海表温度异常影响东亚夏季风年代际变化的数值模拟

采用1950-2000年逐月观测的不同海域(全球、热带外、热带、热带印度洋-太平洋、热带印度洋及热带太平洋)海表温度分别驱动NCAR CAM3全球大气环流模式,进行了多组长时间积分试验,对比ERA-40和NCEP/NCAR再分析资料,讨论了这些海域海表温度异常对东亚夏季风年代际变化的影响。数值试验结果表明:全球、热带、热带印度洋-太平洋和热带太平洋海表温度变化对东亚夏季风的年代际变化具有重要作用,均模拟出了东亚夏季风在20世纪70年代中后期发生的年代际减弱现象,以及强、弱夏季风年代夏季大气环流异常分布的显著不同,这与观测结果较一致,表明热带太平洋是影响东亚夏季风此次年代际变化的关键海区;利用热带印度洋海表温度驱动模式模拟出的东亚夏季风在20世纪70年代中后期发生年代际增强现象,即当热带印度洋海表温度年代际偏暖(冷)时,东亚夏季风年代际增强(减弱),与热带太平洋海表温度变化对东亚夏季风年代际变化的影响相反;热带太平洋海表温度年代际背景的变化对东亚夏季风在20世纪70年代中后期的年代际减弱有重要作用。     

东亚夏季风的年际到年代际变化及其与全球大气环流和海表温度的联系

The East Asian summer monsoon (EASM) underwent an interdecadal variation with interannual variations during the period from 1958 to 1997, its index tended to decline from a higher stage in the mid-1960＇s until it reached a lower stage after 1980＇s. Correlation analysis reveals that EASM is closely related with the global atmospheric circulation and sea surface temperature (SST). The differences between the weak and strong stage of EASM shows that, the summer monsoon circulation over East Asia and North Africa is sharply weakened, in the meantime, the westerlies in high latitudes and the trade--wind over the tropical ocean are also changed significantly. Over the most regions south of the northern subtropics, both air temperature in the lower troposphere and SST tended to rise compared with the strong stage of EASM. It is also revealed that the ocean-atmosphere interaction over the western Pacific and Indian Ocean plays a key role in interannual to interdecadal variation of EASM, most probably, the subtropical Indian Ocean is more important. On the other hand, the ENSO event is less related to EASM at least during the concerned period.     

Responses of East Asian summer monsoon to historical SST and atmospheric forcing during 1950–2000

The East Asian summer monsoon (EASM) circulation and summer rainfall over East China have experienced large decadal changes during the latter half of the 20th century. To investigate the potential causes behind these changes, a series of simulations using the national center for atmospheric research (NCAR) community atmospheric model version 3 (CAM3) and the geophysical fluid dynamics laboratory (GFDL) atmospheric model version 2.1 (AM2.1) are analyzed. These simulations are forced separately with different historical forcing, namely tropical sea surface temperature (SSTs), global SSTs, greenhouse gases plus aerosols, and a combination of global SSTs and greenhouse gases plus aerosols. This study focuses on the relative roles of these individual forcings in causing the observed monsoon and rainfall changes over East Asia during 1950–2000. The simulations from both models show that the SST forcing, primarily from the Tropics, is able to induce most of the observed weakening of the EASM circulation, while the greenhouse gas plus (direct) aerosol forcing increases the land-sea thermal contrast and thus enhances the EASM circulation. The results suggest that the recent warming in the Tropics, especially the warming associated with the tropical interdecadal variability centered over the central and eastern Pacific, is a primary cause for the weakening of the EASM since the late 1970s. However, a realistic simulation of the relatively small-scale rainfall change pattern over East China remains a challenge for the global models.     

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.     

中国东部夏季降水型的年代际变化及其与北太平洋海温的关系

采用中国160站降水资料、NOAA ERSST海温资料以及ERA－40大气再分析资料, 分析了中国东部夏季降水型的年代际变化特征及其与北太平洋海温的可能联系。结果表明: 中国东部夏季降水型在近50年中经历了两次年代际变化, 第1次发生在20世纪70年代中后期, 北太平洋中纬度地区冬季海温由正距平向负距平转变, 太平洋年代际振荡(PDO, Pacific decadal oscillation) 由负位相向正位相转变, 通过影响东亚夏季风环流, 使东亚夏季风减弱, 中国东部夏季降水从北到南呈现出“＋－＋” 转变为“－＋－”的三极分布形态, 这次年代际变化体现了同一模态正负位相的转变; 第2次发生在20世纪80年代末90年代初, 北太平洋海温转变为日本以南西北太平洋的正距平分布, 同时菲律宾群岛附近海温偏暖, 西太平洋副热带高压偏南偏西, 使得中国东部夏季降水由北至南转变成“－＋”的偶极分布形态, 这次年代际变化体现了一种模态向另一种模态的转变。     

