bluuebunny/crossref_metadata_2025 huggingface dataset:
Seasonal variability of sea surface salinity and salt budget of the mixed layer of the north Indian Ocean
Authors: Rao R. R. , Sivakumar R.
Published: May 2003

A subset of the recently published salinity database of the global oceans is utilized to characterize and explain the observed seasonal variability of sea surface salinity of the north Indian Ocean, in greater detail than has been possible previously. The influence of salinity on the seasonal evolution of near‐surface mixed layer depth is highlighted. The relative importance of freshwater flux (evaporation minus precipitation) and horizontal advection in accounting the observed seasonal variability of sea surface salinity is evaluated. The influence of massive river outflow in producing the observed sea surface salinity minima in the coastal northwestern Bay of Bengal during August–September is highlighted. The observed interannual variability of sea surface salinity along two major shipping lanes in the tropical Indian Ocean in relation to El Niño is examined. The annual average of sea surface salinity shows contrasting distributions in the Arabian Sea and the Bay of Bengal due to differences in hydrological forcing. The seasonal variability of sea surface salinity is most pronounced in the coastal region of the northern Bay of Bengal, northwestern Arabian Sea, and the southeastern Arabian Sea. The incorporation of salinity effect reduces the thickness of the near‐surface mixed layer, and this reduction is most pronounced in the Bay of Bengal, where it builds up from June to July and becomes most prominent by February in the following year, when the freshening effects of hydrological forcing through local rainfall and river discharges are felt the most on the near‐surface layers. The salt budget analysis of the mixed layer shows a broad agreement between the patterns of observed and diagnosed seasonal changes caused by freshwater flux and horizontal advection, despite limitations in the accuracy of these estimates. The freshwater input through rainfall and river discharges in the Bay of Bengal far exceeds evaporation, causing surplus freshwater for export. Horizontal advection of salinity is found to be important in the southeastern Arabian Sea during winter and in the western and eastern Arabian Sea during the summer monsoon season and in the Bay of Bengal throughout the year with the exception of premonsoon season. The pronounced dilution observed during the height of the summer monsoon season in the coastal northwestern Bay of Bengal is attributed to peak discharges from major rivers. Historic data along two major shipping lanes in the tropical Indian Ocean have clearly revealed the signature of El Niño in the interannual variability of sea surface salinity.
A semiempirical model of the normalized radar cross‐section of the sea surface 1. Background model
Authors: Kudryavtsev Vladimir , Hauser Danièle , Caudal Gérard , Chapron Bertrand
Published: May 2003

Multiscale composite models based on the Bragg theory are widely used to study the normalized radar cross‐section (NRCS) over the sea surface. However, these models are not able to correctly reproduce the NRCS in all configurations and wind wave conditions. We have developed a physical model that takes into account, not only the Bragg mechanism, but also the non‐Bragg scattering mechanism associated with wave breaking. A single model was built to explain on the same physical basis both the background behavior of the NRCS and the wave radar Modulation Transfer Function (MTF) at HH and VV polarization. The NRCS is assumed to be the sum of a Bragg part (two‐scale model) and of a non‐Bragg part. The description of the sea surface is based on the short wind wave spectrum (wavelength from few millimeters to few meters) developed by Kudryavtsev et al. [1999] and wave breaking statistics proposed by Phillips [1985]. We assume that non‐Bragg scattering is supported by quasi‐specular reflection from very rough wave breaking patterns and that the overall contribution is proportional to the white cap coverage of the surface. A comparison of the model NRCS with observations is presented. We show that neither pure Bragg nor composite Bragg model is able to reproduce observed feature of the sea surface NRCS in a wide range of radar frequencies, wind speeds, and incidence and azimuth angles. The introduction of the non‐Bragg part in the model gives an improved agreement with observations. In Part 2, we extend the model to the wave radar MTF problem.
A semiempirical model of the normalized radar cross section of the sea surface, 2. Radar modulation transfer function
Authors: Kudryavtsev Vladimir , Hauser Danièle , Caudal Gérard , Chapron Bertrand
Published: May 2003

Multiscale composite models based on the Bragg theory are widely used to study the normalized radar cross section (NRCS) over the sea surface. However, these models are not able to correctly reproduce the NRCS in all configurations. In particular, even if they may provide consistent results for vertical transmit and receive (VV) polarization, they fail in horizontal transmit and receive (HH) polarization. In addition, there are still important discrepancies between model and observations of the radar modulation transfer function (MTF), which relates the modulations of the NRCS to the long waves. In this context, we have developed a physical model that takes into account not only the Bragg mechanism but also the non‐Bragg scattering associated with radio wave scattering from breaking waves. The same model was built to explain both the background NRCS and its modulation by long surface wave (wave radar MTF problem). In part 1, the background NRCS model was presented and assessed through comparisons with observations. In this part 2, we extend the model to include a third underlying scale associated with longer waves (wavelength ∼10–300 m) to explain the modulation of the NRCS. Two contributions are distinguished in the model, corresponding to the so‐called tilt and hydrodynamic MTF. Results are compared to observations (already published in the literature or derived from the FETCH experiment). As found, taking into account modulation of wave breaking (responsible for the non‐Bragg mechanism) helps to bring the model predictions in closer agreement with observations. In particular, the large MTF amplitudes for HH polarization (much larger than for VV polarization) and MTF phases are better interpreted using the present model.
Surface salinity budget in oceanic simulation using data from TOGA COARE
Authors: Dourado Marcelo , Caniaux Guy
Published: May 2003

