Mechanism on how the spring Arctic sea ice impacts the East Asian summer monsoon

Observational analysis and purposely designed coupled atmosphere–ocean (AOGCM) and atmosphere-only (AGCM) model simulations are used together to investigate a new mechanism describing how spring Arctic sea ice impacts the East Asian summer monsoon (EASM). Consistent with previous studies, analysis of observational data from 1979 to 2009 show that spring Arctic sea ice is significantly linked to the EASM on inter-annual timescales. Results of a multivariate Empirical Orthogonal Function analysis reveal that sea surface temperature (SST) changes in the North Pacific play a mediating role for the inter-seasonal connection between spring Arctic sea ice and the EASM. Large-scale atmospheric circulation and precipitation changes are consistent with the SST changes. The mechanism found in the observational data is confirmed by the numerical experiments and can be described as follows: spring Arctic sea ice anomalies cause atmospheric circulation anomalies, which, in turn, cause SST anomalies in the North Pacific. The SST anomalies can persist into summer and then impact the summer monsoon circulation and precipitation over East Asia. The mediating role of SST changes is highlighted by the result that only the AOGCM, but not the AGCM, reproduces the observed sea ice-EASM linkage.     

1999年东亚夏季风异常活动的物理机制研究

文中从海-气相互作用的角度探讨了1999年东亚夏季风及与其相联系的雨带异常活动的物理机制。结果表明,由于1998年春季至1999年南海-热带西太平洋出现了近20 a最强的异常暧海温,该地强异常海-气相互作用的维持使得这种局地的热力强迫成为1999年东亚夏季风和降水异常的最主要外强迫机制,并使得1999年的季风活动和降水分布有别于一般的统计情形。从1998年秋到1999年,由于热带大气对南海-西太平洋暧海温所诱发的局地强加热的响应,热带西太平洋地区所出现的Gill模态的异常环流分布从冬季一直发展到夏季,并因此在海洋和大气之间形成了局地的强烈正反馈,不仅使得异常环流得以持续发展,而且也使得暖海温得以维持,成为影响1999年环流异常的最强前期信号。随着从冬到夏的季节演变,大气基本态对上述持续性异常环流的影响导致了冬、夏异常环流呈现出不同的纬向非对称,诱发了盛夏期间东亚到北美沿岸的遥相关波列。在东亚沿岸异常气旋性环流的影响下,大尺度异常东风在东亚沿岸的维持形成了极不利于季风西风在南海北部转向的条件,导致了季风在中国东部北进的异常偏弱和低纬西风转向位置的异常偏东。     

An analysis on the physical process of the influence of AO on ENSO

The influence of the spring AO on ENSO has been demonstrated in several recent studies. This analysis further explores the physical process of the influence of AO on ENSO using the NCEP/NCAR reanalysis data over the period 1958–2010. We focus on the formation of the westerly wind burst in the tropical western Pacific, and examine the evolution and formation of the atmospheric circulation, atmospheric heating, and SST anomalies in association with the spring AO variability. The spring AO variability is found to be independent from the East Asian winter monsoon activity. The spring AO associated circulation anomalies are supported by the interaction between synoptic-scale eddies and the mean-flow and its associated vorticity transportation. Surface wind changes may affect surface heat fluxes and the oceanic heat transport, resulting in the SST change. The AO associated warming in the equatorial SSTs results primarily from the ocean heat transport in the face of net surface heat flux damping. The tropical SST warming is accompanied by anomalous atmospheric heating in the subtropical north and south Pacific, which sustains the anomalous westerly wind in the equatorial western Pacific through a Gill-like atmospheric response from spring to summer. The anomalous westerly excites an eastward propagating and downwelling equatorial Kelvin wave, leading to SST warming in the tropical central-eastern Pacific in summer-fall. The tropical SST, atmospheric heating, and atmospheric circulation anomalies sustain and develop through the Bjerknes feedback mechanism, which eventually result in an El Niño-like warming in the tropical eastern Pacific in winter.     

