青藏高原春季区域性极端大风频次下降成因

On the Causes of Decreased Regional Extreme Wind Frequency over the Qinghai-Xizang Plateau during the Spring Season

极端大风是一种重要的气象灾害,同时亦是一种重要的气候资源。作为“世界屋脊”的青藏高原(Qinghai-XizangPlateau,QXP)区域是全球和区域气候变化的敏感区,其极端大风事件及其气候变化背景下变化特征与机理尚未完全清楚,尤其在川藏铁路建设背景下,加深高原区域极端大风事件的科学认知可为铁路建设和运营提供科学支撑和参考。为探讨高原区域性极端大风长期变化特征及其可能原因,利用地面观测最大风速日值数据,定义了青藏高原区域性极端大风事件,在此基础上,采用趋势分析、EOF分析、合成分析等多元统计方法分析了1982-2021年高原中东部区域性极端大风事件的时空分布和变化特征、发生的环流条件及其频次减少的可能原因。分析结果表明:(1)自20世纪80年代以来,青藏高原中东部区域性极端大风事件呈显著下降趋势,以95和99百分位定义的区域性极端大风日数每10年分别下降44天和11.6天,且变化主要表现为空间一致型,主要集中在高原东北部。(2)区域性极端大风发生期间,中高纬位势高度呈“西高东低”,低纬地区呈“西低东高”的分布形势,主要特征表现为蒙古-西伯利亚高压正异常,蒙古地区气旋式环流异常,高原东北部西风异常以及蒙古地区地表温度负异常。(3)全球变化背景下,春季西伯利亚高压强度减弱、上一个冬季东亚大槽强度加强致使高原南北两侧气压梯度减小、西风急流减弱导致动量下传减弱并且伴随着气旋性环流异常减弱,以及亚洲地表温度非均匀性上升使得南北温度梯度变小。这些大尺度环流和局地热力因子的协同影响可能是高原中东部区域性极端大风事件发生频次减少的重要原因。

Extreme wind events(EWEs)not only are a kind of meteorological disaster,but also serve as an important natural resource.The Qinghai-Xizang Plateau(QXP),well known as the"roof of the world",is vulnerable to global and regional climate change.However,the characteristics and mechanism of variability of extreme wind events over this region and associated mechanisms remain elusive.Under the background of global change,Chinese Sichuan-Tibetan Railway construction,the exploration on changes in the EWEs could deepen our scientific understanding the regional climate.In order to explore the spatial-temporal distribution characteristics of regional extreme wind events(REWs)over the central and eastern QXP for the period of 1982-2021,daily data of maximum wind speed are utilized to define REWs.Further,with the aid of thermal-thermodynamic diagnosis and the multiple statistical methods,such as trend analysis,EOF and composite analysis,we explore the potential causes leading to the decreased frequency of RWEs.The results show that:(1)The REWs over the QXP significantly decreased since 1980,varied consistently throughout the QXP and occurred mostly in the northeastern QXP.The REWs as defined in the 95th and 99th percentiles decline with the rate of 44d/10a and 11.6d/10a.(2)During periods of REWs,the circulation pattern is"high in the west and low in the east"in the middle and high latitudes and"low in the west and high in the east"in the low latitudes.The main characteristics are as follows:the abnormal positive geopotopic in Mongolia-Siberia,the abnormal cyclonic circulation in Mongolia,the abnormal westerly wind in the northeast of the plateau,and the abnormal negative surface temperature in Mongolia.(3)In the context of global change,the weakening of the Siberian high in spring and the strengthening of the East Asian trough in winter leads to decreasing pressure gradient between the north and south sides of the QXP,the weakening of the westerly jet stream leads to decreasing momentum downdraft,and the rise of non-uniform surface temperature in Asia leads to the decreasing temperature gradient between the north and south sides of the QXP.At the same time,the abnormal cyclonic circulation weakened in Mongolia.It may be resulted from the co-influence of these large-scale circulation and local thermal factors that reduce the frequency of regional extreme wind events in the central and eastern parts of the QXP.