bluuebunny/crossref_metadata_2025 huggingface dataset:
Tectonic dewatering and mechanics of protothrust zones: Example from the Cascadia accretionary margin
Authors: Wang Chi‐yuen , Hwang Win‐tsuang , Cochrane Guy R.
Published: February 2004

Recent analyses of multichannel seismic data for the Cascadia margin off central Oregon show a systematic, landward increase in velocity (increase in sediment dewatering) within a protothrust zone about 10 km wide seaward of the toe of the accretionary wedge (Cochrane et al., 1994). Here we analyze the mechanical conditions for the transmission of compression from the rear of the accretionary wedge to the area seaward of the frontal thrust to create the protothrust zone and to cause sediment dewatering. The analysis assumes that sediments are elastoplastic (Coulomb) material; i.e., they behave elastically up to a yield limit at which failure occurs. We show that the necessary conditions for the transmission of compression from the accretionary wedge to the seaward area are (1) the presence of incipient decollements seaward of the frontal thrust and (2) the presence of high pore pressures within the incipient decollement. We also show that the magnitude of pore pressure within the incipient decollement may control the width of the protothrust zone. We suggest that the latter determines the location of the next frontal thrust and hence the ramp spacing between the imbricated thrusts within accretionary wedges. Thus pore pressures within incipient decollements may significantly influence the evolution of the internal structures of accretionary wedges.
Predicting the hydrodynamic permeability of sandstone with a pore‐scale model
Authors: du Plessis J. P. , Roos L. I.
Published: February 2004

Volumetric averaging of the intrapore fluid momentum transport equation yields a corresponding equation for the transport of averaged fluid momentum. This equation contains an integral over the actual fluid‐solid interface and its deterministic quantification is obtained by the introduction of a geometric pore‐scale model for granular media. This result is then used to determine the permeability of sandstone quantitatively. The final results are found to be in very good agreement with experimental data for Fontainebleau, Berea and Massilon sandstone. Correlation could be enhanced even further by inclusion of a low porosity cut‐off criterion.
Analysis of coseismic surface displacement gradients using radar interferometry: New insights into the Landers earthquake
Authors: Peltzer Gilles , Hudnut Kenneth W. , Feigl Kurt L.
Published: February 2004

The map of the coseismic displacement field generated by interferometric processing of synthetic aperture radar (SAR) images taken before and after the June 28, 1992, Landers earthquake sequence brings new insights into the nature of deformation caused by these earthquakes. We use the interferometric map generated by Massonnet et al. (1993) to analyze the surface displacement field in the vicinity of the fault trace. Complexities in the fringe pattern near the fault reflect short‐wavelength variations of the surface rupture and slip distribution, and attest to large displacement gradients. Along two sections of the fault, characteristic fringe patterns can be recognized, contrasting in density and direction with patterns observed away from the rupture. In order to understand the observed fringe patterns, we compute synthetic interferograms in three simple cases: (1) rigid‐body rotations about a vertical axis, (2) about a horizontal axis (tilt), and (3) distributed, simple shear. The orientation and spatial separation of interferometric fringes predicted by these models help constrain near‐field deformation and rupture parameters. Where the Kickapoo fault connects with the Homestead Valley fault, the interferogram shows a clear pattern of parallel N20°W fringes separated by about 160 m. This pattern and vertical offsets measured along the Kickapoo fault suggest that the block between this fault and the Johnson Valley fault may have been tilted, down to the west. A 5‐km block lifted by l m on one side would be tilted by an angle of 0.01° (190 μrad), producing fringes separated by about 160 m, parallel to the tilt axis. Such a tilt, parallel to a N20°W direction, would account for the gradual, northward increase of the vertical slip component observed along the Kickapoo fault. This tilt may also explain the 1 m of reverse slip observed along the “slip gap” section of the Homestead Valley break. Between the southern end of the Johnson Valley fault and the Eureka Peak fault, where no surface rupture has been mapped, the dense pattern of fringes implies distributed shear, probably resulting from fault slip at depth. The density and direction of the fringes in the gap are consistent with a right‐lateral slip of 1.2–3.8 m on a blind fault locked above the depth of 1.5–2 km. Such observations of small wavelength features in the SAR interferogram bring new insights into the near‐field displacement gradient and thus on response of the uppermost crust to seismic rupture.
History of Earth's magnetic field and possible connections to core‐mantle boundary processes
Authors: McFadden Phillip L. , Merrill Ronald T.
Published: February 2004

