Showing papers in "Reviews of Geophysics in 2004"

Heinrich events: Massive late Pleistocene detritus layers of the North Atlantic and their global climate imprint

Abstract: [1] Millennial climate oscillations of the glacial interval are interrupted by extreme events, the so-called Heinrich events of the North Atlantic. Their near-global footprint is a testament to coherent interactions among Earth's atmosphere, oceans, and cryosphere on millennial timescales. Heinrich detritus appears to have been derived from the region around Hudson Strait. It was deposited over approximately 500 ± 250 years. Several mechanisms have been proposed for the origin of the layers: binge-purge cycle of the Laurentide ice sheet, jokulhlaup activity from a Hudson Bay lake, and an ice shelf buildup/collapse fed by Hudson Strait. To determine the origin of the Heinrich events, I recommend (1) further studies of the timing and duration of the events, (2) further sedimentology study near the Hudson Strait, and (3) greater spatial and temporal resolution studies of the layers as well as their precursory intervals. Studies of previous glacial intervals may also provide important constraints.

Mesoscale convective systems

Abstract: The largest convective clouds are mesoscale convective systems, which account for a large portion of Earth's cloud cover and precipitation, and the patterns of wind and weather associated with mesoscale convective systems are important local phenomena that often must be forecast on short timescales. They often produce floods. Mesoscale convective systems are generally much larger than the individual cumulonimbus and lines of cumulonimbus discussed in Chapter 8 . They develop circulations on the mesoscale, which are larger in scale than the updrafts and downdrafts of individual cumulonimbus clouds. The mesoscale circulations produce large regions of stratiform (nimbostratus) precipitation of the type discussed in Chapter 6 . Often the stratiform precipitation regions trail a squall line consisting of convective cells, and a mesoscale convective vortex tends to form in the stratiform region. The heating profile in the stratiform region is positive at upper levels and negative at lower levels due to evaporation and melting of the precipitation particles. The dynamics of mesoscale circulations involve a joint adjustment to the wind shear and thermodynamic stratification of the large scale environment. Gravity-wave dynamics also contribute to the maintenance of mesoscale convective systems. This chapter reviews both the observed structure of mesoscale systems and their unique dynamics.

Climate over past millennia

Abstract: [1] We review evidence for climate change over the past several millennia from instrumental and high-resolution climate “proxy” data sources and climate modeling studies. We focus on changes over the past 1 to 2 millennia. We assess reconstructions and modeling studies analyzing a number of different climate fields, including atmospheric circulation diagnostics, precipitation, and drought. We devote particular attention to proxy-based reconstructions of temperature patterns in past centuries, which place recent large-scale warming in an appropriate longer-term context. Our assessment affirms the conclusion that late 20th century warmth is unprecedented at hemispheric and, likely, global scales. There is more tentative evidence that particular modes of climate variability, such as the El Nino/Southern Oscillation and the North Atlantic Oscillation, may have exhibited late 20th century behavior that is anomalous in a long-term context. Regional conclusions, particularly for the Southern Hemisphere and parts of the tropics where high-resolution proxy data are sparse, are more circumspect. The dramatic differences between regional and hemispheric/global past trends, and the distinction between changes in surface temperature and precipitation/drought fields, underscore the limited utility in the use of terms such as the “Little Ice Age” and “Medieval Warm Period” for describing past climate epochs during the last millennium. Comparison of empirical evidence with proxy-based reconstructions demonstrates that natural factors appear to explain relatively well the major surface temperature changes of the past millennium through the 19th century (including hemispheric means and some spatial patterns). Only anthropogenic forcing of climate, however, can explain the recent anomalous warming in the late 20th century.

