Multidecadal-to-centennial SST variability in the MPI-ESM simulation ensemble for the last millennium

We assess the responses of North Atlantic, North Pacific, and tropical Indian Ocean Sea Surface Temperatures (SSTs) to natural forcing and their linkage to simulated global surface temperature (GST) variability in the MPI-Earth System Model simulation ensemble for the last millennium. In the simulations, North Atlantic and tropical Indian Ocean SSTs show a strong sensitivity to external forcing and a strong connection to GST. The leading mode of extra-tropical North Pacific SSTs is, on the other hand, rather resilient to natural external perturbations. Strong tropical volcanic eruptions and, to a lesser extent, variability in solar activity emerge as potentially relevant sources for multidecadal SST modes’ phase modulations, possibly through induced changes in the atmospheric teleconnection between North Atlantic and North Pacific that can persist over decadal and multidecadal timescales. Linkages among low-frequency regional modes of SST variability, and among them and GST, can remarkably vary over the integration time. No coherent or constant phasing is found between North Pacific and North Atlantic SST modes over time and among the ensemble members. Based on our assessments of how multidecadal transitions in simulated North Atlantic SSTs compare to reconstructions and of how they contribute characterizing simulated multidecadal regional climate anomalies, past regional climate multidecadal fluctuations seem to be reproducible as simulated ensemble-mean responses only for temporal intervals dominated by major external forcings.     

Climate Variability-Observations, Reconstructions, and Model Simulations for the Atlantic-European and Alpine Region from 1500-2100 AD

A detailed analysis is undertaken of the Atlantic-European climate using data from 500-year-long proxy-based climate reconstructions, a long climate simulation with perpetual 1990 forcing, as well as two global and one regional climate change scenarios. The observed and simulated interannual variability and teleconnectivity are compared and interpreted in order to improve the understanding of natural climate variability on interannual to decadal time scales for the late Holocene. The focus is set on the Atlantic-European and Alpine regions during the winter and summer seasons, using temperature, precipitation, and 500 hPa geopotential height fields. The climate reconstruction shows pronounced interdecadal variations that appear to “lock” the atmospheric circulation in quasi-steady long-term patterns over multi-decadal periods controlling at least part of the temperature and precipitation variability. Different circulation patterns are persistent over several decades for the period 1500 to 1900. The 500-year-long simulation with perpetual 1990 forcing shows some substantial differences, with a more unsteady teleconnectivity behaviour. Two global scenario simulations indicate a transition towards more stable teleconnectivity for the next 100 years. Time series of reconstructed and simulated temperature and precipitation over the Alpine region show comparatively small changes in interannual variability within the time frame considered, with the exception of the summer season, where a substantial increase in interannual variability is simulated by regional climate models.     

The Summer Snow Cover Anomaly over the Tibetan Plateau and Its Association with Simultaneous Precipitation over the Mei-yu–Baiu region

The summer snow anomalies over the Tibetan Plateau （TP） and their effects on climate variability are often overlooked,possibly due to the fact that some datasets cannot properly capture summer snow cover over high terrain.The satellite-derived Equal-Area Scalable Earth grid （EASE-grid） dataset shows that snow still exists in summer in the western part and along the southem flank of the TP.Analysis demonstrates that the summer snow cover area proportion （SCAP） over the TP has a significant positive correlation with simultaneous precipitation over the mei-yu-baiu （MB） region on the interannual time scale.The close relationship between the summer SCAP and summer precipitation over the MB region could not be simply considered as a simultaneous response to the Silk Road pattern and the SST anomalies in the tropical Indian Ocean and tropical central-eastern Pacific.The SCAP anomaly has an independent effect and may directly modulate the land surface heating and,consequently,vertical motion over the western TP,and concurrently induce anomalous vertical motion over the North Indian Ocean via a meridional vertical circulation.Through a zonal vertical circulation over the tropics and a Kelvin wave-type response,anomalous vertical motion over the North Indian Ocean may result in an anomalous high over the western North Pacific and modulate the convective activity in the western Pacific warm pool,which stimulates the East Asia-Pacific （EAP） pattern and eventually affects summer precipitation over the MB region.     

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.     