The core‐mantle boundary (CMB) sets boundary conditions for processes occurring within the core. Thus the history of the geomagnetic field is intimately connected with the history of the CMB: information about one can often provide information about the other. It is a simple matter to separate sources of external origin from those of internal origin, but there is no unique way to specify the location of the internal sources. Indirect arguments suggest that sources within Earth's outer core strongly dominate the field for spherical harmonic terms with degree less than about 14 and that crustal sources play an increasing role in harmonics with higher degrees. Building on the pioneering studies of Roberts and Scott and of Backus in the 1960s, the velocity field of the core fluid at the top of the outer core can be estimated from geomagnetic secular variation data. However, even with appropriate assumptions about location of the magnetic field source, there is a serious nonuniqueness in inversion of the secular variation data to velocity field because magnetic field lines are not changed by movement of conductive fluid parallel to those field lines. Additional assumptions about the flow are therefore required, sometimes leading to quite different estimates in the pattern of fluid flow. The resulting velocity field estimates appear to be sensitive to conditions (e.g., small lateral variations in temperature) at the CMB. Consideration of mantle dynamics suggests that large changes in the CMB conditions probably occur on a 10‐ to 100‐m.y. timescale. Secular variations with periods shorter than a million years, but longer than several years, almost certainly originate from processes operating in the outer core; unfortunately, there is not yet consensus as to what those processes are. Longer‐period variations in the paleomagnetic record, including nonstationarities in the rates of magnetic field reversals, in paleointensities, and in paleosecular variation, may reflect changes in CMB conditions. Because these boundary conditions are controlled primarily by mantle dynamics, there have been several speculations regarding causal links between changes in Earth's lithosphere and changes in Earth's magnetic field. Finally, the evidence linking lateral variations at the CMB to perceived systematics in polarity transition data is found to be intriguing, but insufficient.
Evolution of backarc rifting: Mariana Trough, 20°–24°N
Authors: Martínez Fernando , Fryer Patricia , Baker Nancy A. , Yamazaki Toshitsugu
Published: February 2004

The Mariana Trough is an actively opening backarc basin in the western Pacific. The trough formed by extension which longitudinally split an earlier arc massif creating a crescent‐shaped basin between the remnant West Mariana Ridge and eastern active volcanoes of the Mariana Arc. Opening increases southward from the Volcano Islands near 24°N where the two arcs join. At 18°N in the central Mariana Trough the basin is widest and may be opening primarily by seafloor spreading. We present a synthesis of closely spaced shipboard gravity, magnetic, and bathymetry measurements from the northern basin (20°–24°N), an area that undergoes a significant progressive southward increase in extension. We have identified three stages in the evolution of rifting in this area: (1) asymmetric rifting from 24°N to 22°15′N where faulting and magmatism have migrated laterally to remain near the active volcanic arc side of the basin, (2) a “localization” of rifting from 22°15′N to ∼21°N where the primary zone of rifting separates from the active volcanic arc, and (3) a further concentration of rifting from 21°N to 20°N leading to the formation of deep tectonic grabens near the center of the basin. This last stage may be a precursor to (or incipient) seafloor spreading. We describe a new type of magnetic lineation resolved in a three‐dimensional seafloor magnetization inversion of the trough between 20° and 24°N. The detailed character of these lineations, their association with tectonic structures, and other geophysical observations indicate that they are not seafloor spreading lineations but rather result from magnetic intrusions and volcanism emplaced within preexisting, less magnetic rifted arc crust. The development of these well‐defined magnetization bands indicates that the zone of magmatism remains relatively narrow at any one time throughout the opening of the trough, although tectonic deformation appears to be more widely distributed. In addition, the geometry of the magnetization bands with respect to the remnant arc border faults indicates that rifting propagated rapidly northward.
Residual stress and domain structure
Authors: Ye Jun , Merrill Ronald T.
Published: February 2004

Calculations show that as the residual stress increases, the number of magnetic domains in titanomagnetite at first increases and subsequently decreases, and large residual stress causes laminar domain structures in Ti‐rich titanomagnetite, as is often observed. In addition, large residual stress significantly decreases the number of domains, as is also observed. One anticipates that large residual stress also exists in most pure magnetites, but theory and observations are inadequate to confirm this expectation. We conclude that the presence of a large macrostress and possible systematic errors in the observational estimates of the number of domains observed explains the current discrepancy between domain observations and theory for titanomagnetite grains.
Mantle temperature anomalies along the past and paleoaxes of the Galápagos spreading center as inferred from gravity analyses
Authors: Ito Garrett T. , Lin Jian
Published: February 2004

