Trends in evaporation of a large subtropical lake

In order to further investigate the capability of the Standardized Precipitation Index (SPI) to identify flood/drought events, monthly precipitation data from 26 precipitation stations and monthly discharge data from four hydrological stations from 1960 to 2006 in the Minjiang River basin were used to analyze the correlations between multiple time scales of the SPI and river discharge. The SPI series that had a maximum correlation with discharge was chosen to detect flood/drought events in the basin, and the results were compared to historical flood/drought events. The results indicated the following. (1) High Pearson correlations between the SPI and discharge were identified at shorter time scales (1 to 3 months), and the maximum correlation was found on the time scale of 2 months. (2) Five floods among the six largest historical flood events in the Minjiang River basin were identified with the 2-month SPI, but the SPI does have shortcomings in identifying more general floods. The SPI also identified major drought events that were consistent with historical data. This demonstrates that the 2-month SPI is an effective indicator for the identification of major flood/drought events in the Minjiang River basin.     

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.     

Effects of heterogeneous vegetation on the surface hydrological cycle

Using the three-layer variable infiltration capacity (VIC-3L) hydrological model and the successive interpolation approach (SIA) of climate factors, the authors studied the effect of different land cover types on the surface hydrological cycle. Daily climate data from 1992 to 2001 and remotely-sensed leaf area index (LAI) are used in the model. The model is applied to the Baohe River basin, a subbasin of the Yangtze River basin, China, with an area of 2500 km2. The vegetation cover types in the Baohe River basin consist mostly of the mixed forest type (85%). Comparison of the modeled results with the observed discharge data suggests that: (1) Daily discharges over the period of 1992–2001 simulated with inputs of remotely-sensed land cover data and LAI data can generally produce observed discharge variations, and the modeled annual total discharge agrees with observations with a mean difference of 1.4%. The use of remote sensing images also makes the modeled spatial distributions of evapotranspiration physically meaningful. (2) The relative computing error (RCE) of the annual average discharge is ?24.8% when the homogeneous broadleaf deciduous forestry cover is assumed for the watershed. The error is 21.8% when a homogeneous cropland cover is assumed and ?14.32% when an REDC (Resource and Environment Database of China) land cover map is used. The error is reduced to 1.4% when a remotely-sensed land cover at 1000-m resolution is used.     

Incorporating circulation statistics in bias correction of GCM ensembles: hydrological application for the Rhine basin

An adapted statistical bias correction method is introduced to incorporate circulation-dependence of the model precipitation bias, and its influence on estimated discharges for the Rhine basin is analyzed for a historical period. The bias correction method is tailored to time scales relevant to flooding events in the basin. Large-scale circulation patterns (CPs) are obtained through Maximum Covariance Analysis using reanalysis sea level pressure and high-resolution precipitation observations. A bias correction using these CPs is applied to winter and summer separately, acknowledging the seasonal variability of the circulation regimes in North Europe and their correlation with regional precipitation rates over the Rhine basin. Two different climate model ensemble outputs are explored: ESSENCE and CMIP5. The results of the CP-method are then compared to observations and uncorrected model outputs. Results from a simple bias correction based on a delta factor (NoCP-method) are also used for comparison. For both summer and winter, the CP-method offers a statistically significant improvement of precipitation statistics for subsets of data dominated by particular circulation regimes, demonstrating the circulation-dependence of the precipitation bias. Uncorrected, CP and NoCP corrected model outputs were used as forcing to a hydrological model to simulate river discharges. The CP-method leads to a larger improvement in simulated discharge in the Alpine area in winter than in summer due to a stronger dependence of Rhine precipitation on atmospheric circulation in winter. However, the NoCP-method, in comparison to the CP-method, improves the discharge estimations over the entire Rhine basin.     

