Evaluation of the Mechanisms Acting on the Atlantic Meridional Overturning Circulation in CESM2 for the 1pctCO_(2) Experiment

The Atlantic Meridional Overturning Circulation(AMOC)is a crucial component of the Earth’s climate system due to its fundamental role in heat distribution,carbon and oxygen transport,and the weather.Other climate components,such as the atmosphere and sea ice,influence the AMOC.Evaluating the physical mechanisms of those interactions is paramount to increasing knowledge about AMOC’s functioning.In this study,the authors used outputs from the Community Earth System Model version 2 and observational data to investigate changes in theAMOC and the associated physical processes.Two DECK experiments were evaluated:piControl and 1pctCO_(2),with an annual increase of 1%of atmospheric CO_(2).The analysis revealed a significant decrease in the AMOC,associated with changes in mixed layer depth and buoyancy in high latitudes of the North Atlantic,resulting in the shutdown of deep convection and potentially affecting the formation of North Atlantic Deep Water and Antarctic Bottom Water.A vital aspect observed in this study is the association between increased runoff and reduced water evaporation,giving rise to a positive feedback process.Consequently,the rates of freshwater spreading have intensified during this period,which could lead to an accelerated disruption of the AMOC beyond the projections of existing models.

Sensitivity Experiments on Summer Monsoon Circulation Cell in East Asia

The East-Asian summer monsoon meridional circulation (SMMC) cell is simulated together with two vigorous rainbands in terms of a primitive-equation model including in itself a variety of diabatic heating, frictional dissipation and moist processes under the condition of mountains available. Results are comparable to observations. Also, performed are experiments with the reduction of water content, and exclusion of the cumulus convective process and mountain effect. Contrast analyses indicate that the cell is strongly sensitive to the condition of the humidity field in the atmosphere, more intensely at 120°than at 100°E, and the presence (absence) of the cumulus convection has considerable effect on the intensification(weakening) of the cell, with the mountain ranges exhibiting more influence upon the cell at 100° than 120°E. This may suggest that a great difference lies in the cause of the cell for the two meridions.

Atmospheric Circulation Cells Associated with Anomalous East Asian Winter Monsoon

Atmospheric circulation cells associated with anomalous East Asian Winter Monsoon （EAWM） were studied using the 1948/49 to 2002/03 NCEP/NCAR reanalysis and NCAR CAM3 AGCM simulations with monthly global sea surface temperatures from 1950 to 2000. Several atmospheric cells in the Pacific [i.e., the zonal Walker cell （ZWC） in the tropic, the Hadley cell in the western Pacific （WPHC）, the midlatitude zonal cell （MZC） over the central North Pacific, and the Hadley cell in the eastern Pacific （EPHC）] are associated with anomalous EAWM. When the EAWM is strong, ZWC, WPHC, and MZC are enhanced, as opposed to EPHC. The anomalous enhanced ZWC is characterized by air parcels rising in the western tropical Pacific, flowing eastward in the upper troposphere, and descending in the tropical central Pacific before returning to the tropical western Pacific. The enhanced MZC has characteristics opposite those of the enhanced ZWC in the central North Pacific. The anomalous WPHC shows air parcels rising in the western Pacific, as in the case of ZWC, followed by flowing northward in the upper troposphere and descending in the west North Pacific, as in the case of the enhanced MZC before returning to the western tropical Pacific. The anomalous EPHC is opposite in properties to the anomalous WPHC. Opposite characteristics are found during the weak EAWM period. The model simulations and the observations show similar characteristics and indicate the important role of sea surface temperature. A possible mechanism is proposed to link interannual variation of EAWM with the central-eastern tropical Pacific sea surface temperature anomaly （SSTA）.

Uncertainty in projections of the South Asian summer monsoon under global warming by CMIP6 models:Role of tropospheric meridional thermal contrast

Driven by the increase in CO_(2)concentration,climate models reach a consensus that the large-scale circulation of the South Asian summer monsoon(SASM) becomes weakened but with different magnitudes.This study investigates the major uncertainty sources of the SASM response to an abrupt quadrupling of CO_(2)(abrupt-4×CO_(2))in 18 models of phase 6 of the Coupled Model Intercomparison Project.The projected weakening of the SASM indicated by both zonal and meridional monsoon circulation indices is closely linked to decreases in the meridional gradient of upper-tropospheric temperature between Eurasia and the Indian Ocean(EUTT-IUTT).A climate feedback-response analysis method is applied to linearly decompose the uncertainty of changes in EUTT-IUTT into the partial changes due to external forcing and internal processes of the earth-atmosphere column.Results show that the uncertainty of changes in EUTT-IUTT is contributed positively by the dominant atmospheric dynamic process,followed by the cloud shortwave radiative effect,and negatively by the surface latent heat flux and cloud longwave radiative effect.Contributions from CO_(2)forcing and other internal processes including albedo and water vapor feedbacks,oceanic heat storage,and sensible heat flux are found to be minor.

