Atmospheric Structure Observed over the Antarctic Plateau and Its Response to a Prominent Blocking High Event

夏季南极高原大气结构特征及其对强阻塞高压过程的响应

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  • Corresponding author: Libo ZHOU, zhoulibo@mail.iap.ac.cn
  • Funds:

    Supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (XDA19070401), the Second Tibetan Plateau Scientific Expedition and Research (STEP) program (2019QZKK0105), National Natural Science Foundation of China (41830968), and the CAS Key Subordinate Projects (KGFZD-135-16-023 and KFZD-SW-426).

  • doi: 10.1007/s13351-021-1079-x

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  • Studies on the atmospheric structure over the Antarctic Plateau are important for better understanding the weather and climate systems of polar regions. In the summer of 2017, an observational experiment was conducted at Dome-A, the highest station in Antarctica, with a total of 32 profiles obtained from global positioning system (GPS) radiosondes. Based on observational data, the atmospheric temperature, humidity and wind structures and their variations are investigated, and compared with those from four other stations inside the Antarctic circle. Distinguished thermal and dynamic structures were revealed over Dome-A, characterized by the lowest temperature, the highest tropopause, the largest lapse rate and the most frequent temperature and humidity inversion. During the experiment, a prominent blocking event was identified, with great influence on the atmospheric structure over Dome-A. The blocking high produced a strong anticyclone that brought warm and moist air to the hinterland of the Antarctic Plateau, causing a much warmer, wetter and windier troposphere over the Dome-A station. Meanwhile, a polar air mass was forced out of the Antarctic, formed a cold surge extending as far as Southern New Zealand and affected the weather there. Our results proved that there would be a direct interaction between the atmosphere over the hinterland of the Antarctic Plateau and middle latitudes with the action of a blocking high. Further studies are needed to explore the interaction between the atmospheric systems over the Antarctic and middle latitudes under intense synoptic disturbance, with long-term data and numerical modeling.
    我国昆仑站位于南极高原最高点,其大气结构对于正确认识极区大气系统具有特殊意义。本研究基于2017年1月昆仑站的高分辨率探空数据,结合其他四个南极站的同期数据,对夏季南极不同区域大气结构进行对比分析。研究发现,昆仑站上空大气具有独特的热力和动力结构:对流层温度最低,对流层顶最高,温度递减率最大,逆温逆湿最频繁等。观测期间,在一次强阻塞高压过程作用下,中纬度暖湿气流侵入南极高原腹地,对昆仑站上空大气结构产生显著影响,表现为对流层变暖变湿变厚。同时,极地气团也在高压作用下移出南极圈,影响中纬度地区大气。此结果表明,南极内陆与中纬度大气系统可在阻塞高压影响下直接相互作用,相关机理和影响有待进一步研究。
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  • Fig. 1.  Locations of the four radiosonde stations and topography of Antarctica. The Dome-A station is denoted by black triangle, and the other four stations are denoted by the black dot. Each station name is labeled below the marker.

    Fig. 2.  Distribution of the average geopotential height (panel (a), contour), wind vector (panel (a), vectors), temperature (panel (b), shaded) and specific humidity (panel (b), contour) at 500 hPa at 40°−90°S during Jan. 5–20, 2017. The units of geopotential height, wind vector, temperature and specific humidity are gpm, m s-1, °C and g/kg, respectively. The Dome-A station is marked as a black triangle in each panel.

    Fig. 3.  Vertical distributions of air temperature, specific humidity, zonal, and meridional wind speed from the ground surface to 15000 m over Dome-A (a, c, e, g) and over four other Antarctic stations (b, d, f, h), averaged for the observation period from January 5 to 20, 2017. The shaded areas in Fig. 3 (a), (c), (e), and (g) denote one standard deviation.

    Fig. 4.  Daily temperature (a), specific humidity (b), zonal (d) and meridional (d) winds from 4100 m to 15000 m over Dome-A during Jan. 5–20, 2017. Daily tropopause is marked as red dots on Figure 4(a).

    Fig. 5.  Distribution of geopotential height (contour) and temperature (shaded) at 500 hPa in 40°−90°S on January 7–9, 11, and 14, 2017. The intervals of geopotential height and temperature are 50 gpm and 3°C, respectively. The date is printed on the top left of each panel.

    Fig. 6.  Back trajectory of air mass over Dome-A: (a) Air mass arriving on Jan. 7, 2017; (b) Air mass arriving on Jan. 12, 2017; (c) Air mass arriving on Jan. 18, 2017. The back-trajectories date back to 96 hours before, with a time step of 3 h, calculated by the HYSPLIT model using ERA-5 reanalysis data.

    Fig. 7.  The average temperature (a), specific humidity (b), zonal (c) and meridional, (d) wind anomalies from 4000 m to 1500 m over Dome-A during the blocking event (Jan. 8–14, 2017), with respect to the experimental mean (Jan. 5–20, 2017).

