bluuebunny/crossref_metadata_2025 huggingface dataset:
The steady state thermal structure of eroding orogenic belts and accretionary prisms
Authors: Royden Leigh H.
Published: February 2008

An analytical expression, derived in this paper, for the steady state thermal structure of thrust belts and subduction systems shows that surface erosion rate, basal accretion rate, and upper plate heat production are extremely important in controlling the temperature and metamorphic history of orogenic belts and accretionary prisms, especially where upper plate thickness (h) is large. For example, even at rapid erosion and accretion rates and with large upper plate heat productions, temperatures near the toe of a thrust wedge (e.g. at h=5 km) are nearly the same as those that would be present in the absence of erosion, accretion and upper plate heat production; in thicker parts of the same wedge (e.g. at h=30 km) the combined effects of erosion, accretion and upper plate heat production can be dramatic. For sufficiently large magnitudes of upper plate heat production or sufficiently rapid erosion rates inverted (negative) geothermal gradients are produced in the lower part of the upper plate and the upper part of the lower plate, although it is difficult to produce inverted geotherms where h≤20 km. The potential importance of erosion and accretion can be illustrated through two examples: (1) beneath the Himalayas, increasing the upper plate radiogenic heat production from 0 to 2.5 mW/m3 and increasing the rate of surface erosion from 0 to 1 mm/yr raises the computed steady state temperatures on the Main Central Thrust, at h= 30 km, from 265°C to 600°C, and produces a local temperature maximum of 635°C located 6 km above the fault; and (2) within the upper plate of the Peru trench, increasing the rate of surface erosion and basal accretion from 0 to 2.4 mm/yr (or increasing the upper plate radiogenic heat production from 0 to 1.3 mW/m3) increases the computed steady state surface heat flow at h=30 km from 10 to 33 mW/m3, but only increases temperatures at the fault surface from 125°C to 170°C. Shear heating along thrust faults or subduction boundaries is not necessary to produce the high metamorphic temperatures or surface heat flows observed in these two examples, and the generation of large amounts of heat by shear stresses on the faults (more than a few tens of bars) appears to be inconsistent with some of the observations made in these areas. This study indicates that many of the major discrepancies between existing thermal models of overthrust belts and observed temperatures, as inferred from field data, can be attributed to neglect of horizontal and vertical advection of heat caused by surface erosion and by accretion of material from lower plate to upper plate across the fault surface.
Dislocation modelling of the 1989 dike intrusion into the Flank of Mount Etna, Sicily
Authors: Bonaccorso A. , Davis P. M.
Published: February 2008

We have applied a dislocation model, consisting of a dipping tabular body with constant dislocation perpendicular to the plane of the body, to surface displacements related to an intrusion in the south flank of Etna volcano, Sicily. Ground deformation was measured by a trilateration array and automatic tilt measurements during the 1989 eruption of the volcano. Significant line length changes and tilt were measured associated with an episodic 0.8 km propagation of the intrusion into the measurement array. We incorporated the orientation and location of measured surface cracking into a Bayesian nonlinear inversion of the data to determine the geometry of the intrusion. We suggest the intrusion caused a 1‐m‐wide opening of a vertical crack that extended from a depth of 1 km to within a few hundred meters of the surface. We relate the distribution of surface cracking and seismicity associated with the fracture to stresses generated in the slip‐weakening zone associated with the crack edge.
Pervasive remagnetization of paleozoic rocks acquired at the time of Mesozoic folding in the South China Block
Authors: Wang Zhongmin , Van Der Voo Rob
Published: February 2008

A total of 152 paleomagnetic samples was collected from 23 sites distributed over six Paleozoic formations, with ages ranging between Middle Ordovician and Early Permian, in the Tangshan type section in the Nanjing area of South China. Stepwise thermal demagnetization has successfully resolved two magnetization components from these samples. Component A with northerly shallow tilt‐corrected directions resides in approximately 95% of the total number of samples analyzed; this component passes fold tests at the 99% significance level in three formations. The overall mean direction of component A is 350/−3 (tilt‐corrected, a95=7.2). Component B, isolated from 19 samples in two formations (Cl and D3), yields negative fold tests at the 95% significance level and is, therefore, younger than the Mesozoic folding. The overall site mean direction of component B is 354/−4 (in situ, a95=8.4). The tilt‐corrected direction of component A clearly resembles the in situ direction of component B, indicating that both components are likely to be of approximately the same age. We infer that they are both remagnetizations, associated with the deformation. A review of the Paleozoic paleomagnetic data reported by other authors from adjacent areas shows that similar remagnetization situations may prevail. It is concluded that the northeast part of south China has experienced pervasive remagnetizations at the time of folding. The direction of this folding‐induced remagnetization yields a new well‐defined paleopole position (56°N, 314°E) for south China. Although this paleopole is significantly different from Mesozoic poles reported to date, we infer that a Triassic to Jurassic age is most likely for this remagnetization.
Numerical simulation of collapsing volcanic columns
Authors: Dobran Flavio , Neri Augusto , Macedonio Giovanni
Published: February 2008

