Yangtze-Huaihe River Research Articles

The heterogeneity of Meiyu rainfall over Yangtze–Huaihe River valley and its relationship with oceanic surface heating and intraseasonal variability

Increasing heavy concentrated Meiyu precipitation over the Yangtze–Huaihe river valley (YHRV) during recent years has been previously reported. In fact, the concentrated Meiyu rainfall occurring in a small region or a certain period easily results in floods, thus it is worthy to analyze the heterogeneity of Meiyu rainfall over YHRV. In this study, we use both of precipitation concentration period (PCP) and precipitation concentration degree (PCD) based on vector analysis to identify the heterogeneity of Meiyu rainfall over YHRV. On the climatological mean, the concentrated heavy precipitation occurs in late summer over the Yangtze River Delta, where is usually suffered by floods. The dominant two patterns of PCP and PCD variations are northeast–southwest dipole pattern, homogeneous anomalies and homogeneous variation, north–south dipole pattern, respectively. In addition, the relationship on heterogeneity of Meiyu rainfall with sea surface temperature (SST) and the low level summer intraseasonal oscillation (ISO) are investigated. Two key regions of SST activities are found: Bay of Bengal (BOB) and Equatorial eastern Pacific. From BOB, more abundant water vapor has been brought. On the El Nino-Southern Oscillation variation, it is closely relative with PCD–PC1 during the decaying phase of El Nino, while PCP–PC2 is accompanied with developing phase of La Nina events, suggesting a negative feedback of PCP–PC2 on the Nino3.4 SST, and changes to positive during the later winter. On the ISO activities, the robust regions are located over the high-latitude areas, which are closely related with northeastern cold vortex. The north “cold and dry” air southwardly invaded with the lower-level strong warm air in the rainy area, and easily formed an “upper-wet and lower-dry” unstable layer. Under the trigger of the upward motion, the concentrated heavy rainfall easily occurred over YHRV. In all, the homogeneity variation of the concentrated heavy precipitation over YHRV is closely associated with both of the heating forcing (SST) and dynamical atmospheric forcing (low-level ISO).

Characteristics and Changes of Extreme Precipitation in the Yellow–Huaihe and Yangtze–Huaihe Rivers Basins, China

Abstract Many works suggest that the intensity of extreme precipitation might be changing under the background of global warming. Because of the importance of extreme precipitation in the Yellow–Huaihe and Yangtze–Huaihe River basins of China and to compare the spatial difference, the generalized Pareto distribution (GPD) function is used to fit the daily precipitation series in these basins and an estimate of the extreme precipitation spatial distribution is presented. At the same time, its long-term trends are estimated for the period between 1951 and 2004 by using a generalized linear model (GLM), which is based on GPD. High quality daily precipitation data from 215 observation stations over the area are used in this study. The statistical significance of the trend fields is tested with a Monte Carlo experiment based on a two-dimensional Hurst coefficient, H2. The spatial distribution of the shape parameter of GPD indicates that the upper reaches of the Huaihe River (HuR) basin have the largest probability of extreme rainfall events, which is consistent with most historical flood records in this region. Spatial variations in extreme precipitation trends are found and show significant positive trends in the upper reaches of Poyang Lake in the Yangtze River (YaR) basin and a significant negative trend in the mid- to lower reaches of the Yellow River (YeR) and Haihe River (HaR) basins. The trends in the HuR basin and the lower reaches of Poyang Lake in the YaR basin are nearly neutral. All trend fields are significant at the 5% level of significance from the Monte Carlo experiments.

