bluuebunny/crossref_metadata_2025 huggingface dataset:
Changes in the configuration of ice stream flow from the West Antarctic Ice Sheet
Authors: Jacobel Robert W. , Scambos Theodore A. , Raymond Charles F. , Gades Anthony M.
Published: February 2004

Surface‐based ice‐penetrating radar profiles on the northeast flank of Siple Dome support the hypothesis that a curvilinear scar first observed in advanced very high resolution radiometer satellite imagery represents the margin of a formerly active ice stream. The scar defines the southwestern boundary of an ice stream flowing from ice stream C to ice stream D, close to where it enters the Ross Ice Shelf. Our studies show that the scar coincides with a trough and upward step in surface topography approximately 5 km across, underlain by a zone of disturbed internal stratigraphy revealed by the radar. Burial depth of the disturbed zone enables us to calculate the time of shutdown as occurring prior to approximately 1.3 ka. The configuration of the ice streams draining the West Antarctic Ice Sheet into the Ross Ice Shelf evidently changes with time, and attempts to predict the evolution of the ice sheet must incorporate this observation.
The three‐dimensional seismological model a priori constrained: Confrontation with seismic data
Authors: Ricard Yanick , Nataf Henri‐Claude , Montagner Jean‐Paul
Published: February 2004

We compare the predictions of an a priori model of the upper mantle with seismic observations of surface waves and eigenmodes. The 3‐Dimensional Seismological Model A Priori Constrained (3SMAC) has been developed by Nataf and Ricard [1996]. It is based on the interpretation by geodynamicists of the near surface layers of the Earth; on distributions of temperature, pressure, and composition as a function of depth; and then on estimates of seismic parameters (density, velocities, attenuations) from solid state laboratory measurements as a function of temperature and pressure. The 3SMAC predictions are confronted with observations consisting of phase velocities for Love and Rayleigh waves in the period range of 70–250 s [Montagner and Tanimoto, 1990]. We first show that tomographic inversions applied to 3SMAC synthetics induce a strong smoothing of the heterogeneities. This casts doubt on the meaning of the spectra of mantle heterogeneities revealed by tomography. We then show that most of the Love and Rayleigh fundamental mode observations for periods less than 200 s are satisfactorily predicted by 3SMAC. The major differences come from the seismic velocities under the Red Sea and Southeast China, which are much slower than what is estimated from 3SMAC, as well as those under Greenland, which are not as fast as the other cratonic areas. Because the lithosphere is thinner than 100 km under oceans and thinner than 300 km under continents in 3SMAC, we suggest that the existence of deeper lithospheric anomalies as proposed in many tomographic models is mostly due to a spurious effect of the inversion rather than implied by surface wave data. Half of the variance of the degree 2 anomaly mapped by low‐degree eigenmode observations can be explained by lithospheric velocity structures. The other half is highly correlated with the distribution of deep slabs, but its amplitude is a factor of 3 or 4 larger than that predicted by 3SMAC. The lithospheric anomalies present a degree 6 pattern well correlated with the distribution of hotspots even when the thermal anomalies that could be associated with plumes are not included in 3SMAC. Our results emphasize the importance of giving very close attention to “surface corrections” in tomographic models.
Correlation of Cenozoic‐Late Cretaceous geomagnetic polarity time scales: An Internet archive
Authors: Mead Gregory A.
Published: February 2004

Since the development of the first comprehensive Cenozoic geomagnetic polarity time scale in 1968, at least 11 additional time scales have been produced, potentially leading to confusion when comparing geologic studies using different time scales. This compilation correlates 12 different time scales published from 1968 to 1995, graphically and in tabular form, in order to enable geoscientists to compare studies dated on the basis of different time scales. The table and supporting documents are stored electronically on‐line at the World Data Center‐A for Paleoclimatology (National Oceanographic and Atmospheric Administration/National Geophysical Data Center).
Finite element modeling of crustal deformation in the North America‐Caribbean Plate Boundary Zone
Authors: Lundgren P. R. , Russo R. M.
Published: February 2004

We have developed two‐dimensional spherical shell finite element models of elastic displacements in the North America‐Caribbean (NA‐Ca) plate boundary zone (PBZ) in order to quantify crust and fault motions in the PBZ. The models we derive are dependent on both the internal fault constraints and the NA‐Ca Euler pole we used. Since the location and magnitude of the NA‐Ca euler pole are still matters of much debate, we consider three Euler poles [DeMets et al., 1990; Calais and Mercier de Lépinay, 1993; and Deng and Sykes, 1995]. We compare the resulting finite element model displacements to recent seismicity and to geological and geophysical field observations. The model of DeMets et al. [1990], NUVEL‐1, features an NA‐Ca relative velocity across the PBZ which is less than the observed Cayman spreading axis rate, and thus, the finite element model based on it produces fault motions which are inconsistent with observation. The Calais and Mercier de Lépinay [1993] (C&M), and Deng and Sykes [1995] (D&S) models both yield far‐field rates across the PBZ at the point of the Cayman spreading center of approximately 20 mm/yr, a value greater than the observed rate. The greatest differences in the latter two models lie in the motion calculated for the area around Puerto Rico. Both models feature a counterclockwise rotation of the PBZ around Puerto Rico with opening of the Anegada Passage and compression at the Muertos Trough south of eastern Hispaniola. The C&M‐based model produces normal opening of the Anegada Passage fault system, while the D&S‐based model produces left‐lateral transtension across the Anegada Passage fault system, the result of continuum crustal motions which are nearly orthogonal between the two models in the area NE of Puerto Rico. We conclude that the C&M‐based model better matches geological observations of PBZ fault motions and deformation primarily on the basis of the Anegada Passage results. Because of this rotation of the PBZ from Hispaniola to the Anegada Passage fault system, the Puerto Rico Trench includes highly oblique compression in its eastern portion, changing to pure left‐lateral strike‐slip NW of Puerto Rico. The C&M‐based finite element model agrees with preliminary GPS results from Hispaniola and Puerto Rico if the central Septentrional fault is constrained to strike‐slip motion.
Preliminary determinations of geomagnetic field intensity for the last 400 kyr from the Hawaii Scientific Drilling Project core, Big Island, Hawaii
Authors: Garnier Florence , Laj Carlo , Herrero‐Bervera Emilio , Kissel Catherine , Thomas Don M.
Published: February 2004

