bluuebunny/crossref_metadata_2025 huggingface dataset:
Displacement history of a limestone normal fault scarp, northern Israel, from cosmogenic36Cl
Authors: Mitchell Sara Gran , Matmon Ari , Bierman Paul R. , Enzel Yehouda , Caffee Marc , Rizzo Donna
Published: February 2004

The abundance of cosmogenic36Cl, measured in 41 limestone samples from a 9 m high bedrock fault scarp, allows us to construct the 14 kyr fault displacement history of the Nahef East normal fault, northern Israel (300 m above sea level, N33° latitude). The Nahef East fault is one of a series of fault scarps located along the 700 m high Zurim Escarpment, a major geomorphic feature. Samples at the top of the scarp have the highest nuclide concentrations (79×104atoms (g rock)−1); samples at the base have the lowest (11×104atoms (g rock)−1). Using chemical data from the samples, Nahef East fault scarp geometry, and surface and subsurface production rates for the36Cl‐producing reactions, we have constructed a numerical model that calculates36Cl accumulation on a scarp through time, given a series of unique displacement scenarios. The resulting model36Cl concentrations are compared to those measured in the scarp samples. Faulting histories that result in a good match between measured and modeled36Cl abundances show three distinct periods of fault activity during the past 14 kyr with over 6 vertical meters of motion occurring during a 3 kyr time period in the middle Holocene. Smaller amounts of displacement occurred before and after the period of most rapid motion. The episodic behavior of the Nahef East fault indicates that the average displacement rate of this fault system has varied through time.
A model for complex aftershock sequences
Authors: Moreno Y. , Correig A. M. , Gómez J. B. , Pacheco A. F.
Published: February 2004

The decay rate of aftershocks is commonly very well described by the modified Omori law, n(t) ∝ t−p, where n(t) is the number of aftershocks per unit time, t is the time after the main shock, and p is a constant in the range 0.9 < p < 1.5 and usually close to 1. However, there are also more complex aftershock sequences for which the Omori law can be considered only as a first approximation. One of these complex aftershock sequences took place in the eastern Pyrenees on February 18, 1996, and was described in detail by Correig et al. [1997]. In this paper, we propose a new model inspired by dynamic fiber bundle models to interpret this type of complex aftershock sequences with sudden increases in the rate of aftershock production not directly related to the magnitude of the aftershocks (as in the epidemic‐type aftershock sequences). The model is a simple, discrete, stochastic fracture model where the elements (asperities or barriers) break because of static fatigue, transfer stress according to a local load‐sharing rule and then are regenerated. We find a very good agreement between the model and the Eastern Pyrenees aftershock sequence, and we propose that the key mechanism for explaining aftershocks, apart from a time‐dependent rock strength, is the presence of dynamic stress fluctuations which constantly reset the initial conditions for the next aftershock in the sequence.
New gravity data in the Arctic Ocean: Comparison of airborne and ERS gravity
Authors: Childers Vicki A. , McAdoo David C. , Brozena John M. , Laxon Seymour W.
Published: February 2004

New gravity fields from airborne gravimetry and from ERS‐1 and ‐2 satellite altimetry cover extensive portions of the Arctic Ocean. These two data sets may constitute as much as 60% of the data contributions to the Arctic Gravity Project compilation. Here we evaluate the accuracy and resolution of these data and quantify their impact on the compilation. Both gravity determinations compare favorably with Geological Survey of Canada surface measurements in the Beaufort Sea (airborne, 1.86–2.09 mGal rms; ERS, 2.64–3.11 mGal rms). Comparisons between the airborne and ERS data over the Chukchi Borderlands reveal a 4.38 mGal rms difference over the smoother region of the field and 7.36 mGal rms over the rugose field generated by the shallow ridges and deep troughs. Coherency between the two data sets in the Chukchi region implies a resolution of 19 km. Comparison with Science Ice Expedition submarine measurements over Chukchi Plateau suggests that the ERS field resolves even shorter‐wavelength signal than the airborne data, whereas in the Beaufort Sea the airborne data showed better coherence to ground truth data. Long‐wavelength differences exist between the two data sets, expressed as a 2–3 mGal offset over the Chukchi region. This study highlights the respective strengths of the two data sets. The ERS gravity field has the advantage of ubiquitous coverage of the ocean south of 81.5°N, a denser sampling of the gravity field, and a recovery of signal down to ∼15 km. The airborne data cover a significant portion of the polar hole in the satellite coverage, have lower measurement noise, and recover somewhat higher anomaly amplitudes in the 25–100 km wavelength range.
Simulation of seismic cycles of buried intersecting reverse faults
Authors: Kato Naoyuki
Published: February 2004

