Trends in Atmospheric Humidity and Temperature above Dome C, Antarctica Evaluated from Observations and Reanalyses

: The time evolution of humidity and temperature above Dome C (Antarctica) has been investigated by considering data from (1) meteorological radiosondes (2005–2017), (2) the microwave radiometer HAMSTRAD (2012–2017), (3) four modern meteorological reanalyses (1980–2017) and (4) the southern annular mode (SAM) index (1980–2017). From these observations (2005–2017), a signiﬁcant moistening trend (0.08 ± 0.06 kg m − 2 dec − 1 ) is associated with a signiﬁcant warming trend (1.08 ± 0.55 K dec − 1 ) in summer. Conversely, a signiﬁcant drying trend of − 0.04 ± 0.03 kg m − 2 dec − 1 ( − 0.05 ± 0.03 kg m − 2 dec − 1 ) is associated with a signiﬁcant cooling trend of − 2.4 ± 1.2 K dec − 1 ( − 5.1 ± 2.0 K dec − 1 ) in autumn (winter), with no signiﬁcant trends in the spring. We demonstrate that 1) the trends identiﬁed in the radiosondes (2005–2017) are also present in the reanalyses and 2) the multidecadal variability of integrated water vapor and near-surface temperature (1980–2017) is strongly inﬂuenced by variability in the SAM index for all seasons but spring. Our study suggests that the decadal trends observed in humidity and near-surface temperature at Dome C (2005–2017) reﬂect the multidecadal variability of the atmosphere, and are not indicative of long-term trends that may be related to global climate change.


Introduction
The evolution of the Antarctic climate during the second half of the 20th century and the beginning of the 21st century has been intensively investigated, because it directly affects the Antarctic region and indirectly influences the Earth's geo-biophysical system. Based on surface observations, space-borne measurements, meteorological analyses and reanalyses, and climate model output, two essential points can be drawn. (1) One of the largest and most rapid warmings recorded on Earth is occurring over the Antarctic Peninsula (Western Antarctica), with values of~0.5 • C dec −1 since 1950 [1], although the absence of warming during the early 21st century seems consistent with the natural variability of the atmosphere [2,3]. (2) Over the Antarctic Plateau (Eastern Antarctica), the temperature trends depend upon the season, with significant cooling in autumn and significant warming in spring [4]. More generally, temperature trends over Antarctica have been linked to changes in the tropics

Data and Method
In order to study the time evolution of IWV, temperature and H 2 O over Dome C, we employed observations made by (1) the microwave radiometer HAMSTRAD installed in 2009 operating at 60 and 183 GHz [31], which provides temperature vertical profiles and IWV with 7-min time resolution from 2012 to 2017; and (2) the meteorological radiosondes launched every day at 12:00 UTC, to obtain temperature and H 2 O vertical profiles (from which IWV was computed) from 2005 to 2017. An overview describing the temperature and H 2 O vertical profiles, as well as the IWV measured by HAMSTRAD and radiosondes at Dome C, shows that HAMSTRAD IWV estimates (1) are of excellent quality, with a Pearson linear correlation coefficient R against radiosonde IWV higher than 0.98; and (2) are wetter than radiosondes in 2010-2014 by about 12% (from 0.035 kg m −2 in winters (June, July, August-JJA) to 0.092 kg m −2 in summers (December, January, February-DJF)) [32]. It has also been shown that radiosonde observations tend to be biased cold and dry in dry and cold environments [33]. In order to assess the trends and the correlation of some parameters (H 2 O and temperature) along the vertical, we have systematically interpolated each individual radiosonde profile to the 38 HAMSTRAD retrieval vertical levels between 10 and 10,000 m above ground level (see [31]). Below 1000 m, the levels are at: 10,30,50,75,100,125,150,200,250,325,400,475,550, 625, 700, 800, 900 and 1000 m. Note that all the results presented in the following section will be provided at these heights above ground level.
The SAM is the leading mode of variability in Southern Hemisphere atmospheric circulation on month-to-month and interannual timescales. The phase of the SAM is determined by the position of the westerly winds that surround the Antarctic continent, and the occurrence of high or low pressure over Southern Australia. SAM variability has a large impact on Antarctic surface temperatures, ocean circulation, and many other aspects of the Southern Hemisphere climate (see e.g., [9]). Therefore, we can expect that the trends in humidity above Dome C are influenced by the trends in SAM. In our study, the SAM index has been computed according to [30].