New Metrics of the Tropospheric Biennial Oscillation in the East Asian Summer Monsoon and Its Interdecadal Shift

This study designed a simple index for measuring irregular tropospheric biennial oscillation（TBO） activities, which was used to determine that the TBO in the East Asian Summer Monsoon（EASM）, the most important summer precipitation system for China, has strengthened rather than weakened since the late 1970s. The lead/lag correlations between the EASM and tropical Indian-Pacific sea surface temperature（SST） suggest a relationship between interbasin SST and EASM coupling processes and that this alternative correlation pattern is likely related to TBO. Significant correlation occurred only in recent decades, which implies a reinforcement of TBO in the EASM. From records of representative points in the Indian-Pacific, the interdecadal intrinsic SST modes of the areas can be obtained with ensemble empirical mode decomposition owing to its good temporal locality. Statistical results show Indian-Pacific SST interdecadal trends that include out-of-phase and in-phase warming before and after the late 1970s, respectively, which may be responsible for the TBO interdecadal augmentation present since the late 1970s.     

NUMERICAL SIMULATION OF SSTA IMPACTS UPON THE INTERDECADAL VARIATION OF THE CROSS-EQUATORIAL FLOWS IN EASTERN HEMISPHERE

Impacts of regional sea surface temperature(SST)anomalies on the interdecadal variation of the cross-equatorial flows(CEFs)in Eastern Hemisphere are studied using numerical simulations with a global atmospheric circulation model(NCAR CAM3)driven with 1950-2000 monthly SSTs in different marine areas(the globe,extratropics,tropics,tropical Indian Ocean-Pacific,and tropical Pacific)and ERA-40reanalysis data.Results show that all simulations,except the one driven with extratropical SSTs,can simulate the interdecadal strengthening of CEFs around Somali,120oE,and 150oE that occurred in the midand late-1970s.Among those simulated CEFs,the interdecadal variability in Somali and its interdecadal relationship with the East Asian summer monsoon are in better agreement with the observations,suggesting that changes in the SSTs of tropical oceans,especially the tropical Pacific,play a crucial role in the interdecadal variability of CEFs in Somali.The interdecadal change of CEFs in Somali is highly associated with the interdecadal variation of tropical Pacific SST.As the interdecadal warmer(colder)SST happens in the tropical Pacific,a"sandwich"pattern of SST anomalies,i.e."+,-,+"("-,+,-"),will occur in the eastern tropical Pacific from north to south with a pair of anomalous anticyclone(cyclone)at the lower troposphere;the pair links to another pair of anomalous cyclone(anticyclone)in the tropical Indian Ocean through an atmospheric bridge,and thus strengthens(weakens)the CEFs in Somali.     

大气热源年代际异常与20世纪70年代末期东亚夏季风年代际减弱

利用1958-2001年ERA-40再分析资料计算大气热源,统计分析了亚洲季风区及其邻近海域大气热源年代际变异的典型模态;利用线性斜压干模式,模拟了夏季大气对大气热源年代际异常的响应,揭示了大气热源年代际异常与1970s末期东亚夏季风年代际减弱的关系。结果表明:近50 a来亚洲及其邻近海域夏季整层大气热源变异主要表现为年代际变化特征,其年代际位相转换发生在1970s中后期,这与东亚夏季风年代际减弱的时间一致;菲律宾附近海域和中国西南地区是与东亚夏季风年代际减弱有直接联系的两个热源异常关键区;东亚夏季风年代际减弱最直接地表现为这两个关键区热源异常的共同作用,而赤道中东太平洋、赤道印度洋大气热源增强则通过大气遥响应机制影响菲律宾附近海域低层大气环流异常对东亚夏季风变异起相反的贡献。     

