季风的季节、年际和年代际变化

区域季风和全球季风的分布依赖于季风的定义。基于“季风是热带大气水汽中心随越赤道气流向赤道外季节扩展”的定义总结并讨论了东亚季风的区域特征、亚洲季风的季节进程和全球季风的分布以及季风的年际和年代际变化。最后对季风研究的潜在方向作了适当的讨论。

中国北方季风边缘区过去530年降水的时空变化及其驱动机制研究

中国北方季风边缘区位置的移动影响着整个西北地区的旱涝变化和生态建设的发展.树轮和历史文献记录可以为该地区过去几百年气候环境变化和季风变化提供重要的信息.本文选取了8条树轮序列结合31条中国历史文献记录和实测降水量资料,重建了中国北方地区近530年的年降水量演化特征,对比分析了不同特征时期400rmm等降水量线空间位置的差异及其可能的驱动机制.结果表明,在年际尺度上,降水量最丰年400mm等降水量线位置都比近30年平均位置偏北,而干旱事件时期400mm等降水量线位置比近30年平均位置更偏南,且干旱时期的波动要比湿润年份的波动幅度更大;在年代际尺度上,最湿润10年和最干旱10年400mm等降水量线位置同样比最近30年偏北和偏南,但是其空间位置的变化没有年际尺度显著.研究区降水量空间位移变化主要是受北大西洋涛动（NAO）和厄尔尼诺南方涛动（ENSO）的影响.

Variability of Summer Monsoon Rainfall in India on Inter-Annual and Decadal Time Scales

Indian Summer Monsoon Rainfall （ISMR） exhibits a prominent inter-annual variability known as troposphere biennial oscillation.A season of deficient June to September monsoon rainfall in India is followed by warm sea surface temperature （SST） anomalies over the tropical Indian Ocean and cold SST anomalies over the westem Pacific Ocean.These anomalies persist until the following monsoon,which yields normal or excessive rainfall.Monsoon rainfall in India has shown decadal variability in the form of 30 year epochs of alternately occurring frequent and infrequent drought monsoons since 1841,when rainfall measurements began in India.Decadal oscillations of monsoon rainfall and the well known decadal oscillations in SSTs of the Atlantic and Pacific oceans have the same period of approximately 60 years and nearly the same temporal phase.In both of these variabilities,anomalies in monsoon heat source,such as deep convection,and middle latitude westerlies of the upper troposphere over south Asia have prominent roles.

半个世纪来热带海洋风暴对中国大陆的影响

本文利用美国海军台风警报中心(JTWC)提供的1945～2002年热带风暴路径资料统计分析了西北太平洋(NWP)和中国南海(SCS)风暴生成及登陆中国大陆热带风暴的时空演变特征.季节变化上,NWP风暴登陆主要集中于6～11月,SCS风暴影响主要集中在6～9月,但后者登陆总数比前者少.西北太平洋风暴在东南沿海(27°N,120°E)附近登陆的频次最高,在此以北随纬度急剧下降.年际变化时间尺度上,登陆大陆的风暴年总数与来自南海的年风暴数成正比.登陆我国的热带风暴年频数有明显的区域差异和显著的2～7年振荡.长期趋势上,两个海域的风暴年生成频数和登陆大陆的年风暴频数在58年中总体呈线性增长趋势,其中登陆频数增长趋势相对缓慢,但近几年登陆风暴数与生成风暴数都表现出减少的趋势.生成频数和登陆频数都呈现出年代际变化,其年代转换发生在1960、1970年和1990年前后.

