bluuebunny/crossref_metadata_2025 huggingface dataset:
Elastodynamics of a non‐ideal interface: Application to crack and fracture scattering
Authors: Carcione José M.
Published: February 2004

This work introduces a numerical algorithm for solving wave propagation in the presence of an imperfect contact between two isotropic, elastic and heterogeneous media. Non‐ideal interfaces of general type can be modeled as boundary discontinuities of the displacement u and its first time derivative (the particle velocity v). The stress field is continuous, and the quantity [κu + ζv], where the brackets denote discontinuities across the interface, is equal to the corresponding stress component. The specific stiffness κ introduces frequency dependence and phase changes in the interface response. On the other hand, the specific viscosity ζ is related to the energy loss. It is shown here that, in the velocity stress formulation of the wave equation, such a model is described by Maxwell relaxation like functions. I compute the reflection and transmission coefficients in terms of the corresponding incident propagation angle and complex moduli, together with the energy dissipated at the interface. This analysis characterizes the properties of the non‐ideal interface. The numerical method is based on a domain decomposition technique that assigns a different mesh to each side of the interface. As stated above, the effects of the interface on wave propagation are modeled through the boundary conditions that require a special boundary treatment based on characteristic variables. The algorithm solves the velocity‐stress wave equations and two additional first‐order differential equations (in two‐dimensional space) in the displacement discontinuity. For each mesh, the spatial derivatives normal to the interface are solved by the Chebyshev method, and the spatial derivatives parallel to the interface are computed with the Fourier method. The algorithm allows general material variability. The modeling is applied to the problems of crack and fracture scattering.
Transient creep of a composite lower crust: 2. A polymineralic basis for rapidly evolving postseismic deformation modes
Authors: Ivins Erik R.
Published: February 2004

Postseismic horizontal strain and displacement following the June 28, 1992, Landers, California, earthquake (MW 7.3) is broad scale and cannot be explained solely by delayed afterslip located at the rupturing fault trace. Both the observed strain at Piñon Flat Observatory (PFO) and observed Global Positioning System receiver velocities evolve rapidly after the Landers‐Big Bear earthquake sequence. The observed exponential decay of these motions, with timescales of 4–34 days, may reflect a soft creep rheology in the lower crust and brittle‐ductile transition zone or even within the seismogenic crust itself. Here a simple model of a two‐dimensional screw dislocation in a layered Maxwell viscoelastic Earth is employed in conjunction with a composite rheology to demonstrate that the short timescale transient response modes (≈4–34 days) are consistent with the behavior of a biviscous lower crust. The lowest viscosity of this system is derivable from laboratory experimental data on the long‐term creep of natural quarztites, and the highest viscosity is consistent with isostasy‐related lower crustal flow in a continental extensional tectonic environment. The model predicts significant stress relaxation at the base of the seismogenic crust. Near the base of the seismogenic zone, and about 4 km away from the mainshock, the rate of predicted relaxation is of the order of 0.01 MPa d−1 during the first 20 days of postseismic flow. Oblate spheroidal inclusions at 5% concentration levels that are both aligned and fairly flat in shape and that have a viscosity of 3–4 × 1015 Pa s are consistent with both the amplitude and decay time of horizontal crustal strain observed at PFO after the Landers mainshock. It is speculated that the structures exposed in cross sections and in seismic reflection profiles of the lower crust that have mylonitic associations are, in part, the cause of such rapid postseismic evolution in southeastern California. Unmylonitized quartz‐rich rock at sufficiently elevated temperatures could also contribute to the rapid decay modes.
Transient creep of a composite lower crust: 1. Constitutive theory
Authors: Ivins Erik R. , Sammis Charles G.
Published: February 2004

A composite model is proposed to describe the time‐dependent response of the Earth's lower crust. The motivation for such a model is twofold: First, new observations of widespread postseismic deformation indicate that the deep continental crust responds viscoelastically, having both long‐ and short‐term decay times. Second, by any number of observationally based rationales, the lower crust is compositionally and structurally heterogeneous over many length scales. For heterogeneities that have much smaller characteristic lengths than the minimum deformation wavelength of interest, the aggregate rheology can be described by composite media theory. For wavelengths of the order of the thickness of the lower crust (≈25–40 km) and larger, composite theory may be applied to heterogeneities that are smaller than about several hundred meters, or equivalent to the vertical extent of a thick lower crustal mylonitic shear zone. The composite media theory developed here is constructed using both Eshelby‐Mori‐Tanaka theory for aligned generalized spheroidal inclusions and a generalized self‐consistent method. The inclusions and matrix are considered to be Maxwellian viscoelastic: a rheology that is consistent with past homogeneous models of postseismic stress relaxation. The composite theory presented here introduces a transient response to a suddenly imposed stress field which does not appear in homogeneous Maxwell models. Analytic expressions for the amplitude and duration of the transient and for the effective long‐ and short‐term viscosities of the composite are given which describe the sensitivity to inclusion concentration (Φ), to shape, and to ratio of inclusion‐to‐matrix viscosity (R).
The influence of bubble plumes on air‐seawater gas transfer velocities
Authors: Asher W. E. , Karle L. M. , Higgins B. J. , Farley P. J. , Monahan E. C. , Leifer I. S.
Published: February 2004