A three‐dimensional regional model is used to investigate the role of different terms of the salinity budget in the western Pacific during TOGA COARE. The model is a version of the ocean general circulation model (OGCM) developed at the Laboratoire d'Océanographie Dynamique et de Climatologie (LODYC) in Paris and includes open lateral boundaries and a 1.5‐level‐order turbulence closure scheme. The surface atmospheric forcing used, including water flux, comes from a combination of European Center for Medium‐Range Weather Forecasts model output and estimates from bulk parameterization. The data set collected during the Intensive Observation Period enables the initialization, taking into account the lateral boundary conditions, and validating the model outputs. Different atmospheric weather conditions were experienced during the simulation of the variability of the mixed layer. During 25 days, air‐sea fluxes were downward, that is precipitation exceeds evaporation, and increasing with time. In periods of weak winds (December 2–12), strong solar radiation, and shoaling of the oceanic mixed layer, the entrainment is negative and salt is removed from the oceanic mixed layer. Mixed layer currents are low and slightly divergent from the equator toward the south with a mean eastward component. Due to the zonal advection, there is a freshening tendency in the north of the domain. In periods of strong winds (December 21–27) and reduced solar radiation, the oceanic mixed layer is deepening and entrainment is positive and contributes to the salinity increase. The salinity storage term is quite inhomogeneous during the two periods as a consequence of the patchy pattern of advection. Moreover, most of its temporal and spatial variability is strongly correlated with the temporal and spatial variability of the advection term. Currents are largely converging toward the equator and create a band of saltier water in the surface layer.
North Atlantic Ocean surface currents
Authors: Reverdin Gilles , Niiler P. P. , Valdimarsson H.
Published: May 2003

Close to 1800 surface drifters are used to investigate the 15 m circulation of the North Atlantic Ocean. The data are used to describe structures of the average Eulerian circulation and of the associated eddy variability. The data resolve scales on the order of 50 km, which have hitherto not been systematically described, in particular, near shelf breaks and near the most intense currents, the Gulf Stream, the North Atlantic Current (NAC), and the frontal currents of the subpolar gyre. This reveals a complex series of quasi‐permanent eddies, meanders, and recirculation patterns. Gulf Stream intensity is portrayed as changing abruptly near 54°W, east of which, it is identified as two current branches centered near 39° and 41.5°N, the northern one connecting more directly with the NAC, and the southern one with the recirculation gyre and the Azores Current (AC). Many features of the currents are controlled by topography, in particular, currents are often intensified near shelf breaks or parallel to ridges in the subpolar gyre. However, the largest northward branch of the NAC in the Icelandic basin is located near the deepest bathymetry, not near steep bathymetry. Other currents, in particular, in the subtropical gyre, are less clearly related to topography: for instance, the AC is featured as a zonal eastward current extending far west of the Mid‐Atlantic Ridge (MAR) to at least 55°W and possibly 63°W. In the interior and away from topographic features the eddy kinetic energy (EKE) is the largest where the mean currents are the largest. In the subpolar gyre, there are striking differences in EKE between southward flowing currents (the Labrador and east Greenland Current) and the northward flowing currents (west Greenland Current and branches of the NAC), which have higher EKE. The areas of weakest variability are located in the southwest part of the subpolar gyre, northeast of Iceland, and in the eastern Atlantic south of 45°N. The AC eddies and the mesoscales south of the Canary Islands transect this eastern eddy desert. Drifter trajectories are used as realizations of Lagrangian particles in the vicinity of current cores. These illustrate the variety of paths or connections between different current systems and demonstrate cross‐stream dispersions.
Variability in the properties of Shelf Water in the Middle Atlantic Bight, 1977–1999
Authors: Mountain David G.
Published: May 2003

The seasonal and interannual variability in the temperature, salinity, and volume of Shelf Water (SHW) in the Middle Atlantic Bight (MAB) is described for the period 1977–1999. Large interannual variations in the volume, salinity, and, to a lesser extent, temperature of the SHW occurred that were coherent over multiple year time periods. The variations in volume and salinity originated through processes acting outside of the MAB and were advected into the region from the Gulf of Maine. On a decadal average the SHW observed in the 1990s was approximately 1°C warmer, 0.25 PSU fresher, and 1000 km3 more abundant than during the 1977–1987 period. The warming during the 1990s was largest in the southern part of the bight during the winter, when the SHW was more than 2°C warmer then during the earlier decade.
Relative effects of wind stress curl, topography, and stratification on large‐scale circulation in Lake Michigan
Authors: Schwab David J. , Beletsky Dmitry
Published: May 2003

