Spatial B-jump at sudden channel enlargements with abrupt drop

An analytical and experimental study of the hydraulic jump in stilling basins with abrupt drop and sudden enlargement, called the spatial B-jump here, is carried out for finding the sequent depth ratio and resulting energy dissipation. The spatial B-jump studied has its toe downstream of the expansion section, and the stream lines at the toe are characterized by downward curvature. An expression is obtained for the sequent depth ratio based on the momentum equation with suitable assumptions for the extra pressure force term because of the abrupt drop in the bed and sudden enlargement in the basin width. Predictions compare favorably with experiments. It is shown that the spatial B-jump needs less tailwater depth, thereby enhancing the stability of the jump when compared either with spatial jump, which forms in sudden expanding channels, or with B-jump, which forms in a channel with an abrupt drop in bed. It is also shown that there is a significant increase in relative energy loss for the spatial B-jump compared to either the spatial jump or B-jump alone.

Historical evidence of abrupt coastal climatic change in Southern California, 1790-1880

Historical sources of the late-18th and 19th centuries were searched for information on coastal weather conditions in Southern California. Relatively calm winters until 1828 were followed by unusually stormy winters from about 1829 to 1839. Later periods were again predominantly calm, with notable exceptions related to the ENSO events of 1845 and 1878. Following decreases through the stormy 1830s, sizes of kelp forests appear to have rebounded in the 1840s. ENSO occurrences and eruption of the volcano Cosiguina in 1835 are likely causes for changing wind patterns. Our results link the unique AD 1840 Macoma leptonoidea pelecypod shell layer in laminated Santa Barbara Basin sediment ("Macoma event") to abruptly changing oceanographic and weather patterns.

Effects of non-abrupt interface on waveguide properties of sitle/Si MQW photodetector

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Causes and prediction of abrupt climate-driven ecosystem shifts in the North Atlantic

Warming of the global climate is now unequivocal and its impact on Earth’ functional units has become more apparent. Here, we show that marine ecosystems are not equally sensitive to climate change and reveal a critical thermal boundary where a small increase in temperature triggers abrupt ecosystem shifts seen across multiple trophic levels. This large-scale boundary is located in regions where abrupt ecosystem shifts have been reported in the North Atlantic sector and thereby allows us to link these shifts by a global common phenomenon. We show that these changes alter the biodiversity and carrying capacity of ecosystems and may, combined with fishing, precipitate the reduction of some stocks of Atlantic cod already severely impacted by exploitation. These findings offer a way to anticipate major ecosystem changes and to propose adaptive strategies for marine exploited resources such as cod in order to minimize social and economic consequences.

Rapid ecosystem response to abrupt climate changes during the last glacial period in western Europe, 40-16 ka

We present a high-resolution and independently dated multiproxy lake sediment record from the paleolake at Les Echets in southeastern France that displays synchronous changes in independent limnic and terrestrial ecosystem proxies, in concert with millennial-scale climate oscillations during the last glacial period. Distinct lake-level fluctuations, low lake organic productivity, and open, treeless vegetation indicate cold and dry conditions in response to Heinrich events. Alternating phases of higher and low lake organic productivity, stratified surface waters and long-lasting lake ice cover, decreased or increased catchment erosion, and tree-dominated or herb-dominated vegetation resemble Dansgaard-Oeschger interstadial-stadial variability. Transitions between different ecological states occurred in as little as 40-230 yr and seem to have been controlled by the position of the Polar Front. Ecosystem response after 30 ka suggests that local climate conditions became more important. Our results demonstrate that all parts of the terrestrial system responded to the abrupt and dramatic climatic changes associated with Dansgaard-Oeschger and Heinrich events, and that regional factors modulated ecosystem response.

Local Conentration of Waves due to Abrupt Alongshore Variation of Nearshore Bathymetry

Small-scale physical and numerical experiments were conducted to investigate the local concentration of waves (monochromatic and group) due to abrupt change of nearshore bathymetry in alongshore direction. Wave run-up motions along the shoreline were measured using an image analysis technique to compare localized concentration of wave energy, when waves propagate a over bathymetry composing rhythmic patterns of mild/steep slope bottom configurations. Measured alongshore variation of maximum wave run-up heights showed significant peak near the boundary, which has sudden alongshore change of depth, both under monochromatic and group wave trains. This phenomenon is found to be due to interaction of waves with neashore currents, which is further enhanced by excitation of long wave components by breaking of group waves. Furthermore, this paper discusses results of preliminary experiments carried out to test the effectiveness of several shore protection structure layouts in mitigating such wave concentrations. Numerical simulations were performed by using a model developed based on Nwogu (1993) Boussinesq-type equations; coupled with a transport equation to model energy dissipation due to wave breaking.

Northern peatland initiation lagged abrupt increases in deglacial atmospheric CH4

Peatlands are a key component of the global carbon cycle. Chronologies of peatland initiation are typically based on compiled basal peat radiocarbon (14C) dates and frequency histograms of binned calibrated age ranges. However, such compilations are problematic because poor quality 14C dates are commonly included and because frequency histograms of binned age ranges introduce chronological artefacts that bias the record of peatland initiation. Using a published compilation of 274 basal 14C dates from Alaska as a case study, we show that nearly half the 14C dates are inappropriate for reconstructing peatland initiation, and that the temporal structure of peatland initiation is sensitive to sampling biases and treatment of calibrated14C dates. We present revised chronologies of peatland initiation for Alaska and the circumpolar Arctic based on summed probability distributions of calibrated 14C dates. These revised chronologies reveal that northern peatland initiation lagged abrupt increases in atmospheric CH4 concentration at the start of the Bølling–Allerød interstadial (Termination 1A) and the end of the Younger Dryas chronozone (Termination 1B), suggesting that northern peatlands were not the primary drivers of the rapid increases in atmospheric CH4. Our results demonstrate that subtle methodological changes in the synthesis of basal 14C ages lead to substantially different interpretations of temporal trends in peatland initiation, with direct implications for the role of peatlands in the global carbon cycle.