东亚—西北太平洋地区夏季季节内振荡的年代际变化

本文重点分析对比热带夏季季节内振荡(Boreal Summer Intraseasonal Oscillation,BSISO)1987—1995年(P1),1996—2007年(P2)和2008—2017年(P3)三阶段东亚—西北太平洋地区(East Asian-Western North Pacific,EAWNP)5—9月BSISO年代际变化的季节内差异特征。结果表明,在P1和P3两阶段,5—7月EAWNP BSISO强度几乎相同,但P2中每个月均显著增强,表明5—7月EAWNP BSISO经历了P1—P2增强和P2—P3减弱的年代际变化。8月,EAWNP BSISO强度从P1到P3逐渐增强,P3阶段比P1有显著增强,孟加拉湾和东亚副热带区域的BSISO活动增强。和P1相比,南海地区BSISO活动在P2阶段异常活跃,在5—7月强度增强,并且北传显著。在P2阶段,负位相的太平洋年代际(Interdecadal Pacific Oscillation,IPO)对应的赤道西太平洋和印度洋海温增暖,及Walker环流的增强为5—7月BSISO活动提供了水汽和对流发展的有利条件,而南海地区北传对流的叠加作用以及南海海温增暖进一步加强了BSISO的强度和北传。在P3阶段,8月孟加拉湾BSISO活动增强,除了热带印度洋一致增暖和太平洋ENSO型海温为BSISO活动提供水汽和对流发展的条件外,70°~90°E区域局地Hadley环流引起的上升运动也对BSISO的强度增强和北传有贡献。

东亚—西北太平洋海平面气压场的动力特征分析

东亚—西北太平洋地区的海平面气压直接反映了低层大气的环流特征,其动力特征对大气环流形势,气压系统的演变和天气、气候系统的发展等都有显著的影响。因此,深入分析东亚—西北太平洋地区海平面气压场的时空演变特征,对于提高我国的天气和气候预报具有重要意义。为了从非线性动力学的角度进行深入研究,文中使用了一种新方法来定量估算海平面气压吸引子的两个瞬时指标:瞬时维度和瞬时稳定度。瞬时维度表征了吸引子轨道在局部空间上的离散程度,瞬时稳定度则表征了轨道在局部时间上的稳定程度,它们共同反应了海平面气压吸引子的瞬时(逐日)动力特性。本文利用不同大小的指标值与海平面气压场的对应关系,研究了该地区海平面气压场的不同时空特征。主要结论为:(1)海平面气压吸引子的两个指标都是低值时,对应逐日环流场的空间特征表现为:气压结构单一,通常有几个强大的高低气压中心东西对峙;时间特征表现为:环流模态可以稳定的持续10 d左右。(2)当两个指标都是高值时,逐日环流场的空间特征表现为:多个弱的气压中心同时存在且结构混乱;时间特征为:环流场极不稳定,持续性仅在1 d左右。(3)瞬时维度和瞬时稳定度具有一致的年代际变化趋势:20世纪70~90年代呈现明显的下降趋势,90年代末快速上升,2000年后波动变化。

Joint effects of three oceans on the 2020 super mei-yu

An unexpected super mei-yu struck in 2020 in the Yangtze-Huaihe River basin,southern Korea,and southern Japan(hereafter referred to as the mei-yu regions),causing many casualties and huge economic losses.The super mei-yu was characterized by a remarkably early onset(around 1 June),late withdrawal(around 1 August),and intense rainfalI during the mei-yu season.The precipitation in the early onset and late withdrawal stages contributed more than half of the total mei-yu-period precipitation over the mei-yu regions in 2020.In this study,the authors explored the dominant remote forcing of the mei-yu early onset and late withdrawal to understand the mechanisms of this super mei-yu.The early onset can mainly be attributed to an early northward-shifted East Asian jet stream(EAJS).The late withdrawal mainly resulted from the stagnant EAJS and the western North Pacific subtropical high(WPSH) during 10 July to 1 August.Specifically,North Atlantic sea surface temperature anomalies(SSTAs) excited a Rossby wave,which was steered by atmospheric anomalies related to the western North Pacific SSTAs,causing the early northward-shifted EAJS and generating an early onset.The record-breaking warm SSTAs over the North Indian Ocean to South China Sea and the reduced sea-ice concentration(SIC) over the Laptev-East Siberian Sea played important roles in causing the stagnant WPSH and EAJS during July,which led to the late withdrawal.Meanwhile,the SIC anomalies may have caused the inhomogeneous rainfall distribution in the mei-yu regions.Furthermore,projection results suggest that the probability of a late mei-yu withdrawal similar to the 2020 case will increase in the future.Finally,potential predictors of an extreme mei-yu are discussed.