This paper uses the results from two multiseason numerical model simulations of Lake Michigan hydrodynamics to examine the relative effects of wind stress curl, topography, and stratification on large‐scale circulation. The multiseason simulations provide a period long enough to encompass the full range of atmospheric and thermal conditions that can occur in the lake. The purpose of this paper is to diagnose the relative importance of various mechanisms responsible for the large‐scale circulation patterns by analyzing the vorticity balance in the lake on a monthly timescale. Five different model scenarios are used to isolate the predominant mechanisms: (1) baroclinic lake, spatially variable wind stress; (2) barotropic lake, spatially variable wind stress; (3) baroclinic lake, spatially uniform wind stress; (4) barotropic lake, spatially uniform wind stress; and (5) barotropic lake, linearized equations, spatially uniform wind stress. By comparing the results of these five model scenarios it is shown that the cyclonic wind stress curl in the winter and the effect of baroclinicity in the summer are primarily responsible for the predominantly cyclonic flow in the lake. Topographic effects are also important but are not as significant as wind stress curl and baroclinic effects. Nonlinear effects are much smaller.
SSM/I integrated water vapor content measurements in coastal regions: A comparison with shipborne and airborne remote sensing measurements, radiosonde measurements, and NWP model retrievals
Authors: Flamant Cyrille , Pelon Jacques , Eymard Laurence , Tournadre Jean
Published: May 2003

The Special Sensor Microwave Imager (SSM/I) integrated water vapor content (IWVC) estimates in a coastal region during nonstationary cold‐air outbreak conditions are analyzed using IWVC retrievals obtained from shipborne microwave radiometry (DRAKKAR), water vapor differential absorption laser remote sensing (LEANDRE 2), radiosonde measurements, and numerical weather prediction (NWP) model ALADIN of Météo‐France. The study focuses on the 24 March 1998 Tramontane/Mistral event, which occurred over the Gulf of Lion (GoL) (western Mediterranean) in the framework of the flux, état de mer et télédétection en condition de fetch variable (FETCH) experiment. Comparisons of SSM/I and ALADIN IWVCs with collocated shipborne microwave radiometry, at distances from the coast between 50 and 100 km, were carried out on a full diurnal cycle. In the morning, when moister conditions prevailed, the agreement between SSM/I (one passage) and DRAKKAR was good while the bias observed in the afternoon (three passages) was larger (i.e., 2 kg m−2). Reasons for this are investigated. We show that this is due to the fact that the sensitivity limit of the algorithm has been reached during very dry Mistral conditions. This bias is not explained by effects of land contamination, which is shown to remain below 0.2 kg m−2. No systematic bias was observed between ALADIN and DRAKKAR. Nevertheless, the trend observed on the DRAKKAR data between 1430 and 2030 UT was best captured by SSM/I despite the sensitivity issue. SSM/I and ALADIN IWVC estimates were also found to be in good agreement with collocated airborne lidar measurements, thereby reproducing the spatial evolution of the IWVC with the distance to the coast.
A numerical study of wave‐current interaction through surface and bottom stresses: Coastal ocean response to Hurricane Fran of 1996
Authors: Xie L. , Pietrafesa L. J. , Wu K.
Published: May 2003

A three‐dimensional wave‐current coupled modeling system is used to examine the influence of waves on coastal currents and sea level. This coupled modeling system consists of the wave model‐WAM (Cycle 4) and the Princeton Ocean Model (POM). The results from this study show that it is important to incorporate surface wave effects into coastal storm surge and circulation models. Specifically, we find that (1) storm surge models without coupled surface waves generally under estimate not only the peak surge but also the coastal water level drop which can also cause substantial impact on the coastal environment, (2) introducing wave‐induced surface stress effect into storm surge models can significantly improve storm surge prediction, (3) incorporating wave‐induced bottom stress into the coupled wave‐current model further improves storm surge prediction, and (4) calibration of the wave module according to minimum error in significant wave height does not necessarily result in an optimum wave module in a wave‐current coupled system for current and storm surge prediction.
Thin and thinner: Sea ice mass balance measurements during SHEBA
Authors: Perovich Donald K. , Grenfell Thomas C. , Richter‐Menge Jacqueline A. , Light Bonnie , Tucker Walter B. , Eicken Hajo
Published: May 2003

As part of a large interdisciplinary study of the Surface Heat Budget of the Arctic Ocean (SHEBA), we installed more than 135 ice thickness gauges to determine the sea ice mass balance. While installing these gauges during the fall of 1997, we found that much of the multiyear ice cover was only 1 m thick, considerably thinner than expected. Over the course of the yearlong field experiment we monitored the mass balance for a wide variety of ice types, including first‐year ice, ponded ice, unponded ice, multiyear ice, hummocks, new ridges, and old ridges. Initial ice thicknesses for these sites ranged from 0.3 to 8 m, and snow depths varied from a few centimeters to more than a meter. However, for all of their differences and variety, these thickness gauges sites shared a common trait: at every site, there was a net thinning of the ice during the SHEBA year. The thin ice found in October 1997 was even thinner in October 1998. The annual cycle of ice thickness was also similar at all sites. There was a steady increase in thickness through the winter that gradually tapered off in the spring. This was followed by a steep drop off in thickness during summer melt and another tapering in late summer and early fall as freeze‐up began. Maximum surface melting was in July, while bottom ablation peaked in August. Combining results from the sites, we found an average winter growth of 0.51 m and a summer melt of 1.26 m, which consisted of 0.64 m of surface melt and 0.62 m of bottom melt. There was a weak trend for thicker ice to have less winter growth and greater net loss for the year; however, ice growth was also impacted by the snow depth. Considerable variability was observed between sites in both accretion and ablation. The total accretion during the 9‐month growth season ranged from zero for thick ridged ice to more than a meter for young ice. Ponds tended to have a large amount of surface melting, while ridges had considerable bottom ablation.
Sand bed roughness in the nearshore
Authors: Gallagher Edith L. , Thornton E. B. , Stanton T. P.
Published: May 2003