Competition of NAO regime changes and increasing greenhouse gases and aerosols with respect to Arctic climate projections

Regional magnitudes and patterns of Arctic winter climate changes in consequence of regime changes of the North Atlantic Oscillation (NAO) are analyzed using a regional atmospheric climate model. The regional model has been driven with data of positive and negative NAO phases from a control simulation as well as from a time-dependent greenhouse gas and aerosol scenario simulation. Both global model simulations include a quite realistic interannual variability of the NAO with pronounced decadal regime changes and no or rather weak long-term NAO trends. The results indicate that the effects of NAO regime changes on Arctic winter temperatures and precipitation are regionally significant over most of northwestern Eurasia and parts of Greenland. In this regard, mean winter temperature variations of up to 6 K may occur over northern Europe. Precipitation and synoptic variability are also regionally modified by NAO regime changes, but not as significantly as temperatures. However, the climate changes associated with the NAO are in some regions clearly stronger than those attributed to enhanced greenhouse gases and aerosols, indicating that projected global changes of the atmospheric composition and internal circulation changes are competing with each other in their importance for the Arctic climate evolution in the near future. The knowledge of the future NAO trend on decadal and longer time scales appears to be vitally important in terms of a regional assessment of climate scenarios for the Arctic.     

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.     

Decadal trends of main Eurasian oscillations and the Eastern Mediterranean precipitation

Summary ?The role of the two main European low-frequency oscillations – the East Atlantic/West Russian (EA/WR) and the North Atlantic Oscillation (NAO), in controlling the precipitation in the Eastern Mediterranean region is investigated based on the NCEP/NCAR reanalysis and the Israeli precipitation data for 1958–1998. The data on the EA/WR and NAO indices, received from the NCEP Climate Prediction Center, are also adapted. Composite mean sea level and precipitation anomaly patterns are constructed and analyzed. In addition to the widely investigated positive NAO trend, another, also positive EA/WR trend characterized atmospheric developments during the period. During NAO positive months, the EA/WR-associated positive SLP anomaly areas were shifted from the east Atlantic to southwest Europe. The areas were shifted to the north during the NAO-negative months and were located over central and northern Europe. This demonstrates that the use of fixed pressure NAO patterns may be not the optimum way to understand climate variability. Analysis of the NAO, EA/WR patterns, as well as that of their decadal trends, demonstrated a relationship between the main European oscillations and the EM precipitation. The results allow explanation of the observed reduction of the north Israeli precipitation by the EA/WR positive trend during the period. Received April 5, 2001; Revised February 14, 2002     

Extreme value analysis in the Danube lower basin discharge time series in the twentieth century

The daily discharge time series in the lower Danube basin (Orsova) have been considered for the 1900–2005 period. The extreme value theory (EVT) is applied for the study of daily discharges incorporating some covariates. Two methods are applied for fitting the data to an extreme value distribution: block maxima and peaks over thresholds (POT). Using the block maxima approach associated with the use of the generalised extreme value (GEV) distribution, monthly and seasonal maxima of daily discharge for 1900–2005 have been analysed. Separately the monthly maxima of daily discharge for the 1958–2001 was analysed in order to be compatible with atmospheric circulation available from ERA-40. For performing parameter estimation, the maximum likelihood estimation (MLE) method was used. From the three possible types of GEV distribution, a Weibull distribution fits both the monthly and seasonal maxima of the daily discharges very well. The North Atlantic Oscillation (NAO) and the first ten principal components (PC) of the decomposition in multi-variate empirical orthogonal functions (MEOF) of three atmospheric fields (sea level pressure, 500 hPa and 500–1000 hPa thickness) over the Atlantic-European region (ERA-40), have been introduced as covariates. An improvement over the model without the covariate is found by incorporating NAO as the covariate in location parameter, especially for the spring maxima having the NAO as predictor during the winter. Related to atmospheric circulation influence, the most significant results are obtained by incorporating the first 10 PCs of the MEOF in the location parameter of GEV distribution within a month before the month of the discharge level. Regarding the POT approach associated with generalised Pareto distribution (GPD), different thresholds have been tested for daily discharges in the period 1900–2005, where the maxima were fitted by a bounded (or beta) distribution.     

