Showing papers in "Reviews of Geophysics in 1985"

Sea surface temperature anomalies, planetary waves, and air‐sea feedback in the middle latitudes

Abstract: Current analytical models for large-scale air-sea interactions in the middle latitudes are reviewed in terms of known sea-surface temperature (SST) anomalies. The scales and strength of different atmospheric forcing mechanisms are discussed, along with the damping and feedback processes controlling the evolution of the SST. Difficulties with effective SST modeling are described in terms of the techniques and results of case studies, numerical simulations of mixed-layer variability and statistical modeling. The relationship between SST and diabatic heating anomalies is considered and a linear model is developed for the response of the stationary atmosphere to the air-sea feedback. The results obtained with linear wave models are compared with the linear model results. Finally, sample data are presented from experiments with general circulation models into which specific SST anomaly data for the middle latitudes were introduced.

Submesoscale, coherent vortices in the ocean

Abstract: A variety of observations of intense, long-lived oceanic vortices are interpreted as examples of a distinct phenomenon, which is given the name Submesoscale, Coherent Vortices (SCV's). The distinguishing characteristics of SCV's are defined and illustrated by example, and a survey is made of the different SCV types presently known. On the basis of extant theoretical and modeling solutions, interpretations are made of the dynamics associated with SCV existence, movement, endurance, interactions with other currents, generation, and contributions to the transport of chemical properties in the ocean.

SNC meteorites: Clues to Martian petrologic evolution?

Abstract: Shergottites, nakhlites and the Chassigny meteorites (SNC group) may have originated on Mars. The shergottites are medium-grained basalts, the nakhlites are pyroxenites and the Chassigny is a dunite. The SNC group is petrologically diverse but differs from all other known achondrites in terms of mineral chemistry, the redox state, the oxygen isotopic composition and the radiometric ages. The SNC stones are mafic and ultramafic cumulate rocks with mineralogies that indicate rapid cooling and crystallization from tholeiitic magmas which contained water and experienced a high degree of oxidation. The characteristics suggest formation from a large parent body, i.e., a planet, but not earth. The estimated ages for the rocks match the estimated ages for several mapped Martian volcanoes in the Tharsis region. Additionally, the elemental and isotopic abundances of atmospheric gases embedded in melts in the SNC stones match Viking Lander data for the Martian atmosphere. However, reasons are cited for discounting the possibility that a large meteorite(s) collided with Mars about 180 myr ago and served as the mechanism for ejecting the SNC stones to earth.