Measurements made using sonar altimeters mounted on an amphibious surveying vehicle are analyzed for root‐mean‐square (RMS) bedform roughness. Observations for 22 days over a 500 m × 700 m area are used to quantify the cross‐shore distribution of roughness and its variability. Large bedforms (with amplitudes 5–50 cm) occur frequently in shallow water (<2 m water depth). However, the patchiness of these bedforms is also largest in shallow water. This is likely owing to the spatial and temporal variability of the waves and currents, of the sediment grain size distributions, and of the large‐scale nearshore morphology. Large bedforms were observed to exist only for mobility numbers <150, above this threshold the RMS roughness was <2 cm and the bed was interpreted to have transitioned to sheet flow conditions. However, the state of the bed (e.g., large ripples, small ripples, flat bed) was not predictable for mobility numbers between about 30 and 150. Existing models for orbital ripples do not predict the observed large bedforms and have little predictive skill for these data.
Sea ice growth in the eastern Weddell Sea in winter
Authors: Kottmeier C. , Frey K. , Hasel M. , Eisen O.
Published: May 2003

The energy budget and the formation of sea ice in the eastern Weddell Sea west of Maud Rise are investigated on a regional scale on the basis of data obtained from drifting buoys deployed during the ANZFLUX project in 1994. A one‐dimensional model for snow‐covered sea ice and a kinematic‐thermodynamic sea ice model for ice formed in leads are coupled and run under atmospheric and oceanic conditions specified from in situ data. Flooding at the snow‐ice interface and snow ice formation need to be accounted for to achieve reasonable agreement between modeled and measured temperature distributions. The time‐averaged values for the net heat flux are 21 W m−2, some 60% of which originate in refreezing leads, and 9.8 cm for the monthly rate of ice accumulation. Whereas leads contribute a net accumulation of 10.8 cm, the mass balance of consolidated, snow‐covered ice results in a net ablation of −1.0 cm. Sensitivity studies reveal the contribution of processes related to snow cover and ocean heat flux to sea ice formation and heat exchange. Without taking snow ice formation into account, a net ablation of −14.9 cm occurs under snow‐covered ice due to high oceanic heat fluxes. A comparison between a simple two‐layer model and the one‐dimensional thermodynamic model emphasizes the influence of heat storage in sea ice on the development of heat flux and ice thickness. The heat and mass fluxes obtained on a regional scale, and their sensitivity to the parameterizations, elucidate the implication of vigorous small‐scale processes on sea ice development and point out their importance for sea ice models.
A salt flux model for salinity change through ice production in the Greenland Sea, and its relationship to winter convection
Authors: Wilkinson J. P. , Wadhams P.
Published: May 2003

A salt flux model which incorporates ice formation, advection and melt, as well as time‐dependent brine drainage from frazil‐pancake ice, has been developed for the central Greenland Sea in winter. Information from ice drifting buoys suggests that this region can be thought of as similar to a latent heat polynya, with the wind blowing newly formed sea‐ice constantly in a southerly or southeasterly direction such that it melts in a different area from that of its formation. The result is a salt refinement process generating a net positive salt flux over the ice formation region, which may play an important role in stimulating deep open‐ocean convection. By combining buoy data, ECMWF meteorological data and passive microwave ice concentration data the model is used to estimate the sea ice‐derived salt flux over the 1996–1997 winter. Salt fluxes of over 40 kg m−2 were obtained in the center of the region, which corresponds closely an area where a maximum depth of convection is seen in winter cruises. The model gives realistic results when compared with in situ ice and oceanographic data gathered in the region, and suggests that ice formation and drift play an important role in the preconditioning of surface waters for deep water formation events.
Effects of wave‐current interaction on rip currents
Authors: Yu Jie , Slinn Donald N.
Published: May 2003