Probabilistic seasonal prediction of summer rainfall over East China based on multi-model ensemble schemes

The skill of probability density function (PDF) prediction of summer rainfall over East China using optimal ensemble schemes is evaluated based on the precipitation data from five coupled atmosphere-ocean general circulation models that participate in the ENSEMBLES project. The optimal ensemble scheme in each region is the scheme with the highest skill among the four commonly-used ones: the equally-weighted ensemble (EE), EE for calibrated model-simulations (Cali-EE), the ensemble scheme based on multiple linear regression analysis (MLR), and the Bayesian ensemble scheme (Bayes). The results show that the optimal ensemble scheme is the Bayes in the southern part of East China; the Cali-EE in the Yangtze River valley, the Yangtze-Huaihe River basin, and the central part of northern China; and the MLR in the eastern part of northern China. Their PDF predictions are well calibrated, and are sharper than or have approximately equal interval-width to the climatology prediction. In all regions, these optimal ensemble schemes outperform the climatology prediction, indicating that current commonly-used multi-model ensemble schemes are able to produce skillful PDF prediction of summer rainfall over East China, even though more information for other model variables is not derived.

Dry/wet changes and their standing characteristics in China during the past 531 years

Time series of the dryness-wetness (DW) index of 531 yr (AD 1470–2000) at 42 stations in regions A (most of North China and the east of Northwest China) and B (the Yangtze-Huaihe River valley) in China are applied to investigating the historical DW characteristics over various periods of the series with a relatively stationary average value using Bernaola-Galvan (BG) algorithm. The results indicate that region A/B underwent three drought-intensive periods (DIP; 1471–1560, 1571–1640, and 1920–2000/1501–1540, 1631–1690, and 1911–1960) in the last 531 years. In the DIP of the last 130 years, the frequency of DW transition has increased in region A, but not obviously changed in region B in comparison with the other two historical DIPs. The dry period started in about 1920 in region A with severe drought events occurring from the late 1970s to the early 1980s. It lasted for about 50–70 yr in this century, and then a DW shift took place. The wet period in region B might maintain for the coming several decades. The variations of DW in region A are positively correlated with changes in temperature, but in region B, the correlation with temperature is weaker. It is found that the number of DW indices of various categories within a running window is an exponential function of the running window length. The dryness scale factor (DSF) is defined as the reciprocal of the characteristic value of the exponential distribution, and it has a band-like fluctuation distribution that is good for the detection of extreme drought (flood) clustering events. The results show that frequencies of the severe large-scale drought events that concurrently occurred in regions A and B were high in the late 12th century, the early 13th century, the early 17th century, and the late 20th century. This provides evidence for the existence of the time-clustering phenomena of droughts (floods).

Decadal variation of different durations of continuous Meiyu precipitation and the possible cause

The features of the decadal evolution of different durations of continuous Meiyu precipitation and their possible cause from the viewpoint of the low-level intraseasonal oscillation (ISO) are examined in this study. It is found that the total frequency of Meiyu precipitation events of certain duration gradually decreases as the duration of the events increases, and the main center of the events moves from the northern Yangtze-Huaihe River Valley (YHRV) to the southern YHRV. The linear trends of different durations of continuous Meiyu precipitation indicate that longer-duration precipitation has become significantly less frequent since 2000, while 2-day and 3–4-day continuous precipitation processes have become more frequent. At the same time, a close relationship between the variation in the low-level ISO and the decadal evolution of frequency of longer-duration Meiyu precipitation is detected. Clearly, since 2000, the low-level ISO has significantly decreased, and a single peak of longer-duration events has been replaced by several peaks of shorter-duration events, which has resulted in a significant decrease in the frequency of longer-duration precipitation. Meanwhile, low-level baroclinicity increased (decreased) during each period, which is in accordance with the increasing (decreasing) frequency of longer-duration precipitation. It is confirmed that longer-duration precipitation has significantly decreased in recent years.

