bluuebunny/crossref_metadata_2025 huggingface dataset:
An apparent mechanism dependence of radiated seismic energy
Authors: Pérez‐Campos Xyoli , Beroza Gregory C.
Published: February 2004

We develop an extension to the method of Boatwright and Choy [1986] for determining the radiated seismic energy Es that accounts for factors that bias the estimate. We apply our technique to 204 events worldwide during the period 1992–1999 and find that the apparent stress is on average largest for strike‐slip events (0.70 MPa), while for reverse and normal events it is significantly smaller (0.15 and 0.25 MPa, respectively). These results support the mechanism dependence of Es reported by Choy and Boatwright [1995], although we find that once likely sources of bias are accounted for, the mechanism dependence is not as strong as found previously. The source of the mechanism dependence is unclear, but one possibility is that it reflects a mechanism‐dependent difference in the stress drop. This hypothesis is suggested by the scaling of slip with width in large strike‐slip earthquakes and makes two predictions, which could be used to test it. The first is that the discrepancy should disappear for the very largest dip‐slip earthquakes as the length of the fault greatly exceeds the downdip extent. The second is that the discrepancy ought to disappear for smaller earthquakes. The first can not yet be tested due to a lack of recent, very large dip‐slip earthquakes. The second is supported by the lack of mechanism dependence to Es for smaller earthquakes. An alternative hypothesis is that the apparent mechanism dependence could result if faults are opaque during rupture, blocking seismic radiation across them [Brune, 1996]. This could cause radiated seismic energy to be trapped preferentially in the crust near the source volume for dipping faults. There remains, however, a large discrepancy between estimates of Es obtained from teleseismic versus regional data. This discrepancy indicates a problem with teleseismic and/or regional estimates of the seismic energy and must be resolved before a definite conclusion can be drawn.
Pressure‐volume‐temperature equation of state of MgSiO3 perovskite from molecular dynamics and constraints on lower mantle composition
Authors: Marton Frederic C. , Ita Joel , Cohen Ronald E.
Published: February 2004

The composition of the lower mantle can be investigated by examining densities and seismic velocities of compositional models as functions of depth. In order to do this it is necessary to know the volumes and thermoelastic properties of the compositional constituents under lower mantle conditions. We determined the thermal equation of state (EOS) of MgSiO3 perovskite using the nonempirical variational induced breathing (VIB) interatomic potential with molecular dynamics simulations at pressures and temperatures of the lower mantle. We fit our pressure‐volume‐temperature results to a thermal EOS of the form P(V, T) = P0(V, T0) + ΔPth(T), where T0 = 300 K and P0 is the isothermal Universal EOS. The thermal pressure ΔPth can be represented by a linear relationship ΔPth = a + bT. We find V0 = 165.40 Å3, KT0 = 273 GPa, KT0′ = 3.86, a = −1.99 GPa, and b = 0.00664 GPa K−1 for pressures of 0–140 GPa and temperatures of 300–3000 K. By fixing V0 to the experimentally determined value of 162.49 Å3 and calculating density and bulk sound velocity profiles along a lower mantle geotherm we find that the lower mantle cannot consist solely of (Mg,Fe)SiO3 perovskite with XMg ranging from 0.9–1.0. Using pyrolitic compositions of 67 vol % perovskite (XMg = 0.93–0.96) and 33 vol % magnesiowüstite (XMg = 0.82–0.86), however, we obtained density and velocity profiles that are in excellent agreement with seismological models for a reasonable geotherm.
Contrast in crustal structure across the Clipperton transform fault from travel time tomography
Authors: Van Avendonk H. J. A. , Harding A. J. , Orcutt J. A. , McClain J. S.
Published: February 2004

A three‐dimensional (3‐D) seismic refraction study of the Clipperton transform fault, northern East Pacific Rise, reveals anomalously low compressional velocities from the seafloor to the Moho. We attribute this low‐velocity anomaly to intensive brittle deformation, caused by transpression across this active strike‐slip plate boundary. The seismic velocity structure south of the Clipperton transform appears unaffected by these tectonic forces, but to the north, seismic velocities are reduced over 10 km outside the zone of sheared seafloor. This contrast in seismic velocity structure corresponds well with the differences in mid‐ocean ridge morphology across the Clipperton transform. We conclude that the amount of fracturing of the upper crust, which largely controls seismic velocity variations, is strongly dependent on the shallow temperature structure at the ridge axis. Intermittent supply of magma to the shallow crust north of the Clipperton transform allows seawater to penetrate deeper, and the cooler crust is brittle to a greater depth than south of the transform, where a steady state magma lens is known to exist. The crustal thickness averages 5.7 km, only slightly thinner than normal for oceanic crust, and variations in Moho depth in excess of ∼0.3 km are not required by our data. The absence of large crustal thickness variations and the general similarity in seismic structure imply that a steady state magma lens is not required to form normal East Pacific Rise type crust. Perhaps a significant portion of the lower crust is accreted in situ from a patchwork of short‐lived gabbro sills or from ductile flow from a basal magma chamber as has been postulated in some recent ophiolite studies.
Three‐dimensional S wave velocity structure and Vp/Vs ratios in the New Madrid Seismic Zone
Authors: Vlahovic Gordana , Powell Christine A.
Published: February 2004

