Characteristics of the Onset of the Asian Summer Monsoon and the Importance of Asian-Australian "Land Bridge"

Based on summarizing previous achievements and characteristics of Asian summer monsoon and the role using data as long and new as possible, the onset of Asian-Australian ＂land bridge＂ in the onset of summer monsoon are further discussed. In particular, the earliest onset area of Asian summer monsoon is comparatively analyzed, and the sudden and progressive characteristics of the onset of summer monsoon in different regions are discussed, Furthermore, the relationships among such critical events during the onset of Asian summer monsoon as the splitting of subtropical high belt over the Bay of Bengal （BOB）, the initiation of convection over Indo-China Peninsula, the westward advance, reestablishment of South Asian High, and the rapid northward progression of convection originated from Sumatra in early summer are studied. The important impact of the proper collocation of she latent heating over Indo-China Peninsula and the sensible heating over Indian Peninsula on the splitting of the subtropical high belt, the deepening of BOB trough, the activating of Sri Lanka vortex （twin vortexes in the Northern and Southern Hemispheres）, and the subsequent onset of South China Sea summer monsoon are emphasized.

Impacts of Land Process on the Onset and Evolution of Asian Summer Monsoon in the NCEP Climate Forecast System

Impacts of land models and initial land conditions （ICs） on the Asian summer monsoon, especially its onset, were investigated using the NCEP Climate Forecast System （CFS）. Two land models, the Oregon State University （OSU） land model and the NCEP, OSU, Air Force, and Hydrologic Research Laboratory （Noah） land model, were used to get parallel experiments NCEP/Department of Energy （DOE） Global Reanalysis 2 System （GLDAS）. The experiments also used land ICs from the （GR2） and the Global Land Data Assimilation Previous studies have demonstrated that, a systematic weak bias appears in the modeled monsoon, and this bias may be related to a cold bias over the Asian land mass. Results of the current study show that replacement of the OSU land model by the Noah land model improved the model＇s cold bias and produced improved monsoon precipitation and circulation patterns. The CFS predicted monsoon with greater proficiency in E1 Nifio years, compared to La Nifia years model in monsoon predictions for individual years. and the Noah model performed better than the OSU These improvements occurred not only in relation to monsoon onset in late spring but also to monsoon intensity in summer. Our analysis of the monsoon features over the India peninsula, the Indo-China peninsula, and the South Chinese Sea indicates different degrees of improvement. Furthermore, a change in the land models led to more remarkable improvement in monsoon prediction than did a change from the GR2 land ICs to the GLDAS land ICs.

Simulating the Intraseasonal Variation of the East Asian Summer Monsoon by IAP AGCM4.0

ABSTRACT This study focuses on the intraseasonal variation of the East Asian summer monsoon （EASM） simulated by IAP AGCM 4.0, the fourth-generation atmospheric general circulation model recently developed at the Institute of Atmospheric Physics, Chinese Academy of Sciences. In general, the model simulates the intraseasonal evolution of the EASM and the related rain belt. Besides, the model also simulates the two northward jumps of the westem Pacific subtropical high （WPSH）, which are closely related to the convective activities in the warm pool region and Rossby wave activities in high latitudes. Nevertheless, some evident biases in the model were found to exist. Due to a stronger WPSH, the model fails to simulate the rain belt in southern China during May and June. Besides, the model simulates a later retreat of the EASM, which is attributed to the overestimated land-sea thermal contrast in August. In particular, the timing of the two northward jumps of the WPSH in the model is not coincident with the observation, with a later jump by two pentads for the first jump and an earlier jump by one pentad for the second, i.e., the interval between the two jumps is shorter than the observation. This bias is mainly ascribed to a shorter oscillating periodicity of convection in the tropical northwestern Pacific.

