bluuebunny/crossref_metadata_2025 huggingface dataset:
On the stability of barotropic prograde and retrograde jets along a bottom slope
Authors: Li Sheng , McClimans Thomas A.
Published: September 2002

Barotropic slope currents following isobaths are common features of shelf seas in nonequatorial regions. They are usually remotely forced and stabilized locally by the bottom slope. The stability of these barotropic currents is investigated to determine how well they can be modeled numerically as stationary flows. Both prograde jets, with their cyclonic flank in the deeper water, and retrograde jets, with their cyclonic flank in shallower water, are considered. A necessary condition for instability is at least one extremum in the distribution of the potential vorticity across the jet, which is equivalent to the planetary beta stability problem, and corresponds to the well‐known Rayleigh inflection‐point theorem for the stability problem of a nongeophysical flow. A criterion for instability that involves the bottom slope parameter as a stabilizing factor and the velocity shear as a destabilizing factor has been derived. This criterion suggests that even mild slopes are capable of stabilizing both prograde and retrograde jets. For the case of a cosine jet along an exponential topography, neutral solutions are found to be governed by the Mathieu equation. Because of our use of exponential topography our equation contains more terms than the instability problem for planetary beta on a flat bottom, but the difference tends to vanish when the Rossby number is small. Our results also imply that a given bottom slope stabilizes more wave modes for a retrograde jet, which flows in a direction opposite to topographic Rossby waves.
Results of the Sea Ice Model Intercomparison Project: Evaluation of sea ice rheology schemes for use in climate simulations
Authors: Kreyscher Martin , Harder Markus , Lemke Peter , Flato Gregory M.
Published: September 2002

A hierarchy of sea ice rheologies is evaluated on the basis of a comprehensive set of observational data. The investigations are part of the Sea Ice Model Intercomparison Project (SIMIP). Four different sea ice rheology schemes are compared: a viscous‐plastic rheology, a cavitating‐fluid model, a compressible Newtonian fluid, and a simple free drift approach with velocity correction. The same grid, land boundaries, and forcing fields are applied to all models. As verification data, there are (1) ice thickness data from upward looking sonars (ULS), (2) ice concentration data from the passive microwave radiometers SMMR and SSM/I, (3) daily buoy drift data obtained by the International Arctic Buoy Program (IABP), and (4) satellite‐derived ice drift fields based on the 85 GHz channel of SSM/I. All models are optimized individually with respect to mean drift speed and daily drift speed statistics. The impact of ice strength on the ice cover is best revealed by the spatial pattern of ice thickness, ice drift on different timescales, daily drift speed statistics, and the drift velocities in Fram Strait. Overall, the viscous‐plastic rheology yields the most realistic simulation. In contrast, the results of the very simple free‐drift model with velocity correction clearly show large errors in simulated ice drift as well as in ice thicknesses and ice export through Fram Strait compared to observation. The compressible Newtonian fluid cannot prevent excessive ice thickness buildup in the central Arctic and overestimates the internal forces in Fram Strait. Because of the lack of shear strength, the cavitating‐fluid model shows marked differences to the statistics of observed ice drift and the observed spatial pattern of ice thickness. Comparison of required computer resources demonstrates that the additional cost for the viscous‐plastic sea ice rheology is minor compared with the atmospheric and oceanic model components in global climate simulations.
Carbon 13/Carbon 12 ratios of sedimentary organic matter from the Ross Sea, Antarctica: A record of phytoplankton bloom dynamics
Authors: Villinski Jennifer C. , Dunbar Robert B. , Mucciarone D. A.
Published: September 2002

