bluuebunny/crossref_metadata_2025 huggingface dataset:
Influence of a toroidal magnetic field on the nutations of Earth
Authors: Buffett Bruce A.
Published: February 2008

Calculations are presented for the electromagnetic interactions between a thin conducting layer at the base of the mantle and a magnetic field that is parallel to the core‐mantle boundary, the so‐called toroidal field. The effect of such interactions on the nutations of Earth is investigated. The analysis shows that a conducting layer in the mantle can significantly increase the amplitude of the toroidal field near the core‐mantle boundary. The nutations are affected by the resulting magnetic stresses at the core‐mantle boundary, which can arise in one of two ways. One coupling mechanism involves an interaction between the toroidal and radial components of Earth's main field; a perturbation in the radial field caused by the nutations will produce a magnetic stress on the mantle in the presence of a toroidal field. The largest torque results when the dipole field is perturbed in the presence of a quadrupolar toroidal field. The amplitude of this torque, however, is expected to be too small to produce observable changes in the amplitudes of the nutations, which are currently determined with a standard error of 0.04 milliarcseconds (Herring et al., 1991). The other coupling mechanism proves to be more important, but it depends on the presence of nonhydrostatic undulations in the core‐mantle boundary. The toroidal field is perturbed by a steady flow in the core over the undulations in the boundaries. The perturbed field diffuses across the boundary, which gives rise to magnetic stresses when the core flow associated with the nutations is superimposed on the steady flow. Calculations based on an earlier study of Moffatt and Dillon (1976) suggest that a 40 G toroidal field could produce observable changes in the nutations if there is a thin layer at the base of the mantle with a conductivity of 5 × 105 S m−1.
Characteristics of the bed of the Lower Columbia Glacier, Alaska
Authors: Humphrey Neil , Kamb Barclay , Fahnestock Mark , Engelhardt Hermann
Published: February 2008

An unplanned, but unique, experiment has given an in situ measurement of the strength of deforming subglacial till under the central region of a major valley glacier. We report on both planned and unplanned borehole investigations of the subglacial shear zone of Columbia Glacier, southeast Alaska. Basal samples, coring and down‐hole water samples show that the fiord‐filling lower reach of the glacier is underlain by a thin, ∼ 7‐cm, veneer of rock debris. Fluidized debris intruded at least a meter up the borehole. At a higher site, 13 km from the terminus and above the fiord, probing, samples, and the bending of a drill stem, which was stuck in the basal zone for 5 days, showed that the basal till layer was ∼ 65 cm thick. Horizontal velocity of the till decreased monotonically downward from the ice/till interface. Till at the interface moved with the ice velocity. Plastic deformation of the drill stem gave an estimate of the strength of the basal till, which is normally described as a viscoplastic material. If the till is assumed to be either perfectly plastic or Newtonian viscous, then the strengths are as follows; the plastic yield strength of the till was 5.5×103 Pa (0.055 bar) with an upper bound of 1.3 ×104Pa (0.13 bar), while the nominal viscosity was of the order of 2×108 Pa s (2×109 poise), with an upper bound of 5×108Pa s. In neither case is the till “strength” enough to supply the bulk basal shear stress to resist the glacier flow.
Emplacement of mantle rocks in the seafloor at mid‐ocean ridges
Authors: Cannat Mathilde
Published: February 2008

This paper discusses the geological and geophysical data available on mid‐ocean ridges with outcrops of serpentinized mantle peridotites, with the objective of better constraining the modes of emplacement of these rocks in the seafloor. Ridges with serpentinized peridotites outcrops are in most cases characterized by slow‐spreading rates, and in every case by deep axial valleys. Such deep axial valleys are thought, based on geophysical constraints and on mechanical modelling results, to characterize ridges with a thick axial lithosphere. A predictable effect of a thick axial lithosphere is that it should prevent magmas from pooling at crustal depths in a long‐lasting magma chamber: gabbroïc magmas should instead form shortlived dike or sill‐like intrusions. Samples from axial outcrops of serpentinized peridotites are often cut by dikelets of evolved gabbros which are interpreted as apophyses of such dike and sill‐like intrusions. This observation leads to a discontinuous magmatic crust model, in which mantle‐derived peridotites form screens for numerous gabbroïc intrusions. This discontinuous magmatic crust is expected to form in magma‐poor ridge regions, where there is not enough magma to produce a 4‐to 7‐km‐thick magmatic crust, and where the uppermost kilometers of oceanic lithosphere therefore have to be at least partially made of tectonically uplifted mantle material. Because the dimensions of individual mantle‐derived ultramafic screens may be smaller than seismic experiments detection limits, the discontinuous magmatic crust model discussed in this paper may produce a layer 3‐type seismic signature, even without extensive serpentinization of its ultramafic component. It therefore provides an alternative to Hess's [1962] serpentinite layer 3 model, for the geological interpretation of seismic data from oceanic areas with frequent outcrops of deep crustal and mantle‐derived rocks.
Introduction to modeling crustal deformation
Authors: Snay Richard A. , Holdahl Sandford R.
Published: February 2008

