Latitude-Dependent Response of East Asian Summer Monsoon to External Forcing in the Last Millennium

Response of the East Asian summer monsoon(EASM) rainfall to external forcing(insolation,volcanic aerosol,and greenhouse gases) is investigated by analysis of a millennium simulation with the coupled climate model ECHO-G.The model reproduces reasonably realistic present-day EASM climatology.The simulated precipitation variation in East Asia over the last millennium compares favorably with the observed and proxy data.It is found that the features and sensitivity of the forced response depend on latitude.On...     

What drives the global summer monsoon over the past millennium?

The global summer monsoon precipitation (GSMP) provides a fundamental measure for changes in the annual cycle of the climate system and hydroclimate. We investigate mechanisms governing decadal-centennial variations of the GSMP over the past millennium with a coupled climate model’s (ECHO-G) simulation forced by solar-volcanic (SV) radiative forcing and greenhouse gases (GHG) forcing. We show that the leading mode of GSMP is a forced response to external forcing on centennial time scale with a globally uniform change of precipitation across all monsoon regions, whereas the second mode represents internal variability on multi-decadal time scale with regional characteristics. The total amount of GSMP varies in phase with the global mean temperature, indicating that global warming is accompanied by amplification of the annual cycle of the climate system. The northern hemisphere summer monsoon precipitation (NHSMP) responds to GHG forcing more sensitively, while the southern hemisphere summer monsoon precipitation (SHSMP) responds to the SV radiative forcing more sensitively. The NHSMP is enhanced by increased NH land–ocean thermal contrast and NH-minus-SH thermal contrast. On the other hand, the SHSMP is strengthened by enhanced SH subtropical highs and the east–west mass contrast between Southeast Pacific and tropical Indian Ocean. The strength of the GSMP is determined by the factors controlling both the NHSMP and SHSMP. Intensification of GSMP is associated with (a) increased global land–ocean thermal contrast, (b) reinforced east–west mass contrast between Southeast Pacific and tropical Indian Ocean, and (c) enhanced circumglobal SH subtropical highs. The physical mechanisms revealed here will add understanding of future change of the global monsoon.     

基于CCSM4.0长期积分试验评估不同辐射强迫对中国干旱半干旱区降水的影响

本文基于气候系统模式CESM4.0长期积分试验,分析评估了工业革命前（1850年）及当前（2000年）两种辐射强迫作用（分别为太阳活动和温室气体）对中国北方干旱半干旱区降水的影响。结果表明,模拟结果与观测之间尽管存在一定的偏差,但仍能再现降水气候态的空间分布以及季节变化特征;两种辐射强迫下的降水长期变化均无明显趋势,但二者的差异却呈现出70~100年的准周期振荡;由人类活动引起的当前辐射强迫作用对降水的多年际变率幅度有一定影响,造成极端强降水事件出现的概率增多,而由太阳活动引起的辐射强迫作用主要对降水多年代际周期具有一定的调制作用。进一步分析表明,两种辐射强迫下中国北方干旱半干旱区降水年多年代际变率的主要模态基本一致,但人类活动引起的辐射强迫作用会影响降水多年代际变率与热带海温异常的相互作用的强度,从而改变降水多年代际变率的幅度。     

The response of the North Pacific Decadal Variability to strong tropical volcanic eruptions