The Relationship of Land-Ocean Thermal Anomaly Difference with Mei-yu and South China Sea Summer Monsoon

Based on the NCEP/NCAR reanalysis data for the period of 1948-2004 and the monthly rainfall data at 160 stations in China from 1951 to 2004, the relationships among the land-ocean temperature anomaly difference in the mid-lower troposphere in spring （April-May）, the mei-yu rainfall in the Yangtze River- Huaihe River basin, and the activities of the South China Sea summer monsoon （SCSSM） are analyzed by using correlation and composite analyses. Results show that a significant positive correlation exists between mei-yu rainfall and air temperature in the middle latitudes above the western Pacific, while a significant negative correlation is located to the southwest of the Baikal Lake. When the land-ocean thermal anomaly difference is stronger in spring, the western Pacific subtropical high （WPSH） will be weaker and retreat eastward in summer （June-July）, and the SCSSM will be stronger and advance further north, resulting in deficient moisture along the mei-yu front and below-normal precipitation in the mid and lower reaches of the Yangtze River, and vice versa for the weaker difference case. The effects and relative importance of the land and ocean anomalous heating on monsoon variability is also compared. It is found that the land and ocean thermal anomalies are both closely related to the summer circulation and mei-yu rainfall and SCSSM intensity, whereas the land heating anomaly is more important than ocean heating in changing the land-ocean thermal contrast and hence the summer monsoon intensity.

THERMAL INFLUENCES OF LAND-SEA CONTRAST AND TOPOGRAPHY ON SUMMER MONSOON IN 1998

In this work, the SCSMEX data are used to diagnose and compare the local land-sea thermal conditions, with the focus of discussion on possible influences of thermal forcing of the western Pacific and the Tibetan Plateau on the onset and development of summer monsoon in 1998. Results show a close relationship between the distribution of the heat sources and the land-sea contrast. Due to the blocking effect of terrain, main maximum zones of the heat sources in areas with more evident north-south land-sea contrast are more obviously southward located than those exclusively with oceans. The surface heating is characterized with apparent seasonal variation and difference between land and sea. The relationship between the western Pacific and the onset of summer monsoon is reflected in the variations of the sea surface temperature (SST) and the latent heat. The influence mechanism of the Tibetan Plateau during the summer monsoon is different: it is dominated by sensible heating during the South China Sea monsoon and by condensed latent heating during the Indian monsoon.

EAWM-Related Air-Sea-Land Interaction and the Asian Summer Monsoon Circulation

Based on the data analysis, this study further explores the characteristics of East Asian winter monsoon (hereafter, EAWM, for brevity) as well as the related air-sea-land system, and illustrates how and to what degree anomalous signals of the subsequent Asian summer monsoon are rooted in the preceding EAWM activity. We identified an important air-sea coupled mode, i.e., the EAWM mode illustrated in Section 3. In cold seasons, strong EAWM-related air-sea two-way interaction is responsible for the development and persistence of the SSTA pattern of EAWM mode. As a consequence, the key regions, i.e., the western Pacific and South China Sea (hereafter, SCS, for brevity), are dominated by such an SSTA pattern from the winter to the following summer. In the strong EAWM years, the deficient snow cover dominates eastern Tibetan Plateau in winter, and in spring, this anomaly pattern is further strengthened and extended to the northwestern side of Tibetan Plateau. Thus, the combined effect of strong EAWM-related SSTA and Tibetan snow cover constitutes an important factor in modulating the Asian monsoon circulation. The active role of the EAWM activity as well as the related air-sea-land interaction would, in the subsequent seasons, lead to: 1) the enhancement of SCS monsoon and related stronger rainfall; 2) the northward displacement of subtropical high during Meiyu period and the related deficient rainfall over Meiyu rainband; 3) above-normal precipitation over the regions from northern Japan to northeastern China in summer; 4) more rainfall over the Arabian Sea and Northeast India, while less rainfall over southwest India and the Bay of Bengal. The strong EAWM-related air-sea interaction shows, to some degree, precursory signals to the following Asian summer monsoon. However, the mechanism for the variability of Indian summer monsoon subsequent to the strong EAWM years remains uncertain.

Effects of Land-Sea Distribution, Topography and Diurnal Change on Summer Monsoon Modeling

The effects of the land-sea distribution, the topography and the diurnal change of the solar radiation on the summer monsoon modelings are studied by use of a coupled modeling system with a 5-layer primitive equation model of the atmosphere and a 2-layer soil or ocean thermodynamic model which are all solved in a zonal model domain between 60°S and 60°N. The results of numerical simulations show that the quasi-stationary patterns of the mean monsoon circulations are mainly affected by the land-sea distribution and the topography, the effect of the diurnal change is the secondary. However, the inclusion of the diurnal change into the model system may improve the intensity of the simulated monsoon circulation, it can influence the distributive pattern of precipitation to a larger extent,without the diurnal change precipitation in the interior of land would decrease and in the coastal regions it would increase.

