cloud (PSC) occurrence inside the polar vortex and transport of a NOx-rich air mass from the polar vortex boundary region, which can produce additional ClONO2 by reaction of ClO with NO2. The deactivation pathways from active chlorine into reservoir species (HCl and/or ClONO2) were confirmed to be highly dependent on the availability of ambient O3. At 18 km, where most ozone was depleted, most ClO was converted to HCl. At 22 km where some O3 was available, an additional increase in ClONO2 from the prewinter value occurred, similar to the Arctic.

Download Full-text

Chlorine partitioning near the polar vortex edge observed with ground-based FTIR and satellites at Syowa Station, Antarctica in 2007 and 2011

10.5194/egusphere-egu2020-2280 ◽

2020 ◽

Author(s):

Hideaki Nakajima ◽

Isao Murata ◽

Yoshihiro Nagahama ◽

Hideharu Akiyoshi ◽

Takeshi Kinase ◽

...

Keyword(s):

Temporal Variation ◽

Negative Correlation ◽

Michelson Interferometer ◽

Polar Vortex ◽

Boundary Region ◽

The Arctic ◽

Winter Period ◽

Syowa Station ◽

Reservoir Species ◽

Satellite Sensor

We retrieved lower stratospheric vertical profiles of O3, HNO3, and HCl from solar spectra taken with a ground-based Fourier-Transform infrared spectrometer (FTIR) installed at Syowa Station, Antarctica (69.0&#176;S, 39.6&#176;E) from March to December 2007 and September to November 2011. &#160;This was the first continuous measurements of chlorine species throughout the ozone hole period from the ground in Antarctica. &#160;We analyzed temporal variation of these species combined with ClO, HCl, and HNO3 data taken with the Aura/MLS (Microwave Limb Sounder) satellite sensor, and ClONO2 data taken with the Envisat/MIPAS (The Michelson Interferometer for Passive Atmospheric Sounding) satellite sensor at 18 and 22 km over Syowa Station. &#160;HCl and ClONO2 decrease occurred from the end of May at both 18 and 22 km, and eventually in early winter, both HCl and ClONO2 were almost depleted. &#160;When the sun returned to Antarctica in spring, enhancement of ClO and gradual O3 destruction were observed. &#160;During the ClO enhanced period, negative correlation between ClO and ClONO2 was observed in the time-series of the data at Syowa Station. &#160;This negative correlation was associated with the relative distance between Syowa Station and the edge of the polar vortex. &#160;We used MIROC3.2 Chemistry-Climate Model (CCM) results to investigate the behavior of whole chlorine and related species inside the polar vortex and the boundary region in more detail. &#160;From CCM model results, rapid conversion of chlorine reservoir species (HCl and ClONO2) into Cl2, gradual conversion of Cl2 into Cl2O2, increase of HOCl in winter period, increase of ClO when sunlight became available, and conversion of ClO into HCl, was successfully reproduced. &#160;HCl decrease in the winter polar vortex core continued to occur due to both transport of ClONO2 from the subpolar region to higher latitudes, providing a flux of ClONO2 from more sunlit latitudes into the polar vortex, and the heterogeneous reaction of HCl with HOCl. &#160;Temporal variation of chlorine species over Syowa Station was affected by both heterogeneous chemistries related to Polar Stratospheric Cloud (PSC) occurrence inside the polar vortex, and transport of a NOx-rich airmass from the polar vortex boundary region which can produce additional ClONO2 by reaction of ClO with NO2. &#160;The deactivation pathways from active chlorine into reservoir species (HCl and/or ClONO2) were confirmed to be highly dependent on the availability of ambient O3. &#160;At 18 km where most ozone was depleted, most ClO was converted to HCl. &#160;At 22km where some O3 was available, additional increase of ClONO2 from pre-winter value occurred, similar as in the Arctic.

Download Full-text

Polar processing in a split vortex: early winter Arctic ozone loss in 2012/13

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-15-4973-2015 ◽

2015 ◽

Vol 15 (4) ◽

pp. 4973-5029 ◽

Cited By ~ 3

Author(s):

G. L. Manney ◽

Z. D. Lawrence ◽

M. L. Santee ◽

N. J. Livesey ◽

A. Lambert ◽

...