To better understand the effects of hot spots on mid‐ocean ridge thermal structure, we investigate the subsurface density structure of the Galápagos spreading center and nearby lithosphere. Using shipboard gravity and bathymetry data, we obtain maps of mantle Bouguer anomalies (MBA) by removing from the free‐air gravity the attractions of seafloor topography and a 6‐km‐thick model crust. Comparison of observed and theoretical MBA profiles along isochrons for ages 0.0–7.7 Ma suggests that seafloor topography is isostatically compensated by mass anomalies primarily in the upper 100 km of the mantle. This result is consistent with the notion that seafloor topography along the Galapagos spreading center is supported by lateral changes of crustal thickness and upper mantle density, both of which are controlled by temperatures in the upper mantle where decompression melting occurs. Along the ridge axis, the MBA decreases from the east and west toward the Galapagos hot spot by ∼90 mGal, reaching a minimum nearest the hot spot at 91°W. Seafloor topography mirrors the MBA along axis, increasing by ∼1.1 km toward the hot spot. These variations in MBA and bathymetry can be explained by crustal thickening and mantle density variations resulting from a gradual axial temperature increase of 50±25°C toward the hot spot. The predicted crustal thickening of 2–4 km nearest the hot spot accounts for 70–75% of the along‐axis MBA and bathymetry anomalies; mantle density variations account for the rest of the anomalies. From the crustal isochron of age 7.7 Ma to the present‐day axis, the along‐isochron amplitudes of MBA decrease from ∼150 to ∼90 mGal. The corresponding along‐isochron bathymetry anomalies decrease from ∼1.7 to ∼1.1 km. These observations along the paleoaxes of the Galapagos spreading center indicate that the axial temperature anomaly was 70% hotter in the past (86±25°C) and has steadily decreased to 50±25°C as the ridge axis migrated away from the Galapagos hot spot. These along‐isochron temperature anomalies, however, have remained well below that estimated for the hot spot itself (200°C), indicating that the lateral temperature gradient between the hot spot and the ridge axis has remained 10–20 times greater than that along the ridge axis over the past 7.7 m.y.
Paleomagnetism of the Upper Cretaceous strata of Mount Tatlow: Evidence for 3000 km of northward displacement of the eastern Coast Belt, British Columbia
Authors: Wynne P. J. , Irving E. , Maxson J. A. , Kleinspehn K. L.
Published: February 2004

This study contributes the first paleomagnetic data from dated Cretaceous units of the Coast Belt (51.3°N, 123.8°W) in which paleohorizontal bedding surfaces can be identified with certainty. We sampled 38 sites through a 1.5‐km stratigraphic interval of Cenomanian Silverquick sedimentary strata and Cenomanian to Campanian Powell Creek volcanics in the Tatlow syncline in the Tyaughton‐Methow Basin. Magnetizations at 21 sites have high unblocking temperatures (TUB) and pass the tilt test (precision k = 5 before untilting, maximum k = 48 after 100% untilting). Magnetic minerals of volcanics show original textures, mean declinations in volcanic and sedimentary rocks are statistically identical, and polarity is consistently normal. The mean inclination (55.4°±3.4°, probability P = 0.05) of this pretilting magnetization is 20.1°±4.0° shallower than that expected if the rocks were deposited in their present position relative to cratonic North America. Field and laboratory observations rule out other causes for these anomalous shallow inclinations. These rocks therefore have been displaced northward by 3000±500 km relative to cratonic North America since mid‐Cretaceous time. This displacement is almost 3 times greater than that estimated from mid‐Cretaceous volcanics in the now‐adjacent Intermontane Belt. Evidently the two belts were not amalgamated until latest Cretaceous or Early Tertiary time. These results are inconsistent with all previous geologically based tectonic reconstructions of the Cordillera. Magnetizations at eight sites in Silverquick and Powell Creek beds have low TUB and directional characteristics similar to those in three younger diorite intrusions. Reversed and normal polarities occur, the rocks are commonly highly altered, and maximum precision occurs after 40% untilting. The magnetizations therefore are interpreted as overprints of latest Cretaceous age acquired during folding.
Environmental magnetism: Past, present, and future
Authors: Verosub Kenneth L. , Roberts Andrew P.
Published: February 2004

Environmental magnetism involves the application of rock and mineral magnetic techniques to situations in which the transport, deposition, or transformation of magnetic grains is influenced by environmental processes in the atmosphere, hydrosphere, and lithosphere. The first explicit description of environmental magnetism as a distinct field was in 1980. Since that time environmental magnetism has become a broad field that is finding application in an ever‐increasing array of scientific disciplines. In this review of the present state of environmental magnetic studies, we divide the field into three broad, but arbitrary, categories. The first involves the use of mineral magnetic assemblages in the geological record to study physical processes in depositional environments. This category includes the correlation of sediment cores using magnetic susceptibility measurements, studies of geomagnetic field behavior, the analysis of depositional and postdepositional mechanical processes that affect sediments, and the examination of magnetic parameters that might represent proxies for paleoclimatic variation. The second category encompasses studies of the processes responsible for variations in the magnetic minerals brought into a sedimentary environment. These provenance investigations include studies of changes in catchment‐derived sediment in lakes, fluctuations in contributions from terrigenous, aeolian and glaciogenic components in deep‐sea sediments, and the origin of atmospheric particulates. The final category addresses in situ changes and transformations of magnetic minerals in sedimentary environments, including pedogenesis, authigenetic/diagenetic formation of ferrimagnetic phases, dissolution of magnetic minerals due to reductive diagenesis, and contributions of biomagnetism to sedimentary magnetism. Because environmental magnetism can address problems in so many disciplines and because many of these problems may be inaccessible to other techniques, it is likely that the scope of environmental magnetism will continue to expand rapidly. Environmental magnetism is capable of providing important data for studies of global environmental change, climatic processes, and the impact of humans on the environment, all of which are major research initiatives in the international scientific community. These factors suggest that environmental magnetism has a bright and diverse future.
Heat flow at Cajon Pass, California, revisited
Authors: Lachenbruch Arthur H. , Sass J. H. , Clow Gary D. , Weldon Ray
Published: February 2004

