bluuebunny/crossref_metadata_2025 huggingface dataset:
Tests of the rigid‐plate hypothesis and bounds on intraplate deformation using geodetic data from very long baseline interferometry
Authors: Argus Donald F. , Gordon Richard G.
Published: February 2004

The geodetic results of Ma et al. [1994] from very long baseline interferometry are used to assess rates of intraplate deformation. We analyze the resulting set of velocities of 18 plate‐interior sites, nine on the North American plate, four western European sites on the Eurasian plate, three on the Pacific plate, and two on the Australian plate. We formulate three alternative error budgets based partly on the work of Ryan et al. [1993a]: an optimistic error budget, a conservative error budget, and an upper bound error budget. All three budgets assume the existence of a previously unmodeled error that is inversely proportional to the time span of observations at a site. Use of the conservative error budget indicates no significant motion between sites on any one of the four plates. Use of the upper bound error budget, designed to be precisely large enough to be consistent with rigid plates, places a usefully small upper bound on how fast any site might be moving relative to the rest of its plate. Speeds faster than 2 mm/yr are excluded at the 95% confidence level for seven of the 18 plate‐interior sites. Adjustments of the site velocities for motion due to postglacial rebound as predicted from the model of Tushingham and Peltier [1991] give an insignificant improvement to the fit of the horizontal velocities to the rigid plate model. The velocities of six sites near the western margin of the interior of the North American plate were also analyzed. Use of the conservative error budget indicates that a site near Fairbanks (Alaska) has an upper bound of 3 mm/yr on its speed, a site in Nome (Alaska) has an upper bound on its speed of ∼8 mm/yr, and a site in Penticton (British Columbia) has an upper bound on its speed of ∼4 mm/yr. Of three sites on the Colorado Plateau, two move marginally significantly, and one moves insignificantly relative to stable North America. Upper limits on their speeds relative to stable North America range from 4 to ∼5 mm/yr. We conclude that the data do not yet demonstrate significant velocities between sites in the stable interior of any one plate but do provide useful and small upper bounds of 2 mm/yr or less on the speeds of the best observed intraplate sites. Six sites near the western margin of the North American interior have upper bound speeds of 3 to 8 mm/yr.
Analysis of fractures intersecting Kahi Puka Well 1 and its relation to the growth of the island of Hawaii
Authors: Morin Roger H. , Paillet Frederick L.
Published: February 2004

As part of the Hawaii Scientific Drilling Project, Kahi Puka Well 1 penetrated about 275 m of Mauna Loa basalts overlying a sequence of Mauna Kea flow units as it was drilled and cored to a total depth of 1053 m below land surface. A borehole televiewer (BHTV) was run in most of the well in successive stages prior to casing in order to obtain magnetically oriented acoustic images of the borehole wall. A total of 283 individual fractures were identified from this log and characterized in terms of strike and dip. These data are divided into three vertical sections based upon age and volcanic source, and lower hemisphere stereographic plots identify two predominant, subparallel fracture subsets common to each section. Assuming that most of the steeply dipping fractures observed in the BHTV log are tensile features generated within basalt flows during deposition and cooling, this fracture information can be combined with models of the evolution of the island of Hawaii to investigate the depositional history of these Mauna Loa and Mauna Kea basalts over the past 400 kyr. The directions of high‐angle fractures appear to be generally parallel to topography or to the coastline at the time of deposition, as is supported by surface mapping of modern flows. Consequently, an overall counterclockwise rotation of about 75° in the strike of these fractures from the bottom to the top of the well represents a systematic change in depositional slope direction over time. We attribute the observed rotation in the orientations of the two predominant fracture subsets over the past 400 kyr to changes in the configurations of volcanic sources during shield building and to the structural interference of adjacent volcanoes that produces shifts in topographic patterns.
An updated evaluation of the Loop Current's eddy‐shedding frequency
Authors: Vukovich Fred M.
Published: February 2004

The Loop Current's eddy‐shedding frequency was reevaluated using two existing data sets which were updated with 5 years of additional data. The data sets are a 22‐year data set (1972–1993) on the eddy‐shedding period and a 17‐year data set (1977–1993) of the monthly averaged distance between the northern boundary of the Loop Current and the 30°N latitude line. Only Loop Current eddies having diameters larger than 250 km were considered in this study. The histogram of the eddy‐shedding periods indicated that there is a primary mode (9 months) and a secondary mode (14 months). The range in the data is from 6.0 to 17 months, and the average period is 11 months, with a standard deviation of ±3 months. In the histogram of the eddy‐shedding periods the frequency of occurrence for the 11‐month period is smaller than those for the primary and secondary modes. However, the 11‐month period will have a frequency of occurrence identical to the secondary mode, with only one more occurrence of the 11‐month period and identical to the primary mode, with two more occurrences. The spectrum of the Loop Current's northern boundary displacements indicated that the predominant period was 11.1 months, which conformed to the average separation period.
A seasonal model of the Mediterranean Sea general circulation
Authors: Roussenov Vassil , Stanev Emil , Artale Vincenzo , Pinardi Nadia
Published: February 2004