Factors favorable to frequent extreme precipitation in the upper Yangtze River Valley

Extreme precipitation events in the upper Yangtze River Valley (YRV) have recently become an increasingly important focus in China because they often cause droughts and floods. Unfortunately, little is known about the climate processes responsible for these events. This paper investigates factors favorable to frequent extreme precipitation events in the upper YRV. Our results reveal that a weakened South China Sea summer monsoon trough, intensified Eurasian-Pacific blocking highs, an intensified South Asian High, a southward subtropical westerly jet and an intensified Western North Pacific Subtropical High (WNPSH) increase atmospheric instability and enhance the convergence of moisture over the upper YRV, which result in more extreme precipitation events. The snow depth over the eastern Tibetan Plateau (TP) in winter and sea surface temperature anomalies (SSTAs) over three key regions in summer are important external forcing factors in the atmospheric circulation anomalies. Deep snow on the Tibetan Plateau in winter can weaken the subsequent East Asian summer monsoon circulation above by increasing the soil moisture content in summer and weakening the land–sea thermal contrast over East Asia. The positive SSTA in the western North Pacific may affect southwestward extension of the WNPSH and the blocking high over northeastern Asia by arousing the East Asian-Pacific pattern. The positive SSTA in the North Atlantic can affect extreme precipitation event frequency in the upper YRV via a wave train pattern along the westerly jet between the North Atlantic and East Asia. A tripolar pattern from west to east over the Indian Ocean can strengthen moisture transport by enhancing Somali cross-equatorial flow.     

夏季亚洲—太平洋涛动与大气环流和季风降水

利用ERA-40再分析资料和数值模拟,分析了在亚洲-太平洋区域的大气遥相关以及与亚洲季风降水和西北太平洋热带气旋活动气候特征的关系,探讨了青藏高原加热和太平洋海表温度(SST)对遥相关的影响,结果表明:亚洲-太平洋涛动(Asian-Pacific Oscillation,APO)是夏季对流层扰动温度在亚洲与太平洋中纬度之间的一种"跷跷板"现象,当亚洲大陆中纬度对流层偏冷时,中、东太平洋中纬度对流层偏暖,反之亦然;这种遥相关也出现在平流层中,只是其位相与对流层的相反.APO为研究亚洲与太平洋大气环流相互作用提供了一个途径.APO指数也是亚洲-太平洋对流层热力差异指数,它具有年际和年代际的多时间尺度变化特征,在1958-2001年亚洲与太平洋之间的对流层热力差异呈现出减弱趋势,同时也有显著的5.5 a周期.APO形成可能与太阳辐射在亚洲陆地和太平洋的加热差异所造成的纬向垂直环流有关,数值模拟进一步表明:夏季青藏高原加热可以造成高原附近对流层温度升高、上升运动加强,太平洋下沉运动加强、温度下降,从而形成APO现象;而太平洋年代际涛动和赤道东太平洋的厄尔尼若现象对APO的影响可能较小.当夏季APO异常时,南亚高压、欧亚中纬度西风急流、南亚热带东风急流以及太平洋上空的副热带高压都出现显著变化,并伴随着亚洲季风降水及西北太平洋热带气旋活动异常.过去40多年来的长江中上游地区夏季变冷与APO有关,可能是全球大气环流年代际变化在该区域的一种反映.APO异常信号可以传播到南、北两极.此外,亚洲-太平洋之间的这种遥相关型也出现在其他季节.     

Interannual modulation of subtropical Atlantic boreal summer dust variability by ENSO

Dust variability in the climate system has been studied for several decades, yet there remains an incomplete understanding of the dynamical mechanisms controlling interannual and decadal variations in dust transport. The sparseness of multi-year observational datasets has limited our understanding of the relationship between climate variations and atmospheric dust. We use available in situ and satellite observations of dust and a century-length fully coupled Community Earth System Model (CESM) simulation to show that the El Niño-Southern Oscillation (ENSO) exerts a control on North African dust transport during boreal summer. In CESM, this relationship is stronger over the dusty tropical North Atlantic than near Barbados, one of the few sites having a multi-decadal observed record. During strong La Niña summers in CESM, a statistically significant increase in lower tropospheric easterly wind is associated with an increase in North African dust transport over the Atlantic. Barbados dust and Pacific SST variability are only weakly correlated in both observations and CESM, suggesting that other processes are controlling the cross-basin variability of dust. We also use our CESM simulation to show that the relationship between downstream North African dust transport and ENSO fluctuates on multidecadal timescales and is associated with a phase shift in the North Atlantic Oscillation. Our findings indicate that existing observations of dust over the tropical North Atlantic are not extensive enough to completely describe the variability of dust and dust transport, and demonstrate the importance of global models to supplement and interpret observational records.     

