Displays

In France, large multi-decadal variations in river flows have occurred over the instrumental period. These multi-decadal variations, likely of internal origin, could be a major source of uncertainties in the evolution of river flows during the 21st century, and especially during the coming decades, when the climate change signal is weaker. Depending on their phase, these variations might indeed strongly temporarily amplify or weaken (and even possibly reverse) the signal of climate change. From an adaptation perspective, it is crucial that hydrological projections correctly capture the amplitude of these multi-decadal variations, so that the associated uncertainties can be correctly estimated. The realism of hydrological projections in this context lies to a large extent in the realism of climate models, used at the first stage of the vast majority of the studies of the impacts of climate change.

The brevity of the instrumental record makes it difficult to characterize robustly multi-decadal hydro-climate variations, and the lack of observations for important hydrological variables makes it difficult to understand the mechanisms at play. The evaluation of climate models in this context is therefore also particularly challenging. 

In this presentation, I will describe our work to better characterize hydrological variations over France in terms of amplitude and mechanisms, thanks to joint use of newly developed hydrological reconstructions beginning in the mid-nineteenth century, long observations from data-rescue efforts and paleo-climate reconstructions. Based on this work, I will then describe the results of the evaluation of multi-decadal hydrological variations in current global climate models, in terms of amplitude and associated mechanisms, taking into account the very large sampling uncertainties associated with the characterization of multi-decadal variations on relatively short periods. 

How to cite: Boé, J. and Bonnet, R.: Capacity of climate models to capture multi-decadal hydrological variations over France, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3525, https://doi.org/10.5194/egusphere-egu2020-3525, 2020.

The United States (U.S.) Great Plains southerly low-level jet (GPLLJ) is a ubiquitous feature of the summertime climatological flow in the central U.S. contributing to a large percentage of mean and extreme summertime rainfall, the generation of vast quantities of U.S. renewable wind energy, and severe weather outbreaks.  Like other LLJs across the globe, the GPLLJ can be 1) vertically coupled to the large-scale cyclone-anticyclone flow pattern associated with an upper-level jet stream or 2) uncoupled to the large-scale flow but sustained in response to various local land-atmosphere coupling mechanisms.  Many studies have focused on the interactions between teleconnection patterns and associated GPLLJ variability, treating the GPLLJ as a singular phenomenon.  Here, we treat the GPLLJ as two phenomena, coupled and uncoupled to the upper-level flow, and explore the multiscale impacts of SST forced and internally generated modes of variability on the GPLLJ.  With mounting evidence for the low-frequency control on higher frequency GPLLJ variability, the current study analyzes the contribution of the Pacific/North America (PNA) pattern on sub-seasonal timescales and ENSO on interannual timescales to changes in the frequency distributions of both coupled and uncoupled GPLLJs.

This analysis utilizes 1) the Coupled ERA 20th Century (CERA-20C; 1901-2010) reanalysis from ECMWF which provides a large sample of teleconnection conditions and their impacts on GPLLJ variability and 2) a recently developed automated technique to differentiate those GPLLJs that are coupled or uncoupled to the upper-level flow.  Many studies have already shown that two distinct synoptic regimes dominate GPLLJ variability, a western U.S. trough and a central U.S. ridge.  This leads to changes in the frequency ratio of coupled and uncoupled GPLLJ events and ultimately in the location and intensity of precipitation across the GP.  Recently, Burrows et al. (2019) showed that during the Dust Bowl period of 1932-1938, the central and northern GP experienced anomalously high (low) uncoupled (coupled) GPLLJ event frequencies that coincided with a multi-year dry period across the entire region.  Understanding the upscale and lower frequency forcing patterns that lead to these distinct synoptic regimes would lead to greater predictability and forecasting skill.  On sub-seasonal timescales, it is shown that the negative phase of the PNA, which is associated with a southerly displaced Pacific jet stream and a western U.S. trough, leads to increases in the frequency of GPLLJs that are coupled to the upper-level flow, increases in Gulf of Mexico moisture flux and a redistribution of GP precipitation.  On interannual timescales, the location of ENSO events, i.e., eastern or central Pacific, is explored to determine the relationship between tropical forced variability and upper-level coupling to the GPLLJ.  In line with recent studies, it is hypothesized that central Pacific ENSO events may lead to increases in coupled GPLLJ events and precipitation, particularly in the southern GP.

How to cite: Burrows, D. A., Ferguson, C., Agrawal, S., and Bosart, L.: The impacts of ENSO and PNA teleconnections on upper-level coupling to the Great Plains low-level jet, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-504, https://doi.org/10.5194/egusphere-egu2020-504, 2020.

The amount of summer precipitation in the Alpine Region is found to show no linear trend whatsoever over the last 140 years. However, we found significant low frequency periodicity of the interannual variability summer precipitation which synchronizes with the Atlantic Multidecadal Oscillations periodicity of 50 years with a time lag of 17 years. Analyzing atmospheric circulation characteristics over the Alpine Region revealed a see-saw of enhanced/reduced meridional flow which alters the interannual variability of summer precipitation. The polar jet stream appears as a physical mechanism linking atmosphere and oceanic temperature gradients and the meridional/zonal circulation characteristics. Enhanced meridional flow over the Alps induced by a weak jet is increasing precipitation variability through positive soil moisture precipitation feedbacks on the regional scale, whereas enhanced zonal flow is generating less variability through constant moisture flow from the Atlantic and suppressed feedbacks with the land surface. The lagged response to the Atlantic Multidecadal Oscillation is rooted in the spatially inhomogeneous warming/cooling phases which are subject to distinct sea surface temperature gradient patterns.

