青藏高原与极地气象

青藏高原与极地气象研究进展

Progress in the Tibetan Plateau and Polar Meteorology Research

1 高原气象

1 Tibetan Plateau meteorology

1.1 Co-variability of July precipitation between North China and the Kazakhstan-Xinjiang region and its precursory atmospheric signals

The present study investigates the dominant pattern of precipitation variability over the mid-highlatitude Asia during summer for the period 1981–2018 and associated precursory signals in the atmosphere.An in-phase co-variability is identified in July precipitation between the Kazakhstan-Xinjiang region and North China,which is the leading mode of precipitation variability.Geopotential height anomalies over the Mediterranean Sea and Kara Sea regions show persistence from June to July and contribute to July geopotential height and associated precipitation anomalies over the Kazakhstan-Xinjiang region through the dispersion of Rossby wave energy.The geopotential height anomalies over the Kara Sea also affect downstream geopotential height anomalies from the eastern coast of China to the Okhotsk Sea and related precipitation over North China through a Rossby wave from the Kara Sea to the Okhotsk Sea via the Lake Baikal.In addition,the Tibetan Plateau geopotential height anomaly during June may modulate downstream geopotential height anomalies around the Okhotsk Sea during the ensuing July through a slowly-varying northeastward shift and extension of atmospheric anomalies on intra-seasonal timescale.The joint effect of geopotential height anomalies over the three regions during June leads to the co-variability of precipitation between the Kazakhstan-Xinjiang region and North China in July.Based on the above physical connection,we defined a June Kara-Mediterranean-Tibetan Plateau geopotential height index.This atmospheric precursory index shows a high skill in predicting the co-variability of July precipitation.(Li Di,Liu Ge,Wu Renguang,He Kejun,Shi Mingyuan)

1.2 Co-variability of the summer NDVIs on the eastern Tibetan Plateau and in the Lake Baikal region:Associated climate factors and atmospheric circulation

The Tibetan Plateau and Siberia are both crucial regions in which the vegetation dynamics are sensitive to climate change.The variabilities in the normalized difference vegetation index (NDVI) over the two regions have been explored previously,but there have been few studies on the relationship of the NDVI in the two regions.Using the GIMMS-NDVI,GHCNCAMS and NCEP reanalysis datasets and statistical and physical diagnostic methods,we show that the summer (June,July and August) NDVIs over the eastern Tibetan Plateau and Lake Baikal and its adjacent eastern region of Siberia have an in-phase co-variability,especially on an interannual timescale (with a correlation coefficient of 0.69 during the time period 1982-2014).Further analyses show that precipitation and the related cloud cover and solar radiation are responsible for the variability in the NDVI over the eastern Tibetan Plateau,whereas temperature has the more important role in modulating the variability in the NDVI over the Lake Baikal region.A dipole pattern prevails over the Tibetan Plateau-Lake Baikal region and reflects the anomalies in the intensity and location of the South Asian high and the northeast Asian blocking high.This dipole pattern simultaneously modulates precipitation over the eastern Tibetan Plateau and the temperature over the Lake Baikal region and leads to the co-variability of the NDVI between the two regions.A synergistic sea surface temperature index,which reflects sea surface temperature anomalies in the eastern tropical Pacific Ocean,the northwest Pacific Ocean,the northern Indian Ocean and the subtropical north Atlantic Ocean,appears to adjust this Tibetan Plateau–Lake Baikal dipole pattern and is therefore closely related to the co-variability of the NDVI between the eastern Tibetan Plateau and the Lake Baikal region.Our results suggest that vegetation dynamics may not only be a local phenomenon in some areas,but also are likely to remotely link with variations in vegetation over other regions.(He Kejun,Liu Ge,Zhao Junfang,Li Jingxin)

1.3 Relationship between the thermal condition of the Tibetan Plateau and precipitation over the region from eastern Ukraine to north Caucasus during summer

Remarkable climate anomalies occurred in Europe in recent years,but the reasons are not entirely disclosed.This entails further exploring the physical mechanism of anomalous climate variability over Europe on the basis of previous studies.Using NCEP-NCAR reanalysis and CMAP precipitation datasets,we investigated the relationship between the thermal condition of the Tibetan Plateau (TP) and precipitation over Europe during summer and related mechanisms behind through observational analyses and simulation experiments.The results show that the summer TP surface air temperature (SAT) is significantly correlated with the simultaneous precipitation over the region from eastern Ukraine to the north Caucasus (EUNC;43°–51°N,34°–46°E) during the period 1979–2017.The effect of anomalous TP heating plays,to some extent,an active role in linking the summer TP SAT with EUNC precipitation.The variability of the TP SAT is a result of the anomalous TP heating,but it can reflect the variability in temperature of a thick tropospheric air column over the TP well.Corresponding to higher (lower) TP SAT,the higher (lower) temperature anomaly appears over the TP and extends westwards through the transport of anomalous temperature fluxes from the TP to the EUNC regions in the upper troposphere,resulting in the expansion (withdrawal) of the South Asian high (SAH) and associated less (more) precipitation over the EUNC region.Numerical simulations basically reproduce the above-mentioned physical process,which confirms that,through stimulating the variability of upper-tropospheric temperatures over the TP and a larger area to its west and associated upstream large-scale atmospheric circulation,the summer TP’s thermal condition can modulate and expand the impact of the SAH to the EUNC region and therefore affect the concurrent precipitation in situ.This study implies an importance of the thermal anomaly of the TP to the variability of EUNC precipitation during summer.(Chen Junming,Yue Xiaoyuan,Liu Ge,Nan Sulan)

1.4 Diurnal characteristics of gravity waves over the Tibetan Plateau in 2015 summer using 10-km downscaled simulations from WRF-EnKF regional reanalysis