To understand how reverse fault systems interact, I perform a numerical simulation of seismic cycles. The model fault system in a two‐dimensional uniform elastic half‐space consists of a shallowly dipping listric main reverse fault and a subfault with a higher dip angle. A fixed slip rate is imposed on deeper parts of the main fault, and frictional stress on the remaining parts of the main fault and on the subfault obeys a rate‐ and state‐dependent friction law. The main results of the numerical simulation are as follows: (1) Because of fault interactions, there exists moderate scatter in recurrence intervals of large earthquakes on the main fault and the subfault. (2) The cumulative slip amounts on deeper parts of the main fault, on shallower parts of the main fault, and on the subfault are different from one another. Their values are such that the stress concentration at the fault intersection is a minimum. (3) A large earthquake on the main fault increases shear stress and decreases normal stress on the subfault, causing slip on deeper parts of the subfault. A subfault earthquake sometimes induces slip on the main fault. Result 3 is consistent with some seismic observations that two conjugate fault planes were active during an earthquake sequence. Results 1 and 2 may be important for long‐term evaluation of seismic potential.
Crustal uplift in the south central Alaska subduction zone: New analysis and interpretation of tide gauge observations
Authors: Cohen Steven C. , Freymueller Jeffrey T.
Published: February 2004

We have examined tide gauge measurements of apparent sea level height in south central Alaska to determine the history of crustal uplift subsequent to the 1964 Prince William Sound earthquake. There are spatial and temporal variations in the uplift rate since the 1994 earthquake that depend on the location of the tide gauge relative to the coseismic rupture features. At Seward, on the eastern side of the Kenai Peninsula, we find slow uplift that is consistent with elastic strain accumulation at the locked North American‐Pacific Plate boundary. Conversely, at Seldovia and Nikiski, on the western side of the Kenai Peninsula, we find persistent rapid uplift of ∼10 mm yr−1 that may be longterm transient response to the earthquake but that cannot be sustained over the entire several hundred year recurrence interval for a great earthquake. Farther to the southwest, at Kodiak, the rate of uplift is several millimeters per year but has slowed significantly over the past three and a half decades. To the east of the Kenai Peninsula we find subsidence at Cordova and an uncertain behavior at Valdez. At Cordova, and to a lesser extent Valdez, there is a mathematically significant time dependence, although the evidence for the time dependence is less compelling than at Kodiak. At Anchorage, there is little evidence of vertical motion since the earthquake. The along‐strike spatial variability in the relaxation time of the rates of vertical motion since the 1964 earthquake may be related to variations in the updip coseismic slip during the megathrust event.
A two‐phase model for compaction and damage: 2. Applications to compaction, deformation, and the role of interfacial surface tension
Authors: Ricard Yanick , Bercovici David , Schubert Gerald
Published: February 2004

New equations for the dynamics of a two‐phase mixture are derived in a companion paper [Bercovici et al., this issue (a)]. These equations do not invoke a bulk viscosity as most previous papers have done, and use the existence of the pressure difference between the two phases, including the possibility of surface energy at the interface between the phases. In this paper we show how a two‐phase mixture reacts to simple stress fields. As a basic example, we discuss the deformation of a porous material confined by an impermeable jacket and loaded by a porous piston and show that the fluid can never be totally extracted from the matrix. We demonstrate that an unconfined porous sample is stronger under shear deformation than under normal stress. We consider spherically symmetric compaction and show that some unphysical results obtained using a constant matrix bulk viscosity are naturally avoided in our approach. We discuss the problem of compaction of a two‐phase liquid in the presence of surface tension. In a one‐dimensional simulation the surface tension generates porosity instabilities that tend to localize the fluid into narrow sills and dikes that cannot reach the surface.
Tomographic imaging of the earthquake source: Numerical validation in two‐dimensional approximation
Authors: Bindi Dino , Caponnetto Andrea
Published: February 2004

We use numerical methods to solve the two‐dimensional tomographic source problem, and we investigate the accuracy of the slip velocity reconstruction achieved. We perform the inversions of the source time functions (STFs) considering both the effect of errors in the source‐station azimuths and the uncertainties introduced by using an empirical Green function scheme as a method for computing the STF. We also investigate the dependence of the results on the number and arrangement of the stations. We adopt two different iterative schemes to invert the Radon transform. The numerical tests show that even small errors in the data can have significant effect on the results, pointing out the ill‐posedness of the considered inverse problem. The results also show that the introduction in the algorithm of a priori information, such as positivity or boundary for the reconstruction domain, improves the quality of the reconstructions and allows retrieval of qualitative information about the rupture process even in the case of poor azimuthal coverage. We test our method on two recorded earthquakes. The first one occurred in southern California on May 1, 1985 (ML = 2.4), and our results show good agreement with earlier studies. The second one, which occurred in northwestern Italy on August 23, 1996 (MD = 2.9), appears to involve a more complex, possibly bilateral rupture. The inability of the Radon approach to resolve the bilateral rupture, as we show using a synthetic test, does not allow us to draw any definitive conclusion about this earthquake.
Fracturing processes due to temperature and pressure nonlinear waves propagating in fluid‐saturated porous rocks
Authors: Merlani Antonio Luigi , Natale Giuseppe , Salusti Ettore
Published: February 2004