Comparing the spatial structure of variability in two datasets against each other on the basis of EOF-modes

In analysis of climate variability or change it is often of interest how the spatial structure in modes of variability in two datasets differ from each other, e.g. between past and future climate or between models and observations. Often such analysis is based on Empirical Orthogonal Function (EOF) analysis or other simple indices of large-scale spatial structures. The present analysis lays out a concept on how two datasets of multivariate climate variability can be compared against each other on basis of EOF analysis and how the differences in the multivariate spatial structure between the two datasets can be quantified in terms of explained variance in the leading spatial patterns. It is also illustrated how the patterns of largest differences between the two datasets can be defined and interpreted. We illustrate this method on the basis of several well-defined artificial examples and by comparing our approach with examples of climate change studies from the literature. These literature examples include analysis of changes in the modes of variability under climate change for the sea level pressure (SLP) of the North Atlantic and Europe, the SLP of the Southern Hemisphere, the surface temperature of the Northern Hemisphere, the sea surface temperature of the North Pacific and for precipitation in the tropical Indo-Pacific.     

Predictable patterns and predictive skills of monsoon precipitation in Northern Hemisphere summer in NCEP CFSv2 reforecasts

The predictable patterns and predictive skills of monsoon precipitation in the Northern Hemisphere summer (June–July–August) are examined using reforecasts (1983–2010) from the National Center for Environmental Prediction Climate Forecast System version 2 (CFSv2). The possible connections of these predictable patterns with global sea surface temperature (SST) are investigated. The empirical orthogonal function analysis with maximized signal-to-noise ratio is used to isolate the predictable patterns of the precipitation for three regional monsoons: the Asian and Indo-Pacific monsoon (AIPM), the Africa monsoon (AFM), and the North America monsoon (NAM). Overall, the CFSv2 well predicts the monsoon precipitation patterns associated with El Niño-South Oscillation (ENSO) due to its good prediction skill for ENSO. For AIPM, two identified predictable patterns are an equatorial dipole pattern characterized by opposite variations between the equatorial western Pacific and eastern Indian Ocean, and a tropical western Pacific pattern characterized by opposite variations over the tropical northwestern Pacific and the Philippines and over the regions to its west, north, and southeast. For NAM, the predictable patterns are a tropical eastern Pacific pattern with opposite variations in the tropical eastern Pacific and in Mexico, the Guyana Plateau and the equatorial Atlantic, and a Central American pattern with opposite variations in the eastern Pacific and the North Atlantic and in the Amazon Plains. The CFSv2 can predict these patterns at least 5 months in advance. However, compared with the good skill in predicting AIPM and NAM precipitation patterns, the CFSv2 exhibits little predictive skill for AFM precipitation, probably because the variability of the tropical Atlantic SST plays a more important than ENSO in the AFM precipitation variation and the prediction skill is lower for the tropical Atlantic SST than the tropical Pacific SST.     

Climate change features along the Brahmaputra Valley in the past 26 years and possible causes

Through analyzing the changes of averaged temperature and precipitation along the Brahmaputra Valley from 1980 to 2005, and investigating the correlation between atmospheric circulation indices and these changes, the respondence of the Brahmaputra Valley to the global warming had been analyzed in this paper. It is found that the Brahmaputra Valley climate became warmer and wetter from 1980 to 2005, and the change magnitude is higher than that over the whole Tibetan Plateau. Therefore, the respondence of the Brahmaputra valley to the global warming is more evident than that of the whole plateau during the period. The relationship between atmospheric circulation indices and the regional climate change is significant. When North Atlantic Oscillation index is higher in summer, the climate of the Brahmaputra Valley becomes warmer and dryer; vice versa. When South Oscillation index is higher, it becomes warmer and wetter; vice versa.Worthy of note, the results in this study only emphasize some evidence of change in temperature and precipitation over the Brahmaputra Valley during 1980–2005.     