Suspended and sea ice particulate organic carbon (POC) collected from the Ross Sea, Antarctica, shows seasonal enrichments in 13C unrelated to atmospheric pCO2 variations. The carbon isotopic composition of phytoplankton bloom‐related POC (δ13CPOC) collected from the southwestern Ross Sea during a 1992 bloom event is strongly correlated with POC concentration. This correlation may be attributed to high growth rates, species‐specific fractionation, input from sea ice communities enriched in 13C, increased heterotrophic recycling, or high rates of bloom‐related CO2[aq] drawdown in the upper water column. While all these factors likely contribute to POC 13C enrichment, we favor the latter hypothesis because of the link between 13C enrichment and partial isolation of the surface water CO2 pool in areas of sea ice melt‐induced stratification of the upper water column. The CO2 drawdown hypothesis is further supported by dissolved inorganic carbon δ13C enrichments of up to 2–3‰ in the vicinity of phytoplankton blooms. Patterns of isotopic composition and organic carbon concentration in Ross Sea surface sediments resemble that of the upper water column, particularly in areas of frequent, annually recurrent phytoplankton blooms. Overall, surface sediment δ13C values are 1–3‰ enriched relative to surface water POC, with the highest δ13C values found in areas that receive large amounts of phytoplankton bloom detritus. Future downcore carbon isotopic analyses may prove useful as tracers of bloom dynamics in melt‐water‐influenced areas, where large enrichments in 13C coincide with high productivity.
Experiments on barotropic vortex‐wall interaction on a topographic β plane
Authors: Sansón L. Zavala , van Heijst G. J. F. , Janssen F. J. J.
Published: September 2002

The problem of a barotropic cyclonic vortex, moving on a β plane and interacting with a meridional vertical wall, is studied by means of laboratory experiments and a finite difference numerical model. In the laboratory, the vortex is produced in a rectangular rotating tank with a weakly sloping bottom. This so‐called topographic β plane simulates the latitudinal variations of the Coriolis parameter (β effect). On this β plane, the cyclonic vortex moves to the northwest and eventually interacts with the western wall. Two different results are found, depending on the initial strength and zonal position of the vortex. (1) For strong vortices, opposite‐sign vorticity is created at the wall owing to the no‐slip boundary condition, which leads, together with the cyclone, to the formation of a dipole structure that subsequently moves away from the wall in the northeastward direction. New wall interactions may occur when the original vortex recovers its northwestward motion. (2) In the case of weak vortices, the cyclone remains approximately at the same latitude for some time and later drifts slowly southward until it is dissipated by viscous effects. It is proposed that this behavior is a consequence of the vortex dispersion due to the β effect.
Numerical study of the circulation in a steep canyon off the Catalan coast (western Mediterranean)
Authors: Ardhuin Fabrice , Pinot Jean‐Michel , Tintoré Joaquin
Published: September 2002

We use a limited‐area fine‐resolution primitive equation model to carry out a case study of the circulation in the steep and narrow Blanes Canyon, located off the Catalan coast (western Mediterranean). This area is characterized by a permanent along‐slope density‐driven current and strong northerly and easterly wind bursts in autumn. In a first step the slope current is allowed to adjust to the bottom topography, leading to a stationary circulation pattern which presents large vertical variations related to the canyon geometry. In the surface layer the strong stratification allows the current to flow straight over the canyon rim, ignoring bottom topography. In the intermediate layer the canyon is wide, and waters are isolated in a geostrophic anticyclonic eddy maintained by friction at the edge of the slope current. In the deep layer the canyon is narrow, and a topographic upwelling lifts bottom waters toward the canyon head. In a second step the circulation is forced by easterly (along‐shore with the coast on its right) and northerly (seaward) 1‐day wind bursts which induce significant vertical and cross‐shore motions steered by the bottom slope. This ageostrophic circulation is substantially amplified in the canyon where the flow pattern presents large‐amplitude oscillations characterized by a downwelling (upwelling) during the easterly (northerly) wind burst followed by a rebound upwelling (downwelling) as the wind relaxes. The upwelling induced by the northerly wind burst is capable of renewing waters trapped in the canyon eddy. Our results appear to be in fairly good agreement with in situ physical and biogeochemical data collected in Blanes Canyon and comparable canyons of the world ocean. The three‐dimensional features obtained in this study can explain some of the observed patterns of larvae distribution and exchanges between the coastal zone and the open sea.
Sea ice dynamics in the Weddell Sea simulated with an optimized model
Authors: Harder Markus , Fischer Holger
Published: September 2002