How to cite: Haslinger, K., Hofstätter, M., Schöner, W., and Blöschl, G.: Variability of Central European Summer Precipitation forced by Sea Surface Temperature Gradients, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6971, https://doi.org/10.5194/egusphere-egu2020-6971, 2020.

    Over recent decades, extreme events in the context of climate change have become a global concern. In China, floods are the one of the severest disasters, which cause 25.6% of all deaths from disasters and account for 54.4% economic losses of GDP. Hence, understanding the influence of climate variability on floods and flood-related variables is of vast theoretical and practical importance. In this study, daily precipitation data and EM-DAT data across China covering a period of 1961-2014 were analyze to investigate the three indices of climate variability, El Niño Southern Oscillation (ENSO), the North Atlantic Oscillation (NAO), and the Indian Ocean Dipole (IOD) during their positive, negative and neutral phases, and to understand their relationships with frequency and intensity of extreme precipitation, flood frequency, and flood damage. The results indicated that:

    (1) The positive and negative phases of NAO and IOD are associated with seasonal extreme precipitation frequency and intensity over large areas of China. During NAO^N and IOD⁺, extreme precipitation occurred more frequently and distributed more widely, especially in spring and summer.

    (2) The influence of ENSO on the extreme precipitation frequency and intensity in China appeared to be much smaller than the influence of NAO or IOD, only strong during ENSO⁺ in spring.

    (3) ENSO, NAO, IOD show significant relationships with flood frequency and flood damage in one or more phase and/or season.

    The strongest link w observed between NAO and flood-related variables. During NAO⁺ and NAO^-, summer flood occurred more frequently. Besides, during NAO^-, anomalies in flood frequency, total death, total affected and total damage are respectively 12%, 3%, 97%, 93% higher in spring and 8%, 160%, 470%, 167% lower in autumn compared to NAO⁺ phase.

    The impact of ENSO, IOD on flood-related variables is relatively weak. Compared to ENSO^N, autumn flood frequency is lower during ENSO⁺ and ENSO^-, but total death and total damage are 48%, 146% higher during ENSO^-.

    IOD shows different characteristics per seasons. In spring and summer, floods occurred more frequently and flood damage showed low significant difference during IOD+. In autumn, flood occurrence is low, but anomalies in total death and total damage were 51%, 102% higher than IOD+.

    Overall, three indices of climate variability show different degrees of impact on flood-related variables over China, and the potential seasonal variation of climate variability indices plays an important role in forecasting flood disasters, mitigating flood risks and enhancing water resource management in China.

How to cite: Wang, Y. and Du, J.: Influence of the climate variability on extreme precipitation and floods in China, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17560, https://doi.org/10.5194/egusphere-egu2020-17560, 2020.

Since the early 1980s, several studies have noticed an abrupt decrease of inflows in the main reservoirs of the western Mediterranean basin. This decline has been more noticeable in the Iberian Peninsula (IP) during the extended winter season (DJFM) where mean inflows decreased until 40% during 1981-2010 compared to 1951-1980. Higher inflows reductions have been found over the western IP where precipitation is mainly modulated by Atlantic fronts. Several plausible causes have been attributed to this phenomenon; changes in land uses, improved datasets or changes in the atmospheric dynamics, among others.

In this work, we assess what is the role of the changes in the large-scale to induce the eighties abrupt precipitation decrease. The analysis consists on the computation of the Wintertime Circulation Types (WCTs) during 1951-2010 using the SLP from ERA20C Reanalysis over a window encompassing the North Atlantic and the Western Europe (-30W, 30E, 65N, 25S). The precipitation associated to these WCTs is analysed using the high-resolution database SPREAD (Serrano-Notivoli et al., 2018). Results show that retaining a group of WCTs may be enough to represent the synoptic situations during reference period over the target region. The frequency of some anticyclonic WCTs (associated with a high pressure over the Iberian Peninsula) showed a significant positive trend for 1951-2010. In contrast, WCTs associated with Atlantic fronts had a significant negative trend. The WCTs promoting westerly flow lead close to the 50% of the annual precipitation over western and central IP during 1951-2010. Then, an abrupt decrease of the frequency of these WCTs directly affects to the precipitation decline in this region (~200 mm). In contrast, the abrupt increase of the anticyclonic WCTs lead to an increase of the precipitation over the eastern IP (~50 mm). Similar significant abrupt shift in precipitation was observed during WCTS associated with cyclones and anticyclones. These results are in agreement with Gómez-Martínez et al. (2018) who found evident links between an increasing NAO index and the decreasing inflows in two basins of the Iberian Peninsula.