作物生长对流域水文过程与区域气候的影响

利用区域气候模式RegCM3以及考虑作物生长过程的耦合模式RegCM3_CERES对东亚区域进行20年模拟,研究作物生长对流域水文过程与区域气候的影响。结果表明：考虑作物生长过程的耦合模式模拟海河流域、松花江流域、珠江流域多年平均降水效果明显改进,在除黑河流域外的各流域模拟的温度负偏差有所减小,其中在海河流域、淮河流域的夏季改进尤为明显。各流域夏季（6、7、8月）月蒸散量最高,其中长江流域、海河流域、淮河流域、珠江流域的夏季月蒸散量基本上在100 mm左右,并且七大流域蒸散发的季节变化趋势跟总降水基本一致。多数流域考虑作物生长过程的耦合模式模拟得出蒸散发减少且进入的水汽增加,导致局地水循环率减小;黑河流域与黄河流域降水有所增加,其他流域均有不同程度的减小。针对长江流域,比较耦合模式RegCM3_CERES与模式RegCM3模拟结果显示,叶面积指数减少1.20 m²/m²,根区土壤湿度增加0.01 m³/m³,进而导致潜热通量下降1.34 W/m²（其中在四川盆地地区减少16.00 W/m²左右）,感热通量增加2.04 W/m²,从而影响到降水和气温。     

Impact of the North Atlantic Oscillation on Middle Eastern Climate and Streamflow

Interannual to decadal variations in Middle Eastern temperature, precipitationand streamflow reflect the far-field influence of the North Atlantic Oscillation (NAO), a dominant mode of Atlantic sector climate variability. Using a new sea surface temperature (SST) based index of the NAO and availablestreamflow data from five Middle Eastern rivers, we show that the first principal component of December through March streamflow variability reflects changes in the NAO. However, Middle East rivers have two primary flooding periods.The first is rainfall-driven runoff from December through March, regulated on interannual to decadal timescales by the NAO as reflected in local precipitation and temperature.The second period, from April through June, reflects spring snowmelt and contributes in excess of 50% of annual runoff.This period, known locally as the khamsin, displays no significantNAO connections and a less direct relationship with local climatic factors, suggesting that streamflow variability during this period reflects land-cover change, possibly related to agriculture and hydropower generation, and snowmelt.     

索马里越赤道气流与淮河流域夏季降水关系的年代际增强

基于1961~2016年淮河流域172个测站逐日降水资料,分析了淮河夏季降水的多时间尺度变率,发现其具有显著的准两年周期,1990年代以来亦呈现出明显的年代际变化。EOF分析结果显示,淮河夏季降水的异常主要表现为全流域一致偏多或偏少型（第一模态）,其方差贡献高达37%。相比于长江中下游地区,淮河夏季降水与东亚500 hPa位势高度场上的EAP遥相关型关系很弱,但和对流层低层西南水汽输送有更好的对应关系,表现为从索马里半岛至淮河流域的多个正负交替的相关波列。这一波列表明当索马里越赤道气流加强时,通过热带印度洋西风的纽带作用加强了进入淮河流域的西南暖湿气流,并在淮河上空低层形成水汽辐合,造成淮河多雨,反之当索马里越赤道气流强度弱时,淮河夏季降水偏少。索马里越赤道气流和中国台站降水的空间相关同样显示出最显著的相关区位于淮河流域。进一步的分析发现,研究时段内二者关系并非稳定维持,而是表现出显著的年代际变化,近二十年来索马里越赤道气流对淮河流域夏季降水的影响明显增强,且在季节预测上的指示意义也在增强。这一增强的可能原因是索马里越赤道气流与流域南侧的经向水汽输送和西侧的纬向水汽输送的关系均发生了年代际反转,并且这两条水汽输送带对流域夏季降水的影响发生了年代际增强。     

