Key Circulation Characteristics of Spring-to-summer Seasonal Transition Process over Mid- and High-Latitude Asia
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摘要: 亚洲中高纬环流春夏季节转换是亚洲大陆上发生的不同区域春夏季节转换中的一个重要组成部分,它为江淮流域梅雨形势的建立提供必要的中高纬环流条件。但是关于其独特性和关键特征,迄今为止尚没有系统性的总结。本文利用NCEP/NCAR再分析资料I的逐日数据,分析和总结了这一春夏季节转换过程的关键特征。亚洲中高纬环流春夏季节转换以500 hPa东北亚脊和“双阻型”环流形势的依次建立为重要标志。东北亚脊及其相关的海陆温度差异的形成主要归因于东北亚积雪融化和局地强烈增温过程。东北亚低压(850 hPa)的建立是亚洲中高纬环流春夏季节转换的另一个重要标志。当季节转换发生时,青藏高原上空的200 hPa急流轴从35°N向北跳到37°N,与此同时亚洲温带急流彻底消失。伴随着季节转换,亚洲中高纬地区近地面温度经向梯度减弱,高频瞬变斜压扰动随之减弱;与之形成鲜明对比,低频天气系统,包括亚洲阻塞高压和东北冷涡系统,则成为该地区主导天气系统。本文也从春夏季节转换早晚的角度,探讨了亚洲中高纬环流和天气系统的演变特征,由此进一步补充了春夏季节转换的关键信息。Abstract: The spring-to-summer seasonal transition over mid- and high-latitude Asia (MHASST) is an important part of several spring-to-summer seasonal transitions in different regions of the Asian continent. It provides the necessary circulation conditions in the mid- and high-latitudes for the establishment of the Meiyu rainfall in the Yangtze and Huaihe River basins. However, so far, there is no systematic summary of its uniqueness and key characteristics. This paper analyzes and summarizes the key characteristics of the MHASST process based on the daily data of NCEP/NCAR reanalysis data I. The MHASST is symbolized by the establishment of the Northeast Asian ridge at 500 hPa and the “double blocking” circulation pattern. The formation of the Northeast Asian ridge and its related land-sea temperature difference is mainly attributed to the snow melting process and consequently the local strong warming process in Northeast Asia. The establishment of the northern East Asian low (850 hPa) is another important sign of the MHASST. When the MHASST occurs, the 200-hPa Asian jet axis over the Tibetan Plateau jumps northward from 35°N to 37°N, while the Asian temperate jet disappears completely. With the seasonal change, the meridional gradient of the near-surface temperature in the mid- and high-latitude Asia weakens, thus causing the attenuation of high-frequency transient baroclinic disturbances. In contrast, low-frequency weather systems, including the Asian blocking high and the Northeast China cold vortex system, become the dominant weather systems in the same region. From the perspective of the early and late timing of the MHASST, this paper also discusses the evolution features of the circulation and weather system over the mid- and high-latitude Asia. The obtained results further supplement the key information of the climatic MHASST.