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.     

Impact of the western North Pacific subtropical high on the East Asian monsoon precipitation and the Indian Ocean precipitation in the boreal summertime

The western North Pacific subtropical high (WNPSH) is a crucial component of the East Asian summer monsoon (EASM) system and significantly influences the precipitation in East Asia. In this study, distinguished role of WNPSH on the EASM and Indian Ocean monsoon (IOM) are investigated. Based on the boreal summer mean field of 850-hPa geopotential height and its interannual variability, the WNPSH index (WNPSHI) is defined by the areaaveraged geopotential height over the region [110°–150°E, 15°–30°N]. The WNPSHI is significantly related to the precipitation over the East Asian monsoon (EAM) region [105°–150°E, 30°–40°N] and IOM region [70°–105°E, 5°–15°N]. Rainfalls over these two regions have good correlation with WNPSH developments and the geopotential height fields at 850 hPa related to the EAM precipitation and IOM precipitation have remarkably different teleconnection patterns in boreal summer. These features exhibit that EAM and IOM precipitations have different type of development processes associated with different type of WNPSH each other. Focusing on the relationships among the EAM precipitation, IOM precipitation, and the WNPSH variabilities, we assume that WNPSH and EAM precipitation are usually fluctuated simultaneously through the sea surface temperature (SST)-subtropical ridge-monsoon rainfall feedback, whereas the IOM precipitation varies through the different process. To clarify the relationships among WNPSH, EAM, and IOM, two cases are selected. The first one is the case that all of WNPSH, EAM, and IOM are in phase (WE(+)I(+)), and the second one is the case that WNPSH and EAM are in phase and WNPSH/EAM and IOM is out of phase (WE(+)I(?)). These two cases are connected to the thermal forcing associated with SST anomalies over the eastern Pacific and Indian Ocean. This different thermal forcing induces the change in circulation fields, and then anomalous circulation fields influence the moisture convergence over Asian monsoon regions interactively. Therefore, the monsoon rainfall may be changed according to the thermal conditions over the tropics.     

Teleconnected influence of North Atlantic sea surface temperature on the El Ni?o onset

Influence of North Atlantic sea surface temperature (SST) anomalies on tropical Pacific SST anomalies is examined. Both summer and winter North Atlantic SST anomalies are negatively related to central-eastern tropical Pacific SST anomalies in the subsequent months varying from 5 to 13?months. In particular, when the North Atlantic is colder than normal in the summer, an El Ni?o event is likely to be initiated in the subsequent spring in the tropical Pacific. Associated with summer cold North Atlantic SST anomalies is an anomalous cyclonic circulation at low-level over the North Atlantic from subsequent October to April. Corresponded to this local response, an SST-induced heating over the North Atlantic produces a teleconnected pattern, similar to the East Atlantic/West Russia teleconnection. The pattern features two anticyclonic circulations near England and Lake Baikal, and two cyclonic circulations over the North Atlantic and near the Caspian Sea. The anticyclonic circulation near Lake Baikal enhances the continent northerlies, and strengthens the East-Asian winter monsoon. These are also associated with an off-equatorial cyclonic circulation in the western Pacific during the subsequent winter and spring, which produces equatorial westerly wind anomalies in the western Pacific. The equatorial westerly wind anomalies in the winter and spring can help initiate a Pacific El Ni?o event following a cold North Atlantic in the summer.     