The advection of sea ice and associated freshwater/salt fluxes in the Weddell Sea in 1986 and 1987 are investigated with a large‐scale dynamic‐thermodynamic sea ice model. The model is validated and optimized by comparison of simulated sea ice trajectories with observed drift paths of six buoys deployed on the Weddell Sea ice. The skill of the model is quantified by an error function that measures the deviations of simulated trajectories from observed 30‐day sea ice drift. A large number of sensitivity studies show how simulated sea ice transports and associated freshwater/salt fluxes respond to variations in physical parameterizations. The model reproduces the observed ice drift well, provided ice dynamics parameters are set to appropriate values. Optimized values for the drag coefficients and for the ice strength parameter are determined by applying the error function to various sensitivity studies with different parameters. The optimized model yields a mean northward sea ice volume export out of the southern Weddell Sea of 1693 km3 in 1986 and 2339 km3 in 1987. This shows the important role of sea ice transport for the freshwater budget of the Weddell Sea and gives an indication of its high interannual variability.
Mixing induced by the Atlantic equatorial wave activity in an eddy‐resolving OGCM
Authors: Li Xianjin , Chang Ping
Published: September 2002

The variability of equatorial circulation in the middepth Atlantic Ocean from an eddy‐resolving ocean general circulation model (OGCM) was analyzed. Four types of equatorially trapped waves were identified which dominate the variability of the circulation: an annual Rossby wave, a semiannual Rossby wave, a group of 40‐ to 60‐day short Rossby waves, and a group of 30‐day Yanai waves. In particular, the 40‐ to 60‐day short Rossby waves were found near the western boundary to the north of the equator, which may be related to nonlinear instability processes associated with the north Brazil current (NBC) retroflection. The wave‐induced stirring and transport along the equator was investigated by calculating the trajectories of a large ensemble of particles in the middepth equatorial Atlantic Ocean from the eddy‐resolving, model‐simulated velocity field. The distribution of finite‐time Lyapunov exponents derived from the simulated velocity at a depth of ∼2000 m bears a close resemblence to the observed CFC tongue, suggesting that the wave‐induced chaotic stirring and transport may contribute to the formation of the observed CFC tongue in the middepth equatorial Atlantic Ocean along the equator.
West Florida shelf response to upwelling favorable wind forcing: Kinematics
Authors: Li Zhenjiang , Weisberg Robert H.
Published: September 2002

The barotropic responses of the west Florida continental shelf to idealized upwelling favorable alongshore and offshore wind stresses are studied using the three‐dimensional, time‐dependent, primitive equation Princeton Ocean Model (POM). When forced with uniform winds, the shelf circulation evolves quickly to a quasi steady state. A southeastward alongshore wind lowers sea level along the coast and drives a southeastward coastal jet with a relatively weak northwestward return flow farther offshore. A southwestward offshore wind lowers sea level along the west Florida coast and raises sea level along the Panhandle coast. Two independent circulation gyres are set up in association with these regionally different sea level distributions: an anticyclonic gyre off the west Florida coast consisting of a southeastward coastal jet and a broad return flow over the middle shelf and a cyclonic gyre off the Panhandle coast consisting of a strong northwestward coastal jet and an adjacent narrow southeastward undercurrent. These gyres are separated by the Big Bend region. In both cases (alongshore and offshore wind forcing) the circulations are fully three‐dimensional, with opposing surface and bottom boundary layer flows accounting for the across‐shelf transports. The shapes of the coastline and the isobaths are important determinants of the shelf‐wide responses. Several locally maximum upwelling regions are identified for geometrical reasons. These include the Panhandle coast south of Apalachicola Bay, the west Florida coast south of Tampa Bay, along the Florida Keys, and at the shelf break where topographic Rossby waves are evident.
On the determination of characteristics of the interior ocean dynamics from radar signatures of internal solitary waves
Authors: Brandt P. , Romeiser R. , Rubino A.
Published: September 2002