In recent studies of a 3½km borehole near Cajon Pass we showed that the observed high heat flow and its sharp decrease with depth are predictable effects of independently determined erosion history, topography, and radioactivity, leaving little room for the large contribution from frictional heat required by conventional faulting models for the nearby San Andreas fault. We have since discovered an error in our analysis that lowers the predicted surface heat flow from the upper end (∼100 mW/m2) to the lower end (∼90 mW/m2) of the range of measurement uncertainty at this complex site; it permits, but does not require, a source increment of up to 10 mW/m2 not accounted for in the prediction. Better agreement between the prediction and observations at depth confines the permissible extra heat flow to the upper part of the hole, making it difficult to attribute it to a deep frictional source. In any case, however, such a frictional source would be too small to attribute to conventional high‐strength faulting models, and the basic conclusion of the original study is unchanged. The most likely cause of the relatively small discrepancy between predicted and observed heat flow (if it exists) is preferential three‐dimensional flow into the higher‐thermal conductivity rock that occupies the upper part of the borehole.
Extinct spreading center in the Labrador Sea: Crustal structure from a two‐dimensional seismic refraction velocity model
Authors: Osler John C. , Louden Keith E.
Published: February 2004

The Labrador Sea contains a rare example of an abandoned mid‐ocean ridge where active accretion of oceanic crust ceased due to a change in the spreading geometry of lithospheric plates. Seismic refraction data were collected along a line which transversely crosses the extinct spreading center. Two‐dimensional analyses of the refraction data, using ray‐tracing and synthetic seismogram techniques reveal major variations in crustal thickness and velocity in relation to the axis of the extinct spreading center. In the extinct spreading center, a crustal thickness of approximately 4 km is determined, compared with 5.5 km for the flanks. Substantial lateral variations in P wave velocities of the upper and lower crust are observed with a marked decrease within the extinct spreading center. Low velocities are also observed in the uppermost mantle underlying the extinct spreading center and are interpreted as being the result of hydrothermal alteration. The anomalously low crustal velocities and crustal thinning are attributed to a decreasing supply of partial melt and increasing degree of tectonism at the slow spreading rates preceding extinction.
Constraints on mantle 3He fluxes and deep‐sea circulation from an oceanic general circulation model
Authors: Farley K. A. , Maier‐Reimer E. , Schlosser P. , Broecker W. S.
Published: February 2004

We have simulated the steady‐state distribution of helium in the deep sea to investigate the magnitude and spatial and temporal variability of mantle degassing and to characterize deep‐sea circulation and ventilation. The simulation was produced by linking a simple source function for helium injected at mid‐ocean ridges with an oceanic general circulation model (GCM). By assuming that the flux of mantle helium is linearly proportional to the seafloor spreading rate and by using previous estimates for the total flux of mantle helium into the oceans, the GCM yields an oceanic 3He distribution which is in qualitative agreement with observations both in overall magnitude and in general distribution. This provides new evidence that the flux of mantle 3He into the oceans is about 1000 mol/yr and that mid‐ocean ridges are the dominant source of mantle helium. Although the match with observations is good in the Pacific and Indian Oceans, the simulated 3He anomalies throughout the Atlantic Ocean are much higher than has been measured. Because the GCM is thought to reproduce Atlantic circulation reasonably well, this discrepancy suggests an error in the helium source function. Either helium injection is not a linear function of seafloor emplacement rate, or eruption and concomitant degassing are highly episodic at the slow spreading rates characteristic of the Mid‐Atlantic Ridge (MAR). The latter explanation would imply minimal volcanic activity along the entire length of the MAR over the last few centuries. In addition to constraints on the degassing flux, our work provides information on the transport and ventilation of deep ocean waters and constrains the degree to which current GCMs can reproduce deep‐water circulation patterns. While the results generally support the GCM's abyssal circulation, our simulation reveals regions of overly‐intense lateral diffusion and upwelling in the model, particularly in the equatorial Pacific. Similarly, there appears to be insufficient production of He‐ventilated bottom waters in the model Antarctic. These observations suggest that further refinement of the GCM abyssal circulation is required.
The mechanism of phreatic eruptions
Authors: Germanovich Leonid N. , Lowell Robert P.
Published: February 2004

We investigate the mechanism for initiating phreatic eruptions following the emplacement of a shallow magmatic intrusion into water‐saturated permeable rock which contains subsidiary low‐permeability crack networks and disconnected cracks. Heat from the intrusion causes the local groundwater to boil and ascend through the main permeable crack network. As the ascending superheated steam heats the overlying rock, the water in the subsidiary networks and disconnected cracks will boil. The pressure exerted by the vapor in the subsidiary and disconnected cracks can lead to rapid horizontal crack propagation, resulting in an increase in crack length by more than an order of magnitude. According to the model, the eruption process starts near a free surface and migrates rapidly along thermoelastic isostresses as a result of multiple breakage of the thin surface layers above the cracks. For certain crack and rock parameters, however, the crack propagation mechanism, instead of leading to a dynamic eruption, may generate a highly cracked zone that may be removed later by fluid transport processes. The proposed mechanism gives rise to precursory phenomena observed in conjunction with many phreatic eruptions. According to the model developed here, phreatic eruptions are most likely to occur only for a rather restricted set of rock parameters. For example, the country rock should not be too strong (σt ≅ 10 MPa) and should be characterized by two‐scale permeability structure involving a main crack network of relatively high permeability (≳10−12 m2) and a subsidiary crack network with much lower permeability (<10−17 m2). Moreover, the model works better if the mean crack aspect ratio is relatively large (β ∼ 10−1) and the crack concentration is not too low (Ω > 10−2). These restrictions may explain indirectly why phreatic eruptions are not ubiquitous in volcanic regions.
Intergranular solid‐fluid phase transformations under stress: The effect of surface forces
Authors: Heidug Wolfgang K.
Published: February 2004