This paper describes the seasonal characteristics of the Mediterranean Sea general circulation as simulated by a primitive equation general circulation model. The forcing is composed of climatological monthly mean atmospheric parameters, which are used to compute the heat and momentum budgets at the air‐sea interface of the model. This allows heat fluxes to be determined by a realistic air‐sea interaction physics. The Strait of Gibraltar is open, and the model resolution is
in the horizontal and 19 levels in the vertical. The results show the large seasonal cycle of the circulation and its transient characteristics. The heat budget at the surface is characterized by lateral boundary intensifications occurring in downwelling and up welling areas of the basin. The general circulation is composed of subbasin gyres, and cyclonic motion dominates the northern and anticyclonic motion the southern part of the basin. The Atlantic stream which enters from Gibraltar and assumes the form of different boundary current subsystems is a coherent structure at the surface. At depth it appears as current segments and jets around a vigorous gyre system. The seasonal variability is manifested not only by a change in amplitude and location of the gyres but also by the appearance of seasonally recurrent gyres in different parts of the basin. Distinct westward propagation of these gyres occurs, together with amplitude changes. For the first time a Mersa‐Matruh Gyre is successfully simulated due to the introduction of our heat fluxes at the air‐sea interface. The seasonal thermocline is formed each summer, and a deep winter mixed layer is produced in the region of Levantine intermediate water formation. Deep water renewal does not occur, probably due to the climatological forcing used.
Turbulence within inertia‐buoyancy balanced axisymmetric intrusions
Authors: Lemckert Charles J. , Imberger Jörg
Published: February 2004

Data from two detailed field experiments were used to investigate the turbulence within inertia‐buoyancy balanced, axisymmetric intrusive gravity currents entering nearly quiescent stratified water bodies. The intrusions were generated by energetic point source bubble plume systems installed within different freshwater reservoirs, and the mixing activity was recorded with microstructure profilers. Close to the bubble plume, the intrusions were found to be very turbulent. The turbulent activity within the intrusions was observed to decay most rapidly in the intrusion core, but the shear at the upper and lower boundaries of the intrusions maintained the turbulence in these regions for a long distance. The results showed that the turbulence began with a length scale equal to the maximum sustainable overturn length for internally generated shear and hence a turbulent Froude number near unity. As the intrusions moved away from the generation region, the Froude number remained near unity, but the Reynolds number decreased with distance; the shear generated by the intrusion thus sustained an active turbulence field, albeit with ever decreasing length scales.
Observations of wind waves in the Gulf Stream frontal zone
Authors: Kudryavtsev V. N. , Grodsky S. A. , Dulov V. A. , Bol'shakov A. N.
Published: February 2004

Experimental results of a study of wind waves in the Gulf Stream frontal zone are presented. Measurements of two‐dimensional radar spectra of surface waves and surface current were performed during ship crossings of the Gulf Stream under different wind conditions. Waves reflected from the Gulf Stream were observed on the windward side for an along current wind. These effects resulted in a widening of the spectrum angular distribution and a deviation of the mean wave direction from that of the wind. Trapped waves were observed under against current winds. Depending on wave age, the trapped waves were either locally generated (developing waves) or entered into the jet downstream (nearly fully developed waves). Trapped waves significantly increased the wave energy locally. The experimental data are interpreted in terms of wave kinematics and the equation of spectrum evolution in the relaxation approximation.
Biological processes and optical measurements near the sea surface: Some issues relevant to remote sensing
Authors: Cullen John J. , Lewis Marlon R.
Published: February 2004

The advent of remote sensing, the development of new optical instrumentation, and the associated advances in hydrological optics have transformed oceanography: it is now feasible to describe ocean‐scale biogeochemical dynamics from satellite observations, verified and complemented by measurements from optical sensors on profilers, moorings, and drifters. Only near‐surface observations are common to both remote sensing and in situ observation, so it is critical to understand processes in the upper euphotic zone. Unfortunately, the biological principles that must be used to interpret optical variability near the sea surface are weaker than we would like, because relatively few experiments and analyses have examined bio‐optical relationships under high irradiance characteristic of the upper optical depth. Special consideration of this stratum is justified, because there is good evidence that bio‐optical relationships are altered near the surface: (1) the fluorescence yield from chlorophyll declines, leading to bias in the estimation of pigment from fluorometry; (2) the modeled relationship between solar‐stimulated fluorescence and photosynthesis seems to deviate significantly from that presented for the lower euphotic zone; and (3) carbon‐specific and cellular attenuation cross sections of phytoplankton change substantially during exposures to bright light. Even the measurement of primary productivity is problematic near the sea surface, because vertical mixing is not simulated and artifactual inhibition of photosynthesis can result. These problems can be addressed by focusing more sampling effort, experimental simulation, and analytical consideration on the upper optical depth and by shortening timescales for the measurement of marine photosynthesis. Special efforts to study near‐surface processes are justified, because new bio‐optical algorithms will require quantitative descriptions of the responses of phytoplankton to bright light.
Error in predicting hydrosol backscattering from remotely sensed reflectance
Authors: Weidemann A. D. , Stavn R. H. , Zaneveld J. R. V. , Wilcox M. R.
Published: February 2004