In this study, the effects of volcanic forcing on North Pacific climate variability, on interannual to decadal time scales, are examined using climate model simulations covering the last 600?years. The model used is the Bergen Climate Model, a fully coupled atmosphere–ocean general circulation model. It is found that natural external forcings, such as tropical strong volcanic eruptions (SVEs) and variations in total solar irradiance, play an important role in regulating North Pacific Decadal Variability (NPDV). In response to tropical SVEs the lower stratospheric pole–to–equator temperature gradient is enhanced. The North polar vortex is strengthened, which forces a significant positive Arctic Oscillation. At the same time, dipole zonal wind anomalies associated with strong polar vortex propagate downward from the lower stratosphere. Through positive feedbacks in the troposphere, the surface westerly winds across the central North Pacific are significantly weakened, and positive sea level pressure anomalies are formed in the North Pacific. This anomalous surface circulation results in changes in the net heat fluxes and the oceanic advection across the North Pacific. As a result of this, warm water converges in the subtropical western North Pacific, where the surface waters in addition are heated by significantly reduced latent and sensible heat fluxes from the ocean. In the eastern and high–latitude North Pacific the ocean loses more heat, and large–scale decreases in sea surface temperatures are found. The overall response of this chain of events is that the North Pacific enters a negative phase of the Pacific decadal oscillation (PDO), and this negative phase of the PDO is maintained for several years. It is thus concluded that the volcanic forcing plays a key role in the phasing of the PDO. The model results furthermore highlight the important role of troposphere–stratosphere coupling, tropical–extratropical teleconnections and extratropical ocean–atmosphere interactions for describing NPDV.     

南半球环流变化对东亚夏季风的影响

南半球环流是影响东亚夏季风季节内、季节到年际变化的重要因子之一.作者系统综述了南半球环流各系统包括连接两半球的越赤道气流、马斯克林高压和澳大利亚高压、南极涛动和南极海冰等对东亚夏季风环流和中国夏季降水的影响.特别是,近年来的研究揭示了南极涛动是影响东亚夏季风年际变化的强信号.当南极涛动偏强时,马斯克林高压和澳大利亚高压和相关的越赤道气流也趋于偏强.同时,西太平洋副热带高压偏西偏南,强度增强,长江流域降水偏多,其两侧降水偏少.这对中国夏季降水的预测有重要的应用价值.最后提出了一些相关的科学问题以供进一步研究.       

The Combined Effects of the Tropical and Extratropical Quasi-biweekly Oscillations on the Record-setting Mei-yu Rainfall in the Summer of 2020

During June–July 2020, the strongest recorded mei-yu rainfall occurred in the middle and lower reaches of the Yangtze River. The rainfall processes exhibited an obvious quasi-biweekly(biweekly in brief) variability, and there are altogether five cycles. It is found that the biweekly rainfall cycle mainly arises from the collaborative effects of biweekly variabilities from both the tropics and extratropics. As for the tropics, the biweekly meridional march and retreat of the western Pacific subtr...     

Impact of local sea surface temperature anomaly over the western North Pacific on extreme East Asian summer monsoon

In this study, the anomalous characteristics of observed large-scale synoptic fields in the extreme East Asian summer monsoon (EASM) years are analyzed, and the impact of the local sea surface temperature (SST) anomaly over the western North Pacific (WNP) on the extreme EASM is investigated through sensitivity experiments of 28?years EASM simulations to the local SST over the WNP. The observation analysis reveals that the extreme EASM is influenced more by anomalous large-scale atmospheric features such as monsoon circulations and the western North Pacific subtropical high than the local SST anomaly over the WNP. However, the results of the sensitivity experiments show that the local SST anomaly has an implicit impact on the extreme EASM. The patterns of differences in precipitation between the experiment forced by observed SST in each year and the experiment forced by climatological SST over the WNP are opposite to anomaly patterns of observed precipitation in the extreme EASM years. This is because the SST anomaly over the WNP plays a role in reducing precipitation anomaly by changing surface latent heat flux and monsoon circulations. In particular, the local SST anomaly over the WNP decreases anomalies of large-scale circulations, i.e., the local Hadley and the Walker circulations. Thus, the local SST anomaly over the WNP plays a role in decreasing the interannual variability of the EASM.     