North African climate variability. Part 2: Tropical circulation systems

Summary Tropical North African climate variability is investigated using a Sahel rainfall index and streamflow of the Nile River in the 20^th century. The mechanisms that govern tropical North Africa climate are diagnosed from NCEP reanalysis data in the period 1958–1998: spatially – using composite and correlation analysis, and temporally – using wavelet co-spectral analysis. The Sahelian climate is characterised by a decadal rhythm, whilst the mountainous eastern and equatorial regions exhibit interannual cycles. ENSO-modulated zonal circulations over the Atlantic/Pacific sector are important for decadal variations, and create a climatic polarity between South America and tropical North Africa as revealed through upper-level velocity potential and convection patterns. A more localised N–S shift in convection between the Sahel and Guinea coast is associated with the African Easterly Jet.     

Extratropical control of recent tropical Pacific decadal climate variability: a relay teleconnection

Observations indicate that recent tropical Pacific decadal climate variability tends to be associated with the extratropical North Pacific through a relay teleconnection of a fast coupled ocean-atmosphere bridge and a slow oceanic tunnel. A coupled ocean-atmosphere model, forced by the observed decadal wind in the extratropical North Pacific, explicitly demonstrates that extratropical decadal sea surface temperature (SST) anomalies may propagate to the tropics through a coupled wind-evaporative-SST (WES) feedback. The WES feedback cannot only lead to a nearly synchronous change of tropical SST, but also force a delayed adjustment of the meridional overturning circulation in the upper ocean to further sustain the tropical SST change. The study further suggests that the extratropical–tropical teleconnection provides a positive feedback to sustain the decadal changes in both the tropical and extratropical North Pacific.     

Quantifying atmosphere and ocean origins of North American precipitation variability

How atmospheric and oceanic processes control North American precipitation variability has been extensively investigated, and yet debates remain. Here we address this question in a 50 km-resolution flux-adjusted global climate model. The high spatial resolution and flux adjustment greatly improve the model’s ability to realistically simulate North American precipitation, the relevant tropical and midlatitude variability and their teleconnections. Comparing two millennium-long simulations with and without an interactive ocean, we find that the leading modes of North American precipitation variability on seasonal and longer timescales exhibit nearly identical spatial and spectral characteristics, explained fraction of total variance and associated atmospheric circulation. This finding suggests that these leading modes arise from internal atmospheric dynamics and atmosphere-land coupling. However, in the fully coupled simulation, North American precipitation variability still correlates significantly with tropical ocean variability, consistent with observations and prior literature. We find that tropical ocean variability does not create its own type of atmospheric variability but excites internal atmospheric modes of variability in midlatitudes. This oceanic impact on North American precipitation is secondary to atmospheric impacts based on correlation. However, relative to the simulation without an interactive ocean, the fully coupled simulation amplifies precipitation variance over southwest North America (SWNA) during late spring to summer by up to 90%. The amplification is caused by a stronger variability in atmospheric moisture content that is attributed to tropical Pacific sea surface temperature variability. Enhanced atmospheric moisture variations over the tropical Pacific are transported by seasonal mean southwesterly winds into SWNA, resulting in larger precipitation variance.

     

Interdecadal change of the controlling mechanisms for East Asian early summer rainfall variation around the mid-1990s