A three‐dimensional S wave velocity model for the New Madrid Seismic Zone (NMSZ) has been developed using nonlinear travel time tomography. The inversion utilized 5544 S wave arrival times from 720 earthquakes recorded by digital, three‐component stations deployed in the NMSZ over the time period 1989 through 1992. We imaged S wave velocity anomalies ranging from −5% to +8% relative to the starting one‐dimensional velocity model. Lowest S wave velocities are found south of Ridgley, Tennessee, in an area characterized by a high earthquake swarm rate and shallower than normal hypocenters. Two centers of higher than average S wave velocity are located west of the Mississippi river, north of Caruthersville, Missouri. The S wave model is similar to a P wave velocity model generated using the same earthquake data set. The similarity in ray coverage in both the P and S wave solutions allowed calculation of Vp/Vs ratios. Most of the seismicity in the NW trending central arm of the NMSZ is associated with normal Vp/Vs values that border regions with high Vp/Vs. North of Ridgely, high Vp/Vs values are associated with higher than average compressional and shear wave velocities and are interpreted to be due to mafic intrusions along the axis and edges of the Reelfoot rift. SE of Ridgley, the end of the central arm coincides with high Vp/Vs values that are due to a significant shear wave low‐velocity zone and are interpreted to be the result of highly fractured and fluid saturated crust.
Seasonal and interannual variability of the Odden ice tongue and a study of environmental effects
Authors: Comiso Josefino C. , Wadhams Peter , Pedersen Leif Toudal , Gersten Robert A.
Published: February 2004

The Odden ice tongue is an ice cover phenomenon in the Greenland Sea that covers all or part of the area of influence of the Jan Mayen Current and that encloses a bay of open water called Nordbukta. The yearly attributes of the spatial distribution of the Odden from 1979 through 1998 have been quantified in terms of average concentration, standard deviation of ice concentrations, persistence, maximum extent, and anomalies in extent, and the results show vastly different formation characteristics and seasonalities during different years. The monthly average extents of the Odden are shown to have a strong negative correlation with monthly surface air temperatures recorded at Jan Mayen Island and monthly temperatures retrieved from satellite infrared data, with correlation coefficients of 0.74 and 0.89, respectively. Wind effect is also shown to be an important factor at daily and shorter timescales, directly influencing size and shape and sometimes initiating the formation of Nordbukta. The Odden was most extensive in 1979, 1982, 1986, and 1997 and most persistent in 1988, 1989, and 1997 but did not appear in 1984, 1994, and 1995, suggesting decadal periodicity for the 20 year period. With the use of the 75 year temperature record at Jan Mayen Island as a proxy for Odden size, spectral analysis also shows a periodicity of 10–12 years. A correlation study of North Atlantic Oscillation (NAO) indices with extents of the Odden, as derived from satellite data, yields a correlation coefficient of 0.4, which suggests that the interannual variability of the Odden may be influenced but not controlled by NAO. Regression analysis of the Jan Mayen surface temperature record also yields a slight warming (0.06 K decade−1) and a decrease in Odden area (−0.14×104 km2 decade−1) from 1979 to 1996, but during the past 75 years, there has actually been a slight cooling (−0.15 K decade−1), suggesting either that the Odden had actually been smaller in size several decades ago or that it had occurred farther north.
Breaking probabilities for dominant surface waves on water of finite constant depth
Authors: Babanin Alexander V. , Young Ian R. , Banner Michael L.
Published: February 2004

This paper extends our previous study of the breaking probability of dominant deep water gravity surface waves into the finite water depth environment. It reports a unified behavior of the mean breaking statistics once the effects of finite water depth are taken into account. The shallow water wave data that form the basis of this study were acquired at a field experiment site at Lake George, New South Wales, Australia. The breaking events were detected through visual observation of videotaped records of the wave field in combination with acoustic signatures of the breaking waves from a collocated hydrophone. Following Banner et al. [2000], we argue that when constant finite depth bottom influence is operative, nonlinear hydrodynamical effects associated with energy and momentum fluxes within deforming wave groups remain the primary determinant of breaking onset. This underpins our proposed finite depth water parameterization for the environmental dependence of dominant wave breaking probability, given by the average number of breakers passing a fixed point per dominant wave period. The additional influence of bottom interaction with the wind drift current shear and wind forcing are also included in our finite constant depth formulation. This is a natural extension of our recently proposed deep water dependence and reduces to it as the significant wave height becomes much smaller than the water depth. In common with the deep water case we propose that there exists a threshold of the significant peak steepness below which negligible dominant wave breaking occurs. The available data show encouraging agreement with our proposed dependence, with a correlation coefficient approaching 0.9.
Gravity current structure of the Chesapeake Bay outflow plume
Authors: Marmorino G. O. , Trump C. L.
Published: February 2004