The calculation of the linear trends in IWV, near-surface temperature and SAM index and their associated (±1σ) standard deviations [34], were performed for different periods, due to data availability: HAMSTRAD observations over 2012-2017, radiosonde observations over 2012-2017 and 2005-2017 and reanalyses and the SAM index over 1980-2017. In order to obtain trends that can be quantitatively compared, at least in the 21st century, we have considered decadal trends for the entire article.
To calculate decadal trends, the observed, linear, yearly trends evaluated over the periods 2005-2017 or 2012-2017 have been multiplied by a factor 10 to attain decadal trends in IWV (kg m −2 dec −1 ) and in 10-m temperature (K dec −1 ) as seasonal or annual averages (Section 3.2). Then, in order to highlight the multidecadal variability of IWV, near-surface temperature, and the SAM index over the 38-year period, we used a 10-year moving window, within which we calculated the linear decadal trends and their associated (±1σ) errors, from 1980 to 2017, as seasonal and annual averages (Section 3.3). Finally, we assumed that a decadal trend is statistically significant when, in absolute value, it is greater than its 1-σ error (Sections 3.2 and 3.3). Section 3.2 presents the observed decadal trends in temperature and water vapor in the early 21st century. Section 3.3 considers both the multidecadal trends and variability in near-surface temperature and IWV from reanalyses and in the SAM index over 1980-2017, to be compared with the decadal trends in Section 3.2, and to assess whether the observed decadal trends in the 21st century are related to the long-term trends of the atmosphere, or simply reflect the multidecadal variability of the atmosphere.

Seasonal Variations of IWV, Temperature and H 2 O from 2005-2017
We first calculate the decadal trends in humidity and temperature using the available observational data sets at Dome C. The daily-averaged IWV from HAMSTRAD and radiosondes over the period 2009-2017 and 2005-2017, respectively, are presented in Figure 1. Similarly, the daily-averaged 10-m temperatures from HAMSTRAD and radiosondes over the period 2009-2017 and 2005-2017, respectively, are shown in Figure 2.

Seasonal Variations of IWV, Temperature and H2O from 2005-2017
We first calculate the decadal trends in humidity and temperature using the available observational data sets at Dome C. The daily-averaged IWV from HAMSTRAD and radiosondes over the period 2009-2017 and 2005-2017, respectively, are presented in Figure 1. Similarly, the dailyaveraged 10-m temperatures from HAMSTRAD and radiosondes over the period 2009-2017 and 2005-2017, respectively, are shown in Figure 2.  Over the periods considered, the annual cycle indicates a dry (~0.1 kg m −2 ) and cold (~205 K) winter in JJA, followed by a moist (~0.6-0.8 kg m −2 ) and warm (~250 K) summer in DJF, consistent with numerous studies (e.g., [15,35]). In summertime, the atmosphere can be very humid, with observed IWV reaching daily averages greater than 1.0 kg m −2 , and sometimes as high as 1.7 kg m −2 , such as in 2010-2011, 2013-2014 and 2016-2017. The atmosphere was also observed to be occasionally quite warm (~255 K), such as in 2010-2011 and 2013-2014. We note that (1) the IWV intra-seasonal variability is much greater in summer (standard deviation of ±0.5 kg m −2 ) than in winter (±0.2 kg m −2 ); Atmosphere 2020, 11, x FOR PEER REVIEW 4 of 22

Seasonal Variations of IWV, Temperature and H2O from 2005-2017
We first calculate the decadal trends in humidity and temperature using the available observational data sets at Dome C. The daily-averaged IWV from HAMSTRAD and radiosondes over the period 2009-2017 and 2005-2017, respectively, are presented in Figure 1. Similarly, the dailyaveraged 10-m temperatures from HAMSTRAD and radiosondes over the period 2009-2017 and 2005-2017, respectively, are shown in Figure 2.  Over the periods considered, the annual cycle indicates a dry (~0.1 kg m −2 ) and cold (~205 K) winter in JJA, followed by a moist (~0.6-0.8 kg m −2 ) and warm (~250 K) summer in DJF, consistent with numerous studies (e.g., [15,35]). In summertime, the atmosphere can be very humid, with observed IWV reaching daily averages greater than 1.0 kg m −2 , and sometimes as high as 1.7 kg m −2 , such as in 2010-2011, 2013-2014 and 2016-2017. The atmosphere was also observed to be occasionally quite warm (~255 K), such as in 2010-2011 and 2013-2014. We note that (1) the IWV intra-seasonal variability is much greater in summer (standard deviation of ±0.5 kg m −2 ) than in winter (±0.2 kg m −2 ); Over the periods considered, the annual cycle indicates a dry (~0.1 kg m −2 ) and cold (~205 K) winter in JJA, followed by a moist (~0.6-0.8 kg m −2 ) and warm (~250 K) summer in DJF, consistent with numerous studies (e.g., [15,35]). In summertime, the atmosphere can be very humid, with observed IWV reaching daily averages greater than 1.0 kg m −2 , and sometimes as high as 1.7 kg m −2 , such as in 2010-2011, 2013-2014 and 2016-2017. The atmosphere was also observed to be occasionally quite warm (~255 K), such as in 2010-2011 and 2013-2014. We note that (1) the IWV intra-seasonal variability is much greater in summer (standard deviation of ±0.5 kg m −2 ) than in winter (±0.2 kg m −2 ); and (2) the temperature intra-seasonal variability is greater in winter (±10 K) than in summer (±5 K). The intra-seasonal changes in temperature and