摘要:

A region of transition of surface water characteristics from subantarctic to antarctic and an associated eastward flowing Antarctic Circumpolar Current (ACC) have long been recognized to exist as a band around Antarctica. In this review we summarize the most important observational and theoretical findings of the past decade regarding the ACC, identify gaps in our knowledge, and recommend studies to address these. The nature of the meridional zonation of the ACC is only now being revealed. The ACC seems to exist as multiple narrow jets imbedded in, or associated with, density fronts (the Subantarctic and Polar fronts) which appear to be circumpolar in extent. These fronts meander, and current rings form from them; lateral frontal shifts of as much as 100 km in 10 days have been observed. The volume transport of the ACC has been estimated many times with disparate results. Recently, yearlong direct measurements in Drake Passage have shown the mean transport to be approximately 134 × 106m³/s, with an uncertainty of not more than 10%. The instantaneous transport can vary from the mean by as much as 20%, with most of the variation associated with changes in the reference flow at 2500 m rather than in the vertical shear. Meridional exchanges of heat across the ACC are known to be important to the heat balance of the abyssal ocean and consequently to global climate. The most likely candidate process for the required poleward heat exchange seems to be mesoscale eddies, though narrow abyssal boundary currents may also be important. Observations from ships, drifters, and satellites reveal surface mean kinetic energy to be at a maximum along the axis of the ACC and eddy kinetic energy to be large mainly in western boundary regions and off the tip of Africa. Eddy variability in the open ocean is consistent with baroclinic instability of the narrow jets. Calculations using data from Drake Passage show that the necessary conditions for baroclinic and barotropic instabilities are met in the ACC. The basic dynamical balance of the ACC is still not well known, although bottom and lateral topography and dynamic instabilities are shown to be important in balancing wind forcing. The ACC is generally conceded to be driven by the wind, but the coupling of wind and thermohaline circulations have not yet been adequately investigated. The mechanism responsible for the multiple cores of the ACC has not been identified in detail. It is suggested that future studies address: (1) the circumpolar structure and temporal behavior of the Subantarctic Front and Polar Front; (2) the general dynamical balance of the ACC and specific mechanisms for creation and maintenance of the major fronts; (3) the representativeness to the entire ACC of the existing estimates of meridional exchanges of heat and other properties, as well as kinematic and dynamic quantities, made in Drake Passage; (4) the variability of the ACC transport in several places and coherence of its variability; (5) the climatology of fields of atmosphere‐ocean forcing over the southern ocean; and (6) the possibility of identifying and using simple indices as good indicators of the behavior of the ACC or parts ther

ISSN:8755-1209    

DOI:10.1029/RG024i003p00469

年代:1986

数据来源: WILEY

摘要:

This paper reviews the nonlinear interaction calculations for the internal gravity wave field in the deep ocean. The nonlinear interactions are a principal part of the dynamics of internal waves and are an important link in the overall energy cascade from large to small scales. Four approaches have been taken for their analysis: the evaluation of the transfer integral describing weakly and resonantly interacting waves, the application of closure hypotheses from turbulence theories to more strongly interacting waves, the integration of the eikonal or ray equations describing the propagation of small‐scale internal waves in a background of large‐scale internal waves, and the direct numerical simulation of the basic hydrodynamic equations of motion. The weak resonant interaction calculations have provided most of the conventional wisdom. Specific interaction processes and their role in shaping the internal wave spectrum have been unveiled and a comprehensive inertial range theory developed. The range of validity of the resonant interaction approximation, however, is not known and must be seriously doubted for high‐wave number, high‐frequency waves. The turbulence closure calculations and the direct numerical modeling are not yet in a state to be directly applicable to the oceanic internal wave field. The closure models are too complex and rest on conjectures that are not demonstrably justified. Numerical modeling can treat strongly interacting waves and buoyant turbulence, but is severely limited by finite computer resolutions. Extensive suites of experiments have only been carried out for two‐dimensional flows. The eikonal calculations provide an efficient and versatile tool to study the interaction of small‐scale internal waves, but it is not clear to what extent the scale‐separated interactions with larger‐scale internal waves compete with and might be overwhelmed by interactions among like scales. The major shortcoming of all four approaches is that they neglect the interaction with the vortical (=potential vorticity carrying) mode of motion that must be expected to exist in addition to internal waves at small scales. This interaction is intrinsically neglected in all Lagrangian‐based studies and in the non‐rotating two‐dimensional simulations. The most promising approach for the future that can handle both arbitrarily strong interactions and the interaction with the vortical mode is numerical modeling once the resolutio