Preliminary paleointensity determinations using the Thelliers' method have been carried out on 141 samples of core HSDP (Hawaii Scientific Drilling Project), drilled into the Mauna Loa and Mauna Kea volcanoes and spanning the last 400 kyr. Rock magnetic investigations identify pseudo‐single‐domain magnetite as the main carrier of the natural remanent magnetization (NRM) all along the core. In the Mauna Kea sequence ∼75% of the specimens yielded exploitable results with linear NRM/TRM (Thermoremanent Magnetization) plots and positive partial TRM (pTRM) checks generally up to the highest temperatures (550°C). A lower percentage of successful experiments (∼50%) was observed for the Mauna Loa samples and the NRM/TRM diagrams are linear over a shorter temperature interval than for the Mauna Kea samples. These preliminary results, based on only one sample per flow, document a marked geomagnetic field intensity low at about 40 kyr probably related to the Laschamp event. They also suggest that the geomagnetic field at Hawaii might be characterized by large, sharp intensity fluctuations spaced by a few thousand years. The average value of the field intensity suggested by these results for the last 400,000 years is slightly higher than the present field at Hawaii.
A note on infragravity waves in the Arctic Ocean
Authors: Menemenlis Dimitris , Farmer David M. , Czipott Peter V.
Published: February 2004

Long‐period (20–50 s) surface‐gravity waves were recorded in the eastern Arctic (83°N, 10°E) using tiltmeters deployed on the ice and an acoustical current meter deployed a few meters below the ice during the spring of 1989. The waves caused ice tilt of a few microradians and horizontal velocity of the water just beneath the ice of order 10−4 m s−1. The unprecedented sensitivity of the velocity measurements is the result of a path‐averaging technique based on reciprocal transmission along 200‐m horizontal paths. The measurements indicate that infragravity waves and low‐frequency swell penetrate the Arctic Ocean through the Pram Strait and that they are freely propagating and not tied to groups of short waves. It is suggested that these waves could provide a method to measure ice thickness distribution in the Arctic Ocean.
Forcing of intraseasonal Kelvin waves in the equatorial Pacific
Authors: Kessler William S. , McPhaden Michael J. , Weickmann Klaus M.
Published: February 2004

Ten‐year time series of sea surface temperature (SST), 20°C depth, and zonal winds measured by moored buoys across the equatorial Pacific are used to define the intraseasonal (30‐ to 90‐day period) Kelvin waves. The Kelvin waves are observed to be forced west of the date line and propagate at a speed of 2.4 m s−1, with high zonal coherence over at least 10,000 km. They form a major component of thermocline depth variability in the east‐central Pacific. The intraseasonal‐band variance has a low‐frequency modulation both at the annual and interannual frequencies; higher amplitudes are observed in boreal fall/winter and during the onset phase of El Niño warm events. The oceanic intraseasonal variability and its low‐frequency modulation are coherent with atmospheric intraseasonal variations (the Madden‐Julian Oscillation (MJO)), which are known to propagate eastward into the Pacific from the Indian Ocean as part of a planetary‐scale signal. The life cycle of an individual or series of MJOs is determined by a combination of factors including tropical SSTs over the warm pool regions of the Indian and Pacific Oceans and interaction with the planetary‐scale atmospheric circulation. Thus the intraseasonal Kelvin waves should be taken as an aspect of a global phenomenon, not simply internal to the Pacific. The oceanic intraseasonal variability peaks at periods near 60–75 days, while the corresponding atmospheric variations have somewhat higher frequencies (35‐ to 60‐day periods). We show that this period offset is potentially related to the zonal fetch of the wind compared to the frequency‐dependent zonal wavelength of the Kelvin wave response. A simple model is formulated that suggests an ocean‐atmosphere coupling by which zonal advection of SST feeds back to the atmosphere; the model duplicates the steplike advance of warm water and westerly winds across the Pacific at the onset of the El Niño of 1991–1992. The key dynamics of the model is that the atmosphere responds rapidly to the state of the ocean, but the ocean's response to the atmosphere is lagged because it is an integral over the entire wind forcing history felt by the wave. This results in a nonlinear ordinary differential equation that allows a net nonzero lowfrequency ocean signal to develop from zero‐mean sinusoidal forcing at intraseasonal frequencies.
The effect of salinity on density in the California Current system
Authors: Batteen Mary L. , Collins Curtis A. , Gunderson Christopher R. , Nelson Craig S.
Published: February 2004