Role of Ocean Dynamics in the Seasonal Hadley Cell:A Response to Idealized Arctic Amplification

How atmospheric and oceanic circulations respond to Arctic warming at different timescales are revealed with idealized numerical simulations.Induced by local forcing and feedback,Arctic warming appears and leads to sea-ice melting.Deep-water formation is inhibited,which weakens the Atlantic Meridional Overturning Circulation(AMOC).The flow and temperature in the upper layer does not respond to the AMOC decrease immediately,especially at mid-low latitudes.Thus,nearly uniform surface warming in mid-low latitudes enhances(decreases)the strength(width)of the Hadley cell(HC).With the smaller northward heat carried by the weaker AMOC,the Norwegian Sea cools significantly.With strong warming in Northern Hemisphere high latitudes,the long-term response triggers the“temperature-wind-gyre-temperature”cycle,leading to colder midlatitudes,resulting in strong subsidence and Ferrel cell enhancement,which drives the HC southward.With weaker warming in the tropics and stronger warming at high latitudes,there is a stronger HC with decreased width.A much warmer Southern Hemisphere appears due to a weaker AMOC that also pushes the HC southward.Our idealized model results suggest that the HC strengthens under both warming conditions,as tropical warming determines the strength of the HC convection.Second,extreme Arctic warming led by artificially reduced surface albedo decreases the meridional temperature gradient between high and low latitudes,which contracts the HC.Third,a warmer mid-high latitude in the Northern(Southern)Hemisphere due to surface albedo feedback(weakened AMOC)in our experiments pushes the HC northward(southward).In most seasons,the HC exhibits the same trend as that described above.

A Comparison between Numerical Simulations of Forced Local Hadley (Anti-Hadley) Circulation in East Asian and Indian Monsoon Regions

Two numerical simulations of forced local Hadley circulation are carried out based on a linear diagnostic equation to provide an insight into the mechanisms of monsoon evolution in different monsoon regions. One simulation is for the zonal mean Hadley circulation over East Asia (from 95°E to 122.5°E), another over India (from 70°E to 85°E). With the NCEP/ NCAR re-analysis data re—processed by Chinese Academy of Science in Beijing, the former simulation displays a dominant anti—Hadley circulation pattern over East Asia at 1200 UTC May 1, 1994. The simulated circulation pattern is consistent well enough with the circulation pattern plotted directly from the data for lack of the radiation information at each level. Although the simulation over India is not as good as that over East Asia, a dominant Hadley circulation pattern is obvious as data show. Further analysis shows that the defective simulation over India is due to the presence of statically unstable condition at some grid points in the lower troposphere. This circumstance slightly violates the hydrodynamic stability criterion required by the elliptic diagnostic equation for the forced circulation. Since the simulations are reliable enough compared with the given data, the linear equation facilitates a systematic assessment of relative importance of each internally forcing process. The assessment shows that among the internal processes, the horizontal temperature advections account obviously for the Hadley (anti—Hadley) circulation over India (East Asia) at 1200 UTC May 1, 1994 in addition to the process associated with the latent heat releasing. The calculation of latent heat energy is a little bit unreliable due to the unclear cloud physics in the convection processes and the less accurate humidity data. These preliminary results are consistent with the results of previous studies which show that the feature of the seasonal warming in the upper troposphere and the corresponding processes are part of key processes closely related to the evolution of the summer monsoon over East Asia and India. Key words Monsoon circulation - Hadley circulation - Forced meridional circulation This work was supported by the “ National key programme of China for developing basic science” G 1998040900 part 1, NSFC 49675264 and NSFC 49875021.

Discussion of Meridional Propagation Mechanism of Quasi-40-Day Oscillation

Based on researches made by the author in recent years, discussion is made of the quasi-40-day oscillation (QDO) nature and its characteristic propagation, with emphasis on the Southern Hemisphere mill-latitude quasi-periodic cold air forcing on the tropical atmosphere quasi-40-day oscillation along with its effect upon the Northern Hemisphere summer monsoon. It is proposed that the interaction between, or lateral coupling of, meridional circulation systems may serve as the mechanism of the oscillation propagation in a meridional direction.