Measurements made across the edge of the Chesapeake Bay outflow plume in September 1996 are examined for evidence of the frontal head structure expected for a buoyant gravity current. Shipboard acoustic Doppler current profiler and hydrographic data from two distinct, 2‐hour‐long sets of frontal transects are analyzed. One set of observations was acquired just south of the bay mouth (near Cape Henry, Virginia) during late flood tide in water depth of 8 m. These measurements show a nearly steady frontal structure and a frontal head that is approximately 5‐m deep and 20‐m wide. The second data set was acquired about 20‐km southeast of the bay mouth, where the plume bulges horizontally outward over the continental shelf. These measurements show a 9‐m‐deep frontal head (in water depth of 18 m) that ultimately dissipates during early flood tide. In both sets of observations, relative inflow of plume water into the frontal head is confined to a shallow surface layer estimated to be no more than about 2‐m deep. Referenced to this layer, frontal propagation is supercritical (Froude number greater than 1). Also measured was an expected sinking motion (of the order of 3 cm/s) along the front. Overall, the results are consistent with previous measurements made across smaller buoyant plumes and in the laboratory, and they are compatible with the use of fixed values of Froude number and entrainment parameter in two‐layer plume models. Additional study of the plume front during ebb tide is recommended.
Mixing in the meandering Kuroshio Extension and the formation of North Pacific Intermediate Water
Authors: Joyce T. M. , Yasuda I. , Hiroe Y. , Komatsu K. , Kawasaki K. , Bahr F.
Published: February 2004

Synoptic hydrographic and velocity data were obtained in the first meander crest and trough of the Kuroshio Extension (KE) from measurements aboard the R/V Soyo‐maru in late May and early June 1998. Measurements indicated that the eastward flowing KE had significant (up to 20 cm s−1) flow at 1400 m depth and that a subsurface velocity maximum could be seen on the equatorward side of the current. Large salinity differences were found on potential densities from 26.2 to 27.2 kg m−3 that reflected cold, fresh waters of northern origin and warm, salty subtropical waters. Approaching the first meander crest, northward moving water was saltier than southward moving water at the same density on the downstream side of the crest, approaching the first trough. The velocity‐salinity covariance was high and consistent with an eddy diffusivity of order 104 m2 s−2. With an assumption of quasi‐steady meanders, supported by altimeter results, density advection can be used to infer vertical velocities of 50–100 m d−1 in the meanders. Ascending water was found to be saltier than the descending water, especially in a potential density range about 26.8 kg m−3 within the core of North Pacific Intermediate Water (NPIW)). However, the two patterns of horizontal velocity‐salinity covariance and vertical velocity‐salinity covariance were not consistent with one another. The mixing of Oyashio and Kuroshio source waters, thought to be critical to the formation of NPIW, is clearly tied to cross‐frontal flows within the meandering KE, with Kuroshio water ejected northward as water parcels approach the first meander crest and Oyashio water injected southward into the flow as parcels approach the first trough.
Ocean circulation and ice‐ocean interaction beneath the Amery Ice Shelf, Antarctica
Authors: Williams Michael J. M. , Grosfeld Klaus , Warner Roland C. , Gerdes Rüdiger , Determann Jürgen
Published: February 2004

Simulations of the ocean dynamics in the cavity under the Amery Ice Shelf, Antarctica, were carried out using a three‐dimensional numerical ocean model. Two different boundary conditions were used to describe the open ocean barotropic exchange at the ice front. The simulations show that the circulation in the ocean cavity is predominantly barotropic and is generally steered by the cavity topography. The circulation is driven by the density gradient in the cavity, which is strongly influenced by the heat and salt fluxes from melting and freezing processes at the ice‐ocean interface, and by the horizontal exchange of heat and salt across the open ocean boundary at the ice front. The interaction at the ice‐ocean interface allows the basal component of the mass loss of the Amery Ice Shelf to be estimated. In the two simulations the computed losses were 5.8 Gt yr−1 and 18.0 Gt yr−1, values consistent with observations. The bulk of the melting occurred near the southern grounding line of the ice shelf, although substantial melting also occurred in areas where heat transport by horizontal circulation was large. Accretion was restricted to areas where water, from upstream melting, became supercooled as it ascended the ice shelf base.
Budgets of turbulent kinetic energy and scalar variance in the continental shelf bottom boundary layer
Authors: Shaw W. J. , Trowbridge J. H. , Williams A. J.
Published: February 2004

A local turbulent kinetic energy (TKE) budget, in which dissipation balances production minus buoyancy flux, and a nonlocal turbulent scalar variance (TSV) budget, in which dissipation balances production minus TSV flux divergence, are tested observationally. The calculations are based on 6 week long records of velocity and sound speed measured between 0.7 and 5.4 m above bottom on the New England shelf. Estimates of fluxes and gradients are direct, while estimates of dissipation rates are indirectly obtained from inertial subrange spectra. Within 2 m of the bottom, buoyancy flux is not an important component of the TKE budget (flux Richardson number Rf<0.05, on average). At 4.35 m above the bottom, the highest resolved elevation, the direct effect of stratification is more pronounced (Rf = 0.13, on average). The local TKE balance is essentially closed by the measurements within 2 m of the bed (r2>0.8 and regression coefficient = 0.89 ×/÷ 1.06 and 1.08 ×/÷ 1.05 at 1.65 and 0.74 m above the bottom, respectively). The local TKE budget at 4.35 m above the bed is not closed, however. TSV production and dissipation estimates are strongly correlated at all heights (r2>0.64), but dissipation is systematically greater than production, particularly at 0.74 m above the bed (regression coefficient = 2.42 ×/÷ 1.28). Including TSV flux divergence in the balance reduces, statistically significantly, the observed discrepancy. The implications of the results for turbulence closure models and microstructure diffusivity estimates are discussed.
Departures from the local inverse barometer model observed in altimeter and tide gauge data and in a global barotropic numerical model
Authors: Mathers E. L. , Woodworth P. L.
Published: February 2004