Existing work on mineral solubility in fluid‐infiltrated and stressed rock has remained limited in that it has neglected surface forces. These forces are appreciable only when the fluid exists as a thin film, as in the grain‐to‐grain contact zone and in microcracks. Indeed, when the film thickness is of the order of 10−9 m or so, the strength of the forces can be comparable to overburden stress at several kilometers depth. In this contribution we develop the thermodynamics of the phase reaction between nonhydrostatically stressed grains and an intervening water layer by using the concept of the disjoining pressure to account for surface forces acting in the grain‐to‐grain contact zone. Using a thermodynamic extremum principle, we find an extended version of Gibbs's classical condition for the equilibrium of a stressed solid in contact with its solution phase. We then employ nonequilibrium thermodynamics to formulate kinetic equations describing phase boundary migration and intergranular mass transfer. It is demonstrated that surface forces weaken the efficacy with which diffusion removes dissolved material from the grain‐to‐grain contact zone and enhance the tendency of intergranular pressure solution to flatten initially rough surfaces.
Effects of subsurface heterogeneity on the inference of climate change from borehole temperature data: Model studies and field examples from Canada
Authors: Shen P. Y. , Pollack H. N. , Huang S. , Wang K.
Published: February 2004

All existing approaches to the analysis and interpretation of borehole temperatures in terms of a ground surface temperature history are based on the one‐dimensional theory of heat conduction. Deviations from this idealization are manifest as noise in the interpretation. We present numerical experiments that explore the effects of three‐dimensional subsurface heterogeneity on ground surface temperature histories inferred from borehole temperature profiles. Inversions of steady state temperature profiles containing such three‐dimensional “noise” reveal that in an inversion formulation incorporating a priori information, spurious temperature histories can emerge when the a priori constraints on subsurface temperatures and thermophysical properties are too tight, i.e., when arbitrarily small variations in subsurface temperatures and thermophysical properties are interpreted to have significance for the derived climate history. Relaxation of the a priori constraints enables an effective muting of the spurious histories. Similar experiments conducted on synthetic transient signals confirm the steady state results but also reveal that relaxation of the a priori constraints will also lead to a loss of signal if the level of relaxation is excessive. Our experiments suggest that relaxation of constraints on thermal conductivity is more efficient than relaxation of borehole temperatures in suppressing artifacts arising from the three‐dimensional effects. We identify a range of constraints in which reasonable noise suppression and signal recovery are both achieved and then reprocess data from 22 Canadian boreholes for which surface temperature histories had earlier been derived by ourselves and others. Our reprocessed results reveal a remarkably simple surface temperature history common to much of eastern Canada. This history comprises a recent warming interval commencing in the nineteenth century, in which the surface temperature increased by some 1–4°C. Approximately 0.5–1.0°C of this increase was a recovery from a preceding cooler interval in which the temperature was below the long‐term mean. The remainder of the increase represents warming that exceeds the long‐term mean.
Seismic attenuation in partially saturated rock
Authors: Walsh J. B.
Published: February 2004

Measurements of attenuation in the extensional and shear modes for a sandstone (Murphy, 1982) indicate that two separate mechanisms are operative, one at very low saturation and another at saturation levels above 50%. At saturations of less than a few percent, attenuation in extension increases to a peak and then decreases to a plateau. Attenuation in shear behaves similarly, the magnitude being 20–30% less than that in extension. At saturations greater than 50%, attenuation in shear remains at the plateau value, while attenuation in extension rises to a broad peak, finally decreasing to a low value at full saturation. We analyzed a model in which porosity of the sample is assumed to be composed of cracks and equiaxed cavities called pores. At low saturation, attenuation is assumed to be caused by viscous dissipation as fluid in cracks is squished by changes in aperture under the normal component of the wave. At higher saturation, attenuation is assumed to be caused by viscous losses as fluid is forced from completely saturated regions, composed of many pores, to dry regions. Analysis shows that the model simulates the behavior observed by Murphy for Massilon sandstone. Agreement between theory and experiment suggests that the dominant aspect ratio for cracks is of the order of 10−3 and the size of saturated regions is about 1 mm at 90% saturation, decreasing to smaller values as the sample dries.
Thermohaline circulation: Energetics and variability in a single‐hemisphere basin model
Authors: Huang Rui Xin
Published: February 2004

