“丝绸之路”型与东亚—太平洋型遥相关的低频特征及其对江淮流域夏季降水的影响

江淮流域夏季洪涝灾害主要归因于大气环流异常的稳定维持。不同大气遥相关的低频振荡及其协同变化对江淮流域夏季降水的影响机理是大气科学领域热点问题之一。利用ERA5逐日再分析资料和中国气象局提供的逐日站点降水资料,揭示了“丝绸之路”(SR)型和东亚—太平洋(EAP)型遥相关的低频特征及其对江淮流域夏季降水的影响机理。结果表明:SR型和EAP型遥相关的10~30 d低频周期与江淮流域夏季降水关系密切,SR型和EAP型遥相关各低频系统都经历了正负位相转换并逐渐达到峰值(谷值)。其主要表现为:①10~30 d低频200 hPa等压面经向风沿西风急流呈偏北风与偏南风的交替分布,当SR型达到负位相峰值时,江淮流域为偏北风控制,中纬度低频气旋加强,加速西风急流;②10~30 d低频西太平洋副热带高压西伸东退、南亚高压东移西撤,二者相向而行,在峰值位相重叠于对江淮流域降水有利的位置;③由东亚沿岸向西北移动的向外长波辐射异常波列亦表现出显著的10~30 d低频振荡特征,处于峰值位相时,向外长波辐射在江淮流域为负异常,有利于江淮流域降水;④10~30 d低频低层正涡度、高层负涡度的配置有利于低层辐合、高层辐散,导致上升运动增强,进而触发江淮流域低频降水。

中南半岛和华南地区极端降水时空特征

全球变暖使得极端降水事件的强度和频率均呈上升趋势,位于亚洲季风区的中南半岛和华南地区更易受到极端降水影响而发生洪涝灾害。本文利用逐日降水资料对1951~2015年中南半岛和华南地区湿季(5~10月)的极端降水事件进行客观分类,并对每一类极端降水事件的大尺度流型特征及年际、年代际和长期趋势特征进行了分析。结果表明:(1)根据降水中心位置,中南半岛和华南地区的极端降水可客观分为华南类、中南半岛类、缅甸—云南类和华南南部—越南北部类,它们所联系的大尺度流型以中高纬波列和热带偶极子环流为主。其中,华南类的大尺度流型在对流层高层为类似于东亚—太平洋(East Asia–Pacific,简称EAP)遥相关型的“+−+”经向型波列,但中心位置较典型EAP型偏西南,中东部的负异常环流为关键系统,低层为位于低纬度地区的“+−”经向型波列。中南半岛类大尺度流型为热带偶极子异常环流,对流层低层较高层更为显著宽广。其余两类极端降水大尺度流型特征为在对流层高层受Rossby波列影响,低层主要为偏弱的热带偶极子异常环流。(2)中南半岛和华南地区极端降水的频次呈显著上升的趋势,主要来自于华南类和中南半岛类极端降水的贡献。其中,“+−+”经向型波列的频发是造成华南类极端降水增多的原因。(3)中南半岛与华南地区极端降水的空间分布具有反对称特征。

江淮流域持续性极端降水及预报方法研究进展

持续性极端降水过程会引发严重的洪涝灾害,是我国主要的灾害性天气之一,其形成机理和预报理论与方法研究受到广泛关注。近年来,针对持续性极端降水的形成机理和预报方法研究取得了一系列进展,主要包括：开展了我国区域性持续性极端降水事件的自动识别方法研究,研制建立了江淮流域持续性极端降水的大尺度环流概念模型,并提取了1-2周的前兆信号;从东亚一太平洋遥相关型（EAP）角度探究其对持续性极端降水的影响机理,并探讨利用EAP对江淮流域持续性极端降水进行预报的可行性。此外,在上述研究的基础上发展了基于关键影响系统的持续性极端降水的物理统计预报方法。

Impact of two types of La Nia on boreal autumn rainfall around Southeast Asia and Australia

The distinct influences of eastern Pacific （EP） and central Pacific （CP） La Nina on rainfall anomalies over Southeast Asia and Australia in boreal autumn （September to November） are explored in this study. Composite results reveal that CP La Nina gives rise to significant and severe flooding over Southeast Asia and Australia, whereas EP La Nina fails to exert any evident impacts on rainfall over this region. This difference can be attributed to the distinct features of cooling sea surface temperature anomalies （SSTAs） between EP and CP La Nina. With a more westward location and stronger intensity of the negative SSTAs during CP La Nina autumn, the highest and lowest SLP anomalies are substantially enhanced and shift westwards too, further causing intense easterly winds over the western Pacific and westerly anomalies over the Indian Ocean driven by this SLP gradient. Subsequently, robust low-level convergence and high-level divergence is observed over the Maritime Continent and Australia, resulting in significant above-normal rainfall anomalies in those regions. In contrast, weak and eastern Pacific-confined cooling SSTAs during EP La Nina produce correspondingly weak low-level convergence over the Maritime Continent conditions that make it hard for significant rainfall anomalies to arise.

The projected temporal evolution in the interannual variability of East Asian summer rainfall by CMIP3 coupled models

The projected temporal evolution in the interannual variability of East Asian summer rainfall in the 21st century is investigated here,by analyzing the simulated results of 18 coupled models under the 20th century climate experiment and scenario A1B.The multi-model ensemble(MME)mean projects two prominent changes in the interannual variability of East Asian summer rainfall in the 21st century under scenario A1B.The first change occurs around the 2030s,with a small change before and a large increase afterward.The intensity of the interannual variability increases up to approximately 0.53 mm/d in the 2070s,representing an increase of approximately 30% relative to the early 21st century.The second change happens around the 2070s,with a decrease afterward.By the end of the 21st century,the increase is approximately 12% relative to the early 21st century.The interannual variability of two circulation factors,the western North Pacific subtropical high(WNPSH)and the East Asian upper-tropospheric jet(EAJ),are also projected to exhibit two prominent changes around the 2030s and 2070 under scenario A1B,with consistent increases and decreases afterward,respectively.The MME result also projects two prominent changes in the interannual variability of water vapor transported to East Asia at 850 hPa,which occurs separately around the 2040s and 2070s,with a persistent increase and decrease afterward.Meanwhile,the precipitable water interannual variability over East Asia and the western North Pacific is projected to exhibit two prominent enhancements around the 2030s and 2060s and an increase from 0.1 kg/m2 in the early 21st century to 0.5 kg/m2 at the end of the 21st century,implying a continuous intensification in the interannual variability of the potential precipitation.Otherwise,the intensities of the three factors'(except EAJ)interannual variability are all projected to be stronger at the end of the 21st century than that in the early period.These studies indicate that the change of interannual variability of the East Asian summer rainfall is caused by the variability of both the dynamic and thermodynamic variables under scenario A1B.In the early and middle 21st century,both factors lead to an intensified interannual variability of rainfall,whereas the dynamic factors weaken the interannual variability,and the thermodynamic factor intensifies the interannual variability in the late period.