There are some indications that in large measure the density anomaly field of the California Current system (CCS) follows temperature, salinity playing a significantly lesser role. These indications have been used as justification for not taking as extensive salinity as temperature observations in the CCS, as well as for using fixed or prescribed salinity fields in density calculations in dynamic models of the CCS. An extensive data base of temperature‐salinity observations from the CCS is used to show that there are noticeable differences between the dynamic height field calculated with and without the inclusion of salinity variations. Hence the distribution of salinity is important in defining the large‐scale circulation of the CCS. In particular, owing to the combined effect of temperature and salinity on density, the dynamic height fields show a significant equatorward component along the coast of California and a strong offshore component adjacent to Baja California. Based on these observations, it is concluded that descriptively and dynamically, both temperature and salinity are essential to accurately characterize the large‐scale structure of the CCS.
Light response of phytoplankton in the South Atlantic Ocean: Interpretation of obervations and application to remote sensing
Authors: Hood Raleigh R.
Published: February 2004

A simplified, nonspectral derivation of a classical theory in plant physiology is presented and used to derive an absorption‐based primary productivity algorithm. Field observations from a meridional transect (4°N to 42°S) in the Atlantic Ocean are then described and interpreted in this theoretical context. The observations include photosynthesis‐irradiance curve parameters (α and Pmax), chlorophyll a and phaeopigment concentration, and estimated phytoplankton absorption coefficients at λ = 440 nm (aph (440)). Observations near the top (50% I0) and bottom (6% I0) of the euphotic zone are contrasted. At both light levels, α, Pmax, aph (440), and pigment concentration varied similarly along the transect: values were highest at the equator and at the southern end of the transect and lowest in the central South Atlantic. It is concluded that this pattern was related to increased nutrient availability due to equatorial upwelling in the north, and increased wind mixing in the south. At the 50% light level, α increased relative to aph at the southern end of the transect. This result appears to reflect a large‐scale meridional (southward) increase in the average quantum efficiency of the photosynthetic units of the phytoplankton. A correlation analysis of the data reveals that at the 50% light level, variations in Pmax were more closely related to aph(440) than chlorophyll concentration and that phytoplankton absorption explains 90% of the variability in Pmax. In theory, this shows that the ratio of the average quantum efficiency of the photosynthetic units of the phytoplankton to the product of their average absorption cross section and turnover time is relatively constant. This result is used to simplify the absorption‐based primary productivity algorithm derived previously. The feasibility of using this model to estimate production rate from satellite ocean color observations is discussed. It is concluded that an absorption‐based algorithm should provide more accurate production rate estimates than one based upon chlorophyll (pigment) concentration.
Algal pigment distribution and primary production in the eastern Mediterranean as derived from coastal zone color scanner observations
Authors: Antoine David , Morel André , André Jean‐Michel
Published: February 2004

About 300 coastal zone color scanner (CZCS) scenes, gathered over the eastern Mediterranean basin mostly during the years 1979–1981, have been processed from level 1 by using improved pixel‐by‐pixel procedures for the atmospheric correction and pigment retrieval. The seasonal evolution of the upper ocean pigment concentration is described and analyzed within the whole basin and its subbasins. From the chlorophyll concentration in the top layer, and by using statistical relationships, the depth‐integrated pigment content is estimated and used in conjunction with a light‐photosynthesis model to estimate the carbon fixation. The model relies on a set of physiological parameters, selected after the validation of the light‐photosynthesis model and not on locally measured parameters. Additional information needed in the modeling are the photosynthetically available radiation (computed from astronomic and atmospheric parameters, combined with a cloud climatology), sea temperature and mixed‐layer depth (taken from Levitus (1982)). Actually, the model is used to generate look‐up tables in such a way that all possible situations (concerning available radiation, chlorophyll concentration, and temperature) are covered. The appropriate situation associated with any pixel is selected from these tables to generate primary production maps. Despite a relatively good spatial coverage, studying the interannual variability of the pigment distribution and primary production appeared to be impossible. Therefore 12 “climatological” monthly chlorophyll maps have been produced by merging the data corresponding to several years. The carbon fixation rates in each of the subbasins have been computed on a monthly basis, and annual mean values derived thereafter. The primary production values are compared with sparse field determinations. They are also compared with those previously derived for the Western basin, also by using CZCS data (Morel and André, 1991). When put together, these companion works provide a kind of record of the trophic status of the entire Mediterranean Sea in the early 1980s. Ocean color sensors to be launched next, like SeaWIFS, will allow the seasonal and interannual variabilities in the late 1990s to be addressed.
Chlorofluorocarbon‐113 in the northeast Atlantic
Authors: Haine Thomas W. N. , Watson Andrew J. , Liddicoat Malcolm I.
Published: February 2004

