Evolution récente des paramètres cryo-climatiques et des teneurs isotopiques des précipitations dans les Andes Boliviennes: La Paz et Glacier du Zongo

Résumé

A proximité de La Paz, Bolivie, les indicateurs marquants de la tendance climatique sont: (a) une augmentation de la température de ré-analyse à 500 hPa de 0.017°C an^-1 entre 1973 et 2004; (b) à la station météorologique Laïca Cota, entre 1995 et 2004, une élévation de la température de 0.03°C an^-1 et une diminution de l'humidité relative de 0.6 % an^-1; et (c) une augmentation du δ¹⁸O des précipitations de 0.26‰ an^-1 et de leur excès en deutérium de 0.1‰ an^-1. Une des principales conséquences de cette tendance est un recul du front du Glacier Zongo de 12 m an^-1 entre 1991 et 2004 (réduction de 2% par an des surfaces glaciaires de 1997 à 2004: 1.84 km² en 2004). L'analyse temporelle concomitante des données montre que la variabilité des conditions ENSO a des conséquences en phase à l'échelle infra-annuelle pour la température, l'humidité relative, la précipitation, le bilan glaciaire, et des conséquences en déphasage, sur des périodes dépassant l'année, pour le rapport isotopique des précipitations andines.     

LANDSCAPE CHANGE,GLOBAL CHANGE,AND THE WISDOM OF ROY BEDICHEK

Studies of environmental change often belong to one of two traditions, each generating different human responses. Landscape change in the United States, defined as transformations of the country's land surface over the past half-century, typically is characterized as dominating in magnitude and ubiquitous in spatial extent. Two studies based on rephotography suggest otherwise: Stability seems to be characteristic of recent American landscape history. Perhaps the experiences of everyday lives–necessarily concentrated into those landscapes that have most changed in individual human lifetimes–influence human outlooks. By contrast, recent and anticipated global change, exemplified by human-induced climate modification, creates a different impression: People underestimate the magnitude and impact of such environmental modification. Part of this reaction may lie, again, in the limitations of human experience. Nonetheless, several examples suggest that the characteristics of a dynamic nature, structured by abruptness resulting from process thresholds, complicate the anticipation of global change. Whether landscape or global, change often generates a human sense of regret. Yet, the Texas naturalist, Roy Bedichek, demonstrated that human-induced change may generate patterns in the natural world that bring us pleasure. His wisdom may help to soften the dread that many feel for landscape and global change. [Key words: landscape, environmental change, United States.]     

Sediment and Water Discharge Assessment on Santiago and Pánuco Rivers (Central Mexico): The Importance of Topographic and Climatic Factors

This study explores the main factors controlling sediment and water discharge in the Santiago and Pánuco Rivers, the two largest rivers of central Mexico. Both Santiago and Pánuco Rivers are sourced in the Central Plateau of Mexico and flow in an opposite direction. Santiago River flows over a tectonically active margin draining to the Pacific Ocean, and Pánuco River flows into the passive margin of the Gulf of Mexico. Mean annual and monthly values of suspended sediment load and water discharge spanning around 50 years were used to evaluate sediment load and water discharge in these two rivers. Our findings indicated that Santiago River delivers to the ocean around 45% more sediment than Pánuco River. However, we found that Santiago River has about half the water discharge of Pánuco River. The high river gradient along Santiago River is likely to enhance the net erosion and sediment transport capacity. Water discharge at Pánuco Basin is higher than in Santiago Basin because the annual rainfall is higher for the former. The difference in sediment and water discharge for both rivers are also related to El Niño Southern Oscillation events. Our results indicated that water discharge in Santiago River increases during El Niño and La Niña events. In contrast, Pánuco River is mostly affected by La Niña events.     