Monte Carlo simulations are carried out to determine the error in the inversion of backscattering from remotely sensed reflectance when geometrical shape factors of the light field are assumed to be unity. The results show that error in backscattering inversion can vary from a 40% overestimation to a 20% underestimation and is dependent on the solar angle and the hydrosol constituents contributing to backscattering. The simulations also demonstrate that for chlorophyll concentrations ranging from 0.05 to 20 mg m−3 the most dramatic change in the geometrical shape factor occurs near 1.0 to 1.5 mg m−3 chlorophyll. The potential importance of bacteria in influencing the shape factor and the subsequent effect of bacteria on the retrieval of the backscattering from remote sensing reflectance are shown. Quartzlike material's strong impact on geometrical shape factors and errors of retrieval of backscattering at low chlorophyll concentrations are also demonstrated. Remote sensing reflectance inversion schemes must include information about the backscattering function to be successful.
Effects of Raman scattering on the water‐leaving radiance
Authors: Waters Kirk J.
Published: February 2004

The contribution of Raman scattering to the water‐leaving radiance is examined using Monte Carlo simulations. Exit angle information is retained, allowing a comparison of different satellite viewing directions. Chlorophyll values of 0.0, 0.01, 0.1, and 1.0 mg Chl m−3 are simulated. Little directional variability is found, with the exception of the direct solar backscatter direction. The wavelength variability is greatest for low chlorophyll concentrations and is negligible for 1.0 mg Chl m−3. At 550 nm the Raman contribution ranges from approximately 18% of the total water‐leaving radiance for pure water to 3% for 1.0 mg Chl m−3. At 440 nm the range is from 6% to 2%, indicating that Raman scattering will impact radiance ratios for ocean color satellite algorithms.
Current meter observations in the Old Bahama Channel
Authors: Atkinson L. P. , Berger T. , Hamilton P. , Waddell E. , Leaman K. , Lee T. N.
Published: February 2004

Current meter observations were made at 50, 250, and 435 m in 495 m of water at the center of the Old Bahama Channel between November 1990 and November 1991. The mean speed at those depths was 2.6, 50, and 26 cm s−1 toward the Straits of Florida. A maximum speed of 193 cm s−1 was found at the 250‐m level. There was evidence of a speed maximum between 50 and 250 m. Large internal diurnal tidal currents were observed that produced high shears in the water column under some conditions. Transport calculated from the single mooring using simple assumptions varied from −2.4 to +6.6 Sv with a mean of +1.9 Sv toward the Straits of Florida. These values, while based on limited data, are very significant compared to the mean flow in the Straits of Florida. Thus flow through the Old Bahama Channel may be an important component of heat and salt transport in the straits.
Global oceanic emissions of nitrous oxide
Authors: Nevison Cynthia D. , Weiss Ray F. , Erickson David J.
Published: February 2004

The global N2O flux from the ocean to the atmosphere is calculated based on more than 60,000 expedition measurements of the N2O anomaly in surface water. The expedition data are extrapolated globally and coupled to daily air‐sea gas transfer coefficients modeled at 2.8°×2.8° resolution to estimate a global ocean source of about 4 (1.2–6.8) Tg N yr−1. The wide range of uncertainty in the source estimate arises mainly from uncertainties in the air‐sea gas transfer coefficients and in the global extrapolation of the summertime‐biased surface N2O data set. The strongest source is predicted from the 40–60°S latitude band. Strong emissions also are predicted from the northern Pacific Ocean, the equatorial upwelling zone, and coastal upwelling zones occurring predominantly in the tropical northern hemisphere. High apparent oxygen utilization (AOU) at 100 m below the mixed layer is found to be correlated positively both to N2O production at depth and to the surface N2O anomaly. On the basis of these correlations, the expedition data are partitioned into two subsets associated with high and low AOU at depth. The zonally averaged monthly means in each subset are extrapolated to produce two latitude‐by‐month matrices in which monthly surface N2O is expressed as the deviation from the annual mean. Both matrices contain large uncertainties. The low‐AOU matrix, which mainly includes surface N2O data from the North Atlantic and the subtropical gyres, suggests many regions with positive summer deviations and negative winter deviations, consistent with a seasonal cycle predominantly driven by seasonal heating and cooling of the surface ocean. The high‐AOU subset, which includes the regions most important to the global N2O ocean source, suggests some regions with positive winter deviations and negative summer deviations, consistent with a seasonal cycle predominantly driven by wintertime mixing of surface water with N2O‐rich deep water. Coupled seasonal changes in gas transfer coefficients and surface N2O in these important source regions could strongly influence the global ocean source.
A comparative study of tropical Pacific sea surface height variability: Tide gauges versus the National Meteorological Center data‐assimilating ocean general circulation model, 1982–1992
Authors: Enfield David B. , Harris Jesse E.
Published: February 2004