Diurnal variations of gravity waves over the Tibetan Plateau (TP) in summer 2015 were investigated based on high-resolution downscaled simulations from WRF-EnKF (weather research and forecasting model and an ensemble Kalman filter) regional reanalysis data with particular emphasis on wave source,wave momentum fluxes and wave energies.Strong diurnal precipitations,which mainly happen along the south slope of the TP,tend to excite upward-propagating gravity waves.The spatial and temporal distributions of the momentum fluxes of small-scale (10-200 km) and meso-scale (200-500 km) gravity waves agree well with the diurnal precipitation distributions.The power spectra of momentum fluxes also show that the small- and mesoscale atmospheric processes become important during the period of the strongest rainfall.Eastward momentum fluxes and northward momentum fluxes are dominant.Wave energies are described in terms of kinetic energy(KE),potential energy (PE) and vertical fluctuation energy (VE).The diurnal variation and spatial distribution of VE in the lower stratosphere correspond to the diurnal rainfall in the troposphere.(Qian Tingting)

1.5 MAX-DOAS measurements of NO2,SO2,HCHO and BrO at the Mt.Waliguan WMO/GAW global baseline station in the Tibetan Plateau

Mt.Waliguan Observatory (WLG) is a World Meteorological Organization (WMO)/Global Atmosphere Watch (GAW) global baseline station in China.WLG is located at the northeastern part of the Tibetan Plateau(3617°N,10054°E,3816 m a.s.l.) and is representative of the pristine atmosphere over the Eurasian continent.We made long-term ground-based multi-axis differential optical absorption spectroscopy (MAX-DOAS)measurements at WLG during the period 2012-2015.In this study,we retrieve the differential slant column densities (dSCDs) and estimate the tropospheric background mixing ratios of different trace gases,including NO2,SO2,HCHO and BrO,using the measured spectra at WLG.Averaging of 10 original spectra is found to be an “optimum option” for reducing both the statistical error of the spectral retrieval and systematic errors in the analysis.The dSCDs of NO2,SO2,HCHO and BrO under clear sky and low aerosol load conditions are extracted from measured spectra at different elevation angles at WLG.By performing radiative transfer simulations with the model TRACY-2,we establish approximate relationships between the trace gas dSCDs at 1°elevation angle and the corresponding average tropospheric background volume mixing ratios.Mixing ratios of these trace gases in the lower troposphere over WLG are estimated to be in a range of about 7×10−12(January)to 100×10−12(May) for NO2,below 0.5×10−9for SO2,between 0.4 and 0.9×10−9for HCHO,and lower than 0.3×10−9for BrO.The chemical box model simulations constrained by the NO2concentration from our MAXDOAS measurements show that there is a little of net ozone loss (−0.8×10−9per day) for the free tropospheric condition and a little of net ozone production (0.3×10−9per day) for the boundary layer condition over WLG during summertime.Our study provides valuable information and data set for further investigating tropospheric chemistry in the background atmosphere and their links to anthropogenic activities.(Ma Jianzhong)

1.6 The long-term change of latent heat flux over the western Tibetan Plateau

The Tibetan Plateau (TP) has been experiencing warming and wetting since the 1980s.Under such circumstances,we estimated the summer latent heat flux (LE) using the maximum entropy production model driven by the net radiation,surface temperature,and soil moisture of three reanalysis datasets (ERA5,JRA-55,and MERRA-2) at the Ali site over the western TP during 1980–2018.Compared with the observed LE of the Third Tibetan Plateau Atmospheric Scientific Experiment,the coefficient of determination,root-mean-square error,and mean bias error of the estimated summer LE are 0.57,9.3,and −2.25 W m−2during 2014–2016,respectively,which are better than those of LE of the reanalysis datasets.The estimated long-term summer LE presents a decreasing (an increasing) trend of −7.4 (1.8) W m−2per decade during 1980–1991 (1992–2018).The LE variation is closely associated with the local soil moisture influenced by precipitation,glacier,and nearsurface water conditions at the Ali site.The summer soil moisture also presents a decreasing (an increasing)trend of −0.082 (0.022) per decade during 1980-1991 (1992-2018).The normalized difference vegetation index generally shows the consistent trend with LE at the Ali site.(Zhao Ping)

1.7 A revisiting of the parametrization of downward longwave radiation in summer over the Tibetan Plateau based on high-temporal-resolution measurements

The Tibetan Plateau (TP) is one of the research hot spots in the climate change research due to its unique geographical location and high altitude.Downward longwave radiation (DLR),as a key component in the surface energy budget,has practical implications for radiation budget and climate change.A couple of attempts have been made to parametrize DLR over the TP based on hourly or daily measurements and crude clearsky discrimination methods.This study uses 1 min shortwave and longwave radiation measurements at three stations over the TP to parametrize DLR during summer months.Three independent methods are used to discriminate clear sky from clouds based on 1 min radiation and lidar measurements.This guarantees the strict selection of clear-sky samples that is fundamental for the parametrization of clear-sky DLR.A total of 11 clearsky and 4 cloudy DLR parametrizations are examined and locally calibrated.Compared to previous studies,DLR parametrizations here are shown to be characterized by smaller rootmean-square errors (RMSEs) and higher coefficients of determination (R2).Clear-sky DLR can be estimated from the best parametrization with a RMSE of 3.8 W m−2andR20.98.Systematic overestimation of clear-sky DLR by the locally calibrated parametrization in one previous study is found to be approximately 25 W m−2(10%),which is very likely due to potential residual cloud contamination on previous clear-sky DLR parametrization.The cloud base height under overcast conditions is shown to play an important role in cloudy DLR parametrization,which is considered in the locally calibrated parametrization over the TP for the first time.Further studies on DLR parametrization during nighttime and in seasons except summer are required for our better understanding of the role of DLR in climate change.(Zheng Xiangdong)

