Characteristics and spatio-temporal variability of the Amazon River Basin Water Budget

The spatio-temporal variations of the water budget components in the Amazon region are investigated by using a combination of hydrometeorological observations and moisture fluxes derived from the NCEP/NCAR reanalyses, for the period 1970–1999. The key new finding of this study identifies the major differences in the water balance characteristics and variability between the northern and southern parts of the basin. Our results show that there is a seasonality and interannual variability of the water balance that varies across the basin. At interannual time scales, anomalies in the water balance components in the northern Amazon region show relatively stronger links with tropical Pacific interannual variability. Over the entire region, precipitation exceeds evaporation and the basin acts as a sink of moisture (P>E). However, on some occasions the basin can act as a source for moisture (P<E) under extreme conditions, such as those related to deficient rainfall in northern Amazonia during the strong El Niño of 1983. Our estimates of the Amazon regions water balance do not show a closure of the budget, with an average imbalance of almost 50%, suggesting that some of the moisture that converges in the Amazon region is not accounted for. The imbalance is larger over the southern Amazon region than over the northern region, and it also exhibits interannual variability. Large uncertainties are detected in the evaporation and moisture-convergence fields derived from the reanalyses, and in the case of evaporation it can be as large as 10–20% when compared with the few field observations across the basin. Observed precipitation fields derived from station data and from grid-box products also show some discrepancies due to sampling problems and interpolation techniques. The streamflow observed at the mouth of the river is obtained after corrections on the series observed taken at a gauging site almost 200 km inland. However, variability in the evaporation, moisture convergence, and observed rainfall and runoff matches quite well.     

Observational and modelling studies of Amazonia interannual climate variability

The interannual variability of climate in the Amazon basin is studied using precipitation and river level anomalies observed near the March/April rainy season peak for the period 1980–86, supported by satellite imagery of tropical convection. Evaluation of this data in conjunction with the corresponding circulation and sea-surface temperature (SST) anomaly patterns indicates that abundant rainy seasons in Northern Amazonia are characterized by anomalously cold surface waters in the tropical eastern Pacific, and negative/positive SST anomalies in the tropical North/South Atlantic, accelerated Northeast trades and a southward displaced Intertropical Convergence Zone (ITCZ) over the Atlantic sector. Years with deficient rainfall show broadly opposite patterns.General circulation model (GCM) experiments using observed SST in three case studies were aimed at testing the teleconnections between SST and Amazon climate implied by the empirical analysis. The GCM-generated surface fields resemble the corresponding observers fields most closely over the tropical Pacific and, with one exception, over the tropical Atlantic as well. The modeled precipitation features, along the Northwest coast of South America, anomalies of opposite sign to the North and South of the equator, in agreement with observations and results from a different GCM. Similarities in simulations run from different initial conditions, but using the same global SST, indicate broad consistency in response to common boundary forcing.     

Modelling the dominant climate signals around southern Africa

In this study, statistical techniques are employed to decompose climate signals around southern Africa into the dominant temporal frequencies, with the aim of modelling and predicting area-averaged rainfall. In the rainfall time series over the period 1900–1999, the annual cycle accounts for 83% of variance. Residual spectral energy cascades from biennial (42%) to interannual (20%) to decadal bands (3%). Regional climate signals are revealed through a multi-taper singular value decomposition analysis of sea surface temperature and sea level pressure fields over the Atlantic and Indian Oceans, in conjunction with southern Africa rainfall. Rossby wave action in the South Indian Ocean dominates the biennial scale variability. El Niño-Southern Oscillation (ENSO) and related Indian Ocean dipole patterns are important for interannual variability. Significant sea temperature and pressure fluctuations occurring 6–12 months prior to rainfall contribute biennial and interannual indices to a multi-variate model that demonstrates useful predictive skill.     