To help assess the effectiveness of the model‐based analysis and prediction procedures at the National Meteorological Center (NMC), we compare the seasonal and nonseasonal components of sea level from 44 tide gauges in the tropical Pacific with those of the dynamic heights output by two 11‐year model reanalyses (1982–1992) at the same locations, which differ mainly in their wind forcing. Both reanalyses assimilate ocean thermal data and incorporate most of the procedures used by NMC in producing operational ocean analyses and experimental coupled model climate forecasts. The reanalyses reproduce the broad patterns of annual amplitude and phase and of seasonal and nonseasonal variance, except for severe underestimates along the eastern boundary, especially north of the equator. The annual cycles and interannual departures of zonal flow indices estimated from selected island pairs near the dateline show good correspondence for the North Equatorial Countercurrent (NECC) and somewhat flawed and noisy comparisons for the North Equatorial Current (NEC) and South Equatorial Current (SEC). The reanalyses also reproduce the large‐scale time and space patterns of nonseasonal variability in the first three empirical orthogonal functions (EOFs), which together explain about 65% of the anomalous variability and characterize the El Niño‐Southern Oscillation cycle. The first two EOF modes describe the westward migration of three ENSO episodes, and the third mode appears to capture differences between episodes. However, the reanalysis based on the anomalous winds generated by the NMC medium‐range forecast model shows significant discrepancies in the large‐scale spatial and temporal variability. These discrepancies disappear in the reanalysis based on departures of the Florida State University analyzed wind fields. Hence the wind forcing critically affects the reanalysis in spite of the assimilation of ocean thermal data. Future improvements in the atmospheric model to produce a more realistic evolution of the wind field can therefore lead to significantly better model integrations in the analysis and initialization mode (with data assimilation) as well as in the coupled model forecast mode.
Offshore transport of dense shelf water in the presence of a submarine canyon
Authors: Chapman David C. , Gawarkiewicz Glen
Published: February 2004

The formation and offshore transport of dense water over a uniformly sloping shelf crosscut by a submarine canyon is examined using a three‐dimensional primitive‐equation numerical model. A constant negative buoyancy flux is applied in a limited region adjacent to a straight coast to represent brine rejection from ice production in an idealized coastal polynya. A sharp density front forms at the edge of the forcing region, with surface and bottom intensified jets along the front. The flow around the head of the submarine canyon triggers a frontal instability that initially grows only on one side of the canyon. The unstable waves on the other side of the canyon are blocked by a localized barotropic flow that develops near the canyon head. Unstable waves also grow where the forcing region intersects the coast. The frontal waves grow rapidly (with O(1 day) e‐folding timescales) and form eddies with horizontal scales of O(15 km) which extract the densest water from the forcing region and carry it offshore, directly across isobaths. In this way the eddies limit the maximum water density that appears in the model despite continued negative buoyancy forcing. Some dense water descends into the canyon, forming a bottom‐trapped plume that transports the dense water offshore ahead of the eddies. The plume moves relatively slowly (i.e., small Froude number), with little turbulent entrainment, so the advancement and structure of the plume nose can be described successfully as a simple gravity current with an advective‐diffusive heat balance. Eddies may slump into the canyon from the side, altering both the density anomaly and speed of the canyon plume, suggesting that canyon plumes are likely to be highly variable in both space and time.
Polarimetric signatures of sea ice: 2. Experimental observations
Authors: Nghiem S. V. , Kwok R. , Yueh S. H. , Drinkwater M. R.
Published: February 2004

Experimental observations of polarimetric signatures are presented for sea ice in the Beaufort Sea under cold winter conditions and interpreted with the composite model developed in Part 1. Polarimetric data were acquired in March 1988 with the Jet Propulsion Laboratory multifrequency airborne synthetic aperture radar (SAR) during the Beaufort Sea Flight Campaign. The experimental area was located near 75°N latitude and spanned 140°–145°W longitude. Selected sea ice scenes contain various ice types, including multiyear, thick first‐year, and thin lead ice. Additionally, the C band SAR on the first European Remote Sensing Satellite provides supplementary backscattering data of winter Beaufort Sea ice for small incident angles (20°–26°) at vertical polarization. Sea ice characterization and environmental data used in the model were collected at the Applied Physics Laboratory drifting ice station to the northeast of Prudhoe Bay; additional data from field and laboratory experiments are also utilized in this analysis. The model relates sea ice polarimetric backscattering signatures to physical, structural, and electromagnetic properties of sea ice. Scattering mechanisms contributing to sea ice signatures are explained, and sensitivities of polarimetric signatures to sea ice characterization parameters are studied.
On the meridional surface profile of the Gulf Stream at 55°W
Authors: Hallock Zachariah R. , Teague William J.
Published: February 2004