A model is proposed of rock deformation‐fracturing in the subsurface of hydrothermal systems in response to deep fluid‐rock temperature and pore fluid pressure perturbations, carried upward by hot and pressurized fluid fronts. Since during these episodes of unrest one also has to take into account that rock parameters can evolve, a model of fluid diffusivity change as a function of pore fluid pressure is described. Through reformulating the linear thermoporoelastic equations, rock deformation‐fracturing is thus thought of as being associated with migration of thermomechanical nonlinear waves, which travel upward, associated with an increase in concurrent fluid diffusivity. On dynamical grounds it is assumed that on the boundary of the two superimposed horizons the overlying rock suddenly starts rupturing, caused by the arrival of supercritical water from below, which drives up a pore fluid pressure excess. In this connection, the purpose of this analysis is to investigate the general evolution of the subsurface pressure and temperature fields, assuming that the original signal is itself strong enough to generate fracturing processes of the overburden rock on its arrival. A general formulation provides evidence of nonlinear “thermal waves,” “compensated waves,” and “residual pressure Burgers waves,” that can be found for every value of the system parameters. A mechanical analogy is also presented, which is treated analytically and numerically, allowing one to gain intuitive insight into such complex phenomena. A characteristic of these nonlinear processes is that the resulting timescales (of the order of years for the case of the Campi Flegrei and the Izu Peninsula) can be particularly small, corresponding to quick hyperthermal phenomena during the filtrating movement of fluid toward the Earth's surface.
Paleomagnetism of a Late Cretaceous island arc complex from South Sakhalin, East Asia: Convergent boundaries far away from the Asian continental margin?
Authors: Bazhenov Mikhail L. , Zharov Alexander E. , Levashova Natalia M. , Kodama Kazuto , Bragin Nikita Y. , Fedorov Peter I. , Bragina Lyubov' G. , Lyapunov Sergey M.
Published: February 2004

The Hokkaido‐Sakhalin fold system stretches for ∼1500 km along the eastern coast of Asia and consists of several N‐S trending tectonic belts. Studies in South Sakhalin show that the northern part of the Tonino‐Aniva Peninsula (Ozersk unit) is a counterpart of the Tokoro belt on Hokkaido, Japan. In the eastern part of the Ozersk unit, 195 hand samples were sampled at 20 sites from Campanian‐Maastrichtian tuffaceous siltstones and sandstones of the Chayka Formation of island arc affinity. Stepwise thermal demagnetization isolates a postfolding low‐ to intermediate‐temperature component (B) of normal polarity. A high‐temperature component (A) is isolated from about half the samples. Because of strong overlap of unblocking temperature spectra of these two components in other samples, direct observations and remagnetization circles were combined for calculation of site‐mean directions. Component A is mostly of reversed polarity; a few samples of normal polarity are found at three sites. The presence of two polarities with approximately antipodal directions and the positive fold test imply a prefolding, and most probably primary, origin of component A. A formation‐mean inclination of 45.0°±6.4° calculated with the aid of inclination‐only statistics corresponds to a latitude of 26.6°±5.2°N. A similar inclination is derived from a late Cretaceous island arc complex from the Tokoro belt on Hokkaido. Since both mean inclinations are ∼30° lower than the coeval Eurasian reference value, a large‐scale northward transport of the entire Tokoro island arc is inferred. We exclude the possibility of displacement with the Kula plate and coast‐parallel transport; instead, intra‐oceanic motion with the Pacific plate and docking at the Eurasian margin at circa 30 Ma are inferred. Combined paleomagnetic data from the Tokoro belt, the Nemuro belt and Kamchatka region imply that a system of intra‐oceanic island arcs existed in the northwest Pacific in Late Cretaceous time.
Pervasive pressure solution transfer in a quartz sand
Authors: Revil A.
Published: February 2004

The kinetics of deformation of a quartz aggregate by pervasive pressure solution can be, under certain conditions of temperature and grain size, strongly dependent upon the diffusivity of silica into the grain‐to‐grain contacts. An analysis of the factors affecting this key parameter (and less well constrained in the analysis of the problem of rock deformation by pressure solution) is presented. This analysis is based on recent advances on studying silica surfaces and particularly on the existence of a silica gel layer on the silica surfaces undergoing dissolution. By reinvestigating the electroviscous effect occurring at the grain‐to‐grain contact the present analysis shows that the diffusivity of silica at the grain‐to‐grain contacts is likely to be relatively similar to that in the bulk pore water (maybe 1 order of magnitude smaller but not more). This contradicts the previous work by Rutter [1976], which has been the key reference used in many subsequent papers to justify an extremely low value for the diffusivity of silica at the grain‐to‐grain contacts (5 orders of magnitude smaller than the diffusivity of silica in free water). This finding has dramatic implications concerning the deformation rate of quartz sands and sandstones by pressure solution in sedimentary basins with regard to (1) the limiting step affecting the kinetics of the process (diffusion of the solute or dissolution/precipitation chemistry) and (2) the existence of a thermodynamic equilibrium state when deformation by pressure solution occurs over geological timescales. A poroviscoplastic model is used to describe deformation associated with pervasive pressure solution transfer in quartz sands. This model is shown to be consistent with the current state of knowledge of the surface chemistry of silica. In addition, the comparison between this model and both laboratory and field data is rather good.
Testing the accuracy of solar‐reflected radiation corrections applied during satellite shortwave infrared thermal analysis of active volcanoes
Authors: Wooster Martin J. , Kaneko Takayuki
Published: February 2004