Characteristics of spatiotemporal distribution of extreme temperature events over China mainland in different climate states against the backdrop of most probable temperature

Daily max temperature records of National Meteorological Information Center from 1961 to 2009 over China mainland are divided into three climate states of 19611990,19712000,19812009 (just called StateⅠ, StateⅡ, State Ⅲ respectively). Most probable temperature(MPT) and extreme temperature events are defined according to the skewed function. The spatiotemporal distribution characteristics of frequency and strength of extreme temperature events over China mainland in different climate sates against the backdrop of MPT are analyzed. Spatially, frequency and strength of extremly high temperature in summer decrease significantly in the Yangtze-Huaihe river valley and the Yellow river and Huaihe River valley in StateⅠ and increase significantly in the arid-semiarid region and the econormically developed Yangtze River delta in State Ⅲ. The frequency of extremely low temperature in winter reduces remarkably in the north part and the Yangtze River delta in StateⅡ and in the Qinghai-Tibet Plateau, the southeast part of northeast China, the Yangtze River delta in State Ⅲ. The strength of extremely low temperature in winter reduces on the whole and provincial characteristics are not obvious. The frequency and the strength of extreme temperature events are consistont spatially. Temporally, the frequency and the strength of extremely high temperature in summer increase obviously both in StateⅡ and State Ⅲ. The frequency and the strength of extremely low temperature in winter reduce obviously in StateⅡ and the reducing trend slows down in State Ⅲ. Extremely high temperature in summer occurs frequently and extremely low temperature in winter remains stable. Extremity of high temperature in summer is stronger while in winter is stable relatively. The frequency and the strength of extreme temperature events are consistent temporally. During the common time period of States Ⅰ,Ⅱ and Ⅲ, the frequency of extremely high temperature events in the last state was always less than the former while the low temperature events are more, which is in accordance with the fact that the background temperature steps up with the states; the strength of extremely high temperature events in the latter state is less than that of the former while the strength of extremely low temperature events is stronger. For the abrupt change of climate at the end of the 1970 s and the beginning of the 1980 s, the frequency of extreme high temperature events decreases before the change and increases obviously after the change while the ones of extremely low temperature do not change significantly correspondingly; the strength of extremely high temperature events decreassd slightly before the change and increases obviously after the change while the strength of extremely low temperature does not change significantly for the change but the whole strength after the change is slightly lower than the one before the change.

Relationship between the Meiyu over the Yangtze-Huaihe River Basins and the Frequencies of Tropical Cyclone Genesis in the Western North Pacific

The relationship between the Meiyu precipitation over the Yangtze-Huaihe River Basins (YHRB) and the frequencies of tropical cyclone (TC) genesis in the western North Pacific (WNP) for the period 1951-2006 has been investigated in the present paper. The results have shown that there exists a significant negative correlation between them, which is due to the fundamental changes in monsoonal airflows and associated moisture transports over East Asia and the WNP. In the years with fewer TCs genesis, the location of the main body of the subtropical high over the WNP is farther west and south, and its intensity is abnormally stronger. The monsoon trough also shifted farther west, with a weaker intensity. Under this condition, the monsoon flow and the corresponding moisture transport can not extend eastward down to the WNP, but turns to the East Asian region along the western flank of the subtropical anticyclone over the South China Sea (SCS). Meanwhile, the westerlies at mid-latitudes are located farther south with a stronger intensity. This kind of circulation conditions is favorable for the convergence of the dry and cold air from the mid- and high latitudes and the warm and moist air from the low latitudes over the YHRB, and leads to more precipitation there.In the years with more TCs, the subtropical high over the WNP is located farther north and east, with its intensity being weaker. The monsoon trough zonally extends eastward down to the open sea of the WNP. Most of the monsoon flow and the associated moisture transport from the tropical Indian Ocean can all the way reach the WNP, with a smaller proportion turning northward to the East Asian Meiyu region over the SCS, which is favorable for the genesis and development of TC in the WNP. At the same time, the location of the mid-latitude westerlies is farther north and its intensity is weaker, which is unfavorable for the occurrence of the precipitation over the YHRB. This kind of negative correlation relationship will be conducive to the seasonal forecast of the TC and the Meiyu during the rainy season in China.