Quantitative methods for analyzing the roughness of the seafloor

Abstract: The bathymetry of the world ocean has been mapped using a variety of acoustic sounding instruments and traditional contouring methods. Prior to this study, relatively few quantitative studies have been focused on the detailed morphology of the seafloor. The work of most investigators has generally dealt with the analysis of seafloor relief without regard to spatial frequency or concern for obvious morphologic anisotropies. A brief review of previous work is presented and is accompanied by an identification of the specific applications and limitations of those approaches. The availability of more precise digital sounding data coupled with the development of higher-resolution sonar mapping instruments (e.g., DEEPTOW, SEAMARC) now make it possible to investigate quantitatively many important aspects of seafloor relief and the geological processes responsible for its formation. Typical bathymetric contour maps represent a low-frequency deterministic model of the seafloor. To describe the higher-frequency variability or roughness of the seafloor requires the development of an appropriate statistical method for generating a valid stochastic model. New methods are developed herein which allow valid statistical models of the variability of oceanic depths to be derived from existing digital bathymetric soundings. The smooth contoured surface (often preserved as a geographic grid of depths), when supplemented by such a roughness model, can provide an essentially complete statistical description of the relief. Statistical models of seafloor roughness are also valuable tools for predicting acoustic scattering and bottom loss and, in addition, contain a wealth of information for more comprehensive interpretations of deep-sea, relief-forming geological processes. To allow the variability of depths to be described as a function of scale (spatial frequency), the amplitude spectrum is employed as the fundamental statistic underlying the model. Since the validity of the amplitude spectrum depends upon the assumption of a statistically stationary sample space, a computer algorithm operating in the spatial domain was developed which delineates geographic provinces of limited statistical heterogeneity. Within each of these provinces, a spectral model is derived by fitting the amplitude estimates with one or two two-parameter power law functions, using specialized regression techniques. The distribution of resultant model parameters is examined for a large test area adjacent to the coast of Oregon (42°–45°N, 130°–124°W) which includes several contrasting geologic environments. The distribution of roughness corresponds generally with the various physiographic provinces observed in the region. Within some provinces, additional complexities are apparent in the roughness model which cannot be inferred by simply studying the bathymetry. These patterns are related to a variety of geological processes operating in the region, such as the convergence of the continental margin and the presence of a propagating rift on the northern Gorda Rise. In contrast, a very large area of the continental margin off the east coast of the United States was found to have in common a single, distinct amplitude spectrum. This amplitude spectrum is almost identical to that found from the Tufts Abyssal Plain region off the west coast of the United States. Spectra from both of these areas can be clearly separated into two straight-line segments of different slope. The two segments of these spectra are interpreted as reflecting two dominant relief-forming processes, the higher-frequency band representing a sedimentary regime and the lower-frequency band representing an underlying tectonic/volcanogenic regime. In many cases, the calculated roughness statistics are not constant for data collected along different ship track directions; this is due to the anisotropic nature of the seafloor relief. A simple model is developed which describes the roughness statistics as a function of azimuth. The parameters of this model quantify the anistropy of the seafloor, allowing insight into the directionality of the corresponding relief-forming processes and the physical meaning of the derived model statistical parameters. Finally, the model is used to successfully predict the roughness of a surface at scales much smaller than those resolvable by surface sonar systems. The model regression line (derived from a hull-mounted sonar) is compared to data from deep-towed sonars and bottom photographs. The amplitude of roughness is predicted to within half an order of magnitude over five decades of spatial frequency, and this prediction capability can probably be improved even further. The stochastic models presented also demonstrate the potential for closely approximating the full two-dimensional nature of some areas of the seafloor, with only four statistical parameters. These parameters can be estimated from random ship track data, given a sufficient number of tracks and range of headings.

The auroral luminosity structure in the high-latitude upper atmosphere: Its dynamics and relationship to the large-scale structure of the Earth's magnetosphere

Abstract: The concept of the auroral oval was proposed independently by Feldstein and by Khorosheva in the early 1960's, and it has since been widely used as a reference frame for the organization of high-latitude optical, particle, and ionospheric data. More recently, different structural regions associated with the oval have been distinguished, and these have been identified with different magnetospheric regions by different workers. The net result is an inconsistent set of nomenclature and relationships. The literature of this development is reviewed in detail, and an interpretation and terminology that is consistent, in the opinion of the authors at least, are proposed. It is hoped that this proposal will serve as a useful basis for further discussion on the ordering of auroral phenomena in relation to the magnetosphere.

The inference of North Atlantic circulation patterns from climatological hydrographic data

Abstract: We present a review of β spiral techniques which have recently been developed for the determination of absolute velocity profiles from hydrographic observations. A specific technique is then designed and applied to the North Atlantic part of Levitus' (1982) climatological hydrographic atlas with the aim of estimating reference velocities and diffusivities for heat, salt, and potential vorticity. These quantities are determined on a 1° grid from the local gradients of temperature and salinity under the constraints of the thermal wind relations and the conservation of the respective tracers including diapycnic and isopycnic mixing terms. The estimation procedure includes the statistical framework of inverse modeling in the weighting of the constraints by the data noise variances and the determination of the covariances of the model parameters. The resulting circulation pattern bears strong resemblance to the classical view of the North Atlantic circulation as put foreward by Wust (1935) and Defant (1941). The upper layers are dominated by the Gulf Stream/North Atlantic current system with a broad subtropical gyre recirculation. In the lower layer a western boundary current is fed from Norwegian Sea overflow penetrating the Gibbs fracture zone and partly circulating around the Labrador Sea. As a consequence of the climatological averaging the currents appear in broad shape with much reduced velocities, in particular in the upper layer. The vertical structure reveals an almost horizontal level of no motion pattern much along the concepts of Defant (1941). Diffusion coefficients were determined for an upper layer (depth of mixed layer to 800 m depth) and a lower layer (800 m to 2000 m). The spatial pattern of these coefficients correlates with maps of eddy activity, showing higher values in the strong current regimes and low values within the subtropical and subpolar gyre. Average values in the lower layer of the quiet regions are 10−5 m²/s and 10² m²/s for the diapycnal and isopycnal diffusivity, respectively, and 10−1 m²/s for the vertical diffusivity of vorticity (which yields 10² m²/s for the lateral diffusivity of potential vorticity). Toward the regions of strong currents and in the upper layer these values roughly increase by an order of magnitude.