Possible Impacts of the Arctic Oscillation on the Interdecadal Variation of Summer Monsoon Rainfall in East Asia

The influences of the wintertime AO (Arctic Oscillation) on the interdecadal variation of summer monsoon rainfall in East Asia were examined. An interdecadal abrupt change was found by the end of the 1970s in the variation of the AO index and the leading principal component time series of the summer rainfall in East Asia. The rainfall anomaly changed from below normal to above normal in central China, the southern part of northeastern China and the Korean peninsula around 1978. However, the opposite interdecadal variation was found in the rainfall anomaly in North China and South China. The interdecadal variation of summer rainfall is associated with the weakening of the East Asia summer monsoon circulation. It is indicated that the interdecadal variation of the AO exerts an influence on the weakening of the monsoon circulation. The recent trend in the AO toward its high-index polarity during the past two decades plays important roles in the land-sea contrast anomalies and wintertime precipitation anomaly. The mid- and high-latitude regions of the Asian continent are warming, while the low-latitude regions are cooling in winter and spring along with the AO entering its high-index polarity after the late 1970s. In the meantime, the precipitation over the Tibetan Plateau and South China is excessive, implying an increase of soil moisture. The cooling tendency of the land in the southern part of Asia will persist until summer because of the memory of soil moisture. So the warming of the Asian continent is relatively slow in summer. Moreover, the Indian Ocean and Pacific Ocean, which are located southward and eastward of the Asian land, are warming from winter to summer. This suggests that the contrast between the land and sea is decreased in summer. The interdecadal decrease of the land-sea heat contrast finally leads to the weakening of the East Asia summer monsoon circulation.

Weakening of Indian Summer Monsoon in Recent Decades

The analysis of 43 years of NCEP-NCAR reanalysis data and station observations reveals the connections between tropospheric temperature variations and the weakening of the Indian summer monsoon circulation. The Indian summer monsoon variation is strongly linked to tropospheric temperature over East Asia, showing significant positive correlations of mean tropospheric temperature with all-Indian summer rainfall and the monsoon circulation

Response of Asian Summer Monsoon to CO_2 Doubling

Based on simulations of the IPCC 20C3M and SRES A1B experiments in ten coupled models, the Asian summer mon-soon (ASM) response to CO2 doubling and the different responses among models are examined. Nine models show the similar results that the weakening of land-ocean thermal contrast caused by the CO2 doubling contributes to a weaker large-scale ASM circulation. Further analysis in this study also shows that the major ASM components,such as the Somali cross-equatorial flow,the low level India-South China Sea monsoon trough,and the upper level tropical easterly jet stream, weaken as CO2 doubles.However,the ASM rainfall increases as a result of the increased moisture from the warmer Indian Ocean and the South China Sea, and the enhanced northward moisture transport over the ASM region.For the response of enhanced northward moisture transport over South Asia, the positive contribution of moisture content increase in the Indian Ocean is dominant and the negative contribution of the weaker mon-soon circulation is secondary at 850 hPa,but both have positive contribution to the total moisture transport along the East China coast.The paradox of the weaker ASM circulation and the increasing precipitation in CO2 doubling is confirmed.It is found that strengthening of northward moisture transport could intensify the precipitation and atmospheric heat source over the north Arabian Sea and East China,and result in enhanced southwesterly at 850 hPa as global warming occurs.All ten models show significant enhanced southwesterly response over the north Arabian Sea,and six of them show enhanced southwesterly response along the East China coast.

The Response of the East Asian Summer Monsoon to Strong Tropical Volcanic Eruptions

A 600-year integration performed with the Bergen Climate Model and National Centers for Environmental Prediction/National Center for Atmospheric Research （NCEP/NCAR） reanalysis data were used to investigate the impact of strong tropical volcanic eruptions on the East Asian summer monsoon （EASM） and EASM rainfall.Both the simulation and NCEP/NCAR reanalysis data show a weakening of the EASM in strong eruption years.The model simulation suggests that North and South China experience droughts and the Yangtze-Huaihe River Valley experiences floods during eruption years.In response to strong tropical volcanic eruptions,the meridional air temperature gradient in the upper troposphere is enhanced,which leads to a southward shift and an increase of the East Asian subtropical westerly jet stream （EASWJ）.At the same time,the land-sea thermal contrast between the Asian land mass and Northwest Pacific Ocean is weakened.The southward shift and increase of the EASWJ and reduction of the land-sea thermal contrast all contribute to a weakening of the EASM and EASM rainfall anomaly.

