The balance and redistribution of potential vorticity within the ocean

The balance and redistribution of potential vorticity in a turbulent, inhomogeneous ocean is examined analytically. In the first part of the paper, we derive a vorticity conservation law describing the way in which vorticity is lost and gained in a closed ocean basin. We show that the potential vorticity within the ocean can only be altered by molecular diffusion at the bounding surfaces, or by the advective input of vorticity via the turbulent velocity at the free upper surface. An imbalance between these two effects within the body of the ocean leads to a net temporal change in the total potential vorticity, which we are able to express in terms of the temporal changes in the magnitude and orientation of a vector formed by the surface velocity and density and the earth's rotation. Finally, the steady-state balance of integrated vorticity is found to be between the surface wind-stress curl and the molecular diffusion at the solid boundaries.We then consider the localized redistribution of potential vorticity in detail. In recent papers, Green (1970) and Welander (1973) have purported to show that an internal redistribution of potential vorticity can give rise to important forces in the atmosphere (Green) and the ocean (Welander). Using their recipe leads in some cases to such unphysical results as a breakdown of the laws of conservation of angular momentum. Such consequences cause us to question the validity of the argument, and lead us to a close examination' of Welander's mechanism. (Green's argument is more general and does not lend itself to such close examination.) Our investigation reveals important flaws not only in Welander's argument, but also in the classical mixing-length theory of Prandtl (1925) and of Taylor (1915) on which it is modelled. More specifically, the internal redistribution process presented in these theories is found to be incomplete since it fails to take into account the lift force exerted on individual parcels of fluid as they are transported by the turbulent motions. Inclusion of these lift forces not only permits the unification of the classical mixing-length theories, but also shows that the forces claimed by Welander do not in fact arise.     

A numerical study of damping

Diagonal damping matrices were computed for three systems which have non-proportional damping matrices. These diagonal damping matrices were computed on three bases, as follows: 1. After normalizing the equations of motion by the modal matrix, the diagonal terms are retained ignoring the non-diagonal terms. 2. Diagonal damping matrix is established by the optimization algorithm which minimizes the mean square error of the frequency response. 3. Diagonal damping is determined from the normalized differential equation by matching the peaks of the coupled and uncoupled system. The frequency responses for the three cases of one of the three systems are presented together with a comparison of the energy dissipation.     

Volcanic features of the North Rockall Trough: application of visualisation techniques on 3D seismic reflection data

Understanding the volcanic processes operating during continental break-up is hampered by the subsequent burial of the majority of the volcanic pile beneath thick sedimentary sequences currently located in bathymetrically deep offshore regions. Although portions of these volcanic systems are currently exposed on land, a full understanding of the volcanic structure, the eruptive styles and their evolution is not possible as these localities have been partially eroded. Furthermore, as the onshore exposure represents a volumetrically minor part of the entire system, the documented eruptive styles may not be representative. The increasing availability of 3D seismic reflection data has the potential to significantly enhance our understanding of break-up related volcanism, as it allows direct access to detailed information from the buried volcanic succession. However, conventional seismic interpretation methodologies cannot determine lava flow morphologies, and as a result, eruptive styles and their evolution are still largely based on extrapolation from the accessible onshore outcrop data. New 3D seismic volume visualisation techniques allow the buried basalt morphologies to be examined for the first time in a manner similar to outcrop, aerial photographic or satellite-based observations. Applying this new approach for 3D seismic data to the North Rockall Trough, U.K. Atlantic margin, demonstrates that a range of volcanic features indicative of eruptive style can be determined. The data reveal a complex terrain containing lava flows originating from discrete volcanic centres, contemporaneous normal faults, linear fissures a few kilometres long, radial fissures and inflation ridges. Lava flow morphologies that are indicative of tube-fed inflated sheetflows, intracanyon flows and elongate subaerial flows that enter water downslope to produce a large hyaloclastite delta are observed.Electronic Supplementary Material Supplementary material is available in the online version of this article at .Editorial responsibility: J. Stix     

Late Pleistocene and Holocene depositional systems and the palaeogeography of the Dogger Bank, North Sea