Variability of the Atlantic meridional overturning circulation in the last millennium and two IPCC scenarios

The variability of the Atlantic meridional overturning circulation (AMOC) is investigated in several climate simulations with the ECHO-G atmosphere-ocean general circulation model, including two forced integrations of the last millennium, one millennial-long control run, and two future scenario simulations of the twenty-first century. This constitutes a new framework in which the AMOC response to future climate change conditions is addressed in the context of both its past evolution and its natural variability. The main mechanisms responsible for the AMOC variability at interannual and multidecadal time scales are described. At high frequencies, the AMOC is directly responding to local changes in the Ekman transport, associated with three modes of climate variability: El Ni?o-Southern Oscillation (ENSO), the North Atlantic Oscillation (NAO), and the East Atlantic (EA) pattern. At low frequencies, the AMOC is largely controlled by convection activity south of Greenland. Again, the atmosphere is found to play a leading role in these variations. Positive anomalies of convection are preceded in 1?year by intensified zonal winds, associated in the forced runs to a positive NAO-like pattern. Finally, the sensitivity of the AMOC to three different forcing factors is investigated. The major impact is associated with increasing greenhouse gases, given their strong and persistent radiative forcing. Starting in the Industrial Era and continuing in the future scenarios, the AMOC experiences a final decrease of up to 40% with respect to the preindustrial average. Also, a weak but significant AMOC strengthening is found in response to the major volcanic eruptions, which produce colder and saltier surface conditions over the main convection regions. In contrast, no meaningful impact of the solar forcing on the AMOC is observed. Indeed, solar irradiance only affects convection in the Nordic Seas, with a marginal contribution to the AMOC variability in the ECHO-G runs.     

Competing influences of greenhouse warming and aerosols on Asian summer monsoon circulation and rainfall

In this paper, we have compared and contrasted competing influences of greenhouse gases (GHG) warming and aerosol forcing on Asian summer monsoon circulation and rainfall based on CMIP5 historical simulations. Under GHG-only forcing, the land warms much faster than the ocean, magnifying the pre-industrial climatological land-ocean thermal contrast and hemispheric asymmetry, i.e., warmer northern than southern hemisphere. A steady increasing warm-ocean-warmer-land (WOWL) trend has been in effect since the 1950’s substantially increasing moisture transport from adjacent oceans, and enhancing rainfall over the Asian monsoon regions. However, under GHG warming, increased atmospheric stability due to strong reduction in mid-tropospheric and near surface relative humidity coupled to an expanding subsidence areas, associated with the Deep Tropical Squeeze (DTS, Lau and Kim, 2015b) strongly suppress monsoon convection and rainfall over subtropical and extratropical land, leading to a weakening of the Asian monsoon meridional circulation. Increased anthropogenic aerosol emission strongly masks WOWL, by over 60% over the northern hemisphere, negating to a large extent the rainfall increase due to GHG warming, and leading to a further weakening of the monsoon circulation, through increasing atmospheric stability, most likely associated with aerosol solar dimming and semi-direct effects. Overall, we find that GHG exerts stronger positive rainfall sensitivity, but less negative circulation sensitivity in SASM compared to EASM. In contrast, aerosols exert stronger negative impacts on rainfall, but less negative impacts on circulation in EASM compared to SASM.     

The role of precipitation in aerosol-induced changes in northern hemisphere wintertime stationary waves

The coupled climate model EC-Earth2 is used to investigate the impact of direct radiative effects of aerosols on stationary waves in the northern hemisphere wintertime circulation. The direct effect of aerosols is simulated by introducing prescribed mixing ratios of different aerosol compounds representing pre-industrial and present-day conditions, no indirect effects are included. In the EC-Earth2 results, the surface temperature response is uncorrelated with the highly asymmetric aerosol radiative forcing pattern. Instead, the anomalous extratropical temperature field bears a strong resemblance to the aerosol-induced changes in the stationary-wave pattern. It is demonstrated that the main features of the wave pattern of EC-Earth2 can be replicated by a linear, baroclinic model forced with latent heat changes corresponding to the anomalous convective precipitation generated by EC-Earth2. The tropical latent heat release is an effective means of generating stationary wave trains that propagate into the extratropics. Hence, the results of the present study indicate that aerosol-induced convective precipitation anomalies govern the extratropical wave-field changes, and that the far-field temperature response dominates over local effects of aerosol radiative forcing.     