Sea level data from the TOPEX/Poseidon altimeter satellite and from subsets of the global tide gauge and bottom pressure recorder networks have been employed to study the worldwide distribution of departures from the simple local inverse barometer (LIB) model. From a combination of measurement techniques the LIB model was found to provide a reasonable representation of the data (within 5–10%) outside of the tropics for timescales from several days to several weeks for most locations. However, several parts of the Southern Ocean and other higher‐latitude areas were seen from the altimetry to contain significant departures across the range of timescales accessible for study by the technique. Within the tropics the response of sea level to air pressure forcing was observed to be typically 50% of that of the LIB model at timescales of several days, with considerable spatial variation, and to approximate the LIB model more closely at timescales of several weeks. A coarse‐grid global barotropic numerical model was found to reproduce qualitatively the global average response as a function of frequency, to simulate many of the characteristics of the spatial variation of the response, and to demonstrate the relative importance of air pressure and wind forcing at different timescales. Data sets from the Permanent Service for Mean Sea Level and U.K. Meteorological Office were used to investigate LIB departures at intraseasonal and longer timescales along continental coastlines and at ocean islands. Increasingly large departures were observed along many parts of the world coastline as timescales increased from months to seasons.
Geochemistry of suspended particles in the Subantarctic and Polar Frontal zones south of Australia: Constraints on export and advection processes
Authors: Cardinal Damien , Dehairs Frank , Cattaldo Thierry , André Luc
Published: February 2004

Filtered particles from a Southern Ocean meridional transect (from 42°S to 54°S along 142°E) off Tasmania were studied between surface and 600‐m water depth (SAZ 1998 expedition, February‐April 1998). Particulate Ba, Sr, Ca, P, U, Th, Ti, Al, Mn, light rare earth elements, and Zr contents were analyzed by inductively coupled plasma‐mass spectrometry and inductively coupled plasma‐atomic emission spectroscopy. Mesopelagic Baxs (total Ba minus a lithogenic fraction estimated from Al) contents were higher within the Polar Front Zone (PFZ) in comparison with the northern areas, indicating that C export to the intermediate and deep ocean is more important in the PFZ. Average surface Uxs versus latitude covaries with mesopelagic Baxs, contrasting with organic matter proxies such as PP and chlorophyll a. This similar behavior suggests that U might be preferentially adsorbed on organic material, which subsequently becomes involved with barite precipitation. Srp and Cap contents decrease sharply with depth, following total particle content. For the northern area (42°–47°S), surface Ce anomaly reflects essentially a lithogenic signature (0.6<Ce/Ce*<1) ascribed to the supply from the Tasmanian continental shelf. Suspended matter from surface waters at stations south of 47°S shows an enhanced seawater signature (0.3<Ce/Ce*<0.6). In all stations, Ce anomalies increase with depth, toward values close to 1. This is likely due to the increase of lithogenic/authigenic ratio of particles with depth, rather than redox processes. Finally, an advective event is identified at 45°S below 200 m by a strong increase of lithogenic element contents, which affects Baxs.
Wind‐driven transport pathways for Eurasian Arctic river discharge
Authors: Guay Christopher K. H. , Falkner Kelly Kenison , Muench Robin D. , Mensch Manfred , Frank Markus , Bayer Reinhold
Published: February 2004

Distributions of temperature, salinity, and barium in near‐surface waters (depth ≤ 50 m) of the Laptev Sea and adjacent areas of the Arctic Ocean are presented for the summers of 1993, 1995, and 1996. The tracer data indicate that while fluvial discharge was largely confined to the shelf region of the Laptev Sea in the summer of 1993, surface waters containing a significant fluvial component extended beyond the shelf break and over the slope and basin areas north of the Laptev Sea in the summers of 1995 and 1996. These distributions of fluvial discharge are consistent with local winds and suggest two principal pathways by which river waters can enter the central Arctic basins from the Laptev Sea. When southerly to southeasterly wind conditions prevail, river waters are transported northward beyond the shelf break and over the slope and adjacent basin areas. These waters can then enter the interior Arctic Ocean via upper layer flow in the vicinity of the Lomonosov Ridge. Under other wind conditions, river waters are steered primarily along the inner Laptev shelf and into the East Siberian Sea as part of the predominantly eastward coastal current system. These waters then appear to cross the shelf and enter the interior Arctic Ocean via upper layer flow aligned roughly along the Mendeleyev Ridge. The extent to which either pathway is favored in a given year is largely determined by local wind patterns during the summer months, when fluvial discharge is greatest and shelf waters are at the lowest salinity of their annual cycle.
Hydraulics and mixing in controlled exchange flows
Authors: Hogg Andrew M. , Ivey Gregory N. , Winters Kraig B.
Published: February 2004