Nine‐month records from nine inverted echo sounders (IESs) are analyzed to describe the mean baroclinic Gulf Stream at 55°W. IES acoustic travel times are converted to thermocline depth which is optimally interpolated. Kinematic and dynamic parameters (Gulf Stream meridional position, velocity, and vorticity) are calculated. Primary Gulf Stream variability is attributed to meandering and changes in direction. A mean, stream‐coordinate (relative to Gulf Stream instantaneous position and direction) meridional profile is derived and compared with results presented by other investigators. The mean velocity is estimated at 0.84 m s−1 directed 14° to the right of eastward, and the thermocline (12°C) drops 657 m (north to south), corresponding to a baroclinic rise of the surface of 0.87 m. The effect of Gulf Stream curvature on temporal mean profiles is found to be unimportant and of minimal importance overall. The derived, downstream current profile is well represented by a Gaussian function and is about 190 km wide where it crosses zero. Surface baroclinic transport is estimated to be 8.5×104 m2 s−1, and maximum shear (flanking the maximum) is 1.2×10−5 s−1. Results compare well with other in situ observational results from the same time period. On the other hand, analyses (by others) of concurrent satellite altimetry (Geosat) suggest a considerably narrower, more intense mean Gulf Stream.
A comparison of a hydrodynamic lens model to observations of a warm core ring
Authors: Holdzkom J. J. , Hooker Stanford B. , Kirwan A. D.
Published: February 2004

Analysis of advanced very high resolution radiometer satellite images has shown long‐period fluctuations in ring eccentricity for 82‐B. Concurrent shipboard measurements as well as satellite data also suggest that this anticyclonic ring was often paired with one or more cyclonic eddies. Furthermore, satellite image analysis and recent numerical simulations suggest strong deformation in the fluid between the anticyclone and the cyclone. The purpose here is to investigate a causal relation between the time dependent environmental deformation and the eccentricity fluctuations. To this end, we attempt to parameterize the effects of the cyclone as environmental forcing on the anticyclone using a nonlinear, non‐quasi‐geostrophic, hydrodynamic lens model. Our results suggest that the strong deformation in the environmental flow could be responsible for long‐period fluctuations in eccentricity. Other model ring features, such as spin (vorticity/2) and rotation rate of 82‐B, are discussed and compared to observations. Another significant feature of the model results is the presence of superinertial frequencies in the solution fields. Since current mesoscale observation techniques are unable to resolve such high frequencies, it is unknown whether or not this result is realistic.
On the influence of coastline orientation on the steady state width of a latent heat polynya
Authors: Darby Mark S. , Willmott Andrew J. , Somerville Tracy A.
Published: February 2004

This paper investigates under what conditions the steady state shape of a coastal latent heat polynya is influenced by (1) the direction of flow of free‐drifting frazil ice and the consolidated offshore ice sheet and (2) coastline orientation. Analytical solutions are obtained for coastal polynyas when the coastline is straight and when a coastline has a single step. From the solutions it is shown that the asymptotic polynya width is given by the generalized Pease (1987) width, L cos θ, where L is the “classical Pease width” and θ is the angle between the consolidated ice sheet velocity and the normal to the coastline. An expression is also obtained for the alongshore length scale LC, over which the polynya adjusts toward its asymptotic width, and it is shown that LC depends upon the directions of travel of the frazil ice and the consolidated ice sheet. When the frazil ice and the consolidated ice sheet both drift offshore, normal to the coast, LC = 0, in agreement with the classical Pease theory. Variations in the coastline which occur on a length scale much smaller than LC are not reproduced in the polynya shape. Two numerical simulations are presented for Terra Nova Bay, Antarctica, and the St. Lawrence Island Polynya in the Bering Sea. Both examples employ realistic coastline geometry and information about the direction of drift of the consolidated ice sheet. In each example a realistic polynya shape is obtained. The examples highlight the versatility of the model because in the former case the ice sheet drifts almost parallel to the coast, while in the latter it drifts offshore.
Seasonal cycle of temperature and associated wave phenomena in the upper layers of the Bay of Bengal
Authors: Kumar S. Prasanna , Unnikrishnan A. S.
Published: February 2004

The seasonal cycle of temperature in the monthly mean Levitus (1982) data is analyzed by a least squares method fitting the observed temperature with a cosine function having a period of 1 year. It is found that the signal is most pronounced in the subsurface levels (100–200 m), with high amplitudes in the western and eastern parts of the Bay of Bengal and low values in the central region. The peak of the seasonal cycle appears in the westernmost region during April, while in the easternmost region it appears during August. The amplitude of the seasonal cycle of temperature in the western and northern region is influenced by the local wind stress curl. From the values of phase obtained, wavelength and phase speed of annual wave are computed. At 100 m, the computed phase speed along 12.5°N is 21 cm s−1. According to theory (White, 1977), this annual wave, which is considered to be the sum of the effects of local wind stress and westward propagating Rossby waves, propagates westward with a phase speed equal to twice that of nondispersive Rossby waves. At 12.5°N, the theoretical phase speed is 23 cm s−1. There is a reasonable agreement between the computed and theoretical phase speeds at 17.5°N also. From the depth of 20°C isotherm along 12.5°N and 17.5°N, there are signatures of westward propagating Rossby waves. Along 12.5°N, their phase speed is 8 cm s−1, whereas the theoretical value is 11.5 cm s−1. The signatures of northward propagation along the eastern periphery of the Bay of Bengal seen in the plots of the depth of 20°C isotherm do not appear to be locally wind‐driven.
Hydrography and geochemistry of sea surface hydrothermal plumes resulting from Hawaiian coastal volcanism
Authors: Sansone Francis J. , Resing Joseph A.
Published: February 2004

