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新疆夏季降水年际变化与亚洲副热带西风急流
Interannual Variation of Summer Precipitation in Xinjiang and Asian Subtropical Westerly Jet Stream
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摘要: 利用1960—2003年NCEP/NCAR再分析和新疆75个气象站月降水资料, 分析了新疆夏季降水与亚洲副热带西风急流的关系, 新疆夏季降水与西亚急流的南北位置和准静止波活动密切联系。通过波作用量的动力学诊断分析, 研究了新疆降水异常年准静止波活动特征, 新疆降水异常年斯堪的纳维亚半岛向东传播的中高纬静止波传播方式的不同, 从而影响沿副热带西亚西风急流传播的静止波活动, 进而影响新疆夏季降水, 并存在沿60°E自南极高纬低层经向上传至低纬对流层顶部, 并在北半球副热带地区转为经向下传至北半球中纬地区的波列, 该波列活动与西亚急流变化联系。Abstract: The relationships between Asian subtropical westerly jet stream (ASWJS) and summer precipitation in Xinjiang are examined using the NCEP/NCAR reanalysis dataset and 75 stations monthly precipitation data in Xinjiang from 1960 to 2003. Summer precipitation in Xinjiang is closely related to meridional displacement of west Asian jet stream and quasi-stationary wave activity along ASWJS. It is confirmed by Eliasson-Palm flux diagnoses that stationary waves propagation over Scandinavian Peninsulan-middle Europe has important impacts on stationary waves activity along ASWJS. It is found that anomalous summer more (less) rainfall in Xinjiang is related to the West Asian Westerly Jet axis moving to Southern (Northern) rather than normal, and subtropical westerly front also migrates southward (northward) over the whole troposphere. As a result, anomalous convergence and divergence occur over Xinjiang. The composite plots show the troposphere atmospheric steady wave weakens or strengthens over West Asia and strengthens or weakens over Xinjiang which are associated with more or less rainfall in Xinjiang. It is investigated that the horizontal wave vector propagation is obviously different between more and less rainfall years. During more (less) rainfall years, it is found that horizontal wave activity flux divergence center over Scandinavian Peninsula-middle Europe is favorable for moving eastward (westward and southward), and horizontal wave activity propagation strengthens (weakens) toward eastward leading to turning to southeastward near Ural mountain and entering ASWJS at Aral Sea-Xinjiang and the wave propagation strengthens (weakens). In the meantime, the direct wave propagation toward southeastward enters ASWJS in the east Mediterranean and Black Sea and weakens (strengthens) because of which quasi-stationary wave activity along West Asian westerly jet stream weakens (strengthens). Xinjiang summer rainfall is closely related to wave activity and propagation over Scandinavian Peninsula-middle Europe-ASWJS. Furthermore, the meridional wave train along 60°E connecting the two hemispheres is revealed, and this wave train which propagates meridionally upward from lower-troposphere in polar region to low-latitude tropopause and continues turning downward to mid-latitude in northern hemisphere is associated with west Asian jet variation. This wave train EP flux divergence performs opposite the variation near the south and north along 40°N, leading to different west Asian westerly jet stream intense variation near the south and north along 40°N for drought/flood years. EP flux divergence to the south (north) of 40°N and convergence to the north (south) of 40°N enhances (weakens) in flood (drought) years leading to the enhanced (weakened) upper level westerly to the south of 40°N and weakens (enhances) upper level westerly to the north of 40°N.
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图 1 新疆75站夏季降水量标准化序列EOF方差最大的前两个特征模态 (a, b) 及对应的主分量正规化时间序列 (c, d)
Fig. 1 The patterns of the eigenvectors of the first two gravest EOF modes (a, b) and corresponding time series (c, d) of 75 stations normalized rainfall over Xinjiang in summer
图 2 夏季200 hPa纬向风 (单位:m·s-1)(a) 气候平均, (b) 降水偏多年, (c) 降水偏少年
Fig. 2 Zonal winds at 200 hPa in summer (unit:m·s-1)(a) climate mean, (b) in more rainfall years, (c) in less rainfall years
图 3 新疆夏季降水第一主分量时间序列与夏季200 hPa纬向风的相关分布
(阴影区表示通过0.05显著性检验)
Fig. 3 Linear correlation between first mode of Xinjiang summer rainfall EOF and zonal winds in summer
(shaded areas indicate the level exceeding 0.05)
图 6 夏季70°~100°E散度经向-高度分布 (单位:10-6s-1; 阴影为地形高度)(a) 气候平均, (b) 降水偏多年距平, (c) 降水偏少年距平
Fig. 6 Meridion-height cross section averaged over 70°—100°E of divergence in summer (unit:10-6s-1; shaded areas indicate topography)(a) climate mean, (b) anomaly in more rainfall years, (c) anomally in less rainfall years
图 7 夏季200 hPa水平波活动通量 (矢量, 单位:m 2·s -2) 和通量散度 (等值线, 单位:10-6m·s-2)(a) 气候平均, (b) 降水偏多年, (c) 降水偏少年
Fig. 7 The horizontal wave activity flux (vectors, unit:m2·s-2) and its divergence (contours, unit:10-6m·s-2) at 200 hPa in summer (a) climeate mean, (b) in more rainfall years, (c) in less rainfall years
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