东亚夏季风强弱年大气环流和热源异常对比分析

根据黄刚等定义的东亚夏季风指数, 对强、弱东亚夏季风年大气环流、大气热源和外强迫源SST的差异进行分析, 结果表明:强 (弱) 东亚夏季风年前期冬季到夏季, 太平洋SSTA为La Ni?a (El Ni?o) 型分布, 西太平洋暖池SST暖 (冷), 使得暖池附近对流活动较强 (较弱)。与此同时, 南亚大陆从印度半岛、青藏高原南部、中南半岛至华南大气异常加热 (变冷), 并且海陆热力对比加强 (减弱), 有利于出现强 (弱) 的东亚夏季风。此外, 由于暖池附近对流活动强 (弱), 该地区上升气流较强 (弱), Walker环流增强 (减弱), 当强 (弱) 的东亚夏季风向北推进时, 副热带西风急流北撤位置偏北 (南), 副热带高压位置也偏北 (南), 7月至8月华北 (江淮流域) 位于副热带西风急流南侧, 降水偏多, 江淮流域 (华北) 降水偏少。并给出与东亚夏季风年际变异有关的大气环流和SST异常的物理图像。     

西北太平洋低空环流与海温异常关系的季节性差异

通过资料分析与数值模拟研究了西北太平洋低空环流特征及其与海面温度（SST）异常关系的季节性差异,得到如下结论：1）西北太平洋低空环流的空间尺度和位置在春季和夏季存在明显差异,从春季到夏季,异常环流范围缩小且中心位置向西北偏移;2）西北太平洋低空环流与西北太平洋局地海温的相互作用存在季节差异,春季西北太平洋冷海温与上空反气旋异常之间存在相互作用,而夏季则以大气影响海洋为主,异常的反气旋/气旋可以加热/冷却其下垫面的海温,大气超前3～4 d影响海洋;3）夏季异常反气旋环流（WNPAC）的维持主要来自非局地海温异常（北印度洋暖海温与中太平洋冷海温异常）的强迫,这两个海区对WNPAC的影响也存在季节性差异,北印度洋的影响主要体现在晚春至盛夏,而中太平洋则主要在晚夏发挥作用。     

“短期气候预测专刊”

A premonitory sign of an anomalous SST over the eastern equatorial Pacific shows up in the North Pacific Subtropical Mode Water (STMW) 18 months earlier,and the air-sea relationship between the STMW and the anomalous SST over the eastern equatorial Pacific is shown.This premonitory connection involves an air-sea coupling between the longtime persistent mid-latitude sea surface temperature anomaly (SSTA) induced by the remote re-emergence of the STMW and the following spring subtropical atmospheric circulation anomalies.An examination of the air-sea interaction reveals that the following spring subtropical atmospheric circulation,which responds to the longtime persistent SSTA,is dominated by the anomalous negative (positive) geopotential height downstream of the negative (positive) SSTA in the strong (weak) STMW case.Thus,the tropics adjust to these anomalies through coupled dynamics,producing positive (negative) SST anomalies over the eastern equatorial Pacific.A cold water event that occurred over the eastern equatorial Pacific during winter 2008-09 was successfully forecasted by the weak summer STMW index in 2007.The evolution of this process for the air-sea interactions from the autumn of 2007 to December 2008 is presented.     

Winter AO/NAO modifies summer ocean heat content and monsoonal circulation over the western Indian Ocean

This paper analyzes the possible influence of boreal winter Arctic Oscillation/North Atlantic Oscillation (AO/ NAO) on the Indian Ocean upper ocean heat content in summer as well as the summer monsoonal circulation. The strong interannual co-variation between winter 1000-hPa geopotential height in the Northern Hemisphere and summer ocean heat content in the uppermost 120 m over the tropical Indian Ocean was investigated by a singular decomposition analysis for the period 1979–2014. The second paired-modes explain 23.8% of the squared covariance, and reveal an AO/NAO pattern over the North Atlantic and a warming upper ocean in the western tropical Indian Ocean. The positive upper ocean heat content enhances evaporation and convection, and results in an anomalous meridional circulation with ascending motion over 5°S–5°N and descending over 15°–25°N. Correspondingly, in the lower troposphere, significantly anomalous northerly winds appear over the western Indian Ocean north of the equator, implying a weaker summer monsoon circulation. The off-equator oceanic Rossby wave plays a key role in linking the AO/NAO and the summer heat content anomalies. In boreal winter, a positive AO/NAO triggers a down-welling Rossby wave in the central tropical Indian Ocean through the atmospheric teleconnection. As the Rossby wave arrives in the western Indian Ocean in summer, it results in anomalous upper ocean heating near the equator mainly through the meridional advection. The AO/NAO-forced Rossby wave and the resultant upper ocean warming are well reproduced by an ocean circulation model. The winter AO/NAO could be a potential season-lead driver of the summer atmospheric circulation over the northwestern Indian Ocean.     