INTERDECADAL VARIATION OF THE INTENSITY OF SOUTH ASIAN HIGH

The characteristics and possible physical mechanism of interdecadal variation of the intensity of the South Asian High (SAH) in summer are analyzed using the NCEP/NCAR reanalysis data and NOAA extended reconstructed sea surface temperature (SST) data. The results indicate that a remarkable interdecadal transition occurred in the late 1970s that increased the intensity of SAH, or, an abrupt climate change was around 1978. A comparative analysis between the weak and strong period of the SAH intensity shows that the related anomalous patterns of the atmospheric circulation (including wind field, air temperature field and vertical velocity field) are nearly opposite to each other. The surface latent heat flux anomalies over the plateau (especially in the northwest of the plateau) in summer exert great influence on the interdecadal variation of the SAH intensity and the surface sensible heat flux anomalies play a more important role. Consistent with the interdecadal variation of the SAH intensity, the monopole mode of the tropical Indian Ocean SST in summer also experienced a low to high transition in the late 1970s. To some extent, this can reveal the impact of the anomalous monopole mode of the tropical Indian Ocean SST in summer on interdecadal variation of the SAH.     

The long-term variability of Changma in the East Asian summer monsoon system: A review and revisit

Changma, which is a vital part of East Asian summer monsoon (EASM) system, plays a critical role in modulating water and energy cycles in Korea. Better understanding of its long-term variability and change is therefore a matter of scientific and societal importance. It has been indicated that characteristics of Changma have undergone significant interdecadal changes in association with the mid-1970s global-scale climate shift and the mid-1990s EASM shift. This paper reviews and revisits the characteristics on the long-term changes of Changma focusing on the underlying mechanisms for the changes. The four important features are manifested mainly during the last few decades: 1) mean and extreme rainfalls during Changma period from June to September have been increased with the amplification of diurnal cycle of rainfall, 2) the dry spell between the first and second rainy periods has become shorter, 3) the rainfall amount as well as the number of rainy days during August have significantly increased, probably due to the increase in typhoon landfalls, and 4) the relationship between the Changma rainfall and Western Pacific Subtropical High on interannual time scale has been enhanced. The typhoon contribution to the increase in heavy rainfall is attributable to enhanced interaction between typhoons and midlatitude baroclinic environment. It is noted that the change in the relationship between Changma and the tropical sea surface temperature (SST) over the Indian, Pacific, and Atlantic Oceans is a key factor in the long-term changes of Changma and EASM. Possible sources for the recent mid-1990s change include 1) the tropical dipole-like SST pattern between the central Pacific and Indo-Pacific region (the global warming hiatus pattern), 2) the recent intensification of tropical SST gradients among the Indian Ocean, the western Pacific, and the eastern Pacific, and 3) the tropical Atlantic SST warming.     

Phase transition of the Pacific decadal oscillation and decadal variation of the East Asian summer monsoon in the 20th century

This paper focuses on the relationship between the phase transition of the Pacific decadal oscillation (PDO) and decadal variation of the East Asian summer monsoon (EASM) in the twentieth century. The first transition occurred in the 1940s, with an enhanced SST in the North Pacific and reduced SST in the tropical eastern Pacific and South Indian Ocean. In agreement with these SST changes, a higher SLP was found in most parts of the Pacific, while a lower SLP was found in the North Pacific and most parts of the Indian Ocean. In this case, the EASM was largely enhanced with a southerly anomaly in the lower troposphere along the east coast of China. Correspondingly, there was less rainfall in the Yangtze River valley and more rainfall in northern and southern China. An opposite change was found when the PDO reversed its phase in the late 1970s. In the tropical Indian Ocean and western Pacific, however, the SST was enhanced in both the 1940s and 1970s. As a result, the western Pacific subtropical high (WPSH) tended to extend westward with a larger magnitude in the 1970s. The major features were reasonably reproduced by an atmospheric general circulation model (IAP AGCM4.0) prescribed with observed SST and sea ice. On the other hand, the westward extension of the WPSH was exaggerated in the 1970s, while it was underestimated in the 1940s. Besides, the spatial pattern of the simulated summer rainfall in eastern China tended to shift southward compared with the observation.     