A series of numerical experiments have been carried out to explore the variability of the thermohaline circulation and the energetics of halocline catastrophe. It is found that when the amplitude of surface freshwater flux is smaller than a critical value, the thermohaline circulation is in a thermal mode, with deep water formed in the north. When the freshwater flux amplitude is supercritical, the thermohaline circulation does not reach a single steady state. Instead, the model ocean is in a continuous transition between a slow, quasi‐steady saline mode and an energetic, unsteady thermal mode. For cases with no wind stress, the saline mode is characterized by sinking along the equator. For cases with wind stress, the saline mode is characterized by intermediate water formation at midlatitude. During the saline mode phase, the deep water gradually becomes warm and salty. Thus at the end of the saline mode phase, within the northern basin there is cold and relatively fresh water lying on top of warm and salty water. Such a vertical structure is potentially very unstable because small perturbations can grow, supported by the release of potential energy during strong cooling. A quantity called the diabatic available potential energy index is introduced as an indicator for such convective instability. Thus a final equilibrium in the saline mode cannot be reached; instead, the model ocean flips to a very energetic thermal mode in which violent overturning destroys the vertical stratification. After the energy is released, the saline cell supported by precipitation in the subpolar basin advances southward and the model ocean returns to the saline mode. The whole cycle will be repeated.
Western North Atlantic evidence for millennial‐scale changes in ocean circulation and climate
Authors: Keigwin L. D. , Jones G. A.
Published: February 2004

Two late Quaternary series of high resolution percent carbonate data from western North Atlantic sediment drifts (Bermuda Rise and Bahama Outer Ridge) show millennial‐scale oscillations superimposed on the familiar, longer‐period oscillations of orbital origin. The dominant high‐frequency oscillation in these records has a quasi‐period of about 4000 years. These percent CaCO3 changes most likely result from the influence of climate change on the flux of terrigenous material from eastern Canada, the resuspension of continental margin sediment by deep eddy kinetic energy, and carbonate dissolution. Sediment is transported to the Bermuda Rise by deep recirculating gyres and to the Bahama Outer Ridge by the deep western boundary current system. Stable isotope results on foraminifera across several of these oscillations from interstadial climate conditions and from a glacial inception display variability similar to that of percent CaCO3. Oxygen isotope ratios of planktonic foraminifera suggest large variations in near‐surface temperature and/or salinity, and carbon isotope ratios of benthic foraminifera indicate that there were significant oscillations in the flux of North Atlantic Deep Water (NADW). These data support models which couple surface ocean conditions in the North Atlantic, production of NADW, North Atlantic heat flux, and evidence for temperature oscillations in ice cores.
Atmospheric freshwater fluxes and their effect on the global thermohaline circulation
Authors: Zaucker Fritz , Stocker Thomas F. , Broecker Wallace S.
Published: February 2004

Atmospheric water vapor fluxes were derived from a 1‐year data set of horizontal wind speed and specific humidity assimilated from meteorological observations by the European Center for Medium‐Range Weather Forecast (ECMWF). Vertically integrated horizontal freshwater fluxes were compared to those of two data sets based on a climatology [Oort, 1983] and on simulations with an atmospheric general circulation model (AGCM). Zonal transports agree fairly well at all latitudes outside the tropics, where fluxes are about double for the AGCM data set. Meridional fluxes of the AGCM and ECMWF data sets show close agreement, while the climatological fluxes are generally smaller with a considerable northward shift in the southern hemisphere. Atmosphere‐to‐ocean freshwater fluxes were derived from the three data sets. Not only is there substantial disagreement between the data sets, but their zonal averages over the Atlantic, Pacific, and Indian Ocean basins show little resemblance to the respective restoring freshwater fluxes from a 2‐dimensional ocean model. If the ocean model is forced with the observed and modeled atmospheric fluxes, we find that the mode of ocean circulation is determined mostly by the net flux to the high‐latitude oceans and the amount of freshwater exported from the Atlantic basin. The latitudinal structure of the freshwater fluxes in low‐latitudes and midlatitudes has little influence on the modeled thermohaline circulation. The fluxes derived from the climatology and ECMWF permit North Atlantic Deep Water (NADW) formation, but a strong freshwater input to the Southern Ocean inhibits Antarctic Bottom Water formation. The AGCM transports so much moisture to the Arctic Ocean that NADW formation is shut down, resulting in a ocean circulation mode of southern sinking in all three ocean basins. If NADW is formed in the model, the strength of the Atlantic meridional overturning is determined by the net freshwater export from the Atlantic basin. When this export is artificially increased in the model over a range from 0.2 to 1 Sv, the ratio of overturning to freshwater forcing decreases almost linearly.
The global distribution of the seasonal sea level cycle calculated from coastal tide gauge data
Authors: Tsimplis M. N. , Woodworth P. L.
Published: February 2004