INTERDECADAL VARIATIONS OF INTERACTION BETWEEN NORTH PACIFIC SSTA AND EAST ASIAN SUMMER MONSOON

Identification of key SST zones is essential in predicting the weather / climate systems in East Asia. With the SST data by the U.K. Meteorological Office and 40-year geopotential height and wind fields by NCAR / NCEP, the relationship between the East Asian summer monsoon and north Pacific SSTA is studied, which reveals their interactions are of interdecadal variation. Before mid-1970’s, the north Pacific SSTA acts upon the summer monsoon in East Asia through a great circle wavetrain and results in more rainfall in the summer of the northern part of China. After 1976, the SSTA weakens the wavetrain and no longer influences the precipitation in North China due to loosened links with the East Asian summer monsoon. It can be drawn that the key SST zones having potential effects on the weather / climate systems in East Asia do not stay in one particular area of the ocean but rather shift elsewhere as governed by the interdecadal variations of the air-sea interactions. It is hoped that the study would help shed light on the prediction of drought / flood spans in China.

贵州黔西县水西洞石笋记录的末次冰期Heinrich Stadial 4气候突变事件

末次冰期Heinrich Stadial 4气候突变事件(HS4事件)是发生于约40 ka B.P.(B.P.表示Before Present, Present为公元1950年)最为显著的一次海因里希冰阶事件,对其转型特征和精细结构的刻画有助于深入理解千年尺度气候突变事件的机制。本研究基于贵州黔西县水西洞SXG-3石笋的11个高精度230Th年龄和277个δ^(18)O数据,重建了40.77~37.17 ka B.P.时段平均分辨率为13 a的亚洲夏季风强度演变序列。该石笋氧同位素记录清晰地捕捉到了HS4弱季风事件,呈现出三阶段变化的特征,即:第1阶段(39.97~39.13 ka B.P.),石笋δ^(18)O在840±90 a内偏正1.32‰,夏季风缓慢减弱,对应于热带辐合带(Intertropical Convergence Zone,简称ITCZ)的南移和格陵兰气候快速变冷;第2阶段(39.13~38.35 ka B.P.),石笋δ^(18)O整体偏正,平均为-8.34‰,夏季风强度达到最弱,而南美季风达到最强,对应于ITCZ移动至最南端;第3阶段(38.35~37.59 ka B.P.),石笋δ^(18)O在760±89 a的时间内偏负至-9.25‰,对应于ITCZ的向北移动和格陵兰气候快速变暖。水西洞石笋记录的HS4事件三阶段变化特征与福建仙云洞记录十分相似,对应于NEEM(Northern Greenland Eemian Ice Drilling)冰心^(17)O-excess所反映的低纬水文循环过程变化,同时与南美Toca da Boa Vista(TBV)和Toca da Barrigude(TBR)洞穴石笋记录呈“镜像关系”。分析结果表明,在北半球高纬气候触发后,热带海洋和南半球热量的不断积聚及其随后的释放所引起的ITCZ的南北移动是造成这种三阶段变化的主要原因。

SEASONAL VARIATIONS OF NET RADIATIVE HEATING IN THE EARTH-ATMOSPHERIC SYSTEM AND ITS RELATIONS TOASIAN SUMMER MONSOON

Satellite-derived data of the outgoing longwave radiation (OLR), net shortwave radiation at thetropopause (SRT) and circulation information as predicted by NCEP are used in the work to study seasonal variations of net radiative heating in the earth-atmospheric system and its relationship with the Asian summer monsoon. As is shown in the result, the zonal deviations of the zonal deviations of the heating, manifested as mutations in direction between land and sea with seasons, is an indication of the thermal difference between them.Being a month earlier than that in the general circulation from spring to summer, the seasonal reversal of directionmay be playing an essential role in triggering the onset and withdrawal of summer monsoon in Asia.

