bluuebunny/crossref_metadata_2025 huggingface dataset:
Eddy‐ and wind‐forced shelf circulation
Authors: Oey Lie‐Yauw
Published: February 2004

Cochrane and Kelly (1986) proposed a cyclonic gyre as the large‐scale mean circulation on the Louisiana‐Texas (LATEX) shelf, produced by a convergence of coastal currents in the west and a divergence in the east. While currents near the coast are presumably wind and buoyancy driven, the origin of the eastward flow on the outer shelf and shelf break, which forms the seaward limb of the gyre, as well as the near shore convergence and divergence, are not well understood. A numerical model is used to show that the western convergence and shelf break current are driven by collision and stalling of westward propagating Loop Current eddies in the northwest Gulf of Mexico and the divergence in the east is caused by shoreward intrusion in the Mississippi Canyon. The western convergence and shelf break current are modulated by the wind curl, strongest in summer and weakest in winter. On the shelf, westward transport is comparable to that observed (∼0.15 Sv, 1 Sv = 106 m3 s−1) only when the westward wind stress is significant (>0.3 dyn cm−1). A peak transport of 0.21 to 0.25 Sv occurs in autumn, of which 0.1 Sv is due to wind, 0.07 Sv is due to river buoyancy, and 0.04 to 0.08 Sv is due to eddies. Without the mean westward wind, buoyant waters from the Mississippi do not spread onto the LATEX shelf.
A barotropic model of the currents off SE South America
Authors: Glorioso Pablo D. , Flather Roger A.
Published: February 2004

A depth‐averaged numerical model with 13‐km grid resolution was applied to the region defined by the 55°W meridian, the 56°S parallel and the southeast coast of South America. This region is characterized by high biological productivity, and by an exceptional tidal regime that interacts with the subpolar Falkland/Malvinas (FM) current. The results presented contribute to the development of an ecohydrodynamic system intended for the study of the implications of increasing exploitation of natural resources in the region. Detailed tidal charts show the propagation on the continental shelf of tidal waves related to clockwise amphidromes in the South Atlantic, indicating the occurrence of resonance. The averaged velocity field is dominated by the FM current, although tidal residuals around headlands are significant on the Patagonian coast. In contrast, “snapshots” describe the interaction between the tides and the FM current, where the flow patterns displayed are expected to influence the population dynamics of biological species. The mean wind stress induces strong currents and a large anticlockwise circulation cell northeast of Tierra del Fuego. The strong tidal and FM currents generate shelf sea fronts whose locations, revealed by contours of the Simpson‐Hunter parameter, are related to major fishing grounds. Because of the lack of velocity measurements in the region, the FM and wind‐driven currents simulated by the model should be taken as experimental results until proper comparisons with observations can be made.
Numerical simulations of the Arabian Sea using tracers as proxies for phytoplankton biomass
Authors: Gallacher P. C. , Rochford P. A.
Published: February 2004

A set of numerical simulations were conducted to determine the potential interannual variation of phytoplankton biomass in the Arabian Sea. A four‐layer, reduced gravity model was forced with interannually varying, monthly mean winds for the years 1981 to 1990 and a seasonally varying, monthly mean climatological heat flux. Passive tracers, with fixed source and decay rates, were used as proxies for phytoplankton. The tracers were released in the second layer, and upwelling combined with interlayer mixing transported the tracers into the first (the top) layer of the model. In the top layer advection, diffusion and specified decay rate produced distributions of tracer concentration that we compare with coastal zone color scanner (CZCS) images. In addition to maps of tracer distribution, we constructed time series from areal averages of tracer concentration. The time series are particularly useful for analyzing the temporal variability of the tracer concentrations. There was substantial interannual variability in the results and a strong seasonal signal associated with the southwest monsoon. The years of maximal tracer concentration were 1990 and 1985. The first was associated with a decadal maximum in wind speed and the second with a large wind stress curl over the western Arabian Sea. The years with minimal tracer concentration were 1981 through 1983. We compare our results with published analyses of composited CZCS images during the southwest monsoon in 1981 and 1982. We find general agreement between our results and the CZCS images during the peak of the southwest monsoon; however, we find that during the early phase of the monsoon, when the winds are strengthening rapidly with time, the CZCS images are badly aliased by year to year changes in the time periods used to composite available satellite data.
Tidal dissipation and stratification in the Gulf of California
Authors: Argote María Luisa , Amador Alberto , Lavín M. F. , Hunter John R.
Published: February 2004