QANAT SYSTEMS IN IRAN

Abstract

A fuzzy rule-based technique is used for modelling the relationship between climatic forcing and droughts in a Central/Eastern European country, Hungary. Two types of climatic forcing'called premises'are considered: atmospheric circulation patterns (CP) and El Niño Southern Oscillation (ENSO). Both the Hess-Brezowsky CP types and ENSO events influence the occurrence of droughts, but the ENSO signal is relatively weak in a statistical sense. The fuzzy rule-based approach is able to learn the high space—time variability of monthly Palmer Drought Severity Index (PDSI) and results in a proper reproduction of the empirical frequency distributions. The “engine” of the approach, the fuzzy rules, are ascertained from a subset called the learning set of the observed time series of premises (monthly CP frequencies and Southern Oscillation Index) and PDSI response. Then an independent subset, the validation set, is used to check how the application of fuzzy rules reproduces the observed PDSI.     

Moisture transport and seasonal variations in the stable isotopic composition of rainfall in Central American and Andean Páramo during El Niño conditions (2015–2016)

High‐elevation tropical grassland systems, called Páramo, provide essential ecosystem services such as water storage and supply for surrounding and lowland areas. Páramo systems are threatened by climate and land use changes. Rainfall generation processes and moisture transport pathways influencing precipitation in the Páramo are poorly understood but needed to estimate the impact of these changes, particularly during El Niño conditions, which largely affect hydrometeorological conditions in tropical regions. To fill this knowledge gap, we present a stable isotope analysis of rainfall samples collected on a daily to weekly basis between January 2015 and May 2016 during the strongest El Niño event recorded in history (2014–2016) in two Páramo regions of Central America (Chirripó, Costa Rica) and the northern Andes (Cajas, south Ecuador). Isotopic compositions were used to identify how rainfall generation processes (convective and orographic) change seasonally at each study site. Hybrid Single Particle Lagrangian Integrated Trajectory model (HYSPLIT) air mass back trajectory analysis was used to identify preferential moisture transport pathways to each Páramo site. Our results show the strong influence of north‐east trade winds to transport moisture from the Caribbean Sea to Chirripó and the South American low‐level jet to transport moisture from the Amazon forest to Cajas. These moisture contributions were also related to the formation of convective rainfall associated with the passage of the Intertropical Convergence Zone over Costa Rica and Ecuador during the wetter seasons and to orographic precipitation during the transition and drier seasons. Our findings provide essential baseline information for further research applications of water stable isotopes as tracers of rainfall generation processes and transport in the Páramo and other montane ecosystems in the tropics.     

Observed trends and relationships between ENSO and standardized hydrometeorological drought indices in central Chile

Droughts are natural phenomena that severely affect socio economic and ecological systems. In Chile, population and economic activities are highly concentrated in its central region (i.e. between latitudes 29°S and 40°S), which periodically suffers from severe droughts affecting agriculture, hydropower, and mining. Understanding the dynamics of droughts and large-scale atmospheric processes that influence the occurrence of dry spells is essential for forecasting and efficient early detection of drought events. Central Chile's climate is marked by a significant El Niño Southern Oscillation (ENSO) influence that might help to better characterize droughts as well as to identify the effects of ENSO on the spatial and temporal characteristics of meteorological and hydrological droughts in the region. We analysed the behaviour of the Standardized Precipitation Evapotranspiration Index (SPEI) and Standardized Streamflow Index (SSI) time series for 6-month accumulation periods over the austral winter and summer seasons. Multiple linear regression (MLR) and Generalized Linear Models (GLM) showed a significant ENSO influence on dry events for SPEI-6 and SSI-6 during winter and summer. We found that the magnitude of correlation between ENSO and SPEI-6 has changed over the last decades becoming weaker in winter periods and increasing in spring summer periods. Increasing trends in meteorological and hydrological drought events were also identified, along all latitudes, with significant trends during winter in the southern latitudes, and during summer in the semi-arid and Mediterranean zones. These results indicate that drought mitigation actions and policies are necessary to overcome their adverse effects. Given the monthly persistence of ENSO and its relationship to drought indices, there are opportunities for drought monitoring and seasonal forecasting that could become part of national drought management systems.     