This special section on Modeling Crustal Deformation presents a collection of research articles prepared by several participants in the AGU Chapman Conference on Time‐Dependent Positioning. The conference was convened September 23–25, 1991, in Annapolis, Maryland. It provided scientists from various disciplines (geodesy, tectonophysics, seismology, volcanology, hydrology, and geology) with an opportunity to exchange information on alternative modeling concepts and techniques. The conference's scope encompassed user requirements for crustal motion models, the theoretical nature of crustal motion, modeling methodology, and results from recent deformation studies.
Dynamic rupture analysis: Inversion for the source process of the 1990 Izu‐Oshima, Japan, earthquake (M = 6.5)
Authors: Fukuyama Eiichi , Mikumo Takeshi
Published: February 2008

A waveform inversion has been applied to strong motion data using a dynamic shear crack model. We studied the 1990 Izu‐Oshima earthquake (MJMA = 6.5), which has vertical strike‐slip faulting with unilateral rupture propagation. The inversion has two steps, a waveform inversion and a crack inversion, that are applied iteratively. A waveform inversion is used to determine the distribution of rupture starting times and slip dislocations using the slip functions calculated by the initial crack model, or by previous crack inversion. A crack inversion is used to calculate dynamic crack propagation that explains the results of the above inversion. In this step, we use the estimated rupture times as a locking fracture criterion; the maximum shear stress attained before a fault segment breaks gives a lower bound estimate of the peak shear strength at each fault segment. Then the dynamic stress drop distribution is estimated from the slip distribution obtained from waveform inversion assuming a dynamic crack model. From the results, we determine the rise time distribution and the distribution of a dimensionless stress ratio S defined as (strength excess)/(stress drop). Our analysis gives the following picture of the rupture process of the 1990 Izu‐Oshima earthquake: (1) An asperity‐type faulting having large slip and high stress drop was detected in the region around the initiation point of rupture. (2) South of the asperity zone, barrier‐type faulting characterized by incoherent propagating rupture, small slip, long rise time, and high strength excess was detected. This zone corresponds to the intersection of the fault with the 1978 earthquake (MJMA = 7.0).
The three‐dimensional structure of Kilauea Volcano, Hawaii, from travel time tomography
Authors: Rowan Linda R. , Clayton Robert W.
Published: February 2008

A linear, travel time tomography study of the most active shield volcano of the world, Kilauea Volcano, Hawaii, was undertaken to determine the lateral heterogeneities produced by its intricate magmatic and tectonic environment. Kilauea provides an ideal setting to do tomography because of its dense seismograph array and many local earthquakes that allow excellent ray coverage of complex subsurface features. Local P wave data from ∼ 12,295 events were inverted using a one‐dimensional layered velocity model. Inversions were done for two cell sizes (5×5×5 km and 1×1×1 km) to resolve structural regions on different length scales. This study provided a view of the average velocity variations relative to a one‐dimensional velocity model. Analysis and interpretation of the tomographic images allowed us to infer the following model. The main shallow magma reservoir is delineated by a slow velocity region southeast of the summit from 0 to 2 km depth. There is a distinct high velocity region centered northwest of the summit from 0 to 2 km depth that represents a cap of dense, intrusive dikes surrounding the magma chamber. We suggest that the shallow reservoir is a narrow, compartmentalized region of sills and dikes, centered just south‐southeast of Halemaumau caldera. Below the main reservoir, the summit is imaged as a slightly fast region from 5 to 10 km in the coarse model indicating that the main conduit is structurally defined by an intrusive dike complex until about 10 km. The rift zones of Kilauea are imaged as major, high velocity entities, widening to the south with depth until 6 km. These fast anomalies are related to the sheeted dike complexes along the rifts. On a finer scale, slow anomalies suggest the presence of magma pockets centered at 0–2 km depth beneath Mauna Ulu, Makaopuhi and Puu Oo, along the east rift zone (ERZ). Two significant high velocity regions along the lower ERZ near Kalalua and Kaliu are inferred to represent intrusive barriers to magma injection along the shallow (0–4 km) ERZ conduit. The southwest rift zone may have an intrusive barrier related to a high velocity region just southwest of Mauna Iki. The Hilina and Kaoiki fault zones are imaged as slow features at shallow depths (< 5 km) related to the open fractures and scarps along the normal faults. The Koae fault system is imaged as a slightly fast shallow structure (< 6 km) possibly related to intrusive diking from the adjacent rift zones that fill and may even induce the extensional structures associated with this complex fault zone. Continued inversions with the immense amount of seismic data collected for Hawaiian events will allow the detailed development of a three‐dimensional structural model for Kilauea. Such a model will be extremely useful to seismologists and petrologists alike for understanding the tectonic growth and magmatic evolution of this dynamic shield volcano.
The genesis of oceanic crust: Magma injection, hydrothermal circulation, and crustal flow
Authors: Morgan Jason Phipps , Chen Y. John
Published: February 2008