    Table 1.  The statistics of near-surface temperature inversions, including the occurrence (P), average inversion depth (∆H), strength (∆T), temperature at 2 m (T2) and wind speed at 10 m (U10) over the Dome-A, South Pole and Dome-C stations during the observation period (Jan. 5–20, 2017)

    StationsP (%)∆H (m)∆T (°C)T2 (°C)U10 (m s−1)
    Dome-A82.12242.5−31.52.2
    South Pole59.42222.9−25.24.3
    Dome-C56.32351.7−26.93.0
    McMurdo0.0% −1.22.5
    Zucchelli0.0% −1.71.4
    Download: Download as CSV

    Table 2.  The statistics of near-surface humidity inversion, including the inversion occurrence (Pq), the average inversion depth (∆Hq), and strength (∆q) over the Dome-A, South Pole, and Dome-C stations during the observation period (Jan. 5–20, 2017)

    StationsPq (%)∆Hq (m)∆q (g/kg)
    Dome-A1002190.16
    South Pole96.92050.22
    Dome-C100 890.08
    McMurdo35.51010.14
    Zucchelli80.0 660.10
    Download: Download as CSV

    Table 3.  The occurrence (P), average depth (∆H), strength (∆T), bottom temperature (Tb), and top temperature (Tt) of temperature inversion over Dome-A during the blocking high event (Jan. 8–14, 2017) and during the whole experiment (Jan. 5–20, 2017)

    PeriodP (%)∆H (m)∆T (oC)Tb (oC)Tt (oC)
    Blocking high76.92152.8−31.9−29.1
    Experiment82.12242.5−33.9−31.3
    Download: Download as CSV

    Table 4.  The occurrence (Pq), average depth (∆Hq), strength (∆q), bottom temperature (qb), and top temperature (qt) of humidity inversion over Dome-A during the blocking high event (Jan 8–14, 2017) and during the whole experiment (Jan 5–20, 2017)

    PeriodPq (%)∆Hq (m)∆q (oC)qb (oC)qt (oC)
    Blocking high100.01770.200.320.52
    Experiment100.02190.160.260.43
    Download: Download as CSV
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Atmospheric Structure Observed over the Antarctic Plateau and Its Response to a Prominent Blocking High Event

    Corresponding author: Libo ZHOU, zhoulibo@mail.iap.ac.cn
  • 1. LAPC/LAOR, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029
  • 2. University of Chinese Academy of Sciences, No.19(A) Yuquan Road, Shijingshan District, Beijing 100049
Funds: Supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (XDA19070401), the Second Tibetan Plateau Scientific Expedition and Research (STEP) program (2019QZKK0105), National Natural Science Foundation of China (41830968), and the CAS Key Subordinate Projects (KGFZD-135-16-023 and KFZD-SW-426).

Abstract: Studies on the atmospheric structure over the Antarctic Plateau are important for better understanding the weather and climate systems of polar regions. In the summer of 2017, an observational experiment was conducted at Dome-A, the highest station in Antarctica, with a total of 32 profiles obtained from global positioning system (GPS) radiosondes. Based on observational data, the atmospheric temperature, humidity and wind structures and their variations are investigated, and compared with those from four other stations inside the Antarctic circle. Distinguished thermal and dynamic structures were revealed over Dome-A, characterized by the lowest temperature, the highest tropopause, the largest lapse rate and the most frequent temperature and humidity inversion. During the experiment, a prominent blocking event was identified, with great influence on the atmospheric structure over Dome-A. The blocking high produced a strong anticyclone that brought warm and moist air to the hinterland of the Antarctic Plateau, causing a much warmer, wetter and windier troposphere over the Dome-A station. Meanwhile, a polar air mass was forced out of the Antarctic, formed a cold surge extending as far as Southern New Zealand and affected the weather there. Our results proved that there would be a direct interaction between the atmosphere over the hinterland of the Antarctic Plateau and middle latitudes with the action of a blocking high. Further studies are needed to explore the interaction between the atmospheric systems over the Antarctic and middle latitudes under intense synoptic disturbance, with long-term data and numerical modeling.