The effect of tidal mixing upon the distribution of stratification and sea surface temperature (SST) in the Gulf of California is studied by direct observation and by numerical modeling. Data from conductivity‐temperature‐depth surveys made in September and December of 1986 are used to describe the distribution of stratification and SST. A vertically integrated finite differences numerical model was used to obtain the geographical distribution of available turbulent kinetic energy (TKE) from the M2 tide. The following three areas were found to be strongly affected by tidal mixing: the shallow coastal area at the head of the gulf, the shelf south of Tiburón island, and the area of the midgulf archipelago. The first two are vertically well mixed and are therefore similar to other shelf sea frontal areas. Dissipation occurs all around the islands of the archipelago, and the sills are foci of the most intense tidal dissipation. Stratification in this area was reduced with respect to the background, but complete vertical mixing does not occur. Therefore the strong SST fronts that occur in this area do not mark the boundary between well‐mixed and stratified water. The Simpson‐Hujiter criterion (Simpson and Hunter, 1974) for stratification and frontal position
produced 2.75 ≤ λcrit ≤ 3.0 for the frontal position in the shallow northern gulf. This value of λcrit is larger than in other shelf sea fronts, which may be due to horizontal advection, which is not included in the criterion. We found that the effect of advection may be of the same order of magnitude as that of buoyancy flux. In the area over the sills, λ > λcriti the available TKE is not enough to produce complete mixing over the ∼ 400 m of water over the sills.
Kuroshio sea surface height fluctuations observed simultaneously with inverted echo sounders and TOPEX/POSEIDON
Authors: Teague W. J. , Hallock Z. R. , Jacobs G. A. , Mitchell J. L.
Published: February 2004

Sea surface height anomalies measured by inverted echo sounders with pressure gauges (IES/PGs) and TOPEX/POSEIDON are compared at six locations in the northwest Pacific near the Kuroshio where it separates from the Japanese coast. The instruments are positioned along a TOPEX/POSEIDON descending ground track near 35°N, 143°E. Good correlation is found between measurements, with best agreement near the Kuroshio, where the range of sea surface height change is largest. Correlations between IES/PG and TOPEX/POSEIDON sea surface height variations range from 0.70 to 0.97. Tides modeled and removed from the TOPEX/POSEIDON data at the inverted echo sounder sites compare well with tides measured by the IES/PGs. TOPEX/POSEIDON and IES/PG data are complementary in describing mesoscale variability in the Kuroshio region.
The sensitivity of an eddy‐resolving model to the surface thermal boundary conditions
Authors: Xu Weimin , Greatbatch Richard J. , Lin Charles A.
Published: February 2004

We compare the effect of using four different formulations for the surface thermal boundary condition on a primitive equation eddy‐resolving model. The first formulation is the conventional restoring boundary condition. This relaxes the surface temperature of the model to a specified “restoring temperature” on a timescale of 30 to 60 days. The second formulation calculates the surface heat flux interactively by coupling the ocean model to a simple atmospheric model with an effective restoring time of several hundred days. The third formulation (Rahmstorf and Willebrand, 1995) (RW0) is a simplified energy balance model without atmospheric heat transport. The fourth formulation (Rahmstorf and Willebrand, 1995) (RW1) is a linearized energy balance model with atmospheric heat transport parameterized as a diffusion term. The biggest impact is on the vertical structure of the temperature variance. Under the restoring condition the maxima in this variance always occur beneath the surface. Under the other three boundary conditions, maxima are found at the surface and/or subsurface levels, depending on geographical location and in closer agreement with observations. There is also an increase in the magnitude and eastward extension of both the eddy and mean kinetic energy at midlatitudes and in the subpolar gyre region with the use of less constraining surface boundary conditions. We suggest that the use of a conventional restoring surface boundary condition acts to suppress mesoscale variability in eddy‐resolving models. The northward heat transport is also increased by using the RW0 and RW1 formulations. The main reason for the enhancement of eddy variability and northward heat transport using the RW0 and RW1 surface boundary conditions is the release of the mean state.
Direct measurements of transport and water properties through the Bering Strait
Authors: Roach A. T. , Aagaard K. , Pease C. H. , Salo S. A. , Weingartner T. , Pavlov V. , Kulakov M.
Published: February 2004

Four years of temperature, salinity, and velocity data enable a direct computation of volume transport and a temporal description of water properties exchanged through the Bering Strait. The mean volume transport over the 4‐year period (September 1990 through September 1994) is 0.83 Sv northward with a weekly standard deviation of 0.66 Sv. The maximum error in this mean estimate is 30%. Interannual variability in transport is typically 0.1 Sv but can, at times, reach nearly 50% of the mean. The transport of 1.14 Sv during the first 9 months of 1994 is the largest in the last 50 years. The rate of winter salinity increase is very similar from year to year, suggesting regional average ice formation of about 5 cm d−1. The amplitude of the annual salinity cycle is about 2 psu, with salinity reaching a maximum in early April. There can be large interannual variations in the salinity (about 1), particularly in winter. Background autumn salinities average 32.0 in the eastern and 32.6 in the western channel.
Passive microwave airborne measurements of the sea surface response at 89 and 157 GHz
Authors: Guillou C. , English S. J. , Prigent C. , Jones D. C.
Published: February 2004

Microwave measurements of the ocean‐roughened surface have been collected during several recent international experiments using an airborne radiometer observing at 89 and 157 GHz. The purpose of this project is to test and validate the sea emissivity model required for the future humidity sounder, advanced microwave sounder unit B, over a wide range of atmospheric and surface conditions. In this paper, the measurements are statistically analyzed and compared with a geometric optics model with special emphasis on the sensitivity to the input parameters. This model is shown to provide good overall agreement with the data when coupled to the wave slope description of Cox and Munk (1954), the liquid water dielectric permittivity of Liebe et al. (1991), and the foam coverage of Monahan and Lu (1990), after increasing the theoretical predictions by a bias of about 2.5 K at 89 GHz and 1.7 K at 157 GHz. In addition, an empirical emissivity algorithm derived from low‐frequency observations (Hollinger, 1971; Stogryn, 1972) and widely used for satellite retrieval purposes is shown to be inappropriate for use at millimeter frequencies.
Tracking eddies in the southeast Atlantic and southwest Indian oceans with TOPEX/POSEIDON
Authors: Gründlingh Marten L.
Published: February 2004