The entry of lava into the ocean at Kilauea Volcano, Hawaii has produced extensive sea surface hydrothermal plumes. Studies at this site over 1989–1991 showed that particle‐rich, chemically altered thermal plumes of 1–3 m thickness can be mapped for distances up to 2 km from the entries using an array of physical and chemical tracers. Particle concentrations in the plumes were as high as 65 mg L−1, and produced light attenuation anomalies exceeding 11 m−1 the maximum observed seawater temperature was 69°C. The following extremes in plume chemical markers were measured: H2, 2.1 μM; dissolved Mn, 320 nM; dissolved Si, 55 μM; ∑CO2, 1.6 mM; alkalinity, 1.5 meq L−1; and pH, 7.1. Dissolved H2 linearly increased, and ∑CO2, pH, and total alkalinity decreased with increasing seawater temperature. Dissolved Mn and Si also increased with increasing temperature but were better correlated with each other, indicating that heat is not conserved in the surface plumes. Heat and mass inputs to the ocean are comparable to those from mid‐ocean ridge crest vent fields.
The release of brine‐enriched shelf water from Storfjord into the Norwegian Sea
Authors: Schauer Ursula
Published: February 2004

Brine‐enriched water masses are formed through surface cooling, freezing, and subsequent convective mixing and can be accumulated at the bottom of Arctic shelves. Time series from moored instruments over 1 year (1991/1992) in the northwestern Barents Sea reveal the flow of such water from a generation area in the coastal polynya in the Storfjord of Svalbard toward the western shelf edge. A volume of the order of 1012 m3 of cold, brine‐enriched shelf water was released from this site into the Norwegian Sea during 5 months. The salinity of almost the entire water mass (96%) ranged between 34.8 and 35.1 practical salinity units. The source water of the observed outflow was provided by the East Spitsbergen Current advecting Arctic Water during summer and early winter and a mixture of Arctic and Atlantic Water during late winter. Owing to the broad salinity range, brine‐enriched shelf water from the northwestern Barents Sea acts as a freshwater source for intermediate and as a salt source for deep waters of the Norwegian and Nansen Basins.
Observations of steady and seasonal salt, heat, and mass transport through a tidal channel
Authors: Smith Ned P.
Published: February 2004

One‐year time series of current speed, water temperature, and salinity from a tidal channel in the Exuma Cays, Bahamas, are used to investigate the annual mean and time‐varying transport of salt, heat, and mass. Time series of salinity, heat content, density, and volume transport are decomposed into annual mean, low‐frequency (days to seasons), and high‐frequency (tide dominated) components, and perturbation analysis is used to quantify transport. When perturbation products are averaged over the entire year, steady transport dominates the averages of the time‐varying terms. Harmonic analysis reveals distinct seasonal variability. Seasonal cycles in salt, heat, and mass transport arise from forcing over seasonal timescales and from tidal exchanges in the presence of seasonally varying hydrographic gradients between Exuma Sound and Great Bahama Bank. Maximum seasonal import and export values are small relative to the steady transport. Wind forcing plays a central role, acting both directly and indirectly to produce about half of the annual mean volume transport and most of the seasonal variability about the mean.
Offshore wind forcing in the Gulf of Tehuantepec, Mexico: The asymmetric circulation
Authors: Trasviña A. , Barton E. D. , Brown J. , Velez H. S. , Kosro P. M. , Smith R. L.
Published: February 2004

Since the early surveys carried out by the Eastern Tropical Pacific (EASTROPIC) and Scripps Tuna Oceanographic Research (STOR) projects in the tropical Pacific off Mexico, the northerly winds which blow over the Gulf of Tehuantepec were described as an important factor controlling the dynamics of this coastal ocean. In January–February 1989 an international team carried out the experiment Tehuano, designed to study the response of the gulf to these wind pulses. The complete evolution of the coastal ocean after an event of moderate intensity was observed. The forcing is characterized by a mostly symmetric, fanshaped, offshore wind jet, which in turn produces a remarkably asymmetric upper ocean response. While analytical results based on Ekman theory forced by a symmetric offshore wind predict the formation of a symmetric dipolar circulation, the observed flow consists mainly of a large (200 km in diameter) anticyclonic warm‐core eddy in the western gulf, with only a weak cyclonic counterpart in the eastern gulf. Intense surface cooling under the wind jet is caused by entrainment of subsurface water into the upper layer. The thermocline in the west deepens with the development of the eddy, which is formed initially by the advection of warm surface waters from west of the gulf. East of the axis of the wind, the mixed layer deepens due to wind‐induced entrainment, while, at the same time, shoaling and compression of the deeper isotherms by curl‐induced upwelling (Ekman pumping) strengthen the thermocline.
Assimilation of satellite altimeter data into an open ocean model
Authors: Vogeler Armin , Schröter Jens
Published: February 2004