夏季南极高原大气结构特征及其对强阻塞高压过程的响应

我国昆仑站位于南极高原最高点,其大气结构对于正确认识极区大气系统具有特殊意义。本研究基于2017年1月昆仑站的高分辨率探空数据,结合其他四个南极站的同期数据,对夏季南极不同区域大气结构进行对比分析。研究发现,昆仑站上空大气具有独特的热力和动力结构:对流层温度最低,对流层顶最高,温度递减率最大,逆温逆湿最频繁等。观测期间,在一次强阻塞高压过程作用下,中纬度暖湿气流侵入南极高原腹地,对昆仑站上空大气结构产生显著影响,表现为对流层变暖变湿变厚。同时,极地气团也在高压作用下移出南极圈,影响中纬度地区大气。此结果表明,南极内陆与中纬度大气系统可在阻塞高压影响下直接相互作用,相关机理和影响有待进一步研究。
    • Antarctica is the coldest region on Earth, with an area of approximately 1.4 × 107 km2. More than 90% of Antarctica is covered by snow or ice, which can cause distinctive atmospheric thermal and dynamic structures that are different from those over middle and low latitudes. Strong katabatic wind and low-level jets are observed over the Chinese Antarctic Zhongshan Station, located in Prydz Bay and the interior of East Antarctica (Ding et al., 2015). The high boundary layer can be retrieved at Kunlun station, Antarctica, with dry, cold, and stable stratification throughout the layer (Wang et al., 2014). Low-level temperature inversions are frequently observed over Antarctica throughout the year, especially in winter (Connolley, 1996; Tjernström et al., 2005; Tastula and Vihma, 2011; Sterk et al., 2015; Nygård et al., 2017). Near-surface atmospheric humidity inversions also frequently occur over the Antarctic region (Stull, 1988; Bromwich and Liu, 1996; Mahrt, 1999; van den Broeke and van Lipzig, 2003; Seefeldt and Cassano, 2008; Kouznetsov et al., 2013).

      In situ measurements are important for investigating the atmospheric structure over Antarctica. Most of the previous in situ measurements over Antarctica are near-surface observations, accompanied by very few radiosonde observations. For example, only 16 of 120 sites in Antarctica regularly launch radiosondes (Summerhayes, 2008), and most of them are located in low-elevation regions, such as the Antarctic Peninsula and other coastal areas. The atmospheric processes over the high plateau could be significantly different from those at low elevations, which plays an important role in the climate and environment over Antarctica. Wendler and Kodama (1984) analyzed 3-yr surface observations from an automated weather station (AWS) over Dome-C, a top dome in the interior of East Antarctica and compared them with surface observations in the coastal region; the researchers found that the surface temperature is much colder and more stable in the inner Antarctic than in the coastal region, especially during winter. King et al. (2006) compared observed the surface energy budget and boundary layer structure over Dome-C with those over Halley, a coastal station with a latitude similar to that of Dome-C, and found that the diurnal variations in temperature and boundary layer depth in the coastal area were much smaller. Wang et al. (2013) found that there are very strong near-surface inversions inland, while the temperature profile is much straighter over the coast of Antarctica, based on dropsonde observations. Thus, significant differences in near-surface meteorological variables and boundary layer structure can be found between the high- and low-elevation regions of Antarctica, and these differences could be related to the differences in insolation, elevation, terrain slope, and surface thermal properties. In addition, some studies suggested that the local atmospheric processes in the Antarctic region could be affected by large-scale circulation or synoptic processes from lower latitudes (Kejna and Laska, 1999; Braun et al., 2001; Gonzalez et al., 2018; Ambrožová et al., 2020). For example, Braun et al. (2001) found that surface energy fluxes on the King George Island are greatly influenced by large-scale atmospheric conditions. Ambrožová et al. (2020) found that the highest or lowest surface temperature tends to occur when certain types of synoptic patterns occur.

      To investigate the atmospheric structure over the Antarctic Plateau and its relation to synoptic processes at middle latitudes, we conducted an experiment over Kunlun station (Dome-A), the highest Antarctic Plateau, in the summer of 2017. Due to the harsh living and working conditions in Dome-A, such as difficulties in supply, terrain, weather, the experiment could be conducted for only a short period in summer (approximately 16 days) from Jan. 5 to 20, 2017, with a total of 32 profiles obtained from global positioning system (GPS) radiosondes. In this study, the data and methods are described in Section 2. Then, in Section 3, the averaged synoptic conditions and local atmospheric structures over Dome-A are presented based on reanalysis and observational data, and compared with those over four other stations in the Antarctic. The daily variation in the atmospheric structure over Dome-A, and its association with the synoptic disturbance are analyzed and discussed in Section 4. Finally, conclusions are presented in Section 5.

    2.   Data and methods
    • In this study, the observational data are from an experiment conducted at Dome-A station (80°15'00"S, 77°03'36"E, 4093 m) (Figure 1). The station is located at the peak of the Antarctic Plateau, with the surface covered by permanent snow-ice. During the experiment, a GPS sounding system (MW15; Vaisala, Finland) was operated, with 32 radiosonde balloons released twice per day at 0000 and 1200 GMT from January 5 to 20, 2017. Air temperature, relative humidity, wind direction and speed from the ground surface (approximately 4100 m a.s.l) to more than 15000 m were measured, with a vertical resolution of approximately 10 m. All balloons reached altitudes higher than 15 km, which was selected as the top level in this study. For comparison, radiosonde data observed over four other Antarctic stations, with the same observational period and similar data resolutions as those over Dome-A, are selected in this study. The four stations are all inside the 70°S circle, including two plateau stations at high elevation, i.e., the South Pole station (89°59'51"S, 139°16'22"E, 2835 m) and Dome-C station (75°05'59"S, 123°19'56"E, 3233 m), and two coastal stations at low elevation, i.e., the McMurdo station (77°50'53"S, 166°40'06"E, 10 m) and Zucchelli station (74°41'39"S, 164°06'50"E, 15 m).