2004年南海夏季风的爆发及中南半岛陆面过程的可能影响I:诊断分析

南海夏季风的爆发受高原、海洋(海气相互作用)、冷空气和陆地(陆面过程或陆气相互作用)等多种因素的影响,其中中南半岛由于是连接南海夏季风和印度、孟加拉湾季风的关键区,而且孟加拉湾不仅是亚洲最早爆发夏季风的地区,又是副热带高压最早断裂的地区。因此它的陆面过程对南海夏季风的影响是不可忽视的。文章从2004年南海夏季风爆发前后的环流和降水分析其活动特征,并进一步研究中南半岛陆面过程对南海夏季风的爆发日期和强度的影响。2004年南海夏季风于5月19日爆发,利用NCEP再分析资料及地面站点降水资料对这次季风爆发前后的环流形势和降水分布进行分析,结果表明:强对流活动由孟加拉湾移到中南半岛,引起中南半岛的降水增大,导致陆面过程发生改变(包含土壤湿度,感热、潜热通量,向上长波辐射),最终使得中南半岛—南海之间的低层气温差出现符号逆转,为南海夏季风的爆发提供了必要的条件。此外,中南半岛—南海低层气温差同南海夏季风的活跃程度有密切的联系。通常负的温差出现后不久,南海夏季风即进入活跃期或非活跃期,正的温差出现之后则常常是南海夏季风的中断期。

2004年南海夏季风的爆发及中南半岛陆面过程的可能影响Ⅱ:数值试验

在对南海夏季风的爆发及中南半岛陆面过程的可能影响进行了诊断分析的基础上,应用MM5/NOAHLSM模式,研究了中南半岛陆气相互作用对2004年南海夏季风爆发过程的可能影响。结果发现:在南海夏季风爆发前,中南半岛南海地区低层气温差确实出现低值,甚至负值;尽管短期内中南半岛土壤湿度和降水的变化没有引起季风爆发日期的改变,但对季风爆发的强度有影响。土壤湿度和降水变化引起的干异常可导致地表感热通量的增大和地表温度的升高,致使中南半岛与南海之间低层的温差异常(负温差)减小,季风爆发强度减弱;不同的是,湿异常可引起季风爆发强度增强。这一结果说明,在南海夏季风爆发前期,中南半岛上空对流活动和降水异常及其引起的土壤湿度的异常变化在一定程度上会影响到季风爆发的过程。文章还比较了不同温湿地表条件下低层大气状态的差异和地表能量、水分平衡过程的不同,分析了陆气相互作用对季风活动产生影响的物理机制。

我国夏季风过渡区陆-气相互作用研究的新进展

中国夏季风过渡区是全球陆-气相互作用强盛区域之一,也是极端天气灾害频发且易造成严重经济损失的区域,对过渡区陆-气相互作用的进一步认识将有助于提升该区域防灾减灾能力。以近年来中国气象局干旱气候变化与减灾重点实验室为平台开展的夏季风过渡区相关项目群取得的研究成果为基础,对过渡区陆-气相互作用时空分布规律、陆面水分收支对夏季风响应新特征、边界层时空变化特征及发展机制、季风与陆-气相互作用对区域气候影响、陆-气相互作用对作物产量影响以及多因子和多尺度动力学粗糙度参数化方案等方面的新进展进行系统总结,并根据夏季风过渡区陆-气作用研究的发展趋势,提出今后应在侧重加强陆-气交换多循环过程对夏季风年循环响应规律研究基础上,探讨陆-气相互作用对夏季风的多尺度动态响应,建立地表过程和大气边界层关键物理量的气候动力学关系,以改进和提升区域气候模式模拟水平。该工作对推动我国陆-气耦合过程的研究具有重要意义。

Characteristics of land surface thermal-hydrologic processes for different regions over North China during prevailing summer monsoon period