A complex thermo‐fluid dynamic model was employed to model collapsing volcanic columns. The two‐phase flow model accounts for the mechanical and thermal nonequilibrium between the gas and solid particles. The gas phase involves water vapor and air, and the solids phase involves only one particle size class. The particle collisions, which produce particle viscosity and pressure, were modeled by a kinetic theory model in terms of the granular temperature, whereas the gas phase turbulence was modeled by a turbulent subgrid scale model. The partial differential equations of conservation of mass, linear momentum, energy, and granular temperature were numerically solved for an axisymmetric flow configuration with different vent diameters and two‐phase flow conditions. The numerical solutions involved different grid sizes and computational domains in order to assess the adequacy of the model and computational procedure. The results from simulations of collapsing volcanic columns show how after an initial period of fountain building the columns collapse and build radially spreading pyroclastic flows and inward moving column material which is recycled by the columns. For a low‐height collapsing column it was found that the fountain reaches a steady state height, whereas for columns with collapsing heights of several kilometers and fine computational grids the fountain heights vary cyclically with periods which are influenced by the dynamics of material recirculation into the columns. The radially spreading pyroclastic flows of the collapsed columns were found to develop convective instabilities whereby rising clouds of gas and particles are developed on the top of the flows. In very large scale volcanic eruptions the numerical results predicted multiple rising clouds on very thick pyroclastic flows. The results from simulations were shown to be consistent with simple column modeling approaches, laboratory experiments, and field observations.
Remagnetization by basinal fluids: Testing the hypothesis in the Viola limestone, southern Oklahoma
Authors: Elmore R. Douglas , London David , Bagley Don , Fruit David , Gao Guoqiu
Published: February 2008

Migration of orogenic or basinal fluids is a recently invoked mechanism to explain the widespread presence of late Paleozoic secondary magnetizations in the rocks of North America. Paleomagnetic and geochemical results from the Ordovician Viola Limestone in southern Oklahoma are evaluated to assess the role of basinal fluids in leading to secondary magnetizations in the unit. The Viola Limestone contains what we interpret to be a pervasive Pennsylvanian synfolding magnetization residing in magnetite and a localized Permian magnetization which resides in hematite and occurs in alteration zones around mineralized veins. Both secondary magnetizations are interpreted as chemical remanent magnetizations (CRM) based on low burial temperatures and the presence of authigenic magnetic phases. The relative proportion of the Permian CRM in hematite gradually decreases whereas the magnetite CRM increases with distance from the veins. Fluid inclusion and Sr isotope studies indicate that the vein mineralization (calcite with Mississippi‐Valley‐type oxides and sulfides) precipitated from basinal fluids which were warm, saline, and radiogenic. The radiogenic 87Sr/86Sr values of the limestones in the alteration zones, and the fact that there is more significant alteration closer to the veins suggests that the basinal fluids were also responsible for alteration in the limestones. The coincidence of the geochemical and remagnetization trends suggests that the Permian CRM dates the migration of basinal fluids through the veins. Geochemical results from the Viola Limestone containing the pervasive CRM indicate that it is relatively unaltered with no evidence for basinal fluids. The lack of evidence for basinal fluids suggests that other mechanisms for the origin of the pervasive CRM need to be tested. The results of this study indicate that flow of basinal fluids was focused in veins and only locally altered the host limestone.
Paleomagnetic evidence of vertical axis block rotations from the Mesozoic of northern Chile
Authors: Riley Paul D. , Beck Myrl E. , Burmester Russell F. , Mpodozis Constantino , Garcia Alfredo
Published: February 2008

We present paleomagnetic results for three Mesozoic formations from northern Chile: La Ternera Formation (Upper Triassic), Quebrada Monardes Formation (Upper Jurassic); Cerrillos Formation (Upper Cretaceous). Results from the Cerrillos are divided into eastern (Cuesta El Gao (CEG)) and western (Elisa De Bordo (EBD)) localities. Most specimens from La Ternera volcanic and sedimentary rocks are magnetically stable, as shown by alternating field and thermal demagnetization. More complicated but still reliable results were obtained from Quebrada Monardes red beds. Normal and reverse polarities are present in both units; means of both populations are antiparallel at 95% confidence. The Quebrada Monardes Formation also yields positive conglomerate and fold tests. Paleomagnetic poles for La Ternera and Quebrada Monardes are 60.9S, 218.3E (A95, 7.8°), and 66.9S, 191.6E (A95, 12.7°), respectively. Comparison with appropriate reference poles shows that this region of Chile has undergone about 25° of clockwise rotation, with negligible latitudinal transport. Cerrillos CEG results are less reliable and possibly complicated by remagnetization during emplacement of early Tertiary intrusives. Most Cerrillos EDB specimens are stable, but marked increase in scatter upon unfolding suggests remagnetization. Results for the Cerrillos CEG locality, which is contiguous to the sampling area of the La Ternera and Quebrada Monardes formations, show only about half the rotation of those two units, suggesting that rotation commenced after deposition of the Quebrada Monardes rocks in the Late Jurassic and was approximately half complete by the time Cerrillos CEG rocks acquired their magnetization. Cerrillos EDB results come from an area approximately 40 km to the west; these show roughly 45° of clockwise rotation. Dispersion is very low between EDB sites, suggesting that secular variation may not be completely averaged. Nevertheless, the great difference in direction between the two Cerrillos localities suggests that they lie in different structural blocks.
Edge‐driven microplate kinematics
Authors: Schouten Hans , Klitgord Kim D. , Gallo David G.
Published: February 2008