Geosat sea surface height data are assimilated into an eddy‐resolving quasi‐geostrophic open ocean model using the adjoint technique. The method adjusts the initial conditions for all layers and is successful on the timescale of a few weeks. Time‐varying values for the open boundaries are prescribed by a much larger quasi‐geostrophic model of the Antarctic Circumpolar Current (ACC). Both models have the same resolution of approximately 20×20 km (1/3°×1/6°), have three layers, and include realistic bottom topography and coastlines. The open model box is embedded in the African sector of the ACC. For continuous assimilation of satellite data into the larger model the nudging technique is applied. These results are used for the adjoint optimization procedure as boundary conditions and as a first guess for the initial condition. For the open model box the difference between model and satellite sea surface height that remains after the nudging experiment amounts to a 19‐cm root‐mean‐square error (rmse). By assimilation into the regional model this value can be reduced to a 6‐cm rmse for an assimilation period of 20 days. Several experiments which attempt to improve the convergence of the iterative optimization method are reported. Scaling and regularization by smoothing have to be applied carefully. Especially during the first 10 iterations, the convergence can be improved considerably by low‐pass filtering of the cost function gradient. The result of a perturbation experiment shows that for longer assimilation periods the influence of the boundary values becomes dominant and they should be determined inversely by data assimilation into the open ocean model.
Study of seasonal transport variations in the Indonesian seas
Authors: Miyama Toru , Awaji Toshiyuki , Akitomo Kazunori , Imasato Norihisa
Published: February 2004

Seasonal transport variations between the Pacific and Indian Oceans via the Indonesian seas were studied by the Euler‐Lagrangian method. The velocity field was calculated with a fairly high resolution robust diagnostic model. The model well reproduces the features of seasonal variations in the Indonesian seas. The total volume transport of the Indonesian throughflow is 20±3 Sv (1 Sv = 106 m3 s−1), the maximum being from boreal spring to boreal summer and the minimum in boreal winter. The values are similar to those of previous general circulation models with a wide Indonesian passage despite resolution of the presence of the many small islands in the Indonesian seas. Although a large portion of the net transport is contained in the upper layer, deep transport below 1000‐m depth is about 5 Sv. This value corresponds to approximately 25% of the total transport, which means that disregard of the deep transport leads to underestimation of the volume transport of the throughflow. Tracking of numerous labeled particles in the calculated velocity field clarified the sources and pathways of the Indonesian throughflow. The major route is a western one through both the Makassar and Lombok Straits. Most of the North Pacific water supplied from the Mindanao Current passes along this route, entering the Indian Ocean within several months with almost no loss of its properties (intense vertical mixing around the Lombok sill reported by observations could not be reproduced in our model). In contrast, South Pacific water takes the eastern route into the eastern Indonesian seas and subsequently mixes with waters from the North Pacific and Indian Oceans in the Banda Sea, which means that it has a long travel time (at least a few years). Water taking the eastern route therefore loses its original properties before arriving in the Indian Ocean. The transport processes also are significantly affected by seasonal variations in equatorial circulation in the western Pacific. In the surface layer, North Pacific water is vigorously supplied to the western route only from boreal spring to summer in association with the linkage between the current flowing through the Makassar Strait and the Mindanao Current. In other seasons, because the Mindanao Current is strongly linked with the North Equatorial Countercurrent and the New Guinea Coastal Current primarily by northeasterly monsoonal winds, its upper water flows back to the Pacific Ocean. In the subsurface layer, a pronounced inflow of Mindanao Current water into the western route occurs from boreal winter to spring, when the subsurface link between that current and the Equatorial Undercurrent tends to weaken. In the deep, the quasi‐steady transport of Pacific water into the Indian Ocean via the eastern route is fed by the westward deep current in the equatorial Pacific.
Comparison of TOPEX/POSEIDON altimetry and in situ sea level data at Sao Tome Island, Gulf of Guinea
Authors: Verstraete Jean‐Marc , Park Young‐Hyang
Published: February 2004

In situ sea level data from a shallow pressure tide gauge (mean depth 3.21 m) at Sao Tome Island, Gulf of Guinea, and altimeter data from TOPEX/POSEIDON (T/P) were analyzed and compared. The Texas model, a T/P‐derived tide model, is used for the tidal correction of altimeter data. Sea level anomaly time series from both data sets were low‐pass filtered by using a Gaussian recurrent interpolation scheme, suppressing high‐frequency fluctuations of periods much less than 2 months. According to the 1992–1993 Sao Tome tide gauge data, a clear seasonal signal was observed both in 1992 and in 1993, presenting two sea level maxima in February and October and two minima in June and December. These sea level variations are consistent with the known seasonal mean sea level cycle of the area, most of which can be explained by steric sea level changes due to the seasonal cycle of upper 500‐m water column properties in the Gulf of Guinea. On the basis of the 60‐day low‐passed tide gauge time series, significant year to year variations were observed as well, with a 6.3‐cm sea level difference in February (+8.1 cm in 1992 versus +1.8 cm in 1993) and a 4.4‐cm difference in October (+4.6 cm in 1992 versus +9.0 cm in 1993). Minimum sea levels, occurring just prior to the upwelling season (July–August), were almost the same for both years (−9 to −10 cm) but lasted much longer in 1992 (2 months) than in 1993 (2 weeks). During the 15‐month period of concomitant observations, from October 1992 to December 1993, these seasonal and year to year sea level variations are remarkably well reproduced by the T/P sea level time series, with an rms difference of 2.2 cm and a correlation coefficient of 0.88. This result in the equatorial Atlantic is consistent with other intercomparisons in the Pacific and Indian Oceans. This excellent recovery of the oceanic signal is undoubtedly the result of unprecedented high precision of T/P measurements and the reliable T/P‐derived tide model that is used in the present study.
Mean sea level and surface circulation variability of the Mediterranean Sea from 2 years of TOPEX/POSEIDON altimetry
Authors: Larnicol Gilles , Le Traon Pierre‐Yves , Ayoub Nadia , De Mey Pierre
Published: February 2004