海温异常对东亚夏季风影响机理的研究进展

从短期气候预测关注的外强迫信号角度出发,回顾了国内外在海温异常对东亚夏季风和我国汛期降水影响机理方面的主要研究进展,重点评述了热带太平洋ENSO循环、热带印度洋全区一致型海温模态、热带印度洋海温异常偶极子、南印度洋偶极子和北大西洋海温三极子模态的年际变化及其对东亚夏季风年际变率的影响。从研究成果在短期气候预测业务中应用的角度,重点关注海温异常和东亚夏季风年际变率以及我国汛期降水多雨带位置的关系,总结了海温异常作为外强迫信号对我国汛期降水预测的指示意义以及汛期降水预测的难度。最后指出气候预测业务对东亚夏季风影响的机理研究和动力气候模式发展方面的需求。     

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.     

Response of a stochastically-perturbed 1-D surface energy balance model to increased temperature

Summary The response of the climatic system to changes in its radiative forcing has been the subject of much study. Climate models of various complexity have been used to demonstrate that a small increase in the solar constant, or doubling of the atmospheric CO₂, would lead to a warmer surface. Very little scientific attention, however, has been given to the effect such a change in radiative balance might have on climatic variability. That is, would an earth warmed in this way be more temperate or more variable? To move one step closer to answering this question, we employed a simple one-dimensional surface energy balance climate model and forced it with random Gaussian white noise to simulate interannual variability. We integrated the model using 0, 2, and 4% increases in the solar constant. The results of these numerical experiments indicate that, under a warmer surface radiative balance, interannual variability of the surface temperature is reduced.     

东亚夏季风次季节变化研究进展

东亚夏季风次季节（10~90 d）变化是中国夏季持续性强降水、高温热浪等高影响天气事件的重要环流载体,处于天气预报上限和气候季节预测下限之间的预报过渡区。研究表明:东亚夏季风次季节变化是东亚夏季风的固有物理特征,它和季节进程之间的时间锁相关系是东亚夏季风次季节变化潜在可预报性的重要来源。东亚夏季风次季节变化与Madden-Julian振荡（MJO）存在显著差异,试图通过MJO来预测东亚夏季风次季节变化的不确定性较大。东亚夏季风次季节预测的另一重要来源是下垫面外强迫,包括欧亚大陆春季积雪、中国东部春季土壤湿度和厄尔尼诺-南方涛动（ENSO）事件。此外,去趋势偏-交叉相关分析统计方法能够分析东亚夏季风多因子和多时间尺度问题。目前,亟需解决的科学问题包括:东亚夏季风次季节模态的客观定量描述、造成东亚夏季风次季节模态年际变化的关键物理过程、不同外强迫因子对东亚夏季风次季节模态的共同影响。     

Decadal predictability and prediction skill of sea surface temperatures in the South Pacific region

The South Pacific Ocean is a key driver of climate variability within the Southern Hemisphere at different time scales. Previous studies have characterized the main mode of interannual sea surface temperature (SST) variability in that region as a dipolar pattern of SST anomalies that cover subtropical and extratropical latitudes (the South Pacific Ocean Dipole, or SPOD), which is related to precipitation and temperature anomalies over several regions throughout the Southern Hemisphere. Using that relationship and the reported low predictive skill of precipitation anomalies over the Southern Hemisphere, this work explores the predictability and prediction skill of the SPOD in near-term climate hindcasts using a set of state-of-the-art forecast systems. Results show that predictability greatly benefits from initializing the hindcasts beyond the prescribed radiative forcing, and is modulated by known modes of climate variability, namely El Niño-Southern Oscillation and the Interdecadal Pacific Oscillation. Furthermore, the models are capable of simulating the spatial pattern of the observed SPOD even without initialization, which suggests that the key dynamical processes are properly represented. However, the hindcast of the actual phase of the mode is only achieved when the forecast systems are initialized, pointing at SPOD variability to not be radiatively forced but probably internally generated. The comparison with the performance of an empirical prediction based on persistence suggests that initialization may provide skillful information for SST anomalies, outperforming damping processes, up to 2–3 years into the future.     

