Regional modeling of decadal rainfall variability over the Sahel

A regional climate model is used to investigate the mechanism of interdecadal rainfall variability, specifically the drought of the 1970s and 1980s, in the Sahel region of Africa. The model is the National Center for Environmental Prediction’s (NCEPs) Regional Spectral Model (RSM97), with a horizontal resolution of approximately equivalent to a grid spacing of 50 km, nested within the ECHAM4.5 atmospheric general circulation model (AGCM), which in turn was forced by observed sea surface temperature (SST). Simulations for the July–September season of the individual years 1955 and 1986 produced wet conditions in 1955 and dry conditions in 1986 in the Sahel, as observed. Additional July–September simulations were run forced by SSTs averaged for each month over the periods 1950–1959 and the 1978–1987. These simulations yielded wet conditions in the 1950–1959 case and dry conditions in the 1978–1987 case, confirming the role of SST forcing in decadal variability in particular. To test the hypothesis that the SST influences Sahel rainfall via stabilization of the tropospheric sounding, simulations were performed in which the temperature field from the AGCM was artificially modified before it was used to force the regional model. We modified the original 1955 ECHAM4.5 temperature profiles by adding a horizontally uniform, vertically varying temperature increase, taken from the 1986–1955 tropical mean warming in either the AGCM or the NCEP/National Center for Atmospheric Research Reanalysis. When compared to the 1955 simulations without the added tropospheric warming, these simulations show a drying in the Sahel similar to that in the 1986–1955 difference and to the decadal difference between the 1980s and 1950s. This suggests that the tropospheric warming may have been, at least in part, the agent by which the SST increases led to the Sahel drought of the 1970s and 1980s.     

Statistical downscaling of historical monthly mean winds over a coastal region of complex terrain. II. Predicting wind components

A regression-based downscaling technique was applied to monthly mean surface wind observations from stations throughout western Canada as well as from buoys in the Northeast Pacific Ocean over the period 1979–2006. A predictor set was developed from principal component analysis of the three wind components at 500?hPa and mean sea-level pressure taken from the NCEP Reanalysis II. Building on the results of a companion paper, Curry et al. (Clim Dyn 2011, doi:10.1007/s00382-011-1173-3), the downscaling was applied to both wind speed and wind components, in an effort to evaluate the utility of each type of predictand. Cross-validated prediction skill varied strongly with season, with autumn and summer displaying the highest and lowest skill, respectively. In most cases wind components were predicted with better skill than wind speeds. The predictive ability of wind components was found to be strongly related to their orientation. Wind components with the best predictions were often oriented along topographically significant features such as constricted valleys, mountain ranges or ocean channels. This influence of directionality on predictive ability is most prominent during autumn and winter at inland sites with complex topography. Stations in regions with relatively flat terrain (where topographic steering is minimal) exhibit inter-station consistencies including region-wide seasonal shifts in the direction of the best predicted wind component. The conclusion that wind components can be skillfully predicted only over a limited range of directions at most stations limits the scope of statistically downscaled wind speed predictions. It seems likely that such limitations apply to other regions of complex terrain as well.     

Database of glacial lake outburst floods (GLOFs)–IPL project No. 179

This article summarises activities and preliminary results of the International Programme on Landslides Project no. 179 “Database of glacial lake outburst floods (GLOFs)”. This project is planned for 3 years (2013–2015); the main objectives of the first year are (1) to create an online database and (2) to select collaborating partners. A survey of existing and relevant documents, information and organisations has been initiated along with the creation of a website (www.glofs-database.org) and the establishment of international collaboration. The first preliminary results show regional differences in various attributes of GLOF events (e.g. triggers, chronological distribution). These differences should be taken into consideration in regionally focused methods of hazard assessment, mitigation and consequently risk management.     

Late Miocene to Pliocene stratigraphy of the Kura Basin,a subbasin of the South Caspian Basin: implications for the diachroneity of stage boundaries