Decadal climate variability in the Mediterranean region: roles of large-scale forcings and regional processes

We analyze decadal climate variability in the Mediterranean region using observational datasets over the period 1850–2009 and a regional climate model simulation for the period 1960–2000, focusing in particular on the winter (DJF) and summer (JJA) seasons. Our results show that decadal variability associated with the winter and summer manifestations of the North Atlantic Oscillation (NAO and SNAO respectively) and the Atlantic Multidecadal Oscillation (AMO) significantly contribute to decadal climate anomalies over the Mediterranean region during these seasons. Over 30% of decadal variance in DJF and JJA precipitation in parts of the Mediterranean region can be explained by NAO and SNAO variability respectively. During JJA, the AMO explains over 30% of regional surface air temperature anomalies and Mediterranean Sea surface temperature anomalies, with significant influence also in the transition seasons. In DJF, only Mediterranean SST still significantly correlates with the AMO while regional surface air temperature does not. Also, there is no significant NAO influence on decadal Mediterranean surface air temperature anomalies during this season. A simulation with the PROTHEUS regional ocean–atmosphere coupled model is utilized to investigate processes determining regional decadal changes during the 1960–2000 period, specifically the wetter and cooler 1971–1985 conditions versus the drier and warmer 1986–2000 conditions. The simulation successfully captures the essence of observed decadal changes. Model set-up suggests that AMO variability is transmitted to the Mediterranean/European region and the Mediterranean Sea via atmospheric processes. Regional feedbacks involving cloud cover and soil moisture changes also appear to contribute to observed changes. If confirmed, the linkage between Mediterranean temperatures and the AMO may imply a certain degree of regional decadal climate predictability. The AMO and other decadal influences outlined here should be considered along with those from long-term increases in greenhouse gas forcings when making regional climate out-looks for the Mediterranean 10–20?years out.     

基于SPEI的1960—2018年赣江流域干旱特征

基于赣江流域39个气象站点逐月降水和气温数据,计算不同时间尺度标准化降水蒸散发指数(SPEI),采用Mann-Kendall突变检验、主成分分析(PCA)等方法,分析了赣江流域1960—2018年干旱时空变化特征.研究表明:不同时间尺度SPEI均有微弱升高的趋势,干旱形势有所缓解,SPEI能够较好地表征赣江流域旱涝情况.赣江流域中部的轻旱和特旱发生频率要高于其他地区,中旱主要高发地区主要分布在南部和西部区域,重旱主要集中在东部和北部.赣江流域干旱的空间分布具有较好的一致性,旱涝变化整体保持一致,南部与北部旱涝状态存在相反的纵向差异,且中部与南部、北部旱状况涝存在空间差异.     

Interannual and long-term variability in the North Atlantic Oscillation and Indian Summer monsoon rainfall

Summary The interannual and decadal scale variability in the North Atlantic Oscillation (NAO) and its relationship with Indian Summer monsoon rainfall has been investigated using 108 years (1881–1988) of data. The analysis is carried out for two homogeneous regions in India, (Peninsular India and Northwest India) and the whole of India. The analysis reveals that the NAO of the preceding year in January has a statistically significant inverse relationship with the summer monsoon rainfall for the whole of India and Peninsular India, but not with the rainfall of Northwest India. The decadal scale analysis reveals that the NAO during winter (December–January–February) and spring (March–April–May) has a statistically significant inverse relationship with the summer monsoon rainfall of Northwest India, Peninsular India and the whole of India. The highest correlation is observed with the winter NAO. The NAO and Northwest India rainfall relationship is stronger than that for the Peninsular and whole of India rainfall on climatological and sub-climatological scales.Trend analysis of summer monsoon rainfall over the three regions has also been carried out. From the early 1930s the Peninsular India and whole of India rainfall show a significant decreasing trend (1% level) whereas the Northwest India rainfall shows an increasing trend from 1896 onwards.Interestingly, the NAO on both climatological and subclimatological scales during winter, reveals periods of trends very similar to that of Northwest Indian summer monsoon rainfall but with opposite phases.The decadal scale variability in ridge position at 500 hPa over India in April at 75° E (an important parameter used for the long-range forecast of monsoon) and NAO is also investigated.With 4 Figures     