Different impacts of El Niño and El Niño Modoki on China rainfall in the decaying phases

AbstractIn this study, we investigate the different impacts of El Niño and El Niño Modoki on China rainfall in their decaying phases. During spring, in the decaying year of El Niño events, there are positive rainfall anomalies south of the Yangtze River, whereas no obvious rainfall signals are found in the same season for the decaying El Niño Modoki. In the subsequent summer season, the wet signal south of the Yangtze River associated with El Niño continues, while suppressed rainfall now appears in the northern Yangtze–Huaihe River region. In contrast, the rainfall is above normal in the region from the Huaihe River to the Yellow River, and below normal in southern China during the summer of the decaying El Niño Modoki events. The distinct China rainfall anomaly is mainly attributed to the difference between the evolution and location of the anomalous western North Pacific (WNP) anti‐cyclone associated with El Niño and El Niño Modoki events. For the case of El Niño, the WNP anti‐cyclone brings plentiful moisture to southern China; meanwhile it shifts the ridge of sub‐tropical high westward in both spring and summer. These tend to induce positive rainfall anomalies in the southern Chinese region. In contrast, due to fast decaying of El Niño Modoki, the anomalous WNP anti‐cyclone becomes weak so that there are no significant rainfall anomalies in China. In summer, however, the WNP anti‐cyclone re‐invigorates possibly associated with a subsequent developing La Niña and extends more northwestward towards the inland region, compared to its El Niño counterpart. The anomalous moisture transport and sub‐tropical high activity associated with this WNP anti‐cyclone result in different summer rainfall anomalies in China. Copyright © 2010 Royal Meteorological Society

Ensemble projection of 1–3°C warming in China

Studies on the influences of climate change on biogeochemical cycles and on the key vulnerabilities and the risk from climate change suggest that annual surface temperature rise of 1°C, 2°C and 3°C above the present level would lead to changes in extreme weather and climate events, food production, fresh water resources, biodiversity, human mortality, etc. Here two sets of simulations as performed with seventeen atmosphere-ocean general circulation models (AOGCMs) for the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC AR4), i.e. the model outputs from the 20th Century Climate in Coupled Models (20C3M) and from the Special Report on Emissions Scenarios (SRES) emission scenarios B1, A1B and A2, are used to analyze spatial and temporal characteristics of the above values in China over the 21st century. The results indicate that the rate of warming varies from region to region. The above values are reached much later (earlier) when emission amount is lower (higher), and spread of the time when the lower (higher) value is exceeded is narrower (wider) among the three scenarios. As far as the spatial pattern is concerned, the above values are crossed much earlier in northern China and the Tibetan Plateau with respect to the Yangtze-Huaihe River Valley and South China.

Modeling impacts of East Asian Ocean-Land thermal contrast on spring southwesterly winds and rainfall in eastern China

Using the National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) reanalysis dataset, the NOAA’s Climate Prediction Center’s merged analysis of precipitation, and the MM5v3 Meso-scale Model, the impacts of surface temperature differences between the East Asian land and its adjacent oceans on spring southwesterly winds and rainfall over eastern China are studied. The modeling results show that the temperature differences exert strong influence on the occurrence of the southwesterly winds and rainfall over southern China and their northward advances. When surface temperature increases over the land and decreases over the oceans, the temperature gradient with a winter feature earlier changes toward the gradient with a summer feature. Both the low-pressure system east of the Tibetan Plateau and the subtropical high-pressure system over the western Pacific strengthen, accompanying with the strengthening of the lower-tropospheric southwesterly winds over eastern China. Accordingly, the upward motion increases over the Yangtze-Huaihe River (YHR) valleys and decreases over southern China, leading to an increase of spring rainfall over the YHR valleys and a decrease over southern China. Thus, the rain belt over eastern China appears over the YHR valleys but not over southern China. Under a weaker condition of the spring thermal contrast, the rain belt does not occur over eastern China. When the spring thermal contrast pronouncedly strengthens, the rain belt over southern China may advance northward into the YHR valleys during spring, though there is no onset of the tropical monsoon over the South China Sea. This forms a rain belt similar to that of the YHR valleys during the summer Meiyu period.