Northern East Asian low and its impact on the interannual variation of East Asian summer rainfall

The strength of the East Asian summer monsoon and associated rainfall has been linked to the western North Pacific subtropical high (WNPSH) and the lower-tropospheric low pressure system over continental East Asia (EA). In contrast to the large number of studies devoted to the WNPSH, little is known about the variability of the East Asian continental low. The present study delineates the East Asian continental low using 850-hPa geopotential height. Since the low is centered over northern EA (NEA), we refer to it as the NEA low (NEAL). We show that the intensity of the NEAL has large interannual variation, with a dominant period of 2–4 years. An enhanced NEAL exhibits a barotropic structure throughout the whole troposphere, which accelerates the summer-mean upper-tropospheric westerly jet and lower-tropospheric monsoon westerly to its south. We carefully identify the anomalous NEAL-induced rainfall anomalies by removal of the tropical heating effects. An enhanced NEAL not only increases rainfall locally in northern Northeast China, but also shifts the East Asian subtropical front northward, causing above-normal rainfall extending eastward from the Huai River valley across central-northern Japan and below-normal rainfall in South China. The northward shift of the East Asian subtropical front is attributed to the following processes without change in the WNPSH: an enhanced NEAL increases meridional pressure gradients and the monsoon westerly along the East Asian subtropical front, which in turn induces a cyclonic shear vorticity anomaly to its northern side. The associated Ekman pumping induces moisture flux convergence that shifts the East Asian subtropical front northward. In addition, the frequent occurrence of synoptic cut-off lows is found to be associated with an enhanced NEAL. Wave activity analysis indicates that the interannual intensity change of the NEAL is significantly associated with the extratropical Polar Eurasian teleconnection, in addition to the forcing of the tropical WNP heating.     

A multi-model analysis of the role of the ocean on the African and Indian monsoon during the mid-Holocene

We investigate the role of the ocean feedback on the climate in response to insolation forcing during the mid-Holocene (6,000 year BP) using results from seven coupled ocean–atmosphere general circulation models. We examine how the dipole in late summer sea-surface temperature (SST) anomalies in the tropical Atlantic increases the length of the African monsoon, how this dipole structure is created and maintained, and how the late summer SST warming in the northwest Indian Ocean affects the monsoon retreat in this sector. Similar mechanisms are found in all of the models, including a strong wind evaporation feedback and changes in the mixed layer depth that enhance the insolation forcing, as well as increased Ekman transport in the Atlantic that sharpens the Atlantic dipole pattern. We also consider changes in interannual variability over West Africa and the Indian Ocean. The teleconnection between variations in SST and Sahelian precipitation favor a larger impact of the Atlantic dipole mode in this region. In the Indian Ocean, the strengthening of the Indian dipole structure in autumn has a damping effect on the Indian dipole mode at the interannual time scale.     

A comparison of East Asian summer monsoon simulations from CAM3.1 with three dynamic cores