Shortwave infrared (SWIR) satellite imagery is frequently used to study thermal emission from active volcanoes. When using daytime SWIR observations it is necessary to first isolate the thermally emitted signal component, principally by subtracting contributions due to solar‐reflected radiation. Two differing approaches have been used: (1) A “mean” approach based on the average SWIR reflectance of the nonthermally anomalous volcanic background, and (2) a “per‐pixel” approach which estimates the SWIR reflectance of each pixel using a measure of their near infrared reflectance. We assess the accuracy of these using near‐coincident daytime and nighttime Landsat Thematic Mapper (TM) imagery of the active lava dome of Unzen Volcano. Using the nighttime data as a reference we find the most commonly applied mean daytime correction approach to be seriously inadequate, returning results for thermally radiant area 81–94% smaller than the nighttime values. The per‐pixel correction method appears somewhat more effective but still provides values 47–61% smaller than the reference data set. Daytime underestimation of total SWIR thermal spectral radiance is less severe than that of total thermally radiant area because it is primarily pixels having low thermal signals that pose a problem for the daytime approaches. Total thermal spectral radiance is underestimated by 52–79% using the daytime data with the mean correction approach, and by 28–59% with the per‐pixel approach. When applying the TM “dual‐band” technique to determine subpixel hot spot temperatures from the SWIR thermal signals, we find that surfaces cooler than 400°C are poorly represented in the daytime retrievals. However, when using the per‐pixel daytime data correction method the distribution of retrieved hot spots >400°C corresponds quite well to the pattern derived via nighttime imagery. Due to the significant parameter underestimation found to be inherent when using daytime data, this study further indicates the value of nighttime observations to volcanic SWIR thermal studies. If daytime data are used, then we recommend use of the per‐pixel solar‐reflected radiation correction method but would advise caution when quantitatively interpreting the derived thermal parameters, at least at andesitic and dacitic volcanoes. It is possible that daytime retrievals may be more robust at basaltic volcanoes due to their characteristic higher magmatic temperatures and lower surface reflectances.
Surface heat budget in an oceanic simulation using data from Tropical Ocean‐Global Atmosphere Coupled Ocean‐Atmosphere Response Experiment
Authors: Dourado Marcelo , Caniaux Guy
Published: February 2004

A three‐dimensional regional ocean model is used to investigate the response of the upper ocean to different atmospheric weather events encountered during the Tropical Ocean‐Global Atmosphere Coupled Ocean‐Atmosphere Response Experiment (COARE). The basic model is a version of the ocean general circulation model developed at the Laboratoire d'Océanographie Dynamique et de Climatologie in Paris and includes open lateral boundaries and a 1.5 level order turbulence closure scheme. The horizontal grid spacing is 0.1° in longitude and 0.08° in latitude. There are 29 vertical levels, with a resolution of 5 m near the top. The surface atmospheric forcing used comes from a combination of European Centre for Medium‐Range Weather Forecasts model output and estimates from bulk parameterization. The data set collected during the Intensive Observation Period enables a detailed initialization of the model. The analyses of the model output show that most features are in good agreement with the observations. During westerly wind bursts the mixed layer heat budget is negative (−83 W m−2) over a larger part of the COARE domain mainly because of the strong latent heat flux and weak solar radiation. Strong inhomogeneities due to local importance of horizontal advection contrast with relative homogeneous net heat flux. In this period all terms of the heat balance are active in the oceanic mixing layer. During the calm period the budget is weakly positive (15 W m−2) but still remains patchy because of the contribution of horizontal advection.
“Production rates of sea‐spray droplets” by M. J. Pattison and S. E. Belcher: Clarification and elaboration
Authors: Andreas Edgar L. , Pattison Martin J. , Belcher Stephen E.
Published: February 2004

The recent paper by Pattison and Belcher [1999] contains some mathematical errors that led to their misinterpreting how well their spray generation function agrees with one reported by Andreas [1992]. We here correct that mathematical error and show that now, Pattison and Belcher's results agree reasonably well with Andreas's and with other spray generation functions published by Smith et al. [1993] and Andreas [1998]. The main differences among these functions are explained by the fact that the Andreas functions apply right at the sea surface while the Pattison and Belcher and Smith et al. functions are appropriate only for spray droplets reaching heights of at least 1 and 14 m, respectively.
Last interglacial sea stands on Barbados and an early anomalous deglaciation timed by differential uplift
Authors: Johnson R. G.
Published: February 2004