Altimetric data from the first year of the TOPEX/POSEIDON mission and National Oceanic and Atmospheric Administration infrared imagery are used to investigate characteristics of eddies and rings in the oceans around southern Africa. Altogether, 57 eddies were identified. The Agulhas rings in the southeast Atlantic were mostly anticyclonic, drifted at 3–7 cm s−1 in a westerly to northwesterly direction, and it was estimated that five rings are generated annually. Even in close proximity to the Agulhas retroflection, the altimetry provided insight into rings not visible on infrared imagery. The annual heat flux of five rings into the South Atlantic is estimated at 3–8×1013 Watt. Eddy motion in the southwest Indian Ocean seemed more confused where a comparable number of cyclonic and anticyclonic eddies advected mostly in a westerly or northwesterly direction. Eddies were observed to adhere to and entrain water from the offshore edge of the Agulhas Current.
The contribution of TOPEX/POSEIDON to the global monitoring of climatically sensitive lakes
Authors: Birkett C. M.
Published: February 2004

Remote sensing and long‐term monitoring of closed and climatically sensitive open lakes can provide useful information for the study of climatic change. Satellite radar altimetry offers the advantages of day/night and all‐weather capability in the production of relative lake level changes on a global scale. A simple technique which derives relative lake level changes is described with specific relevance to the TOPEX/POSEIDON geophysical data record data set. An assessment of the coverage and global tracking performance of both the NASA radar altimeter and the solid state altimeter over these lakes is discussed, and results are presented for the first 1.75 years of the mission. Lake level time series were acquired for 12 closed lakes, six open lakes, and three reservoirs, providing information in many cases where ground gauge data are unobtainable or the lake is inaccessible. The results, accurate to ∼4 cm rms, mark the beginning of a very accurate lake level data set, showing that TOPEX/POSEIDON can successfully contribute to the long‐term global program.
Thickness, structure, and properties of level summer multiyear ice in the Eurasian sector of the Arctic Ocean
Authors: Eicken H. , Lensu M. , Leppäranta M. , Tucker W. B. , Gow A. J. , Salmela O.
Published: February 2004

In August and September 1991, thickness, structure, and properties of level multiyear ice were studied at 66 locations in the Eurasian sector of the Arctic Ocean. The mean ice thickness was 2.86 m, with 0.31 m of freeboard (including 0.08 m deteriorated ice of mean density 370 kg m−3). On the basis of the study of ice cores, 61% of the ice cover consisted of undeformed columnar ice, the remaining 39% consisted of a mixture of ice types including frazil (18%) and deformed ice (9%). Through microstructural studies, six main classes of pores could be identified. The mean density of the ice cover increased from 720 kg m−3 at the top to >880 kg m−3 below 0.4 m depth. Sea ice salinities (mean value 2.1‰) correlate with ice thickness. On average, salinity profiles exhibit a linear increase from values close to 0‰ at the top to 2‰ at 1 m depth, with less steep salinity gradients below. Sampling from different depths within the ice cover indicates that the brine in summer sea ice is strongly stratified. The influence of meltwater percolation is evident, with salinities around 5‰ and pH values <8 at the top and >15‰ and >8 at greater depths. Brine volumes (ranging from 80 to 150‰) are controlled by the thermodynamic equilibrium between the solid and liquid phases. Gas volumes decrease from >200‰ at the top to <50‰ below 1 m depth. Pore microstructure is highly variable even on small scales. Salinity and other ice properties do not vary to a large degree between different regions. The evolution of level multiyear sea ice is discussed with particular reference to “hidden” occurrence of deformed ice and the importance of ablation processes.
Quasi‐coherent structures in the marine atmospheric surface layer
Authors: Boppe R. S. , Neu W. L.
Published: February 2004

This paper presents a study aimed at identifying burst structures in the marine atmospheric surface layer. Standard ejection detection schemes, including the the quadrant, the variable interval time averaging, and the modified u‐level techniques, were applied to turbulent wind data measured over the ocean. It was found that the proportion of contribution to the Reynolds stress from the four quadrants of the u′w′ plane is in close agreement with the corresponding contributions in laboratory flows. Ejection detection by the quadrant technique followed by the grouping of ejections into bursts yielded a mean burst period of 47 s at a height of 8.2 m above the water surface, where the mean wind velocity was 6.74 m/s. This burst period corresponds well with the peaks obtained from the short‐time autocorrelation of the streamwise velocity signal and the first moment of the stress cospectrum. Phase averages of these events show a structure which is similar to the structure of the events detected in laboratory flows. These events do not scale on the “inner” variables, friction velocity u* and the kinematic viscosity ν, as has been suggested for laboratory flows. Both ejection and burst periods decrease with increasing wind speed, however, the grouping of ejections into bursts breaks down at higher wind speeds. These periods showed no dependence on the surface wave field.
Sea level variations induced by heating and cooling: An evaluation of the Boussinesq approximation in ocean models
Authors: Mellor George L. , Ezer Tal
Published: February 2004