In this paper we discuss two different methods of inferring characteristics of the interior ocean dynamics from radar signatures of internal solitary waves visible on synthetic aperture radar (SAR) images. The first one consists in the recognition and the interpretation of sea surface patterns of internal solitary waves; the second one consists in the analysis of the modulation depth of the normalized radar backscattering cross section (NRCS) associated with internal solitary waves. For this purpose we consider a data set composed of SAR and in situ measurements carried out from 1991 to 1997 in the region of the Strait of Messina. The recognition and the interpretation of sea surface patterns of internal solitary waves in the Strait of Messina can be used to study characteristics of the density distribution in the area: The internal wave field varies with seasonal variations in the vertical density stratification and with remotely induced variations, i.e., variations induced by the larger‐scale circulation, in the horizontal density distribution. In order to inquire into the possibility of inferring parameters of the interior ocean dynamics by analyzing the modulation of the NRCS associated with internal solitary waves, several numerical simulations are carried out using a radar imaging model. These simulations are performed by assuming different wind conditions and internal wave parameters. It is shown that an accurate knowledge of wind conditions is crucial for deriving internal wave parameters and hence parameters of the interior ocean dynamics from the modulation of measured NRCS associated with internal solitary waves.
An assessment of the Geophysical Fluid Dynamics Laboratory ocean model with coarse resolution: Annual‐mean climatology
Authors: Jiang Shi , Stone Peter H. , Malanotte‐Rizzoli Paola
Published: September 2002

The Geophysical Fluid Dynamics Laboratory Modular Ocean Model 2.2 code with coarse resolution (4° × 3°) is assessed by performing three experiments and comparing their equilibrated solutions with recent observationally based analyses (OBAs). The first experiment (E1) uses subgrid‐scale horizontal diffusion and surface boundary conditions which relax surface temperature and salinity toward observations. The second (E2) replaces the physically incorrect heat and moisture flux boundary conditions of E1 by flux conditions taken from OBAs, plus a term relaxing surface temperatures toward observations. The third (E3) uses the same surface boundary conditions as E2 but replaces the horizontal diffusion by the Gent‐McWilliams (GM) parameterization of isopycnal diffusion. Under the restoring surface boundary conditions (E1), the North Atlantic overturning rate is about 17 Sv, smaller than in OBAs, the maximum poleward heat transport in the Northern Hemisphere is 1.2 Petawatts (PW), also smaller than in OBAs, and in the Antarctic Circumpolar Current (ACC) region the poleward heat transport is 1.3 PW, much larger than in OBAs. Under the more realistic flux boundary condition (E2) the overturning rate increases to an unrealistically large level of 40 Sv, and the poleward heat transports are only slightly improved. When the GM parameterization is employed (E3), the overturning is reduced to 28 Sv, and the poleward heat transport in the ACC region is reduced to 0.3 PW; both results are consistent with OBAs. However, there is only a slight further improvement in the poleward heat transport in the Northern Hemisphere, which now has a peak value of 1.6 PW, still about 0.5 PW less than in OBAs. The sea surface temperature errors in E3 are consistent with the conclusion that the heat transport in the Northern Hemisphere is still being underestimated. All the experiments show strong systematic biases in the salinity field.
Toward the elimination of bias in satellite retrievals of sea surface temperature: 1. Theory, modeling and interalgorithm comparison
Authors: Merchant C. J. , Harris A. R. , Murray M. J. , Závody A. M.
Published: September 2002