Data from 1043 tide gauges around the world have been used to map the global distribution of the seasonal cycle of mean sea level at the coast. Large‐scale features, such as the autumn‐spring phase dependence of the annual cycle in each hemisphere, can be seen to be superimposed upon a complexity of regional spatial variability. This variability stems from local meteorological, oceanographic and hydrological (river runoff) forcings. Significant differences in the seasonal cycle are observed at the opposite sides of even small land areas. At ocean islands the annual cycle usually has an amplitude ≤75 mm, while that of the semiannual component is below 20 mm. It is evident that coastal recorders cannot be used to infer the seasonal cycle of sea level over most of the deep ocean and that the future will require joint analyses of tide gauge and satellite altimeter data.
GPS precise tracking of TOPEX/POSEIDON: Results and implications
Authors: Bertiger W. I. , Bar‐Sever Y. E. , Christensen E. J. , Davis E. S. , Guinn J. R. , Haines B. J. , Ibanez‐Meier R. W. , Jee J. R. , Lichten S. M. , Melbourne W. G. , Muellerschoen R. J. , Munson T. N. , Vigue Y. , Wu S. C. , Yunck T. P. , Schutz B. E. , Abusali P. A. M. , Rim H. J. , Watkins M. M. , Willis P.
Published: February 2004

A reduced dynamic filtering strategy that exploits the unique geometric strength of the Global Positioning System(GPS) to minimize the effects of force model errors has yielded orbit solutions for TOPEX/POSEIDON which appear accurate to better than 3 cm (1 σ) in the radial component. Reduction of force model error also reduces the geographic correlation of the orbit error. With a traditional dynamic approach, GPS yields radial orbit accuracies of 4–5 cm, comparable to the accuracy delivered by satellite laser ranging and the Doppler orbitography and radio positioning integrated by satellite (DORIS) tracking system. A portion of the dynamic orbit error is in the Joint Gravity Model‐2 (JGM‐2); GPS data from TOPEX/POSEIDON can readily reveal that error and have been used to improve the gravity model.
Sensitivity of eddy‐induced heat transport to diabatic forcing
Authors: Drijfhout Sybren S.
Published: February 2004

Compensation of the poleward eddy heat transport by the heat transport of an eddy‐induced mean meridional overturning cell is a common feature in many eddy‐resolving ocean models. It has been argued that this is the result of the weak thermal driving of the ocean. As the actual air/sea coupling is scale dependent, it might be questioned whether the approximation of weak thermal driving is relevant for the oceanic eddy field. In this paper the role of diabatic forcing in modifying eddy‐mean flow interaction is investigated. Emphasis has been placed on the sensitivity of the eddy‐induced change in heat transport to the sea surface thermal boundary condition. Experiments have been performed with a multilayer isopycnic primitive equation model of an idealized North Atlantic subtropical and subpolar gyre. For different values of the air/sea coupling, solutions with and without transient eddies have been compared. The air/sea coupling mostly affects the upper ocean thermal and velocity fields. A decrease of the coupling timescale pushes the separation point of the midlatitude jet further northward and induces a tight recirculation southwest of the separated jet. These effects are enhanced by the eddies. In the present model there is compensation of the eddy heat transport for sea surface temperature (SST) relaxation times longer than 150 days; a breakdown of the compensation occurs for SST relaxation times shorter than 50 days (the average upper layer depth is 200 m). In between is a transition regime. For strong thermal driving the eddy‐induced change in total heat transport is of the same order as the eddy heat transport.
On the generation and role of eddy variability in the central North Atlantic Ocean
Authors: Beckmann Aike , Böning Claus W. , Brügge Bernd , Stammer Detlef
Published: February 2004

Sources of near‐surface oceanic variability in the central North Atlantic are identified from a combined analysis of climatology, surface drifter, and Geosat altimeter data as well as eddy‐resolving
and
Community Modeling Effort North Atlantic model results. Both observational and numerical methods give a consistent picture of the concentration of mesoscale variability along the mean zonal flow bands. Three areas of high eddy energy can be found in all observational data sets: the North Equatorial Current, the North Atlantic Current, and the Azores Current. With increasing horizontal resolution the numerical models give a more realistic representation of the variability in the first two regimes, while no improvement is found with respect to the Azores Current Frontal Zone. Examination of the upper ocean hydrographic structure indicates baroclinic instability to be the main mechanism of eddy generation and suggests that the model deficiencies in the Azores Current area are related to deficiencies in the mean hydrographic fields. A linear instability analysis of the numerical model output reveals that instability based on the velocity shear between the mixed layer and the interior is also important for the generation of the mid‐ocean variability, indicating a potential role of the mixed layer representation for the model. The
model successfully simulates the northward decrease of eddy length scales observed in the altimeter data, which follow a linear relationship with the first baroclinic Rossby radius. An analysis of the eddy‐mean flow interaction terms and the energy budget indicates a release of mean potential energy by downgradient fluxes of heat in the main frontal zones. At the same time the North Atlantic Current is found to be supported by convergent eddy fluxes of zonal momentum.
Ocean tides in the Asian semienclosed seas from TOPEX/POSEIDON
Authors: Mazzega P. , Bergé M.
Published: February 2004