Extreme climatic events in the Amazon basin

During 2009 the Amazon basin was hit by a heavy flooding with a magnitude and duration few times observed in several decades. Torrential rain in northern and eastern Amazonia during the austral summer of 2008–2009 swelled the Amazon River and its tributaries. By July 2009, water levels of the Rio Negro, a major Amazon tributary, reached at Manaus harbor a new record, the highest mark of the last 107?years. During the 2008–2009 hydrological year, the rainy season on northern and northwestern Amazonia started prematurely, and was followed by a longer-than-normal rainy season. An anomalously southward migration of the ITCZ during May–June 2009, due to the warmer than normal surface waters in the tropical South Atlantic, was responsible for abundant rainfall in large regions of eastern Amazonia and Northeast Brazil from May to July 2009. We also compared the flood of 2009 with other major events recorded in 1989 and 1999. The hydrological consequences of this pattern were earlier than normal floods in Amazon northern tributaries, which peak discharges at their confluences with the main stem almost coincided with the peaks of southern tributaries. Since the time displacement of the contribution to the main stem of northern and southern Amazon tributaries is fundamental for damping flood waves in the main stem, the simultaneous combinations of peak discharges of tributaries resulted in an extreme flood.     

Simulation of rainfall anomalies leading to the 2005 drought in Amazonia using the CLARIS LPB regional climate models

The meteorological characteristics of the drought of 2005 in Amazonia, one of the most severe in the last 100 years were assessed using a suite of seven regional models obtained from the CLARIS LPB project. The models were forced with the ERA-Interim reanalyses as boundary conditions. We used a combination of rainfall and temperature observations and the low-level circulation and evaporation fields from the reanalyses to determine the climatic and meteorological characteristics of this particular drought. The models reproduce in some degree the observed annual cycle of precipitation and the geographical distribution of negative rainfall anomalies during the summer months of 2005. With respect to the evolution of rainfall during 2004–2006, some of the models were able to simulate the negative rainfall departures during early summer of 2005 (December 2004 to February 2005). The interannual variability of rainfall anomalies for both austral summer and fall over northern and southern Amazonia show a large spread among models, with some of them capable of reproducing the 2005 observed negative rainfall departures (four out of seven models in southern Amazonia during DJF). In comparison, all models simulated the observed southern Amazonia negative rainfall and positive air temperature anomalies during the El Nino-related drought in 1998. The spatial structure of the simulated rainfall and temperature anomalies in DJF and MAM 2005 shows biases that are different among models. While some models simulated the observed negative rainfall anomalies over parts of western and southern Amazonia during DJF, others simulated positive rainfall departures over central Amazonia. The simulated circulation patterns indicate a weaker northeasterly flow from the tropical North Atlantic into Amazonia, and reduced flows from southern Amazonia into the La Plata basin in DJF, which is consistent with observations. In general, we can say that in some degree the regional models are able to capture the response to the forcing from the tropical Atlantic during the drought of 2005 in Amazonia. Moreover, extreme climatic conditions in response to anomalous low-level circulation features are also well captured, since the boundary conditions come from reanalysis and the models are largely constrained by the information provided at the boundaries. The analysis of the 2005 drought suggests that when the forcing leading to extreme anomalous conditions is associated with both local and non-local mechanisms (soil moisture feedbacks and remote SST anomalies, respectively) the models are not fully capable of representing these feedbacks and hence, the associated anomalies. The reason may be a deficient reproduction of the land–atmosphere interactions.     

SST variability in the South Indian Ocean and associated circulation and rainfall patterns over Southern Africa