East Asian (EA) summer monsoon shows considerable differences in the mean state and principal modes of interannual variation between early summer (May–June, MJ) and late summer (July–August, JA). The present study focuses on the early summer (MJ) precipitation variability. We find that the interannual variation of the MJ precipitation and the processes controlling the variation have been changed abruptly around the mid-1990s. The rainfall anomaly represented by the leading empirical orthogonal function has changed from a dipole-like pattern in pre-95 epoch (1979–1994) to a tripole-like pattern in post-95 epoch (1995–2010); the prevailing period of the corresponding principal component has also changed from 3–5 to 2–3 years. These changes are concurrent with the changes of the corresponding El Nino-Southern Oscillation (ENSO) evolutions. During the pre-95 epoch, the MJ EA rainfall anomaly is coupled to a slow decay of canonical ENSO events signified by an eastern Pacific warming, which induces a dipole rainfall feature over EA. On the other hand, during the post-95 epoch the anomalous MJ EA rainfall is significantly linked to a rapid decay of a central Pacific warming and a distinct tripolar sea surface temperature (SST) in North Atlantic. The central Pacific warming-induced Philippine Sea anticyclone induces an increased rainfall in southern China and decreased rainfall in central eastern China. The North Atlantic Oscillation-related tripolar North Atlantic SST anomaly induces a wave train that is responsible for the increase northern EA rainfall. Those two impacts form the tripole-like rainfall pattern over EA. Understanding such changes is important for improving seasonal to decadal predictions and long-term climate change in EA.     

The Decadal Shift of the Summer Climate in the Late 1980s over Eastern China and Its Possible Causes

In this paper, it is pointed out that a notable decadal shift of, the summer climate in eastern China occurred in the late 1980s. In association with this decadal climate shift, after the late 1980s more precipitation appeared in the southern region of eastern China （namely South China）, the western Pacific subtropical high stretched farther westward with a larger south-north extent, and a strengthened anticyclone at 850 hPa appeared in the northwestern Pacific. The decadal climate shift of the summer precipitation in South China was accompanied with decadal changes of the Eurasian snow cover in boreal spring and sea surface temperature （SST） in western North Pacific in boreal summer in the late 1980s. After the late 1980s, the spring Eurasian snow cover apparently became less and the summer SST in western North Pacific increased obviously, which were well correlated with the increase of the South China precipitation. The physical processes are also investigated on how the summer precipitation in China was affected by the spring Eurasian snow cover and summer SST in western North Pacific. The change of the spring Eurasian snow cover could excite a wave-train in higher latitudes, which lasted from spring to summer. Because of the wave-train, an abnormal high appeared over North China and a weak depression over South China, leading to more precipitation in South China. The increase of the summer SST in the western North Pacific reduced the land-sea thermal contrast and thus weakened the East Asian summer monsoon, also leading to more precipitation in South China.     

中国东部夏季气候20世纪80年代后期的年代际转型及其可能成因

指出了中国东部夏季气候在20世纪80年代末出现了一次明显的年代际气候转型.伴随着这次年代际转型,80年代末以后中国东部南方地区降水明显增多,500 hPa西太平洋副热带高压西伸且南北范围变大,西北太平洋上空850 hPa反气旋增强.中国东部夏季80年代后期出现南方多雨的年代际转型与欧亚大陆春季积雪、西北太平洋夏季海面温度的年代际变化存在密切联系,它们也都在80年代末出现年代际转型.从80年代末以后,伴随着欧亚大陆春季积雪明显减少和西北太平洋夏季海面温度明显增高,中国夏季南方降水明显增加.文中分析了欧亚大陆春季积雪和西北太平洋夏季海面温度影响中国降水的物理过程,指出欧亚大陆春季积雪能够在500 hPa激发出大气中的遥相关波列,所激发出的波列可以从春季一直持续到夏季,造成中国北方为高压控制,南方为微弱低压控制,使得降水出现在中国南方.西北太平洋夏季海面温度的升高能够减小海陆热力差异,使得夏季风减弱,导致中国南方地区降水增多.     

北太平洋冬季次表层热状况及其与中国东部夏季气候的关系

利用北太平洋次表层海温及热容量资料和我国160个台站1951-2002年夏季(6-8月)降水、气温资料,分析了北太平洋冬季次表层热状况的特征,研究了中国区域气候与热容量变化之间的关系,并用SVD方法研究了北太平洋热容量变化与中国东部夏季气候的耦合关系.结果表明:北太平洋冬季次表层海温按其特征可分为40 m和240 m两类,两者的年代际特征都很明显;北太平洋冬季热容量异常变化与其相似,并与中国东部夏季气候异常有较好的耦合关系,北太平洋中部偏北和东南部地区冬季热容量异常偏高时,中国华南和华北区夏季降水将会偏多,华南区夏季气温将会偏低;黑潮延续体及北太平洋中部偏东地区冬季热容量异常偏高和北美西海岸地区异常偏低时,中国东北和长江流域地区夏季降水将会偏少,东北和华北地区夏季气温将会偏低,其关系有着明显的年代际特征.     