Laboratory results have demonstrated that bubble plumes are a very efficient air‐water gas transfer mechanism. Because breaking waves generate bubble plumes, it could be possible to correlate the air‐sea gas transport velocity kL with whitecap coverage. This correlation would then allow kL to be predicted from measurements of apparent microwave brightness temperature through the increase in sea surface microwave emissivity associated with breaking waves. In order to develop this remote‐sensing‐based method for predicting air‐sea gas fluxes, a whitecap simulation tank was used to measure evasive and invasive kL values for air‐seawater transfer of carbon dioxide, oxygen, helium, sulfur hexafluoride, and dimethyl sulfide at cleaned and surfactant‐influenced water surfaces. An empirical model has been developed that can predict kL from bubble plume coverage, diffusivity, and solubility. The observed dependence of kL on molecular diffusivity and aqueous‐phase solubility agrees with the predictions of modeling studies of bubble‐driven air‐water gas transfer. It has also been shown that soluble surfactants can decrease kL even in the presence of breaking waves.
Direct evidence for northward flow on the northwestern Bering Sea shelf
Authors: Overland James E. , Stabeno Phyllis J. , Salo Sigrid
Published: February 2004

During the summer of 1994, a satellite‐tracked drifter transited from the southeastern Bering Sea slope through Bering Strait by a route westward along the slope and then northward through Anadyr Canyon and Strait. The trajectory emphasizes the importance of a western location of northward flow on the Bering Sea shelf. The transit time was 2 months from Cape Navarin to Bering Strait with northward drift velocities of 5–40 cm/s.
The circulation in the source area of the South Equatorial Current in the eastern Indian Ocean
Authors: Quadfasel Detlef , Frische Andrea , Cresswell George
Published: February 2004

Direct current measurements and geostrophic transport estimates, using hydrographic data collected in the eastern Indian Ocean, reveal that during October 1987 the South Equatorial Current carried about 22 × 106 m3/s in the upper 500 m of the water column. The Indonesian throughflow from the Pacific Ocean supplied about half of that transport; the other half came from the recirculation of the southern hemisphere subtropical gyre. The mixing of the Indian Central Water and the Australasian Mediterranean Water in the thermocline occurred mainly off the northwest Australian shelf due to enhanced mesoscale current variability in that area.
Modeling the diurnal cycle of carbon monoxide: Sensitivity to physics, chemistry, biology, and optics
Authors: Gnanadesikan Anand
Published: February 2004

As carbon monoxide within the oceanic surface layer is produced by solar radiation, diluted by mixing, consumed by biota, and outgassed to the atmosphere, it exhibits a diurnal cycle. The effect of dilution and mixing on this cycle is examined using a simple model for production and consumption, coupled to three different mixed layer models. The magnitude and timing of the peak concentration, the magnitude of the average concentration, and the air‐sea flux are considered. The models are run through a range of heating and wind stress and compared to experimental data reported by Kettle [1994]. The key to the dynamics is the relative size of four length scales; Dmix, the depth to which mixing occurs over the consumption time; L, the length scale over which production occurs; Lout, the depth to which the mixed layer is ventilated over the consumption time; and Lcomp, the depth to which the diurnal production can maintain a concentration in equilibrium with the atmosphere. If Dmix ≫ L, the actual model parameterization can be important. If the mixed layer is maintained by turbulent diffusion, Dmix can be substantially less than the mixed layer depth. If the mixed layer is parameterized as a homogeneous slab, Dmix is equivalent to the mixed layer depth. If Dmix > Lout, production is balanced by consumption rather than outgassing. The ratio between Dmix and Lcomp determines whether the ocean is a source or a sink for CO. The main thermocline depth H sets an upper limit for Dmix and hence Dmix/L, Dmix/Lout, and Dmix/Lcomp. The models are run to simulate a single day of observations. The mixing parameterization is shown to be very important, with a model which mixes using small‐scale diffusion, producing markedly larger surface concentrations than models which homogenize the mixed layer completely and instantaneously.
Short‐arc calibration of the TOPEX/POSEIDON and ERS 1 altimeters utilizing in situ data
Authors: Murphy C. M. , Moore P. , Woodworth P.
Published: February 2004

A calibration of the TOPEX/POSEIDON and ERS 1 altimeters has been undertaken using altimetry across the English Channel. Data from the tide gauge installed at Newhaven and the satellite laser ranger at Herstmonceux were used in conjunction with local models for the geoid, ocean tide, and storm surge to generate bias values at each subsatellite altimetry point. A range bias value of −17.3 cm was generated for the NASA TOPEX altimeter relative to the four‐parameter sea state bias model calculated by Gaspar et al. [1994]. The corresponding value for the Centre National d'Etudes Spatiales (CNES) POSEIDON solid state altimeter was −0.3 cm. The impact of other parameterizations for the sea state bias on the altimeter bias is studied. The derived results are in close agreement with results from the NASA calibration campaign at Harvest oil platform near California and the CNES experiment at the Italian Lampedusa island. The bias derived for ERS 1 was −39.3 cm relative to an Aston‐derived single‐parameter sea state bias model.
Evolution and structure of a coastal squirt off the Mississippi River delta: Northern Gulf of Mexico
Authors: Walker Nan D. , Huh Oscar K. , Rouse Lawrence J. , Murray Stephen P.
Published: February 2004