Summary A general circulation model is used to study the response of the atmosphere to an idealised sea surface temperature (SST) anomaly pattern (warm throughout the southern midlatitudes, cool in the tropics) in the South Indian Ocean region. The anomaly imposed on monthly SST climatology captures the essence of patterns observed in the South Indian Ocean during both ENSO events and multidecadal epochs, and facilitates diagnosis of the model response. A previous study with this anomaly imposed in the model examined differences in the response between that on the seasonal scale (favours enhancement of the original SST anomaly) and that on the decadal scale (favours damping of the anomaly). The current study extends that work firstly by comparing the response on the intraseasonal, seasonal and interannual scales, and secondly, by assessing the changes in the circulation and rainfall over the adjoining African landmass.It is found that the atmospheric response is favourable for enhancement of the original SST anomaly on scales up to, and including, annual. However, as the scale becomes interannual (i.e., 15–21 months after imposition of the anomaly), the model response suggests that damping of the original SST anomaly becomes likely. Compared to the shorter scale response, the perturbation pressure and wind distribution on the interannual scale is shifted poleward, and is more reminiscent of the decadal response. Winds are now stronger over the warm anomaly in the southern midlatitudes suggesting enhanced surface fluxes, upper ocean mixing, and consequently, a damping of the anomaly.Examination of the circulation and rainfall patterns indicates that there are significant anomalies over large parts of southern Africa during the spring, summer and autumn seasons for both short (intraseasonal to interannual) and decadal scales. It appears that rainfall anomalies are associated with changes in the advection of moist tropical air from the Indian Ocean and its related convergence over southern Africa. Over eastern equatorial Africa, the austral autumn season (the main wet season) showed rainfall increases on all time scales, while parts of central to eastern subtropical southern Africa were dry. The signals during summer were more varied. Spring showed generally dry conditions over the eastern half of southern Africa on both short and decadal time scales, with wet areas confined to the west. In all cases, the magnitude of the rainfall anomalies accumulated over a 90 day season were of the order of 90–180 mm, and therefore represent a significant fraction of the annual total of many areas. It appears that relatively modest SST anomalies in the South Indian Ocean can lead to sizeable rainfall anomalies in the model. Although precipitation in general circulation models tends to be less accurately simulated than many other variables, the model results, together with previous observational work, emphasize the need for ongoing monitoring of SST in this region.With 14 Figures     

The distribution of rainy days in the Aegean area

Summary The object of this study is the determination of the number of rainy days in the area of the Aegean sea, based on data obtained from 14 observation stations during the period 1950–1975.The results showed considerable differences from North to South. The seasonal number of rainy days as well as that of rainy spells were examined for the period 1950–1975. The spells of rainy days play an important role in the agricultural activities, especially over the southern part of the Aegean sea basin, where the annual amounts of rainfall are insufficient.From the data was concluded that during the summer, more than four consecutive rainy days were recorded at the northern most stations only and these of infrequent occurrence; whereas during the winter, it was possible to encounter up to 15 consecutive rainy days.Finally, we give a theoretical distribution for the rainy spells for each station and for each season and year, using Polya's method. We found that this distribution fitted well the 95% of the confidence level in the majority of the cases.With 2 Figures     

An Atlantic influence on Amazon rainfall

Rainfall variability over the Amazon basin has often been linked to variations in Pacific sea surface temperature (SST), and in particular, to the El Niño/Southern Oscillation (ENSO). However, only a fraction of Amazon rainfall variability can be explained by ENSO. Building upon the recent work of Zeng (Environ Res Lett 3:014002, 2008), here we provide further evidence for an influence on Amazon rainfall from the tropical Atlantic Ocean. The strength of the North Atlantic influence is found to be comparable to the better-known Pacific ENSO connection. The tropical South Atlantic Ocean also shows some influence during the wet-to-dry season transition period. The Atlantic influence is through changes in the north-south divergent circulation and the movement of the ITCZ following warm SST. Therefore, it is strongest in the southern part of the Amazon basin during the Amazon’s dry season (July–October). In contrast, the ENSO related teleconnection is through anomalous east-west Walker circulation with largely concentrated in the eastern (lower) Amazon. This ENSO connection is seasonally locked to boreal winter. A complication due to the influence of ENSO on Atlantic SST causes an apparent North Atlantic SST lag of Amazon rainfall. Removing ENSO from North Atlantic SST via linear regression resolves this causality problem in that the residual Atlantic variability correlates well and is in phase with the Amazon rainfall. A strong Atlantic influence during boreal summer and autumn is particularly significant in terms of the impact on the hydro-ecosystem which is most vulnerable during the dry season, as highlighted by the severe 2005 Amazon drought. Such findings have implications for both seasonal-interannual climate prediction and understanding the longer-term changes of the Amazon rainforest.     

