Integrated marine and terrestrial evidence for abrupt Congo River palaeodischarge fluctuations during the last deglaciation

We present a high‐resolution reconstruction of tropical palaeoenvironmental changes for the last deglacial transition (18 to 9 cal. kyr BP) based on integrated oceanic and terrestrial proxies from a Congo fan core. Pollen, grass cuticle, Pediastrum and dinoflagellate cyst fluxes, sedimentation rates and planktonic foraminiferal δ¹⁸O ratios, u₃₇^K′ sea‐surface temperature and alkane/alkenone ratio data highlight a series of abrupt changes in Congo River palaeodischarge. A major discharge pulse is registered at around 13.0 cal. kyr BP which we attribute to latitudinal migration of the Intertropical Convergence Zone (ITCZ) during deglaciation. The data indicate abrupt and short‐lived changes in the equatorial precipitation regime within a system of monsoonal dynamics forced by precessional cycles. The phases of enhanced Congo discharge stimulated river‐induced upwelling and enhanced productivity in the adjacent ocean. Copyright © 2001 John Wiley & Sons, Ltd.     

A further assessment of vegetation feedback on decadal Sahel rainfall variability

The effect of vegetation feedback on decadal-scale Sahel rainfall variability is analyzed using an ensemble of climate model simulations in which the atmospheric general circulation model ICTPAGCM (“SPEEDY”) is coupled to the dynamic vegetation model VEGAS to represent feedbacks from surface albedo change and evapotranspiration, forced externally by observed sea surface temperature (SST) changes. In the control experiment, where the full vegetation feedback is included, the ensemble is consistent with the observed decadal rainfall variability, with a forced component 60 % of the observed variability. In a sensitivity experiment where climatological vegetation cover and albedo are prescribed from the control experiment, the ensemble of simulations is not consistent with the observations because of strongly reduced amplitude of decadal rainfall variability, and the forced component drops to 35 % of the observed variability. The decadal rainfall variability is driven by SST forcing, but significantly enhanced by land-surface feedbacks. Both, local evaporation and moisture flux convergence changes are important for the total rainfall response. Also the internal decadal variability across the ensemble members (not SST-forced) is much stronger in the control experiment compared with the one where vegetation cover and albedo are prescribed. It is further shown that this positive vegetation feedback is physically related to the albedo feedback, supporting the Charney hypothesis.     

A quantitative assessment of changes in seasonal potential predictability for the twentieth century

Changes over the twentieth century in seasonal mean potential predictability (PP) of global precipitation, 200 hPa height and land surface temperature are examined by using 100-member ensemble. The ensemble simulations have been conducted by using an intermediate complexity atmospheric general circulation model of the International Center for Theoretical Physics, Italy. Using the Hadley Centre sea surface temperature (SST) dataset on a 1° grid, two 31 year periods of 1920–1950 and 1970–2000 are separated to distinguish the periods of low and high SST variability, respectively. The standard deviation values averaged for the (“Niño-3.4”; 5°S–5°N, 170°W–120°W) region are 0.71 and 1.15 °C, for the periods of low and high SST variability, respectively, with a percentage change of 62 % during December–January–February (DJF). The leading eigenvector and the associated principal component time series, also indicate that the amplitude of SST variations have positive trend since 1920s to recent years, particularly over the El Niño Southern Oscillation (ENSO) region. Our hypothesis states that the increase in SST variability has increased the PP for precipitation, 200 hPa height and land surface temperature during the DJF. The analysis of signal and noise shows that the signal-to-noise (S/N) ratio is much increased over most of the globe, particularly over the tropics and subtropics for DJF precipitation. This occurs because of a larger increase in the signal and at the same time a reduction in the noise, over most of the tropical areas. For 200 hPa height, the S/N ratio over the Pacific North American (PNA) region is increasing more than that for the other extratropical regions, because of a larger percentage increase in the signal and only a small increase in noise. It is also found that the increase in seasonal mean transient signal over the PNA region is 50 %, while increase in the noise is only 12 %, during the high SST variability period, which indicates that the increase in signal is more than the noise. For DJF land surface temperature, the perfect model notion is utilized to confirm the changes in PP during the low and high SST variability periods. The correlation between the perfect model and the other members clearly reveal that the seasonal mean PP changed. In particular, the PP for the 31 years period of 1970–2000 is higher than that for the 31 years period of 1920–1950. The land surface temperature PP is increased in northern and southern Africa, central Europe, southern South America, eastern United States and over Canada. The increase of the signal and hence the seasonal mean PP is coincides with an increase in tropical Pacific SST variability, particularly in the ENSO region.     

Effects of Testing Conditions on Intact Rock Strength and Variability

The strength of intact rock is a fundamental and integral part of many aspects of engineering and geologic practice. It is necessary to understand how testing conditions influence this strength and its variability, especially for use in reliability-based design. In this study, the effect of sample diameter and moisture content on the strength and its variability was examined in detail. Correlations are given for assessing the effect of sample diameter on the laboratory uniaxial compressive strength (q_u), Brazilian indirect tensile strength (q_t-Brazilian), and point load strength (I_s), and the associated direct correlations among these tests. Correlations also are given for assessing the effect of sample moisture content on q_u, q_t-Brazilian, and I_s. The statistics of data scatter suggest these correlations can introduce large uncertainties. The effect of sample diameter and moisture content on the coefficients of variation was examined subsequently, and it was found to be relatively minor.     