Henceforth, there is a need to fulfil the lack of scientific knowledge regarding this abrupt shift in the hydrological resources of the western Mediterranean basin. Precisely, the results of this study shed some light on the causes for the decrease of inflows and run-off over this area and whether they are driven by changes in the regional atmospheric circulation since the early 1980s, related to the internal variability or a global warming forcing. Hence, these results will enable us to identify mitigation and adaptation policies for optimizing the water management.

References

Gómez-Martínez, G., et al. Water Resources Management, 32(8), 2717–2734, doi:10.1007/s11269-018-1954-0, 2018.

Serrano-Notivoli, R., et al. Earth Syst. Sci. Data, 9, 721-738, doi:10.5194/essd-9-721-2017, 2017.

Acknowledgements

The authors acknowledge the ACEX project (CGL2017-87921-R) of the Ministerio de Economía y Competitividad of Spain. A.H.M. thanks his predoctoral contract FPU18/00824 to the Ministerio de Ciencia, Innovación y Universidades of Spain. R.L.P. thanks to the University of Murcia for her postdoctoral contract, and her contract PTQ2018-010275 with Torres Quevedo Program founded by Ministerio de Ciencia, Innovación y Universidades of Spain.

How to cite: Halifa Marín, A., Lorente-Plazas, R., García-Valero, J. A., Jiménez-Guerrero, P., and Montávez, J. P.: Understanding the climate-driven role in the abrupt eighties shift of Iberian hydrological resources, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16067, https://doi.org/10.5194/egusphere-egu2020-16067, 2020.

Vegetation is an integrated regional indicator of environmental changes related to soil, water and climate. For investigating climate change impacts on ecosystems, the transition zones of vegetations are natural hotspots of historical variations. As a transition zone between humid and arid climates in the northwest region of China, the terrestrial ecosystems of Gansu vary from dense vegetation landscapes in southeast to deserts in northwest. Exploring spatiotemporal vegetation responses to climate variations over Gansu has a great significance to project shifting northwest China vegetation patterns which affect regional water and food security. In this study, the spatiotemporal variations of vegetation were characterised by using the Normalized Difference Vegetation Index (NDVI). Between 2001 and 2019, there was a significant increase of the vegetation cover in almost the whole Gansu, and the increasing trend (around 0.015) was more predominant in the southeast part. Over the whole Gansu, especially at the southeast region, the Webster and Yang Monsoon (WYM) and the North Pacific El Nino oscillation (NP) had significant positive relationships with the extent of vegetation coverage at intra-annual and decadal scales respectively. Although the Central Pacific El Nino oscillation (CP) is only a statistically significant variable for some spotty locations of Gansu, it is negatively related to the vegetation variation over the northwest Gansu at an interannual scale. Based on the above relationships between vegetation and climate variables at different temporal scales, the future vegetation conditions of Gansu were projected based on the Beijing Normal University Earth System Model (BNU-ESM) outputs for the RCP4.5 and RCP8.5 scenarios. In a short term (the 2020s), vegetations in Gansu would increase because of the warmer temperatures along with possible increasing snowmelt water. However, for longer terms (the 2050s and the 2080s), the regional vegetation would significantly decline for both RCP4.5 and RCP8.5 scenarios, due to the depletion of snowmelt water sources resulted from the continuously increasing temperature and less snow accumulation in the region. The vegetation projections revealed the future desertification risk in Gansu. These results have important implications to water and food security in the vegetation transition zones of northwest China, which is a key region of the One Belt One Road initiative, connecting semi-arid regions of central Asian nations.

How to cite: He, Q., Chun, K. P., Pan, X., Chen, L., and Fan, P.: Spatiotemporal vegetation variations and projections driven by atmosphere-ocean oscillations at multiple time scales: a case study in Gansu, China, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4475, https://doi.org/10.5194/egusphere-egu2020-4475, 2020.

Lake Urmia is one of the largest hypersaline lakes on earth with a unique biodiversity. Over the past two decades the lake water level declined dramatically, threatening the functionality of the lake’s ecosystems. There is a controversial debate about the reasons for this decline, with either mismanagement of the water resources, or climatic changes assumed to be the main cause.

During this study we gathered an extensive hydro-meteorological data set, information about the reservoirs and the lake bathymetry. This data served for a quantification of the water budget components of Lake Urmia over the last five decades. Interestingly, a comparison of the temporal patterns of the principal natural boundary conditions of streamflow (precipitation and evaporation) with the inflow to the lake revealed that the variability of the inflow can be well explained its natural drivers. With this we can show that variations of Lake Urmia’s water level during the analyzed period were mainly triggered by climatic changes.

However, under the current climatic conditions agricultural water extraction volumes are significant and often exceed the remaining surface water inflow volumes. This rather simple observation shows that something deeper needs to be dug here. Therefore, we performed a parsimonious hindcast experiment and run a set of development scenarios based on the previously developed water balance. This helped us to better quantify the human impact on the development of the water volume of Lake Urmia. We could show that changes in agricultural water withdrawal would have a significant impact on the lake volume and could either stabilize the lake, or lead to its complete collapse (Schulz et al., 2020).

References

Schulz, S., Darehshouri, S., Hassanzadeh, E., Tajrishy, M. and Schüth, C.: Climate change or irrigated agriculture – what drives the water level decline of Lake Urmia, Sci. Rep., 10(1), 236, doi:10.1038/s41598-019-57150-y, 2020.