Numerical simulations of bidirectional density‐driven exchange flows are used to study the effects of turbulent mixing in these flows. The numerical experiments are designed so that it is possible to specify the intensity of mixing, which allows the investigation of a wide range of flows that are difficult to model in the laboratory. The simulated flows are compared to two analytical solutions, first, the two‐layer hydraulic solution which has no mixing, and second, a solution in which turbulent mixing dominates the flow. We are able to model exchange flows similar to either of these limits by modifying the turbulent mixing, as well as simulating behavior between these two extremes. The simulations demonstrate that the two analytical solutions form the limits of a wide class of problems and that the flow regime in between the limiting solutions can be described by a single nondimensional parameter GrTA2.
Bubble transport in rip currents
Authors: Vagle Svein , Farmer David M. , Deane Grant B.
Published: February 2004

A number of remote and in situ acoustical sensors were mounted in the surf zone off Scripps Pier, La Jolla, California, during March 1997 and used to characterize rip currents and their ability to transport bubbles offshore from the surf zone. The bubble size distributions and air fractions at the injection point were measured using an acoustical resonator and a conductivity sensor right in the surf zone. The offshore moving bubble plume was tracked with a multibeam 100 kHz Doppler sidescan sonar, giving the spatial extent and offshore variability in the rip events as well as information about the surface wave field. Furthermore, three acoustical resonators were mounted at 2.5 m depth 55–60 m farther offshore to probe the bubble field 300–400 s after injection. These results are interpreted using models that include bubble buoyancy and dissolution, rip current advection, and the turbulent boundary layer, which is represented by the Christoffersen and Jonsson [1985] formulation.
Export of Weddell Sea deep and bottom water
Authors: Gordon Arnold L. , Visbeck Martin , Huber Bruce
Published: February 2004

An extensive set of conductivity‐temperature‐depth (CTD)/lowered acoustic Doppler current profiler (LADCP) data obtained within the northwestern Weddell Sea in August 1997 characterizes the dense water outflow from the Weddell Sea and overflow into the Scotia Sea. Along the outer rim of the Weddell Gyre, there is a stream of relatively low salinity, high oxygen Weddell Sea Deep Water (defined as water between 0° and −0.7°C), constituting a more ventilated form of this water mass than that found farther within the gyre. Its enhanced ventilation is due to injection of relatively low salinity shelf water found near the northern extreme of Antarctic Peninsula's Weddell Sea shelf, shelf water too buoyant to descend to the deep‐sea floor. The more ventilated form of Weddell Sea Deep Water flows northward along the eastern side of the South Orkney Plateau, passing into the Scotia Sea rather than continuing along an eastward path in the northern Weddell Sea. Weddell Sea Bottom Water also exhibits two forms: a low‐salinity, better oxygenated component confined to the outer rim of the Weddell Gyre, and a more saline, less oxygenated component observed farther into the gyre. The more saline Weddell Sea Bottom Water is derived from the southwestern Weddell Sea, where high‐salinity shelf water is abundant. The less saline Weddell Sea Bottom Water, like the more ventilated Weddell Sea Deep Water, is derived from lower‐salinity shelf water at a point farther north along the Antarctic Peninsula. Transports of Weddell Sea Deep and Bottom Water masses crossing 44°W estimated from one LADCP survey are 25×106 and 5×106 m3 s−1, respectively. The low‐salinity, better ventilated forms of Weddell Sea Deep and Bottom Water flowing along the outer rim of the Weddell Gyre have the position and depth range that would lead to overflow of the topographic confines of the Weddell Basin, whereas the more saline forms may be forced to recirculate within the Weddell Gyre.
Altimeter observations of the Bering Slope Current eddy field
Authors: Okkonen Stephen R.
Published: February 2004

Topex altimeter data is analyzed to describe the mesoscale eddy field associated with the Bering Slope Current in the central Bering Sea. The observed eddy field is well defined, seasonally modulated, and interannually variable. The most energetic mesoscale eddy activity typically occurs during the spring and summer months and is characterized by a peak along‐isobath wavelength of 212 km and a representative wave period of ∼4 months. Spatial variability of annual period phase indicates that the axis of Bering Slope Current exhibits seasonal offshore migration.
A numerical study of wave‐current interaction through surface and bottom stresses: Wind‐driven circulation in the South Atlantic Bight under uniform winds
Authors: Xie Lian , Wu Kejian , Pietrafesa Leonard , Zhang Chen
Published: February 2004

The influences of surface waves on ocean currents in the coastal waters of the South Atlantic Bight are investigated by using a coupled wave‐current modeling system. The ocean circulation model employed is the three‐dimensional Princeton Ocean Model (POM), and the wave model invoked is an improved third‐generation wave model (WAM). The coupling procedure between the POM and the WAM and the simulated coastal ocean circulation driven by uniform surface winds are presented. The simulated results show that wind waves can significantly affect coastal ocean currents not only through an enhancement of wind stress but also through a modification of bottom stress. Wave‐induced wind stress increases the magnitude of currents both at the surface and near the seabed. On the other hand, wave‐induced bottom stress weakens the currents both at the sea surface and near the seabed. Therefore the net effect of surface wind waves on currents depends on the relative importance of current modulations by wave‐induced wind stress and bottom stress. The results further indicate that at a fixed location, the relative importance of wave‐induced surface and bottom shear stresses in coastal ocean circulation depends on the surface wind field. For the constant wind cases considered in this study, the effect of wave‐induced bottom stress is more significant in along‐shore wind conditions than in cross‐shore wind conditions.
The influence of an alongshelf current on the formation and offshore transport of dense water from a coastal polynya
Authors: Chapman David C.
Published: February 2004