New evidence for a glacioeustatic influence on deep water circulation, bottom water ventilation and primary productivity in the South China Sea

We provide new evidence for the development of a stable estuarine circulation characterized by stagnating water bodies, nutrient recycling and increased primary productivity in the South China Sea (SCS) during glacial intervals. This circulation was caused by the closure of the shallow and narrow straits connecting the SCS in the south and west. Our main evidence is derived from newly measured Mn concentrations and Mn/Al ratios in two sediment cores from the northern and southeastern SCS covering the last 500 ky. Concentrations and Mn/Al ratios of the redox sensitive element Mn show clear glacial–interglacial cycles with maxima during interglacial periods and minima during glacial periods. These cycles indicate ventilation cycles of the bottom water, and are connected to the glacial–interglacial changes in sea level. In contrast, total organic carbon (TOC) concentrations display an opposite pattern with pronounced maxima during glacial times, especially in the southern part of the basin. The variations in TOC can be ascribed to two factors. Firstly, variations in primary productivity are controlled by variations in the intensity of the winter monsoon. Secondly to the degree of preservation of TOC controlled by variations in ventilation, which in turn is ultimately controlled by sea level. Consequently, variations in TOC represent a superimposition of primarily sea level influenced preservation control and winter monsoon driven variations in primary productivity intensity. The decrease in Mn correspond to times when sea level dropped 40–60 m below the present level. The larger amplitude of the variations in TOC and Mn in the southern part of the basin compared to the northern site suggest that oxygen depletion and nutrient recycling was stronger in the parts of the basin situated the furthest away from the only remaining opening to the open Pacific, the Luzon Strait.     

Observed and GCM simulated decadal variability of monsoon rainfall in east China

Variability and associated mechanisms of summer rainfall over east China are identified and described using both observations and a general circulation model (GCM) simulation. The observations include two data sets: the 90-station, 1470–1988 annual drought/flood index and the 60-station, 1889–1988 monthly mean precipitation measurements. The GCM data set is a 100-year equilibrium simulation of the present climate. Spectra of the drought/flood index indicate decadal cycles which decrease from north (47 y) to south (21 y). Correlation coefficients show decadal variability in the relationship between index values along the Yangtse River valley and those over northeast and southeast China. Analysis of the measured data confirms this result; for example, the correlation was small during 1889–1918, but significantly negative during 1930–1959. When compared with precipitation measurements, the GCM better simulates monthly means and variances along the Yangtse River valley. Three distinct 30-year periods of interannual variability in summer rainfall are found over this area. During each period, rainfall is negatively correlated with spring surface temperature over a remote region and is identified with variations in a specific component of the east Asian monsoon circulation: (1) when Eurasian temperatures decrease, the thermal contrast across the Mei-Yu front increases and frontal rainfall intensities; (2) lower temperatures over the Sea of Japan/northwest Pacific Ocean are identified with enhanced easterly flow, moisture transport and rainfall; (3) when tropical east Pacific Ocean temperatures decrease, rainfall associated with the low latitude monsoon trough increases. Given that the GCM generates decadal changes in the relationship between the physical mechanisms, the east Asian monsoon and planetary general circulations and east China rainfall, future studies should focus on the predictability of these changes with the use of improved and much longer GCM simulations.     

Assessment of regional seasonal rainfall predictability using the CPTEC/COLA atmospheric GCM

This is a study of the annual and interannual variability of regional rainfall produced by the Center for Weather Forecasts and Climate Studies/Center for Ocean, Land and Atmospheric Studies (CPTEC/COLA) atmospheric global climate model. An evaluation is made of a 9-member ensemble run of the model forced by observed global sea surface temperature (SST) anomalies for the 10-year period 1982–1991. The Brier skill score and, Relative Operating Characteristics (ROC) are used to assess the predictability of rainfall and to validate rainfall simulations, in several regions world wide. In general, the annual cycle of precipitation is well simulated by the model for several continental and oceanic regions in the tropics and mid latitudes. Interannual variability of rainfall during the peak rainy season is realistically simulated in Northeast Brazil, Amazonia, central Chile, and southern Argentina–Uruguay, Eastern Africa, and tropical Pacific regions, where the model shows good skill. Some regions, such as northwest Peru–Ecuador, and southern Brazil exhibit a realistic simulation of rainfall anomalies associated with extreme El Niño warming conditions, while in years with neutral or La Niña conditions, the agreement between observed and simulated rainfall anomalies is not always present. In the monsoon regions of the world and in southern Africa, even though the model reproduces the annual cycle of rainfall, the skill of the model is low for the simulation of the interannual variability. This is indicative of mechanisms other than the external SST forcing, such as the effect of land–surface moisture and snow feedbacks or the representation of sub-grid scale processes, indicating the important role of factors other than external boundary forcing. The model captures the well-known signatures of rainfall anomalies of El Niño in 1982–83 and 1986–87, indicating its sensitivity to strong external forcing. In normal years, internal climate variability can affect the predictability of climate in some regions, especially in monsoon areas of the world.     