Three major sea level maxima between 137 and 120 kyr B.P. are documented by surveys of pairs of wave‐cut notches and other features associated with the broadly defined last interglacial coral terrace on Barbados. Regression analysis of elevation differences between notch pair members as a function of their uplift rates at seven sites yields the age difference between first and third sea stands and their eustatic sea level difference. These results, when combined with stratigraphic data from the Bahamas, Bermuda, and Western Australia and a chronology based on age limits of corals from the Bahamas and Western Australia, yield an endpoint age and eustatic low‐tide elevation of 136.7 kyr B.P. and +7.4 m, respectively, for the first sea stand, 126 kyr B.P. and +5.6 m for the second, and 120.5 kyr B.P. and +3.0 m for the third. The first sea stand is the result of an anomalous deglaciation that was completed during low northern summer Milankovitch insolation. This is contrary to the Milankovitch hypothesis that orbitally controlled insolation largely determines the variations of glacial ice volume. It implies that internal factors within the Earth's climate system can be much stronger than the effects of insolation variations.
Effects of topographic variability on the formation of upwelling centers off New Jersey: A theoretical model
Authors: Song Y. Tony , Haidvogel Dale B. , Glenn Scott M.
Published: February 2004

A theoretical wind‐driven, continuously stratified coastal ocean model with varying topography is developed to examine the formation of upwelling centers in coastal oceans, such as those observed along the New Jersey coast. In this simple model an Ekman transport layer is induced by a wind‐driven shear friction profile to force the coastal ocean. An analytical perturbation solution, including a component of internal Kelvin waves, is solved for the coastal upwelling system with weak alongshore‐varying topography. Under uniformly upwelling favorable winds, the model solutions show that alongshore‐varying topography leads to the formation of upwelling centers along the coast by increasing upwelling on the downslope sides of topographic highs and decreasing upwelling on the upslope sides of topographic highs, similar to the observed features off the New Jersey coast during summer seasons. A conservation theorem is derived that indicates the topographic variation does not change the total amount of upwelled water but redistributes it unevenly along the coast to form enhanced upwelling centers. The theory suggests that alongshore‐varying topography plays an important role in controlling the formation of upwelling centers as it enhances upwelling at some locations and induces downwelling at others. In addition, the variations of the upwelling fronts are further complicated by the topographically trapped Kelvin waves.
Biogenic silica production in the Australian sector of the Subantarctic Zone of the Southern Ocean in late summer 1998
Authors: Quéguiner Bernard
Published: February 2004

The distribution of orthosilicic acid (Si(OH)4), lithogenic silica (LSi), biogenic silica (BSi), and biogenic silica production rates (ρSi) have been investigated on a transect encompassing the Subantarctic Zone (SAZ) from the Subtropical Zone (STZ) to the northern Polar Frontal Zone (PFZ) in the Australian sector of the Southern Ocean at the end of summer 1998. Si(OH)4 distribution was characterized by a north‐south gradient (surface values: <0.1 μM in the STZ to >2 μM in the northern PFZ). The SAZ and STZ exhibited very low BSi values, while a deep (75–150 m) BSi maximum (up to 2.34 μmol Si L−1) characterized the PFZ. BSi growth rates were higher in the SAZ and STZ (up to 0.46 doubling d−1) as compared to the PFZ (0.01–0.10 doubling d−1) where the deep maximum probably resulted from phytodetritus accumulation at the end of the productive season. A very high KS value (30.7 μM Si(OH)4) was estimated in the PFZ surface waters, and it is concluded that multiple limitations including light, iron, and silicic acid availability act on the different subsystems. The study period is characteristic of the last stage of the seasonal cycle especially in the PFZ where ρSi values were very low (mean value: 2.58 mmol Si m−2 d−1) as compared to previous studies. In the SAZ and the STZ respective ρSi mean values were 1.07 and 0.60 mmol Si m−2 d−1, and the siliceous component does not appear to dominate the phytoplankton community.
Simultaneous observations of rain cells over the ocean by the synthetic aperture radar aboard the ERS satellites and by surface‐based weather radars
Authors: Melsheimer Christian , Alpers Werner , Gade Martin
Published: February 2004

Radar images acquired over the ocean by the C band synthetic aperture radar (SAR) aboard the European Remote Sensing satellites ERS 1 and ERS 2 often show sea surface manifestations of rain cells. We have searched the archives of several weather stations for weather radar data acquired concurrently with ERS SAR data and have found four concurrent data pairs: one in the South China Sea, two in the Baltic Sea and one in the North Sea. The comparison of ERS 1/2 SAR images showing radar signatures of rain cells with weather radar images reveals that the radar signatures of rain cells on ERS SAR images vary considerably, which makes it often difficult to distinguish them from radar signatures of other mesoscale or submesoscale atmospheric and oceanic phenomena. The present analysis, together with results obtained from previous analyses of spaceborne multifrequency SAR data and laboratory data as well as results obtained from theoretical models on radar backscattering at the sea surface suggest the following: C band radar signatures of rain cells with rain rates below 50 mm/h are mainly caused by a modification of the sea surface roughness induced by (1) the raindrops impinging on the sea surface and thus modifying the sea surface roughness and by (2) local wind field variations associated with rain cells (spreading downdrafts). Raindrops impinging on the sea surface generate ring waves as well as turbulence in the upper water layer. Depending on rain rate, drop size distribution, wind speed, and temporal evolution of the rain event, the net effect can be an increase or a reduction of the amplitude of the C band Bragg waves and thus of the backscattered radar power. Thus ocean areas struck by rain can show up on ERS SAR images as areas with higher or lower image brightness than the surroundings.
Pacific Ocean wind stress and surface heat flux anomalies from NCEP reanalysis and observations: Cross‐statistics and ocean model responses
Authors: Auad Guillermo , Miller Arthur J. , Roads John O. , Cayan Daniel
Published: February 2004