Interdecadal Variability of the Afro-Asian Summer Monsoon System

The Afro-Asian summer monsoon is a zonally planetary-scale system, with a large-scale rainbelt covering Africa, South Asia and East Asia on interdecadal timescales both in the past century（1901-2014） and during the last three decades（1979-2014）. A recent abrupt change of precipitation occurred in the late 1990 s. Since then, the entire rainbelt of the Afro-Asia monsoon system has advanced northwards in a coordinated way. Consistent increases in precipitation over the Huanghe-Huaihe River valley and the Sahel are associated with the teleconnection pattern excited by the warm phase of the Atlantic Multidecadal Oscillation（AMO）. A teleconnection wave train, with alternating cyclones/anticyclones, is detected in the upper troposphere. Along the teleconnection path, the configuration of circulation anomalies in North Africa is characterized by coupling of the upper-level anticyclone（divergence） with low-level thermal low pressure（convergence）, facilitating the initiation and development of ascending motions in the Sahel. Similarly, in East Asia, a coupled circulation pattern also excites ascending motion in the Huanghe-Huaihe River valley. The synchronous increase in precipitation over the Sahel and Huanghe-Huaihe River valley can be attributed to the co-occurrences and in-phase changes of ascending motion. On the other hand, the warm phase of the AMO results in significant warming in the upper troposphere in North Africa and the northern part of East Asia. Such warming contributes to intensification of the tropical easterly jet through increasing the meridional pressure gradient both at the entrance region（East Asia） and the exit region（Africa）. Accordingly, precipitation over the Sahel and Huanghe-Huaihe River valley intensifies, owing to ageostrophic secondary cells. The results of this study provide evidence for a consistent and holistic interdecadal change in the Afro-Asian summer monsoon.

IMPACTS OF THE ONSET OF THE SOUTHEAST ASIAN SUMMER MONSOON ON THE BEGINNING OF THE RAINY SEASON IN YUNNAN

Summer monsoon in Southeast Asia can cause large-scale precipitation in the region in early summer, which is featured by prevailing low-level southwesterly from the Bay of Bengal to South China Sea （SCS）. It has characteristics of its own as well as those of Asian monsoons in general. As found in studies over recent years on East Asian monsoons, the earliest onset of the Southeast Asian summer monsoon occurs in early summer over the SCS, among all members of the monsoon system. It then advances westward to India and northward to eastern China, Japan and Korean Peninsula. As pointed out by Lau and Yang121, the end of April is the earliest time when the Asian monsoon sets up at the southern tip of Indo-china Peninsula. Being the earliest signal for the whole summer monsoon system in Asia, it may be of some predictive value for the establishment of Asian summer monsoon （ASM）.

青藏高原及周围地区的平均垂直环流特征

为得到青藏高原及周围地区平均垂直环流的图像，更好地理解该地区的降水气候，利用美国ＮＣＥＰ／ＮＣＡＲ１９６１～１９９０年３０年平均的ｗ、ｕ、ｖ、ｚ等再分析格点资料，计算分析了该地区的平均垂直运动场和经、纬圈环流图，分析了它们的年变化特征和可能的原因。结果表明，该地区冬、夏半年的平均垂直运动场和经、纬圈环流有很大的年变化；揭示了夏季在西北干旱区和华北半干旱区分别存在干旱和半干旱经圈环流，夏季在高原３２．５°Ｎ以南还存在巨大的向西纬圈环流等事实；也指出高原大气热源强度的年变化与随后高原及周边地区平均垂直环流的年变化联系密切。高原及周围地区的平均垂直环流特征也能较好地解释各地的降水气候。

2004年7月北京和上海两次重大气象事件的异同及其科学问题

00 4年 7月 10日北京市出现的局地强降水导致城区交通严重拥堵 ,12日强对流伴随的飑线造成上海市重大的人员伤亡。这两次突发性气象事件都引起了社会公众和政府部门的高度关注。文中给出的观测分析和理论估算初步表明 ,这两次重大气象事件都是由中尺度对流系统 (MCS)引起的。但是在卫星云图上 ,造成北京强降水的对流云团外形不规则、云顶温度较高、云体边缘不光滑并且没有密集的亮温梯度 ,与大多数MCS有很大的不同。此外 ,这两个事件都发生在季风极度北涌 ,同时冷空气又强烈南下的经向环流背景下 ,但两者发生的位置差别很大。文章认为对于这种非典型性MCS的结构和发生条件以及 2 0 0 4年夏东亚季风极度北涌是否与全球环流异常有关等科学问题值得引起重视。