It is known from plate tectonic reconstructions that oceanic microplates undergo rapid rotation about a vertical axis and that the instantaneous rotation axes describing the microplate's motion relative to the bounding major plates are frequently located close to its margins with those plates, close to the tips of propagating rifts. We propose a class of edge‐driven block models to illustrate how slip across the microplate margins, block rotation, and propagation of rifting may be related to the relative motion of the plates on either side. An important feature of these edge‐driven models is that the instantaneous rotation axes are always located on the margins between block and two bounding plates. According to those models the pseudofaults or traces of disrupted seafloor resulting from the propagation of rifting between microplate and major plates may be used independently to approximately trace the continuous kinematic evolution of the microplate back in time. Pseudofault geometries and matching rotations of the Easter microplate show that for most of its 5 m.y. history, block rotation could be driven by the drag of the Nazca and Pacific plates on the microplate's edges rather than by a shear flow of mantle underneath.
Effect of anisotropy on the paleomagnetic contact test for a Grenville Dike
Authors: Hyodo Hironobu , Dunlop David J.
Published: February 2008

We report a detailed paleomagnetic contact test for a mafic dike and its tonalitic gneiss host rock in the Precambrian Grenville Province of Canada. Directions of natural remanent magnetization (NRM) in the country rock were corrected for assumed deflection by the strong magnetic anisotropy due to the rock fabric, using the measured anisotropy of laboratory‐induced thermoremanent magnetization (TRM). The TRM ellipsoids have the same principal axes as the anisotropy of magnetic susceptibility (AMS) ellipsoids, but the anisotropy of remanence is more pronounced than the AMS. Anisotropy correction moves NRM directions in the unbaked zone almost 20°, bringing them close to typical Grenvillian paleofield directions. The anisotropy seems to be controlled by multidomain rather than single‐domain magnetic minerals. NRM directions from the baked gneiss near the dike contact after anisotropy correction agree well with the NRM directions from the chilled margin of the early Cambrian (570 ± 3 Ma) dike. This positive baked contact test is augmented by the presence of a “hybrid zone”, in which country rock NRMs are increasingly overprinted by the dike NRM as one moves from the unbaked toward the baked zone. The dike NRM is therefore primary. However, the virtual geomagnetic pole (VGP) for the dike NRM: 51°N, 131°W (dp=10°, dm=12°, N=24 samples) or 41°N, 153°W (dp=13°, dm=18°, N=4 chilled margin samples) does not fall near most established Cambrian paleopoles. Post‐emplacement tectonic movement of the dike is unlikely; it may be that the dike cooled and acquired TRM during a geomagnetic field excursion because of its geologically short cooling (blocking) period of a few tens of years.
Semiquantitative modeling of strain and kinematics along the Caribbean/North America strike‐slip plate boundary zone
Authors: Calais Eric , De Lépinay Bernard Mercier
Published: February 2008

Recent structural and geophysical studies conducted along the northern Caribbean plate boundary, at sea and on land, have led to a precise description of the geometry and the tectonic regimes along this major transcurrent zone which separates the Caribbean and North American plates. In order to interpret its tectonic features in terms of plate motion, we use a simple numerical model of strike‐slip faulting to test previously proposed kinematic models and to compute new motion parameters. We show that none of the previously proposed models correctly accounts for the observed deformation along the whole plate boundary. On the basis of the deformation pattern obtained from geological data we compute a motion parameters set that integrate rigid plate rotation and “a plate boundary zone deformation component.” Our results show that the deformation along the northern Caribbean plate boundary zone is controled by regional kinematics (i.e., the Caribbean/North America relative motion) rather than by local effects (e.g., small block rotation, intraplate deformation).
Nutrient provinces in the sea: Concentration ratios, reaction rate ratios, and ideal covariation
Authors: Fanning Kent A.
Published: February 2008