Wind stresses and surface heat fluxes over the Pacific Ocean from the National Center for Environmental Prediction (NCEP) reanalysis and the comprehensive Ocean‐Atmosphere Data Set (COADS) (blended with FSU tropical wind stresses) are compared over a common time interval (1958–1997) in their statistics and in the responses that they induce in sea surface temperature (SST) and heat storage when used to force an ocean model. Wind stress anomalies from the two data sets are well correlated in the midlatitude extratropics, especially in the highly sampled North Pacific. In the tropics and subtropics, low correlations were found between the two wind stress data sets. The amplitudes of the stress variations of the two data sets are similar in midlatitudes, but in the tropics NCEP wind stresses are weaker than the COADS/FSU stresses, especially on interannual timescales. Surface heat flux anomalies from the two data sets are well correlated on interannual and shorter timescales in the North Pacific Ocean poleward of 20°N, but they are poorly correlated elsewhere and on decadal timescales. In the extratropics the amplitudes of the heat flux variations of the two data sets are comparable, but in the tropics the NCEP heat fluxes are weaker than those of COADS. Ocean model hindcasts driven by both data sets are also compared. The midlatitude SST hindcasts were superior when using the NCEP flux anomalies while tropical SST hindcasts were equally skillful for the two hindcasts when considering all climatic timescales. The spatial and temporal sampling rates of the COADS observations and their consequent impacts on constraining the NCEP reanalysis appear to be the main factors controlling the results found here.
An s coordinate density evolving model of the northwest European continental shelf: 2. Seasonal currents and tides
Authors: Holt Jason T. , James Ian D. , Jones J. Eric
Published: February 2004

The barotropic tides and the residual currents from a year long run of a three‐dimensional baroclinic model (the Proudman Oceanographic Laboratory Three‐Dimensional Baroclinic B grid model) applied to the northwest European shelf are investigated. M2 currents and elevations differ from observations by typically 0.114 m s−1 and 0.143 m; these errors are dominated by a number of large values at the coast. The density component of the residual currents is derived from the difference between the full model and wind and tide only model runs. This demonstrates the importance of density‐driven currents in a number of regions, particularly in the Norwegian Trench, around the St. George's Channel, to the east of the Shetland Isles, and in the river plumes. However, volume fluxes show the mass budget of the North Sea is primarily determined by the wind‐driven circulation. Calculations of the dynamic balance in the model show the significance of friction and/or advective effects, on a timescale of days, at a number of locations (e.g., the Dover Strait and in the central North Sea). However, the monthly mean currents are in geostrophic balance over virtually the whole domain; ageostrophic components adjust the surface and isopycnal slopes to restore this balance on comparatively short timescales. Particular attention is given to the western boundary, and we demonstrate that variations in the imposed temperature and salinity here have only a limited effect on the shelf. Current meters on the Hebrides shelf show that the model produces reasonably accurate residual currents but also that this agreement is degraded by the boundary condition on Porcupine Bank.
A decade of oceanographic variability in summertime near Elephant Island, Antarctica
Authors: Amos Anthony F.
Published: February 2004

The oceanography of near‐surface (to 750 m) summertime waters surrounding the Elephant Island area of the South Shetland Islands has been surveyed each year from 1990 to 1999 as part of the U.S. Antarctic Marine Living Resources (AMLR) program. Using data from a grid of conductivity‐temperature‐depth stations occupied twice each summer and surface meteorological data collected continuously in the survey area, the mean (and annual departures from the mean) hydrography, circulation, and overlying wind field have been determined. The area is an important krill fishing ground and includes several water mass zones and a major frontal boundary. Differences between middle and late summer (“seasonal”) conditions and interannual variability have been investigated. The mean summer condition is quite representative of conditions encountered in each of the 10 years, and little change occurs from middle to late summer, other than a warming of Antarctic Surface Water, including the Winter Water temperature minimum at 100 m, but there is no discernible trend from middle to late summer at the depth of the Circumpolar Deep Water. Interannual variability of sea temperature, winds, and sea level pressure as measured during these surveys showed no correlation with a sea ice index and a recently devised pressure index, the Drake Passage Oscillation Index (DPOI) [Naganobu et al., 1999], despite the correlations found in that study between AMLR chlorophyll a and krill recruitment indices, sea ice, and the DPOI. No correlation was found between DPOI and annual summertime sea temperature, wind, and surface pressure anomalies in the survey area. The implication is that global‐scale climatic oscillations with periodicities measured in years may not be adequately detected by small‐scale oceanographic surveys even though the surveys may extend over several years unless the surveys are specifically designed for that purpose.
Biologically induced circulation at fronts
Authors: Edwards Andrew M. , Platt Trevor , Wright Daniel G.
Published: February 2004