Controls on tectonic accretion versus erosion in subduction zones: Implications for the origin and recycling of the continental crust

Abstract: [1] Documenting the mass flux through convergent plate margins is important to the understanding of petrogenesis in arc settings and to the origin of the continental crust, since subduction zones are the only major routes by which material extracted from the mantle can be returned to great depths within the Earth. Despite their significance, there has been a tendency to view subduction zones as areas of net crustal growth. Convergent plate margins are divided into those showing long-term landward retreat of the trench and those dominated by accretion of sediments from the subducting plate. Tectonic erosion is favored in regions where convergence rates exceed 6 ± 0.1 cm yr−1 and where the sedimentary cover is 1 km. Large volumes of continental crust are subducted at both erosive and accretionary margins. Average magmatic productivity of arcs must exceed 90 km3 m.y.−1 if the volume of the continental crust is to be maintained. Convergence rate rather than height of the melting column under the arc appears to be the primary control on long-term melt production. Oceanic arcs will not be stable if crustal thicknesses exceed 36 km or trench retreat rates are >6 km m.y.−1. Continental arcs undergoing erosion are major sinks of continental crust. This loss requires that oceanic arcs be accreted to the continental margins if the net volume of crust is to be maintained.

Present‐day sea level change: Observations and causes

Abstract: We investigate climate-related processes causing variations of the global mean sea level on interannual to decadal time scale. Wc focus on thermal expansion of the oceans and continental water mass balance. We show that during the 1990s where global mean sea level change has been measured by Topex/Poseidon satellite altimetry. thermal expansion is the dominant contribution to the observed 2.5 mm/yr sea level rise. For the past decades, exchange of water between continental reservoirs and oceans had a small, but not totally negligible contribution (about 0.2 mm/yr) to sea level rise. For the last four decades, thermal contribution is estimated to about 0.5 mm/yr, with a possible accelerated rale of thermosteric rise during the 1990s. Topex/Posei don shows an increase in mean sea level of 2.5 mm/yr over the last decade, a value about two times larger than reported by historical tide gauges. This would suggest that there has been significant acceleration of sea level rise in the recent past, possibly related to ocean warming.

Dusty plasma effects in Saturn's magnetosphere

Abstract: [1] Comets, planetary rings, exposed dusty surfaces, and the zodiacal dust cloud are all examples of environments where dusty plasma effects establish the size and spatial distributions of small grains. Simultaneously, dust often influences the composition, density, and temperature of the plasma surrounding it. The dynamics of charged dust particles can be surprisingly complex and fundamentally different from the well-understood limits of gravitationally dominated motions of neutral particles or the adiabatic motion of electrons and ions in electromagnetic fields that dominate gravity. In this review we focus on observations that are best explained by theories concerning dusty plasma effects at Saturn. In addition to presenting our current models we also discuss our expectations for new discoveries based on existing observations at Jupiter or on purely theoretical considerations. Our intent is to give an up-to-date overview of dusty plasma effects in Saturn's magnetosphere and to draw attention to several outstanding problems that could be resolved by the Cassini mission.

Polynya dynamics: A review of observations and modeling

Abstract: [1] Polynyas are nonlinear-shaped openings within the ice cover, ranging in size from 10 to 105 km2. Polynyas play an important climatic role. First, winter polynyas tend to warm the atmosphere, thus affecting atmospheric mesoscale motions. Second, ocean surface cooling and brine rejection during sea ice growth in polynyas lead to vertical mixing and convection, contributing to the transformation of intermediate and deep waters in the global ocean and the maintenance of the oceanic overturning circulation. Since 1990, there has been an upsurge in polynya observations and theoretical models for polynya formation and their impact on the biogeochemistry of the polar seas. This article reviews polynya research carried out in the last 2 decades, focusing on presenting a state-of-the-art picture of the physical interactions between polynyas and the atmosphere-sea ice-ocean system. Observational and modeling studies, the surface heat budget, and water mass transformation within these features are addressed.