Global distributions of the ratios of the concentrations of nitrate + nitrite (= [N]) and phosphate (= [P]) are evaluated from Geochemical Ocean Sections Study (GEOSECS) and Transient Tracers in the Ocean (TTO) data sets. If large oceanic regions (or provinces) can be identified on the basis of constant [N]: [P] ratios, then the distribution equation for a reactive variable shows that the ratio of the net reaction rates involving N and P in each one is equal to its concentration ratio. Organisms within the interiors of the provinces would then be in balance with the ratios in which the nutrients are present, producing a non‐fractionated or “ideal” nutrient covariation. Such provinces can be observed throughout the ocean. Notable features are as follows: (1) Between the euphotic zone and 500 m in the west central North Atlantic is a large region in which N‐P regeneration produces very high [N]:[P] ratios: ∼50 mol mol−1. Potential causes are 18° Water formation, coccolithophorid growth, nitrogen fixation, or atmospheric fixed‐nitrogen deposition. (2) Most oligotrophic surface waters seem to have [N]:[P] between 0 and 3 mol mol−1, implying that the net removal ratio of N and P in those waters is 0–3 mol mol−1. (3) Below 600 m, the ocean contains large provinces with N‐P regeneration ratios of 12–18 mol mol−1. The dominant ratio is slightly sub‐Redfield at 14.5–15 mol mol−1, with the entire Indian Ocean below 3000 m being ideally covariant at 14.7 mol mol−1. The northeastern Pacific has provinces with very low regeneration ratios (<14 mol mol−1). Vertical boundaries between deep provinces in the western Pacific and eastern Atlantic suggest that particles from immediately above control regeneration ratios, whereas the more horizontal boundaries between western Atlantic provinces appear to reflect a greater importance of horizontally transported particles in water masses like Antarctic Intermediate Water. N‐P reaction rate ratios along deep isopycnal surfaces are quite variable.
Ozone depletion in Antarctica: Modeling its effect on solar UV irradiance under clear‐sky conditions
Authors: Smith Raymond C. , Wan Zhengming , Baker Karen S.
Published: February 2008

Radiative transfer numerical simulations have been made for quantitatively analyzing the effect of ozone depletion on solar irradiance at the Earth's surface in the UV and visible ranges. Simulations show that ozone's temperature‐dependent absorption and atmospheric multiple scattering play important roles in the UV region. By means of sensitivity analyses, the level of surface UV‐B irradiance under clear‐sky conditions is shown to depend, with a decreasing significance, on solar zenith angle, total column ozone, surface spectral reflectivity, stratospheric temperature, and aerosol variations. The simulation results are compared with the National Science Foundation (NSF)/UV spectroradiometer data obtained at Palmer Station on the Antarctic Peninsula and the Total Ozone Mapping Spectrometer (TOMS) satellite data. In general, the model and data show good agreement, especially in the UV region. Based on simulation results, we develop a UV irradiance ratio method to estimate total column ozone from the NSF/UV spectroradiometer using clear‐sky data obtained during the austral spring of 1988 and 1990. A method for estimating surface UV‐B from TOMS satellite data is also developed. The ATRAD model provides a basis for the accurate estimation of the ratios of UV‐B (280–320 nm) to UV‐A (320–400 nm) to photosynthetic available radiation (PAR, 400–750 nm) for ecological studies concerned with the biological effects of ozone depletion over the Antarctic.
Sediment transport in oscillatory sheet flow
Authors: Dick J. E. , Sleath J. F. A.
Published: February 2008

Motivated by an interest in high rates of sediment transport, an experimental investigation was conducted to examine sediment motion in conditions of intense sediment transport. Fluid velocity and sediment concentration were measured simultaneously in a sheet flow regime generated in a u‐shaped oscillatory water tunnel for two types of noncohesive sediments. A probe was developed to measure the instantaneous variations in sediment concentration based on the electrical conductivity of the fluid‐grain mixture. Unlike previous concentration devices, this probe is nonintrusive and is capable of measuring a wide range of sediment concentrations, from extremely dense concentrations within the bed to low‐concentration suspended load well above the moving layer of sediment. Horizontal fluid velocities were measured simultaneously using laser Doppler anemometry in backscatter mode. The sediment concentration measurements provide a comprehensive data set for sediment transport in sheet flow and near sheet flow regimes. The temporal variation of sediment transport over the half cycle of oscillation was examined. Most of the sediment was transported within the moving bed layer, with the total transport occurring between the stationary bed height and a distance about twice the thickness of the moving bed layer. The temporal variation in sediment transport was found to be proportional to cos(σt). The magnitude of the sediment transport increased with increasing bottom shear stress. Fluid velocity and sediment concentration measurements were used to calculate instantaneous and mean rates of sediment transport, which were compared with predictions from existing models.
Calibrating altimetry to geopotential anomaly and isotherm depths in the western North Atlantic
Authors: Blaha John , Lunde Bruce
Published: February 2008