In early October 1992, satellite‐derived sea surface temperature data revealed a 200 km long and 10‐ to 30‐km‐wide stream of cool water flowing toward the southwest from the Mississippi River delta region. Satellite imagery and in situ measurements have enabled a detailed study of the squirt's kinematics and subsurface characteristics over a 2‐week period. In its early stages, the squirt appeared as a narrow, high‐speed (>75 cm/s) jet of water which flowed westward over the Mississippi Canyon, forcing a semi‐submersible drilling rig to suspend operations from October 2 to 4. After crossing back onto the shelf, the squirt spread laterally, yielding a mushroom‐shaped feature, 75 km wide, which consisted of counter‐rotating vortices. Northeasterly wind forcing (averaging 10–15 m/s) and water level setup east of the delta appear to have been the primary mechanisms for evolution of the high‐velocity currents. Satellite and in situ measurements demonstrate that the dipole eddy was comprised of a cool, low‐salinity, low‐density water mass at least 26 m deep in the center and 16 m deep along its margins. This event demonstrates that strong northeasterly winds over the northern Gulf of Mexico can initiate along‐shelf and off‐shelf flows of cooler coastal waters, contributing significantly to seasonal cooling and freshening of the continental shelf and to shelf/slope exchanges of water. During this event, approximately 100 km3 of inner shelf and river water was transported off the continental shelf, a volume equivalent to 17% of the average annual discharge of the Mississippi and Atchafalaya Rivers.
Low‐frequency currents at the northern shelf edge of the Grand Banks
Authors: DeTracey B. M. , Tang C. L. , Smith P. C.
Published: February 2004

The structure and generation mechanisms of low‐frequency currents at the northern shelf edge region of the Grand Banks are investigated using data from a bottom mounted acoustic Doppler current profiler (ADCP) and an array of conventional current meters collected during the Canadian Atlantic Storms Program II. The vertical structure of the current is determined from an empirical orthogonal function analysis of the ADCP data. The analysis shows that over the shelf edge the low‐frequency currents are predominantly barotropic and polarized in the long‐shelf direction. There is little coherence between the local wind and the barotropic current, while baroclinic currents are moderately coherent with the local wind. In the interior of the shelf, the currents and the local wind are highly coherent. The difference in response to wind forcing over the shelf edge and in the shelf interior suggests that currents in the shelf interior are wind driven, while currents over the shelf edge are generated by mechanisms other than direct wind forcing. Possible generation mechanisms for the barotropic long‐shelf low‐frequency currents are discussed, including meanders and eddy formation in the Labrador Current and continental shelf waves generated by distant storms.
Waves and currents over a fixed rippled bed: 2. Bottom and apparent roughness experienced by currents in the presence of waves
Authors: Mathisen Paul Peter , Madsen Ole Secher
Published: February 2004

This paper is the second of two papers which present the results of an experimental study to verify the use of a single bottom roughness length scale to characterize wave and current boundary layer flows over a rough bed. While the first paper [Mathisen and Madsen, this issue] included analysis of wave attenuation measurements to estimate the bottom roughness experienced by waves in the presence and absence of a current, this paper includes the analysis of time‐averaged velocity profiles to estimate the bottom and apparent roughness experienced by a current in the presence of waves. In this paper, velocity profiles predicted by the Grant and Madsen [1986] model are compared with measured velocity profiles. Apparent hydraulic roughness predictions of the Grant and Madsen [1986] model are shown to underpredict the apparent hydraulic roughnesses experienced by the current. This difference is shown to be a result of an underprediction of the wave boundary layer thickness and of a steady streaming or mass transport which is induced by the wave motion within the wave boundary layer of the combined wave‐current flow. By modifying the wave boundary layer thickness and estimating the wave‐induced mass transport from pure wave experiments, the bottom roughness for pure current, pure wave, and combined wave‐current boundary layer flows is shown to be characterized by a single roughness scale.
Temporal evolution of CFC 11 and CFC 12 concentrations in the ocean interior
Authors: Beining Peter , Roether Wolfgang
Published: February 2004

We study the temporal evolution of concentrations of the chlorofluorocarbons CFC 11 and CFC 12 in the ocean, under the assumption of circulation and mixing being invariant in time. This allows us to define a time‐invariant age distribution for a given point in the ocean, where the age is defined as time since the last contact with the atmosphere occurred. This concept is evaluated for a number of fundamental situations. We deduce a tendency for low CFC 11 and CFC 12 concentrations in advective regimes to increase exponentially in time and for concentrations near to a solubility equilibrium with atmospheric concentrations to increase rather more linearly. The apparent saturations, i.e., the ratios of interior to mixed‐layer CFC concentrations, increase monotonically in time, typical rates being 5–10% per decade. The theoretical results are compatible with time trends found in repeated CFC observations in the ocean. Diagrams on the temporal evolution for different age distributions are presented for the period 1970–2000, which can serve as a general orientation. The diagrams furthermore can provide time corrections for quasi‐synoptic evaluation of CFC observations taken over an extended period of time and assist in constructing time‐dependent CFC boundary conditions for numerical models of ocean circulation.
Interannual variability of the Kuroshio transport in response to the wind stress field over the North Pacific: Its relation to the path variation south of Japan
Authors: Akitomo Kazunori , Ooi Michihiro , Awaji Toshiyuki , Kutsuwada Kunio
Published: February 2004