H 2 O at 10 m were consistent with each other seasonally (i.e., the lowest and highest variability was observed on the same season for both variables). However, the 10-m temperature and IWV did not demonstrate this consistency as the intra-seasonal variability of IWV is mainly governed by the H 2 O variability around 100-200 m, where the maximum in water vapor occurs. The annual cycle of IWV is highly correlated to that of temperature, which is not surprising since [36] already underlined the high correlation coefficient between H 2 O and temperature below 800 m altitude, with R varying from 0.70 to 0.95, depending on the season considered. On average, the atmosphere is slightly wetter in HAMSTRAD observations than in the radiosonde data by 0.05-0.10 kg m −2 (8-15%). This difference in the observations is related to the well-documented dry bias of the radiosondes already observed and commented on in [33]. Note that this bias has not been removed in the present study. The high variability in the summertime IWV can be partially caused by the long-range transport of warm and wet air masses originating from the mid-latitudes, which is more frequent in summer than in winter [21]. In Antarctica, near-surface observations of temperature and water vapor are more accessible than vertical profiles. It is thus crucial to verify whether the time evolution of IWV can be derived from the time evolution of near-surface temperature and/or water vapor.
In order to lessen the intra-seasonal variability of the data, we seasonally-averaged the radiosonde observations  and (2) the temperature intra-seasonal variability is greater in winter (±10 K) than in summer (±5 K). The intra-seasonal changes in temperature and H2O at 10 m were consistent with each other seasonally (i.e., the lowest and highest variability was observed on the same season for both variables). However, the 10-m temperature and IWV did not demonstrate this consistency as the intra-seasonal variability of IWV is mainly governed by the H2O variability around 100-200 m, where the maximum in water vapor occurs. The annual cycle of IWV is highly correlated to that of temperature, which is not surprising since [36] already underlined the high correlation coefficient between H2O and temperature below 800 m altitude, with R varying from 0.70 to 0.95, depending on the season considered. On average, the atmosphere is slightly wetter in HAMSTRAD observations than in the radiosonde data by 0.05-0.10 kg m −2 (8-15%). This difference in the observations is related to the well-documented dry bias of the radiosondes already observed and commented on in [33]. Note that this bias has not been removed in the present study. The high variability in the summertime IWV can be partially caused by the long-range transport of warm and wet air masses originating from the mid-latitudes, which is more frequent in summer than in winter [21]. In Antarctica, near-surface observations of temperature and water vapor are more accessible than vertical profiles. It is thus crucial to verify whether the time evolution of IWV can be derived from the time evolution of nearsurface temperature and/or water vapor. In order to lessen the intra-seasonal variability of the data, we seasonally-averaged the radiosonde observations    Secondly, although summertime H2O at 10 m is close to maximum (~0.20 g m −3 ), for all other seasons the atmosphere is very dehydrated with 10-m H2O of 0.02-0.06 g m −3 , or roughly 50-70% less than the peak value. This is consistent with a surface layer ( Figure 6) that is much colder in winter (216 K) than in summer (241 K). Consequently, and contrarily to what is usually utilized in the tropics, the trend in 10-m H2O in the Eastern Plateau of Antarctica at Dome C cannot be considered as a good proxy for the variation of IWV in the atmosphere. The near-surface measurement of water vapor is thus of limited interest in the region when studying the water cycle. In contrast, the saturation vapor pressure increases with temperature, and thus the water-holding capacity of air correlates with temperature (see e.g., [36]). Combining this thermodynamic relationship between H2O and temperature with the RT-IWV values at 10 m that are, on average, higher than the RH2O-IWV values at 10 m, it can be seen that the near-surface temperature can be considered as a better proxy of the time evolution of IWV in the Dome C atmosphere than the near-surface water vapor. , or roughly 50-70% less than the peak value. This is consistent with a surface layer ( Figure 6) that is much colder in winter (216 K) than in summer (241 K). Consequently, and contrarily to what is usually utilized in the tropics, the trend in 10-m H 2 O in the Eastern Plateau of Antarctica at Dome C cannot be considered as a good proxy for the variation of IWV in the atmosphere. The near-surface measurement of water vapor is thus of limited interest in the region when studying the water cycle. In contrast, the saturation vapor pressure increases with temperature, and thus the water-holding capacity of air correlates with temperature (see e.g., [36]). Combining this thermodynamic relationship between H 2 O and temperature with the R T-IWV values at 10 m that are, on average, higher than the R H2O-IWV values at 10 m, it can be seen that the near-surface temperature can be considered as a better proxy of the time evolution of IWV in the Dome C atmosphere than the near-surface water vapor.