Sixty‐two temperature sections were taken to depths of 800 m by aircraft along Geosat ground tracks within 1–2 days of the satellite orbit. The sections are 500–700 km long and cross the Gulf Stream and its rings. From pairs of sections that repeat on the same ground track, collinear differences of geopotential anomaly (0/25 MPa) and the depth of the 16°C isotherm were collected as along‐track signals of the change of baroclinic conditions. The corresponding collinear differences of sea elevation were collected from the Geosat orbits. These difference signals, from aircraft and altimeter, were fit to one another along common ground track segments by removing residual orbit error as a tilt and bias and proportionally scaling the magnitudes of the signals. The ratio of geopotential anomaly to sea elevation by this method is 0.84 ± 0.10 dyn m m−1. That is, fluctuations of geopotential anomaly which correlate to sea elevation across the Gulf Stream are smaller by this proportional value. The difference of this ratio from unity is consistent with a barotropic signal of about 0.10 m, as estimated from reported measurements of abyssal velocity under the stream. Because the uncertainties in the aircraft data are of the same order as the scatter in the calibration, it is unclear whether greater precision is impeded primarily by these uncertainties or by random barotropic signals. The ratio of the displacement of 16°C to sea elevation is 480 ± 75.
Airborne expendable bathythermograph survey of the Kuroshio Extension and comparison with simultaneous altimeter measurements during the Geosat Exact Repeat Mission
Authors: Horton C. W. , Porter David L. , deWitt P. W. , Rankin W. E.
Published: February 2008

Extensive airborne expendable bathythermograph (AXBT) sections across the Kuroshio Extension during the Geosat Exact Repeat Mission (ERM) were used to assess the ability of the Geosat altimeter to monitor the locations of the Kuroshio and its associated cold‐ and warm‐core eddies. The AXBT sections in themselves provide detailed descriptions of the Kuroshio Extension to about 160°E. These observations demonstrated that surface or near‐surface temperatures in this area may lead to incomplete and deceptive descriptions of the deeper thermal structure. Kuroshio‐eddy interactions in particular were not accurately portrayed by the near‐surface observations. For this reason, accurate mapping of the location of the Kuroshio Extension and of nearby eddies may not be practical from infrared (IR) imagery alone but requires augmentation from other measurement systems such as altimetry. In the present analysis, an estimate of the sea surface height was obtained from the altimeter ranges by time averaging sea surface heights in lieu of a geoid estimate. Sea surface height anomalies computed using the mean altimetric sea surface (i.e., the geoid plus the temporal mean oceanographic signal) can be significantly different from the actual oceanographic topography induced by mesoscale oceanographic features. Nevertheless, it is shown that the positions of the Kuroshio Extension and of the stronger cold‐ and warm‐core eddies associated with the Kuroshio Extension are easily located from these altimetric sea surface anomalies.
The upper layer circulation of the Black Sea: Its variability as inferred from hydrographic and satellite observations
Authors: Oguz Temel , La Violette Paul E. , Unluata Umit
Published: February 2008

Quasi‐synoptic hydrographic data and satellite imagery are used to describe the circulation and the structural variability of the Black Sea with particular emphasis on the Turkish coast. The circulation is indicated to involve a variable cyclonic circulation with no apparent central locus and a well‐defined cyclonic “Rim Current” containing meanders and interacting eddy fields confined to the shelf slope. Interspersed between the coastal eddies are filaments and intense jets, often with dipole eddies at their termina. The extension of these features across the shelf‐slope into the central basin offshore waters implies important dynamical processes related to the shelf‐deep basin exchanges. These features are often steered by the topography and evolve continuously through the mixed baroclinic‐barotropic instability of the Rim Current.
Rings in numerical models of ocean general circulation: A statistical study
Authors: Chassignet Eric P.
Published: February 2008

Because of their importance in the ocean energetics and general circulation, a proper representation of ring generation mechanisms and evolution in numerical models is crucial for an accurate picture of the heat, salt, and energy budgets. Ring locus, lifetime, propagation speed, radius, and interface displacement statistics are derived for four widely used eddy‐resolving numerical models and compared to the 10‐year statistical analysis of Gulf Stream rings based on time series of satellite infrared determinations performed by Brown et al. (1986). The ring formation process and behavior in ocean numerical models depends upon the governing equations, the vertical coordinate, and the boundary conditions used. It is shown that as more terms are retained in the model equations, the mid‐latitude jet becomes more unstable, and its interior penetration as well as the associated number of rings formed are reduced. Rings in the layer model have slower propagation speeds and longer lifetimes than their level counterparts. Such results illustrate the sensitivity of numerical ocean circulation models to the physical and numerical assumptions made.
A chemical indicator of trade wind reversal in corals from the western tropical Pacific
Authors: Shen Glen T. , Linn Laura J. , Campbell Todd M. , Cole Julia E. , Fairbanks Richard G.
Published: February 2008

