Abnormal Warming of the Summer Surface Air Temperature in Central Asia from 1980 to 2019
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摘要: 本文利用1980~2019年NCEP/NCAR全球再分析资料、CRU地表气温资料、积雪覆盖率资料和全球海温资料,分析了中亚夏季地表气温的气候突变及其和北大西洋海温、青藏高原积雪之间的关系。结果表明:中亚夏季地表气温在2005年发生明显的气候突变。标准化的中亚区域平均的气温指数从之前的负位相为主变为之后的正位相为主,表示中亚地区夏季地表气温显著增温。和中亚夏季地表气温异常增温相联系的大气环流场的分析表明,2005年之后,中亚地区西侧的反气旋性环流系统异常增强,该反气旋异常对应的大气下沉增暖以及反气旋异常增强引起的云量减少进而导致向下的短波辐射增加均有利于中亚夏季气温异常升高。进一步的合成分析表明,中亚夏季地表气温在2005年的气候突变和北大西洋中高纬度地区的海表温度的增暖和青藏高原西部积雪的减少有着密切关系。北大西洋中高纬度海表温度增温能激发一个向下游传播的罗斯贝波,青藏高原西部积雪减少能够通过积雪的反照率效应对上空的大气有增温作用,两者均能增强中亚地区上空的反气旋系统,从而有利于中亚夏季地表气温异常偏高。Abstract: This work analyzes the abrupt change in summer surface air temperature (SAT) in Central Asia (CA) and its relationship with sea surface temperature (SST) in the North Atlantic and snow cover in the Qinghai Tibet Plateau between 1980 and 2019 based on NCEP/NCAR reanalysis data, CRU SAT, and snow cover and global SST data. The results reveal a significant summer SAT change in CA in 2005. The standardized regional average temperature index in CA shifts from the previous negative phase to the subsequent positive phase, indicating a significant summer SAT increase in CA. Analysis of the anomalous atmospheric circulations related to interdecadal changes in summer SAT in CA shows the abnormally enhanced anticyclonic circulation system in the west of CA after 2005. The atmospheric subsidence associated with the anomalous anticyclone can cause warming. On the other hand, the reduction in the amount of cloud caused by this anticyclone anomaly enhancement results in the increase in downward short-wave radiation and thus is favorable for the increased summer temperature in CA. Furthermore, the interdecadal summer SAT changes in CA in 2005 are closely related to SST warming in the middle and high latitudes of the North Atlantic and the reduction in snow cover in the west of the Tibet Plateau (TP). The SST increase in the middle and high latitudes of the North Atlantic can stimulate a Rossby wave propagating downstream. The reduction in snow cover in the west of the TP can cause warming to the above atmosphere through the snow albedo effect. The changes in both the North Atlantic SST and the TP snow can strengthen the anticyclone over CA, leading to an abnormally high summer SAT over there.
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Key words:
- Central Asia /
- Summer SAT (Surface Air Temperature) /
- Interdecadal changes /
- SST /
- Snow cover
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图 1 1980~2019年中亚以及周边地区夏季(a)地表气温(填色,单位:°C)叠加850 hPa风场(矢量,单位:m s−1)以及(b)200 hPa纬向风(填色,单位:m s−1)叠加风场(矢量,单位:m s−1)的气候平均分布;(c)1950~2019年中亚地区[(a)中红色框区域;(36°~55°N,40°~90°E)]区域平均的地表气温距平直方图
Figure 1. Climatological summer (a) SAT (Surface Air Temperature, shading, units: °C) and 850-hPa wind (vectors, units: m s−1), (b) 200-hPa zonal wind (shading, units: m s−1) and wind (vectors, units: m s−1) in CA (Central Asia) and the surrounding areas from 1980 to 2019. (c) Regional average surface air temperature anomaly in CA [red box area in (a), (36°−55°N, 45°−90°E)] from 1950 to 2019
图 2 1980~2019年中亚地区(36°~55°N,40°~90°E)夏季地表气温场EOF分析(a)第一模态(EOF1)的空间分布型(单位:°C)和(b)EOF1对应的标准化时间序列(直方图)及其11年的低频滤波(黑色实曲线),图(a)中打点区域表示通过95%的显著性检验,右上角百分数为EOF1的方差解释率。(c)标准化的中亚地区区域平均的地表气温指数(直方图)及其11年的低频滤波(黑色实曲线),虚线表示地表气温指数的正负0.5标准方差
Figure 2. (a) Spatial pattern of the first EOF (EOF1) of summer (June−July−August) SAT (units: °C; shading) over CA (36°−55°N, 45°−90°E) represented by regressing SAT onto the time series of EOF1 from 1979 to 2019. The number on the top right corner indicates the percentage of the variance explained by EOF1. Anomalies significant at the 95% confidence level are dotted. The yellow box in (a) represents CA. (b) Corresponding time series of EOF1 (PC1) (bar charts). The black line represents the low-frequency component of PC1 with periods longer than 10 years. (c) Area-weighted averaged summer temperature index (TI) over CA (represented by the solid bar charts) and low-frequency components of TI (represented by the black solid line). The horizontal long dashed black lines denote the ±0.5 standard deviation