The along‐track scanning radiometer (ATSR), launched in July 1991 on ERS‐1, is an infrared radiometer designed to permit retrieval of skin sea surface temperature (SST) to the accuracy required for many climate research purposes. Using the prelaunch retrieval scheme, this accuracy (0.3 K) was achieved only when observations at 3.7 μm were available, i.e., SSTs derived from nighttime scenes before the failure of this channel in May 1992. Retrievals using only channels at 11 and 12 μm suffered significant biases. First, cold biases of up to 1.5 K arose from the radiative effects of the unanticipated presence of a significant loading of stratospheric aerosol following the eruption of Mount Pinatubo in June 1991. Second, cold biases of up to 0.4 K were associated with regions of high water vapor loading. We solve the first problem by choosing retrieval coefficients to be orthogonal to the modeled changes in brightness temperatures caused by variations in stratospheric aerosol optical depth. We attribute the second problem to deficiencies in radiative transfer modeling of water vapor continuum absorption and show that use of an updated parameterization reduces bias from wet atmospheres. Applying the new retrieval coefficients to ATSR data, we find good consistency between SSTs retrieved with and without the 3.7 μm channels, the global mean and standard deviation of differences between retrievals being of the order of 0.05 K and 0.25 K, respectively. We therefore anticipate that reprocessing ATSR data using our new retrieval scheme will result in a substantially improved record of ATSR SST, in that the following should be reduced to insignificant levels: (1) the artefactual trend (previously ∼0.25 K yr−1 in tropical regions) corresponding to the decaying load of post‐Pinatubo aerosol, (2) the discontinuity in SST retrievals (previously up to 0.7 K) associated with the failure of the 3.7 μm channel, and (3) cold biases (previously ∼0.4 K) in wet tropical regions. Thus this work represents a significant advance in terms of the quality of ATSR SSTs for climate research. The techniques are also applicable to both the ATSR‐2, flying on ERS‐2, and the advanced ATSR (planned for launch on the Envisat platform in 2000). However, we note that even with the improved physical modeling on which the new retrieval coefficients are based, we do not yet meet the stringent requirement of 0.1 K decade−1 stability in retrievals for climate change detection purposes.
A two‐dimensional analytic tidal model for a narrow estuary of arbitrary lateral depth variation: The intratidal motion
Authors: Li Chunyan , Valle‐Levinson Arnoldo
Published: September 2002

An innovative method is introduced to solve a two‐dimensional, depth‐averaged analytic model for narrow estuaries or tidal channels with arbitrary lateral depth variations. The solution is valid if the lateral variation of the amplitude of tidal elevation (|Δa|) is small, i.e., |Δa| ≪ a, where a is the amplitude of the tidal elevation. This assumption is supported by a 60‐day observation of elevation in the James River Estuary using pressure sensors at both sides of a cross section of the estuary. The error introduced by the solution is of the order of |Δa|/a, which has a maximum of ∼5% in the James River Estuary. The propagation of the tidal wave (elevation) is therefore essentially one‐dimensional (along the estuary), regardless of the depth distribution, whereas tidal velocity has a strong transverse shear and is three‐dimensional in general. Dozens of depth functions in six groups of various forms are used to calculate the solution. The tidal velocity is highly correlated with the bathymetry. The largest amplitude of the along‐channel velocity is in the deepest water. The phase of the along‐channel velocity in the shallow water leads that in deep water, causing a delay in time of flood or ebb in the deep water. The transverse velocity is generally small in the middle of a channel but reaches its maximum over the edges of bottom slopes. The depth function has a significant effect on the ellipticity and the sense of rotation of the tidal ellipses. By fitting the observed phase of semidiurnal tide in the James River Estuary to the phase of the momentum equation, we have obtained optimal values of the drag coefficient: 1.5 × 10−3 and 1.8 × 10−3 for the spring and neap tides, respectively. Then we apply these values of the drag coefficient and the model to the James River Estuary using the real bathymetry. Results show remarkable agreement between the observations and the model along the transects for both spring and neap tides. The cross‐channel phase difference of the along‐channel velocity between the channel and the shoal is found to be ∼1 hour, a value consistent with that from the model. The model‐estimated lateral variation of elevation is 2.5% of the tidal amplitude, which is slightly smaller than the observed value.
Characterizing the structure of the surface layer in the Pacific Ocean
Authors: Sprintall Janet , Roemmich Dean
Published: September 2002