This paper examines the sensitivity of CAM3.1 simulations of East Asian summer monsoon (EASM) to the choice of dynamic cores using three long-term simulations, one with each of the following cores: the Eulerian spectral transform method (EUL), semi-Lagrangian scheme (SLD) and finite volume approach (FV). Our results indicate that the dynamic cores significantly influence the simulated fields not only through dynamics, such as wind, but also through physical processes, such as precipitation. Generally speaking, SLD is superior to EUL and FV in simulating the climatological features of EASM and its interannual variability. The SLD version of the CAM model partially reduces its known deficiency in simulating the climatological features of East Asian summer precipitation. The strength and position of simulated western Pacific subtropical high (WPSH) and its ridge line compare more favourably with observations in SLD and FV than in EUL. They contribute to the intensification of the south-easterly along the south of WPSH and the vertical motion through the troposphere around 30° N, where the subtropical rain belt exists. Additionally, SLD simulates the scope of the westerly jet core over East Asia more realistically than the other two dynamic cores do. Considerable systematic errors of the seasonal migration of monsoon rain belt and water vapour flux exist in all of the three versions of CAM3.1 model, although it captures the broad northward shift of convection, and the simulated results share similarities. The interannual variation of EASM is found to be more accurate in SLD simulation, which reasonably reproduces the leading combined patterns of precipitation and 850-hPa winds in East Asia, as well as the 2.5- and 10-year periods of Li?CZeng EASM index. These results emphasise the importance of dynamic cores for the EASM simulation as distinct from the simulation??s sensitivity to the physical parameterisations.     

Past Millennium Contrasting Hydroclimate Patterns Between Monsoonal Northern China and Arid Central Asia: a Modeling Study

Widely distributed proxy records show that there were out-of-phase behaviors of moisture change between arid central Asia (ACA) and monsoonal northern China during the Little Ice Age (LIA) and Medieval Climate Anomaly (MCA). We examined spatial pattern differences between the MCA and LIA to identify contrasting patterns of summer precipitation variability, and to diagnose explanatory mechanisms through the analysis of a 1000-year global climate model simulation driven by natural and anthropogenic forcing. The results show that the model was able to roughly produce the general features of MCA-LIA hydroclimatic spatial differences between monsoonal northern China and ACA, with a relatively wet MCA found in monsoonal northern China and a relatively dry MCA found in ACA. A further analysis of associated circulations shows that increased summer precipitation in monsoonal northern China was caused by the strengthening of summer monsoon, while the decline in summer precipitation in ACA was caused by an anomalous northward displacement of the subtropical westerly jet stream. Our analyses suggest that both effective solar forcing and El-Niño Southern Oscillation (ENSO) may produce these contrasting patterns of precipitation between monsoonal northern China and ACA. Due to a change in the probability of ENSO phases at the centennial time scale found in our experiments may be attributed to solar irradiances, higher effective solar irradiances during the MCA relative to those of the LIA may have been the ultimate forcing mechanism for the simulated precipitation differences between the MCA and LIA.     

亚洲—太平洋夏季风系统的基本模态特征分析

亚洲—太平洋季风区各季风子系统间的相互作用对季风区甚至全球的气候变化都有着显著的影响.整个亚洲—太平洋夏季风系统都处于高层辐散、低层辐合的庞大辐散环流中,从高层辐散中心流出的三支气流分别对推动印度夏季风、东亚副热带夏季风和南海夏季风起着重要的作用,很好地表现了亚洲—太平洋夏季风系统的整体性特征.季风区多种气象要素的基本模态在年代际和年际尺度上都表现出较为一致的变化特征:年代际尺度上亚洲—太平洋夏季风系统整体呈现减弱趋势;年际尺度上存在准2年和准4年的两个周期,其中准2年振荡特征表现为若印度西南季风偏强,则印度季风雨带偏强偏北,导致印度大陆中北部地区降水偏多;同时,由于西太平洋副热带高压的北移和偏强的印度西南季风显著向东延伸,10°N~30°N范围内的西北太平洋地区则表现为异常的气旋性环流,而30°N~50°N之间为反气旋性环流异常,对应东亚夏季风偏强,季风雨带能够北推至我国华北地区.也就是说,当亚洲夏季风中某一季风子系统表现为异常偏强时,另一季风子系统在这一年中也将表现为异常偏强,反之亦然.准2年的振荡周期可能是亚洲—太平洋夏季风系统的一种固有振荡,它从年际尺度上反映了亚洲—太平洋夏季风受热带太平洋—印度洋海温的强迫表现出明显的整体一致特征.