In this paper a sigma coordinate ocean model is modified to remove the commonly used Boussinesq approximation so that the effect of thermal expansion is exactly included in the basic equations in order to cope with the seasonal heating cycle and the detection of climate change through variation in sea level height. Tests are performed to evaluate the differences between Boussinesq and non‐Boussinesq calculations under different heating and cooling conditions and different model domains. For an idealized case of a flat bottom, shallow ocean basin without wind forcing, simulations of a warm eddy show that the non‐Boussinesq dynamics have only a minor effect on the baroclinic current field. However, vertically averaged velocities, though small compared with the baroclinic velocities, are cyclonic for the Boussinesq calculation and anticyclonic for the non‐Boussinesq calculation. The results indicate that global or closed basin Boussinesq models should be able to simulate most of the observed steric sea level changes on seasonal or climate timescales, when corrected by a spatially uniform, time‐dependent factor calculated from the volume‐averaged density change. The seasonal variation of the globally averaged sea level calculated from climatological data is small, about 1 cm. Variations in steric sea level in regional models, both Boussinesq and non‐Boussinesq, may differ from those of global models owing to the unknown transport across their boundaries associated with the local heating and cooling. A spatially uniform, time‐dependent correction, similar to that associated with thermal expansion, is proposed to account for transport across open boundaries of regional models. Variations of sea level obtained from a Boussinesq model of the Atlantic Ocean approximate the seasonal signal due to the heating/cooling cycle of each hemisphere as observed by satellite altimeter data.
Wind speed dependence of acoustic backscattering
Authors: Nützel B. , Herwig H.
Published: February 2004

The results from acoustic scattering measurements conducted in the North Sea during the Synthetic Aperture Radar and X Band Ocean Nonlinearities (SAXON)‐Forschungsplattform Nordsee (FPN) experiment are presented. Scattering strength values were obtained for selected frequencies (5–70 kHz) and for wind speeds up to 20 m/s. The grazing angle dependence was measured for 50 kHz at three different sea state conditions. Two grazing angle regimes were identified (above and below 60°) where the wind speed dependence of acoustic backscattering shows a different behavior. One grazing angle from each regime (normal incidence and 45°) was investigated to explain the physical processes that dominate the backscattering in the different wind speed regimes. In the low grazing angle regime the backscattering strength is caused by the small‐scale surface roughness at low wind speeds and by subsurface bubbles at high wind speeds. There is only weak dependence of backscattering in the intermediate wind speed regime. Scattering and absorption of a random swarm of interacting bubbles inside the propagation range lead to undefined acoustic intensities in front of the surface. Thus scattering from a boundary layer instead of sea surface backscattering has to be considered. The presented acoustic data set is a base for further correlation with radar measurements that were conducted simultaneously. Good correlation between electromagnetic and acoustic backscattering is expected for the wind speed regime in which the scattering mechanisms are the same.
Satellite altimetry from a short‐arc orbit technique: Application to the Mediterranean
Authors: Bonnefond P. , Exertier P. , Schaeffer P. , Bruinsma S. , Barlier F.
Published: February 2004

A short‐arc orbit technique was developed, and a very precise and efficient tool for regional orbit determination with a rigorous error budget is proposed for analyzing the TOPEX/POSEIDON (T/P) and ERS 1 missions. The purpose is to be able to determine very accurately, in some conditions, the local orbit, with a very small error in the radial direction, using satellite laser ranging data. A precision at the level of 2 cm is obtained. With this it was possible to cross validate all the different orbit determination techniques as well as the results of the altimeter calibration, showing the great progress made with the TOPEX/POSEIDON mission. After that the T/P mean sea profiles were computed and improved with high accuracy, thanks also to the short‐arc orbit technique. As a result, a combined mean sea surface using TOPEX/POSEIDON and ERS 1 data has been computed with crossover difference techniques, the T/P mean sea profiles serving as reference. Fine structures of the mean sea surface have been evidenced. Using this combined mean sea surface, the ERS 1 altimeter calibration was performed over the Mediterranean, again using the short‐arc orbit technique. The result is in good agreement with the previous ones (−41.6±7 cm). Finally, the T/P mean sea profiles were computed for different seasons and compared with a yearly averaged mean sea surface. Variations of about 15 cm are evidenced, associated notably with the thermal expansion between fall and winter. The short‐arc orbit technique appears to be well suited for monitoring very accurately the temporal variability of the Mediterranean Sea at a subcentimetric level.
A statistical analysis of extreme events in current variations due to internal waves from the Australian North West Shelf
Authors: Pelinovsky E. , Holloway P. E. , Talipova T.
Published: February 2004