The influence of an ambient alongshelf current on the formation and offshore transport of dense water from an idealized coastal polynya is examined by using a primitive‐equation numerical model. Brine rejection during ice formation within the polynya is modeled by a prescribed surface buoyancy flux over a limited region along a straight coastline. A uniform alongshelf current is imposed at one end of the model domain. The basic ocean response is unchanged by the ambient current; i.e., an unstable density front forms around the polynya perimeter and breaks up into small‐scale (15–30 km) eddies that carry the dense water away from the polynya. The ambient current continuously carries water out of the polynya region, thereby both increasing the alongshelf density flux and reducing the typical density anomaly generated. The total volume of dense water formed is not appreciably altered. The influence of a submarine canyon located downstream of the polynya (in the direction of the imposed alongshelf current) is examined. Almost no dense water enters a canyon oriented normally or diagonally to the coast because the ambient flow tends to follow isobaths and carries virtually all of the dense water around and past the canyon. An appreciable fraction of densified water may flow down a canyon oriented parallel to the coast, the details depending on the canyon geometry and the strength of the ambient current. Thus both the density anomalies produced by coastal polynyas and the pathways of dense water transport are sensitive to ambient shelf currents, potentially providing a source of interannual variability in dense water formation and export from the shelf.
Moored sediment trap measurements of carbon export in the Subantarctic and Polar Frontal zones of the Southern Ocean, south of Australia
Authors: Trull T. W. , Bray S. G. , Manganini S. J. , Honjo S. , François R.
Published: February 2004

Sediment trap moorings were deployed from September 21, 1997 through February 21, 1998 at three locations south of Australia along 140°E: at ∼47°S in the central Subantarctic Zone (SAZ) with traps at 1060, 2050, and 3850 m depth, at ∼51°S in the Subantarctic Front with one trap at 3080 m, and at ∼54°S in the Polar Frontal Zone (PFZ) with traps at 830 and 1580 m. Particle fluxes were high at all the sites (18–32 g m−2 yr−1 total mass and 0.5–1.4 g organic carbon m−2 yr−1 at −1000 m, assuming minimal flux outside the sampled summer period). These values are similar to other Southern Ocean results and to the median estimated for the global ocean by Lampitt and Antia [1997], and emphasize that the Southern Ocean exports considerable carbon to the deep sea despite its “high‐nutrient, low chlorophyll” characteristics. The SAZ site was dominated by carbonate (>50% of total mass) and the PFZ site by biogenic silica (>50% of total mass). Both sites exhibited high export in spring and late summer, with an intervening low flux period in December. For the 153 day collection period, particulate organic carbon export was somewhat higher in all the traps in the SAZ (range 0.57–0.84 gC m−2) than in the PFZ (range 0.31–0.53), with an intermediate value observed at the SAF (0.60). The fraction of surface organic carbon export (estimated from seasonal nutrient depletion, Lourey and Trull [2001]) reaching 1000 m was indistinguishable in the SAZ and PFZ, despite different algal communities.
Modeling salinity distributions in the Indian Ocean
Authors: Han Weiqing , McCreary Julian P.
Published: February 2004

A 4½‐layer model with active thermodynamics and mixed‐layer physics is used to isolate effects on salinity distributions in the Indian Ocean that result from various forcing mechanisms. These forcings include evaporation (ε) and precipitation (P) through the ocean surface and inflows across basin boundaries by river runoff in the Bay of Bengal, the Indonesian Throughflow, the Persian Gulf, and the Red Sea. A suite of solutions is obtained in which each forcing is added sequentially. In the solution forced only by P−ε, salinity patterns in the upper three layers agree qualitatively with the observations, but values tend to be higher throughout most of the basin. When river inflow into the Bay of Bengal is included, salinity values are significantly improved in the upper three layers, especially in the northern Bay and along the east and west coasts of India. In addition, solutions suggest that during the Northeast Monsoon part of the river water flows out of the Bay in the shallow channel between Sri Lanka and India: Only when this channel is opened in the upper layer do solutions develop a strong, across‐shelf salinity gradient along the west Indian coast, consistent with the observations. When the Indonesian Throughflow is added, salinities are lowered in all four layers, especially in the southern tropical ocean. Most of the Throughflow eventually flows out of the Indian Ocean along the western boundary and near Madagascar, but some is advected across the equator by the East African Coastal Current, where it is carried eastward and northward into the central Arabian Sea. Saltier Persian Gulf Water is introduced into the model in layer 3. Some of it subsequently entrains into the surface mixed layer, increasing sea surface salinity by 0.1–0.2 practical salinity units (psu) in a broad region of the Arabian Sea. Saltier Red Sea Water enters the basin in layer 4. It increases layer 4 salinity values throughout much of the Indian Ocean, particularly in the Somali Basin, the interior Arabian Sea, and the central and western equatorial ocean.
Ice shelf advance and retreat rates along the coast of Queen Maud Land, Antarctica
Authors: Kim K. T. , Jezek K. C. , Sohn H. G.
Published: February 2004

