Variations in Summer Precipitation over the Three-River Headwaters Region and the Yarlung Zangbo River Basin and Their Response to the Tibetan Plateau Summer Monsoon
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摘要: 本文利用1981~2020年观测数据和ERA5再分析资料,将青藏高原腹地三江源和东南重要水汽通道河湾区作为典型研究区域,分析了降水不同时间尺度变化特征及其典型强弱年对高原季风环流系统的响应,结果表明:(1)三江源和河湾区降水的季节变化均呈双峰型分布,峰值出现在7月初和8月下旬。夏季降水在21世纪初发生年代际转折,尤其是三江源降水量在近20年增加明显。两个高原季风指数DPMI(Dynamic Plateau Monsoon Index)和ZPMI(Zhou Plateau Monsoon Index)的夏季风爆发时间均超前于河湾区和三江源降水的明显增加期。三江源夏季降水年际变化与两个高原夏季风指数有较好的相关性。三江源与河湾区虽然相邻很近,但三江源夏季降水受高原季风影响程度远大于河湾区。当高原夏季风增强(减弱)时,三江源降水量偏多(少)。(2)三江源降水偏多年,南亚高压偏东偏强,低层高原主体低压异常,有利于西南风和东南风在三江源区域交汇,南方暖湿空气能够深入高原腹地导致水汽辐合偏强。河湾区降水偏多年,河湾区及整个高原主体附近高度场并没有明显异常,河湾区的水汽输送主要有两条路径,一条来自孟加拉湾沿高原南坡的西南路径,另一条来自中亚地区穿过高原上空的西北路径,两条路径在高原东侧汇合继续向东输送。Abstract: Based on precipitation and ERA5 reanalysis datasets from 1981 to 2020, this study analyzed the variation characteristics of precipitation at different time scales over the Three-River Headwaters region (TRHR) and the Yarlung Zangbo River basin (YZRB) and their responses to the Tibetan Plateau summer monsoon. Results are shown as follows: (1) The seasonal variation in precipitation over the TRHR and YZRB shows a bimodal distribution, and the peaks appear in early July and late August. The interdecadal transitions in summer precipitation occur in the early 21st century, especially the TRHR precipitation increases significantly during the recent 20 years. The onset time of summer monsoon in the Dynamic Plateau Monsoon Index (DPMI) and the Zhou Plateau Monsoon Index (ZPMI) is earlier than the precipitation increase period over the TRHR and YZRB. The interannual variation in summer precipitation over the TRHR correlates well with two plateau summer monsoon indices. Although the TRHR is close to the YZRB, the summer precipitation of the TRHR is considerably more affected by the Tibetan plateau monsoon than YZRB. When the Tibetan Plateau summer monsoon strengthens (weakens), the TRHR precipitation is more (less). (2) In wet TRHR years, the South Asian High is stronger and more eastward, while the pressure at low-level over the main body of the plateau is lower than in dry years. These situations are conducive to the intersection of southwest and southeast winds over the TRHR so that the warm and humid air from the South can go deep into the hinterland of the plateau, resulting in stronger water vapor convergence. In wet YZRB years, there is no obvious anomaly in the pressure field near the YZRB or the Tibetan Plateau. The water vapor transport over YZRB mainly has two paths. One is the southwest path from the Bay of Bengal along the south slope of the plateau, and the other is the northwest path from Central Asia and through the plateau. The two paths converge on the east side of the plateau and continue to transport eastward.
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图 1 三江源(TRHR,上黑色框区)、河湾区(YZRB,下黑色框区)及周边气象站点(蓝色圆点)分布和地形(彩色阴影,单位:m)特征
Figure 1. Topography (color shadings, units: m) and the locations of meteorological stations (blue dots) over the Three-River Headwaters region (TRHR, the upper black frame) and the Yarlung Zangbo River basin (YZRB, the lower black frame)
图 5 1981~2020年(a、c)三江源和(b、d)河湾区夏季降水量与550 hPa(a、b)纬向风、(c、d)经向风相关系数空间分布。打点区域表示通过95%置信水平的显著性检验,矢量箭头为气候态(1981~2020年)风场
Figure 5. Spatial distributions of correlation coefficients between the summer precipitation in (a, c) TRHR, (b, d) YZRB and (a, b) zonal wind, (c, d) meridional wind at 550 hPa from 1981 to 2020. Dotted areas represent statistically significant correlation at 95% confidence level, vectors are the climatic (1981–2020) winds
图 6 1981~2020年(a、c)三江源和(b、d)河湾区夏季降水偏多年和偏少年平均的(a、b)100 hPa和(c、d)500 hPa高度场差值(偏多年减偏少年,阴影,单位:dagpm)。实线为气候态(1981~2020年)高度场(单位:dagpm),打点区域为通过95%置信水平的显著性检验
Figure 6. Differences in the geopotential high (shadings, units: dagpm) at (a, b) 100 hPa and (c, d) 500 hPa between more and fewer precipitation years over (a, c) TRHR and (b, d) YZRB from 1981 to 2020. Solid lines are climatic (1981–2020) geopotential high from 1981 to 2020. Dotted areas represent statistically significant correlation at 95% confidence level
图 7 1981~2020年(a)三江源和(b)河湾区夏季降水偏多年和偏少年地面至500 hPa垂直积分的水汽通量(流线)和水汽通量散度(阴影,单位:10−2 g s−1 hPa−1 cm−1)的差值场。打点区域通过95%置信水平的显著性检验
Figure 7. Differences in the water vapor flux (streamline) and divergence (shadings, units: 10−2 g s−1 hPa−1 cm−1) vertically integrated from the surface to 200 hPa between more and fewer precipitation years over (a) TRHR and (b) YZRB from 1981 to 2020. Dotted areas represent statistically significant correlation at 95% confidence level
图 8 1981~2020年夏季20°N~50°N(a)气候态水平风场(单位:m s−1),(b)三江源、(c)河湾区降水偏多年和偏少年水平风场差值(单位:m s−1)沿95°E的垂直剖面。阴影表示地形,红线表示三江源和河湾区位置
Figure 8. Vertical profiles along 95°E for (a) climatic horizontal winds (vectors, units: m s−1) and differences of horizontal winds (vectors, units: m s−1) between more and fewer precipitation years in (b) TRHR and (c) YZRB over 20°N–50°N in summer from 1981 to 2020. Shadings represent the topography, red lines represent the locations of TRHR and YZRB
表 1 1981~2020年三江源和河湾区夏季降水量年际变化、高原夏季风指数之间的相关系数
Table 1. Correlation coefficients between the TRHR and YZRB summer precipitation series and the Tibetan Plateau summer monsoon indices from 1981 to 2020
相关系数 三江源降水量 河湾区降水量 IDPM IZPM 三江源降水量 1 0.45 0.52 0.51 河湾区降水量 \ 1 0.07 0.13 IDPM \ \ 1 0.74 IZPM \ \ \ 1 -
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