北半球冬季温带反气旋活动的统计特征

利用NCEP/NCAR再分析资料,通过客观判定和追踪反气旋的方法统计分析了1948~2013年北半球冬季温带反气旋的时空分布、周期和生命史等气候特征。结果表明,北半球冬季温带反气旋主要活动在东北太平洋、北大西洋、北美落基山脉东部和加拿大、美国东部、欧洲、亚洲中部和东亚地区。太平洋和大西洋上反气旋的生成区分布较分散,于大洋西部生成,在中、东部达到成熟期,最后主要在大洋东部和大陆西岸消亡。大西洋上反气旋消亡区域的大值区从大西洋中部到欧洲西海岸呈西南—东北带状分布,而太平洋上的反气旋消亡区域的两个大值区独立分布。大陆上反气旋多生成于较高纬度和极地地区,主要向东、向东南方移动。北半球各区域反气旋数均具有显著的2~3年周期变化特征,欧亚地区则具有显著的16~18年周期的年代际变化特征。除了欧亚大陆,其他三个区域的反气旋偏强时,其运动轨迹偏北。北美地区反气旋越强,其运动的纬度范围越大;而欧亚大陆反气旋越强,其运动的纬度范围越小。生命史越长的反气旋数比例越少,只有不到10%的反气旋生命史超过一周。

Contrasting two spring SST predictors for the number of western North Pacific tropical cyclones

Recent studies have revealed that two boreal spring sea surface temperature （SST） indices have potential to predict the number of western North Pacific （WNP） tropical cyclones （TCs） in the following peak typhoon season （June-October）： the northern tropical Atlantic （NTA） SST, and the SST gradient （SSTG） between the southwestern Pacific and western Pacic warm pool. The interannua[ and interdecadal variations of NTA SST and SSTG and their relationships to the number ofWNP TCs during 1950-2013 were compared. On the interdecadal timescale, SSTG showed better correlation with the number of WNP TCs than NTA SST. The interdecadal variation of NTA SST was closely associated with the Atlantic Multidecadal Oscillation, while that of SSTG was anti-correlated with the Central Pacific （CP） El Nino index at the interdecadal timescale. On the interannual timescale, both NTA SST and SSTG were modulated by two types of El Nino. The NTA SST revealed significant correlations with the number of WNPTCs beginning from the early 1960s; by contrast, SSTG showed significant correlations after the mid-1970s. Co-variability of NTA SST and SSTG existed after the late 1980s, induced by modulation from CP El Nino.The co-variability of these two spring SST predictors increased their prediction skill after the late 1980s, with enhanced correlation between the number of WNPTCs and the two predictors.

Comparison between the interannual and decadal components of the Silk Road pattern

The Silk Road pattern（SRP）, which is a teleconnection pattern along the Asian upper-tropospheric westerly jet in summer, exhibits both interannual and decadal variabilities. Through the nineyear Gaussian filtering method and regression analyses, this study compares the interannual and decadal components of the SRP. The results indicate that the interannual SRP corresponds to a well-organized wave train of alternate cyclonic and anticyclonic anomalies across the Eurasian continent along the Asian westerly jet, resulting in a similar wave-like pattern of cold and warm surface temperature anomalies. This pattern of temperature anomalies differs from that associated with the original SRP, which is characterized by warmer or cooler temperatures mainly over Europe–West Asia and Northeast Asia, depending on the phase of the SRP. On the other hand, the decadal SRP shows a similar pattern to the interannual one from Europe to Central Asia, but the meridional wind anomalies tend to be weak over East Asia. These circulation anomalies are responsible for the significant temperature anomalies over Europe–West Asia and Northeast Asia but weak anomalies between these two domains.

Possible contribution of the PDO to the eastward retreat of the western pacific subtropical high

Previous studies have demonstrated that the western Pacific subtropical high(WPSH)has experienced an eastward retreat since the late 1970s.In this study,the authors propose that this eastward retreat of the WPSH can be partly attributed to atmospheric responses to the positive phase of the Pacific decadal oscillation(PDO),based on idealized SST forcing experiments using the Community Atmosphere Model,version 4.Associated with the positive phase of the PDO,convective heating from the Indian Peninsula to the western Pacific and over the eastern tropical Pacific has increased,which has subsequently forced a Gill-type response to modulate the WPSH.The resulting cyclonic gyre over the Asian continent and the western Pacific in the lower troposphere is favorable for the eastward retreat of the WPSH.Additionally,the resulting anticyclonic gyre in the upper troposphere is favorable for the strengthening and southward expansion of the East Asian westerly jet,which can modulate the jet-related secondary meridional-vertical circulation over the western Pacific and promote the eastward retreat of the WPSH.