The structure and statistics of the oceanic surface layer are characterized using quarterly surveyed, eddy‐resolving expendable bathythermograph (XBT) and expendable conductivity‐temperature‐depth (XCTD) data collected along a number of routes spanning the Pacific Ocean. This data set consists of more than 18,000 temperature casts to 800 m, with station spacing of 10 to 40 km along transects between Auckland and Seattle (beginning in 1986), San Francisco and Taiwan (1991), Auckland and Valparaiso (1993), and Honolulu and Valdez (1993). The surface layer can assume many different shapes. It can include strongly stratified layers, actively mixed layers, salinity barrier layers, fossil mixed layers, and inversions. The spatial and temporal distribution of these features within the XBT and XCTD profiles is examined. Fossil layers are predominantly a springtime feature and are associated with regions of Subtropical Mode Water formation in the southwest Pacific (near New Zealand) and the northeast Pacific (near San Francisco). Inversions are less seasonally dependent and most commonly related to interleaving of different water masses in the high shear regions of the California Transition Zone and its counterpart eastern boundary systems in the far northeast and southeast Pacific and in the tropical zonal current system. The XCTD casts show a rich and varied surface layer structure in the equatorial and subpolar regions of the Pacific Ocean that is strongly influenced by the salinity stratification. This highlights the need for complementary salinity and density information in these areas to accurately categorize the true nature of the active mixed layer.
Baroclinic instability in a two‐layer model with a free boundary and β effect
Authors: Olascoaga M. J. , Ripa P.
Published: September 2002

The classical Phillips problem of baroclinic instability is generalized, allowing for free deformations of the bottom boundary. The simplicity of the model is exploited to analyze the effects of the variation of the Coriolis parameter with latitude (the so‐called β effect) on the stability/instability problem. Conservation laws of energy, momentum, and vorticity‐related Casimirs are used to establish nonlinear stability conditions. A spectral analysis reveals that unlike the case of Phillips problem, the β effect can either strengthen or weaken the stability of the basic current, depending on the perturbation scale and the slope of the bottom relative to that of the interface. In particular, the maximal instability occurs in the limit of weak stratification when the planetary and the topographic β effects compensate each other. The maximal unstable wave has an intermediate scale between the internal and the external deformation radii. Nonlinear saturation bounds on unstable basics states are also determined using Shepherd's method. It is found that the enstrophy of the most unstable wave can only be bounded by the total enstrophy of the system.
Relationship between air‐sea gas transfer and short wind waves
Authors: Bock Erik J. , Hara Tetsu , Frew Nelson M. , McGillis Wade R.
Published: September 2002

Laboratory studies have been conducted in two circular wind wave flumes to investigate the relationship between air‐sea transfer velocities of weakly soluble, nonreactive gases and wind‐generated surface waves over clean water surfaces and in the presence of surface films. Detailed surface wave measurements have been made using a scanning laser slope gauge. In the circular tanks, longer gravity waves (wavenumber below 12 rad/m) are hardly affected by surfactant, while shorter waves (above 100 rad/m) are significantly reduced. With higher surfactant concentrations, waves above 200–300 rad/m may be completely eliminated. Because of the absence of narrow‐banded fetch‐limited gravity waves, the wave fields in the circular tanks are significantly different from those in linear wind wave flumes. At a given wind friction velocity, the transfer velocity may decrease by as much as 60% because of surface films. Regardless of the surfactant concentrations, the transfer velocity shows a reasonable correlation with the total mean square slope and with the mean square slope of shorter wind waves (wavenumber above 200 rad/m). However, it shows a poor correlation with the mean square slope of longer wind waves (wavenumber below 50 rad/m). These observations suggest that short wind waves play an important role in air‐sea gas exchange.
A new interpretation of multifrequency/multipolarization radar signatures of the Gulf Stream front
Authors: Ufermann Susanne , Romeiser Roland
Published: September 2002

Radar signatures which are observed on SIR‐C/X‐SAR multifrequency/multipolarization synthetic aperture radar images of the Gulf Stream off the U.S. east coast are compared with results of simulations with a numerical radar imaging model. Based on in situ data, current and wind variations are included into the model as well as a variation of the thermal stability of the marine atmospheric boundary layer across the Gulf Stream front. According to our model predictions, all of these parameter variations can cause radar signatures of similar shape and modulation depth. But, due to specific dependencies of radar signatures on variations of surface currents and winds, we show that it is possible to distinguish between radar signatures of oceanic and atmospheric origin in multifrequency/multipolarization images and to estimate the corresponding current and wind variations independently. For one set of radar images we derive a most likely scenario of oceanic and atmospheric parameters during the time of the image acquisition for which good overall agreement between observed and simulated radar signatures is obtained at most radar channels.
Meteorological and oceanographic surface roughness phenomena in the English Channel investigated using ERS synthetic aperture radar and an empirical model of backscatter
Authors: Scoon Alison , Robinson Ian
Published: September 2002