春季北极涛动对盛夏长江流域地表气温的影响

本文基于1958至2002年的ERA-40 月平均再分析资料,利用年际增量方法分析了春季北极涛动(Arctic Oscillation,简称AO)与我国夏季长江流域地表气温的关系。结果表明,在扣除前期冬季ENSO影响后,5月AO指数与8月长江流域地表气温存在显著正相关。通过回归分析发现,5月AO可通过影响中低纬度的海气相互作用进而影响8月长江流域地表气温。当5月AO处于正位相时,在(10°~15°N)及赤道附近产生异常下沉气流,对应着西太平洋局地Hadley环流减弱,对流层底层出现了异常的反气旋性辐散气流。与之对应,赤道西太平洋地区出现了显著的东风异常。由于该东风异常位于5月气候平均的局地海表面温度(SST)极大值中心位置上,该东风异常可通过平流作用使得高海温不断地向西堆积,最终造成赤道西太平洋SST出现显著正异常。当该SST正异常持续至8月时,它通过Gill响应(Gill,1980)在其西北侧激发出气旋性异常环流,它有利于西太平洋副热带高压在我国长江流域的维持,进而造成长江流域地表气温正异常。反之,则相反。     

春季热带南大西洋海表温度异常与夏季亚澳季风区降水异常的联系

利用1979—2019年Hadley中心的海表温度资料、GPCP的降水资料以及NCEP-DOE的再分析资料等,分析了北半球春季热带南大西洋海表温度异常与北半球夏季亚澳季风区降水异常的联系。研究表明,北半球春季热带南大西洋海表温度异常与随后夏季热带西太平洋到南海（澳大利亚东侧海域到热带东印度洋）地区的降水异常为显著负相关（正相关）关系。北半球春季热带南大西洋的海表温度正异常可以引起热带大西洋和热带太平洋间的异常垂直环流,其中异常上升支（下沉支）位于热带大西洋（热带中太平洋）。热带中太平洋的异常下沉气流和低层辐散气流引起热带中西太平洋低层的异常东风,后者有利于热带中东太平洋海表温度出现负异常。通过Bjerknes正反馈机制,热带中东太平洋海表温度异常从北半球春季到夏季得到发展。热带中东太平洋海表温度负异常激发的Rossby波使得北半球夏季热带西太平洋低层出现一对异常反气旋。此时,850 hPa上热带西太平洋到海洋性大陆地区为显著的异常东风,有利于热带西太平洋到南海（澳大利亚东侧海域到热带东印度洋）地区出现异常的水汽辐散（辐合）,导致该地区降水减少（增加）。     

东亚冬夏季风关系在1970s末的年代际转变

利用NCEP/NCAR和Hadley中心的大气与海洋再分析资料,选取具有代表性的东亚冬、夏季风指数,采用滑动相关和线性回归等方法,主要讨论了受ENSO影响的东亚冬季风分量和后期夏季风之间关系的年代际变化,并分析了二者关系发生变化的原因。结果表明:在1965—1979年,受ENSO影响的冬季风与后期夏季风强度的对应关系并不明显。在1980—2004年,受ENSO影响的冬季风强,对应后期的夏季风偏弱,弱冬季风对应的后期夏季风偏强。当受ENSO影响的冬季风较强时,冬季在对流层低层西北太平洋出现了异常气旋并可以维持到次年夏季,低纬地区位势高度偏低,削弱了西太平洋副热带高压,异常气旋西部的偏北气流阻碍了西南风的北进,导致夏季风偏弱。海表温度异常在1980年前后春、夏季不同的分布型可以解释环流在不同时段内的差异。     