Historical levels of manganese have been measured in the annual rings of a reef coral from Tarawa atoll (1°30′N, 173°E) in order to assess the temporal variability of this trace element in surface waters of the western tropical Pacific. Seasonal variability associated with vertical mixing is not apparent as is the case in the eastern Pacific; however, Mn/Ca ratios are highly perturbed during three El Niño‐Southern Oscillation (ENSO) events (1965, 1972, 1976) that occurred within the growth period 1960–1977. For approximately 6 months during each of these events, Mn/Ca ratios increase by 50–80% from a background of 27 nmol Mn/mol Ca. These features co‐occur with negative δ18O pulses recorded in the same coral which result from anomalously abundant rainfall. Wet deposition of Mn, however, does not appear to be an adequate source to account for the inferred surface ocean enrichments. The most plausible explanation has to do with the transient appearance of westerly winds along the equator during ENSO periods. Particularly vigorous westerly wind “bursts” (sometimes averaging 10 m/sec or more over 16 hours) during the aforementioned years may have generated wind waves of sufficient strength to create an aura of particulate and diagenetically remobilized Mn from within the shallow westward facing lagoon of Tarawa. This interpretation is supported by relatively high Mn concentrations measured in pore waters of Tarawa lagoon sediments. The results suggest a useful new chemical tracer of historical ENSO conditions in the western Pacific basin.
Upwelling and internal tides over the inner New South Wales continental shelf
Authors: Griffin David A. , Middleton Jason H.
Published: February 2008

Nearshore variability of the thermocline depth and associated baroclinic currents at Sydney, New South Wales are analyzed for the locally and remotely forced components. Summer and winter data acquired in 65 m of water 3.5 km from the coast include thermistor string and current meter measurements. Cross‐spectral analyses of the data are interpreted with reference to the free and forced modes of a simple two‐layer frictional shelf model. Subinertial frequency (∼ 40 day‐21 hour period) baroclinic variability appears to be locally forced, in contrast with the barotropic variability which is principally remotely forced and acts as one of the local forcing agents of the baroclinic variability through bottom Ekman layer dynamics. The local wind stress appears to be the more important forcing term, despite having less weather band variance than the bottom stress. Strong winds at Sydney are usually from the south and hence lead to downwelling, while bottom stress is responsible for the rarer upwelling events. Near‐inertial variability is substantially inhibited at the study site except during winter when the internal Rossby radius becomes comparable to the distance from the coast. Superinertial frequency (to 6‐hour period) baroclinic variability is dominated by standing internal Poincaré waves, the principal energy source of which is presumed to be tidal currents over the outer shelf. The existence of a standing wave pattern implies that little dissipation occurs upon reflection at the coast.
Air‐sea boundary layer dynamics in the presence of mesoscale surface currents
Authors: Rooth Claes , Xie Lian
Published: February 2008

In the presence of surface currents, a shear stress at the air‐sea interface is induced by the surface currents. In the case of a unidirectional current, a quadratic stress law leads to a stress curl proportional to and opposing the surface current vorticity even with a uniform wind. This causes a spindown effect on the surface vorticity field at a rate proportional to the wind speed. In the steady state, or in slowly varying processes which can be treated as parametrically developing quasi‐steady states, the surface‐layer potential vorticity modulation causes upwelling and downwelling patterns associated with the surface‐current vorticity. These effects are analyzed for an idealized jet current, and for a physical situation characteristic of a Gulf Stream boundary ring along the Florida Keys, where the induced transport patterns may be important for onshore transport of fish and spiny lobster larvae, as well as for onshore transport to the Florida Keys of general flotsam transported past them by the Gulf Stream. The spindown time scale (t*) for a 1.5‐layer system is H/( ρ′cdVa) for a surface jet on the deformation radius scale (where H is the thickness of the surface layer, Va the surface wind speed, ρ′ the air to water density ratio and cd the surface drag coefficient) and increases for large horizontal scales in proportion to the current width squared. For a typical wind speed of 5 m/s and a density normalized drag coefficient ρ′cd= 2 × 10−6, t* is on the order of 1 month for a 30‐m surface layer. In the more general case of a stratified interior water column, the vorticity spindown directly affects only the potential vorticity of the surface layer and generally leads to subsurface velocity and vorticity maxima for mesoscale eddies and jets.
Observations of infragravity waves
Authors: Elgar Steve , Herbers T. H. C. , Okihiro Michele , Oltman‐Shay Joan , Guza R. T.
Published: February 2008

Infragravity‐wave (periods of one‐half to a few minutes) energy levels observed for about 1 year in 8‐m water depth in the Pacific and in 8‐ and 13‐m depths in the Atlantic are highly correlated with energy in the swell‐frequency band (7‐ to 20‐s periods), suggesting the infragravity waves were generated locally by the swell. The amplification of infragravity‐wave energy between 13‐ and 8‐m depth (separated by 1 km in the cross shore) is about 2, indicating that the observed infragravity motions are dominated by free waves, not by group‐forced bound waves, which in theory are amplified by an order of magnitude in energy between the two locations. However, bound waves are more important for the relatively few cases with very energetic swell, when the observed amplification between 13‐ and 8‐m depth of infragravity‐wave energy was sometimes 3 times greater than expected for free waves. Bispectra are consistent with increased coupling between infragravity waves and groups of swell and sea for high‐energy incident waves.
The structure of the southwest monsoon winds over the Arabian Sea during the late Quaternary: Observations, simulations, and marine geologic evidence
Authors: Anderson David M. , Prell Warren L.
Published: February 2008

