bluuebunny/crossref_metadata_2025 huggingface dataset:
Sr isotope geochemistry and hydrothermal alteration of the Oman ophiolite
Authors: Kawahata H. , Nohara M. , Ishizuka H. , Hasebe S. , Chiba H.
Published: February 2004

The Sr isotopic behavior in hydrothermally altered rocks of a complete section through the Wadi Fizh oceanic crust in the Oman ophiolite was investigated in order to evaluate seawater‐rock interaction through the oceanic crust. On the basis of rock‐types, secondary mineral assemblage, alteration temperature, average 87Sr/86Sr of hydrothermal fluids, and degree of alteration, the section is divided into the following 5 sequences: (1) mainly basalts and clay minerals, low alteration temperature, average 87Sr/86Sr 0.70634, and 78% degree of alteration; (2) mainly basalts and chlorite, clay minerals and calcite, fairly low alteration temperature, average 87Sr/86Sr 0.70584, and 76% degree of alteration; (3) basalts and sheeted dike diabase and prehnite‐actinolite facies, moderate alteration temperature, average 87Sr/86Sr 0.70519, and 85% degree of alteration; (4) dike diabase, plagiogranite, metagabbro and epidosite and prehnite‐free greenschist facies, high alteration temperature, average 87Sr/86Sr 0.70514, and 65% degree of alteration; and (5) noncumulate and cumulate gabbro and amphibolite facies, very high alteration temperature, average 87Sr/86Sr 0.70413, and 85% degree of alteration. Alteration (metamorphic) grade deduced from secondary mineral assemblages generally increases versus stratigraphic depth. Only 15% of the rocks have escaped hydrothermal alteration. The estimated strontium isotopic compositions of hydrothermal fluids are much more water‐dominated than those reported previously from modern and ophiolitic hydrothermal systems. On the basis of ideal model analysis we conclude that the whole sequence of oceanic crust in the Wadi Fizh section has experienced seawater recharge (downflow zone). The evidence suggests that the Wadi Fizh section was located close to a segment boundary of the oceanic crust along a spreading axis.
Source parameters and three‐dimensional attenuation structure from the inversion of microearthquake pulse width data: Qp imaging and inferences on the thermal state of the Campi Flegrei caldera (southern Italy)
Authors: de Lorenzo Salvatore , Zollo Aldo , Mongelli Francesco
Published: February 2004

The three‐dimensional P wave attenuation structure of the Campi Flegrei caldera and the estimate of source parameters for 87 local microearthquakes is obtained by the nonlinear inversion of pulse width and rise time measurements by using the method described by Zollo and de Lorenzo (this issue). Source radii represent the better resolved parameters with values ranging from 70 m to 230 m; the dip and strike angles defining fault orientations are usually affected by larger uncertainties and are well constrained only for 11 events. The dip fault is usually confined in the range 30°–60° (with an average uncertainty of 12°); the fault strikes mainly range between −60° and 60° and seem to define preferential directions oriented radially from the symmetry axis of the ground deformation. Stress drop estimates indicate rather low values (0.01–1 MPa) which suggest low strength properties of the incoherent and brittle materials filling the caldera (primarily yellow tuffs). The three‐dimensional Qp images obtained from the inversion of P pulse durations show two significant low‐Qp anomalies between 0 and 1 km of depth, in the north‐eastern sector and at 2–3 km of depth in the central eastern sector of the caldera. The high degree of spatial correlation of the low‐Qp zone and low‐Vs (as inferred by Aster and Meyer (1988)) at 0–1 km in depth and other geophysical and geochemical observations suggest that this anomaly can be related to the presence of densely fractured, porous, and fluid‐filled rocks in the NE sector of the caldera. The deeper low‐Qp anomaly is interpreted as being related to a dominant thermal effect. We used the surface and deep borehole temperature measurements available in the area to obtain a local calibration curve to convert Qp in temperature at Campi Flegrei. The retrieved T(Qp) map shows a high thermal deep disturbance (450°–500°C) at depths between 2 and 3 km in the eastern sector of the caldera, where the most recent eruptive activity is concentrated. The present‐day temperature field retrieved by Qp images has been interpreted by using a three‐dimensional thermal conduction model assuming an extended heat source (initial temperature of 800°C) located underneath the attenuation anomalous region. The results indicate that the Qp‐inferred temperature field can be related to the heat conduction effect of one or more molten bodies whose top should be at about 4‐km depth, consistent with recent seismic estimates of the magma chamber top at Campi Flegrei (Ferrucci et al., 1992). This study suggests that the present thermal state and rock rheology of the inner caldera could be controlled by the cooling of molten bodies that originally intruded at depths of 1.4–1.6 km, during one or more recent (time of <10 kyr) eruptive events.
Influence of depth, focal mechanism, and tectonic setting on the shape and duration of earthquake source time functions
Authors: Houston Heidi
Published: February 2004