Climate modulation on sea surface height in China seas

The climate modulation on the sea surface height （SSH） in China seas is investigated using a China Ocean Reanalysis （CORA） dataset from 1958-2008. The dataset is constructed by assimilating the temperature/salinity profiles derived from the satellite altimetry data and historical observational temperature/salinity profiles. Based on the Empirical Orthogonal Function （EOF）, the CORA sea surface height anomaly （SSHa） is decomposed, and the interannual and decadal variability of the first three leading modes are analyzed. On the interannual timescale, the first principal component （PC1） is significant positively correlated with the E1 Nifio/Southern Oscillation （ENSO）. On the decadal timescale, North Pacific Gyre Oscillation （NPGO） has significant negative correlation with PC 1 whereas Pacific Decadal Oscillation （PDO） is in phase with PC3. Analysis shows that the decadal variability of SSH is mainly modulated by the wind stress curl variability related to the NPGO and PDO. In addition, the effect of net heat flux associated to the NPGO and PDO on SSH is also investigated, with net heat flux variability in the Luzon strait and tropic Pacific found to influence the decadal variability of SSH.

Variability of Tropical Cyclone in High Frequent Occurrence Regions over the Western North Pacific

In this study, three high frequent occurrence regions of tropical cyclones(TCs), i.e., the northern South China Sea(the region S), the south Philippine Sea(the region P) and the region east of Taiwan Island(the region E), are defined with frequency of TC's occurrence at each grid for a 45-year period(1965–2009), where the frequency of occurrence(FO) of TCs is triple the mean value of the whole western North Pacific. Over the region S, there are decreasing trends in the FO of TCs, the number of TCs' tracks going though this region and the number of TCs' genesis in this region. Over the region P, the FO and tracks demonstrate decadal variation with periods of 10–12 year, while over the region E, a significant 4–5 years' oscillation appears in both FO and tracks. It is demonstrated that the differences of TCs' variation in these three different regions are mainly caused by the variation of the Western Pacific Subtropical High(WPSH) at different time scales. The westward shift of WPSH is responsible for the northwesterly anomaly over the region S which inhibits westward TC movement into the region S. On the decadal timescale, the WPSH stretches northwestward because of the anomalous anticyclone over the northwestern part of the region P, and steers more TCs reaching the region P in the greater FO years of the region P. The retreating of the WPSH on the interannual time scale is the main reason for the FO's oscillation over the region E.

Long Term Sea Level Change and Water Mass Balance in the South China Sea

Sea level anomalies observed by altimeter during the 1993-2006 period, thermosteric sea level anomalies estimated by using subsurface temperature data produced by Ishii and SODA reanalysis data, tide gauge records and HOAPS freshwater flux data were analyzed to investigate the long term sea level change and the water mass balance in the South China Sea, The altime- ter-observed sea level showed a rising rate of （3.5±0.9）mmyr-1 during the period 1993-2006, but this figure was considered to have been highly distorted by the relatively short time interval and the large inter-decadal variability, which apparently exists in both the thermosteric sea level and the observed sea level. Long term thermosteric sea level from 1945 to 2004 gave a rising rate of 0.15±0.06 mmyr-1. Tide gauge＇data revealed this discrepancy and the regional distributions of the sea-level trends. Both the ＇real＇ and the ther- mosteric sea level showed a good correspondence to ENSO： decreasing during El Nino years and increasing during La Nina years. Amplitude and phase differences between the ＇real＇ sea level and the thermosteic sea level were substantially revealed on both sea- sonal and interannual time scales. As one of the possible factors, the freshwater flux might play an important role in balancing the water mass.