1.8 夏季青藏高原对流层温度与西北太平洋副热带地区降水的关系

利用1979—2016年CMAP (CPC Merged Analysis of Precipitation)和GPCP (Global Precipitation Climatology Project)降水数据以及ERA-Interim再分析资料，通过统计方法研究了夏季青藏高原地区对流层中上层温度年际变率与同期西北太平洋副热带地区降水的关系及其相关物理过程。结果表明，在年际变化尺度上，夏季高原对流层温度与同期西北太平洋副热带地区降水存在显著的正相关，即当高原对流层温度偏高时，西北太平洋副热带区域的降水偏多，反之亦然。研究指出，当夏季高原对流层温度偏高时，高原上空南亚高压显著增强并且向东扩展至日本地区，高原北部对流层出现异常的上升运动，这一异常上升气流随着高度增加逐渐北偏，并在中高纬度地区沿着异常西风气流向东扩展至日本地区，随后向南下沉至日本南部；受该异常下沉运动影响，日本南部对流层低层出现异常反气旋，其东侧的异常北风与西北太平洋低层的异常气旋、反气旋环流存在紧密联系。西北太平洋地区这种异常环流特征为西北太平洋副热带区域的降水提供了有利的动力和水汽条件，从而使该区域降水增多。（石明远，赵平，刘舸）

1.9 1979-2016年四川盆地低涡的气候特征分析

利用6 h一次、水平分辨率为0.25°×0.25°的ERA-Interim再分析资料，对1979—2016年生成于四川盆地的西南涡的发生和发展进行统计分析。结果表明：四川盆地低涡集中生成于盆地内；在6月生成最多，7月发展最强；按移动情况不同可将其分为5类：东移型、东北移型、东南移型、西移型和少动型；东移型、东南移型、少动型低涡生成个数的峰值在6月，东北移型和西移型低涡生成个数的峰值在7月。夏季5类长生命史四川盆地低涡的结构和降水合成场表明：从发展强度看，东北移型最强，少动型最弱。从成熟期垂直结构看，除西移型外，低涡均随高度向西北或向西倾斜，在对流层低层为冷性结构，中层为暖性结构；东移型、东北移型、西移型低涡的正涡度区在垂直方向伸展更高；除东南移型、西移型低涡的强上升区与其中心重合外，其余类型位于其中心东侧。从降水特征看，除西移型外，其余类型低涡的降水中心均位于其移动路径东侧或东北侧，其中东北移型低涡成熟期6 h累计降水量最大。四川盆地低涡的强上升区、相对湿度大值区、位于对流层低层和中层的辐合辐散中心与降水所在位置有很好的对应关系，各物理量场相互作用共同促进低涡发展。（刘冲，赵平）

2 极地气象

2 Polar meteorology

2.1 A possible mechanism for winter sea ice decline over the Bering Sea and its relationship with cold events over North America

In this study,the mechanism for the sea ice decline over the Bering Sea and its relationship with cold events over North America are investigated based on the daily ERA-Interim data during the winter (December–February) of 1979–2016.The results show that the sea ice decline over the western (eastern) Bering Sea is mainly contributed by (1) the strengthened southerly (southeasterly) wind near the surface,which possibly pushes the sea ice to move northward,and (2) the intensified downward infrared radiation (IR),which is closely related to the local increasing surface air temperature (SAT) and the intensified moisture convergence mostly induced by the anomalous southeasterly wind associated with an anticyclonic anomaly over the Alaska Bay.During the sea ice decline over the Bering Sea,a cold SAT anomaly is simultaneously found over North America.It is proved that the occurrence of such a cold event is driven by the atmospheric internal variation,but not the forcing of sea ice decline over the Bering Sea.This study deepens our understanding of sea ice decline and its relationship with contemporary cold events in winter.（Jiang Zhina）

2.2 Possible links between Arctic sea ice loss events and cold Eurasian anomalies in winter

Recently,there have been two competing perspectives on the links of rapid sea ice retreat over the Barents–Kara Seas (BKS) and in midlatitude severe cold winters over Eurasia.By using the daily ECMWF reanalysis(ERA)-Interim dataset during 1979–2016,we reconcile two contrasting viewpoints; namely,if an upward turbulent heat flux appears and maintains several days after the rapid sea ice loss over the BKS in winter,a dipole structure which consists of a primary positive center of action around the Barents Sea with the other opposite-sign center of action over Eurasian continent is easily amplified,and consequently a cold anomaly over Eurasia will occur,but not vice versa.Our work casts light on the links between the Arctic sea ice loss and Eurasian cold winter anomalies by revealing the different responses of the surface turbulent heat flux (STHF)after rapid sea ice retreat on a daily basis.(Jiang Zhina)

2.3 Projections of Arctic sea ice conditions and shipping routes in the twenty-first century using CMIP6 forcing scenarios

The accelerated decline in Arctic sea ice in recent decades suggests the possibility of future trans-Arctic shipping routes linking the Atlantic and Pacific oceans,with significant implications for the global economy.We present a projection of Arctic sea ice conditions and shipping activities during the 21st century based on 16 CMIP6 models calibrated to remove spatial biases.The multimodel ensemble mean shows that the Arctic is likely to be ice-free in September by 2076 and 2055 under the SSP2-4.5 and SSP5-8.5 scenarios,respectively,whereas the extent of sea ice is >2×106km2throughout the 21st century under the SSP1-2.6 scenario.The Arctic sea ice in September thins over time,leading to a reduction in the area with an ice thickness >120 cm(i.e.the threshold over which sea ice is inaccessible to Type A vessels) by 34%−100% by the late 21st century(2086−2100) under the three scenarios.Given the declines in the extent and thickness of sea ice,the most commonly traversed route along the North West Passage tends to migrate from the southern to the northern route during the 21st century.The optimum route along the Northern Sea Route shifts northward with time,with the Transpolar Sea Route becoming available.Quantitatively,the maritime accessibility to Type A vessels via the Transpolar Sea Route increases from about 6.7% ,4.2% and 2.1% in 2021−2035 to 14.7% ,29.2% and 67.5% in 2086−2100 under the SSP1-2.6,SSP2-4.5 and SSP5-8.5 scenarios,respectively.The season for trans-Arctic shipping extends from 5 to about 7.5 (9) months by the late 21st century under the SSP1-2.6 (SSP2-4.5)scenario and the Arctic becomes navigable all year round under the SSP5-8.5 scenario.These findings may aid in developing strategic planning by governments for the Arctic and providing strategic advice for the global maritime industry.(Wei Ting)