Each summer strong southwest winds produced by the Asian monsoon blow across the NW Arabian Sea, creating a broad region of upwelling characterized by cool sea surface temperatures and enriched phytoplankton biomass. The integrated history of the summer plankton blooms is preserved in the form of distinct plankton fossil assemblages and sediment facies, forming a continuous geologic record of wind‐driven upwelling and thus changes in the SW monsoon winds. Sediments have been recovered from two regions in the NW Arabian Sea, one located along the Oman continental margin, and another located approximately 400 km offshore. Over the late Quaternary, these two sites record time series of upwelling that are substantially different. To understand the source of the differences, we examined the relationship between wind stress and upwelling at these sites using historical observations and model‐simulated wind fields. In the coastal region, upwelling is directly related to the Ekman transport offshore, and is thus linearly proportional to the alongshore wind stress. Farther offshore, open‐ocean upwelling is driven by the positive curl in the wind stress. In historical records, we found little correlation between stronger winds and increased open‐ocean upwelling. In the region offshore where the sediment cores were recovered, the year with the strongest winds was accompanied by weaker wind stress curl and diminished upwelling (indicated by warmer sea surface temperatures and decreased phytoplankton pigment content) because the region of maximum wind stress curl moved NW that year. In atmospheric general circulation model experiments using a range of summer insolation forcing, we found large changes in the wind stress curl occur over some areas of the NW Arabian Sea; however the offshore core locations lies outside of the region where changes in the SW monsoon drive large changes in open‐ocean upwelling. In contrast to the coastal sites where upwelling is directly related to the wind stress, the offshore sites appear to be located in a region where the wind stress curl does not change much over a large range of summer insolation. There is some evidence from both the interannual and model fields that this location is affected more by movements in the position of the low‐level jet than by changes in the over‐all strength of the SW monsoon. Although more data are required to define these patterns, these results indicate that the time series of open‐ocean upwelling observed in the geologic record from offshore sites may differ substantially from the coastal time series.
A simple description of the deepening and structure of a stably stratified flow driven by a surface stress
Authors: Trowbridge J. H.
Published: February 2008

The deepening and structure of a stably stratified flow driven by a surface stress are examined by means of a simple conceptual model, in which local mixing does not occur if the gradient Richardson number Ri is greater than the critical value Ric ≃ ¼ indicating onset of shear instability, and local mixing occurs instantaneously, through a simple exchange of fluid volumes, if Ri is less than Ric. The model leads to a mathematical formulation in which turbulent mixing is a gradient transport process just strong enough to maintain Ri at the critical value Ric throughout the boundary layer. Although unrealistic just beneath the stressed surface, self‐similar solutions based on this formulation plausibly describe the outer part of the boundary layer and the rate at which the layer deepens, provided that appropriately defined bulk Richardson and Reynolds numbers are sufficiently large. A comparison of model computations with existing laboratory measurements and turbulence closure simulations supports the conceptual model as an approximate representation of the processes controlling the deepening of the boundary layer and the qualitative structure in the outer part of the flow. Estimates are obtained of the region within the boundary layer and in parameter space where the formulation is realistic.
Data assimilation with a coupled wind‐wave model
Authors: de las Heras Miriam M. , Janssen Peter A. E. M.
Published: February 2008

A first attempt to assimilate observations into a coupled wind‐wave model system is presented. A special feature of the wind model is that the drag coefficient depends on the wave age, reflecting the dependence of the air flow on the wave state. Usually, both wind and wave data are assimilated. The assimilation is carried out by means of the adjoint technique. This coupled wind‐wave data assimilation not only corrects successfully both model fields, but their dynamical connection is retained. Furthermore, the assimilation of only wave data results in enough information on the atmospheric state at the sea surface that it can correct both wave and wind fields as well. Moreover, there is still some impact of the assimilation during the forecast period. This persistence is found to be dependent on the typical relaxation time scale of the wind model, and also to be related to the swell decay time scale due to wave breaking. The computations reveal the importance of both the coupling and the coupled wind‐wave data assimilation scheme that has been developed.
A model simulation of circulation in the northeast Atlantic shelves and seas
Authors: Oey Lie‐Yauw , Chen Ping
Published: February 2008

A three‐dimensional, primitive‐equation simulation of the circulation in the northeast Atlantic shelves and seas, defined by 51°–76°N latitudes and 20°W–22°E longitudes, has been conducted for the period February‐March 1988. The simulation was initialized from a 585‐day quasi‐equilibrium calculation and included realistic meteorological forcing, inflows/outflows across the open boundaries (inflow of the North Atlantic warm water in particular), tides, coastal and Baltic discharges, and relaxation to wintertime climatology for model depths > 500 m. The calculation is the first part of an overall effort to nest a high‐resolution region for simulation of eddies and fronts in the Norwegian Coastal Current (NCC). This paper presents detailed simulation strategies and discusses results from the coarse‐grid region to study the larger‐scale model response induced by atmospheric forcing, so that its effects on flow dynamics in the nested grid can be better understood. The mean and variability of the simulated flow field are compared, whenever possible, with published observations. In particular, we examine in detail the simulated wind‐induced response in the Skagerrak transport, which produces blocking and outbreak of the Skagerrak and North Sea waters. These transport variabilities can be expected to be important in the development of the NCC meanders and eddies further north.
Three‐dimensional tidal flow around headlands
Authors: Geyer W. Rockwell
Published: February 2008