Relative ages of late Cenozoic stratigraphy throughout the Caspian region are referenced to regional stages that are defined by changes in microfauna and associated extreme (>1000 m) variations in Caspian base level. However, the absolute ages of these stage boundaries may be significantly diachronous because many are based on the first occurrence of either transgressive or regressive facies, the temporal occurrence of which should depend on position within a basin. Here, we estimate the degree of diachroneity along the Akchagyl regional stage boundary within the Caspian basin system by presenting two late Miocene‐Pliocene aged measured sections, Sarica and Vashlovani, separated by 50 km and exposed within the Kura fold‐thrust belt in the interior of the Kura Basin. The Kura Basin is a western subbasin of the South Caspian Basin and the sections presented here are located >250 km from the modern Caspian coast. New U‐Pb detrital zircon ages from the Sarica section constrain the maximum depositional age for Productive Series strata, a lithostratigraphic package considered correlative with the 2–3 Myr‐long regional Eoakchagylian or Kimmerian stage that corresponds to a period of extremely low (>500 m below the modern level) Caspian base level. This new maximum depositional age from the Productive Series at Sarica of 2.5 ± 0.2 Ma indicates that the regionally extensive Akchagyl transgression, which ended the deposition of the Productive Series near the Caspian coast at 3.2 Ma, may have appeared a minimum of 0.5 Myr later in the northern interior of the Kura Basin than at the modern Caspian Sea coast. The results of this work have important implications for the tectonic and stratigraphic history of the region, suggesting that the initiation of the Plio‐Pleistocene Kura fold‐thrust belt may have not been as diachronous along strike as previously hypothesized. More generally, these results also provide a measure of the magnitude of diachroneity possible along sequence boundaries, particularly in isolated basins. Comparison of accumulation rates between units in the interior of the Kura subbasin and the South Caspian main basin suggest that extremely large variations in these rates within low‐stand deposits may be important in identifying the presence of subbasins in older stratigraphic packages.     

Seasonal Rainfall Forecasts for the Yangtze River Basin in the Extreme Summer of 2020※

Seasonal forecasts for Yangtze River basin rainfall in June, May–June–July (MJJ), and June–July–August (JJA) 2020 are presented, based on the Met Office GloSea5 system. The three-month forecasts are based on dynamical predictions of an East Asian Summer Monsoon (EASM) index, which is transformed into regional-mean rainfall through linear regression. The June rainfall forecasts for the middle/lower Yangtze River basin are based on linear regression of precipitation. The forecasts verify well in terms of giving strong, consistent predictions of above-average rainfall at lead times of at least three months. However, the Yangtze region was subject to exceptionally heavy rainfall throughout the summer period, leading to observed values that lie outside the 95％ prediction intervals of the three-month forecasts. The forecasts presented here are consistent with other studies of the 2020 EASM rainfall, whereby the enhanced mei-yu front in early summer is skillfully forecast, but the impact of midlatitude drivers enhancing the rainfall in later summer is not captured. This case study demonstrates both the utility of probabilistic seasonal forecasts for the Yangtze region and the potential limitations in anticipating complex extreme events driven by a combination of coincident factors.     

Ecohydrologic separation alters interpreted hydrologic stores and fluxes in a headwater mountain catchment

Recent studies have demonstrated that compartmentalized pools of water preferentially supply either plant transpiration (poorly mobile water) or streamflow and groundwater (highly mobile water) in some catchments, a phenomenon referred to as ecohydrologic separation. The omission of processes accounting for ecohydrologic separation in standard applications of hydrological models is expected to influence estimates of water residence times and plant water availability. However, few studies have tested this expectation or investigated how ecohydrologic separation alters interpretations of stores and fluxes of water within a catchment. In this study, we compare two rainfall‐runoff models that integrate catchment‐scale representations of transport, one that incorporates ecohydrologic separation and one that does not. The models were developed for a second‐order watershed at the H.J. Andrews Experimental Forest (Oregon, USA), the site where ecohydrologic separation was first observed, and calibrated against multiple years of stream discharge and chloride concentration. Model structural variations caused mixed results for differences in calibrated parameters and differences in storage between reservoirs. However, large differences in catchment storage volumes and fluxes arise when considering only mobile water. These changes influence interpreted residence times for streamflow‐generating water, demonstrating the importance of ecohydrologic separation in catchment‐scale water and solute transport.     

An early warning system for wave-driven coastal flooding at Imperial Beach,CA

Natural Hazards - Waves overtop berms and seawalls along the shoreline of Imperial Beach (IB), CA when energetic winter swell and high tide coincide. These intermittent, few-hour long events flood...     

Historical trends affecting accumulation of sediment and phosphorus in Lake Pepin,upper Mississippi River,USA

This study was conducted to collect historical land use information that would help explain the historical patterns in accumulation of sediment and phosphorus in Lake Pepin documented by Engstrom et al. (J Paleolimnol, this issue). A wide range of historical factors including cropping systems, phosphorus applications from fertilizer and manure, human and animal populations, river flows and phosphorus discharges from waste water treatment plants were studied using statistical methods. Results showed that sediment losses from the Minnesota River basin are significantly correlated with historical increases in river flows, row crop production acreage and basin population. Total phosphorus accumulations in the sediments of Lake Pepin are significantly correlated with increased phosphorus discharges from metropolitan area wastewater treatment plants, and increases in row crop acreage and river flows. Total phosphorus inflows to Lake Pepin are significantly correlated with increases in river flows, row crop acreage and phosphorus fertilizer applied to agricultural lands. This is one of eight papers dedicated to the “Recent Environmental History of the Upper Mississippi River” published in this special issue of the Journal of Paleolimnology. D. R. Engstrom served as guest editor of the special issue.