2.4 Reduction in extreme climate events and potential impacts by the use of technological advances

Technological advances have the potential to balance climate change mitigation and economic development.However,it remains unclear how much technological advances alone can mitigate climate change and the associated economic losses in the future.Through designing a suite of technological advances scenarios and using an earth system model with an integrated assessment model,we illustrate that rapid technological progress without production control might achieve the 2 ℃ global warming target in the 2100s.Relative to a world of stagnant technology,the frequency (intensity) of extreme warm events at the end of the 21st century (2081–2100) would be reduced by 21% (5.5 ℃) via rapid technological advances,with a reduction in extreme precipitation (droughts) by 41% (10 times).Furthermore,fast technological advances may reduce the global economic losses linked with climate change at 2081-2100 by 21% and those in China related to floods(droughts) by 86% (67%).Our results highlight the potential of technological advances to fill the emission gap between the Paris Agreement and unconditional Nationally Determined Contributions and hence to efficiently mitigate global warming.(Wei Ting)

2.5 Towards more snow days in summer since 2001 at the Great Wall station,Antarctic peninsula:The role of the Amundsen sea low

The variation in the precipitation phase in polar regions represents an important indicator of climate change and variability.We studied the precipitation phase at the Great Wall station and Antarctic peninsula(AP) region,based on daily precipitation,synoptic records and ERA-Interim data during the austral summers of 1985−2014.Overall,there was no trend in the total precipitation amount or days,but the phase of summer precipitation (rainfall days versus snowfall days) showed opposite trends before and after 2001 at the AP.The total summer rain days/snow days increased/decreased during 1985−2001 and significantly decreased at a rate of −14.13 days per 10 years/increased at a rate of 14.31 days per 10 years during 2001−2014,agreeing well with corresponding variations in the surface air temperature.Further,we found that the longitudinal location of the Amundsen sea low (ASL) should account for the change in the precipitation phase since 2001,as it has shown a westward drift after 2001 (−41.1° per 10 years),leading to stronger cold southerly winds,colder water vapor flux,and more snow over the AP region during summertime.This study points out a supplementary factor for the climate variation on the AP.(Ding Minghu)

2.6 Year-round record of near-surface ozone and O3 enhancement events (OEEs) at Dome A,East Antarctica

Dome A,the summit of the east Antarctic ice sheet,is an area challenging to access and is one of the harshest environments on the Earth.Up until recently,long-term automated observations from Dome A (DA)were only possible with very low power instruments such as a basic meteorological station.To evaluate the characteristics of near-surface O3,continuous observations were carried out in 2016.Together with observations at the Amundsen-Scott station (South Pole,SP) and Zhongshan station (ZS,on the southeast coast of Prydz Bay),the seasonal and diurnal O3variabilities were investigated.The results showed different patterns between coastal and inland Antarctic areas that were characterized by high concentrations in cold seasons and at night.The annual mean values at the three stations (DA,SP and ZS) were (29.2 ± 7.5)×10−9,(29.9 ± 5.0)×10−9and (24.1 ± 5.8)×10−9,respectively.We investigated the effect of specific atmospheric processes on near-surface summer O3variability,when O3enhancement events (OEEs) are systematically observed at DA (average monthly frequency peaking at up to 64.5% in December).As deduced by a statistical selection methodology,these O3enhancement events (OEEs) are affected by significant interannual variability,both in their average O3values and in their frequency.To explain part of this variability,we analyzed the OEEs as a function of specific atmospheric processes:(i) the role of synoptic-scale air mass transport over the Antarctic Plateau was explored using the Lagrangian back-trajectory analysis hybrid single-particle Lagrangian integrated trajectory(HYSPLIT) method,and (ii) the occurrence of “deep” stratospheric intrusion events was investigated using the Lagrangian tool STEFLUX.The specific atmospheric processes,including synoptic-scale air mass transport,were analyzed by the HYSPLIT back-trajectory analysis and the potential source contribution function (PSCF)model.Short-range transport accounted for the O3enhancement events (OEEs) during summer at DA,rather than efficient local production,which is consistent with previous studies of inland Antarctica.Moreover,the identification of recent (i.e.,4 days old) stratospheric-intrusion events by STEFLUX suggested that deep events only had a minor influence (up to 1.1% of the period,in August) on deep events during the variability in nearsurface summer O3at DA.The deep events during the polar night were significantly higher than those during the polar day.This work provides unique data on ozone variation at DA and expands our knowledge of such events in Antarctica.(Ding Minghu).

2.7 The surface energy balance at Panda 1 station,Princess Elizabeth Land:A typical katabatic wind region in East Antarctica

Using automatic weather station and reanalysis data (ERA5) from 2011 at Panda-1 station,situated in the katabatic region of Princess Elizabeth Land,East Antarctica,the surface energy balance was calculated using a surface temperature iteration method,and the characteristics of each energy component were analyzed.Downward shortwave and longwave radiation were the two primary energy sources during summer days with seasonal means of 346 and 142 W m−2.The turbulent fluxes of sensible and latent heat flux represent smaller heat sources.In the annual mean,reflected shortwave radiation exceeds the upward longwave radiation with a seasonal average values of −287 W m−2.During winter,the shortwave radiation is small,and the main energy input and output terms of the surface energy balance are downward and upward longwave radiation,with seasonal average values of 149 and −159 W m−2,respectively.The combination of high wind speed and a large near-surface humidity gradient during summer resulted in significant frost depositional events.The total surface frost deposition for the whole year was 24 kg m−2,which accounted for 61% of the total accumulation(averaged over 10 years).When a high-pressure ridge blocks cyclones and deflects fronts of low-pressure systems to inland East Antarctica during winter,this has a significant impact on the surface energy balance at the Panda 1 automatic weather station,with daily sensible and latent heat fluxes increasing by as much as 25 and 12 W m−2,respectively.These results still contain uncertainties as we only address a single year,when interannual variability may be considerable,and we do not consider drifting snow sublimation.(Ding Minghu)