Characteristics of rainfall over Brazil: Annual variations and connections with the Southern Oscillation

Summary Annual and interannual variations of rainfall over Brazil are discussed. First, rainy and dry seasons for several stations of Brazil are determined using the data of 21 years (1958–1978). The progressive movement of the Intertropical Convergence Zone seems to be associated with the progresive variation of rainfall seasons in the equatorial eastern Brazil. The annual migration of deep tropical convection from Central and Southern Portion of the Amazon basin in austral summer to the northwestern sector of South America in austral winter seems to be responsible for the annual cycle of rainfall in the Amazon basin. The conncection between the interannual variation of rainfall over Brazil and the Southern Oscillation is also discussed. The correlation coefficient between the Southern Oscillation index and the rainfall is generally small over most of Brazil except over Rio Grande do Sul. The correlation between the spring rainfall of Rio Grande do Sul and the Southern Oscillation index of the same or of the previous season is significantly high and shows prospects for seasonal rainfall prediction.With 5 Figures     

El Ni?o对东亚夏季风和夏季降水季节内变化的影响

基于1979~2012年候平均再分析资料,合成分析了El Ni?o对东亚夏季风和夏季降水季节内变化的影响。结果表明,在El Ni?o衰减年夏季,西太平洋副热带高压(副高)明显偏强,位置偏向西南。副高的这种异常特征随夏季的季节进程有明显变化,初夏异常较弱,盛夏期间异常达到最强。此外,根据东亚夏季风降水呈现阶段式北进的特征,将夏季分为华南前汛期、江淮梅雨期、华北和东北雨期以及华南后汛期来分析东亚夏季风和降水的季节内变化。在上述各个时期,大气对流层低层表现为一致的环流异常型,副高及其以南区域为异常反气旋,其北部为异常气旋。这种异常环流型加强了副高南部偏东风及其北部偏北风,增强了热带水汽输送和高纬度地区冷空气的入侵,二者结合造成主汛期地区降水增加。需要强调的是,上述环流异常型随东亚夏季风逐步向北推移,导致东亚各地区的主汛期降水增加,非主汛期降水减少,降水分布更为集中。     

CMIP5模式对中国东部夏季不同强度降水气候态和年代际变化的模拟能力评估

利用东亚地区逐日降水资料,评估了17个CMIP5气候模式对中国东部夏季不同强度降水的时空分布、不同强度降水对1970年代末中国东部夏季总降水量年代际转折的贡献的模拟能力。从夏季不同强度降水占总降水的比重来看,在中国东北和华北地区,小雨和中雨占主导;而在华南和江淮地区,大雨和暴雨则相对更为重要。CMIP5模式可大致模拟出中国东部小雨、大雨和暴雨占总降水比重的空间分布,但对中雨占比的空间分布模拟较差。总体说来,多数CMIP5模式高估了小雨和中雨的比重,但低估了大雨和暴雨的比重,从而导致大多数模式高估东北和华北的总降水量,而低估华南和江淮的总降水量。对1970年代末我国华北和江淮地区夏季降水量的年代际转折,观测资料表明该转折主要体现为大雨和暴雨雨量的年代际转折;仅有少数CMIP5模式能模拟出华北大雨和暴雨年代际减少的特征,使得这些模式对华北地区总降水的年代际变化也有较好的模拟能力。对于江淮区域,由于大雨和暴雨的比重被严重低估,尽管部分模式能模拟出夏季总降水量年代际增加的特征,但却多以小雨、中雨的年代际变化为主。多模式集合并不能显著提高模式对不同强度降水的空间分布的模拟能力,尤其是降水年代际变化的模拟能力。     