The time evolution of rip currents in the nearshore is studied by numerical experiments. The generation of rip currents is due to waves propagating and breaking over alongshore variable topography. Our main focus is to examine the significance of wave‐current interaction as it affects the subsequent development of the currents, in particular when the currents are weak compared to the wave speed. We describe the dynamics of currents using the shallow water equations with linear bottom friction and wave forcing parameterized utilizing the radiation stress concept. The slow variations of the wave field, in terms of local wave number, frequency, and energy (wave amplitude), are described using the ray theory with the inclusion of energy dissipation due to breaking. The results show that the offshore directed rip currents interact with the incident waves to produce a negative feedback on the wave forcing, hence to reduce the strength and offshore extent of the currents. In particular, this feedback effect supersedes the bottom friction such that the circulation patterns become less sensitive to a change of the bottom friction parameterization. The two physical processes arising from refraction by currents, bending of wave rays and changes of wave energy, are both found to be important. The onset of instabilities of circulations occurs at the nearshore region where rips are “fed,” rather than offshore at rip heads as predicted with no wave‐current interaction. The unsteady flows are characterized by vortex shedding, pairing, and offshore migration. Instabilities are sensitive to the angle of wave incidence and the spacing of rip channels.
Volume, heat, and freshwater transports of the global ocean circulation 1993–2000, estimated from a general circulation model constrained by World Ocean Circulation Experiment (WOCE) data
Authors: Stammer D. , Wunsch C. , Giering R. , Eckert C. , Heimbach P. , Marotzke J. , Adcroft A. , Hill C. N. , Marshall J.
Published: May 2003

An analysis of ocean volume, heat, and freshwater transports from a fully constrained general circulation model (GCM) is described. Output from a data synthesis, or state estimation, method is used by which the model was forced to large‐scale, time‐varying global ocean data sets over 1993 through 2000. Time‐mean horizontal transports, estimated from this fully time‐dependent circulation, have converged with independent time‐independent estimates from box inversions over most parts of the world ocean but especially in the southern hemisphere. However, heat transport estimates differ substantially in the North Atlantic where our estimates result in only 1/2 previous results. The models drift over the estimation period is consistent with observations from TOPEX/Poseidon in their spatial pattern, but smaller in their amplitudes by about a factor of 2. Associated temperature and salinity changes are complex, and both point toward air–sea interaction over water mass formation regions as the primary source for changes in the deep ocean. The estimated mean circulation around Australia involves a net volume transport of 11 Sv through the Indonesian Throughflow and the Mozambique Channel. In addition, we show that this flow regime exists on all timescales above 1 month, rendering the variability in the South Pacific strongly coupled to the Indian Ocean. Moreover, the dynamically consistent variations in the model show temporal variability of oceanic heat transports, heat storage, and atmospheric exchanges that are complex and with a strong dependence upon location, depth, and timescale. Our results demonstrate the great potential of an ocean state estimation system to provide a dynamical description of the time‐dependent observed heat transport and heat content changes and their relation to air–sea interactions.
Evolution of stratification over the New England shelf during the Coastal Mixing and Optics study, August 1996–June 1997
Authors: Lentz Steve , Shearman Kipp , Anderson Steve , Plueddemann Al , Edson Jim
Published: May 2003

To investigate the processes influencing the evolution of stratification over continental shelves a moored array was deployed on the New England shelf from August 1996 to June 1997. Temperature, salinity, and current observations spanning the water column were obtained at four midshelf sites, along with meteorological measurements at a central site to estimate the wind stress and the surface heat and freshwater fluxes. Four processes contributed to the seasonal evolution of the stratification. (1) The breakdown of the seasonal thermocline in fall was primarily due to wind forcing, not surface cooling, and occurred in four discrete steps associated with westward, along‐coast wind stress events. Eastward wind stress events of similar magnitude did not reduce the stratification. (2) The water at midshelf remained stratified throughout most of the winter due to saltier shelf‐slope front water displaced onshore by anomalously strong and persistent eastward alongcoast wind stresses. (3) The gradual redevelopment of the thermocline, beginning in April, was primarily a one‐dimensional response to increasing surface heat flux. (4) Stratification in early April and throughout May was substantially enhanced by low‐salinity water associated with river runoff from southern New England that was driven eastward and offshore by upwelling‐favorable (eastward) wind stresses.
Observations of the semidiurnal internal tide on the southern California slope and shelf
Authors: Lerczak J. A. , Winant C. D. , Hendershott M. C.
Published: May 2003

We give a detailed description of the semidiurnal‐band current and temperature variability observed during the Internal Waves on the Continental Margin (IWAVES) field experiments of 1996 and 1997 off of Mission Beach, California. This variability was dominated by the internal tide, and the structure of the internal tide on the slope and shelfbreak region was different from that on the narrow shelf. On the slope and shelfbreak, the internal tide was dominated by alongshore propagating coastal‐trapped waves. In this region, semidiurnal‐band currents were predominantly oriented in the alongshore direction. In the lower half of the water column at a water depth H of 350 m, current and temperature variability were consonant with a short wavelength (∼8 km) bottom trapped wave propagating in the alongshore direction to the north. In the upper 120 m of the water column (above the depth of the shelfbreak), slope and shelfbreak currents were highly coherent with a zero phase lag; that is, there was no phase propagation in the cross‐shore direction. On the narrow (∼10 km) shelf, cross‐shore currents u were much more energetic than on the slope and had the structure of a mode‐one internal wave. The alongshore currents v decreased monotonically from the surface to the bottom of the water column with a phase that did not change with depth. The near‐bottom u signal propagated toward the coast during all mooring deployments, faster in the summer than in the fall. The near‐bottom u and mid‐column temperature relative phase was neither consistent with a purely progressive nor a purely standing mode‐one internal wave. We conclude that the internal tide on the shelf was partially reflected.
Dynamical analysis of the upwelling circulation off central Chile
Authors: Mesias Jorge M. , Matano Ricardo P. , Strub P. Ted
Published: May 2003