How to cite: Schulz, S., Darehshouri, S., Hassanzadeh, E., and Schüth, C.: Climate change or irrigated agriculture – what drives the water level decline of Lake Urmia?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11448, https://doi.org/10.5194/egusphere-egu2020-11448, 2020.

In this paper, the process-based analytical derivation approach is applied to insight into the nonstationary of the probability distribution of annual runoff in humid and arid watersheds of China. The nonstationary of the runoff generation process in watersheds are captured by the hydrological inputs and model parameters of the process-based analytical derivation approach. The results indicate that climate change and human activities can impact the probability distribution of annual runoff in different ways, for the nonstationary analysis of humid watersheds, climate change leads to changes in hydrological inputs, and human activities leads to changes in model parameters, which leads to nonstationary of the probability distribution of annual runoff. For the nonstationary analysis of arid watersheds, climate change leads to changes in hydrological inputs, the combined action of human activities with climate change leads to changes in model parameters, which leads to nonstationary of the probability distribution of annual runoff.

How to cite: Guo, S.: Appraisal of the process-based analytical derivation approach in calculating the probability distribution of annual runoff in different watersheds, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2121, https://doi.org/10.5194/egusphere-egu2020-2121, 2020.

The hydrological series can no longer meet the stationarity hypothesis due to the influence of climate variability and human activities. The process of runoff and sediment load changed significantly under a changing environment. Analyzing the variations of runoff and sediment load and exploring the main influencing causes leading to their changes will be of great help to understand the dynamic process of water and sediment in river basin. Many studies have considered the effects of rainfall and reservoir on the downstream runoff or sediment: the impact of rainfall on runoff or sediment load is normally performed by comparing the statistical characteristics before and after an extreme weather event (e.g. heavy rain of the Yangtze river in 1998); the effect of reservoirs is usually determined by comparing the pre-dam and post-dam frequencies of runoff or sediment load. In this study, the major influencing factors of annual runoff and sediment load in Wujiang River basin were identified firstly based on the results of trend analysis and change-point diagnosis for runoff and sediment load. Then, Generalized Addictive Models in Location, Scale, and Shape (GAMLSS) is used to describe the rainfall and reservoir impacts on nonstationarity of runoff and sediment load, in which, distribution parameters (including the location, scale and shape parameter) are expressed as a function of the explanatory variables. The results show that: (1) runoff and sediment load of Wujiang River decrease with the intensification of climate change and human activities; (2) runoff is mainly affected by rainfall, the operation of cascade reservoirs has critical effect on the sediment load; (3) the correlation between runoff and sediment closely related to the nonstationarity of sediment load, namely, the sediment load change can directly lead to the alteration of dependence between runoff and sediment.

How to cite: Li, R.: Nonstationary frequency analysis for annual runoff and sediment load of the Wujiang River, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2161, https://doi.org/10.5194/egusphere-egu2020-2161, 2020.

Land use/cover change (LUCC) affects regional climate change not only through its direct changes of land surface properties, but also through its further modifications of land-atmosphere interactions including the surface energy budget, water cycle and carbon cycle. Urban land expansion as a typical case of LUCC, has been widely discussed about its effects on regional climate, notably on temperature and known for urban heat island (UHI). Another important climate variable atmospheric humidity is also seriously affected by LUCC but has not earned as much attention as temperature. We examined atmospheric humidity changes by a series of indicators in the Yangtze River Delta urban agglomeration of China during 1965-2017, and found obvious urban dry land (UDI) effect in the urban cores, as characterized by decreased humidity and increased vapor pressure deficit. Furthermore, we found similar spatial patterns of humidity changes with urban land expansion process and strong correlations of humidity changes with evapotranspiration and leaf area index changes, indicating that LUCC affects regional climate through an ecohydrological way. We suggest that the UDI effect should be paid more attention in future urban planning and landscape design and more quantitative estimations of urban expansion effect on regional and global drying trends are needed.

How to cite: Li, B.: Urban dry island effect and its potential underlying mechanisms in the Yangtze River Delta urban agglomeration, China, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2162, https://doi.org/10.5194/egusphere-egu2020-2162, 2020.