V838 Mon: light echo evolution and distance estimate

Following its 2002 February eruption, V838 Mon developed a light echo that continues to expand and evolve as light from the outburst scatters off progressively more distant circumstellar and/or interstellar material. Multifilter images of the light echo, obtained with the South African Astronomical Observatory (SAAO) 1.0-m telescope between 2002 May and 2004 December, are analysed and made available electronically. The expansion of the light echo is measured from the images and the data compared with models for scattering by a thin sheet and a thin shell of dust. From these model results we infer that the dust is probably in the form of a thin sheet distant from the star, suggesting that the material is of interstellar origin, rather than being from earlier stages in the evolution of the star. Although the fit is uncertain, we derive a stellar distance of ∼9 kpc and a star–dust distance of ∼5 pc, in good agreement with recent results reported from other methods. We also present JHKL  and Cousins UBVRI  photometry obtained at the SAAO during the post-outburst second, third and fourth observing seasons of the star. These data show complex infrared colour behaviour while V838 Mon is slowly brightening in the optical.     

Contribution of the east�Cwest thermal heating contrast to the South Asian Monsoon and consequences for its variability

The focus of this paper is to assess the relative role of the north?Csouth and east?Cwest contrasts in atmospheric heating for the maintenance of the South Asian summer monsoon climatology. The juxtaposition of the Eurasian land mass and the Indian Ocean is responsible for the north?Csouth contrast, while the greater diabatic heating above the western Pacific compared to the one over the African and the tropical South Atlantic Ocean region introduces the east?Cwest gradient. With a series of idealized atmospheric general circulation model experiments, it is found that both contrasts contribute to the maintenance of the South Asian monsoon climatology, but their impact varies at regional scales. The surface atmospheric cyclone and precipitation over northern India are mainly due to the north?Csouth contrast. On the other hand, when the Indian Ocean sea surface temperatures are close to their climatological mean values, the low-level cyclone and consequent rainfall activity in the Bay of Bengal and southern India result from the east?Cwest gradient. The physical mechanism relays on the southern part of the upper-level South Asian monsoon high being forced by the east?Cwest diabatic heating contrast via Sverdrup balance. The east?Cwest heating difference controls also the strength of the Tropical Easterly Jet. Finally, the contribution of the El Ni?o Southern Oscillation to the interannual variability of the Indian monsoon is interpreted as the result of a longitudinal shift of one of the centers of diabatic heating contributing to the east?Cwest contrast.     

The electric field gradient in natural iron-doped chrysoberyl Al<Subscript>2</Subscript>BeO<Subscript>4</Subscript> and sinhalite MgAlBO<Subscript>4</Subscript> single crystals

This work reports on the evaluation of the electric field gradient (EFG) in natural chrysoberyl Al₂BeO₄ and sinhalite MgAlBO₄ using two different procedures: (1) experimental, with single crystal Mössbauer spectroscopy (SCMBS) on the three principal sections of each sample and (2) a “fully quantitative” method with cluster molecular orbital calculations based on the density functional theory. Whereas the experimental and theoretical results for the EFG tensor are in quantitative agreement, the calculated isomer shifts and optical d–d-transitions exhibit systematic deviations from the measured values. These deviations indicate that the substitution of Al and Mg with iron should be accompanied by considerable local expansion of the coordination octahedra.     

Land–sea carbon and nutrient fluxes and coastal ocean CO2 exchange and acidification: Past,present, and future

Epochs of changing atmospheric CO₂ and seawater CO₂–carbonic acid system chemistry and acidification have occurred during the Phanerozoic at various time scales. On the longer geologic time scale, as sea level rose and fell and continental free board decreased and increased, respectively, the riverine fluxes of Ca, Mg, DIC, and total alkalinity to the coastal ocean varied and helped regulate the C chemistry of seawater, but nevertheless there were major epochs of ocean acidification (OA). On the shorter glacial–interglacial time scale from the Last Glacial Maximum (LGM) to late preindustrial time, riverine fluxes of DIC, total alkalinity, and N and P nutrients increased and along with rising sea level, atmospheric PCO₂ and temperature led, among other changes, to a slightly deceasing pH of coastal and open ocean waters, and to increasing net ecosystem calcification and decreasing net heterotrophy in coastal ocean waters. From late preindustrial time to the present and projected into the 21st century, human activities, such as fossil fuel and land-use emissions of CO₂ to the atmosphere, increasing application of N and P nutrient subsidies and combustion N to the landscape, and sewage discharges of C, N, P have led, and will continue to lead, to significant modifications of coastal ocean waters. The changes include a rapid decline in pH and carbonate saturation state (modern problem of ocean acidification), a shift toward dissolution of carbonate substrates exceeding production, potentially leading to the “demise” of the coral reefs, reversal of the direction of the sea-to-air flux of CO₂ and enhanced biological production and burial of organic C, a small sink of anthropogenic CO₂, accompanied by a continuous trend toward increasing autotrophy in coastal waters.     

Projection of low flow conditions in Germany under climate change by combining three RCMs and a regional hydrological model

The present study is aimed to: (a) project future low flow conditions in the five largest river basins in Germany, and (b) to account for the projections uncertainties. The eco-hydrological model SWIM was driven by different regional climate models (REMO, CCLM, and Wettreg) to simulate daily river discharges in each study basin. The 50-year low flow was estimated for the period 1961 to 2000, and its return period was assessed for two scenario periods, 2021–2060 and 2061–2100, using the generalized extreme value distribution. The 50-year low flow is likely to occur more frequently in western, southern, and parts of central Germany after 2061, as suggested by more than or equal to 80% of the model runs. The current low flow period (from August to September) may be extended until late autumn at the end of this century. The return period of 50-year deficit volume shows a similar temporal and spatial pattern of change as for the low flow, indicating slightly less severe conditions with lower confidence. When compared with flood projections for the same area using the same models, the severer low flows projected in this study appear more pronounced, consistent, and have lower uncertainty.