Roles of Indian and Pacific Ocean air–sea coupling in tropical atmospheric variability

Sea surface temperature (SST) variations include negative feedbacks from the atmosphere, whereas SST anomalies are specified in stand-alone atmospheric general circulation simulations. Is the SST forced response the same as the coupled response? In this study, the importance of air–sea coupling in the Indian and Pacific Oceans for tropical atmospheric variability is investigated through numerical experiments with a coupled atmosphere-ocean general circulation model. The local and remote impacts of the Indian and Pacific Ocean coupling are obtained by comparing a coupled simulation with an experiment in which the SST forcing from the coupled simulation is specified in either the Indian or the Pacific Ocean. It is found that the Indian Ocean coupling is critical for atmospheric variability over the Pacific Ocean. Without the Indian Ocean coupling, the rainfall and SST variations are completely different throughout most of the Pacific Ocean basin. Without the Pacific Ocean coupling, part of the rainfall and SST variations in the Indian Ocean are reproduced in the forced run. In regions of large mean rainfall where the atmospheric negative feedback is strong, such as the North Indian Ocean and the western North Pacific in boreal summer, the atmospheric variability is significantly enhanced when air–sea coupling is replaced by specified SST forcing. This enhancement is due to the lack of the negative feedback in the forced SST simulation. In these regions, erroneous atmospheric anomalies could be induced by specified SST anomalies derived from the coupled model. The ENSO variability is reduced by about 20% when the Indian Ocean air–sea coupling is replaced by specified SST forcing. This change is attributed to the interfering roles of the Indian Ocean SST and Indian monsoon in western and central equatorial Pacific surface wind variations.     

On the weakened relationship between spring Arctic Oscillation and following summer tropical cyclone frequency over the western North Pacific: A comparison between 1968–1986 and 1989–2007

This study documents a weakening of the relationship between the spring Arctic Oscillation (AO) and the following summer tropical cyclone (TC) formation frequency over the eastern part (150°-180°E) of the western North Pacific (WNP). The relationship is strong and statistically significant during 1968-1986, but becomes weak during 1989-2007. The spring AO-related SST, atmospheric dynamic, and thermodynamic conditions are compared between the two epochs to understand the possible reasons for the change in the relationship. Results indicate that the spring AO leads to an El Niño-like SST anomaly, lower-level anomalous cyclonic circulation, upper-level anomalous anticyclonic circulation, enhanced ascending motion, and a positive midlevel relative humidity anomaly in the tropical western-central Pacific during 1968-1986, whereas the AO-related anomalies in the above quantities are weak during 1989-2007. Hence, the large-scale dynamic and thermodynamic anomalies are more favorable for TC formation over the eastern WNP during 1968-1986 than during 1989-2007.     

东亚夏季风的强度与前期西太平洋暖池热含量异常的关系

本文利用日本气象厅提供的历史海温资料、Hadley海温资料以及NCEP/NCAR再分析资料(1951~2010年)等探讨了东亚夏季风的强度与前期暖池热含量异常的关系。结果表明,西太平洋暖池热含量可以作为东亚夏季风强度的前期预测因子,两者正相关关系显著。本文选取相关系数更大、持续性更好的前期冬季暖池关键区(-5.5°~5.5°N;157.5°~170.5°E)热含量来进行预报。将暖池热含量指数和东亚夏季风指数均回归到夏季大气环流场上,发现在暖水年次年夏季西太副高偏弱、位置偏北,菲律宾以东以北洋面为气旋性环流,对流上升运动增强,赤道西太平洋地区为显著的西风距平,日本岛以东洋面为反气旋环流,对流下沉运动增强,日本岛以南、黄海至我国中东部地区为显著的东风距平,且前期2月西风带位置偏北,引起夏季海陆热力差异较大,最终导致东亚夏季风强度异常偏高;冷水年则相反。综上所述,当前期冬季西太平洋暖池热含量异常偏高(低)时,会造成次年东亚夏季风强度偏强(弱)。      