Source time functions of 255 moderate to great earthquakes obtained from inversions of teleseismic body waves by Tanioka and Ruff [1997] and coworkers were compared in a systematic way. They were scaled to remove the effect of moment and to allow the direct comparison and averaging of time function shape as well as duration. Time function durations picked by Tanioka and Ruff [1997] are proportional to the cube root of seismic moment if moments from the Harvard centroid moment tensor catalog are used. The average duration of scaled time functions is shorter and the average shape has a more abrupt termination for deeper events than shallower ones, with a distinct change occurring at ∼40 km depth. The complexity of the time functions, as quantified by the number of subevents, appears to decrease below ∼40 km depth. Furthermore, among events shallower than 40 km, the average duration of scaled time functions is shorter, and their average shape has a more abrupt termination (1) for events with strike‐slip focal mechanisms compared to thrust events and (2) for the few thrust events associated with an intraplate setting compared to the majority associated with an interplate (subduction) boundary. In each of these cases, events in more technically and seismically active settings have a longer duration and a more gradual termination. This can be interpreted in terms of lower stress drops and/or slower rupture velocities at active plate boundaries, suggesting that fault rheology depends on slip rate and may evolve as total fault slip accumulates. Furthermore, differences in average time function shape and duration associated with different subduction zones suggest that differences exist in the rheology on the plate boundaries at the various subduction zones.

Supporting data table is available via Web browser or via Anonymous FTP from ftp://kosmos.agu.org, directory “append” (Username = “anonymous”, Password = “guest”); subdirectories in the ftp site are arranged by paper number. Information on seraching and submitting electronic supplements is found at http://www.agu.org/pubs/esupp_about. html.
Slip accumulation and lateral propagation of active normal faults in Afar
Authors: Manighetti I. , King G. C. P. , Gaudemer Y. , Scholz C. H. , Doubre C.
Published: February 2004

We investigate fault growth in Afar, where normal fault systems are known to be currently growing fast and most are propagating to the northwest. Using digital elevation models, we have examined the cumulative slip distribution along 255 faults with lengths ranging from 0.3 to 60 km. Faults exhibiting the elliptical or “bell‐shaped” slip profiles predicted by simple linear elastic fracture mechanics or elastic‐plastic theories are rare. Most slip profiles are roughly linear for more than half of their length, with overall slopes always <0.035. For the dominant population of NW striking faults and fault systems longer than 2 km, the slip profiles are asymmetric, with slip being maximum near the eastern ends of the profiles where it drops abruptly to zero, whereas slip decreases roughly linearly and tapers in the direction of overall Aden rift propagation. At a more detailed level, most faults appear to be composed of distinct, shorter subfaults or segments, whose slip profiles, while different from one to the next, combine to produce the roughly linear overall slip decrease along the entire fault. On a larger scale, faults cluster into kinematically coupled systems, along which the slip on any scale individual fault or fault system complements that of its neighbors, so that the total slip of the whole system is roughly linearly related to its length, with an average slope again <0.035. We discuss the origin of these quasilinear, asymmetric profiles in terms of “initiation points” where slip starts, and “barriers” where fault propagation is arrested. In the absence of a barrier, slip apparently extends with a roughly linear profile, tapered in the direction of fault propagation.
Liquefaction and dynamic poroelasticity in soft sediments
Authors: Bachrach Ran , Nur Amos , Agnon Amotz
Published: February 2004

In this paper we present a model that can explain earthquake‐induced dynamic liquefaction in unconsolidated sediments without the need for irreversible compaction. We study the behavior of a poroelastic layer subjected to a periodic cyclic stress using dynamic poroelasticity formulation. We show that in such a layer pore pressure may increase due to a resonant mode of Biot's type II wave, an attenuated mode whose wavelength is short and can resonate inside a layer of few meters thickness. We show that in sediments such as sand, where the shear modulus is less than 0.3 GPa, pore pressure can exceed the total stress. This will cause unconsolidated material to liquefy. We also show that as the elastic coefficient of the sediment decreases, the pore pressure induced by Biot's type II wave increases. Thus, in a material where the elastic moduli are pressure dependent (e.g., Hertzian material) the increased pore pressure reduces the stiffness and thus liquefaction is more likely to occur.
Detection of shallowest submarine seismicity by acoustic coupled shear waves
Authors: Sugioka Hiroko , Fukao Yoshio , Okamoto Taro , Kanjo Kenji
Published: February 2004

An outstanding slow wave train similar to T wave in appearance has been observed on records of an ocean bottom seismometer array, off Boso, central Japan. These slow wave trains are not observed on land, are usually spindle‐shaped and dominant in vertical component, and have apparent velocities of ∼1.3 km/s along the array. In order to understand the observations, we calculate synthetic seismograms by a finite difference method for horizontally stratified layers of seawater, sediment, and basement, which show a well‐defined wave train consistent with the observations only if the source is placed above the basement layer. This result and normal mode calculations indicate that the slow wave train comprises essentially the sediment‐trapped shear waves coupled with ocean acoustic waves. The observed events of slow wave trains occur near the boundary between the outer arc high and the forearc basin, where vertical differential motion has been suggested to occur. We suggest that these small events represent the shallowest crustal activity unique to the triple junction off Boso.
A comparison of surface eddy kinetic energy and Reynolds stresses in the Gulf Stream and the Kuroshio Current systems from merged TOPEX/Poseidon and ERS‐1/2 altimetric data
Authors: Ducet N. , Le Traon P.‐Y.
Published: February 2004