The empirically derived ERS scatterometer model CMOD4 is used to examine the relationship between radar backscatter measured by synthetic aperture radar (SAR) and wind speed in shallow waters. It is demonstrated that the model can be used to obtain accurate mean wind speed information from ERS SAR, using meteorological synoptic charts to provide wind direction; this approach could enable submesoscale wind variations to be monitored. Results from the wind speed analysis were used to investigate the small‐scale (<200 m) variability of the surface roughness detectable by SAR, the aim being to characterize the effects of parameters additional to the mean wind speed which contribute to the modulations of the backscatter signal. It is shown that the natural small‐scale fluctuation of the SAR‐derived mean wind field barely exceeds the radiometric noise limitations of the instrument. Where the small‐scale variability of the calculated “apparent” wind speed was above this level, the cause could be linked to dynamic oceanographic parameters.
Spray production by air bubbles bursting on a water surface
Authors: Rossodivita A. , Andreussi P.
Published: September 2002

The size and two components of the ejection velocity of the jet droplets generated by bursting bubbles on a water surface have been measured by a phase Doppler particle analyzer. The experiments are for bubbles of diameters in the range 200–1000 μm, which produced drops in the range of 10–150 μm. A careful analysis of raw data allowed the top and second jet drop velocities at the ejection position to be derived. These data agree well with previous measurements and show very good consistency for data taken at distances of 3, 18.5, and 30 mm from the water surface. The analysis of data for the horizontal component of the ejection velocity shows that the mean value of the ejection angle (inclination with respect to the vertical) for droplets in the most populated size band (50–60 μm) is ∼8°–9° and increases with droplet size.
Steady models of Arctic shelf‐basin exchange
Authors: Goldner Daniel R.
Published: September 2002

Steady state exchanges between the Arctic shelves and the central basins are estimated using an inverse box model. Estimates based only on mass, heat, and salt conservation are highly uncertain and cannot determine how the shelf‐basin exchange is partitioned between dense overflows and surface layer exchange. Inclusion of δ18O conservation greatly reduces the uncertainty. With the δ18O constraints, dense water fluxes from the shelves of 0.2±0.1 Sv in the Canadian Basin and 1.4±0.3 Sv in the Eurasian Basin with a small (O(0.1 Sv)) surface layer transport from shelves to basins best satisfy the requirements of steady state. Up to 3.8 Sv of shelfbreak upwelling is consistent within error bars with steady state, but none is required. Water leaving the shelves is replaced primarily from the Bering Strait, the Barents Sea inflow, and runoff and not from the basin surface layer; proper representation of these external sources for the shelves is important for capturing the full range of plausible steady solutions. The steady state solutions identify the balances which, if upset, could lead to Arctic change.
Characteristics of wind‐generated rings in the eastern tropical Pacific Ocean
Authors: Müller‐Karger Frank E. , Fuentes‐Yaco César
Published: September 2002