Links between long-lived hot spots, mantle plumes, d″, and plate tectonics

Abstract: [1] The existence, spatial distribution, and style of volcanism on terrestrial planets is an expression of their internal dynamics and evolution. On Earth a physical link has been proposed between hot spots, regions with particularly persistent, localized, and high rates of volcanism, and underlying deep mantle plumes. Such mantle plumes are thought to be constructed of large spherical heads and narrow trailing conduits. This plume model has provided a way to interpret observable phenomena including the volcanological, petrological, and geochemical evolution of ocean island volcanoes, the relative motion of plates, continental breakup, global heat flow, and the Earth's magnetic field within the broader framework of the thermal history of our planet. Despite the plume model's utility the underlying dynamics giving rise to hot spots as long-lived stable features have remained elusive. Accordingly, in this review we combine results from new and published observational, analog, theoretical, and numerical studies to address two key questions: (1) Why might mantle plumes in the Earth have a head-tail structure? (2) How can mantle plumes and hot spots persist for large geological times? We show first that the characteristic head-tail structure of mantle plumes, which is a consequence of hot upwellings having a low viscosity, is likely a result of strong cooling of the mantle by large-scale stirring driven by plate tectonics. Second, we show that the head-tail structure of such plumes is a necessary but insufficient condition for their longevity. Third, we synthesize seismological, geodynamic, geomagnetic, and geochemical constraints on the structure and composition of the lowermost mantle to argue that the source regions for most deep mantle plumes contain dense, low-viscosity material within D″ composed of partial melt, outer core material, or a mixture of both (i.e., a “dense layer”). Fourth, using results from laboratory experiments on thermochemical convection and new theoretical scaling analyses, we argue that the longevity of mantle plumes in the Earth is a consequence of the interactions between plate tectonics, core cooling, and dense, low-viscosity material within D″. Conditions leading to Earth-like mantle plumes are highly specific and may thus be unique to our own planet. Furthermore, long-lived hot spots should not a priori be anticipated on other terrestrial planets and moons. Our analysis leads to self-consistent predictions for the longevity of mantle plumes, topography on the dense layer, and composition of ocean island basalts that are consistent with observations.

Nonlinear multivariate and time series analysis by neural network methods

Abstract: [1] Methods in multivariate statistical analysis are essential for working with large amounts of geophysical data, data from observational arrays, from satellites, or from numerical model output. In classical multivariate statistical analysis, there is a hierarchy of methods, starting with linear regression at the base, followed by principal component analysis (PCA) and finally canonical correlation analysis (CCA). A multivariate time series method, the singular spectrum analysis (SSA), has been a fruitful extension of the PCA technique. The common drawback of these classical methods is that only linear structures can be correctly extracted from the data. Since the late 1980s, neural network methods have become popular for performing nonlinear regression and classification. More recently, neural network methods have been extended to perform nonlinear PCA (NLPCA), nonlinear CCA (NLCCA), and nonlinear SSA (NLSSA). This paper presents a unified view of the NLPCA, NLCCA, and NLSSA techniques and their applications to various data sets of the atmosphere and the ocean (especially for the El Nino-Southern Oscillation and the stratospheric quasi-biennial oscillation). These data sets reveal that the linear methods are often too simplistic to describe real-world systems, with a tendency to scatter a single oscillatory phenomenon into numerous unphysical modes or higher harmonics, which can be largely alleviated in the new nonlinear paradigm.