2.8 Characteristics of the atmospheric vertical structure with different sea ice covers over the Pacific sector of the Arctic Ocean in summer

During the Chinese National Arctic Research Expeditionin 2018 (CHINARE 2018),84 radiosondes were released over the Pacific sector of the Arctic Ocean.Using these data,the characteristics of the atmospheric vertical structure with diffierent sea ice covers were studied.The results showed that (1) Typical temperature inversions were found in the low-level atmosphere with various characteristics over diffierent regions.The inversions over the open water were the weakest and elevated.Oppositely,the inversions over the very pack ice zone were the strongest and located at the lowest altitude.Warm air advection,radiative cooling and surface melt together provided favorable conditions for the occurrence of an inversion over the very close pack ice zone.As a transition region,the marginal ice zone presented relatively moderate inversions that were mainly controlled by warm air advection and surface melt.(2) The upper-level jet over the very close pack ice zone was the weakest,and large diffierences were observed in the characteristics of low-level jets(LLJs) over diffierent regions.With increasing sea ice,the frequency and strength of the LLJ increased and tended to enhance,respectively.Most LLJs were located below the tops of the inversions over the very close pack ice zone.Additional observation data are required to confirm that sea ice has significant impacts on the atmospheric vertical structure.(Tian Zhongxiang,Zhang Dongqi,Song Xiaojiang,Zhao Fu)

2.9 Measurements of atmospheric Δ14CO2 along the R/V Xuelong cruise track from Zhongshan station (Antarctica) to Shanghai

During January-March 2011,29 atmospheric Δ14CO2samples were measured along the R/V Xuelong cruise track from Zhongshan station (69.37°S,76.38°E; Antarctica) to Shanghai.The extent of fossil fuel contamination in the samples was evaluated by comparison with contemporary hemispheric-scale averages of Δ14CO2,together with transport analysis and consideration of the δ13CO2levels in the samples.Generally,the Δ14CO2levels along the route south to 40.67°S were lower than the hemispheric-scale average.Circumpolar deep water (CDW) upwelling,stratosphere–troposphere transport (STT),and regional transport associated with the Fukushima nuclear power plant (NPP) accident in Japan on 12 March 2011 were found to have influenced Δ14CO2levels at the sampling sites.In comparison with the atmospheric Δ14CO2level (45.7 ± 3.5‰ (1σ))measured in the coastal region of East Antarctica,CDW upwelling caused reduction of about 10‰ in Δ14CO2in two cross sections:62.0°–64.0°S along 74.0°E and 51.7°–54.0°S along 80.5°–84.0°E.Conversely,STTenhanced Δ14CO2was about 7‰ (3‰) at Zhongshan (around 45.0°S).The linkage was supported by a certain Model-3/CMAQ simulation that indicated dispersion of radionuclides from the Fukushima NPP accident over the East Asian region,which explained the individual high Δ14CO2(52‰) level on 29 March 2011 in Shanghai.（Zheng Xiangdong）

2.10 AIRS卫星温度和臭氧廓线在南极的验证

利用2008年中山站、Amundesen-Scott （South Pole）站和Neumayer站为期一年的温度和臭氧探空数据，对AIRS第6版温度和臭氧垂直廓线产品在南极的精度进行了验证。结果表明，AIRS温度与探空温度总体上两者具有显著的一致性，其中对流层偏差最小（RMSe 2 ℃），近地面温度由于受到下垫面影响偏差略大（RMSe为2 ℃），平流层偏差较大（2 ℃ RMSe 3 ℃），AIRS温度平均低于探空观测且受季节变化影响显著，秋冬季偏差整体上高于春夏季。AIRS臭氧反演精度在平流层（RMSe为25%）要优于对流层（RMSe为30%），RMSe最大值出现在UT-LS区域（可达40%）且在“臭氧洞”期间明显增大。AIRS产品精度在南极沿岸和内陆存在差异，由于南极地区探空资料较少且主要位于沿海，故在南极内陆地区进行探空观测对于提高卫星资料精度，改善该区域天气预报能力具有重大意义。（张雷，丁明虎，卞林根等）

2.11 北极西北航道海冰变化以及通航情况评估

使用不莱梅大学 AMSR-E（Advanced Microwave Scanning Radiometer for EOS）和 AMSR2（Advanced Microwave Scanning Radiometer 2）日尺度海冰密集度数据，计算了 2002—2018年加拿大北极群岛 7—9月的平均海冰面积，研究了 9月平均海冰密集度变化特征; 结合商船破冰能力确定海冰密集度阈值，选取西北航道关键区域，统计了西北航道的通航窗口，探讨了西北航道在实际商业通航方面的可能性。研究发现，在过去 17年加拿大北极群岛的 7—9月海冰面积整体呈下降趋势但有明显波动性，9月的海冰分布年际变化复杂，差异较大；在西北航道可通航的年份中，可通航的开始日期一般在 8月，结束日期在 9月底至 10月初，南路可通航时间最短 14 天，最长达到 80 天。总的来说，西北航道可通航年份和时间缺乏规律性。(汪楚涯，杨元德，张建，田彪，丁明虎)

2.12 南极中山站太阳紫外辐射测值比较

比较了2017年南极中山站3种仪器测量地面太阳紫外B（UVB）波段和紫外A（UVA）波段的辐照度。以Brewer光谱仪测值为参考，国产宽波段FSUVB日射表在UVB（波段280～315 nm）的辐照度相对误差为（55 ± 75）%，误差随大气臭氧总量增加呈上升趋势，但在南极“臭氧洞”期间偏低。Yankee UVB宽波段日射表在UVB（波段280～320 nm）的辐照度相对误差为（-31 ± 22）%；国产宽波段FSUVA日射表在UVA（波段315～400 nm）的辐照度相对误差为（23 ± 5.9）%。太阳天顶角低于80°的晴天以对流层紫外和可见光辐射模式计算结果为参考时，FSUVB、Yankee UVB和FSUVA辐照度平均相对误差分别为（30 ± 37）%、（-22 ± 19）%和（27 ± 6.4）%，Brewer相对误差未超过3.5%。国产宽波段UV日射表测值偏高，表明波长较长的杂散光对太阳辐照度测值影响明显。（郑向东）

