Utilization of the energy in ocean waves

The characteristics of ocean wind waves place certain constraints upon devices designed to convert their energy to a useful form. Here we consider the nature of these constraints and the theoretical analysis of a wave power generator that conforms to the design criteria. We also present the results of field tests with several models of the wave power generator. The experimental results support the theory and indicate that such a wave pump is suitable for power generation in a variety of circumstances.     

Hydraulic geometry and maximum flow efficiency as products of the principle of least action

Basic flow relationships have previously been seen to be insufficient to explain the self‐adjusting mechanism of alluvial channels and as a consequence extremal hypotheses have been incorporated into the analyses. In contrast, this study finds that by introducing a channel form factor (width/depth ratio), the self‐adjusting mechanism of alluvial channels can be illustrated directly with the basic flow relations of continuity, resistance and sediment transport. Natural channel flow is able to reach an optimum state (Maximum Flow Efficiency (MFE), defined as the maximum sediment transporting capacity per unit available stream power) with regard to the adjustment of channel form such that rivers exhibit regular hydraulic geometry relations at dominant or bankfull stage. Within the context of MFE, this study offers support for the use of the concepts of maximum sediment transporting capacity (MSTC) and minimum stream power (MSP). Furthermore, this study indicates that the principle of least action is able to provide a physical explanation for the existence of MFE, MSTC and MSP. Potential energy is minimized and consequently sediment transport is maximized in alluvial channels. Copyright © 2000 John Wiley & Sons, Ltd.     

Intensification of decadal and multi-decadal sea level variability in the western tropical Pacific during recent decades

Previous studies have linked the rapid sea level rise (SLR) in the western tropical Pacific (WTP) since the early 1990s to the Pacific decadal climate modes, notably the Pacific Decadal Oscillation in the north Pacific or Interdecadal Pacific Oscillation (IPO) considering its basin wide signature. Here, the authors investigate the changing patterns of decadal (10–20 years) and multidecadal (>20 years) sea level variability (global mean SLR removed) in the Pacific associated with the IPO, by analyzing satellite and in situ observations, together with reconstructed and reanalysis products, and performing ocean and atmosphere model experiments. Robust intensification is detected for both decadal and multidecadal sea level variability in the WTP since the early 1990s. The IPO intensity, however, did not increase and thus cannot explain the faster SLR. The observed, accelerated WTP SLR results from the combined effects of Indian Ocean and WTP warming and central-eastern tropical Pacific cooling associated with the IPO cold transition. The warm Indian Ocean acts in concert with the warm WTP and cold central-eastern tropical Pacific to drive intensified easterlies and negative Ekman pumping velocity in western-central tropical Pacific, thereby enhancing the western tropical Pacific SLR. On decadal timescales, the intensified sea level variability since the late 1980s or early 1990s results from the “out of phase” relationship of sea surface temperature anomalies between the Indian and central-eastern tropical Pacific since 1985, which produces “in phase” effects on the WTP sea level variability.     

Heating of Jupiter’s thermosphere by the dissipation of upward propagating acoustic waves

Thunderstorms in Jupiter’s atmosphere are likely to be prodigious generators of acoustic waves, as are thunderstorms in Earth’s atmosphere. Accordingly, we have used a numerical model to study the dissipation in Jupiter’s thermosphere of upward propagating acoustic waves. Model simulations are performed for a range of wave periods and horizontal wavelengths believed to characterize these acoustic waves. The possibility that the thermospheric waves observed by the Galileo Probe might be acoustic waves is also investigated. Whereas dissipating gravity waves can cool the upper thermosphere through the effects of sensible heat flux divergence, it is found that acoustic waves mainly heat the Jovian thermosphere through effects of molecular dissipation, sensible heat flux divergence, and Eulerian drift work. Only wave-induced pressure gradient work cools the atmosphere, an effect that operates at all altitudes. The sum of all effects is acoustic wave heating at all heights. Acoustic waves and gravity waves heat and cool the atmosphere in fundamentally different ways. Though the amplitudes and mechanical energy fluxes of acoustic waves are poorly constrained in Jupiter’s atmosphere, the calculations suggest that dissipating acoustic waves can locally heat the thermosphere at a significant rate, tens to a hundred Kelvins per day, and thereby account for the high temperatures of Jupiter’s upper atmosphere. It is unlikely that the waves detected by the Galileo Probe were acoustic waves; if they were, they would have heated Jupiter’s thermosphere at enormous rates.     

A multi-proxy study of Holocene environmental change in the Maya Lowlands of Peten, Guatemala

We used multiple variables in a sediment core from Lake Peten-Itza, Peten, Guatemala, to infer Holocene climate change and human influence on the regional environment. Multiple proxies including pollen, stable isotope geochemistry, elemental composition, and magnetic susceptibility in samples from the same core allow differentiation of natural versus anthropogenic environmental changes. Core chronology is based on AMS ¹⁴C measurement of terrestrial wood and charcoal and thus avoids the vagaries of hard-water-lake error. During the earliest Holocene, prior to 9000 ¹⁴C yr BP, the coring site was not covered by water and all proxies suggest that climatic conditions were relatively dry. Water covered the coring site by 9000 ¹⁴C yr BP, coinciding with filling of other lakes in Peten and farther north on the Yucatan Peninsula. During the early Holocene (9000 to 6800 ¹⁴C yr BP), pollen data suggest moist conditions, but high ¹⁸O values are indicative of relatively high E/P. This apparent discrepancy may be due to a greater fractional loss of the lake's water budget to evaporation during the early stages of lake filling. Nonetheless, conditions were moist enough to support semi-deciduous lowland forest. Decrease in ¹⁸O values and associated change in ostracod species at 6800 ¹⁴C yr BP suggest a transition to even moister conditions. Decline in lowland forest taxa beginning 5780 ¹⁴C yr BP may indicate early human disturbance. By 2800 ¹⁴C yr BP, Maya impact on the environment is documented by accelerated forest clearance and associated soil erosion. Multiple proxies indicate forest recovery and soil stabilization beginning 1100 to 1000 ¹⁴C yr BP, following the collapse of Classic Maya civilization.     