Midlatitude droughts are affected by the tropical disturbances, which are linked to sea-surface temperature patterns in the Pacific and Atlantic Oceans. The combined effects of these two ocean basins manifest themselves in the variation of streamflows, from land surface filtering. In this study, we have developed a framework to explore the effects of global sea surface temperature variations along with atmospheric teleconnection patterns, on local hydroclimatic conditions related to droughts over the Seine catchment, a main waterway in northern France. Using the Standardized Runoff-discharge Index (SRI) to quantify hydrological drought conditions over the Seine, the North Atlantic Oscillation (NAO) index was found to be a significant driver for the upstream dryness between 2001 and 2015. The El Nino Southern Oscillation (ENSO) index was also found to be a significant forcing variable, but for the Seine downstream. The Atlantic Multidecadal Oscillation (AMO) and the West Mediterranean Sea (WMED) indices were significant over almost the whole Seine River basin. Results show that the drought spatial patterns of the Seine River vary differently with the atmospheric and oceanic oscillations from interannual to decadal scales. Over a small catchment with a drainage area around 78,700 square kilometres, the spatial drought variations in the Seine catchment appear to be usual, and they are likely to be related to regional conditions which drive local land surface mechanisms linked with microclimates or geological processes. In general, during the negative phase of AMO and the positive phase of ENSO, the sea surface temperature of the North Atlantic Ocean is low. The positive phase of NAO also lowers sea surface temperatures of the North Atlantic Ocean and the West Mediterranean Sea. Droughts are likely to occur at the Seine during the negative phase of AMO and the positive phase of NAO, because the cold North Atlantic Ocean has less evaporation and provides less moisture to France. Based on these results, a statistical downscaling model is developed to relate SRI to atmospheric and oceanic oscillation indices, which are derived from the Institut Pierre Simon Laplace climate model (IPSL-CM5) outputs. Using this statistical downscaling model and scenarios of Representative Concentration Pathways (RCP4.5 and RCP8.5), the drought conditions of the Seine are projected for the mid- and long-term future (2050s and 2080s). Diverse drought results are obtained. Based on relative importance of oscillation indices, the implications of diverse projections for general drought managements in midlatitude regions related to tropical sea surface temperature disturbances and atmospheric teleconnections are discussed.

How to cite: Chun, K. P., He, Q., Dieppois, B., Massei, N., and Fournier, M.: Analysing and projecting spatial drought conditions of the Seine catchment based on ocean-atmosphere oscillations over interannual and decadal scales , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4385, https://doi.org/10.5194/egusphere-egu2020-4385, 2020.

Large-scale modes of climatic variability, or teleconnections, influence global patterns of climate variability and provide a framework for understanding complex responses of the global water cycle to global climate. Here, we examine how Terrestrial Water Storage (TWS) responds to 14 major teleconnections (TCs) during the 2003–2016 period based on data obtained from the Gravity Recovery and Climate Experiment (GRACE). By examining correlations between the teleconnections and TWS anomalies (TWSA) data, we find these teleconnections significantly influence TWSA over more than 80.8% of the global land surface. The El Niño-Southern Oscillation (ENSO), the Pacific Decadal Oscillation (PDO), and the Atlantic Multidecadal Oscillation (AMO) are significantly correlated with TWSA variations in 55.8%，56.2% and 60% the global land surface, while other teleconnections affect TWSA at regional scales. We also explore the TCs’ effect on three key hydrological components, including precipitation (P), evapotranspiration (ET) and runoff (R), and their contribution to TWSA variations in 225 river basins. It’s found the TCs generally exert the comprehensive but not equally impact on all three components (P, ET and R). Our findings demonstrate a significant and varying effect of multiple TCs in terrestrial hydrological balance.

How to cite: Li, T.: Links between global terrestrial water storage and large-scale modes of climatic variability, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6613, https://doi.org/10.5194/egusphere-egu2020-6613, 2020.

The modern Arctic is becoming warmer and more humid, and the Arctic Ocean is increasingly free of ice in summer. One of the feedbacks of global warming in the arctic part of the climate system is an increase of downward long-wave radiation inflow to the surface of snow and ice due to an increase of the content of water vapor in the atmosphere of the Arctic. The source of the increase of the water vapor content in the arctic atmosphere is the atmospheric branch of the freshwater cycle, including moisture transport from low latitudes and inflow from the ocean surface. Moisture from low latitudes is transferred not only to the Arctic, but also to the adjacent continent of Eurasia, from where its excess is transferred by river flow to the Arctic Ocean. Strengthening of zonal transports of heat and moisture from oceanic regions to continents and meridional transports from low latitudes of the World Ocean to temperate and high latitudes is shown using the proposed indices of the zonal and meridional circulation. The indices were calculated according to the NCEP, ERA-Interim reanalysis data. It has been established that the increase in transports is manifested, in particular, in an increase of air temperature, in an increase of the total moisture content in the atmosphere over the area of Siberian rivers flow formation, in an increase of precipitation and, as a result, in an increase of the run-off of rivers flowing into the Arctic Ocean. The connection between the indices and surface air temperature, precipitation, atmospheric moisture content in the regions of catchment areas of three main Siberian rivers, Ob, Lena and Yenisei, confirmed the influence of atmospheric transports in the cold part of the year. Assessment of the relationship between changes of climatic conditions in the catchment areas and interannual changes of river runoff parameters indicated that annual runoff increases and mostly is affected by increase of average annual precipitation. The study was carried out with the support of the Russian Foundation for Basic Research (Project 18-05-60107).

How to cite: Vyazilova, A., Genrikh, A., Natalya, K., and Natalya, G.: Impact of global warming on the conditions of the Siberian rivers discharge formation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9410, https://doi.org/10.5194/egusphere-egu2020-9410, 2020.

It is now widely recognized that El Nino-Southern Oscillation (ENSO) occurs in more than one form, e.g. eastern and central Pacific ENSO. Given that these various ENSO flavours may contribute to climate variability and trends in different ways, this study presents a framework that treats ENSO as a continuum to examine its impact on precipitation, and to evaluate the performance of the last two generations of global climate models (GCMs): CMIP5 and CMIP6.