Interannual variability of the Meiyu onset over Yangtze-Huaihe River Valley and analyses of its previous strong influence signal

Meiyu onset (MO) over Yangtze-Huaihe River Valley (YHRV) possesses obvious characteristics of interannual variations. Based on NCEP/NCAR reanalysis data sets, NOAA OLR and ERSST data, the interannual variability of MO(IVMO) and its previous strong influence signal (PSIS) are investigated. The possible mechanisms that the PSIS affecting IVMO are also discussed. The results show that the previous CP-ENSO (Central Pacific El Nino/Southern Oscillation) event is the PSIS affecting IVMO and it has a better accuracy rate of short-term climate prediction and practicality. The MO is most likely to be late (early) with the warm (cold) phase of CP-ENSO in previous boreal February and spring. CP-ENSO affects MO mainly by means of EAP (East Asian-Pacific) or JP (Japanese-Pacific) teleconnection, in which the tropical western North Pacific anticyclone plays an important role. In the years of CP-ENSO warm phase, the tropical warm wet water vapor transportation to YHRV is late. The anomalous positive sea surface temperature near the equatorial central Pacific results in late northward jump of the western Pacific subtropical high and late establishment of Indian southwest monsoon via air-sea interaction, which leads to late seasonal transition of the atmospheric circulations over East Asia from boreal spring to summer. Late seasonal transition of the atmospheric circulations and late tropical warm wet water vapor transport to YHRV are the primary reasons that cause the late MO. The situations are directly opposite in the years of CP-ENSO cold phase.

Relationship between precipitation and the infiltration depth over the middle and lower reaches of the Yellow River and Yangtze-Huaihe River Valley

The purpose of the paper is to extract and make the best use of rainfall information contained in the Chinese historical archives of Qing Dynasty, and reconstruct the precipitation series during 1736–1911. The study followed the Yu-Fen-Cun (rainfall infiltration depth) observation method in Qing Dynasty, to conduct soil infiltration experiment under the natural rainfall conditions during 2004–2006. The related parameters, such as precipitation, intensity, initial soil moisture content and infiltration depth have been measured at 10 stations (east of 105°E, 30°–40°N) with representative climatic conditions and soil physical texture in the middle and lower reaches of the Yellow River and Yangtze-Huaihe River Valley. The statistical results indicate that precipitation is the primary influencing factor to infiltration depth and is positively correlated with the infiltration depth. Additionally, the effects of every factor on infiltration depth are significantly different between the middle and lower reaches of the Yellow River and Yangtze-Huaihe River Valley. Finally, the relationship equations of precipitation and infiltration depth are established for each site, most of which can pass over 60% of the variance explanations and provide quantitative models to reconstruct the precipitation series using Yu-Fen-Cun records in Qing Dynasty.

Potential vorticity structure and inversion of the cyclogenesis over the Yangtze River and Huaihe River valleys

In this paper, the potential vorticity structure and inversion of the cyclogenesis over the Yangtze River and Huaihe River valleys during 21–23 June 2003 are investigated with a potential vorticity (PV) framework. The cyclogenesis is manifested by a lower-tropospheric PV anomaly over the Yangtze River and Huaihe River valleys at early stages mainly due to latent heat release, which greatly affects the evolution of the associated lower-tropospheric geopotential height and wind fields as demonstrated by piecewise PV inversion. At later stages, an upper-tropospheric PV anomaly develops, resulting in the growth of ridges over the cyclone in both the upstream and downstream, which provide a favorable background field for the low-level cyclone development. But the effect of a surface thermal anomaly always impedes the development of the cyclone to different extents during this cyclogenesis. It is further demonstrated that the position and the strength of the PV anomaly are closely related to the low-level cyclone development, and the lower-tropospheric PV anomaly seems to constitute the most significant feature, for instance, contributing about 60% to the low-level jet (LLJ).