Measurements of current fluctuations due to internal waves obtained from two fixed point moorings on the Australian North West Shelf have been analyzed with the aim of calculating the probability of short‐scale, large‐amplitude internal waves exceeding particular values. Such waves are generated as a result of breaking or hydrodynamical instability of the semidiurnal internal tide. The statistical stationarity of each record has been checked, and it is shown that the records can be used for the calculation of the frequency by which currents exceed a particular value for wave heights (double amplitudes) of less than 60 cm s−1. The theoretical forms for exceedance frequency from the theory of extreme statistics have been checked, and it is shown that for one location a power asymptotic provides a slightly better descriptor of the statistics than an exponential asymptotic and for the other location an exponential asymptotic is found to provide the better fit. The exceedance frequency for internal waves near the Australian North West Shelf is found to be less than 1.3 waves per hour. The exceedance probability for each location has been calculated using Poisson statistics.
CFC‐113 shows Brazil eddy crossing the South Atlantic to the Agulhas retroflection region
Authors: Smythe‐Wright D. , Gordon A. L. , Chapman P. , Jones M. S.
Published: February 2004

Using the new ocean tracer CFC‐113 and other hydrographic and chemical data, we have identified and “aged” a Brazil eddy transported across the South Atlantic to the Cape Basin. This is the first observation of such an eddy so far east. It was observed during two RRS Discovery cruises in January and May 1993, and, using TOPEX‐POSEIDON altimeter data, we have been able to track it between the two locations. TS and nutrient characteristics show that the eddy could not have an eastern Atlantic origin. Comparison with eddies found in the Brazil/Falklands Confluence suggests that this is the most likely source. From the CFC‐113:CFC‐11 ratio it appears that the eddy is in excess of 4 years old, and from the two sightings and the altimeter data we have estimated a transport rate for the eddy and given credence to the CFC‐113:CFC‐11 and CFC‐113:CFC‐12 ageing technique in surface waters.
Ocean tide model obtained from TOPEX/POSEIDON altimetry data
Authors: Matsumoto Koji , Ooe Masatsugu , Sato Tadahiro , Segawa Jiro
Published: February 2004

Tidal corrections are very important for oceanographic study of altimetry because tidal signals contaminate the low‐frequency part of raw altimetric signals. Accurate tidal predictions are also necessary for geophysical research, such as with the superconducting gravimeter (SCG), which needs to distinguish very weak signals from the overwhelming tidal signals. The aim of this paper is to derive accurate global ocean tide models from the TOPEX/POSEIDON sea surface height data of 5 cm accuracy, more accurate than any other previous altimeter data. TOPEX geophysical data records of cycles 9–94 were processed and analyzed. We describe response analysis of tidal waves at crossover points and application of hydrodynamical interpolation. The tidal solutions with 0.5° grid system are obtained for eight major constituents (i.e., M2, S2, N2, K2, K1, O1, P1, and Q1) and compared with the Schwiderski (1980a, b, c) model, the Cartwright and Ray (1991) model, and the eight‐constituent version of the Egbert et al. (1994) model or TPXO.2. The accuracy of the models is checked using island and bottom tide gauge data in open ocean and using SCG data in Antarctica and Japan.
The meridional circulation in the Southern Ocean and its seasonal variability as seen by the fine‐resolution Antarctic model
Authors: Döös Kristofer
Published: February 2004

A fine‐resolution primitive equation model is used to investigate the seasonal variability of the interocean mass exchange via the Southern Ocean. The meridional overturning is calculated on both depth levels and density layers. The interocean mass exchange is estimated from the meridional stream functions in the different oceans. The annual average is found to be mainly governed by the thermohaline circulation in the so‐called oceanic conveyor belt. The seasonal variability is mainly due to changes in the wind stress, which are not necessarily local. The largest seasonal variability is found in the Pacific Ocean around 30°S and around 34°S in the Indo‐Atlantic. In the Indo‐Atlantic there are two meridional cells merged into each other at the surface, but driven by the northward Ekman transport at two different latitudes. The deepest one, which is part of the conveyor belt, has a seasonal variability driven by the changes in the northward Ekman transport at 35°S. The shallower one (Deacon Cell) is driven further south at 43°S, where the zonal wind stress reaches its maximum. Simple relationships with the meridional Ekman transport, the meridional thermohaline transport, and the total interocean exchange of water masses are found among the three world oceans. The meridional stream function, simulated by an analytical model, shows that seasonal variability of the fine‐resolution Antarctic model is barotropic and its baroclinic circulation is mainly stationary.
Intensive surveys of the Azores Front: 1. Tracers and dynamics
Authors: Rudnick Daniel L. , Luyten James R.
Published: February 2004

The hypothesis that fronts are sites of active subduction is examined using density, temperature, salinity, and horizontal velocity data from a trio of surveys of the Azores Front done in May 1991 and March 1992. These surveys were made using a SeaSoar equipped with a conductivity‐temperature‐depth profiler and a shipboard acoustic Doppler current profiler. The potential density and potential voracity indicate that dense water from the north side of the front may be sliding down beneath the surface outcrop. This apparently subducting isopycnal has a great deal of temperature and salinity variability. Horizontal velocity is nearly parallel to isopycnals, indicating that the time rate of change and vertical advection must be small. The thermal wind balance is observed to be valid, especially in the region of the largest horizontal density gradients. Shear at the base of the mixed layer is likely due to near‐inertial motions. The potential vorticity is dominated by the planetary vorticity, except at the front, where vertical shears (the tilting term) become large. The tilting term acts to reduce the magnitude of the potential vorticity at the front, in agreement with simple theoretical models. The magnitude of the tilting term is similar to the total vorticity in the seasonal thermocline.
Search for proxy indicators of young sea ice thickness
Authors: Zabel I. H. H. , Jezek K. C. , Gogineni S. P. , Kanagaratnam P.
Published: February 2004