Consider a frontal region that has high phytoplankton biomass on one side and low biomass on the other. Irradiance penetrates deeply through the water column on the low‐biomass side but is attenuated nearer the surface on the biomass‐rich side because of absorption by phytoplankton. Thus the near‐surface water is heated more on the biomass‐rich side than on the clearer side, resulting in lower‐density surface water on the biomass‐rich side. At greater depths the situation is reversed, with lower‐density water occurring on the biomass‐poor side. We model this situation and examine the resulting perturbations to the frontal circulation. Our aim is to provide an order‐of‐magnitude estimate of the feedbacks from the biological component of the ecosystem to the current field. The model consists of the steady state momentum equations, including Coriolis, pressure gradient, and viscous effects. We compute induced vertical velocities of up to 0.2 mm s−1, commensurate with field measurements and previous modeling estimates of vertical velocities at fronts. The horizontal along‐frontal velocities are of order 2 cm s−1 or less and so will not represent a major contribution to the overall flow field; however, such values are certainly not insignificant.
Decadal deep‐water variability in the subtropical Atlantic and convection in the Weddell Sea
Authors: Stössel Achim , Kim Seong‐Joong
Published: February 2004

This study suggests a link between convection in the Weddell Sea and deepwater flows in the (sub)tropical Atlantic. Employing a global sea ice‐ocean general circulation model, events of enhanced convection are found to create enhanced outflow of Antarctic Bottom Water (AABW), the associated anomaly of which propagates rapidly along the model's deep western boundary. The propagation timescale of the order of a few years to reach the tropical Atlantic matches baroclinic Kelvin waves with phase speed corrected for the model's grid resolution. This feature is consistent with earlier findings on the baroclinic adjustment to perturbations of the rate of North Atlantic Deep Water (NADW) formation. The mean deep ocean transports, as well as the timescales and amplitudes of the deep ocean anomalies, are in good agreement with observation‐based estimates. Upon arrival in the deep western tropical Atlantic, the model AABW anomalies induce strong anomalies in NADW outflow. This behavior suggests that the actual magnitude of NADW outflow across 30°S is highly susceptible to external perturbations on the baroclinic adjustment timescale. The implied higher susceptibility is supported by recent observations that NADW turns mostly eastward between 20°S and 30°S. The NADW anomalies eventually propagate southward and join the Antarctic Circumpolar Current. There is weak indication that the associated anomalies enter the eastern Weddell Gyre to eventually feed back on the critical convection site in the Weddell Sea, possibly sustaining the oscillation. However, there is stronger evidence that the anomalies initially created by the convective events are advected with the model's Weddell Gyre, the advective timescale of which (10–12 years) is consistent with the periodicity of the convective events.
General circulation and intergyre dynamics in the eastern North Atlantic from a regional primitive equation model
Authors: Penduff Thierry , de Verdière Alain Colin , Barnier Bernard
Published: February 2004

The mean circulation of the eastern North Atlantic is investigated using a regional 0.8° × cos(latitude)‐resolution configuration of the SPEM primitive equation sigma coordinate model, forced by seasonal and monthly surface fluxes. The computational domain is surrounded by three self‐adapting open boundaries which evacuate the outgoing perturbations and laterally control the baroclinic modes in inflow regions, but let the model adjust the barotropic mode to a large extent. The final solution is stable and reproduces most features of the basin's mean circulation well: A realistic Azores Current, the observed paths and transports of subpolar currents, of the branches of the North Atlantic Current, of the intergyre zone modal and intermediate water masses down to about 2000 m. Some unrealistic circulation features, attributed to the modest resolution and to certain limitations of the sigma coordinates, are found below 2000 m. The buoyancy and vorticity balances are investigated in the intergyre zone. The Subpolar Mode Water (SPMW), subducted at a realistic rate, continues its southward journey toward the ocean's interior in accordance with the ventilated thermocline theory, with eddy diapycnal fluxes exerting a moderating effect. The poleward motion observed between 300 and 1000 m is well reproduced north of 45°N and governed by similar dynamics, but is absent south of 45°N within the upper Mediterranean Water.
An analytical solution for stratified tidal lagoons: Application to the validation of a numerical open boundary condition
Authors: Eliason Donald E. , Bourgeois Alfred J.
Published: February 2004

An analytical solution is presented for the case of a stratified, tidally forced lagoon. This solution, especially its energy and mass balances, is useful for the validation of numerical shallow water models under stratified, tidally forced conditions. The utility of the analytical solution for validation is demonstrated for a simple finite difference numerical model. The energy, mass, and their balances from the numerical model are shown to converge to the analytical solution with a power law dependence as spatial and temporal resolution are increased. The power law convergence of the numerical results to the analytical solution validates both the finite difference scheme and the open boundary conditions used for the tidal forcing. The open boundary conditions work well because they are consistent with the characteristics of the analytical solution.
Effects of iron, silicate, and light on dimethylsulfoniopropionate production in the Australian Subantarctic Zone
Authors: DiTullio G. R. , Sedwick P. N. , Jones D. R. , Boyd P. W. , Crossley A. C. , Hutchins D. A.
Published: February 2004