We have investigated the interannual variability in the Kuroshio transport, focusing on the baroclinic response of the upper ocean to the long‐term variation of the wind stress field over the North Pacific from 1961 to 1987. First, temporal change of the Kuroshio transport at the western boundary from 17° to 31°N has been estimated on the assumptions that the Kuroshio is a baroclinic flow confined in the upper layer and that the effect of interior wind stress curl on the volume transport at the western boundary is transferred by nondispersive baroclinic Rossby wave. Peak values in the estimated transport appear in its beginning region (17°–21°N) when or just before the Kuroshio settles into a meandering state south of Japan in 1975, 1981, and 1986. These peaks are due to negative wind stress curl anomalies which are induced by the intensified trade winds in the central and eastern tropical North Pacific on and just after an El Niño event and in the western tropical North Pacific months to years before the next El Niño event. Such peaks are not found in the barotropic Sverdrup transport. In downstream regions, i.e., in the East China Sea and south of Japan (north of 23°N), on the contrary, the estimated transports (through baroclinic and barotropic responses) do not have such good correlation to the path variation. Temporal change in thermocline depths estimated from bathythermograph data clearly shows the westward propagations of positive anomalies by nondispersive baroclinic Rossby waves at latitudes of the beginning of the Kuroshio (south of 21°N), which is in phase with peaks of the Kuroshio baroclinic transport at the western boundary. On the other hand, there are only the local propagations of the anomalies in the downstream regions, as previous studies reported. However, it is emphasized that temporal change of the geostrophic transport in the East China Sea (∼28°N) is coincident with that of the baroclinic transport at the beginning of the Kuroshio, not with that at the same latitude. A numerical experiment with a nonlinear reduced‐gravity model confirms that temporal changes of the transport in the East China Sea and south of Japan are due to the advective (nonlinear) effect of the Kuroshio itself, while the linear baroclinic response to the wind stress field is dominant in its beginning region.
Seasonal variability of wind and thermohaline‐driven circulation in the Black Sea: Modeling studies
Authors: Oguz Temel , Malanotte‐Rizzoli Paola
Published: February 2004

The seasonal variability of the Black Sea circulation is studied using an eddy‐resolving primitive equation model. A series of numerical experiments is carried out to determine the relative importance of wind stress, air‐sea thermohaline fluxes, and river‐induced lateral buoyancy forcing in driving the circulation on the monthly and seasonal timescales. A synthesis is made of the results with those obtained under yearly climatological conditions by Oguz et al. [1995] to assess whether the major circulation features are a response to the yearly forcings or are dominated by the seasonal cycle. The model experiments indicate that under all forcing mechanisms, the overall basin circulation is characterized by a very strong seasonal cycle dominating the yearly signal described by Oguz et al. [1995]. The purely wind‐driven circulation reveals most of the observed circulation features including a well‐defined meandering boundary current system and subbasin scale cyclonic gyres forming the interior flow structure of the basin. Topography obviously remains a crucial factor in controlling the pattern of the persistent rim current system all year long. The dynamical instabilities of the rim current produce strong meandering and mesoscale eddies which often modulate the basin and subbasin scale structures of the circulation. The surface thermohaline fluxes generate simpler circulation patterns with a comparable strength but mostly in the opposite direction to the wind‐driven circulation. Two important by‐products emerge from the present work. First is the necessity of reanalyzing the heat flux climatology. The existing surface thermohaline fluxes, even though not affecting critically the general characteristics of the surface circulation patterns, may induce rather unrealistic horizontal temperature distributions and water mass properties in the surface layer. Second, the role of the northwestern shelf in the cold intermediate water (CIW) mass formation process is shown to be secondary during moderate‐to‐high winter discharge conditions from the northwestern rivers. In these conditions the freshwater outflow reduces the density of the cold water formed on the shelf by about 1 kg/m3 as compared with that of the basin interior, which is the major reservoir for the formation of the winter CIW.
Evidence of boundary reflection of Kelvin and first‐mode Rossby waves from TOPEX/POSEIDON sea level data
Authors: Boulanger Jean‐Philippe , Fu Lee‐Lueng
Published: February 2004