In this study we construct a thermal and mechanical model for the genesis of oceanic crust. Magma is halted in its ascent within the oceanic crust when it reaches a freezing horizon, where the dilational volume change associated with magma freezing leads to viscous stresses that favor magma ponding near the freezing horizon. To model the steady state thermal impact of crustal accretion via dike injection and pillow flows, we treat all crustal accretion in rocks cooler than a magma “solidus” to occur in a narrow 250‐m‐wide dike‐like region centered about the ridge axis. The rest of the oceanic crust is modeled to be emplaced as a steady state magma lens directly beneath the “solidus” freezing horizon where the steady state emplacement rate is determined by the constraint that this lens supply all crust that is not emplaced through diking/extrusion above the magma lens. If hydrothermal heat transport within crustal rocks cooler than 600°C removes heat 8 times as efficiently as heat conduction, then we find that a steady state magma lens will only exist within the crust for ridges spreading faster than a 25 mm/yr half rate. The depth dependence of the magma lens with spreading rate is in good agreement with seismic observations. These results suggest that a fairly delicate balance between magmatic heat injection during crustal accretion and hydrothermal heat removal leads to a strongly different Crustal thermal structure at fast and slow spreading ridge axes. Our results support the hypothesis that median valley topography is due to extension of strong ridge axis lithosphere; it is the difference in thermal regime that is directly responsible for the striking difference between the typical median valley seen at slow spreading ridges (e.g., Mid‐Atlantic Ridge) and the axial high seen at fast spreading ridges (e.g., East Pacific Rise). This paradigm for the origin of a median valley at a slow spreading ridge predicts that along‐axis variations in median valley topography of a slow spreading center reflect variations in recent magmatic heat input along a segment, that is, that the axial topography is a good time‐averaged indicator of the relative importance of hydrothermal cooling and magmatic injection along a given section of a ridge segment. We determine the accumulated crustal strain associated with lower crustal flow which supports the hypothesis that the Oman Ophiolite crust was created at a paleo‐analogue to a fast spreading ridge and also suggests that crustal strain, and not cumulate layering, may be the dominant physical process that generates “layered gabbros” within the Oman Ophiolite.
Deep mantle viscous structure with prior estimate and satellite constraint
Authors: Ivins E. R. , Sammis C. G. , Yoder C. F.
Published: February 2008

Radial viscosity profiles are constructed for the mantle using plausible temperature profiles and high‐temperature creep models in olivine. While it is possible to specifically design mantle thermal profiles that produce relatively constant viscosity, temperatures deduced from parametric and/or full numerical simulation of convection predict steep increases of viscosity with depth. The most ubiquitous trend is an increase in viscosity from about 800 to 900 km depth to the top of the D″ layer. Predicted increase of viscosity in this portion of the lower mantle ranges from a factor of 102 to 104. Precise estimates are impossible due uncertainty in determining activation volume V* and activation energy E*. Zeroth‐order extrapolation of a creep law to lower mantle conditions requires the assumption that diffusion of a single ionic species controls dislocation mobility. The extrapolation is useful in spite of its great uncertainty. Our rationale consists of two essential elements: (1) Creep laws with depth‐dependent prefactor A(r) and depth‐dependent activation volume V*(r), energy E*(r), and enthalpy S*(r) can be parameterized using estimates of diffusion constants based on elasticity. (2) Secular changes in Earth's gravity field, particularly the zonal coefficients Jl are now detected in the orbits of artificial satellites such as LAGEOS and Starlette. These secular changes are believed to be dominated by postglacial rebound, although there is some contribution from present‐day glacial melting. These gravity field measurements, when combined with constraints on glacial melting, provide a sensitive set of constraints on lower mantle viscous response to the Last glacial epoch and, hence, provide a test of a priori estimates. A radially stratified and incompressible Earth model is used to test sensitivity of data to viscosity increases with depth and convective boundary layer structure. Prior estimates and observed nontidal J2(−C20) are consistent with a layered lower mantle viscosity. Details of this layering are examined by comparing predicted and observed J3, J4. Speculation that a high‐viscosity layer exists above D″ is considered. With a 650‐km‐thick deep high‐viscosity layer (η ≈ 6.0 × 1023 Pa s) the remaining lower mantle is in one of two ranges: 1.5 to 3.5 × 1020 or 3.5 to about 10. × 1022 Pa s. Observational bounds on secular gravity coefficients C30, C40, C50, and C70 using LAGEOS, Starlette, or Etalon satellite data could eliminate the ambiguity between these two viscosity ranges.
Bubble populations and gas exchange in hypersaline solutions: A preliminary study
Authors: Shatkay Michal , Ronen Daniel
Published: February 2008

Bubble populations in hypersaline solutions of NaCl (up to 5 M), MgSO4 (up to 1.5 M), MgCl2 (up to 4.0 M) and Dead Sea brine (ionic strength of about 9 M) produced by a ceramic diffuser were analyzed. Different population parameters were observed for the different salts. It was found that for solutions of MgCl2 and MgSO4 the degree of coalescence generally decreased with increasing concentrations of salt, but not monotonically. Bubble size distributions are not simple functions of the ionic strength, the viscosity or the surface tension of the studied solutions. It is suggested that the present observations can be explained by an interplay between four mechanisms: (1) enhancement of water structure, (2) variation of surface potential, (3) “bubble flocculation,” and (4) ion complexation. This preliminary study indicates that the bubble‐mediated transport of gas into a liquid phase is more significant in the case of a hypersaline solution as compared with fresh water and might play a significant role in determining the regime of dissolved gases in natural brines.
Statistics of small‐scale activity in the upper ocean
Authors: Dugan J. P. , Stalcup B. W. , DiMarco R. L.
Published: February 2008