Mesosphere inversion layers and stratosphere temperature enhancements

Abstract: [1] It has been known for at least 30 years that vertically narrow thermal layers form within the middle atmosphere Two types of temperature enhancements, the low-latitude to midlatitude mesosphere inversion layer (MIL) and the high-latitude winter stratosphere temperature enhancement (STE), have both received much attention within the atmospheric science community because of their unexplained formation mechanisms and potential impacts on the middle-atmosphere global circulation Numerous experimental, numerical, and theoretical studies have attempted to explain certain aspects of these respective thermal layers, but no one theory consistently and satisfactorily describes all the features observed We present a review of the literature and explicitly propose a classification scheme based on the different formation mechanisms suspected to cause these events For the MIL we demonstrate that there are two subtypes The first one is tidally driven and tends to occur above ∼85 km This MIL originates from large-amplitude tidal waves propagating into the mesosphere and their subsequent nonlinear interactions with gravity waves, which can often create the appearance of a “double MIL” separated by approximately one vertical tidal wavelength (∼25 km) The other subtype of MIL is formed by a climatological planetary wave dissipation mechanism that occurs at a zero-wind line The dissipation of the planetary wave tends to generate a mesoscale (∼1000 km) inversion layer in the range of 65–80 km These two formation mechanisms explain a host of observed characteristics, including the reason behind the downward progression of some MILs and not others, the different climatological nature of the two forms of MIL events, and the relative scarcity of MIL observations at high latitudes The STE is believed to be generated by an altogether different process, namely, the nonlinear interaction between the polar vortex and planetary waves/Aleutian High The induced temperatures typically peak around 40 km and often exceed 300 K, generating what appears to be a “low, hot stratopause” When vertical temperature profiles are combined with synoptic analyses, one observes that the STE is the consequence of high-latitude vortex interactions creating a baroclinic atmosphere, ie, a downward adiabatic compression induced by an ageostropic flow We summarize the details of the relationship between this feature and sudden stratospheric warmings, as well as the potential for in situ gravity wave generation We close with a review of currently unexplained MIL/STE features and offer new directions for future middle-atmosphere thermal layer research

Instability layers and airglow imaging

Abstract: [1] For over 30 years it has been recognized that atmospheric gravity waves (AGWs) in the 80–110 km region significantly perturb the basic atmospheric state, perhaps causing instability regions and subsequent turbulence. It has also been recognized for nearly as long that AGWs cause strong fluctuations in the airglow emissions that originate in this altitude region. Airglow images have been obtained since 1973, and they have shown structures that have mainly been attributed to the passage of AGWs through this region as predicted theoretically. The AGWs have been assumed to originate largely in the troposphere because of either convective activity or the flow of air over large mountain ranges. However, intensive analysis of the properties of a class of small-scale features currently known as ripples [Taylor et al., 1997; Nakamura et al., 1999] suggests that these features are not AGWs but are rather instability features generated in situ. The basis for this hypothesis is examined in this review, and it is concluded that while there is support for the instability hypothesis as the origin of ripple features, at present the exact nature of the instabilities causing these features is not known.

Stochastic fractal-based models of heterogeneity in subsurface hydrology: Origins, applications, limitations, and future research questions

Abstract: [1] Modern measurement techniques have shown that property distributions in natural porous and fractured media appear highly irregular and nonstationary in a spatial statistical sense. This implies that direct statistical analyses of the property distributions are not appropriate, because the statistical measures developed will be dependent on position and therefore will be nonunique. An alternative, which has been explored to an increasing degree during the past 20 years, is to consider the class of functions known as nonstationary stochastic processes with spatially stationary increments. When such increment distributions are described by probability density functions (PDFs) of the Gaussian, Levy, or gamma class or PDFs that converge to one of these classes under additions, then one is also dealing with a so-called stochastic fractal, the mathematical theory of which was developed during the first half of the last century. The scaling property associated with such fractals is called self-affinity, which is more general that geometric self-similarity. Herein we review the application of Gaussian and Levy stochastic fractals and multifractals in subsurface hydrology, mainly to porosity, hydraulic conductivity, and fracture roughness, along with the characteristics of flow and transport in such fields. Included are the development and application of fractal and multifractal concepts; a review of the measurement techniques, such as the borehole flowmeter and gas minipermeameter, that are motivating the use of fractal-based theories; the idea of a spatial weighting function associated with a measuring instrument; how fractal fields are generated; and descriptions of the topography and aperture distributions of self-affine fractures. In a somewhat different vein the last part of the review deals with fractal- and fragmentation-based descriptions of fracture networks and the implications for transport in such networks. Broad conclusions include the implication that models based on increment distributions, while more realistic, are inherently less predictive than models based directly on stationary stochastic processes; that there is presently an unresolved ambiguity when a measurement is attempted in a medium that exhibits property variations on all scales; the strong possibility that log(property) increment distributions that appear to be described by the Levy PDF are actually superpositions of several PDFs of finite variance, one for each facies; that there are apparent similarities in the transport behavior of heterogeneous porous media and fractured rock at the field scale that appear to be related to the existence of a few preferential flow paths in both types of media; and finally, that additional carefully collected data sets are needed to clarify and advance the fractal-based theories, particularly in the case of three-dimensional fracture networks where few data are available. Further refinement is needed also in the understanding of instrument spatial weighting functions in heterogeneous media and how measurements in media exhibiting variations on all scales should be interpreted.