东亚夏季风环流与ENSO循环的关系

采用NCEP/NCAR再分析资料和NCAR海温资料 ,对ENSO循环不同阶段东亚夏季风环流的变化进行了分析。计算了各年夏季风环流的强度系数及其与多年平均夏季风环流的相似系数和差异系数 ,分析它们与海温变化的关系。结果表明 :东亚夏季风环流强度有明显的年际变化和年代际变化 ,且与赤道东太平洋SST有较好的负相关关系 ,其中又以与三个月前的海温变化关系最好。在春季 (3～4月 )Nino 1+2区为冷、暖水时 ,当年夏季 (6～ 7月 )东亚季风区中 85 0hPa等压面上 >2m·s-1的经向风北伸纬度和东亚季风区的垂直经圈环流都有明显差异 ,在冷水期 >2m·s-1的经向风北伸纬度比暖水期高 ,季风环流圈的上升支北移 ,东亚夏季风环流较暖水期强。

东亚副热带季风特征及其指数的建立

基于大气对流活动和非绝热加热的分析 ,给出东亚夏季风区域变化特征 ,客观地确定东亚副热带季风及热带季风对流强度 ,并由此建立其与大尺度季风环流的内在联系 ,提出分别用所对应区域经向风垂直切变来构造季风指数。对比分析显示 ,上述季风指数在反映夏季环流和降水等方面效果显著 ,能很好地刻划东亚夏季风强度。同时指出 ,东亚副热带季风指数与西太平洋副热带高压及长江中下游降水密切相关 ,在空间场上表现出东亚 /太平洋型 (EAP型 )遥相关特征。高指数年副高偏南 ,长江中下游为涝 ;低指数年则相反。

东亚季风经向环流数值模拟及结果分析 Ⅰ.算法设计

着重介绍局地径向环流诊断方程的数值求解 ,内容包括数值模式格式的设计。

东亚季风涌对我国东部大尺度降水过程的影响分析

将NECP/NCAR资料中850 hPa纬向风分量进行30~60天带通滤波,用大气季节内振荡(ISO)来表征东亚夏季风涌的活动特征。研究表明,长江中下游地区和淮河流域分别出现涝年时,都伴有很显著的季风涌向北传播的特征。季风涌的北传实质就是季风中来自热带地区ISO的向北传播。季风涌北传过程中,得到来自东西两侧纬向传播的ISO补充。纬向传播的ISO在110°E^120°E附近汇合后增强自热带地区向北传的季风涌,使得季风涌在经向上可以继续向北传播,并对我国东部地区夏季大尺度降水过程产生一定的影响。纬向补充的ISO特征差异对我国东部大尺度降水位置会产生一定的差异。

中国东部夏季暴雨的年代际跃变及其大尺度环流背景

本文利用1960-2011年中国东部地面测站的逐日降水资料和JRA-55再分析资料探讨了夏季暴雨分布的年代际跃变及其相关联的大尺度环流异常特征。基于暴雨频数和占比（夏季暴雨占比是指5-8月暴雨降水量对总降水量的贡献百分比）的分析结果表明：中国东部夏季暴雨分布在20世纪70年代末和90年代初经历两次反相的经向＂三极子＂跃变。中国东部夏季暴雨的年代际演变过程可分为三个时段：1960-1979年为华南和华北暴雨偏多、江淮流域暴雨偏少的经向＂三极子＂分布;1980-1991年为南方和华北暴雨偏少、江淮流域暴雨偏多的经向＂三极子＂分布;1992-2011年为南方暴雨显著偏多、华北暴雨持续偏少,逐渐形成经向＂偶极子＂分布,并导致近十多年我国夏季＂南涝北旱＂的整体格局。1970年代末（1990年代初）跃变相关联的大尺度环流异常配置：东亚夏季风的减弱（增强）,西太平洋副高的增强西伸但南撤（北抬）,南亚高压的减弱南缩（增强东扩）,以及蒙古高原中低层的气旋式（反气旋式）环流异常。与此同时,低层局地环流也发生调整：华北和黄淮地区以及华南和江南地区均为反气旋式（气旋式）环流异常,而江淮流域和四川盆地受控于风场切变式辐合（辐散）异常;涡度场发生相应变化,南北方大部分地区的负（正）涡度异常不（有）利于低涡的发展,而江淮流域和四川盆地的正（负）涡度异常有（不）利于低涡的发展,进而引发江南和华南暴雨减少（增加）、江淮流域和四川盆地暴雨增加（减少）、黄淮和华北暴雨减少（增加）的经向＂三极子＂跃变。