The TOPEX/POSEIDON sea level data lead to new opportunities to investigate some theoretical mechanisms suggested to be involved in the El Niño‐Southern Oscillation phenomenon in the tropical Pacific ocean. In particular, we are interested in studying the western boundary reflection, a process crucial for the delayed action oscillator theory, by using the TOPEX/POSEIDON data from November 1992 to May 1995. We first projected the sea level data onto Kelvin and first‐mode Rossby waves. Then we estimated the contribution of wind forcing to these waves by using a single baroclinic mode simple wave model forced by the ERS 1 wind data. Wave propagation was clearly observed with amplitudes well explained by the wind forcing in the ocean interior. Evidence of wave reflection was detected at both the western and eastern boundaries of the tropical Pacific ocean. At the eastern boundary, Kelvin waves were seen to reflect as first‐mode Rossby waves during the entire period. The reflection efficiency (in terms of wave amplitude) of the South American coasts was estimated to be 80% of that of an infinite meridional wall. At the western boundary, reflection was observed in April–August 1993, in January–June 1994, and, later, in December 1994 to February 1995. Although the general roles of these reflection events in the variability observed in the equatorial Pacific ocean are not clear, the data suggest that the reflections in January–June 1994 have played a role in the onset of the warm conditions observed in late 1994 to early 1995. Indeed, during the January–June 1994 period, as strong downwelling first‐mode Rossby waves reflected into downwelling Kelvin waves, easterly wind and cold sea surface temperature anomalies located near the date line weakened and eventually reversed in June–July 1994. The presence of the warm anomalies near the date line then favored convection and westerly wind anomalies that triggered strong downwelling Kelvin waves propagating throughout the basin simultaneously with the beginning of the 1994–1995 warm conditions.
Weddell Sea ice drift: Kinematics and wind forcing
Authors: Vihma Timo , Launiainen Jouko , Uotila Juha
Published: February 2004

Ice drift in the Weddell Sea was studied on the basis of positional and meteorological data from Argos buoys drifting in 1990–1992 and surface pressure analyses from the European Centre for Medium Range Weather Forecasts (ECMWF). The drift kinematics showed differences between the eastern and western parts of the Weddell Sea. Close to the Antarctic Peninsula, the ice drifted as an almost nonrotating uniform field at a low speed, having reduced small‐scale motions with little meandering, compared to regions further to the east. Inertial motion was detected from the ice drift in areas east of 35°W and in the region of the Antarctic Circumpolar Current. On timescales of days, wind was the primary forcing factor for the drift. A linear model between the wind and ice drift explained 40–80% of the drift velocity variance. The degree of explanation was higher in the central Weddell Sea (around 40°W) and lower closer to the Antarctic Peninsula. The geostrophic wind was found to provide almost as good a basis for the general drift estimation as the surface wind observed by the buoys, although strong cyclones were not well detected by the ECMWF analyses. The data suggest a dependency upon atmospheric stability such that stable stratification reduces the wind forcing on the drift. For 60–80% of the time the direction of the drift deviated less than 45° from the geostrophic wind and for 45–70% of the time less than 45° from the ocean current. Ice transport through a transect crossing the Weddell Sea from the Antarctic Peninsula tip to Kapp Norwegia was estimated on the basis of the geostrophic winds, the drift's observed response to the wind, and literature‐based information on ice concentration and thickness. The estimated annual mean net export in 1992–1994 varied from 8000 to 22,000 m3/s. Most of the net export took place in winter and spring, export prevailing west of 35°W and import east of it.
Seasonal patterns of wind stress and wind stress curl over the Gulf of Mexico
Authors: de Velasco Guillermo Gutiérrez , Winant Clinton D.
Published: February 2004

Meteorological observations from an array of stations deployed along the periphery of the Gulf of Mexico, between 1990 and 1993, are used to describe the seasonal fluctuations in patterns of atmospheric variables from a contemporary set of measurements. Seasonal maps of wind stress based on these measurements resemble wind stress maps based on ship observations, as published by Elliott [1979], rather than maps based on analyses of numerical weather forecasts, as published by Rhodes et al. [1989], particularly near the western boundary of the gulf. Seasonal maps of wind stress curl are characterized by positive curls over the western and southwestern gulf. The central result of this study is to document the important role of the mountain chain which extends along the southwestern section of the gulf in channeling the wind toward the Isthmus of Tehuantepec.
Eddies of newly formed upper Labrador Sea water
Authors: Pickart Robert S. , Smethie William M. , Lazier John R. N. , Jones E. Peter , Jenkins William J.
Published: February 2004

A new type of submesoscale eddy has been observed south of the Labrador Sea during late winter, embedded in equatorward flow along the western boundary. The eddy radius is 20 km, with a weak dynamic signature (swirl speed of 0.5 cm/s). The center of the eddy is characterized by weak stratification, elevated concentrations of oxygen and anthropogenic tracers, and low tracer ages, all indicative of newly ventilated water. Strong lateral intrusions distort the shape of the feature. The water mass contained in the eddy is not classical Labrador Sea water (from the central Labrador Sea) but is significantly fresher and hence lighter. It is of the correct density to be the source of the high‐chlorofluorocarbon layer of the shallow deep western boundary current observed further south and hence is termed upper Labrador Sea water. Using a combination of hydrographic data sets along the western boundary to implement a simple lateral diffusion model, it is shown that such eddies decay of the order of several months and are difficult to observe equatorward of the Grand Banks of Newfoundland. This is in contrast to deeper lenses of classical Labrador Sea water which persist further equatorward. Tracer‐derived ages of the upper Labrador Sea water eddy range from 3 to 5 years, much older than the lifetime deduced from their lateral diffusion. A simple convection model of tracer age shows that this age discrepancy is caused by gas exchange being unable to maintain equilibrium between the deep convecting mixed layer and the atmosphere during formation.
Modeling nutrient and plankton processes in the California coastal transition zone: 2. A three‐dimensional physical‐bio‐optical model
Authors: Moisan J. R. , Hofmann E. E. , Haidvogel D. B.
Published: February 2004