Orographic distillation and spatio‐temporal variations of stable isotopes in precipitation in the North Atlantic

Little is known about the spatial and temporal variability of the stable isotopic composition of precipitation in the North Atlantic and its relationship to the North Atlantic Oscillation (NAO) and anthropogenic climate change. The islands of the Azores archipelago are uniquely positioned in the middle of the North Atlantic Ocean to address this knowledge gap. A survey of spatial and temporal variability of the stable isotope composition of precipitation in Azores is discussed using newly presented analyses along with Global Network of Isotopes in Precipitation data. The collected precipitation samples yield a new local meteoric water line (δ²H = 7.1 * δ¹⁸O + 8.46) for the Azores region and the North Atlantic Ocean. The annual isotopic mean of precipitation shows a small range for the unweighted and precipitation mass‐weighted δ¹⁸O‐H₂O values. Results show an inverse relation between the monthly δ¹⁸O‐H₂O and the amount of precipitation, which increases in elevation and into the interior of the island. Higher amounts of precipitation (from convective storm systems) do not correspond to the most depleted values of stable isotopes in precipitation. Precipitation shows an orographic effect with depleted δ¹⁸O‐H₂O values related to the Rayleigh effect. Monthly δ¹⁸O‐H₂O values for individual precipitation sampling stations show little relationship to air temperature. Results show a local source of moisture during the summer with the characteristics of the first vapour condensate. The stable isotope composition of precipitation is strongly correlated to the NAO index, and δ¹⁸O‐H₂O values show a statistically significantly trend towards enrichment since 1962 coincident with the increased air temperature and relative humidity due to climate change. Results are in line with observations of increasing sea surface temperature and relative humidity.     

Relationship between historical sea-surface temperature variability and climate change-induced coral mortality in the western Indian Ocean

Many of the world’s coral reefs suffered high coral mortality during the 1998 ENSO, with the highest mortality in the western Indian Ocean (WIO). A meta-analysis of field data on change in coral cover across the 1998 ENSO event was conducted for 36 major reef areas in the WIO, and relationship of the change with the historical sea-surface temperature (SST) variability investigated. WIO reefs were categorized into three major SST groups of differing coral cover change. Cover change was negatively associated with standard deviation (SD) SST until about SD 2.3, with increasing flatness of the SST frequency distributions. It increased with further increase in SD as the SST distributions became strongly bimodal in the Arabian/Persian Gulf area. The study indicates that environmental resistance/tolerance to extreme anomalous events could be predicted and management priorities directed accordingly for a warmer and more variable future climate.     

Assessing changes in extreme sea levels: Application to the English Channel, 1900–2006

A recently extended and spatially rich English Channel sea level dataset has been used to evaluate changes in extreme still water levels throughout the 20th century. Sea level records from 18 tide gauges have been rigorously checked for errors and split into mean sea level, tidal and non-tidal components. These components and the interaction between surge and tide have been analysed separately for significant trends before determining changes in extreme sea level. Mean sea level is rising at 0.8–2.3 mm/year, depending on location. There is a small increase (0.1–0.3 mm/year) in the annual mean high water of astronomical tidal origin, relative to mean sea level, and an increase (0.2–0.6 mm/year) in annual mean tidal range. There is considerable intra- and inter-decadal variability in surge intensity with the strongest intensity in the late 1950s. Storm surges show a statistically significant weak negative correlation to the winter North Atlantic Oscillation index throughout the Channel and a stronger significant positive correlation at the boundary with the southern North Sea. Tide–surge interactions increase eastward along the English Channel, but no significant long-term changes in the distribution of tide–surge interaction are evident. In conclusion, extreme sea levels increased at all of the 18 sites, but at rates not statistically different from that observed in mean sea level.     

Spatio-temporal analysis of the melt onset dates over Arctic sea ice from 1979 to 2017

The melt onset dates(MOD) over Arctic sea ice plays an important role in the seasonal cycle of sea ice surface properties, which impacts Arctic surface solar radiation absorbed by the ice-ocean system. Monitoring interannual variations in MOD is valuable for understanding climate change. In this study, we investigated the spatio-temporal variability of MOD over Arctic sea ice and 14 Arctic sub-regions in the period of 1979 to 2017 from passive microwave satellite data. A set of mathematical and ...