Keyword(s):

Nitric Acid ◽

Lower Stratosphere ◽

Polar Vortex ◽

The Arctic ◽

Early Winter ◽

Sudden Stratospheric Warming ◽

Ozone Loss ◽

Chlorine Monoxide ◽

Polar Stratospheric Cloud ◽

Vortex Split

Abstract. A sudden stratospheric warming (SSW) in early January 2013 caused the polar vortex to split. After the lower stratospheric vortex split on 8 January, the two offspring vortices – one over Canada and the other over Siberia – remained intact, well-confined, and largely at latitudes that received sunlight until they reunited at the end of January. As the SSW began, temperatures abruptly rose above chlorine activation thresholds throughout the lower stratosphere. The vortex was very disturbed prior to the SSW, and was exposed to much more sunlight than usual in December 2012 and January 2013. Aura Microwave Limb Sounder (MLS) nitric acid (HNO3) data and observations from CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations) indicate extensive polar stratospheric cloud (PSC) activity, with evidence of PSCs containing solid nitric acid trihydrate particles during much of December 2012. Consistent with the sunlight exposure and PSC activity, MLS observations show that chlorine monoxide (ClO) became enhanced early in December. Despite the cessation of PSC activity with the onset of the SSW, enhanced vortex ClO persisted until mid-February, indicating lingering chlorine activation. The smaller Canadian offspring vortex had lower temperatures, lower HNO3, lower hydrogen chloride (HCl), and higher ClO in late January than the Siberian vortex. Chlorine deactivation began later in the Canadian than in the Siberian vortex. HNO3 remained depressed within the vortices after temperatures rose above the PSC existence threshold, and passive transport calculations indicate vortex-averaged denitrification of about 4 ppbv; the resulting low HNO3 values persisted until the vortex dissipated in mid-February. Consistent with the strong chlorine activation and exposure to sunlight, MLS measurements show rapid ozone loss commencing in mid-December and continuing through January. Lagrangian transport estimates suggest ~ 0.7–0.8 ppmv (parts per million by volume) vortex-averaged chemical ozone loss by late January near 500 K (~ 21 km), with substantial loss occurring from ~ 450 to 550 K. The surface area of PSCs in December 2012 was larger than that in any other December observed by CALIPSO. As a result of denitrification, HNO3 abundances in 2012/13 were among the lowest in the MLS record for the Arctic. ClO enhancement was much greater in December 2012 through mid-January 2013 than that at the corresponding time in any other Arctic winter observed by MLS. Furthermore, reformation of HCl appeared to play a greater role in chlorine deactivation than in more typical Arctic winters. Ozone loss in December 2012 and January 2013 was larger than any previously observed in those months. This pattern of exceptional early winter polar processing and ozone loss resulted from the unique combination of dynamical conditions associated with the early January 2013 SSW, namely unusually low temperatures in December 2012 and offspring vortices that remained well-confined and largely in sunlit regions for about a month after the vortex split.

Download Full-text

Influence of transport and mixing in autumn on stratospheric ozone variability over the Arctic in early winter

Atmospheric Chemistry and Physics ◽

10.5194/acp-12-7921-2012 ◽

2012 ◽

Vol 12 (17) ◽

pp. 7921-7930

Author(s):

D. Blessmann ◽

I. Wohltmann ◽

M. Rex

Keyword(s):

Transport Model ◽

Stratospheric Ozone ◽

Vortex Formation ◽

Polar Vortex ◽

The Arctic ◽

Early Winter ◽

Mixing Ratios ◽

Ozone Amount ◽

High Fraction ◽

Transport And Mixing

Abstract. Early winter ozone mixing ratios in the Arctic middle stratosphere show an interannual variability of about 10%. We show that ozone variability in early January is caused by dynamical processes during Arctic polar vortex formation in autumn (September to December). Observational data from satellites and ozone sondes are used in conjunction with simulations of the chemistry and transport model ATLAS to examine the relationship between the meridional and vertical origin of air enclosed in the polar vortex and its ozone amount. For this, we use a set of artificial model tracers to deduce the origin of the air masses in the vortex in January in latitude and altitude in September. High vortex mean ozone mixing ratios are correlated with a high fraction of air from low latitudes enclosed in the vortex and a high fraction of air that experienced small net subsidence (in a Lagrangian sense). As a measure for the strength of the Brewer-Dobson circulation and meridional mixing in autumn, we use the Eliassen-Palm flux through the mid-latitude tropopause averaged from September to November. In the lower stratosphere, this quantity correlates well with the origin of air enclosed in the vortex and reasonably well with the ozone amount in early winter.