      Figure 1.  Locations of the four radiosonde stations and topography of Antarctica. The Dome-A station is denoted by black triangle, and the other four stations are denoted by the black dot. Each station name is labeled below the marker.

      In addition to the observational data, ERA-5 reanalysis products from European Centre for Medium-Range Weather Forecasts (ECMWF) are also used in this study, including geopotential height, temperature, specific humidity, and zonal and meridional winds (Hersbach et al., 2020). The reanalysis data have a horizontal resolution of 0.75° × 0.75°, with 37 vertical levels ranging from 1000 hPa to 1 hPa.

      Tropopause height is a critical parameter for atmospheric structure studies. The tropopause is defined as the lowest height when the rate of temperature decrease reaches below 2°C km−1 or less, provided that the average lapse rate between this level and all higher levels within 2 kilometers does not exceed 2°C km−1 (WMO, 1992).

    3.   Average synoptic conditions and local atmospheric structures
    • To better understand the atmosphere over the Dome-A station, we first need to know the average synoptic conditions over the Antarctic and adjacent areas during the experiment. Figure 2 presents the average status of geopotential height, temperature, specific humidity and winds at 500 hPa from 40°S to 90°S during January 5−20, 2017. The geopotential height at 500 hPa over the Antarctic was mostly below 5300 gpm, much lower than those at middle latitudes (Fig. 2a). The sharp contrast in geopential height between the Antarctic and the middle latitudes generated strong westerlies surrounding the polar area, which nearly isolated the atmosphere over the Antarctic from outside. The distribution of temperature and humidity at 500 hPa revealed that the atmosphere over the Antarctic was distinctly colder and drier than those at middle latitudes, with a large gradient occurring on the edge of the Antarctic, which proved the isolation effect of the strong westerlies again. Therefore, under the average synoptic conditions during the experiment, the atmosphere over the Antarctic was hardly involved in exchange with the outside, due to the isolation of the strong circumpolar winds.

      Figure 2.  Distribution of the average geopotential height (panel (a), contour), wind vector (panel (a), vectors), temperature (panel (b), shaded) and specific humidity (panel (b), contour) at 500 hPa at 40°−90°S during Jan. 5–20, 2017. The units of geopotential height, wind vector, temperature and specific humidity are gpm, m s-1, °C and g/kg, respectively. The Dome-A station is marked as a black triangle in each panel.

    • Figure 3 shows the average temperature, specific humidity, and zonal and meridional winds from the ground surface to 15000 m over Dome-A during the experiment, in comparison with those over four other Antarctica stations. The average air temperature decreased from the near-surface upwards, with a lapse rate of 5.91°C km−1 in the troposphere, reached a minimum of -53.9°C at a height of approximately 8800 m, and then increased upwards. Near-surface temperature inversions were found from 4100 m to 4300 m, with an average strength of 0.8°C. The average tropopause height over Dome-A was 9217 m, much higher than those over the other four Antarctic stations. Compared with the other four Antarctica stations, the air temperature over Dome-A exhibited a distinguished structure, with the lowest tropospheric temperature, the highest tropopause height, the largest tropospheric temperature lapse rate and the strongest near-surface temperature inversions. Atmospheric water vapor over Dome-A was most prevalent below 10 km, with a maximum specific humidity of 0.39 g kg−1 at 4300 m. A strong near-surface humidity inversion was clearly seen from 4100 m to 4300 m in the average humidity profile over Dome-A, with a strength of 0.12 g kg−1. In comparison, the air mass over Dome-A was the driest among all the five Antarctica stations. The humidity inversion over Dome-A had a depth and strength similar to those over the South Pole station but much larger than those over the other three Antarctica stations. A weak easterly prevailed in the troposphere over Dome-A, and changed to a weak westerly above the tropopause, with a maximum wind speed of 2.7 m s−1 at a height of 4200 m. The peak value of zonal wind at approximately 4200 m over Dome-A should be associated with the katabatic wind, as mentioned in previous studies (Kobayashi and Yokoyama, 1976; Renfrew and Anderson, 2002; Ding et al., 2015). A northerly dominated the whole troposphere and lower stratosphere over Dome-A, with a maximum wind speed of 5.9 m s−1 near the tropopause, during the experiment. Compared with other stations, the local wind speed over Dome-A is much weaker and exhibits a smaller vertical variation. Therefore, the atmosphere over Dome-A showed distinct properties from those at the other four Antarctic stations during the experiment, including the lowest temperature and specific humidity, the largest lapse rate and relative low wind speed in the troposphere. The atmospheric characteristics over Dome-A should be attributed to its special location, i.e., the peak of the Antarctic Plateau.