Decadal Trends of IWV and 10-m Temperature
For Table 1, we have calculated the linear decadal trends in IWV (kg m −2 dec −1 ) and 10-m temperature (K dec −1 ) from HAMSTRAD observations over the period 2012-2017, and from radiosonde observations over the periods 2012-2017, to align with HAMSTRAD data availability, and 2005-2017, for the entire radiosonde record. The decadal trends in humidity above Dome C in the beginning of the 21st century demonstrate strong seasonality. In all periods and datasets considered, the atmosphere moistens in summer and spring, while it dries in winter and autumn, although the trends are not always statistically significant. The moistening trend is larger in summer (0.04-0.09 kg m −2 dec −1 ) than in spring (0.01-0.02 kg m −2 dec −1 ) within both observational datasets. The moistening trend is only significant in summer for the radiosonde data over the period 2005-2017 (0.08 ± 0.06 kg m −2 dec −1 ). The drying trend is of the same order of magnitude in winter and autumn (from −0.04 to −0.08 kg m −2 dec −1 ) in both datasets, but is only significant in the radiosonde data in winter (−0.05 ± 0.03 kg m −2 dec −1 ) and autumn (−0.04 ± 0.03 kg m −2 dec −1 ) from 2005-2017, and in autumn only (−0.08 ± 0.07 kg m −2 dec −1 ) from 2012-2017. Note that, in the HAMSTRAD data set, no significant moistening or drying trend is found, probably due to 1) the short period under consideration (6 years) producing a 1-σ error much larger than those in the data with longer time period (13 years); and 2) for the 6-year time period, a 1-σ error in the IWV measurements that is 1-2 times larger in the HAMSTRAD data than in the radiosonde observations. If we now consider the annual average IWV (column labelled as 'Year' in Table 1), the linear trends are only significant in the radiosonde data over the shortest period 2012-2017: −0.07 ± 0.05 kg m −2 dec −1 . Table 1. Decadal trends in IWV (kg m −2 dec −1 ) and in 10-m temperature (K dec −1 ) along with 1-σ error for HAMSTRAD ('HAMS') and radiosondes ('RS'), based either on seasonally-or annually-averaged data over 2012-2017, and over 2005-2017 for radiosondes only. Significant trends are highlighted in green. As expected from the high correlation between temperature and humidity (see previous section), the decadal trends in temperature above Dome C in the beginning of the 21st century are also seasonally-dependent (Table 1). For all periods and datasets evaluated, a warming trend is observed in summer whereas a cooling trend is found in winter and autumn. In spring, the radiosonde observations show a cooling trend (−0.30 ± 1.35 K dec −1 in 2005-2017 and −1.99 ± 3.24 K dec −1 in 2012-2017), and thus disagree with HAMSTRAD, which shows a warming trend (2.69 ± 11.42 K dec −1 ). However, none of these trends are significant. The summer warming trend (0.8-8.7 K dec −1 ) is greater in the HAMSTRAD data than in the radiosonde data by a factor 2-8, but is only significant in the radiosonde data sets over the period 2005-2017 (1.08 ± 0.55 K dec −1 ). The cooling trend is 1.  Table 1), all the data show a significant linear trend but not of the same sign. A warming trend is identified in the HAMSTRAD data (1.33 ± 0.96 K dec −1 ), while a cooling trend is calculated from radiosondes over both 2012-2017 and 2005-2017: −2.65 ± 0.72 and −1.80 ± 1.54 K dec −1 , respectively. The fact that HAMSTRAD and radiosonde trends are opposite over the same period (2012-2017) might be a consequence of the much larger summer warming trend in HAMSTRAD than in the radiosondes (a factor~10), combined with the much larger autumn cooling trend in the radiosondes than in HAMSTRAD (a factor~3). This discrepancy calls for further investigation into the sensitivity and variability of the two data sources in the future.