The determination of young sea ice thickness from space remains an elusive goal for those interested in the interaction of the oceans and the atmosphere, the thermal and chemical state of the ocean, and sea ice dynamics. Recent experiments and models have shown relationships between active and passive microwave signatures of new, growing ice and ice thickness. The two processes that dominate in determining the microwave signature are changes in dielectric properties and changes in surface roughness. In this paper we investigate the competition between these two processes in determining radar backscatter, the usefulness of surface roughness as an indicator of young ice thickness, and the optimum sensor parameters for observing changes in scattering linked to ice thickness. We present simulations that are based on radar observations made on laboratory‐grown saline ice. These observations confirm that surface scattering dominates over volume scattering for 13.9 GHz radar backscatter from young, rough ice at most angles and for young, smooth ice below 30°. Although rms roughness and backscatter (at 5.3 and 13.9 GHz, 23° incidence, and VV polarization) increase together after about 10 cm of ice growth under quiet conditions, it is unlikely that surface roughness and ice thickness are simply connected in real sea ice, where surface roughness can change rapidly due to the action of wind, waves, and snow. Simulations show, however, that formation of frost flowers is detectable by spaceborne radar and can serve to classify ice of roughly 5–20 cm thickness since it is a distinct, transient event that occurs under physical conditions that constrain the thickness of the ice. Our experimental data show that future sensors operating near 12° incidence may offer potential for probing the relationship between near‐surface dielectric properties and ice thickness, since the effects of variability in roughness and snowfall are minimized near this angle.
Sensitivity of the heat budget in a midlatitude ocean model to variations in atmospheric forcing
Authors: O'Brien Enda W. , Horsfall Fiona
Published: February 2004

The rate of change of total heat storage in the ocean is determined primarily by the net heat fluxes across the air‐sea interface. In the context of a wind‐ and heat‐driven ocean model the long‐term mean heat budget was examined in a series of perturbation experiments in which atmospheric forcing parameters (surface winds and surface air temperature) were systematically varied. The thermodynamic effect of increasing surface wind speed is to take heat out of the mixed layer and thus cool the ocean. However, this wind‐induced cooling is overwhelmed by wind‐induced mechanical deepening of the mixed layer. A mixed layer deepened in this way is also cooler, so heat loss to the atmosphere is reduced. This ultimately leads to a warmer vertically integrated ocean. A change in wind stress forcing has little effect on total heat storage. Surface air temperatures are typically cooler than sea surface temperatures, so warmer air temperatures result in less heat flux out of the ocean, a warmer mixed layer, and more oceanic heat storage. The seasonal cycle is dominated by changes in radiative forcing to which the model responds exactly as it does to surface air temperature changes. As a result the model ocean stores less heat in winter (February–April) than in summer (August–October). The dependence of this seasonal change on mixed‐layer depth, mixed‐layer temperature and deep ocean temperature is quantified. Mixed‐layer depth is by far the most important controlling factor. Comparisons with climatological data from the North Atlantic and North Pacific Oceans confirm this general picture, although the comparisons suggest that the model mixed‐layer depth varies too much between seasons. The climatological data also indicate that the northern oceans store more heat in winter than in summer around 25°N. Possible explanations for this are discussed.
Measurement of 223Ra and 224Ra in coastal waters using a delayed coincidence counter
Authors: Moore Willard S. , Arnold Ralph
Published: February 2004

We describe a nuclear detector system for measuring low activities of 223Ra and 224Ra in natural waters based on an original design of Giffin et al. (1963). Samples are obtained by adsorbing 223Ra and 224Ra onto a column of MnO2 coated fiber (Mn fiber). The short‐lived Rn daughters of 223Ra and 224Ra which recoil from the Mn fiber are swept into a scintillation detector where alpha decays of Rn and Po occur. Signals from the detector are sent to a delayed coincidence circuit, which discriminates decays of the 224Ra daughters, 220Rn and 216Po, from decays of the 223Ra daughters, 219Rn and 215Po. The system is calibrated using 232Th and 227Ac standards with daughters in equilibrium adsorbed on Mn fiber. Results of samples from Tampa Bay, Florida, and the Atchafalaya and Mississippi Rivers mixing zones are reported. The method is extendible to measurements of 227Ac, 231Pa, 228Th, and 228Ra.
Intercalibration of wave and wind data from TOPEX/POSEIDON and moored buoys off the west coast of Canada
Authors: Gower J. F. R.
Published: February 2004