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图 1 (a)P28、(b)P30、(c)P32和(d)P34气候平均500 hPa位势高度(Z500)场(等值线,间隔:50 gpm)及其纬偏场(填色,gpm)。粗实线为5450 gpm等值线,每图最低点为(20°N,90°E)
Figure 1. Climatological 500-hPa geopotential height (contours; drawn every 50 gpm) and its zonal difference (color shaded; units: gpm) for (a) P28, (b) P30, (c) P32, and (d) P34 in the Northern Hemisphere. The heavy black line represents 5450 gpm and the lowest point of the map is (20°N, 90°E)
图 3 (a、d)P30、(b、e)P32和(c、f)P34时气候平均的850 hPa位势高度场(等值线,间隔为15 gpm)、水汽通量场(矢量,单位:g s−1 cm−1 hPa−1)、温度场(填色,单位:°C)分布(左列)以及水汽通量增量场(矢量,单位:g s−1 cm−1 hPa−1)分布(右列)。红色粗实线为0°C线,框区为东北亚低压关键区(45°~60°N,110°~140°E),灰色表示高于1500 m地形
Figure 3. Climatological 850-hPa geopotential height (contours; drawn for every 15 gpm), water vapor flux (vectors; units: g s−1 cm−1 hPa−1) and temperature (color shaded; units: K) for (a) P30, (b) P32, and (c) P34. (d–f) as in (a–c), but for water vapor flux increment (vectors; units: g s−1 cm−1 hPa−1). Red heavy lines represent 0°C. The gray shading indicates the topography higher than 1500 m and the box area denotes the key area of northeastern Asian low (45°–60°N, 110°–140°E)
图 4 (a)P28、(b)P30、(c)P32和(d)P34期间气候平均表面温度(SAT;等值线,间隔:5 K)及其增量场(填色,单位:K)。浅蓝、深蓝以及紫色等值线分别代表275 K、280 K以及285 K等温线,灰色表示高于1500 m的地形,每图最低点为(20°N,90°E)
Figure 4. Climatological surface air temperature (contours; drawn every 5 K) and its increment (color shaded; units: K) for (a) P28, (b) P30, (c) P32, and (d) P34. Light blue, dark blue, and purple lines represent 275 K, 280 K, and 285 K, respectively. The altitude of the gray area is higher than 1500 m. The lowest point of the map is (20°N, 90°E)
图 6 (a)P25至P36期间亚洲副热带急流轴线的位置以及(b)P28、(c)P30、(d)P32和(e)P34期间气候平均200 hPa纬向风速U(等值线间隔为10 m s−1;斜线区为U>20 m s−1的区域)。灰色阴影表示高于1500 m的地形
Figure 6. (a) Position of the climatological axis of the jet stream over Asia from P25 to P36. The climatological 200-hPa zonal wind (contours drawn every 10 m s−1, slash area larger than 20 m s−1) for (b) P28, (c) P30, (d) P32, and (e) P34. The altitude of the gray area is higher than 1500 m
图 7 (a、e)P28、(b、f)P32、(c、g)P34和(d、h)P38期间气候平均mAPV阻塞高压频率(左列;等值线,间隔:5%)以及300 hPa瞬变涡动动能(EKE300;右列;等值线,间隔为10 m2 s−2)
Figure 7. Climatological mAPV block frequency (contours; drawn every 5%) for (a) P28, (b) P32, (c) P34, and (d) P38. (e–h) are the same as (a–d), but for the 300-hPa EKE (contours; drawn every 10 m2 s−2)
图 8 东北亚关键区(50°~75°N,70°~160°E)平均mAPV阻塞频率(红线)、东北冷涡关键区(35°~55°N,115°~140°E)平均EKE300(黑线,单位:m2 s−2)和东北冷涡天数(蓝线,单位:d)的季节性演变曲线。横坐标为时间(P25至P39)
Figure 8. The average mAPV block frequency (50°–75°N,70°–160°E; red line), average 300-hPa EKE (35°–55°N,115°–140°E; black line; units: m2s−2) and average number of northeastern cold vortex days (blue line; units: d). The x-coordinate is the pentad from P25 to P39
图 9 (a、d)P30、(b、e)P32和(c、f)P34季节转换偏早年(左列)和偏晚年(右列)的Z500场(等值线,间隔为40 gpm)及其距平场(填色,单位: gpm)。粗实线为5450 gpm等值线,打点区为显著性水平超过0.1的地方,每图最低点为(20°N,90°E)
Figure 9. Composite 500-hPa geopotential height (contours; drawn every 50 gpm) and its anomaly (color shaded; units: gpm) for (a, d) P30, (b, e) P32, and (c, f) P34 in years of early summer onset (left column) and late summer onset (right column). Areas above the 90% confidence level are dotted; heavy lines represent 5450 gpm. The lowest point of the map is (20°N, 90°E)
图 10 (a、d)P30、(b、e)P32和(c、f)P34季节转换偏早年(左列)和偏晚年(右列)的mAPV阻塞频率(等值线,间隔为10%)及其距平(填色)。打点区为显著性水平超过0.1的地方。每图最低点为(35°N,90°E)
Figure 10. Composite mAPV block frequency (contours; drawn every 10 %) and its anomaly (color shaded) for (a) P30, (b) P32, and (c) P34 in years of early summer onset (left column) and late summer onset (right column). Areas above the 90% confidence level are dotted. The lowest point of the map is (35°N, 90°E)
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