A three‐dimensional (3‐D) primitive equation model, developed to simulate the circulation features (filaments) observed in the California coastal transition zone (CTZ), was coupled to a nine‐component food web model and a bio‐optical model. The simulated flow fields from a 3‐D primitive equation model are used to advect the constituents of the food web model, which include silicate, nitrate, ammonium, two phytoplankton size fractions, copepods, doliolids, euphausiids, and a detritus pool. The bio‐optical model simulates the wavelength‐dependent attenuation of the subsurface irradiance field. The overall objective of this modeling study was to understand and quantify the processes that contribute to the spatial and temporal development of nutrient and plankton distributions in the CTZ. The resulting simulated 3‐D nutrient, plankton and submarine light fields agree well with those observed within the CTZ. Specifically, high nutrient and plankton biomass occur onshore and within the core of the simulated filament. Variations in the depth of the 1% light level, which result from the simulated plankton distributions, shallows to less than 30 m in regions of high phytoplankton biomass, and deepens to greater than 75 m in regions of low phytoplankton biomass. The onshore and offshore surface carbon flux patterns are similar in shape due to the meander‐like flow patterns of the filament; however, the net across‐shore area‐integrated carbon flux is predominantly offshore. The total 20‐day integrated carbon transport for the model domain varies with distance from shore and is highest (35 × 109 g C) in the region where the filament circulation pattern develops into an anticyclonic and cyclonic pair of eddies. The annual integrated carbon transport by filaments along the California coast is estimated to be 1.89 × 1012 g C.
Nitrogen fixation within a tropical upwelling ecosystem: Evidence for a Redfield budget of carbon/nitrogen cycling by the total phytoplankton community
Authors: Walsh John J.
Published: February 2004

Recent measurements and paradigms suggest that (1) the uptake of dissolved carbon and nitrate by phytoplankton may be greater than the Redfield ratio of 6.6 and (2) the oceans may be loosing nitrogen from an imbalance in the global rates of nitrogen fixation and denitrification. An analysis of concurrent ΔDIC/ΔNO3 depletion ratios within the Venezuelan and Peruvian upwelling ecosystems, indeed, suggests that values of 10.1–28.6 may pertain to these tropical eutrophic habitats. Nitrogen fixation may provide a Redfield balance in at least the former system, with 34–77% of the new production attributed to assimilation of N2. Independent confirmation of such new production on the Venezuelan shelf is provided by the interannual increases of H2S and DIC within the adjacent Cariaco Trench.
Bubbles entrained by mechanically generated breaking waves
Authors: Loewen M. R. , O'Dor M. A. , Skafel M. G.
Published: February 2004

A photographic technique was used to make measurements of the size distributions of large bubbles entrained by mechanically generated breaking waves in freshwater and saltwater. Digital video images of the region immediately beneath and behind the breaking wave crest were analyzed. This imaging technique was accurate for bubble radii, r ≥ 0.8 mm. The magnitude and shape of the bubble size distributions were approximately the same in freshwater and saltwater. The bubble size distributions are well represented by an exponential equation of the form N ∼ e−1.5r or by a power law equation of the form N ∼ r−3.7. There were no significant differences observed between the depth distribution of the bubbles in saltwater and freshwater. In both cases the data can be represented by N = No e−z/h, where z is the distance below the free surface, h is the entrainment depth, and No is the total bubble concentration at the free surface. No and h were found to be 4 × 105 m−3 and 1.0 cm, respectively. The fact that significant numbers of large bubbles are entrained by relatively small breaking waves (wavelength ∼ 1.5 m) suggests that gas transfer due to large entrained bubbles may be important even at low wind speeds.
Partitioning of solar energy in melt ponds from measurements of pond albedo and depth
Authors: Podgorny Igor A. , Grenfell Thomas C.
Published: February 2004

An explicit inverse radiative transfer algorithm is developed in order to determine solar energy redistribution in a plane‐parallel melt pond from direct measurements of pond albedo and depth. The algorithm is based on the exact analytical solution to the forward problem that is found by ignoring scattering in the melt water and assuming a cardioidal upwelling radiance distribution at the pond bottom. Sensitivity of the algorithm to errors in measurements and effects of inaccuracy involved in the direct solution are examined for a broad range of simulation parameters typical for actual melt ponds. Finally, the algorithm is illustrated using data sets relevant to young and old melt ponds under clear and cloudy sky conditions.
Tracing the conveyor belt in the Hamburg large‐scale geostrophic ocean general circulation model
Authors: Drijfhout Sybren S. , Maier‐Reimer Ernst , Mikolajewicz Uwe
Published: February 2004