An automated gas Chromatographic technique to measure the concentrations of chlorofluorocarbon 113 (CFC‐113:CCl2FCClF2) dissolved in seawater has been developed. The method also quantifies chlorofluorocarbons 11 and 12 (CFC‐11:CCl3F and CFC‐12:CCl2F2). Seawater collected from Niskin bottles in ground‐glass syringes is stripped by a gas stream and concentrated on a cryogenic trap in the manner of Bullister and Weiss (1988) and Gammon et al. (1982). By isolating and heating the trap, the chlorofluorocarbon compounds are reliberated and injected onto a high‐resolution capillary gas Chromatographie column, followed by electron‐capture detection. The analysis time for each sample is less than 15 min. Surface seawater precisions are 2.9%, 2.4%, and 1.2% for CFC‐113, CFC‐11, and CFC‐12, with detection limits of 0.003–0.004, 0.02, and 0.03‐0.05 pmol L−1, respectively. Although these statistics do not compare favorably with other CFC‐11 and CFC‐12 techniques (precision ∼1%, detection limit ∼0.005 pmol L−1 (Bullister and Weiss, 1988)), the dynamic ranges of the CFC‐113:CFC‐11 and CFC‐113:CFC‐12 “ventilation ages” are 20:1, better than that of the best CFC‐11:CFC‐12 age, albeit with inferior precisions. Estimates of the solubility ratios of CFC‐113:CFC‐11 and CFC‐113:CFC‐12 are 0.303 and 1.22, disagreeing with the work of Wisegarver and Gammon (1988), whose CFC‐113 results are believed to be boosted by coelution with methyl bromide. The optimum tracer ventilation age resolution is ±0.9 years for both CFC‐113:CFC‐11 and CFC‐113:CFC‐12 from a cast considered in the northeastern Atlantic. A plot of CFC‐113:CFC‐12 ventilation age is presented on an outcropping isopycnal. A strong correlation with pressure and dissolved oxygen concentration is noted and an oxygen utilization rate between 4.2 and 5.5±0.4 μmol L−1 yr−1 is implied, depending on the choice of CFC‐113 atmospheric history.
Seasonal circulation under the eastern Ross Ice Shelf, Antarctica
Authors: Hellmer Hartmut H. , Jacobs Stanley S.
Published: February 2004

An annual cycle of shelf water temperatures and salinities measured at depth near the eastern Ross Ice Shelf front is used to force a two‐dimensional thermohaline circulation model adapted to different subice paths in the vicinity of Roosevelt Island. These paths were assumed to have constant water column thicknesses of 160, 200, and 240 m and lengths of 460–800 km. Additional simulations with the longer cavity included a 80‐m thick interior water column in order to approximate conditions closer to the grounding line. Model results were compared with other long‐term measurements that showed outflow from beneath the ice shelf. Shelf water flowing into the cavity west of Roosevelt Island appears to follow a cyclonic route around the island. The ice shelf base loses mass at a rate of 18–27 cm yr−1, with seasonal forcing increasing the spatial and temporal variability of circulation and property distributions in the larger cavities. Shallow cavities reduce the influence of shelf water variability with increasing length. Introducing a transient shelf water temperature rise of 0.01°C yr−1 for 100 years increases the melt rate by 4–5 times. However, this increase is smaller if salinity also decreases over the same period of time, as might be expected from the added meltwater component.
Path‐averaged measurements of turbulence beneath ice in the Arctic
Authors: Menemenlis Dimitris , Farmer David M.
Published: February 2004

Two‐way acoustic propagation measurements obtained along 200‐m horizontal paths, a few meters below ice in the eastern Arctic, are used to estimate energy dissipation rate and momentum transfer between the ice and the ocean. Forward scattered sound propagating through the turbulent fluid of the boundary layer under ice produces characteristic scintillation patterns at the receivers which are interpreted in terms of the refractive index fine structure of the flow. Reciprocal travel time measurements yield information about the velocity variability along each acoustic path. By combining these two types of measurements, the contributions of sound speed and water‐particle velocity fluctuations to forward acoustic scatter can be separated. For the particular case of the under‐ice boundary layer, sound speed variability is negligible. Therefore, under the assumptions of a Kolmogorov inertial subrange and Taylor's frozen field hypothesis, the one‐way and the reciprocal acoustical measurements provide two independent estimates of turbulent kinetic energy dissipation rate averaged over the propagation paths. Both these methods yield a value of 8×10−8 W kg−1 6 m from the bottom of the ice for a 7‐hour period during which the mean ice‐relative velocity is 0.11 m s−1. Surface layer approximations then yield a value of 0.03 N m−2 for turbulent stress near the surface. The acoustical measurements also indicate that circulation around ice keels can cause horizontal “lift forces” of order 104 N.
Aleutian Basin of the Bering Sea: Thermohaline, oxygen, nutrient, and current structure in July 1993
Authors: Roden Gunnar I.
Published: February 2004