Analyses of surface eddy kinetic energy (EKE) inferred from 5 years of merged TOPEX/Poseidon and ERS‐1/2 altimetric data are presented in the Gulf Stream (GS) and the Kuroshio (KS) Current systems, focusing on anisotropy and EKE seasonal, interannual, and long‐term variations, as well as Reynolds stresses. For both GS and KS a descriptive analysis of the EKE field shows that it is anisotropic along the path of the currents and assesses the close relationship between the spatial distribution of EKE high levels and bottom topography. A striking ocean feature is found in the GS, shaped as a zonal “double‐blade” structure in the EKE field, centered at 37°N, 71.5°W, that likely corresponds to a regular and permanent lateral displacement of the mean path of the current at this geographical location. Maps of EKE seasonal means for years 1993, 1996, and 1997 provide a detailed description of the surface variability (meanders, pinched‐off eddies, etc.) and show strong interannual variations in the KS. EKE monthly mean changes in the KS reveal a well‐marked seasonal cycle during the first 3 years (1993–1995), with EKE peaking in July/August. This is followed by a constant increase of its level, from the beginning of year 1996 till the beginning of 1997. During this increase the zonal velocity variance 〈u′2〉 is up to 30% higher than its meridional counterpart, suggesting a zonal acceleration of the eastward flow. EKE seasonal changes are also found in the KS Current (30°–35°N, 130°–140°E) and also show a regular annual cycle during the first 3 years, which precedes by 2 months the EKE seasonal cycle in the KS Extension (maximum in May/June). The maximum in EKE in both regions precedes by 2 months the maximum surface transport found by Qiu et al. [1991] and Zlotnicki [1991], occurring in July and late September, respectively. No such annual cycle is found in the GS region during the first 3 years. Year 1996 is anomalous in both current systems, with mean EKE level that increases by more than 20% compared to the other years. Also, EKE monthly mean changes in the KS reveal a well‐marked positive tendency in the eastern part of the current of more than 300 cm2 s−2 yr−1 and a negative tendency both in its eastern part and in the KS Current. This feature is absent from the GS. At last, eddy/mean flow interaction is presented. Both the spatial averaging of turbulent quantities and the relative contribution of the different terms of the horizontal momentum fluxes are discussed. A tentative explanation is given for the anomalously large 〈u′2〉 at the end of 1996 in the KS Extension.
Video measurements of near‐surface bubble spectra
Authors: Bowyer Peter A.
Published: February 2004

Large bubbles >300μm radius have been observed at depths of between 10 cm and 1.2 m using a small video camera attached to a surface following float. In salt water, bubble spectra are presented for both limited fetch and open sea conditions and in fresh water for limited fetch conditions. The void fractions and areas of the observed bubbles are calculated. Measurements were made in a limited range of wind conditions (greater than Beaufort force 5) and depths (between 10 cm and 1.2 m). Video records were analyzed by eye which introduced some uncertainty in the measured size and limited the number of sampled bubbles. Bubble populations are represented by power law spectra
. Near the surface, 2 < b < 3 on average; the distribution is very patchy: near breaking waves there are relatively more large bubbles: here 1 < b < 2. Away from the surface, 3.5 < b < 5.
Current measurements near Ronne Ice Shelf: Implications for circulation and melting
Authors: Foldvik A. , Gammelsrød T. , Nygaard E. , Østerhus S.
Published: February 2004

We present the first year‐long current meter records ever obtained near the floating Filchner‐Ronne Ice Shelf in the Weddell Sea. The currents are steered along the ice front, but in the lower layer where the bottom topography is descending toward the west the current has a component toward the ice front of about 3 cm s−1. During winter the temperature stayed near the surface freezing point, while the salinity increased, indicating that ice was formed and brine released. The seasonal variation in salinity was 0.15±0.05 psu, corresponding to the formation of 1–2 m of ice on a shelf depth of 400 m. The transport of High‐Salinity Shelf Water (HSSW) into the ice shelf cavity was found to be of the order 0.5×106 m3 s−1. The production of this water due to oscillating tides and off shelf winds was found to be of the same order of magnitude. In contact with glacial ice at great depths, and because of the depression of the freezing point, the HSSW is transformed to Ice Shelf Water (ISW) by cooling and melting processes. The melting rate was estimated to 1×1011 ton yr−1. This corresponds to the melting of 0.2 m ice per year if the melting is evenly distributed over the Filchner‐Ronne Ice Shelf. If the melting is concentrated along a path from the Berkner Shelf around the Berkner Island to the Filchner Depression, then melting rates up to 7 m yr−1 must be expected. A comparison of HSSW characteristics in the Ronne Depression, our winter observations on the Berkner Shelf, and the ISW flowing out of the Filchner Depression indicates that very little water passes through the cavity from the Ronne to the Filchner Depression. It appears that most of the ISW originating from processes on the Berkner Shelf escapes the cavity in the Filchner Depression. This leaves the Berkner Shelf as the important source of ISW and subsequently of the Weddell Sea Bottom Water formed from ISW.
Formation process of intermediate water in baroclinic current under cooling
Authors: Yoshikawa Yutaka , Akitomo Kazunori , Awaji Toshiyuki
Published: February 2004