We describe the circulation and mean sea level variations of the Mediterranean Sea from 2 years of TOPEX/POSEIDON altimetric data. It is first shown that the response of the Mediterranean Sea to atmospheric pressure forcing is close to an inverse barometer (except at high frequencies) which means that the adjustment is accompanied by a flow through the Straits of Sicily and Gibraltar. We then use TOPEX/POSEIDON to study the mean sea level variations, representing steric effects and integrated large‐scale changes of the mass of the Mediterranean Sea. We observe an annual cycle with a fast drop during winter. Steric effects account for about half of the observed variations. The remaining signal is believed to be driven by evaporation minus precipitation (E ‐ P) forcing and internal hydraulic control in the Straits of Gibraltar. Using suboptimal space‐time objective analysis, the classic components of the Mediterranean surface circulation are recovered, despite low signal‐to‐noise ratio (the rms of sea level variability is less than 10 cm). The variable Mediterranean circulation is seen as a complex combination of mesoscale and large‐scale variations. The surface circulation is more complex in the eastern basin than in the western basin. In the east it is composed of subbasin‐scale gyres, such as the so‐called Mersa‐Matruh and Shikmona gyres, which do not have an obvious recurrence period. We also observe an intensification of the large‐scale cyclonic winter circulation in the western and in the Ionian basins. Several mesoscale structures, such as the Alboran gyres and the Ierepetra gyre, show a clear seasonal cycle, with a maximum in summer. The good qualitative and quantitative agreement of the results with previous data from the Mediterranean illustrates the improved accurary of TOPEX/POSEIDON over its predecessors.
Effects of Indo‐Pacific throughflow on the upper tropical Pacific and Indian Oceans
Authors: Verschell M. A. , Kindle J. C. , O'Brien James J.
Published: February 2004

The dynamics and influence of the connection between the Pacific and Indian Oceans in the Indonesian Archipelago (PACIO) on the upper tropical Pacific and Indian Oceans are investigated using a reduced gravity global model. Two different model geometries are used; the first has an open PACIO region (open case) that permits normal transport, the second has a closed PACIO region (closed case) that allows no transport. The model is forced by either monthly 1980–1989 European Centre for Medium‐Range Weather Forecasting wind stresses or by an imposed Rossby wave pulse. The average upper layer thickness of the Indian Ocean is significantly greater and more variable because of the influence of PACIO transport. The effect in the Pacific Ocean is much smaller, and there appears to be no appreciable effect of PACIO transport on interannual variability in the eastern tropical Pacific Ocean. RMS differences between the open and closed cases show areas of low‐frequency (> ½ year) variability over most of the southern Indian Ocean, the western boundary of the Indian Ocean, the region of the Mindanao Eddy, and the eastern equatorial Pacific waveguide. Areas of significant high‐frequency variability are the East African Coastal Current, the Mindanao Eddy region, the Coral Sea Basin, and in the eastern equatorial Pacific waveguide. The large effect on the southern Indian Ocean is linked to off‐equatorial Rossby waves generated by transport through the PACIO region.
A tracer study of mixing in the Santa Cruz Basin
Authors: Ledwell James R. , Bratkovich Alan
Published: February 2004

A tracer release experiment to study diapycnal and isopycnal mixing was performed in Santa Cruz Basin in 1988–1989. Approximately 1450 g of sulfur hexafluoride were injected close to an isopycnal surface at 1500 m depth in May 1988. This depth is midway between the bottom of the basin at 1920 m and the sill at 1084 m. The diapycnal dispersion measured during the first few months, while the tracer was still mixing to the walls of the basin, yielded a diapycnal eddy diffusivity of 1.0 cm2 s−1, within a factor of 1.5. This is approximately 4 times larger than the diffusivity found by Ledwell and Watson (1991) in the interior of Santa Monica Basin, where the buoyancy frequency was 5 times greater. The tracer was well mixed to the walls 6 months after the injection, and at that time the diapycnal spread of the tracer was greater near the walls than in the interior. The rate of diapycnal dispersion of the tracer increased dramatically after the tracer had mixed to the walls of the basin, indicating enhanced boundary mixing. An effective basin‐wide diapycnal diffusivity of 10 cm2 s−1 would be required to support the fluxes of tracer observed. A one‐dimensional model for tracer and heat diffusion does not simulate the results very well, however, probably because of the finite mixing time between the boundary region and interior and because of the complexity of the processes in the boundary region.