Revisiting wintertime cold air intrusions at the east of the Andes: propagating features from subtropical Argentina to Peruvian Amazon and relationship with large-scale circulation patterns

This study investigates the spatial and temporal characteristics of cold surges that propagates northward along the eastern flank of the Andes from subtropical to tropical South America analysing wintertime in situ daily minimum temperature observations from Argentina, Bolivia and Peru and ERA-40 reanalysis over the 1975–2001 period. Cold surges usually last 2 or 3 days but are generally less persistent in the southern La Plata basin compared to tropical regions. On average, three to four cold surges are reported each year. Our analysis reveals that 52 % of cold episodes registered in the south of La Plata basin propagate northward to the northern Peruvian Amazon at a speed of around 20 m s^?1. In comparison to cold surges that do not reach the tropical region, we demonstrate that these cold surges are characterized, before they reach the tropical region, by a higher occurrence of a specific circulation pattern associated to southern low-level winds progression toward low latitudes combined with subsidence and dry condition in the middle and low troposphere that reinforce the cold episode through a radiative effect. Finally, the relationship between cold surges and atmosphere dynamics is illustrated for the two most severe cold intrusions that reached the Peruvian and Bolivian Amazon in the last 20 years.     

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     

Summer Sahel-ENSO teleconnection and decadal time scale SST variations

The correlation between Sahel rainfall and El Niño–Southern Oscillation (ENSO) in the northern summer has been varying for the last fifty years. We propose that the existence of periods of weak or strong relationship could result from an interaction with the global decadal scale sea surface temperature (SST) background. The main modes of SST variability have been extracted through a principal component analysis with Varimax rotation. The correlations between a July-September Sahel rainfall index and these SST modes have been computed on a 20-year running window between 1945 and 1993. The correlations with the interannual ENSO-SST mode are negative, not significant in the 1960s during the transition period from the wet climate phasis to the long-running drought in the Sahel, but then were significant since 1976. During the former period, the correlations between the Sahel rainfall index and the other SST modes (expressing mostly on quasi and multi-decadal scales) are the highest, in particular correlations with the tropical Atlantic “dipole”. Correlations between Sahel and Guinea Coast rainfall are also significantly negative. After 1970, the Sahel-Guinea Coast rainfall correlations are no longer significant, and the ENSO-SST mode becomes the only one significantly correlated with Sahel rainfall, especially due to the impact of warm events. The partial correlations between the ENSO-SST mode and the Sahel rainfall index, when the influence of the other SST modes are eliminated, are significant over all the 20-year running periods between 1945 and 1993, suggesting that this summer teleconnection could be modulated by the decadal scale SST background. The NCEP/NCAR reanalyses reproduce accurately the interannual variability of the atmospheric circulation after 1968. In particular a regional West African Monsoon Index (WAMI), combining wind speed anomalies at 925 and 200?hPa, is highly correlated with the July-September Sahel rainfall index. A warm ENSO event is associated both with an eastward mean sea level pressure gradient between the eastern tropical Pacific and the tropical Atlantic and with a northward pressure gradient along the western coast of West Africa. This pattern leads to enhanced trade winds over the tropical Atlantic and to weaker moisture advection over West Africa, consistent with a weaker monsoon system strength and a weaker Southern Hemisphere Hadley circulation. The NCEP/NCAR reanalyses do not reproduce accurately the decadal variability of the atmospheric circulation over West Africa because of artifical biases. Therefore the impact of the decadal scale pattern of the atmospheric circulation has been investigated with atmospheric general circulation model (AGCM) sensitivity experiments, by forcing the ARPEGE-Climat model with different combinations of an El Niño-like SST pattern with the pattern of the main mode of decadal scale SST variability where the hightest weights are located in the Pacific and Indian basins. AGCM outputs show that the decadal scale SST variations weakly affect Sahel rainfall variability but that they do induce an indirect effect on Sahel rainfall by enhancing the impact of the warm ENSO phases after 1980, through an increase in the fill-in of the monsoon trough and a moisture advection deficit over West Africa.     