The formation process of intermediate water in baroclinic current under cooling is investigated using a nonhydrostatic numerical model. After baroclinic instability develops into finite amplitude in a short time, strong downdrafts with a horizontal scale of 1 km are generated near the density front and subduct surface water to depths (∼400 m) along isopycnals. As a result, patches of ventilated water of 10∼20 km horizontal scale with anticyclonic circulation are formed at intermediate depths. Combined effects of baroclinic instability and convection are key dynamics for these phenomena. Convection acts as an initiator for baroclinic instability at the onset and accelerates its subsequent growth by reducing stratification. Developed baroclinic wave forms an intense density front, and downdraft along isopycnals is generated through the frontogenetic process. Density change due to convection intensifies this frontal downdraft by strengthening the geostrophic forcing (tendency to destroy the geostrophic balance) and by reducing potential vorticity (static stability). Further, symmetric instability induced by the density change due to convection and intensified by the frontogenetic process drives slantwise convection, which in turn, enhances the frontal downdraft. Consequent downward velocity becomes >20 times as large as that of the frontal downdraft without convection (cooling) and twice larger than that of pure convection. Since the intensified frontal downdraft moves its position with time and induces divergent flow at depths, a patch of ventilated water with a horizontal scale much larger than that of the frontal downdraft is formed. The role of convection (cooling) in the formation process of intermediate water in this context is to enhance the frontal downdraft rather than to deepen the mixed layer. This scenario is quite different from the one realized when baroclinic instability and convection do not coexist.
Ocean circulation influences on sea surface temperature in the equatorial central Pacific
Authors: Wang Chunzai , Weisberg Robert H.
Published: February 2004

Velocity data from an array of acoustic Doppler current profilers moored about 0°, 140°W from May 1990 through June 1991 during the Tropical Instability Wave Experiment are used in conjunction with Tropical Atmosphere‐Ocean array data and a blended sea surface temperature (SST) product to study the processes that control SST variations. The horizontal velocity data allow us to calculate the vertical velocity component by vertically integrating the continuity equation. Given the three‐dimensional temperature flux divergence, we examine the role of the ocean circulation in SST. Upwelling and downwelling are found to be associated with cooling and warming, respectively, suggesting that a vertical velocity component of either sign affects SST. Both the temperature flux divergence and advective formulations for the ocean circulation's influence on the temperature budget show times when the ocean circulation appears to provide the primary control on SST and times when this is not the case, with the flux divergence formulation performing better than the advective formulation. Statistically, within a bandwidth encompassing the tropical instability waves and the intraseasonal variations, roughly half of the SST variation is accounted for by the ocean circulation. These results are encouraging, given that data sets with different spatial and temporal scales have been used. They suggest that future field experimentation which utilizes a flux divergence array with velocity and temperature data sampled at the same spatial and temporal scales will yield quantitatively improved results. The analyses also show that the ocean circulation, on average, provides a cooling effect requiring the net surface heat flux to be positive to maintain the mean background state. The cooling effect is mainly controlled by mean ocean circulation and temperature fields.
Accuracy of in situ sea surface temperatures used to calibrate infrared satellite measurements
Authors: Emery W. J. , Baldwin D. J. , Schlüssel Peter , Reynolds R. W.
Published: February 2004

The present computation of sea surface temperature (SST) from infrared satellite measurements requires a coincident sample of in situ (drifting buoy and/or ship) SST measurements, to compute by regression the algorithmic coefficients for the infrared data. Ignoring the fundamental difference between satellite‐measured “skin SST” and buoy/ship measured “bulk SST,” we analyze past buoy and ship SST data to better evaluate the errors involved in the routine computation of SST from operational satellite data. We use buoy and ship SST data for 2 years (1990 and 1996) from the Comprehensive Ocean‐Atmosphere Data Set as well as 2 years of previously cloud‐cleared satellite radiances with matching drifting/moored buoy SST data from the NASA Pathfinder SST data set. We examine the in situ SST data for geographic distribution, accuracy, and self‐consistency. We find that there are large geographic regions that are frequently not sampled by the present drifting buoy network, a natural consequence of the fact that most buoys are not deployed to measure in situ SST for satellite infrared SST calibration. Comparisons between drifting buoy SSTs suggest an error of ∼0.4°C for nearly coincident buoy SSTs. Comparing moored with adjacent drifting buoy SSTs, we find that drifting and moored buoy SSTs are samples from the same population. Ship SSTs are noisier and have a significant warm bias relative to drifting buoy SSTs. We explore the SST measurement accuracy changes that occur with variations in sampling coverage used for the SST algorithm regression. We both vary the total amount of points and restrict the regression data to regional sampling biases. Surprisingly the total number of calibration SST values can be quite small if they cover all latitudes. We conclude that buoy SSTs can have residual bias errors of ∼0.15°C with RMS errors closer to 0.5°C.
On the decay of a baroclinic jet flowing along a continental slope
Authors: Hetland Robert , Hsueh Ya , Yuan Dongliang
Published: February 2004