An overview is given of the surface‐to‐bottom thermohaline, oxygen, nutrient, and baroclinic flow structures in the Aleutian Basin of the Bering Sea and the adjacent North Pacific, based on high‐resolution field measurements in July 1993. A basic four‐layer vertical structure is observed in the upper 1500 m. The 30‐m top layer is warm, low in salinity and nutrients, and high in oxygen. Beneath it a 100‐ to 150‐m thick temperature minimum layer is found, believed to be a remnant of winter convection. This is followed by a 300‐m thick warm layer in which the main halocline, oxycline, and nutricline are located. Below it lies a layer of minimum oxygens and maximum nitrates and phosphates, centered at a depth of 900 m in the Bering Sea but rising to 400 m south of the Aleutians. This basic structure is disrupted in the vicinity of Amchitka Pass due to strong tidal mixing. The deep and abyssal waters of the Aleutian Basin differ substantially from those at comparable depth in the adjacent Pacific by being warmer ( Δ = 0.1°C), less saline ( Δ = 0.01 ‰) less dense ( Δ = 0.01 kg/m3), poorer in oxygen ( Δ = 50 μmol/kg), and richer in silicates ( Δ = 80 μmol/kg), nitrates ( Δ = 1.0 μmol/kg), and phosphates ( Δ = 0.3 μmol/ kg). The highest silicate concentrations (>240 μmol/kg) occur at the foot of the Bering shelf. Unusual hydrographic conditions exist in an isolated tidal mixing basin in Amchitka Pass, where the bottom mixed layer is 600 m thick, 0.7°C warmer, 0.14‰ less saline, and 21 μmol/kg less oxygenated than at comparable depths outside. Baroclinic currents indicate strong, deep, jetlike flows on both sides of the Aleutian ridge, weak shallow eddies in the middle of the Aleutian Basin, and somehat enhanced flows near the Bering shelf edge. The Alaska Stream in July 1993 was 100 km wide, had a double core with a top speed of 0.54 m/s, and had a westward volume transport of 38 Sv, with 14 Sv below 1000 dbar. The north Aleutian current had a speed of 0.2 m/s and an eastward transport of 10 Sv, with 4 Sv below 1000 dbar. Off the Bering slope, both eastward and westward flows were observed, with volume transports near 7 Sv.
Salinity variability in the surface layer of the tropical western Pacific Ocean
Authors: Tomczak Matthias
Published: February 2004

Observations of surface layer salinity and temperature, rainfall, wind and upper ocean currents from two surveys during the Tropical Ocean Global Atmosphere‐Coupled Ocean‐Atmosphere Response Experiment are presented. The surveys covered a region of 25×25 n. mi (46.3×46.3 km) with hydrographic sections of 5 n. mi (9.3 km) separation, using a Seasoar towed conductivity‐temperature‐depth profiler. One survey was dominated by the presence of several rain‐induced patches of low‐salinity water with slightly reduced temperature. These patches extended to the bottom of the mixed layer, which at the time, was 20–30 m deep, and appeared to be the result of convection driven by their reduced temperature. Below the mixed layer was a 15 to 20 m thick barrier layer. By distributing the freshwater over the mixed layer almost instantly, the convecting patches appeared to contribute to the maintenance of the barrier layer. This survey is typical for conditions found during most of the observation period. The second survey was dominated by a salinity front and advection of low‐salinity water with the large‐scale westerly wind that prevailed at the time. A single event of very heavy rain produced a shallow lens of freshened water; but the reduction in salinity was so strong that the associated cooling was insufficient to initiate convection. No barrier layer was seen ahead of the front but the observations indicate the development of a barrier layer after the passage of the front. The work demonstrates the difficulties in establishing a freshwater budget for the tropical ocean. Ship borne rain gauge measurements, even though becoming more accurate, seem not to be representative for an area much beyond a few kilometers or less. Sometimes the surface salinity distribution shows better visual correlation with the wind gust pattern than with the rainfall distribution, indicating that the wind gusts associated with atmospheric convection cells are less intermittent and more coherent in space than the rain.
Submesoscale anomalies in the North Pacific Subarctic Front
Authors: Maximenko N. , Yamagata T.
Published: February 2004

Data analysis of the experiment “Megapolygon‐87”, targeted at large‐scale and mesoscale currents, revealed a submesoscale coherent eddy with a core of anomalously cold water south of the North Pacific Subarctic Front (SF). TS characteristics of the core water are similar to the ones of the Subarctic water mass north of SF. The core occupies a depth range from 950 through at least 1500 m and has an outer diameter of 40 km. The maximum value of thermal anomaly of −0.5°C is located at 1000 m depth. Anticyclonic orbital velocity reaches 30 cm/s at 10‐km radius at the edge of the internal core, rotating nearly as a solid body. The 7 cm/s translating eddy is governed by the larger‐scale currents. Significant seawater density anomalies above the lens are explained by the interaction of the lens with a mean horizontal flow with strong vertical shear. The lens was probably formed either by upper layer cooling during the previous winter‐spring season, able to generate deep convection processes and move surface water to the middle and lower part of the main thermocline, or by instability of the countercurrent existing below SF. No direct observations of those processes in the northwestern Pacific are available at present. Other similar submesoscale anomalies have been found in the area south of SF.
An indirect estimate of the diurnal cycle in upper ocean turbulent heat fluxes at the equator, 140°W
Authors: Bond Nicholas A. , McPhaden Michael J.
Published: February 2004