Significant wave height (SWH) and wind speed data from the merged geophysical data record (MGDR) of the TOPEX/POSEIDON satellite altimeters, for the first 47 cycles of satellite coverage (September 1992 to December 1993), are compared with measurements from an array of instrumented buoys moored along and off the west coast of Canada. SWH values from the satellite and buoys cover a range of 0.1 to 8 m, in one case exceeding 12 m. The comparison shows TOPEX waves, on average, low by 5% relative to the buoys, with rms scatter about the mean relation of 30 cm. For buoys lying within 10 km of the satellite tracks, this scatter is reduced to about 15 cm. TOPEX tracks pass sufficiently close to each of the buoys for the satellite altimeter to be used as a “transfer standard,” showing a small difference between the two types of buoys in the array and occasional problems with the buoy measurements. A significant difference was found between the satellite and buoy wind speeds, with NASA altimeter winds being high by about 10% relative to the buoys. This can be explained by known factors affecting the buoy data. The small amount of POSEIDON data shows the winds in the MGDR to be 12% low compared with the buoys. The difference between POSEIDON and NASA altimeter winds is confirmed by computing large‐area spatial averages over the Pacific Ocean. The rms scatter for wind speed about the best fit regression was near 2 m/s, reducing to 1.4 m/s when data were restricted to distances less than 10 km from deep water buoys. Atmospheric pressure values, inserted in the satellite data from a global weather model, were found to agree with buoy measurements to within about 1 mbar. Apart from the discrepancy in wind speeds, the results are consistent with both buoy and satellite data meeting or exceeding their design specifications. The effects of spatial variations of wave climate and changes in atmospheric stability are assessed. The comparison demonstrates how satellite and buoy observations can be used for mutual calibration and performance monitoring.
Radium as internal tracer of Mediterranean Outflow Water
Authors: Schmidt Sabine , Reyss Jean‐Louis
Published: February 2004

Radium‐226 and 228Ra data are presented for the Western Mediterranean Sea and the NE Atlantic. In the NW Mediterranean Sea, the near‐shore surface waters of the Ligurian Current contain much higher 228Ra concentrations than surface waters collected in the open Ligurian seawater. The gradient of 228Ra along a Villefranche ‐ Calvi transect has been used to calculate values of the horizontal eddy diffusion coefficient perpendicular to the flow direction of the Ligurian Current of about 106 cm2 s−1. The observed decay of 228Ra, as the Mediterranean Deep Water flows toward the Gibraltar Straits, is in good agreement with the assumption of a rapid turnover of the Mediterranean basin. Concentration profiles of 226Ra and 228Ra were measured in the Mediterranean Outflow Water (MOW) off the Gibraltar Straits in the NE Atlantic basin at two stations, 36°30′N ‐ 15°35′W and 36°27′N ‐ 10°35′W. These sites showed the hydrographic characteristics of Meddies (MOW which propagates as eddies in the NE Atlantic) at intermediate depth. At the center of the Meddies, around 1000 m depth, the activity of the short half‐life 228Ra increases up to 0.9 × 10−2 disintegrations per minute per liter, higher than usually found in North Atlantic Deep Water. This “young” 228Ra results from the contact, a few months earlier, of the MOW with the Spanish continental shelf in the vicinity of the Gibraltar Straits. The decay of 228Ra, used as an internal tracer of water mass motion, leads to a transit time of the Meddy Yseult of less than 1 year since its formation.
Long‐term temperature and circulation changes off eastern Australia
Authors: Ridgway K. R. , Godfrey J. S.
Published: February 2004

Long‐term changes in the thermocline circulation have been observed in the southwest Pacific Ocean from a combined data set of hydrographic and expendable bathythermograph data. The data were divided up into a network of 5° × 5° bins (with some exceptions to allow for coastal features) and mean values of temperature at several depths were obtained for the four pentads between 1970 and 1989. Differences in temperature between pairs of pentads show features which are coherent over scales as large as the Coral Sea. Since there is a high correlation between the temperature at 450 dbar (T450) with steric height in the Tasman and Coral Seas, we have focused our study on the behavior of T450. The 1985–1989 pentad appears to be cooler at 450 m depth than the preceding 15 years over most of the region of study (by more than 1°C in some bins). Applying an empirical orthogonal function analysis to the data shows that the first two modes explain 47% and 25% of the variance, respectively. The first mode consists of a warming from 1970 to 1976, followed by a steady cooling period to the latter part of the 1980's (below the starting temperature); its spatial structure shows that it is centered just to the west of New Caledonia and has a Coral Sea‐wide scale, reminiscent of a weakening of the Sverdrup wind‐driven gyre. The second mode has a more complex spatial character, which is most pronounced in the EAC region. It is qualitatively similar to that found in a recent study of the effect of blocking the Indonesian Throughflow in a global ocean model. It shows a warming from 1970, which plateaus from 1976 to 1983 and then decreases slightly. There are some statistically significant changes in the T‐S relations for the region, but the associated steric height variations are at the noise level. Futhermore, the long‐term vertical profile of temperature change in the top 450 m is shown to be similar to that associated with the energetic changes due to eddy movements. Using the relationship of steric height with T450, the data suggest that there were quite large changes in the circulation over this period. We estimate that the zonal geostrophic inflow to and outflow from the Coral and Tasman Seas at 162°E weakened by 15 and 10 Sv respectively, in the period between the 1975–1979 and 1985–1989 pentads. This is large compared to a similarly estimated long‐term mean inflow of 27 Sv. However, little evidence of broad‐scale, in‐phase El Niño‐Southern Oscillation‐related changes are found south of about 10°S.
Intermediate layer water masses in the western tropical Atlantic Ocean
Authors: Bub Frank L. , Brown Wendell S.
Published: February 2004