Uncertainties in the location and intensity of observed El Nino and La Nina events are assessed in various observational and satellite-derived products (ERSSTv5, COBESSTv2, HadSST1 and OISSTv2). The probability distributions of El Nino and La Nina event locations, and intensities, slightly differ from one observational data set to another. For instance, La Nina events are more intense and more likely to occur in the central Pacific using COBESSTv2. All these products also depict consistent decadal variations in the location and intensity of ENSO events: i) central Pacific ENSO events were more likely in the 1940/50s and from the 1980s; ii) eastern Pacific ENSO events were more likely in the 1910/20s and 1960/70s; iii) La Nina events have become more intense during the 20^th and early 21^st centuries.

These fluctuations in ENSO location and intensity are found to impact precipitation consistently across diverse global precipitation products (CRUv4.03, GPCCv8 and UDELv5.01). Over southern Africa, for instance, more intense eastern (central) Pacific El Nino events are found to favour drought conditions over northern (southern) regions during austral summer. By contrast, over the same regions, more intense La Nina events favours wet conditions, while the location of these events has little effect on precipitation. Over West Africa, ENSO locations favour a zonal (E-W) rainfall gradient in precipitation during boreal summer, while changes in ENSO intensity modulate the strength of the meridional (N-S) rainfall gradient.

Using both historical and pi-Control runs, we demonstrate that most CMIP5 and CMIP6 models favour either eastern or central Pacific ENSO events, but very few models are able to capture the full observed ENSO continuum. Regarding ENSO impacts on worldwide precipitation, contrasted results appear in most models.

How to cite: Dieppois, B., Eden, J., Monerie, P.-A., Pohl, B., Crétat, J., and Chun, K. P.: ENSO continuum and its impacts on worldwide precipitation: Observation vs. CMIP5/6 , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9606, https://doi.org/10.5194/egusphere-egu2020-9606, 2020.

Hydrological processes vary over long time-scales, which are originating from large-scale climate. It is not always straightforward, however, to identify how large-scale climate variability can affect regional or local-scale hydrological processes, as such relationships are not linear. Taking the example of the Seine river watershed (northern France), we study the modalities of precipitation and chalk aquifer groundwater level (GWL) variability, focusing on interannual (4-12 years) and interdecadal (12-23 years) scales. We propose a methodological approach for analysing and discussing potential large-scale relationships and forcings on hydrological systems.

163 GWL and 13 precipitation monthly time series, covering the northern half of metropolitan France between 1964 and 2015, were analysed using continuous and discrete multiresolution wavelet transforms. GWL time series all revealed statistically significant oscillating components on interannual and interdecadal scales, but with different amplitudes in space. All precipitation time series displayed the same oscillating components across the watershed with rather constant amplitudes spatially, contrary to GWL time series, which suggest an impact of local physical watershed properties to filter some parts of the climate signal. Using precipitation and GWL time series available over one century, as well as the NOAA 20CR reanalysis, we then analysed the relationship with the North Atlantic atmospheric circulation at both the interannual and interdecadal scales. On interannual scale, using sea-level pressure and geopotential height at 200 hPa, we found that precipitation and GWL variability would be linked to pronounced Rossby wave-like patterns. On interdecadal scale, the patterns obtained correspond to clear west-circulation patterns, which are very similar to the patterns associated with Atlantic Multidecadal Oscillation (AMO). Interdecadal precipitation variability are indeed also found to be consistent with the positive and negative phases of the AMO, suggesting potential impacts on hydrological variability. Examining both precipitation and GWL, major droughts occured during low levels of interannual and interdecadal components. This study therefore demonstrates that such extreme events would then be: i) linked to a weakened western circulation with strong North-South jet stream oscillations on interannual scale; ii) modulated by western circulation associated with the AMO on interdecadal scale.

How to cite: Massei, N., Fournier, M., Chun, K. P., He, Q., Dieppois, B., Lattelais, C., and Dupont, J.-P.: Modulation of regional precipitation and groundwater level variability by large-scale oceanic/atmospheric circulation over interannual and interdecadal scales, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10353, https://doi.org/10.5194/egusphere-egu2020-10353, 2020.