The flow which constitutes the conveyor belt in the Hamburg large‐scale geostrophic ocean general circulation model has been investigated with the help of a particle tracking method. In the region of North Atlantic Deep Water formation a thousand trajectories were calculated backward in time to the point where they upwell from the deep ocean. Both the three‐dimensional velocity field and convective overturning have been used for this calculation. Together, the trajectories form a representative picture of the upper branch of the conveyor belt in the model. In the Atlantic Ocean the path and strength (17 Sv) of the conveyor belt in the model are found to be consistent with observations. All trajectories enter the South Atlantic via Drake Passage, as the model does not simulate any Agulhas leakage. Large changes in water masses occur in the South Atlantic midlatitudes and subtropical North Atlantic. Along its path in the Atlantic the water in the conveyor belt is transformed from Antarctic Intermediate Water to Central North Atlantic Water. On the average the timescale on which the water mass characteristics are approximately conserved is only a few years compared to the timescale of 70 years for the conveyor belt to cross the Atlantic. The ventilation of thermocline waters in the South Atlantic midlatitudes is overestimated in the model due to too much convective deepening of the winter mixed layer. As a result the fraction of the conveyor belt water flowing in the surface layer is also overestimated, along with integrated effects of atmospheric forcing. The abnormally strong water mass transformation in the South Atlantic might be related to the absence of Agulhas leakage in the model.
Coupling between SW monsoon‐related surface and deep ocean processes as discerned from continuous particle flux measurements and correlated satellite data
Authors: Rixen T. , Haake B. , Ittekkot V. , Guptha M. V. S. , Nair R. R. , Schlüssel P.
Published: February 2004

Particle flux data obtained by time series sediment traps deployed at water depths of approximately 3000 m in the western, central, and eastern Arabian Sea since 1986 were compared with wind speeds derived from measurements made by microwave radiometer flying on polar orbiting satellites and sea surface temperatures (SSTs) provided by the Physical Oceanography Distributed Active Archive Center at Jet Propulsion Laboratory. This comparison has allowed us to trace the link between the oceanographic and biological processes related to the development of the SW monsoon with the pattern and interannual variability of particle fluxes to the interior of the Arabian Sea. We could recognize the well‐known upwelling systems along the coasts of Somalia and Oman as well as open ocean upwelling at the beginning of the SW monsoon. Both open ocean upwelling and coastal upwelling off Oman cause a cooling of surface waters at our western and central Arabian Sea stations. When SSTs fall below their long‐term average, an increase in fluxes which are dominated by coccolithophorid‐derived carbonates occurs. The timing of this increase is determined by the rate of surface water cooling. Further intensification of upwelling as the SW monsoon progresses causes additional increases in biogenic opal fluxes denoting diatom blooms in the overlying waters. The total fluxes during this period are the highest measured in the open Arabian Sea. At the central Arabian Sea location the fluxes are only randomly affected by these blooms. The particle flux in the eastern Arabian Sea is as high as in the central Arabian Sea but is influenced by a weaker upwelling system along the Indian coast. The observed interannual variability in the pattern of particle fluxes during the SW monsoons is most pronounced in the western Arabian Sea. This is controlled by the intensity of the upwelling systems on the one hand and the transport of cold, nutrient‐poor, south equatorial water into the Oman region on the other. The latter effect, which is strongest during the SW monsoon with highest recorded wind speeds, reduces the influence of upwelling and the related particle fluxes at the western Arabian Sea station, where highest fluxes occur during SW monsoons with moderate wind speeds. Thus coastal and open ocean upwelling are most effective in transferring biogenic matter to the deep sea during the SW monsoons of intermediate strength.
Assimilation studies of open‐ocean flows: 1. Estimation of initial and boundary conditions
Authors: Gunson James Reginald , Malanotte‐Rizzoli Paola
Published: February 2004

The problem of estimating boundary and initial conditions for a regional open‐ocean model is addressed here. With the objective of mimicking the Synoptic Ocean Prediction (SYNOP) experiment in the Gulf Stream system, a meandering jet is modeled by the fully nonlinear barotropic vorticity equation. Simulated velocity observations are taken using current meters and acoustic tomography; twin experiments are then performed in which the adjoint method is used to reconstruct the flow field. The estimated flow is forced to resemble the true flow by minimizing a cost function with respect to some control variables. First, the vorticity initial conditions are used as control variables, and the boundary conditions are specified. The strong flow is found to induce strong dependence of the model/data misfit upon the specified boundary conditions. Second, the boundary values of stream function and vorticity are then included among the control variables. Various choices of a priori information about the control variables are employed, using various observational strategies. The major new result obtained is the successful estimation of the complete set of initial and boundary conditions, which is necessary to integrate the vorticity equation forward in time. From a time‐invariant first guess for the boundary conditions the assimilation is able to create temporal variations at the boundaries that make the interior flow match well the velocity observations.
Physical, structural, and isotopic characteristics and growth processes of fast sea ice in Lützow‐Holm Bay, Antarctica
Authors: Kawamura T. , Ohshima K. I. , Takizawa T. , Ushio S.
Published: February 2004