In this article we analyze the momentum and vorticity balances of a numerical simulation of the upwelling circulation off central Chile (34°–40°S) and its response to interannual local wind changes. Our analysis indicates that the path of the upwelling jet is strongly controlled by the bottom topography. This topographic steering causes the jet to separate from the coast at the Punta Lavapie cape (∼37°S). Although the zeroth‐order momentum balance is dominated by the geostrophic terms, the circulation is also affected by nonlinear processes, which lead to the formation of large meanders and the shedding of cyclonic eddies north of Punta Lavapie during periods of wind relaxation. The relative contributions of the zeroth‐order vorticity balance and the advective terms are also strongly controlled by changes in the coastline geometry and the bottom topography. Vorticity is created along the current axes and transported toward the coast and the Peru‐Chile Trench, where it dissipates. South of Punta Lavapie the across‐shelf transports are weaker with equatorward flows that are more stable than in the north. Additional numerical simulations indicate that during periods with El Niño conditions, the area is affected with a general weakening of the currents and upwelling activity, although the northern region still shows the formation of eddies. During years with relatively stronger winds, in contrast, the upwelling activity and across‐shelf transport processes are significantly increased. The results show that the Punta Lavapie cape has a large effect on the spatial and temporal variability of the coastal currents in the region off central Chile.
Barotropic eddy generation by flow instability at the shelf edge: Sensitivity to open boundary conditions, inflow and diffusion
Authors: Davies Alan M. , Xing Jiuxing , Gjevik Bjorn
Published: May 2003

A shelf edge model is used to examine the stability of an along shelf flow to changes in potential vorticity of the inflow. Calculations with a two‐dimensional vertically integrated hydrodynamic model, (containing the nonlinear momentum advection terms), using different inflow jet positions and magnitudes show that barotropic instabilities in the along shelf flow can give rise to both stable and unstable shelf modes with associated eddies. The influence of the form of boundary condition, and horizontal viscosity term (parameterized using either a bi‐harmonic or Laplacian operator) upon the solution is also examined. In initial calculations a large region model is used to produce a “benchmark” solution against which limited domain calculations can be tested and the sensitivity of the solutions to open boundary conditions assessed. Calculations are used to examine the influence of a range of inflow and outflow boundaries upon the accuracy of the solution. This involves comparing total solutions and results from an Empirical Orthogonal Function (EOF) analysis. The EOF analysis also yields significant insight into flow features. Comparisons show that Sommerfeld type outflow boundary conditions remain stable and cause very little false reflection provided the horizontal viscosity coefficient in the biharmonic formulation is increased close to the open boundary. In the case of an inflow that produces strong barotropic instabilities then the form of the inflow boundary can influence the solution. If a Laplacian rather than a biharmonic horizontal diffusion is used, then excessive smoothing can occur.
Tracing Pacific water in the North Atlantic Ocean
Authors: Jones E. P. , Swift J. H. , Anderson L. G. , Lipizer M. , Civitarese G. , Falkner K. K. , Kattner G. , McLaughlin F.
Published: May 2003

In the Arctic Ocean, Pacific source water can be distinguished from Atlantic source water by nitrate‐phosphate concentration relationships, with Pacific water having higher phosphate concentrations relative to those of nitrate. Furthermore, Pacific water, originally from the inflow through Bering Strait, is clearly recognizable in the outflows of low‐salinity waters from the Arctic Ocean to the northern North Atlantic Ocean through the Canadian Arctic Archipelago and through Fram Strait. In the Canadian Arctic Archipelago, we observe that almost all of the waters flowing through Lancaster and Jones sounds, most of the water in the top 100 m in Smith Sound (containing the flow through Nares Strait), and possibly all waters in Hudson Bay contain no water of Atlantic origin. Significant amounts of Pacific water are also observed along the western coast of Baffin Bay, along the coast of Labrador, and above the 200‐m isobath of the Grand Banks. There is a clear signal of Pacific water flowing south through Fram Strait and along the east coast of Greenland extending at least as far south as Denmark Strait. Pacific water signature can be seen near the east coast of Greenland at 66°N, but not in data at 60°N. Temporal variability in the concentrations of Pacific water has been observed at several locations where multiple‐year observations are available.
Electromagnetic bias estimation using in situ and satellite data: 2. A nonparametric approach
Authors: Millet Floyd W. , Arnold David V. , Gaspar Philippe , Warnick Karl F. , Smith Justin
Published: May 2003