Data Sets
To summarize, over 2005-2017, the radiosonde observations at Dome C show a drying trend of the atmosphere with a cooling trend in winter and autumn, and a moistening trend of the atmosphere combined with a warming trend in summer. All trends in these three seasons are statistically significant. However, in spring, no significant trends were found. The trend significance increases with the length of the period considered. In HAMSTRAD, over 2012-2017, a significant cooling trend was only found in winter. Compared with the trends obtained at the South Pole station [24] in 2005-2018, we note that those obtained at Dome C: 1) agree with South Pole trends in summer highlighting a warming trend (1.3 K dec −1 ) at the surface and a moistening trend (0.02 g kg −1 dec −1 ) for heights below 5 km but 2) disagree in autumn and winter, since a warming (0.6 and 1.5 K dec −1 , respectively) and moistening trend (<0.02 g kg −1 dec −1 ) for heights below 5 km were observed at South Pole, whereas the opposite was seen in the Dome C data. For the trend in annual averages, only radiosonde data averaged over 2012-2017 show significant cooling and drying of the atmosphere.

Decadal Trends in H 2 O and Temperature Profiles
The decadal trends in H 2 O and temperature seasonal averages as a function of height above the surface have been explored using the radiosonde observations from 2005-2017 (Figures 7 and 8, respectively). Note that, in contrast to IWV, the vertical distribution of the HAMSTRAD temperature (0-10 km) and absolute humidity (0-4 km) is subject to known biases [32] that prevent the calculation of trends along the vertical for HAMSTRAD. Consistent with the results obtained for IWV, the H 2 O decadal trends ( Figure 7) exhibit: (1) significant drying in winter (−0.20 ± 0.10 g m −3 dec −1 ) with a less intense trend in autumn (−0.10 ± 0.10 g m −3 dec −1 ), and (2) significant moistening in summer (+0.15-0.20 ± 0.10 g m −3 dec −1 ), although it is less intense in spring (about +0.05 ± 0.10 g m −3 dec −1 ). Note that, below 100 m, the absolute trends in H 2 O are weaker than those above 100 m, regardless of season.
The decadal trends in temperature ( Figure 8) below 100 m mimic the trends observed at 10 m (Table 1) in all seasons except spring, although the changes are larger closer to the surface than at 100 m. There is warming in summer, cooling in autumn and winter, and either cooling or warming in spring below or above 100 m, respectively. Above 100 m, the decadal trends appear to be constant along the vertical for all seasons. In summer, above 100 m, an insignificant warming of 0.50 ± 0.75 K dec −1 is observed whilst, below 100 m, the warming is significant (1.00-0.80 ± 0.75 K dec −1 ). In winter, the cooling observed above 100 m is significant (−1.80 ± 0.80 K dec −1 ), as is the cooling below (−2.0 to −5.0 ± 0.80 K dec −1 ). In autumn, above 100 m, there is cooling (−0.75 ± 0.75 K dec −1 ), but only the change below 100 m is significant (from −0.80 to −2.50 ± 0.75 K dec −1 ). In spring, above 100 m, there is an insignificant warming (0.5-1.0 ± 1.2 K dec −1 ) whilst below 100 m, there is insignificant cooling −0.5 to 0.0 ± 1.2 K dec −1 ). Note that analysis of the near-surface (2-m) temperature from the Automated Weather System (AWS) at Dome C (not shown) confirms the results obtained with the radiosondes at 10 m with warming in DJF (1.9 ± 0.7 K dec −1 ) and SON (0.9 ± 1.0 K dec −1 ), and cooling in JJA (−2.9 ± 1.7 K dec −1 ) and MAM (−0.9 ± 1.1 K dec −1 ). Atmosphere 2020, 11, x FOR PEER REVIEW 10 of 22 The decadal trends in temperature ( Figure 8) below 100 m mimic the trends observed at 10 m (Table 1) in all seasons except spring, although the changes are larger closer to the surface than at 100 m. There is warming in summer, cooling in autumn and winter, and either cooling or warming in spring below or above 100 m, respectively. Above 100 m, the decadal trends appear to be constant along the vertical for all seasons. In summer, above 100 m, an insignificant warming of 0.50 ± 0.75 K dec −1 is observed whilst, below 100 m, the warming is significant (1.00-0.80 ± 0.75 K dec −1 ). In winter, the cooling observed above 100 m is significant (−1.80 ± 0.80 K dec −1 ), as is the cooling below (−2.0 to −5.0 ± 0.80 K dec −1 ). In autumn, above 100 m, there is cooling (−0.75 ± 0.75 K dec −1 ), but only the change below 100 m is significant (from −0.80 to −2.50 ± 0.75 K dec −1 ). In spring, above 100 m, there is an insignificant warming (0.5-1.0 ± 1.2 K dec −1 ) whilst below 100 m, there is insignificant cooling −0.5 to 0.0 ± 1.2 K dec −1 ). Note that analysis