3D seismic data from the Dogger Bank, North Sea, allow the mapping of Late Pleistocene and Holocene depositional systems in unprecedented detail. The data demonstrate that glacial processes resulted in the development of incised tunnel valley systems during the Weichselian and that these were subsequently modified by fluvial processes in a pro-glacial setting. Subsequently, the Dogger Bank formed an emergent plain during the Holocene with a complex meandering river system, associated tributary or distributary channels and lakes, dominating the region. Prior to the sea level rising sufficiently to submerge the Dogger Bank around 7500 yr ago, the meandering river system was replaced by a dendritic channel network of potential fluvial, estuarine or intertidal origin. As the Holocene depositional features bear no systematic relationship to the bathymetry this study demonstrates that previously published bathymetry-based models for the Holocene palaeogeographic development of the North Sea require modification.     

Postmagmatic cooling and late Cenozoic denudation of the North Patagonian Batholith in the Los Lagos region of Chile, 41°−42°15′S

Zircon and apatite fission track (FT) thermochronology was applied to investigate the history of cooling and denudation of the Southern Andes between 41° and 42°15′S in relation to the late Cenozoic activity of the Liquiñe-Ofqui fault zone (LOFZ) and the northward migration of the Chile Triple Junction (CTJ). Fifty-six zircon and 51 apatite FT ages, plus 37 apatite confined track-length distributions were obtained mainly from plutonic rocks of the North Patagonian Batholith (NPB) in the main Andean Cordillera. Apatite FT ages and track lengths indicate a stage of rapid cooling at ∼5--3 Ma along both sides of the LOFZ, whereas older Miocene ages with monotonous cooling histories were obtained further away from the fault. Zircon FT ages range from Cretaceous to Pliocene, with marked differences observed along and across the LOFZ. Three different types of temperature-time histories characterise the post-magmatic cooling of the NPB in the region: deep intrusions with moderate and steady cooling rates, intrusions in the upper crust with very slow cooling rates following a stage of initial rapid cooling, and rapidly cooled and exhumed shallow intrusions, the latter with younger ages towards the fault zone. The most prominent denudation episode along the LOFZ is late Miocene to Pliocene, coeval with plate tectonic reconstructions for the arrival and subduction of the Chile Rise beneath the Taitao Peninsula.     

Simulation of Time Series of Concentration Fluctuations in Atmospheric Dispersion Using a Correlation-distortion Technique

Short-duration fluctuations in the concentration of airborne substances can be important in a variety of atmospheric dispersion problems, especially when assessing the risks posed by harmful materials. This paper discusses a simulation technique for generating individual realisations of fluctuating concentration time series in dispersing plumes based on target probability distributions and spectral functions. The scheme uses a correlation-distortion approach to simulate these time series. Gaussian processes with modified spectral characteristics are generated and then transformed to yield non-Gaussian processes with the desired spectral characteristics. The simulation scheme is initially developed for a single receptor, and is then extended to model pairs of correlated time series at two receptors. In fact, the modelling technique can be generalised to an arbitrary number of receptors and this provides, in principal, an approach that is applicable to a wide class of similar problems (such as the modelling of instantaneous puff releases or the response of line-of-sight detection systems). The simulation technique is illustrated using observations made during recent field experiments, conducted both in the United Kingdom and in the U.S.A., investigating the short-range dispersion of a passive tracer.     

Ducted whistler-mode signals received at two widely spaced locations

Whistler-mode signals from a single VLF transmitter that have propagated in the same duct, have been observed simultaneously at Faraday, Antarctica (65°S, 64°W) and Dunedin, New Zealand (46°S, 171°E). The signals received have group-delay times that differ in the order of 10 ms, which can be explained by the differences in southern-hemisphere sub-ionospheric propagation time from duct exit region to receiver for the two sites. This difference has been used to determine the location of the duct exit region, with confirmation provided by arrival-bearing information from both sites. The whistler-mode signals typically occur one or two days after geomagnetic activity, with K_p\geq5. The sub-ionospheric-propagation model, LWPC, is used to estimate the whistler-mode power radiated from the duct exit region. These results are then combined with estimated loss values for ionospheric and ducted transmission to investigate the role of wave-particle amplification or absorption. On at least half of the events studied, plasmaspheric amplification of the signals appears to be needed to explain the observed whistler-mode signal strengths.