The Relationship between the Interdecadal Variation of Summer Precipitation and Its Interannual Variability over the Middle and Lower Reaches of the Yangtze River Valley

The intensity of summer precipitation interannual variability is strongest over the middle and lower reaches of the Yangtze River Valley(MLYRV). The variability is larger than 1.5 mm d–1 and up to 35%–40% of the climatological mean summer precipitation. The relationship between the interdecadal change in the intensity of summer precipitation and its interannual variability over this area is investigated, by analyzing five gauged and re-constructed precipitation datasets. The relationship is found to be very weak over the MLYRV, with a correlation coefficient of only approximately 0.10. The Pacific Decadal Oscillation influences the western North Pacific subtropical high, which is responsible for the interdecadal change in summer precipitation over the MLYRV. However, the precipitation interannual variability is closely related to the ENSO events in the preceding winter due to its impact on the meridional displacement of the East Asian westerly jet. Different physical mechanisms cause different interdecadal variation in the intensity of summer precipitation and its interannual variability, and thus result in a poor relationship.

Analysis of Decadal Climate Variability in the Tropical Pacific by Coupled GCM

This study presents the spatial and temporal structures of the decadal variability of the Pacific from an extended control run of a coupled global climate model (GCM).The GCM used was version-g2.0 of the Flexible Global Ocean Atmosphere Land System (FGOALS-g2.0) developed at LASG/IAP.The GCM FGOALS-g2.0 re-produces similar spatial-temporal structures of sea surface temperature (SST) as observed in the Pacific decadal os-cillation (PDO) with a significant period of approximately 14 years.Correspondingly,the PDO signals were closely related to the decadal change both in the upper-ocean temperature anomalies and in the atmospheric circulation.The present results suggest that warm SST anomalies along the equator relax the trade winds,causing the SSTs to warm even more in the eastern equatorial Pacific,which is a positive feedback.Meanwhile,warm SST anomalies along the equator force characteristic off-equa-torial wind stress curl anomalies,inducing much more poleward transport of heat,which is a negative feedback.The upper-ocean meridional heat transport,which is asso-ciated with the PDO phase transition,links the equatorial to the off-equatorial Pacific Ocean,acting as a major mechanism responsible for the tropical Pacific decadal variations.Therefore,the positive and negative feedbacks working together eventually result in the decadal oscilla-tion in the Pacific.

The boreal spring stratospheric final warming and its interannual and interdecadal variability

Based on the daily NCEP/DOE reanalysis II data,dates of the boreal spring Stratospheric Final Warming(SFW) events during 1979–2010 are defined as the time when the zonal-mean zonal wind at the central latitudes(65°–75°N) of the westerly polar jet drops below zero and never recovers until the subsequent autumn.It is found that the SFW events occur successively from the mid to the lower stratosphere and averagely from the mid to late April with a temporal lag of about 13 days from 10 to 50 hPa.Over the past 32 years,the earliest SFW occurs in mid March whereas the latest SFW happens in late May,showing a clear interannual variability of the time of SFW.Accompanying the SFW onset,the stratospheric circulation transits from a winter dynamical regime to a summertime state,and the maximum negative tendency of zonal wind and the strongest convergence of planetary-wave are observed.Composite results show that the early/late SFW events in boreal spring correspond to a quicker/slower transition of the stratospheric circulation,with the zonal-mean zonal wind reducing about 20/5 m s-1 at 30 hPa within 10 days around the onset date.Meanwhile,the planetary wave activities are relatively strong/weak associating with an out-of-/in-phase circumpolar circulation anomaly before and after the SFW events in the stratosphere.All these results indicate that,the earlier breakdown of the stratospheric polar vortex(SPV),as for the winter stratospheric sudden warming(SSW) events is driven mainly by wave forcing;and in contrast,the later breakdown of the SPV exhibits more characteristics of its seasonal evolution.Nevertheless,after the breakdown of SPV,the polar temperature anomalies always exhibit an out-of-phase relationship between the stratosphere and the troposphere for both the early and late SFW events,which implies an intimate stratosphere–troposphere dynamical coupling in spring.In addition,there exists a remarkable interdecadal change of the onset time of SFW in the mid 1990s.On average,the SFW onset time before the mid 1990s is 11 days earlier than that afterwards,corresponding to the increased/decreased planetary wave activities in late winter-early spring before/after the 1990s.