Intermediate layer water masses are defined according to temperature‐salinity relationships derived from conductivity‐temperature‐depth (CTD) observations measured during four 1990–1991 Western Tropical Atlantic Experiment (WESTRAX) hydrographic surveys. The intermediate layer, bounded by density surfaces of sigma theta 26.00 and 27.65 (approximately 150 and 1300 m deep, respectively), is conveniently divided into upper and lower layers by the relatively low salinity Antarctic Intermediate Water (AAIW), which is centered at sigma theta 27.25 (approximately 700 m deep). Approximately 60 ± 5% of the region's waters are traced to a southern hemisphere origin, indicating the importance of AAIW in the western tropical Atlantic's water mass structure. The southern source water masses, South Atlantic Central Water and AAIW, enter the WESTRAX region (west of 44°W and between the equator and 9°N) as part of the subthermocline North Brazil Current. Depending on the season, all or part of these southern waters retroflect anticyclonically through the region and flow eastward into the North Equatorial Undercurrent. The primary northern source water mass, North Atlantic Central Water, enters the northeastern corner of the WESTRAX region as part of a cyclonic branch of the North Equatorial Current (NEC) and converges with the southern water. This meeting produces mixture water masses which make up 45 ± 4% of the region in volume and are predominantly of a southern nature. The patterns of the mixture water masses which fill the areas between the source water masses suggest the importance of lateral mixing in this ocean region. Further, some of the mixture water in the upper layer appears to be part of the NEC, suggesting southern water recirculation in the tropical Atlantic gyre. A time dependent water mass box model of advective and mixing transports is used to suggest that lateral mixing dominates vertical mixing by a ratio of approximately 10 to 1. Typical box model results for the fall–winter 1990–1991 period indicate that 13 Sv of mixture water masses are produced through mixing (a sum of 9 Sv and 4 Sv from southern and northern source water masses, respectively), while a net 17 Sv of mixture water masses are exported from the region.
Flow modification and shelf‐slope exchange induced by a submarine canyon off the northeast Spanish coast
Authors: Alvarez A. , Tintoré J. , Sabatés A.
Published: February 2004

We present results from a study of the modifications induced by the short‐timescale response of a shelf break baroclinic jet to the presence of a downstream canyon. The field site was the Canyon of Palamós, located in the northeastern edge of Spain. To determine the short time scale variability of the area, three intensive surveys were carried out at approximately 10‐day intervals. Data indicate that the jet's location, usually over the slope, is subjected to significant onshore‐offshore displacements. Analysis of the data has shown the importance of a small‐scale temporal variability induced by river runoff and the climatology of the area. This variability generates on(off)shore displacements of the jet which interacts with the head(mouth) of the canyon. The different interactions with the canyon induced by these displacements produce different regimes of shelf‐slope exchange in the canyon domain that strongly modify the biomass patterns in the area.
Monthly period oscillations in the Pacific North Equatorial Countercurrent
Authors: McPhaden Michael J.
Published: February 2004

Monthly period oscillations in temperature and velocity in the vicinity of the North Equatorial Countercurrent along 140°W are examined using moored time series data collected during 1988–1989. These oscillations, which presumably derive their energy from instability of the large‐scale shear of the zonal equatorial current system, are found to be energetic across a broad band of periods of about 15–50 days. Along 7°N, wavelike motions at these periods propagate westward with a phase speed of about 30–40 cm s−1and a zonal wavelength of 750–1150 km. The oscillations are approximately in geostrophic balance with the thermal field, with the velocity vector rotating anticyclonically around westward propagating high sea level centers. Zonal volume transport variations associated with these waves between 5° and 9°N at 20–180 m depth have a standard deviation of about 4.1 Sv, with peak‐to‐peak changes in some instances approaching 20 Sv. Maximum temperature variance along 140°W occurs between 5° and 7°N at 100–150 m depth in a region of strong mean vertical and meridional temperature gradient associated with a thermocline sloping upward to the north. At the surface, maximum temperature variability occurs from 2° to 5°N in the vicinity of the strong sea surface temperature front north of the equator. Analysis of the upper ocean temperature balance at 7°N, 140°W reveals that meridional advection is important both in the surface layer and in the thermocline. Vertical advection, estimated using vertical velocity calculated from a simplified linear vorticity balance, is also important in the thermocline, where it contributes a variance comparable to that of meridional advection. Zonal advection is likely to be significant in the upper 80 m, below which it is probably of secondary importance compared to meridional and vertical advection. One consequence of advection in the temperature balance is that variance is shifted toward lower frequencies in temperature vis‐a‐vis velocity spectra. This “red shift” may account for the apparent difference between periods for instability waves inferred from near‐equatorial meridional velocity spectra, and slightly longer periods reported for oscillations of the sea surface temperature front north of the equator based on satellite imagery.