Comparison of products from ERA-40, NCEP-2, and CRU with station data for summer precipitation over China

Summer precipitation products from the 45-Year European Centre for Medium-Range Weather Forecast (ECMWF) Reanalysis (ERA-40), and NCEP-Department of Energy (DOE) Atmospheric Model Intercomparison Project (AMIP-H) Reanalysis (NCEP-2), and Climatic Research Unit (CRU) TS 2.1 dataset are compared with the corresponding observations over China in order to understand the quality and utility of the reanalysis datasets for the period 1979–2001. The results reveal that although the magnitude and location of the rainfall belts differ among the reanalysis, CRU, and station data over South and West China, the spatial distributions show good agreement over most areas of China. In comparison with the observations in most areas of China, CRU best matches the observed summer precipitation, while ERA-40 reports less precipitation and NCEP-2 reports more precipitation than the observations. With regard to the amplitude of the interannual variations, CRU is better than either of the reanalyses in representing the corresponding observations. The amplitude in NCEP-2 is stronger but that of ERA-40 is weaker than the observations in most study domains. NCEP-2 has a more obvious interannual variability than ERA-40 or CRU in most areas of East China. Through an Empirical orthogonal function (EOF) analysis, the main features of the rainfall belts produced by CRU agree better with the observations than with those produced by the reanalyses in the Yangtze-Huaihe River valley. In East of China, particularly in the Yangtze-Huaihe River valley, CRU can reveal the quasi-biennial oscillation of summer precipitation represented by the observations, but the signal of ERA-40 is comparatively weak and not very obvious, whereas that of NCEP-2 is also weak before 1990 but very strong after 1990. The results also suggest that the magnitude of the precipitation difference between ERA-40 and the observations is smaller than that between NCEP-2 and the observations, but the variations represented by NCEP-2 are more reasonable than those given by ERA-40 in most areas of East China to some extent.

Estimations of precipitable water and its characteristics during the HUBEX/IOP 1998

GMS-5 satellite data at channels of infrared split windows and water vapor are analyzed to retrieve the precipitable water (PW) distributions under cloud-free conditions. Radiosonde data and surface station data are applied to estimate the PW distributions under cloudy conditions. These two methods are then merged to obtain the PW distributions under all-weather conditions during the Huaihe River Basin Energy and Water Cycle Experiment (HUBEX). The results of the all-weather PW distributions from these methods demonstrate that this new merging technique may be applied to derive large-scale or global maps of PW. It is revealed that the atmospheric water vapor over the Yangtze-Huaihe River Basins came mainly from the southwest during the 1998 prevailing period of Meiyu. Sufficient atmospheric PW is a necessary condition for ground rainfall. Under certain dynamic conditions, it can be partially transformed into surface precipitation. Several types of rain are displayed and their PW conditions and characteristics, as well as atmospheric dynamic conditions, are analyzed. It is demonstrated that surface precipitation centers usually appear not at the high PW centers but on their downwind sides.

Analysis of the Large-Scale Circulation Features and Synoptic Systems in East Asia During the Intensive Observation Period of GAME/HUBEX.