Interdecadal Variability of the East Asian Summer Monsoon and Associated Atmospheric Circulations

Based on the National Centers for Environmental Prediction and National Center for Atmospheric Research （NCEP/NCAR） reanalysis data from 1950-1999, interdecadal variability of the East Asian Summer Monsoon （EASM） and its associated atmospheric circulations are investigated. The EASM exhibits a distinct interdecadal variation, with stronger （weaker） summer monsoon maintained from 1950-1964 （1976-1997）. In the former case, there is an enhanced Walker cell in the eastern Pacific and an anti-Walker cell in the western Pacific. The associated ascending motion resides in the central Pacific, which flows eastward and westward in the upper troposphere, descending in the eastern and western ends of the Pacific basin. At the same time, an anomalous East Asian Hadley Cell （EAHC） is found to connect the low-latitude and mid-latitude systems in East Asia, which strengthens the EASM. The descending branch of the EAHC lies in the west part of the anti-Walker cell, flowing northward in the lower troposphere and then ascending at the south of Lake Baikal （40°-50°N, 95°- 115°E） before returning to low latitudes in the upper troposphere, thus strengthening the EASM. The relationship between the EASM and SST in the eastern tropical Pacific is also discussed. A possible mechanism is proposed to link interdecadal variation of the EASM with the eastern tropical Pacific SST. A warmer sea surface temperature anomaly （SSTA） therein induces anomalous ascending motion in the eastern Pacific, resulting in a weaker Walker cell, and at the same time inducing an anomalous Walker cell in the western Pacific and an enhanced EAHC, leading to a weaker EASM. Furthermore, the interdecadal variation of summer precipitation over North China is found to be the south of Lake Baikal through enhancing and reducing strongly regulated by the velocity potential over the regional vertical motions.     

Impact of global SST on decadal shift of East Asian summer climate

East Asia experienced a significant interdecadal climate shift around the late 1970s, with more floods in the valley of the Yangtze River of central-eastern China and more severe drought in North China since then. Whether global SST variations have played a role in this shift is unclear. In the present study, this issue is investigated by ensemble experiments of an atmospheric general circulation model (AGCM), the GFDL AM2, since one validation reveals that the model simulates the observed East Asian Summer Monsoon (EASM) well. The results suggest that decadal global SST variations may have played a substantial role in this climate shift. Further examination of the associated atmospheric circulation shows that these results are physically reasonable.     

东亚夏季风的自然变率——NCARCam3模拟结果分析

利用季节循环的全球观测海表温度及海冰驱动NCARCam3全球大气环流模式的100a模拟结果,通过定义东亚夏季风指数,分析了模拟的大气内部变化中东亚夏季风的变化特征。结果表明：模拟的东亚夏季风自然变率主要表现为3—7a较显著的年际周期,并具有较明显的年代际变化特征。在弱夏季风年代,亚洲大陆海平面气压增强,日本附近及东亚沿海地区海平面气压降低;500hPa位势高度上,欧洲地区为负高度距平,里海附近地区为正高度距平,日本及其以东太平洋为负高度距平,易形成类似欧亚（EU）型的遥相关波列。在强夏季风年代,其环流异常分布基本与弱夏季风年代相反。模拟的东亚夏季风变化与夏季大气内部500hPa高度场上EU型遥相关波列的关系密切。     

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.     

太平洋—日本遥相关型的年代际变化特征及其成因

本文首先利用NCEP/NCAR和ERA-40再分析资料以及中国753站降水资料对太平洋—日本(Pacific-Japan,简称P-J)遥相关型在上世纪70年代末期气候突变前后的年代际变化特征进行了分析研究。结果表明,在气候突变前后,P-J遥相关型的位置发生了显著的变化,气候突变以后其位置明显向西向南偏移。这种位置的变化同样也反映在纬向风场、高度场上。研究结果还表明,气候突变前后P-J遥相关型的年代际变化与热带太平洋和印度洋海温变化有关。气候突变之前,P-J遥相关型的变化与前期热带太平洋和印度洋海温不存在显著的相关;但在气候突变之后,P-J遥相关型与前期冬春季的热带太平洋、印度洋海温之间存在大范围的显著相关区。这种P-J遥相关型与热带太平洋、印度洋海温相关关系的年代改变可能与1970年代中期以后赤道中东太平洋海温变化振幅明显增强有关。随后,本文采用一个高分辨率的大气环流模式,通过一系列的数值试验也进一步证实了1970年代末期热带太平洋和印度洋海温的年代际变化确实可致使P-J遥相关型位置发生相应的改变。     