A jet flowing in the Kelvin wave propagation direction along the seaward side of a continental slope is found to decrease in strength as it leaks mass onto the continental shelf. As a result, a shelf break jet flowing in the opposite direction is induced on the shelf. The combination of these two flows creates a decaying, high‐pressure tongue centered about the shelf break. An idealized, reduced gravity model is developed to examine the two‐flow combination. In the model a baroclinic open ocean jet flows next to a continental shelf over which the flow is barotropic and dominated by a linear bottom friction proportional to along‐shelf velocity. The far‐field depth of the thermocline is assumed to be much greater than the depth of the shelf break. A solution is found in the limit of small cross‐slope flow. The mathematical problem of finding the flow on the shelf is equivalent to that of heat conduction on a semi‐infinite rod with a finite heat source placed at the end of the rod. A numerical shelf/slope junction model is devised to allow the requirement of small cross‐slope flow to be relaxed. The numerical model, which incorporates higher‐order dynamics, confirms the qualitative results obtained from the analytic model. The motivation for the development of the theory lies in observational evidence of a northward flowing jet along the continental slope off the West Florida Shelf, apparently stemming from the Loop Current. The structure of the flow combination over the shelf break is confirmed with historical hydrographic data and advanced very high resolution radiometer imagery.
Reconstruction of the undiffused seasonal oxygen isotope signal in central Greenland ice cores
Authors: Bolzan John F. , Pohjola Veijo Allan
Published: February 2004

The climatologically important seasonal signal in ice core oxygen isotope records is altered by diffusion with time. Here we use the diffusion model of Whillans and Grootes [1985] to calculate the effects of isotopic diffusion in nine shallow ice cores recovered over a 150 × 150 km grid centered on Summit, Greenland. Two different computational methods give similar results for the back‐diffused δ18O seasonal extrema. Although back diffusing the ice core δ18O records improves the correlations between the individual ice core oxygen isotope and Greenland coastal temperature records; the correlations remain poor, particularly for the low‐accumulation sites. Stacking the back‐diffused isotopic records from several cores results in much higher correlations, with a strong winter linkage between west coast stations and central Greenland. These correlations are also significantly greater than the correlations between the stacked measured (diffused) isotopic profiles and coastal temperatures. Our results indicate that the use of ice core isotopic records to identify anomalous seasons requires that diffusion effects be accounted for.
Microscale wave breaking and air‐water gas transfer
Authors: Zappa C. J. , Asher W. E. , Jessup A. T.
Published: February 2004

Laboratory results showing that the air‐water gas transfer velocity k is correlated with mean square wave slope have been cited as evidence that a wave‐related mechanism regulates k at low to moderate wind speeds [Jähne et al., 1987; Bock et al., 1999]. Csanady [1990] has modeled the effect of microscale wave breaking on air‐water gas transfer with the result that k is proportional to the fractional surface area covered by surface renewal generated during the breaking process. In this report we investigate the role of microscale wave breaking in gas transfer by determining the correlation between k and AB, the fractional area coverage of microscale breaking waves. Simultaneous, colocated infrared (IR) and wave slope imagery is used to verify that AB detected using IR techniques corresponds to the fraction of surface area covered by surface renewal in the wakes of microscale breaking waves. Using measurements of k and AB made at the University of Washington wind‐wave tank at wind speeds from 4.6 to 10.7 m s−1, we show that k is linearly correlated with AB, regardless of the presence of surfactants. This result is consistent with Csanady's [1990] model and implies that microscale wave breaking is likely a fundamental physical mechanism contributing to gas transfer.
The δ15N of nitrate in the Southern Ocean: Nitrogen cycling and circulation in the ocean interior
Authors: Sigman D. M. , Altabet M. A. , McCorkle D. C. , Francois R. , Fischer G.
Published: February 2004

We report analyses of the nitrogen isotopic composition of nitrate in the eastern Indian and Pacific sectors of the Southern Ocean. In this paper, we focus on the subsurface data as well as data from the deep waters of other ocean basins. Nitrate δ15N is relatively invariant in much of the abyssal ocean (i.e., below 2.5 km), with a value of 4.8±0.2‰ observed in Lower Circumpolar Deep Water, North Atlantic Deep Water, and central Pacific deep water. The isotopic invariance of deep ocean nitrate stems fundamentally from the completeness of nitrate utilization in most of the global surface ocean, the Southern Ocean surface being an important exception. In the Subantarctic Zone (north of the Polar Frontal Zone) the nitrate δ15N of Upper Circumpolar Deep Water is ∼0.7‰ greater than that of Lower Circumpolar Deep Water. This isotopic enrichment appears to result from denitrification in the low‐latitude water masses with which Upper Circumpolar Deep Water communicates. The isotopic enrichment of Upper Circumpolar Deep Water is diminished in the Antarctic, probably because of the remineralization of sinking organic N, which has a low δ15N in the Antarctic. Relative to the other water masses of the Southern Ocean, the Subantarctic thermocline has a very low nitrate δ15N for its nitrate concentration because of exchange with the low‐latitude thermocline, where this isotopic signature appears to originate. This signature of the low‐latitude thermocline has two probable causes: (1) mixing with low‐nitrate surface water and (2) the oxidation of newly fixed N.
Zonal circulation in the NW Atlantic and Caribbean from a meridional World Ocean Circulation Experiment hydrographic section at 66°W
Authors: Joyce Terrence M. , Hernandez‐Guerra Alonso , Smethie William M.
Published: February 2004