For the most recent satellite altimeter (Jason‐1), the largest single error budget contribution is the electromagnetic (EM) bias. Nonparametric models have been proposed to reduce the variability of EM bias estimates. In previous work, nonparametric models have been estimated using satellite crossover differences. Using tower data, we show that nonparametric models using wind speed and significant wave height provide some improvement over parametric models. In support of Part I of this paper [Millet et al., 2003], inclusion of the RMS long wave slope improves nonparametric EM bias estimation error values by over 50%. In addition, nonparametric models reduce the historical discrepancy between satellite and tower EM bias measurements.
Antarctic sea ice variability during 1958–1999: A simulation with a global ice‐ocean model
Authors: Fichefet Thierry , Tartinville Benoît , Goosse Hugues
Published: May 2003

A hindcast simulation is carried out with a global ice‐ocean model driven by the National Centers for Environmental Prediction (NCEP)–National Center for Atmospheric Research (NCAR) reanalysis daily surface air temperatures and winds in order to document the variability of the Antarctic sea ice cover during 1958–1999. The model consists of an ocean general circulation model coupled to a thermodynamic‐dynamic sea ice model with viscous‐plastic rheology. Its horizontal resolution is of 1.5° × 1.5°. Both the mean state and variability of the Antarctic ice pack over the satellite observing period are reasonably well reproduced by the model. In particular, from November 1978 to September 1998, the model produces an overall increasing trend in ice area (defined as the total area of ice‐covered ocean) of 11,400 ± 2300 km2 yr−1, which falls within the range of current uncertainty. The model annual mean, area‐averaged ice thickness displays no significant trend over 1958–1999. In contrast, a retreat of the pack is simulated during the second half of the 1970s and the beginning of the 1980s, leading to a loss of ice cover of roughly 0.5 × 106 km2 between 1958–1976 and 1982–1999. The marked weakening of the Antarctic semiannual oscillation that occurred since the mid‐to‐late 1970s seems to be partially responsible for this behavior. Part of the modeled decline might however be spurious and due to the 1979 change in the observing systems utilized in the NCEP–NCAR reanalysis. The simulation also reveals a pronounced decadal variability (8–10 years) in both ice area and thickness. Locally, the changes in ice thickness can reach 1 m during a particular cycle.
Sensitivity of sea ice to physical parameterizations in the GISS global climate model
Authors: Liu Jiping , Schmidt Gavin A. , Martinson Douglas G. , Rind David , Russell Gary , Yuan Xiaojun
Published: May 2003

The GISS coupled model is used to investigate the sensitivity of sea ice to each of the following parameterizations: (1) two sea ice dynamics (CF: cavitating fluid; VP: viscous‐plastic), (2) the specification of oceanic isopycnal mixing coefficients in the Gent and McWillams isopyncal mixing (GM), and (3) the wajsowicz viscosity diffusion (WV). The large‐scale sea ice properties are highly sensitive to sea ice dynamics. With the inclusion of resistance to shear stress, VP captures the major observed sea ice drift features and improves the simulations of sea ice concentrations, thickness, and export through Fram Strait relative to CF. GM significantly improves the simulation of vertical temperature distributions in the Southern Ocean, although it leads to a dramatic reduction of Antarctic sea ice cover. The reduced oceanic isopycnal mixing coefficients lead to Arctic sea ice that tends to be less and thinner in almost the entire Arctic except in the North Pacific and Labrador Sea, while Antarctic sea ice that extends more equatorward throughout the circumpolar regions. The responses of sea ice to WV show an enlargement and thickening of sea ice in the Arctic, within the ice packs around the Antarctic and a reduction and thinning of sea ice in the northern Weddell and Ross Seas. On the basis of these experiments, two composite experiments with the best parameterizations are investigated. The atmospheric responses associated with sea ice changes are discussed. While improvements are seen, there are still many unrealistic aspects that will require further improvements to sea ice and ocean components.
An investigation of altimeter sea state bias theories
Authors: Gommenginger C. P. , Srokosz M. A. , Wolf J. , Janssen P. A. E. M.
Published: May 2003

Two theoretical formulations of the sea state bias (SSB) by Srokosz [1986] (hereafter S86) and Elfouhaily et al. [2000] (hereafter E00) are applied to directional ocean wave spectra from WAM and from NDBC moored buoys collocated with Topex altimeter data. S86 SSB computations obtained for WAM with idealized ocean current fields reveal a quasi‐linear relationship between the SSB coefficient and the r.m.s. ocean slope, which remains valid for a wide range of wind/wave/current conditions. The SSB coefficient is shown to be better parameterized in terms of slope, instead of the significant wave height and wind speed presently used in empirical models. The theoretical SSB for the buoys' 2‐D wave spectra are compared with estimates from five recent empirical SSB models calculated from the collocated Topex data. The magnitude of the E00 SSB shows a strong sensitivity to the choice of high frequency tail model used to extend the 2‐D wave spectra. We find that the E00 theory can adequately model the radar frequency dependence of the SSB given a suitable choice of high frequency tail. However, the E00 theory is found to systematically underestimate the magnitude of the SSB coefficient with respect to empirical SSB models. In contrast, the long‐waves‐only S86 theory displays good agreement with the empirical SSB estimates. Thus our calculations suggest that the sea state bias may be primarily governed by the slope of long gravity waves, presumably through its well‐documented influence on the modulation of short waves, as also suggested by recent empirical findings.
North Atlantic thermal persistence and coherent evolution
Authors: Eshel Gidon
Published: May 2003