Variations of winter precipitation over southeastern China in association with the North Atlantic Oscillation

The synoptic-scale winter precipitation variations over southeastern China (22°–32°N, 105°–125°E) and their association with the North Atlantic Oscillation (NAO) during 1951–2007 are investigated in this paper. The variability of wintertime precipitation is characterized by meridional displacement of its maximum center. Two precipitation regimes, with maximum centers located over the Yangtze and Pearl River basins, are identified via cluster analysis. Time-lagged analyses suggest that the two precipitation regimes are connected with the decaying phases of positive NAO (NAO+) events of different amplitudes. A strong (medium) NAO+ event is defined as one when the maximum amplitude of the NAO index exceeds 1.0 (in the range of 0.7–1.0) for at least 4 consecutive days and drops to less than 0.3 within 7 days following the peak index. After the peak of a strong NAO+, southerly winds expand northward to the Yangtze River (about 30°N), a northeast–southwest-tilted trough migrates to east of Lake Baikal, and cold air intrudes into central eastern China; thus, precipitation is strengthened over the Yangtze River basin where warm and cold air masses converge. In comparison, during the decaying phase of medium NAO+ events, the southerly winds are relatively weak, and precipitation tends to be enhanced at lower latitudes (around 25°N). Further analysis indicates that downstream Rossby-wave propagation may account for the latitudinal expansion of the southerly wind anomalies over the eastern coastal area of China during the decaying phase of NAO+ events of different strengths.     

THE INTERANNUAL AND DECADAL VARIABILITY OF PRECIPITATION FOR YUNNAN PROVINCE IN RAINY SEASON AND ITS RELATIONSHIP WITH TROPICAL UPPER LAYER HEAT CONTENT

Based on the monthly precipitation data of 126 observation stations from 1961 to 2000 in Yunnan Province, the interannual and decadal variability of precipitation in rainy seasons are studied by using wavelet analysis. It is shownthat there is a 2 - 6 year oscillation at the interannual time scales and a quasi-30 year oscillation at the decadal time scales. These periodic oscillations relate to the distribution of tropical heat content. When the precipitation is much more (less) than normal, the upper seawater is colder (warmer) in almost all the tropical Indian Ocean, and warmer (colder) in the western Pacific as well ascolder (warmer) in the eastern Pacific. The key areas of the anomaly heat content distribution that have significant correlation to the Yunnan precipitation inrainy season are in the southern hemispheric Indian Ocean with a dipole patternin the winter as well as in the deep basin of the South China Sea (SCS) before the Yunnan rainy season begins. Therefore, the anomalous distributions of the heat content in the southern Indian Ocean and the SCS in winter are good indicators for predicting drought or flood in Yunnan Province in the following rainy season.     

On the interannual variability of precipitation and runoff in the Mackenzie drainage basin

An analysis is presented of extratropical cyclone frequency, the 500 hPa height standard deviation field and the monthly 700 hPa height field in the Northern Hemisphere, together with precipitation in the Mackenzie basin and the Mackenzie River runoff. Spatial and temporal variability in the data are examined for the period 1965 to 1989, and a cross-correlation analyses is performed to determine the relationship between the runoff and the precipitation variations, and between the precipitation and the atmospheric circulation anomalies. It is found that precipitation fluctuations in the Mackenzie River drainage basin are strongly linked to variations in the Mackenzie River runoff and in the North Pacific storm tracks, with the time scale of variability ranging from interannual to decadal. The results are discussed in relation to the interdecadal Arctic climate cycle proposed by Mysak, Manak and Marsden, and revised by Mysak and Power. In particular, the latter authors hypothesized that, as part of this cycle, air-sea interactions and synoptic scale processes over the northwestern North Atlantic influenced, via cyclone movements in the Labrador Sea and Baffin Bay, precipitation in northern Canada and hence river runoff into the Arctic. The results of this study indicate that such influences on the precipitation in the Mackenzie basin are small, and hence that the Mysak-Power feedback loop which describes this climate cycle needs further revision.     