20世纪90年代初东亚夏季风的年代际转型

利用1979—2009年JRA-25和NCEP/NCAR再分析资料,通过复矢量经验正交方法揭示了东亚地区夏季850 hPa风场变率的优势模态。结果表明:两套再分析资料所揭示的东亚夏季风在20世纪90年代初均发生了年代际转型,与我国夏季降水的年代际转型时间一致。伴随着东亚夏季风的年代际转型,我国北方大部分地区夏季降水减少,尤其是我国东北北部和长江、黄河之间105°E附近区域显著减少,而华南地区和淮河流域降水显著增加。从动力上解释我国夏季降水年代际转型特征,夏季500 hPa高度场两个时段 (1993—2009年和1979—1992年) 的差值分布显示为欧亚大陆北部准纬向遥相关波列,夏季850 hPa风场差值分布表现为贝加尔湖东南侧和日本以南地区存在两个异常反气旋式环流,而我国南方地区和鄂霍次克海附近均为异常气旋式环流。夏季西北太平洋、北印度洋以及部分中高纬度海洋的海温和春季欧亚大陆积雪在20世纪90年代初出现显著变化,春季北极海冰的年代际转型发生在20世纪90年代初,都可能成为东亚夏季风年代际转型的原因。     

Diversity of the Coupling Wheels in the East Asian Summer Monsoon on the Interannual Time Scale: Challenge of Summer Rainfall Forecasting in China

Two types of three-dimensional circulation of the East Asian summer monsoon(EASM) act as the coupling wheels determining the seasonal rainfall anomalies in China during 1979–2015. The first coupling mode features the interaction between the Mongolian cyclone over North Asia and the South Asian high(SAH) anomalies over the Tibetan Plateau at 200 hPa. The second mode presents the coupling between the anomalous low-level western Pacific anticyclone and upperlevel SAH via the meridional flow over Southeast Asia. These two modes are responsible for the summer rainfall anomalies over China in 24 and 7 out of 37 years, respectively. However, the dominant SST anomalies in the tropical Pacific, the Indian Ocean, and the North Atlantic Ocean fail to account for the first coupling wheel's interannual variability, illustrating the challenges in forecasting summer rainfall over China.     

影响中国夏季降水的前冬北半球中高纬大气环流年际变异主模态

本文利用观测和再分析资料,通过奇异值分解(Singular Value Decomposition, SVD)分析,发现北极涛动(Arctic Oscillation, AO)是显著影响中国夏季降水年际异常的前冬中高纬大气环流变异的主模态。AO在冬季发展成熟,在春季衰亡,在夏季发生位相反转。AO会导致华北、东北、长江中下游和华南夏季降水异常呈现三极型分布。伴随正位相的AO,在黄海至日本海上空的异常低压伴随的东北风异常引起华北和东北水汽通量异常辐散及降水减少,而西北太平洋的异常高压不仅增强其北侧的西南风水汽输送,和北部异常低压共同作用导致长江中下游水汽通量异常辐合及降水增加,而且使得华南水汽通量异常辐散,降水减少。因此,本文发现的前冬AO模态与我国夏季三极型异常降水分布的关系可为我国夏季旱涝预测提供一个重要的中高纬前期因子。     

中国华南春季季风及其与大尺度环流特征的关系

定义了中国华南春季季风，并用NCEP／NCAR再分析资料研究了春季风的气候特征以及春季风降水和大尺度环流在年际变化上的关系。结果表明，从降水和大气环流的变化来看，华南春季风在气候上发生于4月和5月；与华南春季风相联系的大气环流特征与夏季风和冬季风所对应的大气环流特征完全不同。华南春季风降水的年际变化主要与太平洋北部的异常环流相关联，而这种异常环流又与亚洲北部的西风急流和极地涡旋有联系；华南春季风降水的年际变化还与太平洋的海表温度异常有关；而亚洲热带大气环流的年际变化与华南春季风降水的变化关系不大。