The diurnal cycle in the heat budget of the upper equatorial Pacific Ocean is studied using 26 days of special high‐resolution oceanographic and meteorological observations from a current meter mooring at 0°, 140°W. The focus is on the turbulent heat fluxes at the base of the mixed layer, which are calculated as a residual in the mean diurnal heat budget. The inferred downward heat flux at the base of the mixed layer is ∼150 W m−2, averaged over the mean diurnal cycle, and ranged from near zero in the morning and early afternoon to ∼200 W m−2 during the late afternoon until dawn. The magnitude and phasing of the mean diurnal cycle were similar for different definitions of the mixed layer depth. The inferred residual turbulent fluxes were used also to estimate the mean diurnal cycles in vertical eddy diffusivity and turbulent dissipation rate. These turbulence parameters and the heat fluxes from which they were derived have magnitudes that are broadly consistent with those estimated previously from direct microstructure measurements.
Departures from the local inverse barometer model at periods of 5 days in the central South Atlantic
Authors: Woodworth P. L. , Windle S. A. , Vassie J. M.
Published: February 2004

Tide gauge data from Ascension and St. Helena in the central South Atlantic, supplemented by altimeter data from the TOPEX/POSEIDON mission, suggest that the local inverse barometer (LIB) model for the forcing of sea level variability by means of air pressure changes is not applicable within a narrow band of the variability spectrum with periods of approximately 5 days. This is in contrast to other sections of the wider frequency band spanning approximately 0.05 to 0.5 cycles/d, wherein the data are either consistent with approximate LIB response or at least suggest no departures from the LIB model of similar magnitude. These results have similarities to findings from Pacific gauges obtained over a decade ago, but so far, no comparisons have been to data from other ocean areas. They are also qualitatively similar to predictions for the region from a recent global barotropic numerical model. The anomalous part of the spectrum coincides in frequency with the Madden and Julian (1972, 1973) 5‐day pressure wave in the atmosphere. If that global scale forcing is responsible for both the Atlantic and Pacific departures from the LIB model around 5 days, then similar features should be observable in tide gauge data from other tropical areas.
Low‐frequency variability in regions of the Kuroshio Extension and the Gulf Stream
Authors: Wang Liping , Koblinsky Chester J.
Published: February 2004

Using sea surface height data collected by the TOPEX/POSEIDON altimeter, we studied the low‐frequency variability in the Kuroshio Extension and Gulf Stream. The most significant signals are the basin‐wide background seasonal cycle and low‐frequency wave activity generated over major bottom bathymetry and subsequent westward propagation in both regions. The spatial structures of the seasonal cycle in the two regions are quite different. In the Kuroshio Extension the seasonal cycle has a northeast‐southwest orientation and lies closer to the western boundary, whereas it has an east‐west orientation and lies more offshore in the Gulf Stream. The close relationship between the low‐frequency wave activity and major bottom bathymetry features is discussed through both empirical orthogonal functional (EOF) analysis and calculation of eddy kinetic energy and Reynolds stress. The spatial structures of these eddy statistics are fundamentally different from those of high‐frequency mesoscale variability. They are not in the proximity of maximum time‐mean flow, but rather, they lie mostly to the west of major bottom bathymetry. The close association of the clusters of local maximum eddy properties with major bottom topographic features in both regions, wave generation over the major bottom topographic features and subsequent westward propagation, and difference of the eddy statistics from that associated with high‐frequency mesoscale variability all suggest that unlike in the case with high‐frequency mesoscale variability generation in which internal dynamics of the background mean flow such as nonlinear instability plays a fundamental role, external forcing through bottom topography must play a fundamental role in generating the low‐frequency wave activity.
The effects of channels and shoals on exchange between the Chesapeake Bay and the adjacent ocean
Authors: Valle‐Levinson Arnoldo , Lwiza Kamazima M. M.
Published: February 2004

The role of bathymetric changes in determining the transport of water and salt in the lower Chesapeake Bay (LCB) was investigated using high‐resolution acoustic Doppler current profiler (ADCP) and conductivity‐temperature‐depth profiles. A cross‐channel transect was repeated eight times during neap tides on October 6–7, 1993, which illustrated the lateral structure of the longitudinal and transverse flow fields and the intratidal variations in the flow structure across the LCB. Amplitude and phase of the M2 tidal component, as well as the mean flow velocity, were calculated using least squares fitting at every point of a uniform grid obtained from the ADCP data. The results differ from the classical two‐layer pattern of estuarine circulation modified by Coriolis effects but are consistent with recent hydrographic observations. Semidiurnal flow was highest over the navigational channels, and lateral gradients were strongest in regions of sharp bathymetric changes. The phase lag of the semidiurnal flow also showed lateral and vertical gradients that represented advances at the bottom with respect to the surface and over the shoals in relation to the channels. The section of the water column measured indicated a mean outflow of 0.7×104 m3/s and a mean inflow of 1.3×104 m3/s. The apparent gain of water by the estuary during the period of observation can be explained by meteorologically forced net barotropic inflow. The depth‐averaged mean longitudinal flow consisted of inflow in the navigational channels and outflow over the shoals. The mean transverse flow showed near‐surface convergence over the channels. We propose a possible explanation for the observed flow and density structure as follows: the barotropic and baroclinic forcing interact with the bathymetry to extend the inflow from the bottom to the surface, thereby inducing a transverse circulation that yields near‐surface convergence over the channels.
Assimilation of TOPEX sea level measurements with a reduced‐gravity, shallow water model of the tropical Pacific Ocean
Authors: Fukumori Ichiro
Published: February 2004