Observations are presented on the statistical distribution of temperature fluctuations on horizontal scales from 10 cm to 1 km in the seasonal thermocline. This range is especially interesting because it includes internal wave processes on the long end and it approaches microscale processes on the short end. A measure of the activity level is computed as the variance of horizontal gradients of temperature or conductivity normalized by the square of the mean vertical gradient, essentially a pseudo Cox number, in eight wavelength bands across this range of scales. The activity levels are found to be distributed approximately lognormally in these bands, with the mean level increasing with decreasing wavelength in conformance with previously published temperature and displacement spectra. Root‐mean‐square slopes inferred from the mean activity level are found to be slightly less than 1% for wavelengths longer than 300 m and increase only slightly on shorter scales until a wavelength of 2 to 3 m, where they increase sharply. The variance also increases significantly near 3 m and is significantly greater near the peak density gradient under the base of the mixed layer than deeper in the seasonal thermocline. Both horizontal and vertical correlation lengths of the log of activity level are calculated, providing estimates of “patch” sizes which are about 100 m long and about l m high for the shortest wavelengths. These correlation lengths are used with local estimates of the variance of activity level to conclude that there is significant inhomogeneity and/or nonstationarity in the data set.
The seasonal characteristics of the circulation in the north Mediterranean basin and their relationship with the atmospheric‐climatic conditions
Authors: Astraldi M. , Gasparini G. P.
Published: February 2008

The greater heat losses sustained by the Ligurian‐Provençal basin in winter appear to be the major factor leading to the periodic intrusions of the warmer Tyrrhenian water from the east. This is the main conclusion of a 3‐year oceanographic investigation carried out in the eastern Ligurian‐Provençal basin in order to monitor the long‐term characteristics of its exchanges with the Tyrrhenian Sea. The analysis of the whole data set collected in this period shows that two currents converge north of Corsica after having flowed along the northwestern side of Corsica (the West Corsican Current) and through the Corsican Channel (the Tyrrhenian Current). While the former does maintain nearly steady properties year round, the latter crosses the strait only during the coldest season, thus showing a clear seasonal trend that begins again every year. The comparison with the meteorological parameters of the Ligurian‐Provençal basin indicates that the winter intrusions of the Tyrrhenian current are tightly associated with the heat and water losses through the rise induced in the upper basin density. In addition to reintegrating the heat lost to the atmosphere, the Tyrrhenian flow might also restore the volume of the basin upper layer involved in the deep water formation processes. This enables us to infer that a unique circulation pattern, mostly driven by atmospheric‐climatic conditions, involves the northern part of the western Mediterranean Sea.
Nudging satellite altimeter data into quasi‐geostrophic ocean models
Authors: Verron Jacques
Published: February 2008

This paper discusses the efficiency of several variants of the nudging technique (derived from the technique of the same name developed by meteorologists) for assimilating altimeter data into numerical ocean models based on quasi‐geostrophic formulation. Assimilation experiments are performed with data simulated in the nominal sampling conditions of the Topex‐Poseidon satellite mission. Under experimental conditions it is found that nudging on the altimetric sea level is as efficient as nudging on the vorticity (second derivative in space of the dynamic topography), the technique used thus far in studies of this type. The use of altimetric residuals only, instead of the total altimetric sea level signal, is also explored. The critical importance of having an adequate reference mean sea level is largely confirmed. Finally, the possibility of nudging only the signal of sea level tendency (i.e., the successive time differences of the sea level height) is examined. Apart from the barotropic mode, results are not very successful compared with those obtained by assimilating the residuals.
Turbulent heat flux in the upper ocean under sea ice
Authors: McPhee Miles G.
Published: February 2008

Turbulence data from three Arctic drift station experiments demonstrate features of turbulent heat transfer in the oceanic boundary layer. Time series analysis of several w′T′ records shows that heat and momentum flux occur at nearly the same scales, typically by turbulent eddies of the order of 10–20 m in horizontal extent and a few meters in vertical extent. Probability distribution functions of w′T′ have large skewness and kurtosis, where the latter confirms that most of the flux occurs in intermittent “events” with positive and negative excursions an order of magnitude larger than the mean value. An estimate of the eddy heat diffusivity in the outer (Ekman) part of the boundary layer, based on measured heat flux and temperature gradient during a diurnal tidal cycle over the Yermak Plateau slope north of Fram Strait, agrees reasonably well with the eddy viscosity, with values as high as 0.15 m2 s−1. An analysis of measurements made near the ice‐ocean interface at the three stations shows that heat flux increases with both temperature elevation above freezing and with friction velocity at the interface. It also reveals a surprising uniformity in parameters describing the heat and mass transfer: e.g., the thickness of the “transition sublayer” (from a modified version of the Yaglom‐Kader theory) is about 10 cm at all three sites, despite nearly a fivefold difference in the under‐ice roughness z0, which ranges from approximately 2 to 9 cm. A much simplified model for heat and mass transfer at the ice‐ocean interface, suggested by the relative uniformity of the heat transfer coefficients at the three sites, is outlined.
Bound infragravity waves
Authors: Okihiro Michele , Guza R. T. , Seymour R. J.
Published: February 2008