Download Full-text

Persistence of ozone anomalies in the Arctic stratospheric vortex in autumn

Atmospheric Chemistry and Physics ◽

10.5194/acp-12-4817-2012 ◽

2012 ◽

Vol 12 (11) ◽

pp. 4817-4823 ◽

Cited By ~ 3

Author(s):

D. Blessmann ◽

I. Wohltmann ◽

R. Lehmann ◽

M. Rex

Keyword(s):

Potential Temperature ◽

Initial Perturbation ◽

Vortex Formation ◽

Polar Vortex ◽

Quantitative Measure ◽

The Arctic ◽

Early Winter ◽

Dynamical Processes ◽

Vertical Range ◽

Formation Phase

Abstract. Dynamical processes during the formation phase of the Arctic stratospheric vortex in autumn (from September to December) can introduce considerable interannual variability in the amount of ozone that is incorporated into the vortex. Chemistry in autumn tends to remove part of this variability because ozone relaxes towards equilibrium. As a quantitative measure of how important dynamical variability during vortex formation is for the winter ozone abundances above the Arctic we analyze which fraction of an ozone anomaly induced during vortex formation persists until early winter (3 January). The work is based on the Lagrangian Chemistry Transport Model ATLAS. In a case study, model runs for the winter 1999–2000 are used to assess the fate of an ozone anomaly artificially introduced during the vortex formation phase on 16 September. In addition, runs with reduced resolution explore the sensitivity of the results to interannual changes in transport, mixing, temperatures and NOx. The runs provide information about the persistence of the induced ozone anomaly as a function of time, potential temperature and latitude. The induced ozone anomaly survives longer inside the polar vortex than outside the vortex. Half of the initial perturbation survives until 3 January at 550 K inside the polar vortex, with a rapid fall off towards higher levels, mainly due to NOx induced chemistry. Above 750 K the signal falls to values below 0.5%. Hence, dynamically induced ozone variability from the early vortex formation phase cannot significantly contribute to early winter variability above 750 K. At lower levels increasingly larger fractions of the initial perturbation survive, reaching 90% at 450 K. In this vertical range dynamical processes during the vortex formation phase are crucial for the ozone abundance in early winter.

Download Full-text

Chlorine partitioning in the lowermost Arctic vortex during the cold winter 2015/2016

10.5194/acp-2019-370 ◽

2019 ◽

Author(s):

Andreas Marsing ◽

Tina Jurkat-Witschas ◽

Jens-Uwe Grooß ◽

Stefan Kaufmann ◽

Romy Heller ◽

...

Keyword(s):

Chemical Evolution ◽

Global Climate ◽

Model Performance ◽

Polar Vortex ◽

Climate Impact ◽

Lagrangian Model ◽

The Arctic ◽

Reservoir Species ◽

Local Measurements ◽

Catalytic Cycles

Abstract. Activated chlorine compounds in the polar winter stratosphere drive catalytic cycles that process ozone and methane, whose abundances are highly relevant to the evolution of global climate. The present work introduces a novel dataset of in situ measurements of relevant chlorine species in the Arctic lowermost stratosphere from the aircraft mission POLSTRACC/GWLCYCLE/SALSA during winter 2015/2016. The major stages of chemical evolution of the lower polar vortex are presented in a consistent series of high resolution mass spectrometric observations of HCl and ClONO2. Simultaneous measurements of CFC-12 are used to derive total inorganic chlorine (Cly) and active chlorine (ClOx). The new data highlight an altitude dependent shift in the pathway of chlorine deactivation through the recovery of the reservoir species from ClONO2 to HCl in the lowermost vortex below the 380 K isentropic surface. Further, we show that the Chemical Lagrangian Model of the Stratosphere (CLaMS) is generally able to reproduce the chemical evolution of the lower polar vortex chlorine budget, except from a bias in HCl concentrations. The model is used to relate local measurements to the vortex-wide evolution. The results are aimed at fostering our understanding of the climate impact of chlorine chemistry, providing new observational data to complement satellite data and assess model performance in the climate sensitive upper troposphere and lower stratosphere region.