Combined impact of in-phase and out-of-phase variation between the northern East Asian low and western North Pacific subtropical high on East Asian summer rainfall

East Asian summer rainfall is affected by both the continental northern East Asian low （NEAL） and the western North Pacific subtropical high （WNPSH） in the lower troposphere. This study investigates the joint effect of the two circulation factors on East Asian summer rainfall. It is found that the rainfall in East Asia behaves differently in the years with in-phase and out-of-phase variation between the NEAL and WNPSH. When the NEAL and WNPSH vary in phase, i.e. when they are both stronger, the rainfall anomaly shows a dipole pattern in East Asia and displays opposite changes between north and south of 30°N. When the two circulation factors vary out of phase, the rainfall anomaly is concentrated in the Yangtze River valley.

Relationships between the Oxygen Isotopes in East Asian Stalagmites and Large-Scale Atmospheric and Oceanic Modes

The stalagmite δ18O record is known to be associated with the climate, but the specifics of the relationship remain unclear. It may not represent variation in air temperature or precipitation, but instead reflect integral changes of monsoon circulation, especially water vapor sources(the so-called "circulation effect"). Since large-scale atmospheric-oceanic modes such as the Atlantic Multidecadal Oscillation(AMO), Pacific Decadal Oscillation(PDO), and North Atlantic Oscillation(NAO) exert significant effects on Asian monsoon, in this paper the authors investigate the relationships of the East Asian stalagmite δ18O record with these modes. The last three centuries form the focus of our study, for which the authors use reconstructed as well as instrumental data. Considering the impacts of human activity, our analysis is conducted with respect to two periods—the pre- and post-industrial periods. The results show significant lead-lag connections: a positive correlation peaks when the PDO leads East Asian stalagmite δ18O by 3 years, which is persistent over the past 300 years; while the relationships of the AMO and NAO with the East Asian stalagmite δ18O record show significant differences in the post-industrial relative to the pre-industrial period. This implies that the East Asian stalagmite δ18O record may primarily reflect the PDO signal.

The Second Decadal Leading Mode of East Asian Summer Monsoon Rainfall

The first decadal leading mode of East Asian summer rainfall(EASR) is characterized by rainfall anomalies along the East Asian subtropical rain belt. This study focuses on the second decadal leading mode(2DLM), accounting for 17.3% of rainfall decadal variance, as distinct from the other two neighboring modes of EAMR, based on the state-of-the-art in-situ rainfall data.This mode is characterized by a South-China-wet–HuaiheRiver-dry pattern, and is dominated by a quasi-30-yr period. Further analysis reveals the 2DLM corresponds to an enhanced lower-level monsoon jet, an eastward extension of the western North Pacific subtropical high, and a weakened East Asian upper-level westerly jet flow. The Tibetan Plateau surface temperature and Pacific Decadal Oscillation(PDO) are closely linked with the 2DLM. The regressed SST pattern indicates the PDO-like pattern of sea surface temperature anomalies may have a teleconnection relationship with the 2DLM of EASR.

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.

Impact of El Nio on atmospheric circulations over East Asia and rainfall in China： Role of the anomalous western North Pacific anticyclone

This paper presents a review on the impact of El Nio on the interannual variability of atmospheric circulations over East Asia and rainfall in China through the anomalous anticyclone over western North Pacific(WNPAC). It explains the formation mechanisms of the WNPAC and physical processes by which the WNPAC affects the rainfall in China. During the mature phase of El Nio, the convective cooling anomalies over western tropical Pacific caused by the weakened convections trigger up an atmospheric Rossby wave response, resulting in the generation of the WNPAC. The WNPAC can persist from the winter when the El Nio is in its peak to subsequent summer, which is maintained by multiple factors including the sustained presence of convective cooling anomalies and the local air-sea interaction over western tropical Pacific, and the persistence of sea surface temperature anomalies(SSTA) in tropical Indian and tropical North Atlantic. The WNPAC can influence the atmospheric circulations over East Asia and rainfall in China not only simultaneously, but also in the subsequent summer after an El Nio year, leading to more rainfall over southern China. The current paper also points out that significant anomalies of atmospheric circulations over East Asia and rainfall over southern China occur in El Nio winter but not in La Nio winter, suggesting that El Nio and La Nio have an asymmetric effect. Other issues, including the impact of El Nio diversity and its impact as well as the relations of the factors affecting the persistence of the WNPAC with summer rainfall anomalies in China, are also discussed. At the end of this paper some issues calling for further investigation are discussed.