A World Ocean Circulation experiment Hydrographic Program section along 66°W in the North Atlantic was made in 1997. In addition to the usual variables (hydrographic and tracer) measured in WOCE onetime sections, we made lowered acoustic doppler current profiler (LADCP) measurements at nearly all of the stations. The section closed off a portion of the western North Atlantic to the west of the line, making a closed volume for constraining the circulation. In addition, the deep portions of the Caribbean do not communicate with the rest of the basin. By combining mass, silica, and LADCP information as constraints an inverse calculation obtained reveals a strong eastward transport in the Gulf Stream bounded by westward flowing water on either side. Within these energetic flows we see evidence for recently ventilated Classical Labrador Sea Water, which has not reached the Deep Western Boundary Current north of Puerto Rico in any significant amounts. Within the Caribbean our major new finding is a deep cyclonic circulation below sill depth in excess of 100 times the inflow of deep Atlantic water through the major deep sill: the Anegada‐Jungfern Passage. The signature of the deep Atlantic source water is most prominent in CFCs and both a bottom and mid‐depth maximum are present in the Caribbean to the south of Puerto Rico. Off the coast of Venezuela, however, only the deeper CFC maximum is found. For the entire section the net overturning circulation, heat flux, and freshwater fluxes are all consistent with expectations based on water mass formation and air‐sea exchanges to the west of our section, but the annual mean air‐sea fluxes of heat and freshwater from Comprehensive Ocean‐Atmosphere Data Sets appear somewhat too small in comparison with our single‐section result.
Seasonal nutrient depletion and carbon export in the Subantarctic and Polar Frontal zones of the Southern Ocean south of Australia
Authors: Lourey Martin J. , Trull Thomas W.
Published: February 2004

Seasonal surface water nitrate and phosphate depletions were calculated for the Subantarctic Zone (SAZ) and Polar Frontal Zone (PFZ) of the Southern Ocean along the World Ocean Circulation Experiment SR3 repeat section south of Tasmania, Australia, and three other transects slightly to the west. The depletions were calculated between a single winter (July) section and eight other sections from October to March in the years 1991‐1998 and corrected for frontal movements and changes in water masses between the cruises using observed quasi‐linear nutrient‐salinity relationships. Arranged in a seasonal progression, depletion began in approximately October and continued through March, in good agreement with satellite biomass‐based estimates of production. Nitrate depletion in the SAZ (510 mmol N m−2) was approximately twice that of the PFZ (250 mmol N m−2), but phosphate depletions were similar (29 and 24 mmol P m−2 for the SAZ and PFZ, respectively). The SAZ exhibited a Redfield N/P depletion ratio (15.1±2.3), but the PFZ ratio was much lower (8.3±5.4). While the SAZ exhibits greater seasonal nutrient depletion, the relative magnitudes of export in the two zones remain uncertain because supply of nutrients from south of the Polar Front may lower the observed nutrient depletions in the PFZ. However, comparisons with sediment trap and production measurements for the region south of Australia suggest the SAZ and PFZ differences derive from differences in production. Assuming a Redfield C/N ratio (6.6), carbon export for the longest period examined (July‐March) averaged 3400±430 mmol C m−2 in the SAZ, with a range between 2700 and 3900 mmol C m−2 depending on latitude. In the PFZ, carbon export averaged 1600±350 mmol C m−2, with a range of 1100 to 2000 mmol C m−2. These averages represent minimum annual export production estimates because neither resupply nor export outside the July‐March observational period is included.
Summer transport through the Tsushima‐Korea Strait
Authors: Jacobs G. A. , Perkins H. T. , Teague W. J. , Hogan P. J.
Published: February 2004

Transport variations through the Korea‐Tsushima Strait are examined from two lines of bottom‐moored acoustic doppler current profilers (ADCPs) deployed southwest and northeast of Tsushima Island in May 1999. Almost full‐depth velocity profiles are measured. An optimal interpolation (OI) scheme is used to interpolate the data spatially and to provide error estimates along each section. The strong northeastward current core through the southern section lies approximately in the center of the strait, and small southwestward flows occur sporadically near both the Korea and Japan coasts. Much of the flow through the northern line occurs near the Korea and Japan coasts, with a weak southwestward mean flow and large variability in the strait center on the leeside of Tsushima Island. The estimated mean transport is 2.9 sverdrups (Sv) through the southern line and 2.5 Sv through the northern line. The northern line does not extend close to the Korea coast where there is significant flow. Expected errors in the transport estimates at any time are about 0.5 Sv RMS for each mooring line. The gradual transport increase through the summer is carried through the center of the southern line and on the Korea side of the strait through the northern line.
Seasonal variation of physical and biogeochemical features in eastern Caribbean Surface Water
Authors: Corredor Jorge E. , Morell Julio M.
Published: February 2004