A review of applications to constrain pumping test responses to improve on geological description and uncertainty

Abstract: [1] This review examines the single-phase flow of fluids to wells in heterogeneous porous media and explores procedures to evaluate pumping test or pressure-response curves. This paper examines how these curves may be used to improve descriptions of reservoir properties obtained from geology, geophysics, core analysis, outcrop measurements, and rock physics. We begin our discussion with a summary of the classical attempts to handle the issue of heterogeneity in well test analysis. We then review more recent advances concerning the evaluation of conductivity or permeability in terms of statistical variables and touch on perturbation techniques. Our current view to address the issue of heterogeneity by pumping tests may be simply summarized as follows. We assume a three-dimensional array (ordered set) of values for the properties of the porous medium as a function of the coordinates that is obtained as a result of measurements and interpretations. We presume that this array of values contains all relevant information available from prior geological and geophysical interpretations, core and outcrop measurements, and rock physics. These arrays consist of several million values of properties, and the information available is usually on a very fine scale (often <0.5 m in the vertical direction); for convenience, we refer to these as cell values. The properties are assumed to be constant over the volume of each of these cells; that is, the support volume is the cell volume, and the cell volumes define the geologic scale. In this view it is implicit that small-scale permeability affects the movement of fluids. Although more information than porosity is available, we refer to this system as a “porosity cube.” Because it is not economically possible to carry out computations on a fine-scale model with modern resources on a routine basis, we discuss matters relating to the aggregation and scale-up of the fine-scale model from the perspective of testing and show that specific details need to be addressed. The focus is on single-phase flow. Addressing the issue of scale-up also permits us to comment on the application of the classical or analytical solutions to heterogeneous systems. The final part of the discussion outlines the inversion process and the adjustment of cell values to match observed performance. Because the computational scale and the scale of the porosity cube are different, we recommend that the inversion process incorporate adjustments at the fine scale. In this view the scale-up process becomes a part of the inversion algorithm.

Nonlinear dynamics in flow through unsaturated fractured porous media: Status and perspectives

Abstract: [1] The need has long been recognized to improve predictions of flow and transport in partially saturated heterogeneous soils and fractured rock of the vadose zone for many practical applications, such as remediation of contaminated sites, nuclear waste disposal in geological formations, and climate predictions. Until recently, flow and transport processes in heterogeneous subsurface media with oscillating irregularities were assumed to be random and were not analyzed using methods of nonlinear dynamics. The goals of this paper are to review the theoretical concepts, present the results, and provide perspectives on investigations of flow and transport in unsaturated heterogeneous soils and fractured rock, using the methods of nonlinear dynamics and deterministic chaos. The results of laboratory and field investigations indicate that the nonlinear dynamics of flow and transport processes in unsaturated soils and fractured rocks arise from the dynamic feedback and competition between various nonlinear physical processes along with complex geometry of flow paths. Although direct measurements of variables characterizing the individual flow processes are not technically feasible, their cumulative effect can be characterized by analyzing time series data using the models and methods of nonlinear dynamics and chaos. Identifying flow through soil or rock as a nonlinear dynamical system is important for developing appropriate short- and long-time predictive models, evaluating prediction uncertainty, assessing the spatial distribution of flow characteristics from time series data, and improving chemical transport simulations. Inferring the nature of flow processes through the methods of nonlinear dynamics could become widely used in different areas of the earth sciences.