Field measurements of tidal flow around a headland indicate secondary circulation induced by flow curvature. The secondary flow, defined to be the flow in the plane normal to the direction of the vertically averaged current, is directed toward the headland near the bottom and seaward near the surface, consistent with theoretical predictions. The strength of the secondary flow varies from 5 to 15% of the streamwise flow. It is strongest when the water column is stratified because of both the enhanced shear of the streamwise flow and the reduced frictional damping of the secondary flow. The transverse exchange accomplished by the secondary flow significantly influences the structure of the streamwise flow, causing a broadening of the transverse shear and changing the horizontal and vertical distributions of momentum. It may also have a significant influence on horizontal dispersion in the vicinity of headlands.
Manganese dynamics in surface waters of the eastern Caribbean
Authors: Waite T. David , Szymczak Ron
Published: February 2008

The rate of photoreductive dissolution of manganese oxide in eastern Caribbean waters appears to be relatively rapid with an approximate linear dependency of dissolution rate on organic content of these waters. From the magnitude of derived absolute dissolution rates it would appear that reductive rather than oxidative processes should dominate transformations of manganese in surface waters though further insight into the reactivity of “naturally occurring” oxides is needed. Oxidation of Mn(II) added to surface water samples is observed and occurs at rates between those observed for coastal and open ocean surface waters by other investigators. Obvious correlations between oxidation rates and water column characteristics such as total bacterial numbers, dissolved (or particulate) manganese concentrations or organic content of surface waters are not evident. Transformations of manganese in the surface waters of the eastern Caribbean appear to be masked by the supply of fresh particulate material presumably, in part at least, from the Orinoco River.
The monsoonal heat budget of the hydrosphere‐atmosphere system in the Indian Ocean sector
Authors: Hastenrath Stefan , Greischar Lawrence
Published: February 2008

The heat budget of the combined hydrosphere‐atmosphere system in the Indian Ocean sector (30°N to 30°S, 30°–120°E) is studied on the basis of surface ship observations and subsurface temperature casts to 400 dbar, analyses of the atmospheric heat and moisture transport in the layer surface to 100 mbar, published estimates of precipitation, and satellite measurements of net all‐wave radiation at the top of the atmosphere. During boreal winter (November‐April) the hydrosphere of the southern tropical Indian Ocean stores (+2 × 1014 W) and exports (7 × 1014 W) heat, of which about half (3 × 1014 W) is carried northward across the equator; of similar magnitude (9 × 1014 W) is the heat export in the atmosphere accomplished primarily in the upper tropospheric westerlies. In the northern portion of the basin the hydrosphere stores heat (+8 × 1014 W), in part supplied by the hydrospheric heat import from the southern hemisphere; the atmospheric column exports heat (4 × 1014 W), primarily owing to a southward moisture transport. In boreal summer (May‐October) the water body of the northern Indian Ocean cools drastically (−8 × 1014 W), with the net heat gain through the ocean surface being overcompensated by the larger (12 × 1014 W) southward heat transport across the equator, which in turn results in a large heat import (8 × 1014 W) to the hydrosphere of the southern Indian Ocean. This and the depletion of the oceanic heat content (−2 × 1014 W) supply about a third of the energy required for the vigorous evaporation from the southern tropical Indian Ocean (34 × 1014 W), which is further favored by the strong southeast trade winds peaking at this season of the year. The evaporation sustains the large latent heat export from the overlying atmospheric column (+14 × 1014 W). The latent heat surplus from the southern portion of the basin is primarily carried northward across the equator by the lower atmospheric boreal summer monsoon airflow, and in the June‐August core of the season this latent heat flux becomes as large as the southward cross‐equatorial heat transport in the ocean (13 × 1014 W). Associated with the summer monsoon airflow, latent heat import takes place in the northern portion of the basin (−2 × 1014 W) and southern Asia (−6 × 1014 W). In context, hydrosphere and atmosphere cooperate in the energetics of the Indian summer monsoon. The water body to the north of the equator cools (despite the summer season) and exports heat to the southern Indian Ocean, where this energy helps sustain the large evaporation associated with the strong winds, which further carry the moisture northward to supply the abundant monsoon rainfall and latent heat release that fuels the atmospheric portion of the monsoon machine. It is during boreal winter that the northern Indian Ocean water body replenishes its heat content, in preparation for the next summer monsoon.