Shipboard bottle incubation experiments were performed to investigate the effects of iron, light, and silicate on algal production of particulate dimethylsulfoniopropionate (DMSPp) in the Subantarctic Zone (SAZ) south of Tasmania during March 1998. Iron enrichment resulted in threefold to ninefold increases in DMSPp concentrations relative to control treatments, following 7 and 8‐day incubation experiments. Additions of Fe and Si preferentially stimulated the growth of lightly‐silicified pennate diatoms and siliceous haptophytes, respectively, to which we attribute the increased DMSPp production in the incubation bottles. Both of these algal groups were previously believed to be low DMSPp producers; however, our experimental data suggest that addition of iron and silicate to the low‐silicate low‐iron waters of the SAZ will result in increased production of DMSPp by lightly silicified diatoms and siliceous haptophytes, respectively. Increased irradiance enhanced DMSPp production in iron‐amended treatments with both low (0.5 nM) and high (5 nM) concentrations of added iron. However, the role of light in stimulating DMSPp production was apparently of secondary importance compared to the effects of iron addition. The combination of high irradiance and high iron enrichment produced the highest DMSPp production in the experiments, suggesting that iron and light may have a synergistic effect in limiting algal DMSPp production in subantarctic waters.
Oceanic carbon transport carried by freshwater divergence: Are salinity normalizations useful?
Authors: Robbins Paul E.
Published: February 2004

The transport of chemical properties by ocean currents, and thus the divergence/convergence of properties within ocean regions, is strongly influenced by the global hydrological cycle. Specifically, a significant flux of properties such as inorganic carbon and total alkalinity may accompany the oceanic freshwater transport. Previous investigators have proposed the use of “salinity normalization” to eliminate the impact of the poorly known freshwater flux on the calculation of the meridional transport of inorganic carbon and dissolved oxygen. The validity of these normalizations is examined. Two different forms of “salinity normalization” are found in the literature. Analysis presented here shows that neither of these methods results in a normalized property whose transport and flux divergence is fully independent of the oceanic freshwater flux. Employing these previous attempts as guidance, a new form of salinity normalization is found which does satisfy the goal of yielding an accurate estimate of the net oceanic flux of property without explicit calculation of the oceanic freshwater divergence. The validity of this normalization, however, requires an ocean circulation consistent with the large‐scale salt balance. The calculation is implicitly dependent on the freshwater cycle and the investigator must still carefully examine the implied freshwater transports. Furthermore, consideration of the uncertainties involved in estimating the oceanic freshwater flux reveals that the use of salinity normalizations do not decrease the impact of errors in the freshwater flux on the estimation of oceanic carbon transports. Thus, while a salinity normalization can be properly defined, it offers no advantage, either for convenience or increased accuracy, over an explicit calculation of the oceanic property transport associated with the freshwater flux.
Seasonal axis migration of the upstream Kuroshio Extension associated with standing oscillations
Authors: Tatebe Hiroaki , Yasuda Ichiro
Published: February 2004

The TOPEX/Poseidon altimeter data from October 1992 to October 1998 are analyzed to investigate the seasonal cycle of the sea surface height (SSH) in the upstream region of the Kuroshio Extension. After removing steric height changes caused by surface buoyancy fluxes, two maxima of the seasonal SSH amplitude are found around the crest and trough of the quasi‐stationary meanders as standing waves, accompanied with nodes of amplitude minima. Seasonal variations are identified in the subsurface temperature at 400 m depth similar to those of SSH in amplitude and phase. Associated with the standing waves, the current follows a straight path in fall and then changes to a meandering path in spring. From a composite map of the seasonal Kuroshio Extension axes, nodes of minimum displacement are found. One node is located over the Izu Ridge, while the others are located in the areas where meridional acceleration is large. The seasonal axis migration of the upstream Kuroshio Extension is regulated by these nodes.
Interannual variability in the hydrography of Atlantic water northwest of Svalbard
Authors: Saloranta Tuomo M. , Haugan Peter M.
Published: February 2004

Using data from 1910 to 1997, we study the mean structure as well as the interannual variability in the autumn hydrography of the part of the West Spitsbergen Current (WSC) confined over the upper continental slope and of its continuation, the Svalbard branch. This is known to provide a major pathway for warm and saline Atlantic water to the Arctic Ocean. We focus our study on the area northwest of Svalbard, around 80°N. The time series of subsurface hydrography (100–300 m layer) show interannual variations which are, in the 1980s and 1990s, quite coherent with hydrographic time series obtained upstream in the WSC around 76°N as well as with variations in the large‐scale atmospheric patterns, as indicated by the North Atlantic Oscillation (NAO) index. However, in the 1970s, the coherence is poorer. Our results suggest that it may be important to focus on the narrow domain of the part of the WSC confined over the upper continental slope in order to get consistent estimates of the Arctic Ocean inflow signal in the Fram Strait. The warming in this inflow signal (in 1970–1996) does not seem to have been as strong as previously suggested.