Saturn's icy satellites: Thermal and structural models

Thermal histories of the small icy Saturnian satellites Mimas, Tethys, Dione, Rhea, and Iapetus are constructed by assuming that they formed as homogeneous ice-silicate mixtures. The models include effects of radiogenic and accretional heating, conductive and subsolidus convective heat transfer, and lithospheric growth. Accretional heating is unlikely to have melted the water ice in the interiors of these bodies and solid state creep of the predominately ice material precludes melting by radiogenic heating. Mimas is so small that its thermal evolution is essentially purely conductive; at present it is a cold, nearly isothermal body. Any subsolidus convection or thermal activity in Mimas would have been confined to a brief period in its early history and would have been due to a warm formation. The four largest satellites are big enough and contain sufficient heat-producing silicates that solid state convection beneath a rigid lithosphere is inevitable independent of initial conditions. Dione and Rhea have convective interiors for most of their thermal histories, while Tethys and Iapetus have mainly conductive thermal histories with early periods of convective 0activity. The thermal histories of the five satellites for the last 4 by are independent of initial conditions; at present they have cold, conductive interiors. The model thermal histories are qualitatively consistent with the appearances of these satellites: Mimas has an ancient heavily cratered surface, Tethys and probably Iapetus have both heavily cratered and more lightly cratered areas, and Dione and Rhea have extensively modified surfaces. Because of their similar sizes and densities, Mimas and Enceladus are expected to have similar surfaces and thermal histories, but instead Enceladus has the most modified surface of all the small icy Saturnian satellites. Our results suggest a heat source for Enceladus, in addition to radiogenic and accretional heating; tidal dissipation is a possibility. Because the water ice in these bodies does not melt, resurfacing must be accomplished by the melting of a low-melting-temperature minor component such as ammonia hydrate.     

Contrasting transient and steady-state rivers crossing active normal faults: new field observations from the Central Apennines, Italy

We present detailed data on channel morphology, valley width and grain size for three bedrock rivers crossing active normal faults which differ in their rate, history and spatial distribution of uplift. We evaluate the extent to which downstream changes in unit stream power correlate with footwall uplift, and use this information to identify which of the channels are likely to be undergoing a transient response to tectonics, and hence clarify the key geomorphic features associated with this signal. We demonstrate that rivers responding transiently to fault slip-rate increase are characterised by significant long-profile convexities (over-steepened reaches), a loss of hydraulic scaling, channel aspect ratios which are a strong non-linear function of slope, narrow valley widths, elevated coarse-fraction grain-sizes and reduced downstream variability in channel planform geometry. We are also able to quantify the steady-state configurations of channels, that have adjusted to differing spatial uplift fields. The results challenge the application of steady-state paradigms to transient settings and show that assumptions of power-law width scaling are inappropriate for rivers, that have not reached topographic steady state, whatever exponent is used. We also evaluate the likely evolution of bedrock channels responding transiently to fault acceleration and show that the headwaters are vulnerable to beheading if the rate of over-steepened reach migration is low. We estimate that in this setting the response timescale to eliminate long-profile convexity for these channels is ∼1 Myr, and that typical hydraulic scaling is regained within 3 Myr.     

Effects of salinity on the condition and survival of zebra mussels (Dereissena polymorpha)

The objectives of this study were to determine the tolerance of various life stages of zebra mussels to salinity; determine the extent to which acclimation events in estuarine systems, affect tolerance of zebra mussels; and determine the effects of salinity on health or condition of adult zebra mussels. At high temperatures (18–20°C), the condition of zebra mussels is reduced at salinities above 1%.. However, at lower temperatures (3–12°C), the optimum salinity for zebra mussels is 2–4%.. The incipient lethal salinity of post-veligers is near 2%., of larger adults (5–15 mm) between 2%. and 4%., and of veliger larvae near 4.5%.. Zebra mussels are able to acclimate to slowly changing salinities (i.e., 1%. d^?1) such that the time to 50% mortality of a population should be greater than 1 yr at temperatures near 20°C and salinites up to 8%.     

Appraisal of the future climate of the Holocene in the Rocky Mountains

Consideration of the history of Holocene climate in the Rocky Mountains indicates that the over-all trend during the past 2500 yr has been toward increasing warmth, interrupted by cooler times of minor advances of cirque glaciers. Comparison of Holocene climatic history with the record of past interglacials in the region suggests that the present interglacial is not complete and that the climate may become first warmer and subsequently wetter before it is completed. Correlation of the timing of the regional glacial-interglacial record for the past 140,000 yr with the record of major sea level changes and with the calculated changes in the earth's insolation suggest that the present interglacial may be completed within a few millenia and that it may be followed by a significant cooling of the climate.