We describe vertical and temporal variation of water mass structure, dissolved nutrients, phytoplankton chlorophyll a (Chl a), and dissolved organic matter south of the island of Puerto Rico at 67°W, 17°36′N with particular emphasis on variability of Caribbean Surface Water (CSW) features. CSW at the Caribbean Time Series (CaTS) station experiences yearlong continental riverine influence as evidenced by surface salinity consistently below that of Tropical Atlantic Surface Water and elevated silicate content. Salinity exhibits close correlation to average rainfall over the Orinoco River basin with a 3–4 month offset, and salinity and silicate are in turn closely correlated. Calculations using a simple mixing model and based on known end‐members for salinity and silicate indicate that rainwater accounts for 0–3.1% of CSW, while river water accounts for 0.6–5.5% at CaTS, indicating that surface waters of the NE Caribbean are under the influence of riverine water throughout the year. No significant correlation was found between salinity and phytoplankton Chl a, but the depth of the Chl a maximum was statistically correlated to seasonal salinity fluctuations. This observation may account for the apparent seasonal Chl a increase in these waters deduced from remote‐sensing estimates. Analysis of historical data shows that seasonal shoaling of the Chl a maximum results in a net increase in phytoplankton carbon biomass during the period of influence of the Orinoco River plume.
Bio‐optical properties of oceanic waters: A reappraisal
Authors: Morel André , Maritorena Stéphane
Published: February 2004

The apparent optical properties (AOPs) of oceanic case 1 waters were previously analyzed [Morel, 1988] and statistically related to the chlorophyll concentration ([Chl]) used as a global index describing the trophic conditions of water bodies. From these empirical relationships a bio‐optical model of the upper layer was developed. With objectives and structure similar to those of the previous study the present reappraisal utilizes AOPs determined during recent Joint Global Ocean Flux Study cruises, namely, spectral attenuation for downward irradiance Kd(λ) and irradiance reflectance R(λ). This revision also benefits from improved knowledge of inherent optical properties (IOPs), namely, pure water absorption coefficients and particle scattering and absorption coefficients, and from better pigment quantification (via a systematic use of high‐performance liquid chromatography). Nonlinear trends, already observed between optical properties and algal biomass, are fully confirmed, yet with numerical differences. The previous Kd(λ) model, and subsequently the R(λ) model, is modified to account for these new relationships. The R(λ) values predicted as a function of [Chl] and the predicted ratios of reflectances at two wavelengths, which are commonly used in ocean color algorithms, compare well with field values (not used when developing the reflectance model). This good agreement means that semianalytical ocean color algorithms can be successfully applied to satellite data. Going further into purely analytical approaches, ideally based on radiative transfer computations combined with a suite of relationships between the IOPs and [Chl], remains presently problematic, especially because of the insufficient knowledge of the phase function and backscattering efficiency of oceanic particles.
Meridional distribution of arsenic species in the subantarctic zone of the Southern Ocean, south of Australia
Authors: Featherstone Alison M. , Butler Edward C. V. , O'Grady Barry V.
Published: February 2004

Distribution of the arsenic species total inorganic arsenic [As(V+III)], arsenite [As(III)], monomethyl arsenic (MMA), and dimethyl arsenic (DMA) was studied in the Subantarctic Zone (SAZ) of the Southern Ocean, south of Australia, during the austral autumn (March 1998). As(V) was the dominant arsenic species in both vertical profiles and surface waters along the meridional transect 42°–55°S, 141°30′E. It was also the only species observed at depths >600 m. Concentrations of the reduced arsenic species (As(III), MMA, and DMA) were low in these waters compared with other oceanic sites with similar concentrations of chlorophyll a. As(III) concentrations could not be reliably quantified at any sites (<0.04 nM). The greatest conversion of As(V) to “biological” species was found at the surface in the Subtropical Convergence Zone (2.5%) and decreased heading southward to 1% in the Polar Front (PF). While the decline in methyl arsenic concentrations was broadly associated with water temperature and measures of biological production, slightly different trends were found in the SAZ and PF. North of the Subantarctic Front (SAF), methyl arsenic concentrations were well correlated with water temperature, while south of the front, no such relation existed. In addition, the ratio DMA/MMA increased south of the SAF, associated with a change in the microalgal community composition. Low water temperature, phosphate‐replete conditions, and low biological productivity in the Southern Ocean all contribute to the concentrations of biologically produced arsenic species in this region being among the lowest reported for oceanic waters.
Tsunami waveform inversion by adjoint methods
Authors: Pires Carlos , Miranda Pedro M. A.
Published: February 2004