图 3 1980~2019年(a)中亚夏季地表气温场EOF1对应的标准化的时间序列和(b)中亚地区区域平均的地表气温指数的MK(Mann–Kendall)检验。黑色实线为前向曲线(UF),蓝色实线为后向曲线(UB)。水平虚线为95%显著性线,垂直虚线标示UF和UB的交点。
Figure 3. Mann–Kendall (MK) statistic curves of (a) the PC1 of the EOF1 of summer SAT over CA and (b) the area-weighted averaged summer TI over CA. The solid blue and solid black lines are the forward (UF) and backward (UB) curves, respectively. The horizontal long dashed black lines denote the 95% confidence level, and the dashed vertical line denotes the intersection of the UF and UB
图 4 中亚夏季暖年和冷年的(a)地表气温(填色,单位:°C)与850 hPa风场(箭头,单位:m s−1)、(b)500 hPa垂直速度场(单位:Pa s−1)、(c)200 hPa风场(矢量,单位:m s−1)叠加风速场(填色,单位:m s−1)和(d)总云量覆盖率的合成差值场。图中打点区域表示通过95%的显著性检验
Figure 4. Composite differences of the (a) SAT temperature (shading, units: °C) and 850-hPa wind (vectors, units: m s−1), (b) 500-hPa vertical velocity (shading, units: Pa s−1), (c) 200h-Pa wind (vectors, units: m s−1) and wind speed (shading, units: m s−1), and (d) total cloud cover (shading) between the warm and cold years in CA. Anomalies significant at the 95% confidence level are dotted
图 6 中亚夏季暖年和冷年的(a)大西洋海温(填色;单位:°C)的合成差值场。(b)标准化的北大西洋中高纬度(35°~80°N,70°W~10°E)区域平均海温指数(直方图)及其11年的低频滤波(黑色曲线)。北大西洋中高纬度区域平均海温指数对(c)夏季地表气温(填色,单位:°C)和850 hPa风场(矢量,单位:m s−1)以及(d)200 hPa风场(矢量,单位:m s−1)和风速场(填色,单位:m/s)的异常回归场。图中打点区域均表示通过95%的显著性检验
Figure 6. (a) Composite differences of the summer SST (shading, units: °C) between the warm and cold years in CA. (b) Normalized SST index obtained by the area-weighted average of SST over the key region of the North Atlantic (35°–80°N, 70°W–10°E; represented by the solid bar charts) and low-frequency components of the index (represented by the black dotted line). Anomalies in the summer (c) SAT (shading, units: °C) and 850-hPa wind (vectors, units: m s−1) and (d) 200-hPa wind (vectors, units: m s−1) and wind speed (shading, units: m s−1) obtained by regression against the sea surface TI from 1980 to 2019. Anomalies significant at the 95% confidence level are dotted
图 7 中亚夏季暖年和冷年的(a)青藏高原积雪覆盖率的合成差值场。(b)标准化的青藏高原西部(32°~44°N,70°~80°E)区域平均积雪指数(直方图)及其11年的低频滤波(黑色曲线)。(c)青藏高原西部积雪指数对地表气温(填色,单位:°C)与850 hPa风场(箭头,单位:m s−1)的回归图。(d)青藏高原西部积雪指数对200 hPa风场(矢量,单位:m s−1)与风速(填色,单位:m s−1)的回归图。图中的打点区域表示通过95%的显著性检验
Figure 7. (a) Composite differences of the snow cover extent (SCE, shading) between the warm and cold years in CA. (b) Normalized snow index obtained by the area-weighted average of the SCE over the key region of the western TP (32°–44°N, 70°–80°E; represented by the solid bar charts) and low-frequency components of the index (represented by the black dotted line). Anomalies in the summer (c) SAT (shading, units: °C) and 850-hPa wind (vectors, units: m s−1), (d) 200-hPa wind (vectors, units: m s−1), and wind speed (shading, units: m s−1) obtained by regression against the snow index from 1980 to 2019. Anomalies significant at the 95% confidence level are dotted
图 8 青藏高原西部积雪指数对(a)地表气温(填色;单位:°C)与850 hPa风场(箭头,单位:m s−1),(b)200 hPa风场(矢量,单位:m s−1)和风速场(填色,单位:m s−1)去掉北大西洋海温指数的偏回归图。打点区域表示通过95%的显著性检验
Figure 8. Anomalies in the summer (c) SAT (shading, units: °C) and 850-hPa wind (vectors, units: m s−1) and (b) 200-hPa wind (vectors, units: m s−1) and wind speed (shading, units: m s−1) obtained by partial regression against the snow index after removing the North Atlantic sea surface TI from 1980 to 2019. Anomalies significant at the 95% confidence level are dotted
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