Model predictions of bound (i.e., nonlinearly forced by and coupled to wave groups) infragravity wave energy are compared with about 2 years of observations in 8‐ to 13‐m depths at Imperial Beach, California, and Barbers Point, Hawaii. Frequency‐directional spectra of free waves at sea and swell frequencies, estimated with a small array of four pressure sensors, are used to predict the bound wave spectra below 0.04 Hz. The predicted total bound wave energy is always less than the observed infragravity energy, and the underprediction increases with increasing water depth and especially with decreasing swell energy. At most half, and usually much less, of the observed infragravity energy is bound. Bound wave spectra are also predicted with data from a single wave gage in 183‐m depth at Point Conception, California, and the assumption of unidirectional sea and swell. Even with energetic swell, less than 10% of the total observed infragravity energy in 183‐m depth is bound. Free waves, either leaky or edge waves, are more energetic than bound waves at both the shallow and deep sites. The low level of infragravity energy observed in 183‐m depth compared with 8‐ to 13‐m depths, with similarly moderate sea and swell energy, suggests that leaky (and very high‐mode edge) waves contribute less than 10% of the infragravity energy in 8–13 m. Most of the free infragravity energy in shallow water is refractively trapped and does not reach deep water.
Evidence of the barrier layer in the surface layer of the tropics
Authors: Sprintall Janet , Tomczak Matthias
Published: February 2008

Comparisons between isothermal depth to the top of the thermocline, and the mixed layer depth based on a σt criterion were undertaken for the tropical world oceans. In three equatorial regions, a shallower mixed layer than isothermal layer occurs, implying the presence of a strong halocline above the thermocline. This distance separating the top of the thermocline and the bottom of the mixed layer is referred to as the “barrier layer”, in relation to its impediment to vertical heat flux out of the base of the mixed layer. Different mechanisms are responsible for maintaining the barrier layer in each of the three regions. In the western equatorial Pacific Ocean a salinity budget confirmed that heavy local precipitation most likely results in the isothermal but salt‐stratified layer. In the northwest equatorial Atlantic, it is hypothesized that high salinity waters are subducted at the subtropics during winter and advected westward as a salinity maximum in the upper layers of the tropics, resulting in the barrier layer. In the eastern equatorial Indian Ocean, monsoonal related rainfall and river runoff contribute significantly to the freshwater flux, producing salt stratification in the surface. These results suggest the need to include the effects of salinity stratification when determining mixed layer depth.
Acoustic Doppler current profiler measurements of possible lee waves south of Key West, Florida
Authors: Marmorino G. O. , Trump C. L.
Published: February 2008

Acoustic Doppler current profiler measurements made while underway in 200–240 m of water south of Key West, Florida, show wavelike signals with about 1 km horizontal wavelength and 3–5 cm s−1 amplitude vertical velocity. Vertical displacements of about 5 m are inferred from coherent oscillations in surfaces of constant acoustic backscatter, in features associated with the width of the peak in the Doppler spectrum, and in fine structure of vertical current shear. While the sampling was limited, all the measurable waves occurred over irregular bathymetry in the presence of an east‐northeastward mean flow U > 1 m s−1 (averaged over 30‐ to 90‐m depth). The peak strain rate associated with the waves was nearly 1 × 10−3 s−1 at 12‐m depth, and this could have produced bands of smooth and rough water observed at the surface. We tentatively identify the waves as being topographically forced by the Florida Current flowing over a rough continental margin.
Brazil‐Malvinas Confluence: Water mass composition
Authors: Maamaatuaiahutapu Keitapu , Garçon Véronique C. , Provost Christine , Boulahdid Mostefa , Osiroff Ana Paula
Published: February 2008

A quantitative analysis of water masses in the Brazil‐Malvinas Confluence zone is performed with a least squares multiple tracer analysis using data from the confluence winter 1989 cruise. The purpose is to find the mixture of source water types that best describes the composition of a given water sample. This method is valuable in regions involving strong mixing among various source water types, as is the Brazil‐Malvinas Confluence zone. Seven main core layers are identified in this region, and all are retained for the analysis: the Thermocline Water (TW), the Subantarctic Surface Water (SASW), the Antarctic Intermediate Water (AAIW), the Upper Circumpolar Deep Water (UCDW), the North Atlantic Deep Water (NADW), the Lower Circumpolar Deep Water (LCDW), and the Weddell Sea Deep Water (WSDW). Tracers selected are temperature, salinity, dissolved nutrients, and oxygen. The results show the proportion of each source water type along four east‐west sections (35.4°S, 36.5°S, 37.9°S, 39°S). They are accurate to within 20% for all sources. The solution presents evidence of local recirculation of AAIW largely influenced by the two strong currents, Brazil and Malvinas. Southward TW and NADW separate from the coast, NADW turning eastward at a higher latitude than TW.
Thermocline and intermediate water communication between the south Atlantic and Indian oceans
Authors: Gordon Arnold L. , Weiss Ray F. , Smethie William M. , Warner Mark J.
Published: February 2008