2.13 2015年冬季北极极端气旋影响中国寒潮天气的初步研究

从北大西洋中高纬度进入北极的极端气旋会引起北极异常增暖，与中高纬度极端天气事件关系密切，危害极大。利用ERA Interim再分析资料和中国地面气象站观测资料，探讨了2015年1—2月两个极端气旋（C1，C2）影响中国天气的物理过程和机制。结果表明：当极端气旋生成并北移，附近大气低层和高层均出现异常增暖，中高纬度大气环流表现为乌拉尔阻塞形势形成、极涡断裂、低压槽加深南压，我国发生寒潮天气；且极端气旋伴随的异常增暖加强Rossby波能量频散，使中高纬度的槽和脊发展。对比发现，C1和C2的生成地和路径均存在差异，相比于C2，C1生成纬度较高且路径偏东，对应低温寒潮天气范围更大，但强度比C2略弱。这些结果均表明，极端气旋的生成和移动是中国寒潮天气发生的重要原因之一。（张琳,吕俊梅,丁明虎）

2.14 一种实现快速观测的雪反照率/粒径仪

影响冰气能量交换的最重要因子是反照率，但是目前极地的反照率产品质量仍然较低，其主要原因是无法大范围或快速获取地面实测数据，最先进的便携式地物光谱仪仍存在过于笨重的缺点。申请人通过大量试验，研制了950 nm红外波长为技术核心的便携式雪粒径仪（在这个波长范围内，雪粒径与反射率关系最好，并具有较低的温度系数，减少温度对测量结果的影响），其体积仅为手掌大小，可在科考过程中快速观测地面雪反照率。并于2019/2020年度南极内陆考察中试验成功。（丁明虎，杜福嘉，温海焜，王飞腾，田彪）

2.15 走航式大气水汽同位素监测系统研发

2010年之后，波长扫描光腔衰荡光谱（CRDS）和离轴积分腔输出光谱（OA-ICOS）等技术的发展使得水同位素相关研究快速提升，但早期没有商业化的装置和方法对其测量大气水汽δ18O和δD的精度进行客观评价，尤其是便携式观测的校正更是难中之难。在2011年，申请人开发了一种可以快速、动态和准确地测量野外自然条件下水汽中氢氧稳定同位素比率、梯度及其分馏过程的方法和装置，并在第29次南极考察中成功实现大气水汽δ18O和δD的便携式观测，获得了高质量的研究数据，并指导研究生发表多篇研究论文。随后，为了提高观测效率，提升观测精度，同时降低观测成本，申请人开发了一种能以简易的方式产生不同水汽浓度梯度的校准方案，并成功在第8次北极考察和33次南极考察中应用。（丁明虎，张通，张东启，效存德，柳景峰，李传金）

2.16 极地业务服务

选派了中国气象科学研究院朱孔驹、湖北省局祝伟和黑龙江省局罗斌祥3位同志前往南极执行两站地面气象观测、大气成分本底观测和臭氧探测等任务，完成大气成分本底站二期升级任务，并按考察站统一规定发布极地月报。全年未发生错报、漏报等问题。张文千参加中国第11次北极考察任务，考察期间完成1套冰基自动气象站架设、24次GPS探空以及大气化学走航观测。

3 其他

3 Others

3.1 Measuring the vertical profiles of aerosol extinction in the lower troposphere by MAX-DOAS at a rural site in the North China Plain

Ground-based multi-axis differential optical absorption spectroscopy (MAX-DOAS) measurements were performed during the summer (13 June to 20 August) of 2014 at a rural site in the North China Plain.The vertical profiles of aerosol extinction (AE) in the lower troposphere were retrieved to analyze the temporal variations of AE profiles,near-surface AE,and aerosol optical depth (AOD).The average AOD and nearsurface AE over the period of study were (0.51 ± 0.26) and (0.33 ± 0.18) km−1during the effective observation period,respectively.High AE events and elevated AE layers were identified based on the time series of hourly AE profiles,near-surface AEs and AODs.It is found that in addition to the planetary boundary layer height(PBLH) and relative humidity (RH),the variations in the wind field have large impacts on the near-surface AE,AOD,and AE profile.Among 16 wind sectors,higher AODs or AEs occur mostly in the directions of the cities upstream.The diurnal variations of the AE profiles,AODs and near-surface AEs are significant and influenced mainly by the source emissions,PBLH,and RH.The AE profile shape from MAX-DOAS measurement is generally in agreement with that from light detection and ranging (lidar) observations,although the AE absolute levels are different.Overall,ground-based MAX-DOAS can serve as a supplement to measure the AE vertical profiles in the lower troposphere.(Cheng Siyang)

3.2 Roles of synoptic to quasi-monthly disturbances in generating two pre-summer heavy rainfall episodes over South China.