Numerical Study of the Impacts of Urban Expansion on Meiyu Precipitation over Eastern China

The Yangtze River Delta(YRD) has experienced significant urban expansion in recent years, while the Meiyu belt of China has demonstrated a decadal northward shifting trend. Thus, it is of interest to assess how urban expansion affects Meiyu precipitation and hopefully to reveal the underlying physical mechanisms involved. In this study, the urban extents over the YRD in 2001 and 2010 are derived based on land use/land cover(LULC) category data and nighttime light image data. Two parallel groups of10-summer(2001–2010) numerical simulations are carried out with the urban extents over the YRD in2001 and 2010, respectively. The results show that the urban expansion in the YRD tends to result in increased(decreased) Meiyu precipitation over the Huaihe River(Yangtze River) basin with intensities of0.2–1.2 mm day-1. Further analysis indicates that the spatiotemporal pattern of the Meiyu precipitation change induced by the urban expansion resembles the third empirical orthogonal function(EOF) mode of the observed Meiyu precipitation. Analyses of the possible underlying physical mechanisms reveal that urban expansion in the YRD leads to changes in the surface energy balance and warming(cooling) of tropospheric(stratospheric) air temperature over eastern China. Anomalous upward(downward) motion and moisture convergence(divergence) over the Huaihe River(Yangtze River) basin occur, corresponding to the increases(decreases) of the Meiyu precipitation over the Huaihe River(Yangtze River) basin.     

长江流域径流对全球升温1.5℃与2.0℃的响应

全球变暖影响着以流域径流要素为主导的水文水资源系统的变化。长江流域未来水资源量的时空分布对长江大保护与长江经济带的发展意义重大。为探究全球升温1.5℃和2.0℃对长江流域径流变化的影响,使用基于偏差校正的气候模式集合数据驱动两参数月水量平衡模型,比较两种升温情景下径流量的响应差异。结果表明：基于偏差校正的气候模式集合数据可以较好地代表长江流域历史时期（1976—2005年）的年平均降水和年平均蒸散发情势。两参数月水量平衡模型与参数区域化方法相结合能较好地模拟长江流域各子流域的月径流量。升温1.5℃时,无论是年径流量还是季节径流量均呈上升趋势,与历史时期相比,50%以上三级子流域的增幅超过5%;升温2.0℃时,增幅超过8%。这表明升温2.0℃情景下长江流域水资源量将进一步增加。相对于历史时期,升温1.5℃与2.0℃情景下长江流域北部降水量增幅较大;径流量增幅分布格局基本与降水量一致。汉江流域是全流域径流量增幅最显著的区域。     

THE INTERANNUAL AND DECADAL VARIABILITY OF PRECIPITATION FOR YUNNAN PROVINCE IN RAINY SEASON AND ITS RELATIONSHIP WITH TROPICAL UPPER LAYER TEAT CONTENT

~~THE INTERANNUAL AND DECADAL VARIABILITY OF PRECIPITATION FOR YUNNAN PROVINCE IN RAINY SEASON AND ITS RELATIONSHIP WITH TROPICAL UPPER LAYER TEAT CONTENT@郑春怡$Meteorological Observatory of Yunnan Province, Kunming 650034 China @黄菲$Department of Marine Meteorology, Ocean University of China, Qingdao 266003 China @普贵明$Meteorological Observatory of Yunnan Province, Kunming 650034 China~~…