The observation stations of Northern China are divided into three regions:the arid Northwest China,the Loess Plateau,and the cool Northeast China.The consistencies,differences,and associated mechanisms of land surface thermal-hydrologic processes among the three regions were studied based on the normalization of major variables of land surface thermal-hydrologic processes,using data collected during prevailing summer monsoon period(July and August,2008).It is shown that differences of surface thermal-hydrologic processes are remarkable among the three regions because of different impacts of summer monsoon.Especially their soil wet layers occur at different depths,and the average albedo and its diurnal variations are distinctly different.Surface net short-wave radiation in the Loess Plateau is close to that in the cool Northeast China,but its surface net long-wave radiation is close to that in the arid Northwest China.And the ratio of net radiation to global solar radiation in the cool Northeast China is higher than the other two regions,though its temperature is lower.There are obvious regional differences in the ratios of surface sensible and latent heat fluxes to net radiation for the three regions because of distinct contribution of sensible and latent heat fluxesto land surface energy balance.The three regions are markedly different in the ratio of water vapor flux to pan evaporation,but they are consistent in the ratio of water vapor flux to precipitation.These results not only indicate different influences of climate and environmental factors on land surface thermal-hydrologic processes in the three regions,but also show that summer monsoon is important in the formation and variation of the pattern of land surface thermal-hydrologic processes.

Asian summer monsoon responses to the change of land‒sea thermodynamic contrast in a warming climate:CMIP6 projections

It is of practical significance to use the updated Coupled Model Intercomparison Project Phase 6(CMIP6)models to study the impact of changes in land‒sea thermodynamic contrast(TC)on the Asian summer monsoon under different scenarios and to compare the similarities and differences of the impact mechanisms between different monsoon regions.In this study,we investigated future changes of the Asian summer monsoon under four Shared Socioeconomic Pathway(SSP)scenarios using 19 CMIP6 models.The intensity of the South Asian summer monsoon(SASM)is projected to decrease by 2.6%,6.3%,10.1%,and 11.1%,while the East Asian summer monsoon(EASM)intensity is projected to increase by 4.6%,7.9%,7.4%,and 9.8%until the end of the 21st century for SSP126,SSP245,SSP370,and SSP585 scenarios,respectively.Moreover,summer precipitation in Asia is projected to increase remarkably in 2015-2099 under all four scenarios.The inconsistent warming trends over the Tibetan Plateau(TP),Northwest Pacific,and tropical Indian Ocean would greatly impact the monsoon circulations.The upper-troposphere warming trend over the surrounding oceans is remarkably greater than that over the TP,while the near-surface warming trend over the surrounding oceans is smaller than that over the TP.The decrease of upper-troposphere TC between the TP and tropical Indian Ocean results in a weakening of the SASM circulation.The enhancement of the lower-troposphere TC between the TP and Northwest Pacific would strengthen the EASM circulation.Moisture budget analysis shows that the water-vapor would increase in the future,which would thermodynamically enhance summer precipitation through the anomalous vertical moisture transport associated with mean flow.The strengthening of the meridional circulation of the EASM would increase monsoon precipitation,while the weakening of zonal circulation of the SASM would dynamically reduce South Asian summer precipitation.

Possible mechanism of the effect of convection over Asian-Australian “land bridge” on the East Asian summer monsoon onset

The Asian-Australian “land bridge” is an area with the most vigorous convection in Asian monsoon region in boreal spring, where the onset and march of convection are well associated with the onset of East Asian summer monsoon. The convection occurs over Indo-China Peninsula as early as mid-April, which exerts critical impact on the evolution of monsoon circulation. Before mid-April there are primarily sensible heatings to the atmosphere over Indo-China Peninsula and Indian Peninsula, so the apparent heating ratios over them decrease with height. However, after mid-April it changes into latent heating over Indo-China Peninsula due to the onset of convection, and the apparent heating ratio increases with height in mid- and lower troposphere. The vertical distribution of heating ratio and its differences between Indo-China Peninsula and Indian Peninsula are the key factors leading to the splitting of boreal subtropical high belt over the Bay of Bengal. Such mechanism is strongly supported by the fact that the evolution of the vertical heating ratio gradient above Indo-China Peninsula leads that of 850 hPa vorticity over the Bay of Bengal. Convections over Indo-China Peninsula and its surrounding areas further increase after the splitting. Since then, there is a positive feedback lying among the convective heating, the eastward retreat of the subtropical high and the march of monsoon, which is a possible mechanism of the advance of summer monsoon and convection from Indo-China Peninsula to South China Sea.