      Figure 3.  Vertical distributions of air temperature, specific humidity, zonal, and meridional wind speed from the ground surface to 15000 m over Dome-A (a, c, e, g) and over four other Antarctic stations (b, d, f, h), averaged for the observation period from January 5 to 20, 2017. The shaded areas in Fig. 3 (a), (c), (e), and (g) denote one standard deviation.

      Near-surface temperature and humidity inversions can be found even in the average profiles over Dome-A during the experiment (Fig.3a and Fig.3b), indicating a fairly high occurrence of these two types of inversions. Table 1 presents the temperature inversion characteristics over the five Antarctic stations, including the inversion occurrence frequency, depth and strength. The average occurrence of temperature inversion over the three plateau stations, i.e., Dome-A, South Pole and Dome-C, was 82.1%, 59.4%, and 56.3%, respectively, while those over the two coastal stations (McMurdo and Zucchelli) were both 0.0%. The average depths of temperature inversion over the three plateau stations were close to each other, but the average inversion strength over the Dome-C station was slightly smaller than those over the other plateau stations. The higher occurrence of temperature inversion over the three plateau stations (South Pole, Dome-A, and Dome-C) compared to those over the two coastal stations (McMurdo and Zucchelli), should be related to the different physical properties of the ground surface. The plateau stations were covered by snow/ice causing strong radiative cooling and stable air stratification, while the coastal stations were close to the relatively warm sea surface, with less snow cover, stronger surface heating and more active turbulent mixing (see the 2-m temperature in Table 1). Among the three plateau stations (South Pole, Dome-A, and Dome-C), the Dome-A station is located at the peak of the Antarctic Plateau, with the most stable stratification and the weakest wind near the surface, while the other two stations are located at the steep slope of the plateau with stronger wind (Table 1). Previous studies found that the strength of the temperature inversion in polar regions was much smaller on days with strong wind than those on calm days, because strong winds could enhance turbulent mixing and weaken the inversion (Van de Wiel et al., 2017; Vignon et al., 2017; Baas et al., 2019). Therefore, the high occurrence of temperature inversion over Dome-A could be attributed to surface physical properties, topography, and near-surface meteorological conditions.

      StationsP (%)∆H (m)∆T (°C)T2 (°C)U10 (m s−1)
      Dome-A82.12242.5−31.52.2
      South Pole59.42222.9−25.24.3
      Dome-C56.32351.7−26.93.0
      McMurdo0.0% −1.22.5
      Zucchelli0.0% −1.71.4

      Table 1.  The statistics of near-surface temperature inversions, including the occurrence (P), average inversion depth (∆H), strength (∆T), temperature at 2 m (T2) and wind speed at 10 m (U10) over the Dome-A, South Pole and Dome-C stations during the observation period (Jan. 5–20, 2017)

      Table 2 presents the characteristics of humidity inversion over the five stations in the Antarctic during the experiment. The average occurrence of humidity inversion was 100.0%, 96.9%, and 100.0% over the three plateau stations, Dome-A, South Pole, and Dome-C, respectively. Apparently, the three plateau stations all had very high occurrences of humidity inversion. However, the situations over the two coastal stations differed much, although they were located close to each other. The average occurrence of humidity over Zucchelli was 80% during the experiment, while that over McMurdo was only 35.5% during the experiment. The average depths of humidity inversion over Dome-A and South Pole were more than 200 m, much larger than those over the other three stations. The average strength of humidity over South Pole was the largest (0.22 g/kg), and that over Dome-A was the second largest (0.16 g/kg). Previous studies showed that a large portion of humidity inversions in the polar regions were accompanied by temperature inversions (Nygård et al., 2013; Naakka et al., 2018). On the other hand, humidity inversion could probably be generated by warm and moist air advection in coastal areas in polar regions (Nygård et al., 2013; Naakka et al., 2018; Gorodetskaya et al., 2020). Therefore, the occurrence of humidity inversion at the five stations could be influenced by many factors, such as temperature inversion, local distribution of humidity and winds, which requires detailed analysis in further investigation.

      StationsPq (%)∆Hq (m)∆q (g/kg)
      Dome-A1002190.16
      South Pole96.92050.22
      Dome-C100 890.08
      McMurdo35.51010.14
      Zucchelli80.0 660.10

      Table 2.  The statistics of near-surface humidity inversion, including the inversion occurrence (Pq), the average inversion depth (∆Hq), and strength (∆q) over the Dome-A, South Pole, and Dome-C stations during the observation period (Jan. 5–20, 2017)