东亚大陆冬季风的低频振荡特征

本文对东亚大陆冬季风期间的地面气温、气压的数年资料做了３０～６０天的低频波、准双周振荡等分析。结果表明，东亚大陆地区地面气温、气压的０～６０天波和准双周波具有重要的作用，它们多年平均的低频波方差相对贡献分别为５９．２％（气温）和４９．８％（气压）左右。并且伴有显著的年际差异，如华北气温３０～６０天波强的年份其方差相对贡献可达５０．２％，而最弱的年只占１１．９％左右。另外这两种频率振荡强弱还有明显的地区性，３０～６０天波一般华北和长江流域（２８°Ｎ以北）比其南部地区强；冬季风期间，东亚大陆地区的３０～６０波的经向传播主要是由北向南传播的。大多数年份可以传播至东亚大陆的南端，历时约３～６天，但是这种传播在长江流域以北地区表现为更清楚，在长江以南地区，一方面受到了冬季风本身强弱的影响，另方面还受到来自热带地区以致南半球夏季风低频波北传等因素的影响使得经向传播往往变得比较复杂。

1984年南海夏季风经向环流强迫因子的诊断分析

利用纬向平均的局地经向环流模式,对1984年南海地区4月28日～5月31日每日两个时次的局地平均经向环流进行模拟。模拟结果与观测结果吻合,反映出1984年南海夏季风形势的建立是一个经历反复的过程。此外,用诊断分析的方法通过计算各动力和热力强迫因子对季风经向环流的贡献,发现驱动1984年南海夏季风经向环流的首要因子是潜热加热项,主要由北部凝结加热,南部蒸发冷却的经向不均匀加热场分布使得潜热加热对环流的作用为正。温度垂直对流输送项的作用次之,该项的分布在南海北部的值为正、南部值近似为零,这种分布形势也使该因子能单独引起一个正季风环流。第三大因子是纬向温度平流输送项,在南海地区北部有暖平流,南部冷平流的温度平流分布形势对季风环流也起正的作用。而对季风中断期的因子分析表明:潜热加热及纬向温度平流的作用与正季风环流建立期完全相反,温度垂直对流的正作用也大大减弱。

东南亚地区夏季风爆发对云南雨季开始的影响

东南亚地区的夏季风在亚洲季风系统中首先爆发后,通过风场与对流场的变化对周边地区造成影响。通过对亚洲近地面层经向水汽通道的研究,发现在前期影响云南雨季开始的早迟最重要的一支南风水汽通道位于孟加拉湾(80～90°E)附近。该南风水汽通道一般在春季开始建立,它的建立有助于夏季风在东南亚地区爆发。在季风建立前,当这一水汽通道中南风水汽输送异常偏强时,相应地中南半岛附近的对流也增强,对应着云南的雨季开始偏早,初夏降水偏多,反之则初夏降水偏少。研究表明,该水汽输送的强弱可以作为预报云南雨季开始期及初夏雨量的一个重要信号。研究还发现,前期赤道中东太平洋的冷海温有利于孟加拉湾经向水汽输送的加强,从而影响到云南的雨季开始和初夏的雨量。

南海夏季风经向环流的20年平均4～6月演变机制

用NCEP再分析资料和纬向平均的局地经向环流线性诊断模式 ,模拟了 1 980～1 999年 4～ 6月南海夏季风经向环流的建立和演变过程 ,并结合统计方法分析了模式输出的各物理因子和外界影响 (主要为越赤道气流 )在激发南海夏季风经向环流过程中的作用和贡献。结果表明 :( 1 )依 1 980～ 1 999年 4～ 6月经向环流指数的逐日演变情况判断得出 ,南海地区 4～ 6月经向风中有 48%为地转成分 ,非地转成分占 5 2 % ;若从 2 0年平均的经向环流指数逐候演变情况来看 ,4月 1候到 6候南海地区经向风中有 94%为非地转成分 ,而且 5月份非地转经向环流指数与总经向环流指数的变化趋势也比较一致。因此表明 ,引起经向风地转偏差的因子在南海夏季风的酝酿和爆发过程中的作用尤其重要。 ( 2 )对南海地区非地转经向环流指数的贡献 ,主要来自外界影响 (主要由越赤道气流体现 )和 4个物理因子 ,即与温度层结和垂直运动有关的热量垂直输送 ,纬向温度平流 ,潜热加热作用以及西风动量经向输送 ,其余 1 2个物理因子贡献较小。( 3) 2 0年中有 1 5年与外界影响有关的经向环流指数在南海夏季风爆发前一周内出现最明显的突变性加强 ,剩下的 5年则是与热量垂直输送作用有关的经向环流指数出现较明显的突变性加强 ,其余