Yangtze River and Yellow River are the two most important rivers in China. Long-term observation shows that runoff ratio (i.e., runoff/precipitation, denoted as RR) in the headwater of both Yangtze River (HYZR) and Yellow River (HYER) has experienced significant decrease and then increase trend (referred as V-change) during the period 1980-2015. Over the whole period, RR of the HYER shows significant decreasing trend (-0.02/10a, p < 0.05), while it is not significant for the HYZR. Changes in RR in both HYZR and HYER pose great challenge on runoff predication and water management in the downstream. However, driven mechanisms underlying the V-change of RR are still unclear. Here, based on ground-based and remote sensing datasets, both terrestrial and atmospheric water budgets are investigated to understand the evolution of RR in the headwater regions of Yangtze River and Yellow River. Terrestrial water budgets are for evaporation estimation and water cycle analysis. Atmospheric water budgets are used to calibrate the estimated evaporation. Results show that TWS-REC agrees well with observed total water storage (TWS-GRACE) in both HYZR (r = 0.94, NSE = 0.83) and HYER (r = 0.93, NSE = 0.83) over the period of 2003-2012. Estimated evaporation from both terrestrial water balance and atmospheric water balance method also agree well with each other in the HYZR (r = 0.89, NSE = 0.80) and in the HYER (r = 0.88, NSE = 0.79) over the period of 2000-2015. It suggests that reconstructed TWS and estimated evaporation are reliable for analyzing long-term water cycle in the study areas. Both the ratio of the estimated evaporation to precipitation (ER) in two basin increase first and then decreased during the study period. The correlation coefficients between ER and RR in the HYZR and HYER are -0.63 and -0.79, respectively, presenting that RR variability could be mainly caused by the evolution ER. Meanwhile it also indicates the nonignored role of total water storage (TWS) changes in RR variability in the two basin. TWS-REC in both regions have experienced significant increasing with rate of 26 mm/10a (HYZR, p < 0.05) and 17mm/10a (HYER, p < 0.05), later of which is the main reason of downward trend of RR in HYER. Further analysis indicates that changes in ER are resulted from comprehensive effects of precipitation variability (26.4mm/10a, p < 0.05 in HYZR and 3.5mm/10a p > 0.1 in HYER) and of dramatic climate warming (0.6℃/10a, p < 0.05 in HYZR and 0.5℃/10a, p < 0.05 in HYER). TWS changes in both basin are positively related with dramatic temperature rising and significant vegetation greening. It means that annual fluctuation of precipitation-runoff process (i.e., V-change RR) has affected negatively by climate warming and vegetation greening in the HYZR and HYER. These findings can advance our understanding of the runoff ratio evolution and water cycle in the headwater of Yangtze River and Yellow River and it is also important for ecological conservation strategy and downstream water resources management.

How to cite: Xu, Z., Cheng, L., and Liu, P.: Water cycle changes in the headwater regions of the Yangtze River and Yellow River basins during the past three decades, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10055, https://doi.org/10.5194/egusphere-egu2020-10055, 2020.

Understanding the impact of climate variability on aquifer systems is important to improve future projections of groundwater availability, particularly in the context of increasing water scarcity. Coastal aquifers in Mediterranean regions are particularly sensitive to inter-annual and seasonal water storage fluctuations linked to climate forcing and climate-induced pumping. This comparative study examines the implications of climate variability modes on groundwater levels in coastal aquifers of California and Portugal. Piezometric levels in selected aquifers in Portugal (Leirosa-Monte Real and Campina de Faro) and California (Coastal Basins aquifers), spanning a period from 1988 to 2018, are analyzed using wavelet transform methods and principal component analysis. The methods expose not only the impact of the individual climate modes (AMO, PDO, ENSO, PNA in California and NAO, EA and SCAND in Portugal) but also the existence of complex transitive couplings among modes. Together, the climate modes are responsible for most (80%) of the inter-annual variability in groundwater storage in both Portugal and California coastal aquifers. However, our most important result is the recognition that transitive couplings greatly affect the hydrological responses both in Portugal and California. Coupled phases are linked to extreme piezometric levels and are associated with shifts in mode-interaction patterns. The authors would like to acknowledge the financial support FCT through project UIDB/50019/2020 – IDL.

How to cite: Malmgren, K., Neves, M. D. C., and Gorge, T.: Comparing the impacts of climate modes of variability on coastal aquifers of Portugal and California, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10999, https://doi.org/10.5194/egusphere-egu2020-10999, 2020.

Despite the large evidence on teleconnections between the El Niño South Oscillation (ENSO) phenomenon and the hydroclimatology of Central-Southern Chile, the propagation of the ENSO signal through the hydrological cycle is still unclear, because of the complex hydrological processes and compensatory/amplifying effects of meteorological anomalies on the hydrology. In this work, we examine the sensitivity of hydrological responses to contrasting ENSO phases – El Niño, La Niña or neutral phase – against local climatic conditions across 55 near-natural catchments, whose location provides a strong north-south latitudinal dry-wet gradient, with the Andes Cordillera acting as a longitudinal elevation control. We analyze the dependence of four meteorological variables and four hydrological signatures with ENSO phases across three different hydrological regimes – snowmelt-dominated, rainfall-dominated and mixed –. Additionally, we calculate the sensitivity of hydrological signatures to meteorological variables as the total derivative, to assess the full interactions of the system.

Our results confirm statistically significant anomalies of streamflow and meteorological variables at the catchment-scale according to ENSO phases. Hydrological regimes (i.e. seasonality) are enhanced during El Niño years, showing a clear latitudinal gradient. We obtain negative (positive) sensitivities of non-dimensional annual streamflow to increased mean winter temperature at higher (lower) elevations, but positive sensitivities to mean winter storm temperature in the entire study domain. We advise these different sensitivities of non-dimensional annual streamflow – to mean winter storm temperature compared to the whole winter mean temperature – may depict different hydrological implications. We confirm positive (negative) streamflow anomalies with El Niño (La Niña) phases, and we note concomitancy in snowmelt-dominated basins with lesser (higher) runoff ratio anomalies. In rainfall-dominated basins, we obtain higher (lower) runoff ratio anomalies with El Niño (La Niña) phases. This is, for snowmelt-driven/semiarid basins, we note compensatory anomalies between annual streamflow and runoff ratio, and our results suggest that runoff ratio anomalies may be driven by precipitation anomalies rather than by temperature anomalies.