A conductivity‐temperature‐depth and tracer chemistry section in the southeast South Atlantic in December 1989 and January 1990 presents strong evidence that there is a significant interocean exchange of thermocline and intermediate water between the South Atlantic and Indian oceans. Eastward flowing water at 10°W composed of South Atlantic Central (thermocline) Water is too enriched with chlorofluoromethanes 11 and 12 and oxygen to be the sole source of similar θ‐S water within the northward flowing Benguela Current. About two thirds of the Benguela Current thermocline transport is drawn from the Indian Ocean; the rest is South Atlantic water that has folded into the Benguela Current in association with the Agulhas eddy‐shedding process. South Atlantic Central water passes in the Indian Ocean by a route to the south of the Agulhas Return Current. The South Atlantic water loops back to the Atlantic within the Indian Ocean, perhaps mostly within the Agulhas recirculation cell of the southwest Indian Ocean. Linkage of Atlantic and Indian Ocean water diminishes with increasing depth; it extends through the lower thermocline into the Antarctic Intermediate Water (AAIW) (about 50% is derived from the Indian Ocean) but not into the deep water. While much of the interocean exchange remains on an approximate horizontal “isopycnal” plane, as much as 10 × 106 m3 s−1 of Indian Ocean water within the 25 × 106 m3 s−1 Benguela Current, mostly derived from the lower thermocline and AAIW, may balance deeper Atlantic export of North Atlantic Deep Water (NADW). The addition of salt water from the evaporative Indian Ocean into the South Atlantic Ocean thermocline and AAIW levels may precondition the Atlantic for NADW formation. While AAIW seems to be the chief feed for NADW, the bulk of it enters the subtropical South Atlantic, spiked with Indian Ocean salt, within the Benguela Current rather than along the western boundary of the South Atlantic.
Theoretical heights of buoyant convection above open leads in the winter Arctic pack ice cover
Authors: Serreze Mark C. , Maslanik James A. , Rehder Mark C. , Schnell Russell C. , Kahl Jonathan D. , Andreas Edgar L.
Published: February 2008

A fetch‐dependent boundary‐layer model, driven by observed temperature sounding data, is used to examine theoretical heights of buoyant convection (H) above open leads in the wintertime pack ice of the central Arctic. Assuming wet adiabatic ascent with no entrainment or friction, H is estimated as the height at which the model‐predicted equivalent potential temperature at saturation above a lead (θel) intersects with the same value of equivalent potential temperature at saturation (θe) derived from vertical sounding profiles. H increases with increasing lead width. For a 1000‐m lead, the widest which can be reasonably expected for the central Arctic, the median value of H is approximately 1000 m, slightly below the median top of the low‐level Arctic temperature inversion layer. While H shows large variability, events of convection up to 4 km, as recently observed from lidar backscatter data, appear to be fairly rare. First, these events require an open lead of at least 10,000 m. Second, while H tends to be largest under conditions of low surface wind speed, low surface temperature, and a weak low‐level temperature inversion, this combination appears to be atypical of Arctic conditions. Third, while the meteorological conditions that should favor the development of open leads tend to minimize H, conditions favoring large H are also those in which any newly developed leads will quickly ice over.
A toy model linking atmospheric thermal radiation and sea ice growth
Authors: Thorndike A. S.
Published: February 2008

A highly simplified analytical model of sea ice growth is presented in which the atmosphere is in thermal radiative equilibrium with the ice. This makes the downwelling longwave radiation reaching the ice surface an internal variable rather than a specified forcing. The ice is characterized by only two variables, thickness and surface temperature. The seasonal cycle is resolved into four steps, in which either the thickness or the temperature can vary. An ocean mixed layer of fixed depth but variable temperature provides a heat sink if the ice melts away during the summer. Analytical results show how the ice state depends on properties of the ice and on the externally specified climate. The main result is that this bare bones physics is enough to produce an annual cycle of temperature and thickness not unlike today's conditions. The results also suggest how much the climate must change to produce qualitatively different cycles. Reducing the atmospheric poleward heat transport by 20 W m−2 is enough to make the ice so thick (>12 m) that it never warms up to the melting point in summer. Increasing the poleward flux by 30 W m−2 is enough to produce an arctic ocean which stores enough heat in summer to prevent any ice forming in winter. The model predicts that today's climate can support either a perennial ice pack or an ice free Arctic. However, a winter ice pack which melts completely each summer is not possible under any climate.
Isotopic equilibration between dissolved and suspended particulate lead in the Atlantic Ocean: Evidence from 210Pb and stable Pb isotopes
Authors: Sherrell Robert M. , Boyle Edward A. , Hamelin Bruno
Published: February 2008