ISSN:8755-1209    

DOI:10.1029/RG024i003p00493

年代:1986

数据来源: WILEY

摘要:

During the Mesozoic most of the Arabian Peninsula, Persian Gulf, south‐western Iran, and eastern Iraq constituted the Arabian platform. Deformation of the Musandam Peninsula in the Late Cretaceous and mid‐Tertiary by compression (subduction) from the east and southwest, collision of the Arabian platform and Eurasian plate along the Zagros Crush zone during the Oligocene or early Miocene, and emplacement of the Zagros Mountains by gravitational sliding during the Neogene and Pleistocene have reduced the platform area to the Persian Gulf. Other factors that contributed to the reduction of the Arabian platform include the uplift of the Arabian Peninsula during the opening of the Red Sea in the Tertiary, tectonism of the Infracambrian Hormuz salt, upwarp of the platform sediment cover by basement uplift and/or salt tectonics, and a 600‐ to 400‐m drop in sea level since the Cretaceous. At present, tectonism in the region is restricted to the northern edge of the Gulf of Oman where the Arabian plate is subducting the Eurasian plate from the south and to the Zagros Crush zone where the Arabian and Eurasian plates are colliding with one

ISSN:8755-1209    

DOI:10.1029/RG024i003p00537

年代:1986

数据来源: WILEY

摘要:

Near‐infrared reflectance spectra of the lunar highlands are synthesized to obtain global information about the composition of the lunar near‐side crust. Immature exposed surfaces from small fresh craters, mountains and massifs, and large craters with central peaks are included in a survey of rock types across the lunar near side. Compositions recognized in these data include anorthosite, noritic, gabbric, and troctolitic mineral assemblages. Three quarters of the areas studied that represent samples from the upper 1–2 km of lunar crust exhibit noritic compositions with different amounts of pyroxene and/or brecciation alteration. Norites are ubiquitous across the near side and show no spatial clustering associated with any of the major basins. In contrast to this noritic megaregolith of the upper crust, material representing stratigraphically deeper zones (5–15 km) of the lunar crust is dominated by gabbros, anorthosites, and troctolites, with less than a quarter of the areas studied exhibiting a noritic composition. The origin of the noritic megaregolith and its relation to deeper crustal material is thus somewhat of an enigma. The current limited statistics for areas identified as anorthosites and troctolites are insufficient to derive global spatial distributions. Gabbroic areas, however, are notably concentrated in the western hemisphere. Although incomplete for a global assessment of crustal materials, these data demonstrate that the lunar near‐side crust is clearly mineralogically heterogeneous, both laterally and vertically, and not well mixed bel

ISSN:8755-1209    

DOI:10.1029/RG024i003p00557

年代:1986

数据来源: WILEY

摘要:

Despite steady effort over the last century, continuously recording tiltmeters and strainmeters have not yet been successful except for earth tide measurements. This article reviews the techniques and instrument designs that have been developed in this field. The continuous measurement of true tectonic motions, which have rates of 10−14s−1, requires extremely high instrument stability, but more rapid motions (tides and seismic waves apart) are even smaller and not much more easily detectable. Much past effort has been spent improving the transducers used in these instruments, which usually measure small displacements; the commonest choices are optical (the most accurate), capacitive (the most sensitive), and inductive (the most rugged). Nowadays building a transducer is the easiest part; a much harder task, equally important to a good design, is attaching the instrument to the ground so that it can detect the signal wanted. This part of the design includes choosing a location. A large underground opening is thermally stable but distorting; a surface installation is easy to build but noisy because of soil weathering; and a borehole is inaccessible to weather but also to the operator. All such installations (and indeed the whole field of strain and tilt measurement) of course rely on the assumption that the deformations of interest can be measured over baselengths of a few hundred meters or less; this is true for tides and seismic waves, but unproven for tectonic motions. Appropriate attachment techniques differ for each location, and many mundane details (such as cementing techniques for borehole instruments) remain to be worked out. The goal of measuring tectonic and tidal tilt has created many tiltmeter designs. Most are relatively small instruments that use some form of pendulum or bubble level; a few use a liquid surface to measure tilts over a long baseline. Repeated experience has shown the latter type, when well built, to be superior to the former, proving end‐monument motions to be a dominant source of noise. The same appears to hold true for strainmeters, the best results coming from long‐base laser instruments, though strainmeters using rigid or flexible material length standards are useful for special purposes, and borehole instruments show promise. Because achieving a good design has been so difficult, it is important to select the goals an instrument must meet and be prepared to compromise on others. The difficulty of validating results from this type of instrument makes comparison tests essential; such tests as have been made so far show that good results cannot be gotten at low cost or without careful attention to

ISSN:8755-1209    

DOI:10.1029/RG024i003p00579

年代:1986

数据来源: WILEY

摘要:

The surface energy balance (SEB) of a freshwater body must incorporate shortwave (solar) and long‐wave (terrestrial) radiation, together with evaporation and sensible heat energy fluxes. For inclusion in a prognostic water quality model it is vital to be able to calculate, rather than measure, these energy terms and to relate them to routinely observed meteorological parameters such as air temperature and cloud cover. Here various formulae are analyzed and compared by using two climatologically different data bases. For shortwave radiation the formulae recommended for use, in the dearth of local observations, are those of Prescott (1940) (for temporal scales of weeks or months) and Tucker (1982) (for shorter time scales). The shortwave albedo can be given by the parametrization of Pivovarov (1972), the incoming long‐wave atmospheric radiation by a blackbody representation modified by an effective atmospheric emissivity (Raphael, 1962), and the evaporative flux by the equation of Sill (1983), with the sensible heat flux being related to the evaporative energy flux via the Bowen ratio under conditions of neutral atmospheric stabil

ISSN:8755-1209    

DOI:10.1029/RG024i003p00625

年代:1986

数据来源: WILEY

摘要:

Although a great deal of work has been done on the prediction of the sunspot number 1–10 years in advance, very little has been done on the related problem of predicting the level of geomagnetic activity. It is sometimes assumed, apparently, that a prediction of sunspot number is a fairly good guide to the level of geomagnetic activity to be expected, but this is far from the case. However, long‐term prediction of geomagnetic activity is important both for space mission planning and to test our empirical understanding of the relationship of the solar wind to the solar cycle. In this review we bring together the work that has been done in this field to provide a ready reference both for the specialist in solar‐terrestrial relations and for others requiring geomagnetic prediction capability. We find, however, that surprisingly little attention has been paid to this problem, and it is our hope that this review will stimulate further

ISSN:8755-1209    

DOI:10.1029/RG024i003p00650

年代:1986

数据来源: WILEY

摘要:

This paper summarizes our present, preencounter understanding of the physical and chemical processes controling the inner (r<1000 km) region of cometary atmospheres. Special emphasis was attached to compiling a self‐consistent set of governing equations. We are aiming this review at readers who want to understand the present status of the mantle and coma regions and/or who want to develop new, next generation models which will be needed as the large volume of new observational data will become available in the near futur

ISSN:8755-1209    

DOI:10.1029/RG024i003p00667

年代:1986

数据来源: WILEY