of the near-surface (2-m) temperature from the Automated Weather System (AWS) at Dome C (not shown) confirms the results obtained with the radiosondes at 10 m with warming in DJF (1.9 ± 0.7 K dec −1 ) and SON (0.9 ± 1.0 K dec −1 ), and cooling in JJA (−2.9 ± 1.7 K dec −1 ) and MAM (−0.9 ± 1.1 K dec −1 ). If we combine the trends obtained at 10 m, along the vertical, and integrated along the vertical, we can state that, over the period 2005-2017, above Dome C: (1) a significant moistening trend associated with a warming trend is calculated in summer; (2) a significant drying trend associated with a significant cooling trend is calculated in autumn and winter; and (3) neither significant moistening/drying nor significant warming/cooling trends are demonstrated in spring. Significant variations in the vertical distribution of temperature and associated decadal trends below 1000 m are If we combine the trends obtained at 10 m, along the vertical, and integrated along the vertical, we can state that, over the period 2005-2017, above Dome C: (1) a significant moistening trend associated with a warming trend is calculated in summer; (2) a significant drying trend associated with a significant cooling trend is calculated in autumn and winter; and (3) neither significant moistening/drying nor significant warming/cooling trends are demonstrated in spring. Significant variations in the vertical distribution of temperature and associated decadal trends below 1000 m are well correlated with the transition altitude (100-200 m) between the planetary boundary layer and the free troposphere. In this section, we analyze the multidecadal trends and variability of IWV, near-surface temperature and SAM index over the period 1980-2017 in order to determine whether the decadal trends observed using the radiosondes in the early 21st century are due to a long-term, multidecadal change in atmospheric properties or simply reflect its multidecadal variability above Dome C.

Data Sets
In summary, over the period 1980-2017, the only significant cooling and drying trends at Dome C associated with a significant positive trend in the SAM index occur in winter, in the ERA-Int and ERA5 reanalyses.

Multidecadal Variability over Dome C
The multidecadal variability in the SAM index, as well as in the IWV and near-surface temperature, calculated using a 10-year moving window on the four reanalyses (ERA-Int, ERA5, MERRA2, and JRA-55) above Dome C for 1980-2017, are presented in Figures 9-13 as seasonal and annual averages, respectively (see also Section 2). The 10-year running trend algorithm is also applied to the radiosonde observations, though it only provides 1-3 points for the period, depending on the season considered. The radiosonde trends are of similar magnitude to those derived in Section 3.2.1. In the following, each 10-year window will be referred to by its center. For instance, the date of 1985 will correspond to the 1980-1990 period. The linear Pearson correlation coefficients between IWV and near-surface temperature (R IWV-T ), IWV and the SAM index (R IWV-SAM ), and near-surface temperature and the SAM index (R T-SAM ) calculated from the four reanalysis and radiosonde datasets, as seasonal or annual averages, are presented in Figure 14.  The summer SAM index multidecadal variability from 1980 to 2017 (Figure 9a) is, on average, The autumn SAM index multidecadal variability (Figure 10a) is anti-correlated with that of IWV ( Figure 14), but is significant only in 1991-1997 (about 1.5 ± 1.0 index dec −1 ) and 2011-2012 (about 1.5 ± 0.8 index dec −1 ). R IWV-T is on average slightly lower in autumn than in summer ( Figure 14    The spring near-surface temperature multidecadal variability (Figure 12c) is consistently weak within all the data sets prior to 1995, except for a slight, significant cooling trend observed in ERA-Int and ERA5 in 1987-1988 (−1.8 ± 1.5 K dec −1 ). After 1995, the four reanalyses diverge to within 3.0-5.0 K dec −1 , without demonstrating any general trend (cooling/warming) in this period, except in 1996, when a significant warming trend is calculated, of 1.5 ± 1.0 K dec −1 (ERA-Int, ERA5 and JRA-55) The spring near-surface temperature multidecadal variability (Figure 12c) is consistently weak within all the data sets prior to 1995, except for a slight, significant cooling trend observed in ERA-Int and ERA5 in 1987-1988 (−1.8 ± 1.5 K dec −1 ). After 1995, the four reanalyses diverge to within 3.0-5.0 K dec −1 , without demonstrating any general trend (cooling/warming) in this period, except in 1996, when a significant warming trend is calculated, of 1.