Sea surface height variability measured by TOPEX is analyzed in the tropical Pacific Ocean by way of assimilation into a wind‐driven, reduced‐gravity, shallow water model using an approximate Kalman filter and smoother. The analysis results in an optimal fit of the dynamic model to the observations, providing a dynamically consistent interpolation of sea level and estimation of the circulation. Nearly 80% of the expected signal variance is accounted for by the model within 20° of the equator, and estimation uncertainty is substantially reduced by the voluminous observation. Notable features resolved by the analysis include seasonal changes associated with the North Equatorial Countercurrent and equatorial Kelvin and Rossby waves. Significant discrepancies are also found between the estimate and TOPEX measurements, especially near the eastern boundary. Improvements in the estimate made by the assimilation are validated by comparisons with independent tide gauge and current meter observations. The employed filter and smoother are based on approximately computed estimation error covariance matrices, utilizing a spatial transformation and an asymptotic approximation. The analysis demonstrates the practical utility of a quasi‐optimal filter and smoother.
Direct evidence for an Ekman balance in the California Current
Authors: Chereskin T. K.
Published: February 2004

Moored acoustic Doppler current profiler velocity estimates and buoy wind observations made during a period of moderate southward winds were used to test the Ekman balance at a site in the California Current. As in prior studies, the wind‐driven flow was separated from the total flow by subtraction of a deep reference current. The wind‐driven flow was shown to be in an Ekman balance on daily timescales over a period of several months. The mean observed transport was to the right of the wind and agreed to within 3% in magnitude and 4° in phase with the predicted Ekman transport, although the error bar was about 20%. The mean velocity profile was a smooth spiral, qualitatively similar (although flatter) than the theoretical Ekman spiral. Prom the observed mean momentum balance, profiles of the turbulent stress and eddy viscosity were inferred. Eddy viscosity estimates within the wind mixed layer were O(100 cm2 s−1).
Dynamics of the equatorial Atlantic from altimetry
Authors: Katz Eli Joel , Carton James A. , Chakraborty Amal
Published: February 2004

The first 700 days of TOPEX/POSEIDON altimetric data of the equatorial Atlantic are selectively investigated to yield a basin‐wide description of the zonal pressure gradient along the equator and the meridional gradient across the North Equatorial Countercurrent (NECC). In the first analysis the gravest empirical orthogonal function mode is found to be zonally constant in midbasin and a quasi‐stationary linear relationship between the basin‐wide, averaged zonal wind stress and the zonal pressure gradient is demonstrated (correlation coefficient of 0.70 with a zero lag). In the second analysis the zonal variation of the depth of the NECC trough across the basin is described and it and its annual cycle are reproduced with a reduced‐gravity model calculation (correlation coefficient of 0.75). A secondary result of both analyses is an estimate of the adjustment to the working model of the local geoid gradients needed to bring the mean altimetric signal into agreement with the dynamic models and modeled wind field.
Hydrography above the Mid‐Atlantic Ridge (33°–40°N) and within the Lucky Strike segment
Authors: Wilson Cara , Speer Kevin , Charlou Jean‐Luc , Bougault Henri , Klinkhammer Gary
Published: February 2004

As part of the French‐American Ridge Atlantic program the French‐American Zero‐Angle Photon Spectrometer and Rocks (FAZAR) cruise conducted water column studies between 33 and 40°N along the Mid‐Atlantic Ridge (MAR) to detect hydrothermal activity and map its influence. This paper describes the large‐scale hydrography within the axial valley, with particular emphasis on the hydrothermally active Lucky Strike segment (37°17′N). The FAZAR study area is affected by the presence of the Azores Current and Mediterranean Water (MW). Although the MW core has been mapped as far north as 50°N off the ridge, the northern boundary of the MW within the MAR in the FAZAR study area exists as a strong front south of the Azores platform. This front is most likely caused by the shallower ridge crest becoming a physical barrier to the MW. The Lucky Strike segment lies within this front and, as a result, has complicated hydrography which can obscure hydrothermal temperature and salinity anomalies.
Outflow of dense water from the Storfjord in Svalbard: A numerical model study
Authors: Jungclaus Johann H. , Backhaus Jan O. , Fohrmann Hermann
Published: February 2004

Recent investigations on deep water renewal point to the important role of dense water formed on the continental shelves surrounding the Arctic Ocean. In this investigation a hydrostatic, reduced gravity, primitive equation model for the simulation of the spatial and temporal evolution of gravity plumes on a continental slope is applied and compared to the observed outflow of a plume of dense water, originally formed in Storfjorden (Svalbard), into the Greenland Sea toward Fram Strait. The vertically integrated, nonlinear, rotational model accounts for the entrainment of water mass properties from a spatially structured but stagnant ambient water body. This study reveals that part of the dense water, in accordance with earlier estimates, spreads northward along the eastern side of Fram Strait. Another branch of the plume, guided by the topography, flows into a deep trench east of the Knipovitch Ridge to the west of Svalbard. During its descent to depths of more than 2000 m the plume entrains three different water masses (East Spitsbergen Water, Atlantic Water, and Norwegian Sea Deep Water) and hence changes its water mass characteristics. The volume of deep water produced by the mechanism discussed here depends not only on the amount of initially formed brine‐enriched shelf water, but also on the water mass characteristics of the latter.
Comparison of two models of aggregation in continental‐shelf bottom boundary layers
Authors: Hill P. S. , Nowell A. R. M.
Published: February 2004