Decreased consumption of leaded gasoline in the United States over the past two decades has drastically altered the flux and isotopic composition of Pb entering the western North Atlantic from the atmosphere. Here we exploit the resulting temporal changes in the distribution and isotopic composition of oceanic Pb to investigate interactions between dissolved and particulate Pb in the oceanic water column. Measurements of dissolved Pb isotopic composition on samples collected in 1987 in the upper water column near Bermuda demonstrate that surface water 206Pb/207Pb decreased from ∼1.203 to ∼1.192 since 1983 and that a measurable change propagated to 300–500 m since the 1984 profile of Shen and Boyle (1988). The first accurate measurements of suspended particulate Pb in an open ocean profile show concentrations of 1–3 pmol/L, equal to 2–4% of total Pb. Vertical profiles of (1) the stable lead isotopic composition and (2) the ratio of total Pb to 210Pb in suspended particles closely track contemporaneous depth variations in these ratios for dissolved Pb throughout the upper 2000 m of the Sargasso Sea near Bermuda. Thus suspended particles reach isotopic equilibrium with ambient sea water Pb on a time scale which is shorter than their residence time with respect to vertical removal, in agreement with equilibrium scavenging hypotheses based on interpretations of Th isotope distributions. A simple flux model suggests that the effect of deep ocean scavenging processes on the flux and isotopic composition of Pb sinking on large particles was minor throughout the preanthropogenic and most of the anthropogenic era but has become important as surface inputs decrease to pre‐leaded gasoline levels and may exceed the contribution of surface‐derived Pb flux in the next decade.
Sea surface mean square slope from Ku‐band backscatter data
Authors: Jackson F. C. , Walton W. T. , Hines D. E. , Walter B. A. , Peng C. Y.
Published: February 2008

Near‐nadir, quasi‐specular backscatter data obtained with a 14‐GHz airborne radar altimeter are analyzed in terms of the surface mean square slope (mss) parameter. The raw mss data, derived from a least squares fitting of a ray optical scattering model to the return waveform, show an approximately linear wind speed dependence over the wind speed range of 7–15 m s−1, with a slope of about one half that of the optically determined mss. Further analysis based on a simple two‐scale scattering model indicates that, at the higher wind speeds, ∼20% of this apparent slope signal can be attributed to diffraction from waves shorter than the estimated diffraction limit of ∼0.10 m. The present slope data, as well as slope and other data from a variety of sources, are used to draw inferences on the structure of the high wavenumber portion of the wave spectrum. The data support a directionally integrated model height spectrum consisting of wind speed dependent k−5/2 and classical Phillips' k−3 power law subranges in the range of gravity waves, with a transition between the two subranges occurring around 10 times the peak wavenumber, and a Durden and Vesecky wind speed dependent spectrum in the gravity‐capillary wave range. With a nominal value of the spectral constant Au = 0.002 in the first k−5/2 subrange, this equilibrium spectrum model predicts a mss wind speed dependence that accords with much of the available data at both microwave and optical frequencies.
Seasonal fluctuations in sea level on the South Carolina shelf and their relationship to the Gulf Stream
Authors: Noble Marlene A. , Gelfenbaum Guy R.
Published: February 2008

Near‐bottom pressure measurements obtained over a 9‐month period in 1978 on the outer continental shelf off South Carolina document that sea level rose 35 to 50 cm between July and October. Records of coastal sea level showed a similar rise. When the pressure and coastal sea level records were corrected for the effects of wind stress and temperature, the largest portion of the rise in the two variables remained. The amplitude of the change in residual sea level was largest at the shelf break, 28 cm, and decayed to 22 cm at the coast. The relatively small magnitude of the decay suggests that alongshelf pressure gradients imposed at the shelf break have minimum alongshelf wavelengths of 2000 km. The changes in residual sea level were seasonal and associated with seasonal changes in the transport of the Gulf Stream. The 3 years of residual sea level records examined here indicate that the amplitude of a typical decrease in transport of the Gulf Stream off South Carolina between July and October has a strong interannual variability. The change in transport can be 50% lower to over 100% higher than the historical average. The residual sea level records also suggest that a change in transport of the Gulf Stream is not strongly related to the onshore/offshore position of the Gulf Stream.
World ocean circulation diagnostically derived from hydrographic and wind stress fields: 1. The velocity field
Authors: Fujio Shinzou , Kadowaki Toshimitsu , Imasato Norihisa
Published: February 2008

Using a robust diagnostic model, the steady circulation in the world ocean is determined from climatological annual mean fields of hydrographic data and wind stress. The upper circulation reflects the surface geopotential anomaly of the input hydrographic data. In addition, the “warm water route” is prominent: the water passing through the Indonesian seas flows westward across the Indian Ocean to return to the North Atlantic. In the deep Atlantic Ocean, the North Atlantic Deep Water is transported southward by the dominant deep western boundary current in the depth range from about 1500 m to the bottom, whereas the Weddell Sea Deep Water does not extend northward from the Argentine Basin. An important result is a westward current appearing near 25°N in the middepth Atlantic Ocean. Such a current has been inferred from tracer distributions but hardly detected with objective methods. The velocity maximum at middepth suggests that the current is not a lower part of a wind gyre but is a baroclinic flow caused by a thermohaline effect. In the Indian Ocean a swift middepth current appears along the western boundary, but it may be questionable because the resulting heat transport does not agree with earlier estimates. The deep water flowing into the southern basins from the Antarctic Circumpolar Current does not flow further northward even in the Madagascar Basin. The cause of these unsuccessful results is probably that the present model does not take into account large seasonal variations in the Indian Ocean.
Baroclinic transport of the Kuroshio Extension between 141°E and 165°E
Authors: Clifford Melody A. , Horton Charles W.
Published: February 2008