Assimilation of Radiosondes
The assimilation of Dome C radiosonde observations starting in 2005 by the reanalyses may have caused some marked discontinuity in the IWV and near-surface temperature decadal trends. In IWV, if we except the season of DJF when ERA-Int shows much drier trends in 2006 (−0.20 ± 0.05 kg m −2 dec −1 ) than all the other reanalyses (−0.05 to −0.10 ± 0.05 kg m −2 dec −1 ), all the data sets exhibit a consistent behavior within ±0.05 kg m −2 dec −1 in MAM, ±0.03 kg m −2 dec −1 in JJA and SON, and ±0.02 kg m −2 dec −1 in the annual average. On average, no discontinuity can be observed in the time series of IWV trends within the four data sets. However, we note that the spread among the four reanalysis datasets is reduced over the last decade of the analysis. The assimilation of the radiosonde data at Dome C is starting to impact the decadal trend calculation starting in 2000, with decadal trends from 2010 fully benefitting from radiosonde data. That is to say that the 2000-2010 period can be regarded as a progressive transition from zero-radiosonde data to fully-incorporated radiosonde data at Dome C, which corresponds to the progressive reduction in the spread between the reanalyses. Such a behavior is not observed in near-surface temperature decadal trends.
In near-surface temperature, the JRA-55 data set obviously departs from the other data sets starting in 2005 for most seasons and in the annual average, with trends 1 to 5 K dec −1 greater than the trends calculated with the other datasets. The largest difference is found in JJA: 0.0 ± 1.5 K dec −1 in JRA-55 and about −5.0 ± 1.5 K dec −1 for the other datasets. At the present stage, we cannot attribute the cause of this difference to the use or not of the radiosondes from Dome C in the data assimiliation system of JRA-55 since the Japanese system relies on the same radiosonde data base as ERA5 [29]. Finally, we must note that JRA-55 provides surface air temperature although the other reanalysis data sets provide 2-m temperature, which may potentially impact the trend calculation.

Decadal Trends
Based on reconstructed reanalyses of near-surface temperatures from 1958 to 2012, warming trends of about 0.1 K dec −1 calculated by [4] in the Eastern Antarctic Plateau were not statistically significant in summer, winter and as annual averages. In their study, in spring, the long-term trend was statistically significant at 0.2-0.3 K dec −1 whilst, in autumn, a not statistically significant cooling trend of about −0.1 K dec −1 was derived. A cooling trend in autumn is also found in our study, but is not statistically significant. The main difference appears to be in winter, when, in our study, the multidecadal trends from the four reanalyses are all negative although, over 1958-2012 [4], they are positive underlining the impact of the selection of the analysis period on the sign and magnitude of multidecadal trends. We also recall (see Section 3.2.1) that winter decadal trends in temperature were observed to be significantly negative at 10 m at Dome C (−5.  [6]. The link between the multidecadal variability of the SAM index and the near-surface temperature over Antarctica has already been presented by [4], based on a reconstruction of Antarctic monthly mean near-surface temperatures from reanalyses spanning 1958-2012. The Pearson linear coefficient correlation (R T-SAM ) is shown to be statistically significant and less (in absolute value) than −0.6 in the vicinity of the Dome C station. More precisely, in summer and autumn, R T-SAM appears to be less than −0.7, whilst, in winter and spring and as an annual average, it is around −0.6. These results, over a period 20 years longer than the period selected in our study, are consistent with our findings above Dome C, although 1) they are smaller by −0.1 to −0.2 (more anticorrelated) than our correlation coefficients; and 2) their winter correlation coefficient is greater by 0.1 to 0.2 (reduced anticorrelation) than our winter results. A positive SAM index is usually associated with low temperatures for the East Antarctic Plateau, which is attributed to 1) reduced meridional heat exchange within the troposphere, and 2) reduced downward turbulent heat fluxes near the ice sheet's surface [37]. The IWV and near-surface temperature multidecadal variability can also be indirectly influenced by 1) the southern Baroclinic Annular Mode (BAM); 2) the Pacific-South American (PSA) patterns [38]; and 3) the tropically-forced variability induced by El Niño-Southern Oscillation (ENSO) since, in summer and winter, the SAM trends have been shown to be more closely linked to long-term trends in tropical Pacific sea surface temperatures [9].