North Atlantic thermal persistence and coherent evolution are examined. A region near Cape Cod and the Gulf of Maine is identified as the North Atlantic (NA)'s most thermally persistent, with a unique combination of high‐amplitude, long‐lasting sea surface temperature (SST) anomalies. A mechanism is advanced, whereby that region's surface ocean reflects deep upper ocean dynamics, with thermal evolution that tracks heat advection by the slowly evolving upper ocean. This is consistent with the subtropical gyre playing an active and central role in NA climate variability. The proposed interpretation is supported by broad, high lag correlations between the Bermuda potential vorticity time series (representing the subtropical gyre state) and surface anomalies. Using the Bermuda potential vorticity time series as a “dynamical filter”, successive lag correlation patterns reveal surface oscillations that are not apparent in the raw surface data.
Analysis and comparison of three ecosystem models
Authors: Newberger P. A. , Allen J. S. , Spitz Y. H.
Published: May 2003

Three‐component (NPZ), four‐component (NPZD), and five‐component (NNPZD) nitrogen‐based ecosystem models are compared. The fixed points of the zero‐dimensional systems, with no spatial variation except light attenuation by water, are determined. A linear‐stability analysis shows that unstable steady solutions exist for all three models. Time‐periodic solutions are found in these regions. It is shown that the choice of the values of the parameters in the zooplankton equation is critical in determining steady state concentrations. One‐dimensional model studies allowing variation with depth, self‐shading by phytoplankton, and vertical diffusion show that the differences among the models increase when the available light is influenced by the presence of phytoplankton. With vertical diffusion, periodic solutions, such as those in the zero‐dimensional case, are not found. Finally, a set of one‐dimensional depth‐integrated models, with variability in time and across‐shore coordinate, are formulated. These models include horizontal advection and diffusion and the sinking of detritus (or phytoplankton). Application of a constant offshore advection velocity and coastal boundary conditions consistent with upwelling, produces many of the features seen in two‐dimensional experiments [Spitz et al., 2003]. The dependence of the resulting spatial distributions of phytoplankton and zooplankton on the values of the biological parameters is determined, helping to explain the two‐dimensional results. Significant differences are found between the NPZ and the other models as a result of the sinking of phytoplankton rather than detritus.
Implementation of a multivariate data assimilation scheme for isopycnic coordinate ocean models: Application to a 1993–1996 hindcast of the North Atlantic Ocean circulation
Authors: Brankart Jean‐Michel , Testut Charles‐Emmanuel , Brasseur Pierre , Verron Jacques
Published: May 2003

A 1993–1996 hindcast of the North Atlantic circulation is presented and analyzed. Multivariate satellite observations (sea surface height and sea surface temperature) are assimilated into an isopycnic coordinate ocean model (Miami Isopycnic Coordinate Ocean Model [MICOM]) using a reduced order Kalman filter (the SEEK filter). The objective is to validate the assimilation system in hindcast mode before running a near real‐time exercise as part of the European DIADEM project. The analysis scheme is upgraded to make the statistical parameterization as realistic as possible, and this is shown to be particularly important as soon as a multivariate data set is used. Second, the paper describes the modifications to the SEEK filter that are necessary to meet the requirements of a layered model and to benefit from the advantages of the isopycnic coordinate. In particular, the need for an initialization procedure is explained, and this dynamical adjustment is shown to be more natural and robust in the context of the isopycnic coordinate. The hindcast experiment is validated by comparisons with observations, by studying innovation statistics, and by comparing the results with independent observations (expendable bathythermograph [XBT] profiles).
The FETCH experiment: An overview
Authors: Hauser D. , Branger H. , Bouffies‐Cloché S. , Despiau S. , Drennan W. M. , Dupuis H. , Durand P. , Durrieu de Madron X. , Estournel C. , Eymard L. , Flamant C. , Graber H. C. , Guérin C. , Kahma K. , Lachaud G. , Lefèvre J.‐M. , Pelon J. , Pettersson H. , Piguet B. , Queffeulou P. , Tailliez D. , Tournadre J. , Weill A.
Published: May 2003

The “flux, etat de la mer, et télédétection en conditions de fetch variable” (FETCH) was aimed at studying the physical processes associated with air–sea exchanges and mesoscale oceanic circulation in a coastal region dominated by frequent strong offshore winds. The experiment took place in March–April 1998 in the northwestern Mediterranean Sea (Gulf of Lion). Observations were collected with the R/V L'Atalante, with an air–sea interaction spar (ASIS) buoy, with waverider buoys, and with research aircraft equipped for in situ and remote sensing measurements. The present paper is an introduction to the following special section, which groups 12 papers (including this one) presenting results on turbulent flux measurements at the ocean surface, on the behavior of the marine atmospheric boundary layer, on the ocean waves characteristics, on the ocean circulation, and on remote sensing of surface parameters. This overview presents the background and objectives of FETCH, the experimental setup and operations, and the dominant atmospheric and oceanic conditions and introduces the different papers of the special section.