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南海夏季风北推时间及相关环流变化特征
The Period of South China Sea Summer Monsoon Advanced Northward with the Related Circulation Change Features
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摘要: 利用1958—2004年NCEP/NCAR逐日再分析资料和我国730站降水资料分析了南海夏季风爆发后影响到华南地区的时间差异及其环流变化特征。结果表明：南海夏季风向北推进影响到华南地区的时间存在明显差异，最早的可以1 d就推进影响到华南地区，最晚的却要42 d，并且这种变化具有明显的年代际变化特征，即20世纪70年末以前，南海夏季风影响到华南地区的时间总体上要偏早，而70年代末以后，南海夏季风影响到华南地区的时间总体上要偏晚；当南海夏季风建立后，若东亚大槽较深，冷空气活动较活跃，索马里越赤道气流形成的西南风、110°~120°E地区越赤道气流形成的偏南风以及副热带高压西侧边缘的偏南风均偏弱，南亚高压和东亚地区急流位置偏南，就会使得南海夏季风影响到华南地区的时间偏晚，反之，则偏早；南海夏季风推进影响到华南地区的时间偏晚（早）年期间，索马里、105°E和130°E越赤道气流输送的水汽通量和西太平洋副热带高压南部的东南气流水汽输送均较弱（强），华南地区前汛期的锋面降水较强（弱）。Abstract: The Southern China is one of the most rainy areas in the country, which is affected by both the tropical and subtropical monsoon.From April to June, the Southern China experiences the first rainy period, namely the pre rainy season, which is also the first phase that the annual rain belt starts appearing and pushes northward.For the whole Southern China, heavy rain during the target months uses to cause a flood, which accounts for 40%—50% or even more of the whole year precipitation.Because they are concentrated and severe, the rains during the pre rainy season always get more attention and have been studied a lot.But, currently some researches always consider the pre rainy season as a whole.In fact, the pre rainy season can be divided into two phases i, e., the frontal rainfall and the monsoon rainfall.But now, there are not yet any analytical studies on it.The time when the South China Sea summer monsoon breaking out does not mean the commencement of the Southern China summer monsoon precipitation until the South China Sea summer monsoon pushes northward to Southern China, which causes the sec ond peak of the pre rainy season.Therefore, the study can distinguish the frontal rainfall and monsoon rainfall reasonably and get the time when the South China Sea summer monsoon affects the Southern China and the characteristic of the related circulation change, it will be beneficial to providing a new way of thinking for improving precipitation forecasting of the pre rainy season in Southern China.Based on European Center for Medium range Weather Forecast (ECMWF) and NCEP daily reanalyzed dataset from 1958 to 2004 and the surface air temperature/rainfall records from 730 stations in China, the results show that the time exists obvious differences.Also, this variety has obvious inter decadal change characteristic, namely that the time the Southern China summer monsoon affects the Southern China is early before 1970s and late in the late 1970s.When the Southern China summer monsoon breaks out, if the East Asian trough is deeper, the cold air activity is more active, the south west wind from the crossing equatorial currents of Somali and around 110°—120 °E areas and the west part of the subtropical high is weaker, and the position of the South Asia high and the East Asian jet is more southward, it would cause the time to be later, otherwise it would cause the time to be earlier.During the period that the Southern China summer monsoon affected the Southern China later (earlier), the transportation of the moisture from the crossing equatorial currents of Somali and around 105°E and 130°E areas and the southeast part of the west Pacific subtropical high is weaker (stronger), the frontal precipitation of the pre rainy season rainfall in the Southern China is stronger (weaker).