In this study,power spectral analysis and bandpass filtering of daily meteorological fields are performed to explore the roles of synoptic to quasi-monthly disturbances in influencing the generation of pre-summer heavy rainfall over South China.Two heavy rainfall episodes are selected during the months of April−June 2008−2015,which represent the collaboration between the synoptic and quasi-biweekly disturbances and the synoptic and quasi-monthly disturbances,respectively.Results show that the first heavy rainfall episode takes place in a southwesterly anomalous flow associated with an anticyclonic anomaly over the South China Sea (SCS) at the quasi-biweekly scale with 15.1% variance contributions,and at the synoptic scale in a convergence zone between southwesterly and northeasterly anomalous flows associated with a southeastwardmoving anticyclonic anomaly on the leeside of the Yungui Plateau and an eastward propagating anticyclonic anomaly from higher latitudes with 35.2% variance contribution.In contrast,the second heavy rainfall episode takes place in southwest-to-westerly anomalies converging with northwest-to-westerly anomalies at the quasimonthly scale with 23.2% variance contributions to the total rainfall variance,which are associated with an anticyclonic anomaly over the SCS and an eastward-propagating cyclonic anomaly over North China,respectively.At the synoptic scale,it occurs in south-to-south westerly anomalies converging with a cyclonic anomaly on the downstream of the Yungui Plateau with 49.3% variance contributions.In both cases,the lowertropospheric mean south-to-southwesterly flows provide ample moisture supply and potentially unstable conditions; it is the above synoptic,quasi-biweekly or quasimonthly disturbances that determine the general period and distribution of persistent heavy rainfall over South China.(Jiang Zhina)

3.3 A modeling study of the regional representativeness of surface ozone variation at the WMO/GAW background stations in China

Six WMO/Global Atmosphere Watch (GAW) stations are located at background sites in China,which constitute a network for monitoring surface ozone,including its long-term temporal variation.In this study,we evaluated the seasonal variation in surface ozone simulated by the ECHAM/MESSy Atmospheric Chemistry(EMAC) general circulation model with the measurements made at these WMO/GAW background stations for the period of 2010–2012,and investigated the regional representativeness of each station with respect to the seasonal variation in surface ozone.We determined the contributions of ozone originating from various tropospheric and stratospheric latitude bands based on EMAC simulations using a tagged tracer approach.The results showed that the predominant contribution to surface ozone was from the troposphere in northern middle-high and tropical latitudes.For each station there was a large surrounding area in which the yearly ozone maximum occurred in the same month as it did at the station,reflecting the regional characteristics of the seasonal variation in surface ozone recorded at the WMO/GAW background stations.Surface ozone in China was found to generally peak in summer over non-monsoon and mid-latitude monsoon regions and in spring over low-latitude monsoon regions,indicating that the regional representativeness of ozone variability in China is significantly influenced by the Asian summer monsoon.(Liu Ningwei,Ma Jianzhong)

3.4 Distinct growth rates of the two ENSO types

El Niño-Southern Oscillation (ENSO) events can be grouped into two types based on their sea surface temperature anomaly spatial patterns,that is,eastern Pacific (EP) and central Pacific (CP) types.In this study,we propose a new method to empirically diagnose linear growth rates of the EP and CP ENSO types due to a limitation of applying the traditional Bjerknes stability index method simply to these two types.We spatially project the peak-time symmetric component of the mixed-layer oceanic heat budget terms onto the ENSOrelated sea surface temperature anomaly patterns to estimate the ENSO grow rate.After validating the method by comparing it with the Bjerknes stability index,we show that the growth rate is positive for the CP ENSO type and weakly negative for the EP ENSO type,contributed by different feedback terms.Based on the Coupled Model Intercomparison Project Phase 5,the multimodel ensemble mean shows similar characteristics as the reanalysis for the EP ENSO type rather than the CP ENSO type.(Ren Hongli,Wang Run)

3.5 Understanding key roles of two ENSO modes in spatiotemporal diversity of ENSO

El Niño-Southern Oscillation (ENSO) events exhibit a diversity of amplitudes,spatial patterns,and life cycles,with the main ENSO periods concentrated in the 3–7 years (low-frequency,LF) and 2–3 years(quasi-biennial,QB) bands.In this study,the spatiotemporal diversity of ENSO is quantitatively examined by extracting the two ENSO modes,namely,the LF and QB components of ENSO,from the traditional Niño-3.4 index and connecting them with the spatial types of ENSO.El Niño events can be regrouped as the QBdominated central-Pacific ENSO-like (QB-CP),LF-dominated eastern-Pacific ENSO-like (LF-EP),and LFdominated mixing (LF-mixing) types.La Niña events with vague spatial patterns can also have the same categorization.The QB-CP and LF-EP El Niño types both have a high-amplitude QB component.Meanwhile,the former is less affected by its powerless LF component,but the latter is controlled by its strong LF component.Ocean dynamics of the two El Niño types are distinct from each other.The thermocline feedback dominates the growth of the two El Niño types and contributes to the phase transition of the LF-EP type,while the zonal advective feedback is of increasing importance in the QB-CP El Niño and mainly contributes to the phase transitions of the two El Niño types.Additionally,the LF-mixing type with ambiguous spatial features and complex life cycles is distinguished from the other two types.These results indicate that the two ENSO modes coexist in the tropical Pacific air-sea system,and their combination with changing amplitude is the key to explaining the spatiotemporal diversity of ENSO.(Wang Run,Ren Hongli)

3.6 Role of eddy-heat feedback in modulating the winter-mean NAO-related thermodynamic structure.

The thermodynamic structure of the North Atlantic Oscillation (NAO),in terms of the pattern of the winter-mean potential temperature (PT) anomaly in the troposphere,is a crucial indicator of regional climate variations and can be influenced by synoptic-scale eddy heat (EH) fluxes.The role and mechanism of EH feedback in modulating the winter-mean thermodynamic structure of the NAO are investigated.We show that the low-frequency NAO-related flow can barotropically regulate EH flux and give rise to an EH transport that is down-gradient (up-gradient) of the NAO-related PT anomalies,thus creating negative (positive) EH feedback on PT anomalies in the middle-lower (upper) troposphere.Using eddy structure decomposition,we calculated the two primary terms of anomalous EH flux:the BA term is the product of basic eddy velocity and anomalous eddy PT,and the AB term is the product of anomalous eddy velocity and basic eddy PT.The meridional BA EH flux is caused by the meridional shift of eddy PT structure under NAO.It is a major contributor of the anomalous EH flux and forcing,which weaken NAO-related PT anomalies.The zonal AB EH flux is smaller in magnitude and is induced by the zonal slant of the eddy streamfunction structure in the negative NAO phase.It creates EH feedback upstream of the NAO flow.In addition,the positive NAO phase has a negatively stronger EH feedback than the negative phase; this is related to the higher persistence of the anomalous PT pattern under negative NAO.(Zhao Shuo,Ren Hongli,Zhou Fang,Gao Li)