An adjoint method for tsunami waveform inversion is proposed, as an alternative to the technique based on Green's functions of the linear long wave model. The method has the advantage of being able to use the nonlinear shallow water equations, or other appropriate equation sets, and to optimize an initial state given as a linear or nonlinear function of any set of free parameters. This last facility is used to perform explicit optimization of the focal fault parameters, characterizing the initial sea surface displacement of tsunamigenic earthquakes. The proposed methodology is validated with experiments using synthetic data, showing the possibility of recovering all relevant details of a tsunami source from tide gauge observations, providing that the adjoint method is constrained in an appropriate manner. It is found, as in other methods, that the inversion skill of tsunami sources increases with the azimuthal and temporal coverage of assimilated tide gauge stations; furthermore, it is shown that the eigenvalue analysis of the Hessian matrix of the cost function provides a consistent and useful methodology to choose the subset of independent parameters that can be inverted with a given dataset of observations and to evaluate the error of the inversion process. The method is also applied to real tide gauge series, from the tsunami of the February 28, 1969, Gorringe Bank earthquake, suggesting some reasonable changes to the assumed focal parameters of that event. It is suggested that the method proposed may be able to deal with transient tsunami sources such as those generated by submarine landslides.
Wave‐formed sand ripples at Duck, North Carolina
Authors: Hanes Daniel M. , Alymov Vadim , Chang Yeon S. , Jette Chris
Published: February 2004

A recently developed acoustic multiple transducer array was utilized to measure small‐scale bed forms in the nearshore and inner shelf regions at Duck, North Carolina. Two populations of wave‐formed ripples were observed: short wave ripples (SWR) with heights ranging from 3 mm to 2 cm and lengths ranging from 4 to 25 cm and long wave ripples (LWR) with heights ranging from 3 mm to 6 cm and lengths ranging from 35 to 200 cm. The SWR were only present sometimes, and their presence or absence was determined by a critical value of the near‐bed mobility number. The SWR were highly dynamic, sometimes flattening during wave groups and reforming over several incident wave periods. The LWR, in contrast, were almost always present. They were longer and lower relief than predicted by models or generally observed previously. Both SWR and LWR were often observed to migrate shoreward but were rarely observed to migrate seaward. The dimensions of the SWR, when they were present, were predictable by the Nielsen [1981] model or the Wiberg and Harris [1994] model to within approximately a factor of 2.
Oceanographic conditions south of Berkner Island, beneath Filchner‐Ronne Ice Shelf, Antarctica
Authors: Nicholls K. W. , Østerhus S. , Makinson K. , Johnson M. R.
Published: February 2004

We have made oceanographic measurements at two sites beneath the southern Filchner‐Ronne Ice Shelf. Hot‐water drilled access holes were made during January 1999, allowing conductivity‐temperature‐depth (CTD) profiling and the deployment of instrument moorings. The CTD profiles show that the entire water column is below the surface freezing point. We estimate the (summer) flux of water between the two sites to be 2×106 m3 s−1. The summer potential temperature‐salinity properties of the water column suggest that this flow is part of a recirculation in the deepest part of the subice shelf cavity and the Filchner Depression. The recirculation is driven by a combination of the melting of deep basal ice and the freezing that results from the depressurization of the cold buoyant water as it ascends the ice shelf base. The source of the water was high‐salinity shelf water (HSSW) produced in the Ronne Depression. This is the water that provides the external heat necessary for the strong melting at the deep grounding lines in the vicinity of Foundation Ice Stream. Instruments moored at the drill sites show that during the winter HSSW formed on the Berkner Shelf flows beneath the ice shelf and largely displaces the recirculating water from the two sites. This provides an externally driven through flow that is warmer (nearer the surface freezing point) and slower than the internal recirculation and which is low enough in density to escape the Filchner Depression.
Variation of sea surface height at periods of 65–220 days in the subtropical gyre of the North Pacific
Authors: Kobashi Fumiaki , Kawamura Hiroshi
Published: February 2004

Variations of sea surface height (SSH) in the North Pacific Subtropical Countercurrent (STCC) region have been investigated using altimeter‐derived SSH anomaly data during 1992–1999. It is shown by spectral analyses that the dominant temporal scale of SSH variations in the STCC region is 65–220 days. The energy of the SSH variations in this period band is mostly confined to the subtropical gyre and enhanced in the STCC, the Kuroshio and the Kuroshio Extension east of the Shatsky Rise. The broad energetic band in the STCC region can be divided into two regions: one is located in the latitudinal range of 21°–26°N and extending zonally from the Philippine Sea to ∼190° along 22°N, and the other extends from around 145° to the west of the Hawaiian Islands along 19.5°N, slightly tilting northward toward the east. The axes of these two high‐variability tongues are in agreement with those of the STCC multiple bands estimated from climatological hydrographic data. The 65–220 day variations in the STCC region are characterized by westward propagating mesoscale disturbances with wavelengths of 400–1800 km. In terms of ratio of the advective speed to the phase speed the mesoscale variations can be classified into two types, i.e., nonlinear eddies with wavelengths of ∼500–700 km and quasi‐linear wave motions with longer wavelengths of ∼1200 km.