Download Full-text

Influence of transport and mixing in autumn on stratospheric ozone variability over the Arctic in early winter

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-12-15083-2012 ◽

2012 ◽

Vol 12 (6) ◽

pp. 15083-15113

Author(s):

D. Blessmann ◽

I. Wohltmann ◽

M. Rex

Keyword(s):

Transport Model ◽

Stratospheric Ozone ◽

Vortex Formation ◽

Polar Vortex ◽

The Arctic ◽

Early Winter ◽

Mixing Ratios ◽

Ozone Amount ◽

High Fraction ◽

Transport And Mixing

Abstract. Early winter ozone mixing ratios in the Arctic middle stratosphere show a fair amount of interannual variability. We show that ozone variability in early January is caused by dynamical processes during Arctic polar vortex formation in autumn (September to December). Observational data from satellites and ozone sondes are used in conjunction with simulations of the Chemistry and Transport Model ATLAS to examine the relationship between the meridional and vertical origin of air enclosed in the polar vortex and its ozone amount. For this, we use a set of artificial model tracers to deduce the origin of the air masses in the vortex in January in latitude and altitude in September. High vortex mean ozone mixing ratios are related to a high fraction of air from low latitudes enclosed in the vortex and a high fraction of air that experienced small net subsidence. As a measure for the strength of the Brewer-Dobson circulation and meridional mixing in autumn, we use the Eliassen-Palm flux through the mid-latitude tropopause averaged from August to November. In the lower stratosphere, this quantity correlates well with both the ozone amount in early winter and the origin of air enclosed in the vortex.

Download Full-text

Ozone loss in middle latitudes and the role of the Arctic polar vortex

Philosophical Transactions of the Royal Society of London Series A Physical and Engineering Sciences ◽

10.1098/rsta.1995.0067 ◽

1995 ◽

Vol 352 (1699) ◽

pp. 241-245 ◽

Cited By ~ 7

Keyword(s):

Lower Stratosphere ◽

Polar Vortex ◽

Middle Latitude ◽

The Arctic ◽

Ozone Loss ◽

Ozone Destruction ◽

Middle Latitudes ◽

Modelling Studies

There have been a number of different suggestions as to the cause of the observed ozone decline over middle latitudes. Here, the particular impact of polar processes on the middle latitude lower stratosphere is discussed. Recent studies suggest that air, recently activated and then torn from the edge of the polar vortex, contributes to the observed ozone decrease. For example, observational and modelling studies both indicate that there is an important role for filaments of vortex air being stripped away from the vortex edge. However, there appears to be little support for the idea of the vortex as a massive ‘flowing processor’ through which large quantities of air, primed for ozone destruction, are transported.

Download Full-text

Impact of the Quasi-Biennial Oscillation on the Northern Winter Stratospheric Polar Vortex in CMIP5/6 Models

Journal of Climate ◽

10.1175/jcli-d-19-0663.1 ◽

2020 ◽

Vol 33 (11) ◽

pp. 4787-4813 ◽

Cited By ~ 4

Author(s):

Jian Rao ◽

Chaim I. Garfinkel ◽

Ian P. White

Keyword(s):

Polar Vortex ◽

The Arctic ◽

Early Winter ◽

Flux Divergence ◽

Quasi Biennial Oscillation ◽

Stratospheric Polar Vortex ◽

Northern Winter ◽

P Flux ◽

The Impact ◽

Biennial Oscillation

AbstractUsing 16 CMIP5/6 models with a spontaneously generated quasi-biennial oscillation (QBO)-like phenomenon, this study investigates the impact of the QBO on the northern winter stratosphere. Eight of the models simulate a QBO with a period similar to that observed (25–31 months), with other models simulating a QBO period of 20–40 months. Regardless of biases in QBO periodicity, the Holton–Tan relationship can be well simulated in CMIP5/6 models with more planetary wave convergence in the polar stratosphere in easterly QBO winters. This wave polar convergence occurs not only due to the Holton–Tan mechanism, but also in the midlatitude upper stratosphere where an Elissen–Palm (E-P) flux divergence dipole (with poleward E-P flux) is simulated in most models. The wave response in the upper stratosphere appears related to changes in the background circulation through a directly excited meridional–vertical circulation cell above the maximum tropical QBO easterly center. The midlatitude upwelling in this anticlockwise cell is split into two branches, and the north branch descends in the Arctic region and warms the stratospheric polar vortex. Most models underestimate the Arctic stratospheric warming in early winter during easterly QBO. Further analysis suggests that this bias is not due to an overly weak response to a given QBO phase, as the models simulate a realistic response if one focuses on similar QBO phases. Rather, the model bias is due to the too-low frequency of strong QBO winds in the lower stratosphere in early winter simulated by the models.

Download Full-text