Using a water‐mass‐based climatology of temperature and salinity, geostrophic transports derived from dynamic height differences across the width of the Kuroshio Extension are presented. The dynamic height differences are obtained by calculating dynamic heights in the warmer water mass south of the Kuroshio and the cooler water mass north of the Kuroshio. The mean baroclinic transport is examined between 141°E and 165°E relative to several pressure levels. The baroclinic transports are sensitive to the selection of the northern and southern boundaries. The baroclinic transports increase from 141°E to about 145°E and decline east of 145°E. The computed seasonal variation of the longitudinally averaged baroclinic transport shows a minimum during spring and a broad maximum from July to November.
An improved special sensor microwave imager water vapor correction for Geosat altimeter data
Authors: Minster J.‐F. , Jourdan D. , Normant E. , Brossier C. , Gennero M.‐C.
Published: February 2008

A wet tropospheric correction for the Geosat altimeter data is derived from the special sensor microwave imager (SSM/I) water vapor products by sub‐objective mapping. The correction presents wavenumber spectral characteristics close to those of the original SSM/I data in the 200‐ to 2000‐km wave band, and is filtered in the short wavelength domain. In both respects, it behaves significantly better than corrections derived from 4‐day averages of the SSM/I data interpolated quadratically under the Geosat track. As an example, the along‐track rms mesoscale variability is lowered by 3 cm for some tracks in the northeast Atlantic Ocean when this correction is applied rather than the correction of Wentz [1990]. This suggests that it indeed improves the altimeter data.
Edge waves on nonplanar bathymetry and alongshore currents: A model and data comparison
Authors: Oltman‐Shay Joan , Howd Peter A.
Published: February 2008

The numerical solutions for the dispersion of linear, shallow‐water edge waves are compared with observed alongshore wavenumber‐frequency spectra of edge wave variance to examine their prediction of the effects of nonplanar bathymetry and moderate mean alongshore currents. Field observations were made at the two Southern California Nearshore Sediment Transport Study field sites, Torrey Pines (1978) and Leadbetter (1980) beaches. On Torrey Pines beach, where the bathymetry is very planar except for a concave (steepening landward) beach face, observed departures from the analytical plane beach dispersion solution, attributed to the concave face, are predicted by numerical dispersion solutions using the measured cross‐shore depth profile. On both beaches, asymmetry in observed dispersion curves of upstream and downstream progressing edge waves are well predicted by numerical dispersion solutions using the cross‐shore profile of measured mean alongshore current. Spectra of the alongshore velocity component from both beaches show changes in edge wave mode dominance with increased frequency. Oltman‐Shay and Guza (1987) demonstrated that mode dominance transition with frequency, as observed from a fixed offshore location, is associated with the seaward decay of edge waves and that the approximate frequency of mode transition is predicted by the profile solutions for edge wave variance and the assumption of equal shoreline elevation variance amongst modes. A submerged concave beach face at Torrey Pines beach is shown to alter the predicted cross‐shore scaling of the edge wave profiles from that predicted when mean water is below the concave beach face, and therefore to alter the mode transition frequency predictions. Different mode transition frequencies are observed in spectra from data runs with mean water levels both above and below the concave beach face, and the differences are shown to be reasonably predicted using edge wave profile solutions for the measured beach profile. For these data, observed and predicted mode transition frequencies did not change appreciably in the presence of mean alongshore current, implying that large changes in cross‐shore scaling of the edge wave profiles did not occur. However, as has been previously shown by Howd et al. (1992), there are also local edge wave profile shape changes because of the cross‐shore shear of the mean alongshore current. Verification of these predicted local shape changes can not be made with these data. Confidence in cross‐shore profile solutions is of practical importance for estimates of shoreline variances from offshore measurements. On these two beaches, estimates using both numerical solutions for the measured depth and current profiles, and analytical solutions for plane beach approximations typically differ by only 10 to 40%.
Formation of Denmark Strait overflow water by mixing in the East Greenland Current
Authors: Strass Volker H. , Fahrbach Eberhard , Schauer Ursula , Sellmann Lutz
Published: February 2008

Hydrographic data and current meter records from the East Greenland Current, collected during the period 1987 to 1989 around the latitude of the central Greenland Sea, reveal the formation of a water mass having the characteristics of Denmark Strait Overflow Water (DSOW). The downstream (southwestward) transport of this newly formed water mass can be substantial, amounting to roughly half of the outflow of DSOW through Denmark Strait as reported in the literature. The formation of DSOW in the East Greenland Current results from isopycnic mixing of recirculated Atlantic Water and Upper Arctic Intermediate Water. Substantial formation occurs when the necessary conditions for baroclinic instability are satisfied and the current variability due to mesoscale eddies is high; this is subject to significant changes from year to year.