Comparison of two models of aggregation and disaggregation of fine sediment in continental‐shelf bottom boundary layers delineates conditions under which these processes can be modeled simply. Model 1 predicts the evolution of the particle size distribution based on layer‐averaged mean sediment concentrations. Model 2 avoids the simplifying assumptions of model 1 by representing suspension dynamics with a Monte Carlo simulation of the joint probability density function of number concentration in the various particle size classes. In contrast to model 1, this technique bases aggregation rates on local concentrations and explicitly treats the effect of concentration correlations on aggregation rates. Each model produces a time series of particle size distribution in a steady, horizontally uniform, nondepositing, continental‐shelf bottom boundary layer. Comparisons are made for suspensions that are initially polydisperse and monodisperse, for shear velocities of 0.01 and 0.005 m s−1, for fractal dimensions of 1.92 and 2.4, for initial layer‐averaged sediment concentrations of 1, 2.5, 10, and 25×10−3 kg m−3, and for sticking efficiencies of 0.1 and 1.0. In general, model 1 is accurate for suspensions that are initially polydisperse and do not develop strong vertical structure. Results suggest that as long as maximal settling velocity does not exceed roughly 0.6u*, vertical gradients in sediment concentration do not degrade substantially the accuracy of layer‐averaged mean models for concentrations typical of the continental shelf. When initial suspensions are monodisperse and/or vertical gradients are large, transport‐limited aggregation can slow overall aggregation rates of model 2 relative to model 1, or important interactions can occur between particle types that both show strong vertical structure, speeding overall aggregation rates in model 2. In the future the effects on the accuracy of layer‐averaged mean models of short‐ and long‐term unsteadiness, erosion, and deposition will be explored.
The absolute velocity field of Agulhas eddies and the Benguela Current
Authors: Clement Amy C. , Gordon Arnold L.
Published: February 2004

Acoustic Doppler current profiler (ADCP) data referenced to Global Positioning System navigation were obtained in May 1993 from the Royal Research Ship Discovery within the Benguela Current as part of the Benguela Source and Transport (BEST) project. These data are used in combination with hydrographic data collected during the cruise to investigate the absolute geostrophic velocities in the Benguela Current and the transient eddies. Four anticyclonic eddies were encountered during the cruise, of which three were determined to be Agulhas Retroflection eddies of various ages and one was determined to be an eddy derived from the Brazil Current. ADCP velocities averaged between conductivity‐temperature‐depth stations have a high linear correlation with geostrophic velocities derived from the hydrographic data (correlation coefficient of 0.93) along the entire cruise track. The magnitudes of the two velocity estimates, however, were notably different within the eddies. It was determined that these discrepancies are probably due to a significant barotropic component of the flow near the eddy center. As much as 50% of the total flow in the eddy is barotropic. The horizontal length scale (radius of maximum velocity) of this eddy determined from both the ADCP data and the thermal field was found to be approximately 60 km, considerably smaller than previous estimates, which are about 120 km. The barotropic component in Agulhas eddies leads to an equal partitioning of total mechanical energy between available potential and kinetic energy. It is also expected to have a significant effect on the climatically important exchange of mass between the Indian and South Atlantic Oceans. Total geostrophic velocities were computed for the Benguela Current usine the averaged ADCP at 250 m as a reference. The ADCP referenced geostrophic transport across 30°S of water warmer than 9°C in the Benguela Current was found to be 17 Sv (1 Sv= 106 m3 s−1) to the north and that of the upper kilometer was 25 Sv to the north. These values are largely consistent with previous estimates, suggesting that the upper layer flow across this section is dominated by the baroclinic field.
Temperature variability beneath Ronne Ice Shelf, Antarctica, from thermistor cables
Authors: Nicholls Keith W.
Published: February 2004

Thermistor cables have been deployed at two sites beneath Ronne Ice Shelf, Antarctica. One site is to the east of a submarine ridge that delineates the eastern boundary of the Ronne Depression, and the other is 100 km to the north, above the eastern slope of the depression. Long records from the cables (up to 22 months) indicate a large difference in the temperature variability at the two sites, being an order of magnitude greater in the Ronne Depression (site 2). Although the records appear otherwise similar, there is no significant correlation between them. The high variability in the site 2 record has allowed the construction of a simple descriptive model of the local oceanographic regime. Winter freezing in the open water north of the ice front generates Western Shelf Water (WSW), a type of High Salinity Shelf Water, which travels southwest beneath the ice shelf, appearing at site 2 as a slope‐trapped current at the bottom of the water column. Baroclinic instability in the flow manifests itself in the site 2 temperature record as oscillations on time scales of 5 to 15 days. The disturbances cause a periodic east‐west advection of water masses across the Ronne Depression. Site 2 is on the eastern slope of the depression, where the wave‐induced eastward motion forces Ice Shelf Water to rise, resulting in periodic ice‐platelet formation in the water column, as surmised from conductivity‐temperature‐depth measurements at the site. The depth of the WSW layer decreases by 40 to 60 m during a 100‐day period, starting some 4 months after the beginning of the summer. Assuming an absence of significant WSW production during the summer, the 4‐month delay implies a minimum average speed of WSW flow of about 0.02 m s−1. The WSW flux into the Ronne Depression is estimated at 3 × 105 m3s−1.