Interannual and decadal variations of snow cover over Qinghai-Xizang Plateau and their relationships to summer monsoon rainfall in China

Interannual and decadal variations of winter snow cover over the Qinghai-Xizang Plateau (QXP) are analyzed by using monthly mean snow depth data set of 60 stations over QXP for the period of 1958 through 1992. It is found that the winter snow cover over QXP bears a pronounced quasi-biennial oscillation, and it underwent an obvious decadal transition from a poor snow cover period to a rich snow cover period in the late 1970’s during the last 40 years.It is shown that the summer rainfall in the eastern China is closely associated with the winter snow cover over QXP not only in the interannual variation but also in the decadal variation. A clear relationship exists in the quasi-biennial oscillation between the summer rainfall in the northern part of North China and the southern China and the winter snow cover over QXP. Furthermore, the summer rainfall in the four climate divisions of Qinling-Daba Mountains, the Yangtze-Huaihe River Plain, the upper and lower reaches of the Yangtze River showed a remarkable transition from drought period to rainy period in the end of 1970’s, in good correspondence with the decadal transition of the winter snow cover over QXP.     

Recent variations in 700 hPa geopotential heights in summer over Europe and the Middle East,and their influence on other Meteorological factors

Summary Recent variations in atmospheric circulation in the eastern Mediterranean are analyzed and discussed. Interdecadal differences in mean monthly 700 hPa geopotential heights for June, July, and August in the period 1951–1980 show a trend of decreasing pressure of the subtropical high pressure belt over the Sahara Desert. The decrease is observed in the magnitude of the high pressure, in its areal extent, and in its northward position. Broader variations in other meteorological variables, such as rainfall regimes, temperature fields, wind variability, and evapotranspiration rates, are discussed in relation to variations in pressure fields and in indices of circulation such as the North Atlantic Oscillation. The trend from the 1950s through the 1970s was towards more temperate summer climate in the region.With 5 Figures     

Observed precipitation in the Paraná-Plata hydrological basin: long-term trends, extreme conditions and ENSO teleconnections

The Paraná-Plata basin is the second largest hydrological basin in South America and is of great importance for the countries of the region (Argentina, Bolivia, Brazil, Paraguay and Uruguay). The present study focuses on the long-term trends in basin-scale precipitation with special emphasis on the role of distribution changes in extreme large-scale precipitation events and on the characteristics and evolution of ENSO teleconnections over the last 50 years. First, we defined a Paraná-Plata basin total precipitation index (PTPI) as the precipitations spatially averaged over the hydrological basin. On interannual time scales, such an index is mainly representative of anomalous monsoon precipitations in the northern part of the basin and large convective precipitation anomalies in the center of the basin (Paraguay-southern Brazil-Uruguay-northern Argentina) typical of the canonical ENSO teleconnection pattern. Our major findings clearly highlight a positive trend in yearly averaged PTPI mainly from the late 1960s to the early 1980s with a strong dependence from month-to-month. The largest precipitation increase is observed from November to May in southern Brazil and Argentina. A close examination of PTPI distributions during the two halves of the period 1950–2001 shows that the changes in the mean state from 1950–1975 to 1976–2001 result from significant changes in each calendar month mean state and in the tails of the PTPI anomaly distributions in May with lesser and weaker large-scale dry events and stronger large-scale wet events. Further studies will be needed to assess whether the observed trend in large-scale extreme precipitation conditions can be related to natural climate variability or anthropogenic activities and whether it is associated to changes in local/regional extreme events. The stronger wet conditions in different months seem to be associated to changes in ENSO characteristics (amplitude, propagation, spatial structure, ...) since the 1982–1983 El Niño. Indeed, spatial ENSO teleconnections (stronger in November and April–May) have greatly evolved from 1950–1975 to 1976–2001. Moreover, we demonstrate that there is a strong modulation and displacement of the teleconnection patterns from one event to another, impeding the definition of robust statistical relationship between ENSO and precipitation in the Paraná-Plata basin (except maybe over a very limited area near the common border between Paraguay, Argentina and Brazil). Finally, the non-antisymmetrical patterns of precipitation between El Niño and La Niña conditions and the non-linear relationship between precipitation and either Niño3.4 or Niño1+2 sea surface temperature indices show that linear statistical forecast systems are actually of very limited use for impact predictions on society on a local or regional scale.