We mapped ice shelf margins along the Queen Maud Land coast, Antarctica, in a study of ice shelf margin variability over time. Our objective was to determine the behavior of ice shelves at similar latitudes but different longitudes relative to ice shelves that are dramatically retreating along the Antarctic Peninsula, possibly in response to changing global climate. We measured coastline positions from 1963 satellite reconnaissance photography and 1997 RADARSAT synthetic aperture radar image data for comparison with coastlines inferred by other researchers who used Landsat data from the mid‐1970s. We show that these ice shelves lost ∼6.8% of their total area between 1963 and 1997. Most of the areal reduction occurred between 1963 and the mid‐1970s. Since then, ice margin positions have stabilized or even readvanced. We conclude that these ice shelves are in a near‐equilibrium state with the coastal environment.
A persistent subsurface chlorophyll maximum in the Interpolar Frontal Zone south of Australia: Seasonal progression and implications for phytoplankton‐light‐nutrient interactions
Authors: Parslow John S. , Boyd Philip W. , Rintoul Stephen R. , Griffiths F. Brian
Published: February 2004

A subsurface chlorophyll maximum (SCM), composed principally of large diatoms, has been consistently observed in summer and autumn between 53° and 58°S along 140°E on Australian World Ocean Circulation Experiment and Joint Global Ocean Flux Study cruises from 1994 to 1998. This region lies in a zone of weak northward flow, between two branches of the Polar Front (hence the Interpolar Frontal Zone (IPFZ)). In the IPFZ, mixed layer nitrate concentrations are high (>24 μM) year‐round, while mixed layer silicic acid is intermediate (about 15 μM) in winter but is depleted to 2 μM or less in late summer. Dissolved Fe concentrations, only available for summer, are low (<0.2 nM). Mixed layer chlorophyll concentrations are generally <0.3 μg L−1 and surface waters thus fit the high‐nitrate low‐chlorophyll (HNLC) definition. In spring and early summer, the SCM is relatively shallow (about 60 m), intense (up to 1.5 μg chl a, L−1), and contributes 30–50% of column production. By March the SCM is deep (100 m or greater), less intense (about 0.5 μg chl a L−1), and contributes at most 20% of column production. The existence of a SCM in a HNLC region is surprising, and we consider a number of possible explanations. The SCM may be partly explained by changes in C:chl a ratios, but phytoplankton species composition in the SCM also differs from that in the mixed layer. Sinking of diatoms, mediated by Fe and/or silicic acid availability, appears to play an important role in the formation and maintenance of the SCM.
Deterministic modelling of driving and dissipation for ocean surface gravity waves
Authors: Willemsen Jorge F.
Published: February 2004

Consider two questions. What qualitative features should a spectrum of wind driven wave amplitudes possess in deep water? Then, is it possible to compute such a spectrum ab initio? In answer to the first question, at the least the spectrum should exhibit a spectral peak determined by the acting wind; an asymptotic power law tail; and an angular dependence between the dominant wind and wave directions. The principal result of this paper is that a model to calculate a wind‐driven sea spectrum that satisfies the first two requirements starting from a broad suite of initial conditions has been constructed and exercised. To this end, wind driving mechanisms and models for dissipation caused by wave breaking are investigated. This study does not include detailed hydrodynamic calculations, but rather an evaluation of physically plausible model interaction terms. These are appended to Hamilton's equations for a wave field in deep water. This methodology leads to deterministic ordinary differential equations for the evolution of the wave field in which three and four wave nonlinear interactions are incorporated. The deterministic form of the equations is preserved through the introduction of nonstochastic driving and dissipation terms. The time evolution results presented fulfill the qualitative expectations desired for the spectrum. The calculations also yield much more information. Full phase information is retained. The relative magnitudes of the nonlinear interaction terms may be assessed as functions of time. The same applies for the magnitudes of the driving and dissipating terms. This information will be used to improve the model to where it is ready to confront experimental data.
Accumulation and uptake of anthropogenic CO2 in the Southern Ocean, south of Australia between 1968 and 1996
Authors: McNeil Ben I. , Tilbrook Bronte , Matear Richard J.
Published: February 2004