A sea‐ice/ocean study was conducted off Queen Maud Land and Enderby Land, Antarctica, from 1990 to 1991 by the Japanese Antarctic Research Expedition. Observations of multiyear land fast sea ice were made in Lützow‐Holm Bay over a period of 2 years to determine the snow and ice characteristics and ice growth processes. The snow depth in the bay reached large values of 1.0 to 1.5 m during the winter season at offshore locations. From the analysis of ice thickness measurements, it is confirmed that the fast ice with deep snow cover grew little in winter but substantially thickened during the summer months. On the basis of ice core structure, salinity, and stable isotopic composition, we conclude that the summer growth was caused by upward growth at the top of the ice to which snow ice and superimposed ice formation contribute. These processes were the primary contributors to sea‐ice growth and characteristics only where the snow accumulation was large. In areas of low snow accumulation, there was no surface growth. Superimposed ice formation on sea ice in Antarctica has not been reported previously. Evidence for snow cover melting, which is a prerequisite for superimposed ice formation, was also found.
Boundary undercurrent and water mass changes in the Lincoln Sea
Authors: Newton John L. , Sotirin Barbara J.
Published: February 2004

Oceanographic measurements taken between 1989 and 1994 in the Lincoln Sea describe the currents and water mass structure along the continental slope of the Arctic Basin between the Canadian and Eurasian Basins. The measurements included periodic conductivity and temperature versus depth (CTD) profiles from ice camps, CTD cross sections normal to the slope, and year‐round current measurements. Analysis of this data describes the hydrographic structure of the waters over the continental slope and identifies some significant interannual variations. An undercurrent, confined to the continental slope, with a width of about 50 km and speeds of 5–9 cm s−1 is shown in geostrophic current cross sections. The presence of this easterly flow at depth is confirmed by current meter measurements, and its existence has also been observed in the Beaufort Sea and the Barents Sea [Aagaard, 1989]. The waters within this undercurrent exhibit temperature‐salinity (TS) characteristics similar to Canadian Basin waters, suggesting a boundary current system which is continuous along the continental slope north of Alaska and Canada. Significant interannual variations in temperature and salinity profiles may be related to variations in the large‐scale circulation of the Arctic. Examination of the upper pycnocline waters over the slope between 1991 and 1994 indicated a relative temperature maximum overlying a minimum, TS characteristics similar to waters attributed to Bering Sea origin found in the Canadian Basin. These characteristics were not seen, however, during 1989 and 1990, suggesting a recent increase in the transport of surface waters from the Canadian Basin into the Eurasian Basin.
Observations of strong inertial oscillations after the passage of Tropical Cyclone Ofa
Authors: Firing Eric , Lien Ren‐Chieh , Muller Peter
Published: February 2004

One week after Tropical Cyclone Ofa passed near Samoa, near‐inertial oscillations of almost 1 m s−1 in amplitude were observed with a shipboard acoustic Doppler current profiler (ADCP). The velocity was nearly uniform with depth in the upper 80 m. The strongest currents in this layer were found about 300 km from Ofa's track, farther than expected from previous observations and models. Below 80 m was a layer 50–100 m thick in which the shear exceeded 0.01 s−1. Strong near‐inertial oscillations, with currents over 0.6 m s−1 from 130 to 200 m, extended through the shear layer to at least 300 m, the limit of the ADCP measurements. The phase of the oscillations propagated upward, consistent with downward propagation of energy. A simple slab model driven by an idealized wind field approximately matches the observed amplitude and phase of the oscillations in the surface layer within 150 km of Ofa's track, but not the stronger current 300 km from the track.
Dispersion processes and residence times in a semi‐enclosed basin with recirculating gyres: An application to the Tyrrhenian Sea
Authors: Buffoni Giuseppe , Falco Pierpaolo , Griffa Annalisa , Zambianchi Enrico
Published: February 2004

In this paper we investigate the dispersion of a tracer in a semi‐enclosed basin characterized by a steady flow with recirculations. In particular, we focus on the macroscopic behavior of the system, characterized by the total concentration of the tracer in the basin, C(t), and by its residence time T. As a case study, the circulation in an idealized basin mirroring some of the major characteristics of the Tyrrhenian Sea is considered, with a northward current connecting the inflow and the outflow regions of the basin, and with a main cyclonic gyre in the northern part of it. Numerical simulations are performed from several release points in the basin and for two different values of the diffusivity coefficient K. Two independent models for dispersion are used, an Eulerian and a Lagrangian one, allowing us to validate both the basic formalism and the numerical results. The experiments show that the macroscopic properties of dispersion are largely influenced by the presence of the main gyre, while they do not depend strongly on K in the considered range. Namely, after a first phase which depends on the initial conditions, the tracer tends to be concentrated in the region of the northern gyre, and this influences the trend of C(t) versus time and the value T. For almost all the simulations, the decrease of C(t) in time can be approximated by an exponential decay, indicating a constant probability of tracer escaping the basin. The e‐folding timescale of the decay is the inverse of the principal eigenvalue of the advection‐diffusion operator, and it can be computed a priori knowing the flow field; this allows us to compute also an a priori estimate for the residence time T. Only when the initial release is very close to the outflow, is the initial decay of C(t) distinctly different from an exponential, and a more detailed analysis is necessary. The basic results appear generalizable to a number of other similar systems with recirculations.