全球海表温度年代际尺度的空间分布及其对长江中下游梅雨的影响

使用统计诊断的方法,探讨了近百年全球海表温度年代际尺度的空间分布结构与长江中下游梅雨异常变化的可能联系.采用三次样条函数拟合的方法将1885～2000-全球海表温度场和长江中下游梅雨雨量百分比序列的年代际变化分量分离出来,在分析各自年代际变化特征基础上,研究了全球海表温度的年代际尺度分布结构对长江中下游梅雨异常变化的影响.结果表明(1) 全球海表温度年代际尺度变化分量清晰地表征出气候背景的分布状态,其中太平洋年代际振荡(PDO)型态表现突出,特别是1976年以后太平洋的气候背景呈现暖事件增强的趋势.同时,印度洋及大西洋中部海域的海表温度也表现出明显的升温趋势.(2) 长江中下游梅雨年代际尺度变化趋势与全球海表温度的年代际变化趋势基本一致,特别是与PDO典型分布型态的变化趋势有很好的对应,当PDO暖事件趋势处于较强时期时,长江中下游梅雨为偏多的趋势,反之亦然.其中20世纪70胩代中期PDO出现暖位相增强的突变,长江中下游梅雨也在此时期转入增多的趋势.同时,印度洋、大西洋部分地区的海表温度的年代际变化与梅雨的年代际变化之间也有一定的关联.(3) PDO指数与西太平洋副热带高压面积指数的年代际变化趋势一致的统计事实,从一个侧面说明海洋的-代际变化最终通过副热带高压的变动影响梅雨的异常变化的可能性.     

Polar WRF模式海冰密集度方案对北极海雾模拟效果的个例研究

全球变暖的背景下,北极航线的常规通航甚至商业运营有望实现,而海雾会严重影响航道上船只的航行安全。海冰的存在使海气之间相互作用变得更为复杂,是研究北极海雾不可忽略的因素。船载观测发现,与中纬度常见平流冷却雾形成时气温下降速度往往超过海水降温速度不同,北极海雾发生时海冰的存在还会使海水降温速度超过空气降温速度。然而目前海冰分布是否会影响模式模拟海雾的准确性还不得而知,因此本文利用Polar WRF（Polar Weather Research and Forecasting）模式模拟了中国第七次北极考察中观测到的一次海雾过程,并进行海冰密集度敏感性试验。通过与船载观测和欧洲中期天气预报中心再分析数据比对发现,在低浮冰区内（海冰密集度小于50%）考虑海冰分布时可以更加准确地刻画潜热通量与水汽通量,模拟出与观测事实相符的表层空气降温与增湿过程以及相对湿度的变化,因此能够更好地刻画海雾的三维结构及其生消演变。     

Simulation of the asian monsoon by IAP AGCM coupled with an advanced land surface model (IAP94)

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Numerical Experiment on Impact of Anomalous SST Warming in Kuroshio Extension in Previous Winter on East Asian Summer Monsoon

The impact of anomalous sea surface temperature (SST) warming in the Kuroshio Extension in the previous winter on the East Asian summer monsoon (EASM) was investigated by performing simulation tests using NCAR CAM3.The results show that anomalous SST warming in the Kuroshio Extension in winter causes the enhancement and northward movement of the EASM.The monsoon indexes for East Asian summer monsoon and land-sea thermal difference,which characterize the intensity of the EASM,show an obvious increase during the onset period of the EASM.Moreover,the land-sea thermal difference is more sensitive to warmer SST.Low-level southwesterly monsoon is clearly strengthened meanwhile westerly flows north (south) of the subtropical westerly jet axis are strengthened (weakened) in northern China,South China Sea,and the Western Pacific Ocean to the east of the Philippines.While there is an obvious decrease in precipitation over the Japanese archipelago and adjacent oceans and over the area from the south of the Yangtze River in eastern China to the Qinling Mountains in southern China,precipitation increases notably in northern China,the South China Sea,the East China Sea,the Yellow Sea,and the Western Pacific to the east of the Philippines.North China is the key area where the response of the EASM to the SST anomalous warming in the Kuroshio Extension is prominent.The surface air temperature shows a warming trend.The warming in the entire troposphere between 30oN and 50oN increases the land-sea thermal contrast,which plays an important role in the enhancement of the EASM.Atmospheric circulation and precipitation anomalies in China and its adjacent regions have a close relationship with the enhancement of the Western Pacific subtropical high and its northward extension.