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图 1 华南地区大气中的水汽、热力及动力状况变化 (a) 1971—2000年100 hPa气候平均华南前汛期纬向风垂直分布 (单位: m/s, 阴影区为东风; 粗点线为850 hPa假相当位温, 单位: K), (b) 大气柱水汽含量变化 (单位: mm), (c) 110°~120°E平均假相当位温的垂直切变 (700 hPa减去925 hPa, 单位: K), (d) 110°~120°E我国大陆测站实测降水 (单位: mm/d), (e) CMA P资料气候平均降水量 (单位: mm/d, 虚线箭头是锋面降水, 实线箭头是季风降水)
Fig. 1 The water vapor, thermal and dynamical variations over South China (a) vertical cross section of 1971—2000 climatological mean zonal winds during pre-rainy season (unit:m/s, easterlies region is shaded; thick line marked by dots is θse at 850 hPa, unit:K), (b) predictable w ater content (unit:mm), (c) vertical shear of θse averaged for 110°—120°E (θse at 700 hPa minus θse at 925 hPa, unit:K; regions less than-6 K are shaded), (d) climatological mean precipitation over 110°—120°E during the pre-rainy season from surface stations in China mainland (unit:mm/d), (e) mean precipitation during the pre-rainy season from CMAP data (unit:mm/d, dashed arrows denote frontal rain and solid arrows denote summer monsoon rain)
图 2 季风推进影响到华南时间距平图
Fig. 2 Time anomalies of the South China Sea summer monsoon advancing to South China
图 3 推进偏晚年和偏早年由南海夏季风爆发至华南季风降水开始日期时间差异段850 hPa风场差值图
(单位: m/s, 晚减早, 阴影区和实线所围区域分别为纬向风和经向风通过α=0.05的显著性检验区)
Fig. 3 850 hPa wind differences by late years composite and early ones (unit:m/s, late years mean relative long time from South China Sea summer monsoon onset to beginning of monsoon rain over South China, and early years mean relative short time during the process; areas shaded and enclosed by solid line denote the regions that zonal winds and meridional winds exceed α=0.05 significance test, respectively)
图 4 推进偏晚年和偏早年由南海夏季风爆发至华南季风降水开始日期时间差异段500 hPa位势高度场合成差值图 (单位: gpm)
( 晚减早, 阴影区为通过α =0. 05 的显著性检验区)
Fig. 4 Same as Fig.3 except for 500 hPa geopotential height (unit:gpm)
图 6 推进偏晚年 (a) 和偏早年 (b) 由南海夏季风爆发至华南季风降水开始日期时间差异段的高低层假相当位温差值图
(单位: K, 其中高、低层分别以600~300 hPa和1000~700 hPa的平均值代替)
Fig. 6 θse differences between upper (600—300 hPa) and lower troposphere (1000—700 hPa) (a) late years composite, (b) early years composite
图 7 推进偏晚年和偏早年由南海夏季风爆发至华南季风降水开始日期时间差异段的平均地表至300 hPa气柱水汽通量合成和差值图 (a) 偏晚年, (b) 偏早年, (c) 偏晚年与偏早年差值 (a和b的阴影区为水汽通量大于1 kg/(cm·s) 区域, c的阴影区为通过α =0.05的显著性检验区)
Fig. 7 Water vapor fluxes from surface to 300 hPa (a) late years composite, (b) early years composite, (c) differences (shaded regions denote water vapor fluxes are larger than 1 kg/(cm·s) in (a) and (b), and represent the areas passing α=0.05 significance test in (c))
图 8 推进偏晚年 (a) 和偏早年 (b) 由南海夏季风爆发日至华南季风降水开始日期之间得到的时间差异段的区域 (20°~40°N, 100°~120°E) 站点降水合成图
(单位: mm, a和b的阴影区分别为日降水量大于3 mm和1.5 mm区域)
Fig. 8 Precipitation composite from surface stations data in the region of 20°—40°N, 100°—120°E (a) late years, (b) early years (unit:mm, daily precipitations larger than 3 mm and 1.5mm are shaded in (a) and (b), respectively)
表 1 1958—2004年南海夏季风爆发日期、华南季风降水开始日期和季风推进影响到华南地区时间
Table 1 Dates of South China Sea summer monsoon onset and beginning of monsoon rain over South China and time intervals between them
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