Marine proxy evidence linking decadal North Pacific and Atlantic climate

Decadal- to multidecadal variability in the extra-tropical North Pacific is evident in 20th century instrumental records and has significant impacts on Northern Hemisphere climate and marine ecosystems. Several studies have discussed a potential linkage between North Pacific and Atlantic climate on various time scales. On decadal time scales no relationship could be confirmed, potentially due to sparse instrumental observations before 1950. Proxy data are limited and no multi-centennial high-resolution marine geochemical proxy records are available from the subarctic North Pacific. Here we present an annually-resolved record (1818–1967) of Mg/Ca variations from a North Pacific/Bering Sea coralline alga that extends our knowledge in this region beyond available data. It shows for the first time a statistically significant link between decadal fluctuations in sea-level pressure in the North Pacific and North Atlantic. The record is a lagged proxy for decadal-scale variations of the Aleutian Low. It is significantly related to regional sea surface temperature and the North Atlantic Oscillation (NAO) index in late boreal winter on these time scales. Our data show that on decadal time scales a weaker Aleutian Low precedes a negative NAO by several years. This atmospheric link can explain the coherence of decadal North Pacific and Atlantic Multidecadal Variability, as suggested by earlier studies using climate models and limited instrumental data.     

Influence of the circumglobal wave-train on European summer precipitation

We investigate European summer (July–August) precipitation variability and its global teleconnections using the NCEP/NCAR reanalysis data (1950–2010) and a historical Coupled Model Intercomparison Project climate simulation (1901–2005) carried out using the ECHAM6/MPIOM climate model. A wavelike pattern is found in the upper tropospheric levels (200 hPa) similar to the summer circumglobal wave train (CGT) extending from the North Pacific to the Eurasian region. The positive phase of the CGT is associated with upper level anomalous low (high) pressure over western (eastern) Europe. It is further associated with a dipole-like precipitation pattern over Europe entailing significantly enhanced (reduced) precipitation over the western (eastern) region. The anomalous circulation features and associated summer precipitation pattern over Europe inverts for the negative CGT phase. Accordingly, the global teleconnection pattern of a precipitation index summarizing summer precipitation over Western Europe entails an upper level signature which consists of a CGT-like wave pattern extending from the North Pacific to Eurasia. The imprint of the CGT on European summer precipitation is distinct from that of the summer North Atlantic Oscillation, despite the two modes of variability bear strong similarities in their upper level atmospheric pattern over Western Europe. The analysis of simulated CGT features and of its climatic implications for the European region substantiates the existence of the CGT-European summer precipitation connection. The summer CGT in the mid-latitude therefore adds to the list of the modes of large-scale atmospheric variability significantly influencing European summer precipitation variability.     

基于MTM-SVD方法的黄河流域夏季降水年际变化及其主要影响因子分析

应用中国气象台站积雪日数资料和NCEP/NCAR再分析资料以及多锥度—奇异值分解方法 (MTM-SVD),分析了近50年来黄河流域夏季降水的时空变化及其影响因子.发现黄河流域夏季降水存在显著的2～3年周期.在准3年周期上黄河流域夏季降水对前冬青藏高原东部积雪日数有很好的响应,当前冬高原积雪日数以正 (负) 异常为主时,接下来的夏季黄河流域降水偏少 (多).这种响应存在年代际变化,在1983年之前最为明显,1983~1993年是个调整时期,1993年以后又开始明显.在准2年周期上黄河流域夏季降水对前冬西太平洋暖池SST有很好的响应,当前冬西太平洋暖池SST偏高 (低) 时,接下来的夏季黄河流域降水表现为东多 (少)西少 (多) 型.这一响应同样存在年代际变化.前冬高原积雪和西太平洋暖池SST是影响黄河流域夏季降水的重要因子.