Mars: Thickness of the lithosphere from the tectonic response to volcanic loads

Abstract: The response to loading of the elastic lithosphere of Mars by seven large volcanic features is estimated based on the hypothesis of a flexural origin for a definable set of load-concentric graben. From the locations of such graben, or from their absence, the lithospheric thickness and flexural rigidity are inferred. For the Tharsis montes, Alba Patera, and Elysium Mons, elastic lithospheric thicknesses at the time of loading range from 20 to 50 km, assuming a Young's modulus of a trillion dyn/sq cm. The thickness exceeded 120 km beneath Olympus Mons and Isidis Planitia. The corresponding ranges in flexural rigidity are approximately 10 to the 30th to 31st dyn cm and greater than 10 to the 32nd dyn cm, respectively. These results indicate a local thinning of the lithosphere beneath portions of the central regions of the Tharsis and Elysium volcanic provinces at the time of loading-induced fracturing.

Impact melt products of chondritic material

Abstract: Experimental data concerning impact melting processes in chondritic material are reviewed. It is shown that a large variety of objects in chondritic meteorites could have formed as a result of impact melting, including: shock veins; metal-troilite mixtures; metal and sulfide nodules; melt pockets and vugs. The type of object produced in an impact melt is related to the interaction of the shock waves with the particular target rock. It is suggested that various iron meteorites (including groups IAB, IIICD, and IIE, as well as several ungrouped irons) were formed from individual melt pools in chondritic regoliths. The small-scale structure of impact-melted metallic Fe,Ni and troilite in Weston (H chondrite regolith breccia) is illustrated in a photograph.

The computation of body wave synthetic seismograms in laterally homogeneous media

Abstract: Synthetic seismograms, computed for realistic, horizontally stratified media, are now routinely used as an aid to seismic interpretation. This paper reviews the theoretical background to these methods and presents comparisons of the two popular algorithms, reflectivity and WKBJ seismograms, for a variety of earth models. The transformed wave equations are developed from the equations for a spherical, gravitating medium in a symmetric form suitable for body wave calculations. Four methods of solving these equations in general, inhomogeneous layers are described: the WKBJ and Langer asymptotic expansions and the WKBJ and Langer iterative solutions. Together with the earth-flattening transformation and the ray expansion, transformed solutions for body waves can then be obtained for realistic layered media. Four methods of inverting the frequency and wave number transformations are also described: the real and complex spectral and slowness methods. Although realistic seismic models are normally sufficiently complicated that numerical calculations are essential, before proceeding with numerical comparisons we have included a review of the canonical signals included in body wave seismograms. These analytic results for direct rays, partial and total reflections, turning rays on forward and reversed branches, head waves, interface waves, Airy caustics, and Fresnel and interface shadows are useful to anticipate and understand numerical problems and results. Finally, a comparison of Green's functions for crustal, mantle, and whole earth models, calculated using the WKBJ and reflectivity algorithms, is included.

Review of the critical ionization velocity effect in space

Abstract: Laboratory experiments have shown under a variety of conditions that when a neutral gas passes through a magnetized plasma with a relative velocity perpendicular to the magnetic field that is greater than a critical velocity, anomalously high ionization of the neutrals occurs. The conditions under which the same effect is to be expected in space plasmas is still unclear. The experimental evidence for the occurrence of the critical ionization velocity effect in space is summarized, and various areas in which it has been proposed that the effect should be significant are discussed.