      Additionally, we compared the atmospheric thermal and moisture structures over Dome-A at 0000 GMT and 1200 GMT during the experiment. In general, the near-surface temperature and humidity inversions have a higher occurrence frequency and larger inversion depth and strength at 0000 GMT than at 1200 GMT, which was related to the strong surface cooling at midnight or early morning over the Dome-A station. For example, the near-surface temperature inversion occurred each day at 0000 GMT (with an occurrence frequency of 100%) but only had a 64.3% occurrence at 1200 GMT. Although the solar heating persists throughout the day in polar regions during the summer season, near-surface radiation, and temperature also shows distinct diurnal variation, especially on clear sky days (Kodama et al., 1989; Sodemann and Foken, 2005; Wang and Zender, 2011; Pietroni et al., 2014). According to Wang and Zender’s (Wang and Zender, 2011) research, there are two causes for the diurnal variation in near-surface temperature: the first is the diurnal variation in solar radiation due to the varying solar zenith angle, and the second is the diurnal variation in surface albedo associated with the solar azimuth angle and local topographic features. The average difference in the occurrence of the temperature inversion over Dome-A at the two times should be attributed to these diurnal variations at the near-surface, as the average 2-m temperature was much higher at 1200 GMT (−28.3°C) than that at 00:00 GMT (−32.4°C) during the experiment, owing to the different solar heating at the two times.

      The averaged inversion depth and strength reach 245 m and 5.5°C at 0000 GMT and 203 m and 5.2°C at 1200 GMT, respectively. For the near-surface humidity inversion, the occurrence frequency is the same at both times, and the inversion depth and strength are 278 m and 0.18 g/kg at 0000 GMT and 157 m and 0.15 g/kg at 1200 GMT, respectively. The humidity inversion was hardly influenced by the solar heating, thus showing little diurnal variation.

    4.   Impact of synoptic disturbance
    • The daily temperature, specific humidity, and zonal and meridional winds from 4100 m to 15000 m over Dome-A during the experiment are presented in Figure 4. The daily tropospheric temperature showed a low-high-low variation during Jan. 5–20, with two valleys occurring on Jan. 8 and Jan. 18, and a peak occurring on Jan. 12 (Figure 4a). The tropopause varied with the tropospheric temperature, i.e., fell with the decreased temperature and rose with the increased temperature. Similar to temperature, the daily specific humidity over Dome-A also showed a low-high-low variation, with a peak value occurring on Jan. 12. The daily zonal wind over Dome-A was weak westerlies most of the time during the experiment, except on Jan. 5 and during Jan. 9–12, the zonal wind was strong easterlies and strong westerlies, respectively. The daily meridional wind also exhibited great fluctuation during January 9–12, with anomalous strong northerlies occurring in the middle and upper troposphere. Therefore, the atmospheric structure over Dome-A showed significant daily variation during the experiment, especially on January 9–12, when the temperature, humidity, zonal and meridional winds all exhibited great abnormalities.

      Figure 4.  Daily temperature (a), specific humidity (b), zonal (d) and meridional (d) winds from 4100 m to 15000 m over Dome-A during Jan. 5–20, 2017. Daily tropopause is marked as red dots on Figure 4(a).

    • To determine the cause of the drastic change in the atmospheric structure over Dome-A from Jan. 9–12, 2017, the synoptic situations during the experiment are obtained and analyzed based on the ERA-5 reanalysis data. Figure 5 shows the distribution of geopotential height and temperature at 500 hPa at 40°−90°S on January 7–9, 11 and 14, 2017. On Jan. 7, the distribution of geopotential height at 500 hPa over 40°−90°S was similar to that of the average status (see Fig. 2) during the experiment, with geopotential height in the Antarctic much lower than that at middle latitude (Fig. 5a). The temperature over the Antarctic was also distinctly lower than that of the outside. As mentioned earlier, the large gradient of geopotential height on the border of middle and high latitudes would produce strong westerlies surrounding the Antarctic, and isolate the air inside from the outside, thus keeping the air cold and dry within the polar circle. On Jan. 8, a high ridge formed to the southeast of Dome-A, with two low-pressure centers on the left and right (Fig. 5b). Accompanied by the high ridge, a warm air mass with a temperature above -20°C from middle latitudes invaded the Antarctic, generating a distinct warm tongue toward the Antarctic Plateau. The atmospheric structure over Dome-A began to change under the influences of the high ridge and the warm tongue, as shown in Fig. 4. On Jan. 9, the high-pressure system moved southward and developed into a prominent blocking high over the Antarctic continent, with two cut-off low to its lower left and right (Fig. 5c). The geopotential height at the center of the blocking high was higher than 5350 m, while those at the center of the two cutoff low were lower than 5050 m. As indicated by the geostrophic wind relationship, a powerful anticyclone was generated around the blocking high, causing strong northerlies over Dome-A, which was observed by the radiosonde (see Fig. 4c). Driven by the intense southeasterly between the blocking high and the cutoff low on the right side, the warm tongue continued moving toward the Antarctic Plateau and approached Dome-A. The tropospheric temperature and humidity increased rapidly under the influence of the warm tongue, as shown in Fig. 4a and 4b. On Jan. 11, the blocking ridge and the warm tongue moved to the south of Dome-A, both extending from north to south (Fig. 5d). At this point, the influence of the blocking high on the atmosphere over Dome-A reached a maximum, causing drastic changes in the troposphere there, such as sharply risen tropospheric temperature, an intense tropospheric westerly and a highly elevated tropopause, as shown in Fig. 4. On Jan. 14, the high ridge became weak and moved to the northwest of Dome-A. The warm tongue still existed but with much smaller strength (Fig. 5e). From this day on, the atmospheric structure over Dome-A gradually returned to normality, as shown in Fig. 4. Therefore, the analysis of the synoptic situation confirmed that the strong variation in atmospheric structure over Dome-A during the experiment was caused by a prominent blocking high over eastern Antarctica.