An analysis of the large-scale circulation features, major synoptic systems, precipitation, and atmospheric heating fields during the intensive observation period (IOP) of the GAME/HUBEX identified eight features. 1) The beginning and maintenance of the Meiyu season over the Yangtze-Huaihe River Basin in both 1998 and 1999 occurred under the condition of the northward advance and enhancement of the East Asian summer monsoon. 2) The circulation features characterized with two ridges in the region to west of the Baikal Lake and the Sea of Okhotsk, respectively, and one trough in between (in the region of the Baikal Lake) in the middle and high latitudes set a stage for the development of Meiyu seasons. Especially, the quasi-stationary Okhotsk blocking high and the upper-level vortex over East China/the Sea of Japan was a couplet of circulation systems under which the Meiyu front over the Yangtze-Huaihe River Basins was stably maintained. 3) The subtropical high in the western Pacific assumed a significant westward and northward shift before arrival of Meiyu seasons in 1998 and 1999. During the concentrating period of Meiyu precipitation for 1998, its meridional oscillation was closely related to variations of episodes of heavy rainfalls in this region. 4) The low-level easterly aircurrent to the north of the low-level southerly jet played an important role in maintenance of the frontal zone, which is also one of major factors leading to the persistent heavy rainfalls in middle and lower reaches of the Yangtze River. 5) The formation of the low-level jets is different in the synoptic conditions for 1998 and 1999. This difference mainly resulted from the different zonal positions of the subtropical high and activities of the summer monsoon. 6) There existed a succession of meso-scale disturbances (at least 2 and 3 meso-scale vortices for 1998 and 1999, respectively) along the Meiyu frontal zones, which mostly originated in the eastern flank of the Tibetan plateau and migrated eastward, with vortex centers located to the north of the low-level jet axis and heavy rainfalls in the southeast of the disturbances. 7) Along the Meiyu front, there was a high humidity and lower temperature conditions, while relatively dry and higher temperature conditions existed to south and north of the Meiyu front. This kind of sandwich-like temperature and humidity condition is important for the Meiyu maintenance; Zonally, the temperature field was characterized by lower temperature in the west and higher temperature in the east along the Meiyu frontal zone. 8) During the processes of heavy rains in 1998 and 1999, the patterns and the magnitudes of Q1 and Q2 are nearly similar to each other, indicating that the heating over the heavy rain regions was created by the moisture condensation process, due mainly to convective activities. However, it is now not clear how this prosess of energy and water cycle may in turn affect events of heavy rainfalls through feedback effect. The above indications of the large-scale circulation features and synoptic systems provide a starting point for further investigation of the energy and water cycle over the Huaihe River Basin.

Decadal variation of summer rainfall in the yangtzehuaihe river valley and its relationship to atmospheric circulation anomalies over east asia and western north pacific

Decadal variations of summer rainfall during 1951 through 1990 are analyzed by using summer rainfall data of 160 stations in China. Four major patterns of decadal variations are identified. The decadal variations of summer rainfall showed northward shift in the eastern China from South China through the Yangtze-Huaihe River to North China. Summer rainfall in the Yangtze-Huaihe River valley underwent two obvious decadal transitions during the 40 years: one from rainy period to drought period in the end of the 1950's, the other from drought period to rainy period in the late 1970's. Correspondingly, the atmospheric circulation over East Asia through the western North Pacific showed two similar obvious transitions. The East Asian / Pacific (EAP) pattern switched from high index to low index in the end of the 1950's and from low index to high index in the late 1970's, respectively. Hence, summer rainfall in the Yangtze-Huaihe River valley is closely associated with the EAP pattern not only in the interannual variation but also in the decadal variation.

Barotropic local instability and severe storm process

By means of barotropic model, the characteristic and initial value problems are investigated to reveal the local two-dimensional barotropic instability of the nonuniform current to the dynamic mechanism of the formation of the Yangtze-Huaihe River severe storm in July 1991. Analytical theory and numerical experiment show that (i) the unstable developing modes are chiefly the two periods of about 44 d and 10 d, which are fundamentally consistent with that of the precipitation change of the Yangtze-Huaihe River. (ii) The growth rate of the local perturbation is dominated by the meridional wave numbern = 1–5 and zonal wave numberk = 1–12, i.e. the severe storm over the Yangtze-Huaihe River results from the interaction of the systems at different latitudes and waves of different scales, (iii) The perturbation over the Yangtze-Huaihe River possesses the property of local intensification, which slowly migrates westward over the lower and middle reaches of the Yangtze-Huaihe River. (iv) The growth rate of the instability, especially the propagation velocity of the perturbation, is sensitive to the external parameters ū and α.