To summarize, from the multidecadal variability of IWV and near-surface temperature based on the four reanalyses above Dome C and of the SAM index since 1980, we demonstrated that the summer moistening/warming trends and the autumn and winter drying/cooling trends observed in the beginning of the 21st century by the radiosondes agreed with the reanalyses. We also showed that these trends were not systematically found since the end of the 20th century. Instead, periods of moistening/warming alternated with periods of drying/cooling whatever the season considered, and even in the annual averages. The multidecadal variability in IWV and near-surface temperature was clearly anticorrelated with that in the SAM index for all seasons but spring (SON). The anticorrelation with the SAM index was stronger for IWV than for temperature. This is probably due to the fact that IWV is more representative of the whole atmosphere than near-surface temperature. Finally, our study suggests that the decadal trends observed at Dome C since the beginning of the 21st century in humidity and near-surface temperature as seasonal averages simply reflect the multidecadal variability of the atmosphere, and are not due to long-term trends of atmosphere properties.

Conclusions
Decadal trends in atmospheric humidity (IWV and H 2 O profiles) and near-surface temperature (surface/2-m/10-m temperature and profiles) above Dome C (Antarctica) have been evaluated using 1) observations performed at the site (radiosondes and HAMSTRAD) in the beginning of the 21st century (2005-2017 and 2012-2017, respectively); and 2) reanalyses since the end of the 20th century , including the ERA-Int, ERA5, MERRA2 and JRA-55 data sets.
Observations show a clear seasonal cycle in both IWV and 10-m temperature with minima in winter (JJA) and maxima in summer (DJF). The linear Pearson correlation coefficient R between IWV and 10-m temperature or 10-m H 2 O shows that, on average, the time evolution of IWV is more related to the time evolution of 10-m temperature than to that of 10-m H 2 O. This is due to 1) the thermodynamic relationship between H 2 O and temperature; and 2) the vertical profile of H 2 O that does not peak at the surface but around 100-200 m, and dramatically decreases below this layer down to 10 m. As a consequence, the time evolution of 10-m temperature can be considered a better proxy than the time evolution of 10-m water vapor for tropospheric humidity above Dome C.
The decadal trends in IWV, as well as temperature and H 2 O at various heights, from the observations performed at Dome C since the beginning of the 21st century (2005-2017 in radiosondes and 2012-2017 in HAMSTRAD), show three main features. 1) In summer, significant moistening (0.08 ± 0.06 kg m −2 dec −1 ) is associated with significant warming (1.08 ± 0.55 K dec −1 ). 2) In autumn and winter, significant drying (−0.04 and −0.05 ± 0.03 kg m −2 dec −1 , respectively) is associated with significant cooling (−2.4 ± 1.2 and −5.1 ± 2.0 K dec −1 , respectively). 3) In spring, we did not observe any significant moistening/drying trend associated with significant warming/cooling trend.
Finally, we investigated, based on seasonal and annual averages, the multidecadal trends and variability of IWV, near-surface temperature and SAM index over the much longer period 1980-2017, above Dome C, utilizing the four reanalyses. Our study showed that the summer moistening/warming trends and the autumn and winter drying/cooling trends observed in the beginning of the 21st century with the radiosondes 1) agreed with the reanalyses, but 2) were not consistently present at the end of the 20th century. Instead, periods of moistening/warming alternated with periods of drying/cooling in seasonal and annual averages. The SAM index multidecadal variability was clearly anticorrelated to those in IWV and near-surface temperature (R varying from −0.5 to −0.75 and from −0.30 to −0.60, respectively) for all the seasons but spring (SON), with a larger anticorrelation for IWV than for temperature. Finally, our study suggests that the decadal trends observed in humidity and near-surface temperature at Dome C since the beginning of the 21st century simply reflect the multidecadal variability of the atmosphere and, thus, are not indicative of the secular atmospheric trends that may be related to global climate change.
Funding: This research received no external funding.