Convective cloud downdraft structure: An interpretive survey

Abstract: Observational and modeling studies dealing with different aspects of convective clouds are reviewed and interpreted to construct a generalized description of the structure, dynamics, and thermodynamics of convective cloud downdrafts. Observational studies reveal that downdraft speeds and sizes range from typical values of several meters per second and several hundred meters in nonprecipitating cumulus congestus clouds to typical values of 5–10 m s−1 and several kilometers in precipitating cumulonimbi. Maximum measured downdraft speeds appear to be limited to ∼20 m s−1. Different types of downdrafts appear to exist within precipitating convective clouds. Penetrative downdrafts common to nonprecipitating convective clouds and upper regions of precipitating convective clouds exhibit maximum horizontal dimensions of ∼1 km. These downdrafts emerge when subsaturated environmental air is entrained or mixed into the cloud. A second type, cloud edge downdrafts, appear in both observations and in cloud model results. Although their driving mechanisms are not fully understood, such downdrafts may be forced by cloud edge evaporational cooling and localized updraft mass flux compensation. Overshooting downdrafts comprise a third type and are typically associated with intense convection in which updraft air surpasses an equilibrium level of neutral buoyancy, cools upon further ascent, and then descends but remains within a few kilometers of cloud top. Finally, the precipitation-associated downdraft is one forced at low levels by precipitation loading, evaporation, and melting. This downdraft may attain relatively large scales, of the order of the horizontal dimension of precipitating regions within the lowest several kilometers. Such large scales provide a clear distinction from (penetrative-type) downdrafts of ∼1 km maximum scale within nonprecipitating convection. There is evidence from both observational and modeling studies that downdraft dynamical and thermodynamical processes are strongly influenced by static stability, wind shear profiles, cloud microphysical processes, and precipitation characteristics. However, the degree to which downdraft structure depends on such interrelated controlling factors has not yet been determined.

Acoustic properties of sediments: An update

Abstract: This paper will summarize our knowledge of the propagation of sound in marine sediments. This knowledge is important to a wide range of acoustic problems, extending from the high-frequency applications of detection of objects buried in the bottom sediment to the very low frequency application characterizing the propagation of acoustic energy over hundreds of kilometers. In these applications the ocean bottom becomes an important part of the acoustic waveguide. The desirable goal of a complete theoretical understanding of propagation has not been achieved. However, recent measurements of acoustic attenuation in the frequency range of a few hundred hertz are almost an order of magnitude smaller than have previously been measured, and these results may now permit progress in a more complete characterization of the acoustic properties of sediments. (The research performed on this topic by the author under the guidance of R. Reid was published in 1967 (“Acoustic Properties of Sediments,” J. Acoust. Soc. Am., 42, 882–890, 1967). This paper will briefly review aspects of that work and trace the progress in this area of research since that publication.)

On optimal estimation of gravity from gravity gradients at aircraft altitude

Abstract: Least squares collocation in physical geodesy is now a standard technique for estimating gravity-related quantities from discrete observations in a limited region. It is an optimal method because estimation errors due to measurement noise and the discreteness and finite extent of observations are minimized. However, for large observation sets, the computational problem of collocation (i.e., the problem of solving a large, possibly ill-conditioned, linear system of equations) is also well known and is very clearly illustrated with airborne gravity gradiometry. Natural alternatives to rigorous least squares collocation are therefore examined and are categorized as being either nonoptimal, suboptimal, or virtually optimal. All choices of estimation method are directed at reducing the computational requirements with an awareness of the desired estimation accuracy. An example of a nonoptimal procedure is the use of integral formulas of the Stokes or Vening-Meinesz type; suboptimal estimation is exemplified by frequency domain collocation; and a case of virtual optimality is the approximate collocation estimator obtained with a few iterations of the conjugate gradient method used to invert the autocovariance matrix.