3.7 What caused the extreme Indian Ocean dipole event in 2019?

An extreme positive Indian Ocean dipole (IOD) event occurred in 2019 boreal autumn,which has induced severe climate impacts around the Indian Ocean basin.In this study,the cause for 2019 IOD event and the related mechanism are explored.We find that the remarkable strengthening of Australian high and the weakening of sea level pressure over South China Sea/Philippine Sea have been evidently visible since May 2019.Such a record-breaking inter-hemispheric pressure gradient (IHPG) induced northward cross-equatorial flow over the western Maritime Continent,which triggered strong wind-evaporation-SST and thermocline feedbacks in 2019.In addition to 2019 case,historical IOD events are highly correlated with IHPG from boreal late spring to summer.We show that skillful IHPG prediction can be made in March by the European Centre for Medium-Range Weather Forecasts seasonal forecast system,which makes it quite possible for the early warning of extreme IOD events by two to three seasons ahead.(Lu Bo,Ren Hongli)

3.8 Enhanced mid-to-late winter predictability of the storm track variability in North Pacific as a contrast with North Atlantic.

The storm tracks are a major driver of regional extreme weather events.Using the daily output of reanalysis and a latest generation ensemble seasonal forecasting system,this study examines the interannual variability and predictability of the boreal winter storm tracks in the North Pacific and North Atlantic.In both basins,the leading mode of storm track variability describes a latitudinal shifting of the climatological storm tracks.The shifting mode is closely connected with the extratropical large-scale teleconnection patterns (i.e.Pacific-North America teleconnection and North Atlantic Oscillation).The main predictability source for the shifting mode of the North Pacific storm tracks are the ENSO-related sea surface temperature anomalies.Assessment of the seasonal prediction skill further shows that the shifting mode of the North Pacific storm tracks is in general better predicted than that of the North Atlantic storm tracks likely due to stronger ENSO effects.Our analyses also find that,through the modulations of ENSO and the subtropical jet,the shifting mode of the North Pacific storm tracks is likely due to stronger ENSO effects.Our analyses also find that,through the modulations of ENSO and the subtropical jet,the shifting mode of the North Pacific storm tracks exhibits a mid-to-late winter predictability enhancement.During El Niño phases,the North Pacific subtropical jet shifts equatorward and becomes strongest in mid-to-late winter,which dominates the upper-level flow and guides the storm track most equatorward.We argue that the intensification and equatorward shift of the North Pacific subtropical jet in mid-to-late winter of El Nino years provide the main reason for the increased midto-late winter predictability for the storm tracks.The results imply that good representation of the background subtropical jet in models is important for winter climate prediction of storm tracks.(Nie Yu,Ren Hongli)

3.9 Multi-model ensemble sub-seasonal forecasting of precipitation over the maritime continent in boreal summer

The maritime continent (MC) is a critical region with unique geographical conditions and significant monsoon activities that plays a vital role in global climate variation.In this study,the weekly prediction of precipitation over the MC during boreal summer (from May to September) was analyzed using the 12-year reforecasts data from five sub-seasonal to seasonal (S2S) models,including the China Meteorological Administration (CMA),the European Centre for Medium-Range Weather Forecasts (ECMWF),Environment and Climate Change Canada (ECCC),the National Centers for Environmental Prediction (NCEP),and the Met Office (UKMO).The result shows that,compared with the individual models,our newly derived median multi-model ensemble (MME) can significantly improve the prediction skill of sub-seasonal precipitation in the MC.Both the temporal correlation coefficient skill and the pattern correlation coefficient skill reached 0.6 in lead week 1,dropped the following week,did not exceed 0.2 in lead week 3,and then lost their significance.The results show higher prediction skill near the Equator than in the north at 10° N.It is difficult to make effective predictions with the models beyond three weeks.The prediction ability of the median MME improves significantly as the total number of model members increases.The prediction performance of the median MME depends not only on the diversity of models but also on the number of model members.Moreover,the prediction skill is particularly sensitive to the intensity and phase of boreal summer intraseasonal oscillation 1 with the highest skills appearing at initial phases 1 and 5.(Wang Yan,Ren Hongli)

3.10 热带和副热带环流对东亚低纬度冬季风强度影响

东亚冬季风具有南北一致变化和南北反相变化两种主要模态。与第一模态反映的南北贯穿的冬季风整体强弱变化不同，第二模态体现了低纬度（中国南方地区）冬季风强弱变化不依赖于中高纬度（中国北方地区）冬季风强弱、甚至与之相反的变化状态。本文利用经验正交函数分析、相关分析、偏相关分析等方法重点研究了在第二模态背景下，低纬度（中国南方地区）冬季风强弱变化对应的热带和副热带环流异常特征。研究发现：热带辐合带是影响低纬度冬季风的一个重要系统。当热带辐合带加强并向北推进时，热带西太平洋及南海地区对流上升运动相应加强。这一上升支可能强迫出低层偏北风异常，从而引起低纬度冬季风加强。此外，副热带高空急流是影响低纬度冬季风的另一个重要系统。急流轴上风速加强会造成入口区准地转偏北风的异常，它强迫出的正次级环流也会相应加强，对应急流北侧的异常下沉和南侧的异常上升，并促使低层产生偏北风异常，也即促进了低纬度冬季风加强。进一步考察热带辐合带对流活动和副热带急流风速异常对低纬度冬季风的独立和协同影响发现，前者的影响相对更为重要。而在二者同时增强的综合作用下，可引起中国南部35°N 以南地区的偏北风异常显著增强，反之亦然。上述结果揭示，冬季低纬度风场的变化不仅受到北方冷空气爆发的影响，它还受制于热带、副热带环流系统异常的共同调控作用。(彭京备，刘舸，孙淑清，何柯俊)