Increases in the anthropogenic CO2 inventory for Southern Ocean waters to the south of Australia were estimated by comparing measurements made 28 years apart in 1968 and 1996. For this period, the deepest penetration of anthropogenic CO2 is consistent with the depth of 28 year old water from CFC‐11 age estimates. Significant accumulation of anthropogenic CO2 (13±10 μmol kg−1) was found in Antarctic Bottom Water (AABW) from 500–4500 m south of 58°S suggesting AABW to be an important water mass for the storage of anthropogenic CO2 in the deep ocean. Deep penetration of anthropogenic CO2 (<1900 m) was found in the sub‐Antarctic zone (SAZ) between 48°S and 51°S while little accumulation (<400 m) was observed south of 53°S. A simple one dimensional box model was used to investigate the anthropogenic CO2 uptake potential in the SAZ based on observed CFC‐11 concentrations and the calculated anthropogenic CO2 inventory. Formation and export of sub‐Antarctic Mode Water (SAMW) was estimated by reproducing the observed present day SAMW CFC‐11 concentration using time histories of atmospheric CFC‐11 concentrations. The anthropogenic CO2 uptake for the SAZ (45°–50°S) ranged from 0.73 to 0.86 μmol kg yr−1. The calculated range of SAMW formation and export rates only increased the anthropogenic CO2 uptake potential for the SAZ by as much as 18% (<0.13 μmol kg−1 yr−1). Extrapolating our estimate to the circumpolar SAZ gives an anthropogenic CO2 uptake of 0.07–0.08 GTC yr−1. Even though SAMW formation does not considerably enhance anthropogenic CO2 uptake in the SAZ, it provides an important mechanism for transporting anthropogenic CO2 into the ocean interior. SAMW therefore may be important for anthropogenic CO2 uptake on a larger regional scale.
Control of phytoplankton growth by iron and silicic acid availability in the subantarctic Southern Ocean: Experimental results from the SAZ Project
Authors: Hutchins D. A. , Sedwick P. N. , DiTullio G. R. , Boyd P. W. , Quéguiner B. , Griffiths F. B. , Crossley C.
Published: February 2004

Subantarctic Southern Ocean surface waters in the austral summer and autumn are characterized by high concentrations of nitrate and phosphate but low concentrations of dissolved iron (Fe, ∼0.05 nM) and silicic acid (Si, <1 μM). During the Subantarctic Zone AU9706 cruise in March 1998 we investigated the relative importance of Fe and Si in controlling phytoplankton growth and species composition at a station within the subantarctic water mass (46.8°S, 142°E) using shipboard bottle incubation experiments. Treatments included unamended controls; 1.9 nM added iron (+Fe); 9 μM added silicic acid (+Si); and 1.9 nM added iron plus 9 μM added silicic acid (+Fe+Si). We followed a detailed set of biological and biogeochemical parameters over 8 days. Fe added alone clearly increased community growth rates and nitrate drawdown and altered algal community composition relative to control treatments. Surprisingly, small, lightly silicified pennate diatoms grew when Fe was added either with or without Si, despite the extremely low ambient silicic acid concentrations. Pigment analyses suggest that lightly silicified chrysophytes (type 4 haptophytes) may have preferentially responded to Si added either with or without Fe. However, for many of the parameters measured the +Fe+Si treatments showed large increases relative to both the +Fe and +Si treatments. Our results suggest that iron is the proximate limiting nutrient for chlorophyll production, photosynthetic efficiency, nitrate drawdown, and diatom growth, but that Si also exerts considerable control over algal growth and species composition. Both nutrients together are needed to elicit a maximum growth response, suggesting that both Fe and Si play important roles in structuring the subantarctic phytoplankton community.
Control of phytoplankton growth by iron supply and irradiance in the subantarctic Southern Ocean: Experimental results from the SAZ Project
Authors: Boyd P. W. , Crossley A. C. , DiTullio G. R. , Griffiths F. B. , Hutchins D. A. , Queguiner B. , Sedwick P. N. , Trull T. W.
Published: February 2004

The influence of irradiance and iron (Fe) supply on phytoplankton processes was investigated, north (47°S, 142°E) and south (54°S, 142°E) of the Subantarctic Front in austral autumn (March 1998). At both sites, resident cells exhibited nutrient stress (Fυ/Fm<0.3). Shipboard perturbation experiments examined two light (mean in situ and elevated) and two Fe (nominally 0.5 and 3 nM) treatments under silicic acid‐replete conditions. Mean in situ light levels (derived from incident irradiances, mixed layer depths (MLDs), wind stress, and a published vertical mixing model) differed at the two sites, 25% of incident irradiance I0 at 47°S and 9% I0 at 54°S because of MLDs of 40 (47°S) and 90 m (54°S), when these stations were occupied. The greater MLD at 54°S is reflected by tenfold higher cellular chlorophyll a levels in the resident phytoplankton. In the 47°S experiment, chlorophyll a levels increased to >1 μg L−1 only in the high‐Fe treatments, regardless of irradiance levels, suggesting Fe limitation. This trend was also noted for cell abundances, silica production, and carbon fixation rates. In contrast, in the 54°S experiment there were increases in chlorophyll a (to >2 μg L−1), cell abundances, silica production, and carbon fixation only in the high‐light treatments to which Fe had been added, suggesting that Fe and irradiance limit algal growth rates. Irradiance by altering algal Fe quotas is a key determinant of algal growth rate at 54°S (when silicic acid levels are nonlimiting); however, because of the integral nature of Fe/light colimitation and the restricted nature of the current data set, it was not possible to ascertain the relative contributions of Fe and irradiance to the control of phytoplankton growth. On the basis of a climatology of summer mean MLD for subantarctic (SA) waters south of Australia the 47° and 54°S sites appear to represent minimum and maximum MLDs, where Fe and Fe/irradiance, respectively, may limit/colimit algal growth. The implications for changes in the factors limiting algal growth with season in SA waters are discussed.