      Figure 5.  Distribution of geopotential height (contour) and temperature (shaded) at 500 hPa in 40°−90°S on January 7–9, 11, and 14, 2017. The intervals of geopotential height and temperature are 50 gpm and 3°C, respectively. The date is printed on the top left of each panel.

      To further confirm that the warm and wet air mass from middle latitudes could reach deep to the hinterland of the Antarctic Plateau under the influence of the blocking high, a back-trajectory method was applied to trace the air mass source over Dome-A. The back-trajectory is calculated by the HYSPLIT model (Stein et al., 2015; https://www.arl.noaa.gov/hysplit/), using the ERA-5 reanalysis data. Each back trajectory is calculated backward from 0 to -96 h, with a time step of 3 h. For comparison, the air mass trajectories on Jan. 7, 12, and 18, 2017, are also presented in Fig. 6. On Jan. 7, 2017, the air mass over Dome-A came from the edge of the Antarctic continent (north of Wilkers Land), which is cold and dry. On Jan. 12, 2017, the air mass over Dome-A originated from the Southern Pacific (north of 60°S), with an altitude of approximately 1500 m a.s.l. (see the altitude variations in Fig. 6b), which is relatively warm and wet. On Jan. 18, 2017, the air mass at Dome-A came from Wilkers Land on the Antarctic continent (within 70°S), with cold and dry conditions. The air mass on Jan. 12 was from the middle latitudes, while the air masses on Jan. 7 and Jan. 18 were from the local Antarctic atmosphere. From Fig. 6b, an air mass originated over the Southern Pacific (with a lower altitude of approximately 1500 m) on Jan. 8, 2017, moved westwards to the edge of the Antarctic continent on Jan. 10, 2017, entered eastern Antarctica and arrived at the Dome-A region on Jan. 12, 2017. The air mass passage was consistent with the evolution of synoptic situations in Fig. 5, which verified the air flow intrusion from middle latitudes to inner Antarctica during Jan. 8–14, 2017.

      Figure 6.  Back trajectory of air mass over Dome-A: (a) Air mass arriving on Jan. 7, 2017; (b) Air mass arriving on Jan. 12, 2017; (c) Air mass arriving on Jan. 18, 2017. The back-trajectories date back to 96 hours before, with a time step of 3 h, calculated by the HYSPLIT model using ERA-5 reanalysis data.

      Previous studies found that blocking high events at high latitudes have a notable effect on the weather and climate at middle latitudes in Southern Hemisphere, such as causing persistent frost and modulating precipitation in Oceania and Southern America (Müller and Berri, 2007; Mendes et al., 2008; Tozer et al., 2018; Risbey et al., 2019). In Fig. 5c, at the same time as the warm tongue stretched into the Antarctic Plateau, a cold air mass was forced out of the polar circle by the strong southerlies on the left side of the blocking high. Therefore, the blocking high not only drove the warm and humid air to the Antarctic, but also pushed the cold and dry air out of the polar circle in the meantime. Consequently, the polar air mass pushed out would greatly impact the weather and climate at middle latitudes. In Fig. 5d, the cold surge moved as far as to southern New Zealand, causing decreased tropospheric temperature and tropopause height there (figure omitted).

      Based on the above analysis and discussion, we recognize that under the influence of a blocking high, the atmospheric system over the hinterland of the Antarctic Plateau could build direct connection to that at middle latitudes. The blocking high provides an opportunity for interaction between the atmospheric systems in and out of the southern polar circle, with thousands of kilometers distance apart, thus greatly impacting the weather and climate at high and middle latitudes on Southern Hemisphere.

    • To further illustrate the impact of the blocking high event, we compared the average atmospheric properties over Dome-A during January 8–14, with the average of the experiment. Figure 7 shows the temperature, specific humidity, and zonal and meridional wind anomalies during the blocking high event, with respective to the experimental mean. During the blocking high event, the temperature was approximately 1°C higher than the experimental mean in the whole troposphere (below 8000 m) over Dome-A, but colder than the experimental mean above troposphere (Fig. 7a). The temperature anomaly above the troposphere may be attributed to the planetary wave anomaly associated with the blocking high in the stratosphere, as pointed out in previous studies (Zou and Huang, 1989; Colucci, 2001; Nath et al., 2014). Impacted by the air intrusion driven by the blocking high from lower latitudes, the specific humidity over Dome-A was 0.05 g/kg higher than the experimental mean in the lower troposphere, i.e., approximately 16% higher than the experimental mean (Fig. 3c and Fig. 7b). Additionally, the anticyclone caused by the blocking high brought much stronger westerlies and northerlies over Dome-A than the experimental mean, as shown in Fig. 7c and 7d. Therefore, under the influence of the blocking event, the atmospheric structure undergoes significant changes, including greatly increased tropospheric temperature, specific humidity, westerlies and northerlies.

