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Atmospheric Low-frequency Oscillation over the Tibetan Plateau During 1997-1998 and Its Effects on Precipitation
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摘要: 利用1979—1998年NCEP/DOE逐日再分析资料和国家气象信息中心的常规观测站资料，研究了1997/1998年冬季、1998年夏季青藏高原 (简称高原) 季风的低频振荡特征，研究夏季高原和周边区域高低层大气低频环流系统的配置及其与我国降水的联系。结果表明：1997/1998年冬季和1998年夏季，高原季风不仅表现出很强的30~60 d的周期振荡特征，还伴随有较强的准双周低频振荡；相应区域对流层上层200 hPa上的环流系统则是30~60 d为主的周期变化。1998年夏季，高原地面气压也存在两个频带的低频振荡变化，且其强度存在明显的经向变化，即自南向北30~60 d低频振荡信号有逐渐减弱趋势，准双周信号则呈增强趋势。对30~60 d的低频信号而言，高原夏季风低频信号较强 (弱) 时，高原地面表现为低频低 (高) 压环流系统，在同纬度带的我国东部地区和西太平洋沿岸，是较强的低频北 (南) 风和低 (高) 压环流系统；相应地，在80°~90°E之间，自孟加拉湾到我国西北中部地区，是低频反气旋-气旋-反气旋的经向低频波列；受低频环流系统影响，高原东部、长江中下游地区降水偏多 (少)、川西高原、云南西南部降水偏少 (多)。Abstract: Based on NCEP/DOE daily reanalysis data and conventional observations of National Meteorological Information Center, characteristics of the atmospheric low-frequency oscillation of the Tibetan Plateau monsoon in the winter of 1997-1998 and in the summer of 1998 are studied. Furthermore, the configuration of the upper and lower atmospheric low-frequency circulation system on and around the Plateau and its connection to the precipitation of China are also studied. Results mainly show that the Plateau monsoon exhibits not only a strong periodic oscillation of 30-60 days, but also a quasi-biweekly low-frequency oscillation feature, and the relevant upper troposphere circulation system in the same areas at 200 hPa mainly shows a periodic variation of 30-60 days. In the summer of 1998, there are two low-frequency oscillations of surface pressure on the Plateau and its strength has significant longitudinal change, which means that the periodic oscillation of 30-60 days gradually reduces from south to north, while the signal of quasi-biweekly oscillation becomes stronger.As for low-frequency signal of 30-60 days, when the Plateau summer monsoon is stronger (weaker), there is an obvious low-frequency cyclonic convergence (anti-cyclonic divergence) circulation system from the Plateau to the western Pacific between 25°N and 35°N, but the low-frequency anti-cyclonic (cyclonic) circulation system exists in the region south to the Plateau, from the northern Indian subcontinent and the Bay of Bengal to the northern part of South China Sea. Within the longitude scope of the Plateau (between 80°E and 90°E), there is a low-frequency wave chain from the Bay of Bengal to the eastern region of Xinjiang, which ranks as low-frequency anticyclone (cyclone)-low-frequency cyclone (anticyclone)-low-frequency anticyclone (cyclone). Within the latitude scope of the Plateau, there is low-frequency anti-cyclonic (cyclonic) circulation system at 200 hPa from the western part of the Plateau to the Sea of Japan.Influenced by low-frequency circulation system, when Plateau summer monsoon is strong, the low-frequency circulation system configuration within the latitude scope of the plateau converge on the low-level and diverge on the high-level of 200 hPa, which cause more precipitation over the eastern part of the Plateau and the middle and lower reaches of the Yangtze, while cause less precipitation over the western Sichuan Plateau and southwestern Yunnan. When the Plateau summer monsoon is weak, the low-frequency circulation system configuration diverges on the low-level and converge on the high-level, which leads to less rainfall in many parts of the contral-eastern part of the Plateau and the eastern part of China. At this time, the easterly flux from the South China Sea, southwesterly flux from the Bay of Bengal and northerly flux from the Plateau converge in the southwest of Yunnan and corresponding low-frequency circulation system of water vapor transportation also provide moisture conditions in this region, as a result, the precipitation in the southwest of Yunnan becomes more.
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图 1 1997/1998年冬季 (a) 和1998年夏季 (b) 高原季风指数与其季节变化趋势间偏差的小波变换以及冬季 (c) 和夏季 (d) 小波方差
Fig. 1 The wavelet transform of the difference between the Plateau monsoon index and its seasonal change tendency in the winter of 1997-1998 (a) and in the summer of 1998 (b) with wavelet variances during the winter of 1997-1998 (c) and the summer of 1998 (d)
图 2 1997/1998年冬季 (a) 和1998年夏季 (b) 200 hPa区域平均位势高度场与其季节变化趋势偏差的小波变换以及冬季 (c) 和夏季 (d) 的小波方差
Fig. 2 The wavelet transform of the difference between 200 hPa regional average geopotential height field and its seasonal change tendency in the winter of 1997-1998 (a) and in the summer of 1998 (b) with the wavelet variances during the winter of 1997-1998 (c) and the summer of 1998 (d)
图 3 1998年夏季那曲站地面气压与其季节变化趋势偏差的小波变换
Fig. 3 The wavelet transform of the difference between surface pressure and its seasonal tendency at Naqu Station in the summer of 1998
图 4 1998年拉萨站 (a)、当雄站 (b) 和那曲站 (c) 地面气压与其季节变化趋势偏差小波变换在不同频域上的方差
Fig. 4 The wavelet variance on different frequency domain of the wavelet transform of surface pressure at Lhasa Station (a), Dangxiong Station (b) and Nagqu Station (c) in the summer of 1998
图 5 1998年5—9月600 hPa高原季风指数30～60 d低频振荡的4个不同位相合成的600 hPa低频风场 (A代表低频反气旋，C代表低频气旋) (a) 第1位相，(b) 第2位相，(c) 第3位相，(d) 第4位相
Fig. 5 600 hPa low-frequency wind field of four phases based on 30-60 d filtered 600 hPa Plateau monsoon index from May to Sep in 1998 (A denotes low-frequency anticyclone, C denotes low-frequency cyclone) (a) phase 1, (b) phase 2, (c) phase 3, (d) phase 4
图 6 1998年5—9月200 hPa区域平均位势高度场30～60 d低频振荡的4个不同位相合成的200 hPa低频风场
(A代表低频反气旋，C代表低频气旋) (a) 第1位相，(b) 第2位相，(c) 第3位相，(d) 第4位相
Fig. 6 200 hPa low-frequency wind field of four phases based on 30-60 d filtered 200 hPa regional average geopotential height field from May to Sep in 1998
(A denotes low-frequencg anticyclone, C denotes low-frequency cyclone) (a) phase 1, (b) phase 2, (c) phase 3, (d) phase 4
图 7 1998年5—9月600 hPa高原季风30～60 d低频振荡信号最强位相 (a) 和最弱位相 (b) 合成的我国夏季日平均降水分布
Fig. 7 The summer precipitation in our country during the strong (a) and weak (b) phases based on 30-60 d filtered 600 hPa Plateau monsoon index in the Tibetan Plateau from May to Sep in 1998
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