The eight leading ocean tides (M2, S2, N2, K2, K1, O1, P1, and Q1) have been mapped in Asian semienclosed seas by inverting combined sets of tide gauge harmonic constants and a reduced set of TOPEX/POSEIDON satellite altimeter data. The tidal maps are given on a 0.5°×0.5° grid with their formal error estimates. Numerical experiments conducted in the South China Sea have shown the inverse solutions to be quite insensitive to changes in the parameters of the a priori covariance functions for both the tidal signals and data residual errors. Once the various parameters of the inversion scheme are fixed, the algorithm is applied to the Sea of Okhotsk, the Sea of Japan, the “East China” Sea (including the Bo Hai and Yellow Seas) and the Indonesian Seas. A set of tide gauge constants, not used in our solutions, is then used for comparison. Though these accuracy estimates may be biased because of the uneven stations coverage, we conclude that the inversion of only 21 cycles of TOPEX/POSEIDON data leads to solutions with an accuracy comparable to the Schwiderski (1980b, c) semiempirical model and the Cartwright and Ray (1990) model derived from Geosat altimetry.
Structure of two hydrothermal megaplumes
Authors: D'Asaro Eric , Walker Sharon , Baker Edward
Published: February 2004

The dynamic signatures of two megaplumes above the Juan de Fuca Ridge are analyzed. The chemical properties of these two lenslike masses of water were described by Baker et al. (1989) and clearly indicate that they were generated by massive and rapid ventings of hot hydrothermal fluid from the ridge. Both are nearly circular with radii of about 6.5 km. The isopycnals bow upward around these cores of anomalous water, leading to an anticyclonic circulation. A cyclogeostrophic balance gives maximum currents at the edge of the core of 0.11 m s−1 for the first megaplume (MP1) and 0.07 m s−1 for the second megaphone (MP2). Currents extend beyond the core to a radius of 12–15 km. The centers of the cores are in nearly solid body rotation with relative voracities of −0.5ƒ(MP1) and −0.3ƒ(MP2) and potential vorticity anomalies, expressed in units of equivalent relative vorticity, of −0.8ƒ(MP1) and −0.6ƒ(MP2), where ƒ is the Coriolis frequency. The aspect ratio of each megaplume gives a Burger number of 0.22. In terms of these nondimensional numbers, the megaplumes are very similar to eddies of Mediterranean water found in the eastern Atlantic (meddies), despite their very different origin.
TOPEX/POSEIDON: The 2‐cm solution
Authors: Cheney Robert , Miller Laury , Agreen Russell , Doyle Nancy , Lillibridge John
Published: February 2004

The unprecedented accuracy of TOPEX/POSEIDON (T/P) altimeter data warrants a new evaluation of the methods typically used to form time series of sea level change. Whereas explicit removal of orbit error has always been required as a first step in altimeter data processing, the T/P analysis presented here is based simply on unadjusted, monthly averages. This approach has the advantage of retaining the large‐scale ocean signal, which would be distorted by orbit adjustment. Using 16 months of data, we have evaluated the T/P monthly means on spatial scales ranging from mesoscale to global. In the tropical Pacific, comparisons with 17 island tide gauge records and dynamic height derived from 36 thermistor moorings show that the altimeter data have an accuracy of approximately 2 cm when averaged over spatial scales of a few hundred kilometers. On basin scales in the northern hemisphere, similar agreement is found between the T/P data and the dynamic height climatology of Levitus (1982). These new altimeter observations are thus providing the first reliable view of global sea level changes on seasonal‐to‐interannual timescales.
Yield curves and flow rules of pack ice
Authors: Ukita Jinro , Moritz Richard E.
Published: February 2004

A theoretical framework is developed, which relates small‐scale pack ice energy transformations dominated by ridging and sliding processes to large‐scale dynamics described by internal ice stress and strain rate. The framework consists of an energy equation, a kinematic model, and the minimization of maximum shear stress. From the kinematic model and energy equation we derive an expression for the maximum shear stress and compressive stress in terms of a deformational pattern and geometry of cracks. For each choice of a kinematic model the minimization principle applied to this expression gives an explicit constitutive relationship as follows: a yield curve, a flow rule, and the directional relationship between stress and strain rate. The theory provides an energetic explanation for different yield curves and flow rules. The cavitating fluid is realized with the absence of energetic transformations on shear and divergent motions. Hibler's (1979) constitutive relationship corresponds to the energetic function whose form depends on the strain rate magnitude, reflecting the viscous‐plastic coupling. It is found that for a class of energy functions associated with smooth and strictly convex yield curves the flow rule is normal. The Mohr‐Coulomb yield criterion is identified as a special case of the normal flow rule. Preferred orientation of cracks is also explained by the same minimization of the maximum shear stress. The theory predicts that the preferred orientation depends on ice geometry, as well as the energetic contribution from sliding relative to ridging. For uniform square‐and diamond‐shaped floes the maximum shear stress associated with ridging obtains its minimum value when the axes of symmetry of the floes coincide with the principal axes of the strain rate. A field of isotropically oriented square‐shaped floes is simulated, resulting in a sine lens‐shaped yield curve.