      Figure 7.  The average temperature (a), specific humidity (b), zonal (c) and meridional, (d) wind anomalies from 4000 m to 1500 m over Dome-A during the blocking event (Jan. 8–14, 2017), with respect to the experimental mean (Jan. 5–20, 2017).

      Since the tropospheric temperature and humidity increased greatly over Dome-A in the blocking high event, we can guess that the temperature and humidity inversion may also be changed. Tables 3 and 4 shows the characteristics of the temperature and humidity inversions over Dome-A during the blocking high event and the whole experiment. The occurrence, average depth and strength of the temperature inversion over Dome-A during the blocking high event were nearly the same as those during the experiment. The bottom and top temperature of the inversion layer rose about 2°C during the blocking high, indicating that the air intrusion evenly warmed the air over Dome-A in the lower troposphere. For the humidity inversion, the average depth during the blocking high event was smaller than the experimental average, but the average strength during the blocking high event was larger. During the blocking high event, the specific humidity at the top of the inversion layer increased much more than that at the bottom, thus causing stronger humidity inversion. This indicated that the humidity advection by the air intrusion was not evenly distributed vertically in the lower troposphere over Dome-A. In addition, the bottom height of the humidity inversion increased, and the top height decreased during the blocking high event, leading to a thinner inversion depth. Therefore, the temperature inversion over Dome-A experienced little change except for the temperature increase during the blocking high event, while the humidity inversion became stronger but thinner during the event.

      PeriodP (%)∆H (m)∆T (oC)Tb (oC)Tt (oC)
      Blocking high76.92152.8−31.9−29.1
      Experiment82.12242.5−33.9−31.3

      Table 3.  The occurrence (P), average depth (∆H), strength (∆T), bottom temperature (Tb), and top temperature (Tt) of temperature inversion over Dome-A during the blocking high event (Jan. 8–14, 2017) and during the whole experiment (Jan. 5–20, 2017)

      PeriodPq (%)∆Hq (m)∆q (oC)qb (oC)qt (oC)
      Blocking high100.01770.200.320.52
      Experiment100.02190.160.260.43

      Table 4.  The occurrence (Pq), average depth (∆Hq), strength (∆q), bottom temperature (qb), and top temperature (qt) of humidity inversion over Dome-A during the blocking high event (Jan 8–14, 2017) and during the whole experiment (Jan 5–20, 2017)

    5.   Conclusions
    • In January 2017, an observational experiment was conducted at the Dome-A station, the highest station in Antarctica, with a total of 32 profiles obtained from GPS radiosonde measurements. Based on the observational data, the atmospheric thermal and dynamic structures over Dome-A are analyzed and compared with those over the other stations inside the Antarctic circle. During the experiment, large daily fluctuations in the atmospheric thermal and dynamic structure were observed over the Dome-A station. Synoptic analysis confirmed that the daily variation in atmospheric structure over Dome-A was closely associated with a prominent blocking high event. The conclusions of this study are described as follows:

      1) The atmospheric structure over Dome-A showed properties distinct from those at the other four Antarctic stations during the observational experiment, including the lowest tropospheric temperature and specific humidity, and the highest occurrence of temperature and humidity inversions among the five stations. These structural characteristics could be related to the special location and ground surface physical properties of the Dome-A stations.

      2) During the experiment, strong daily variation was observed in atmospheric structure over Dome-A. The synoptic analysis reveals that intense fluctuations in tropospheric temperature, humidity and winds were closely associated with a blocking high event. A prominent blocking occurred over eastern Antarctica during Jan. 8–14, 2017, causing strong air intrusion from middle latitudes to the Antarctic Plateau, and making the troposphere over Dome-A much warmer, wetter and windier than normal.

      Additionally, analysis shows that the blocking high not only produced air intrusion from lower latitudes to the Antarctic, but also forced a polar air mass out of the polar circle and impacted the atmospheric system at middle latitudes in the Southern Hemisphere. This indicates that under the impact of a prominent blocking high, the atmospheric system over the hinterland of the Antarctic Plateau could have direct interaction with that at middle latitudes. Further studies are needed to explore the interactions between the atmosphere over the Antarctic and middle latitudes under intense synoptic disturbance, with long-term data and numerical modeling.

      Acknowledgments. We appreciate having access to the ECMWF ERA-5 datasets through the Climate Data Store (https://cds.climate.copernicus.eu). The authors appreciate all the researchers for their hard work in attending the 33rd China Antarctica Expedition Team. We also thank the NOAA Air Resources Laboratory for their user-friendly HYSPLIT model.

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