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东亚夏季风强弱年大气环流和热源异常对比分析
Contrast Analysis on Atmospheric Circulation and Heat Source Anomalies in Strong and Weak Years of East Asian Summer Monsoon
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摘要: 根据黄刚等定义的东亚夏季风指数, 对强、弱东亚夏季风年大气环流、大气热源和外强迫源SST的差异进行分析, 结果表明:强 (弱) 东亚夏季风年前期冬季到夏季, 太平洋SSTA为La Niña (El Niño) 型分布, 西太平洋暖池SST暖 (冷), 使得暖池附近对流活动较强 (较弱)。与此同时, 南亚大陆从印度半岛、青藏高原南部、中南半岛至华南大气异常加热 (变冷), 并且海陆热力对比加强 (减弱), 有利于出现强 (弱) 的东亚夏季风。此外, 由于暖池附近对流活动强 (弱), 该地区上升气流较强 (弱), Walker环流增强 (减弱), 当强 (弱) 的东亚夏季风向北推进时, 副热带西风急流北撤位置偏北 (南), 副热带高压位置也偏北 (南), 7月至8月华北 (江淮流域) 位于副热带西风急流南侧, 降水偏多, 江淮流域 (华北) 降水偏少。并给出与东亚夏季风年际变异有关的大气环流和SST异常的物理图像。Abstract: Based on East Asian summer monsoon (EASM) index defined by Huang Gang et al., the differences of atmospheric circulation, atmospheric heat source and external forcing (i. e., SST) between strong and weak monsoon years are discussed.It is found that SSTA in Pacific appears La Niña pattern in winter preceding a strong EASM until the summer. That is to say, the SST in the equatorial central and eastern Pacific is low, while that in the Western Pacific Warm Pool is high.The convective activities over the equatorial eastern Indian Ocean, Sumatra and Warm Pool become obviously stronger than weak monsoon years.Meanwhile the atmosphere over South Asia, extending from the Indian subcontinent, through the southern part of the Tibetan Plateau, the Indochina Peninsula to South China, is abnormally heated.Furthermore, the analysis of mean tropospheric temperature anomalies shows that the heat contrasts between sea and land intensify.It is advantageous for the occurrences of strong EASM.In addition, the ascendant flow near Warm Pool is strong because of the active convection.So the Walker circulation becomes intense. The Asian subtropical westerly jet and subtropical high are located northward while the strong EASM advances northward.North China is located at the south side of the Asian subtropical westerly jet during July and August.So the precipitation over North China is abundant.Another rain belt is located at South China.But the precipitation over the Yangtze and Huaihe River Basins is short.On the other hand, winter SSTA in Pacific shows El Niño pattern preceding a weak EASM.The SST in the equatorial central and eastern Pacific and tropical Indian Ocean is high whereas that in the Warm Pool stretching through northwestern Pacific is low.This SSTA pattern lasts until the summer.As a result, the convective activity near Warm Pool becomes weak.Moreover, from April to July the atmosphere over the equatorial central and eastern Pacific is abnormally heated.But the atmosphere over South Asia, extending from the Indian subcontinent to South China, is cold.At the same time, the land-sea thermal difference between Asian land and the ocean weakens.It means a weak EASM.The center of convection activity shifts eastward into the central Pacific.Accordingly the Walker circulation becomes subdued.The ascendant flow over Indochina Peninsula and Philippine is weak.The Asian subtropical westerly jet and subtropical high are located southward while the weak EASM advances northward.The Yangtze and Huaihe River Basins are located at the south side of the Asian subtropical westerly jet during July and August, the precipitation over this region is abundant.Also, the physical scenario of atmosphere and SST anomalies associated with the interannual variability of the EASM are given.
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图 1 强东亚夏季风年合成风场沿105°~125°E平均的经向-高度剖面图 (a) 4月, (b) 6月, (c) 7月, (d) 8月
(矢量的两个分量为经向风v, 单位: m·s-1, 垂直速度ω, 单位: 10-4 hPa·s-1; 等值线表示纬向风u, 单位: m·s-1, 当u>5 m·s-1用阴影表示, 粗的垂直虚线表示西风急流轴所在的位置, 粗的曲线表示105°~125°E平均的降水随纬度的变化, 单位: mm·d-1)
Fig. 1 The composite meridian-altitude sections of wind through 105°—125°E in April (a), June (b), July (c) and August (d) for strong East Asian summer monsoon years
(the components of vectors are meridional wind, unit: m·s-1, vertical velocity, unit : 10-4 hPa·s-1; the contours refer to zonal wind, unit : m·s-1; shaded area indicates zonal wind exceeding 5 m·s-1; the vertical thick dashed line represents the axes of westerly flow; the thick solid line denotes the mean precipitation averaged between 105°E and 125°E, unit : mm·d-1)
图 3 东亚夏季风强弱年OLR (单位: W·m-2) 差值 (a) 4月, (b) 5月, (c) 6月, (d) 7月, (e) 8月
(图中阴影表示超过90%信度检验的区域)
Fig. 3 The difference of OLR (unit : W·m-2) between the strong and weak years of East Asian summer monsoon in April (a), May (c), June (c), July (d) and August (e)
(regions over 90% confidence level are shaded)
图 4 东亚夏季风弱年 (a) 和强年 (b) 夏季对流层 (850~200 hPa) 平均气温的距平合成
(单位: ℃; 图中阴影表示超过90%信度检验的区域)
Fig. 4 The composites of mean tropospheric temperature anomalies averaged from 850 hPa to 200 hPa for weak (a) and strong (b) years of East Asian summer monsoon
(unit : ℃; regions over 90% confidence level are shaded)
图 5 东亚夏季风强年对流层垂直积分 (地面~100 hPa) 的大气视热源Q1的距平合成 (单位: W·m-2) (a)4月, (b) 5月, (c) 6月, (d) 7月
(图中阴影表示超过95%信度检验的区域)
Fig. 5 The composites of Q1 anomalies (unit:W·m-2) integ rated vertically from surface to 100 hPa in April (a), May (b), June (c) and July (d) for strong years of East Asian summer monsoon
(regions over 95% confidence level are shaded)
图 6 东亚夏季风弱年对流层垂直积分 (地面~100 hPa) 的大气视热源Q1的距平合成 (单位: W·m-2) (a)4月, (b) 5月, (c) 6月, (d) 7月
(图中阴影表示超过95%信度检验的区域)
Fig. 6 Same as in Fig. 5, but for East Asian summer monsoon in weak years
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