How to cite: Hernandez, D., Ricchetti, F., Mendoza, P., and Vargas, X.: Teleconnections between ENSO and hydrological response at the catchment-scale in Central-Southern Chile, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12131, https://doi.org/10.5194/egusphere-egu2020-12131, 2020.

Understanding hydrological extremes is becoming increasingly important for future adaptation strategies to global warming. Hydrologic extremes affect food security, water resources, natural hazards, and play an important role in the context of erosional processes and landscape evolution. The Pacific region is strongly affected by large-scale climatic anomalies induced by the El Niño-Southern Oscillation (ENSO). How these climatic anomalies translate into hydrological extremes is complex, because both temperature and precipitation deviate from normal conditions and the effect of this simultaneous change on hydrological processes in river catchments (e.g., snowmelt, evapotranspiration) is challenging to understand.

In this study, we investigate the effect of ENSO on mean precipitation, mean temperature, mean stream flow, and streamflow variability in Chile. We have applied extensive quality control on a large hydrological dataset from the Dirección General de Aguas in Chile, resulting in ~200 good quality streamflow stations. The dataset envelopes the extent from semi-arid climate in the north (~28°S) to humid climate in the south (~42°S). Additionally, the dataset includes low elevation catchments located in the Coastal Cordillera and high elevation catchments in the Andes. We used the monthly Multivariate ENSO Index (MEI) to classify the 5 strongest El Niño and La Niña years, and 5 non-ENSO years after 1975. Changes in mean streamflow and streamflow variability were calculated based on the monitored data from the streamflow stations. For each river catchment, we calculated mean seasonal precipitation using the 0.25°-resolution gridded dataset from the Global Precipitation Climatology Centre (GPCC) and mean seasonal temperature using the 0.5°-resolution global temperature dataset from the Climatology Prediction Centre (CPC).

The precipitation, temperature, and discharge patterns show seasonal variation, varying in strength over the north-south gradient and between low and high elevation catchments. Mean annual precipitation generally increases significantly during El Niño events, and slightly decreases during La Niña events. For both El Niño and La Niña events the mean temperature predominantly changes between 28°S and 35°S and shows increasing temperatures in the Andes and decreasing temperatures in the low elevation Coastal Cordillera. The mean annual streamflow increases during El Niño events, and shows similarities to the pattern of increased mean annual precipitation. However, at the seasonal level, there is a time-lag between precipitation and streamflow, which is regulated by slower snowmelt processes. During La Niña events, the mean annual streamflow increases in the north (28°S-34°S) and decreases in the south (34°S-42°S). Interestingly, the mean annual precipitation and mean annual streamflow patterns behave inversely in the northern Andes. Mean streamflow increases, whereas mean precipitation decreases. This possibly results from enhanced snowmelt because of increased temperatures, but this needs to be further investigated. Finally, the magnitude and frequency of extreme floods predominantly increases in the northern Andean catchments and decreases towards the south for both El Niño and La Niña events. This study shows that large-scale climatic phenomena like ENSO affect catchment hydrology through both anomalies in precipitation and temperature.

How to cite: van Dongen, R., Scherler, D., Wendi, D., Deal, E., Meier, C., Marwan, N., and Mao, L.: El Niño-Southern Oscillation (ENSO) controls on mean streamflow and streamflow variability in Central Chile, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14011, https://doi.org/10.5194/egusphere-egu2020-14011, 2020.

Currently, there are changes in the hydroclimatic system, with most of Europe affected by droughts. Recent reconstructions on historical precipitation and temperature fields can be used for determination of impacts of meteorological, hydrological and agricultural droughts. Those reconstructions are available for European continent in gridded form (Casty et al.,2007). Aridity index, defined as a fraction of potential evapotranspiration and precipitation, can be used for characterization of humid – wet -- and arid – dry -- regions. It represents the ratio between energy availability and water availability. This study deals with conditional probabilities of transitions from arid to humid environment and vice versa. The aridity index was used to determine the transitions annual basis for the European continent for the period 1766 - 2015. The probabilities were calculated for each year, and for 10-year, 20-year and 30-year periods. It is shown that the recent droughts followed the drying of substantial part of Europe starting in 2014 (Hanel et al., 2018). The changes are most pronounced in Northern and Central Europe.

references:

Casty C., Raible Ch. C., Stocker T. F., Wanner H., Luterbacher J., 2007: A European pattern climatology 1766-2000. Climate Dynamics 29. 791-805.

Hanel, M., Rakovec, O., Markonis, Y., Máca, P., Samaniego, L., Kyselý, J., Kumar, R., 2018: Revisiting the recent European droughts in a long-term perspective. Scientific Report 8, 9499.

How to cite: Bešťáková, Z., Máca, P., Kyselý, J., Singh, U., Markonis, Y., and Hanel, M.: Conditional probabilities of transition from arid to humid environment and vice versa in Europe during the period 1766 -2015, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20779, https://doi.org/10.5194/egusphere-egu2020-20779, 2020.