Eddies are generated in the eastern tropical Pacific (3°S–23°N, 75°–105°W) by winds blowing through Central American mountain passes from the Atlantic. We used Coastal Zone Color Scanner (CZCS) and advanced very high resolution radiometer (AVHRR) satellite imagery complemented with monthly in situ sea surface temperature and wind series from the Comprehensive Ocean‐Atmosphere Data Set (COADS) to study these eddies and their effect on pigment concentrations in the region. Pigment values in the Gulf of Tehuantepec generally reach higher values in November–March before those in the Gulf of Papagayo. The eddies generated in the Gulf of Tehuantepec are associated with passages of cold fronts across the Gulf of Mexico from the north, while the eddies off Papagayo and Panamá are associated with increases in trade wind intensity. CZCS images showed larger numbers of eddies per season than have been previously reported on the basis of in situ and AVHRR observations or numerical simulations. We counted 13 eddies in 1979–1980, 8 in 1984–1985, and 6 in 1985–1986. The eddies transfer both energy and biological constituents from the continental margin to the offshore tropical Pacific. The eddies frequently moved distances in excess of 1500 km from their point of origin. Both anticyclonic and cyclonic eddies are generated, but in general, there are more anticyclones. Anticyclonic eddies generally moved to the southwest. Some cyclonic eddies moved to the south and southeast along the Central American coast and appeared to be trapped by the cyclonic Costa Rica thermal dome. Eddies traveled at speeds varying between 9 and 21 cm s−1 and had diameters of 100–500 km. Phytoplankton concentrations associated with the eddies varied from ∼2 to >10 mg m−3 within ∼70 km of the coast to ∼1 mg m−3 up to 600 km of the coast. Between late April and October, fewer eddies were observed, and phytoplankton concentrations were lower (<0.25 mg m−3) and more uniform over the region.
Study of the formation of North Pacific Intermediate Water by a general circulation model and the particle‐tracking method: 2. Formation mechanism of salinity minimum from the view of the “critical gradient” of the Oyashio mixing ratio
Authors: Kobayashi Taiyo
Published: September 2002

In order to avoid the “false North Pacific Intermediate Water (NPIW)” problem a general circulation model is driven by the winter‐like North Pacific wind that can separate the modeled Kuroshio from the coast around 36°N. In the model a salinity minimum appears around 26.8σθ in the North Pacific subtropical region, which does not outcrop in the North Pacific. Formation processes of the salinity minimum are examined. Comparing them with observations and discussing them sufficiently, this study clarified that NPIW salinity minimum is formed just above the less saline water influenced largely by the Oyashio water. The formation of NPIW is basically controlled by the rapid increase of the Oyashio layer thickness around 26.5–26.8σθ, which layer structure is determined by Okhotsk Sea Mode Water pycnostad at 26.8–26.9σθ and the winter sea surface density of ∼26.5σθ. For NPIW salinity minimum formation it is most important that the gradient of the Oyashio mixing ratio exceeds the “critical gradient” around 26.4–26.7σθ. NPIW salinity minimum density is almost determined by the heaviest density of this range (26.7σθ), which is ∼0.1σθ less dense than the bottom of the rapid increase of the Oyashio layer thickness. Then cabbeling leads to an increase in its density to 26.8σθ. The Sea of Okhotsk is very important for the formation of NPIW through the formation of the Oyashio thick layers, and it practically determines NPIW salinity minimum density.
Artificial neural networks for modeling the transfer function between marine reflectance and phytoplankton pigment concentration
Authors: Gross Lidwine , Thiria Sylvie , Frouin Robert , Mitchell B. Greg
Published: September 2002

A neural network methodology is developed to estimate the near‐surface phytoplankton pigment concentration of case I waters from spectral marine reflectance measurements (ocean color) at the Sea‐viewing Wide Field‐of‐view Sensor (SeaWiFS) visible wavelengths. The advantages of neural network approximation, i.e., association of nonlinear complexity, smoothness, and reduced sensitivity to noise, are demonstrated. When applied to in situ California Cooperative Oceanic Fisheries Investigations data, the neural network algorithm performs better than the reflectance ratio algorithms. Relative rms errors on pigment concentration are reduced from 61 and 62 to 38%, and absolute rms errors are reduced from 4.43 and 3.52 to 0.83 mg m−3. When applied to Sea WiFS‐derived imagery, there is statistical evidence that the neural network algorithm filters residual atmospheric correction errors more efficiently than the standard SeaWiFS biooptical algorithm.
Using satellite‐derived sea level and temperature profiles for determining the salinity variability: A new approach
Authors: Maes Christophe , Behringer David
Published: September 2002

We propose a new approach to estimate the vertical variability of the salinity field. The method is based on combined vertical modes of temperature T and salinity S and reconstructs salinity profiles via a weighted least squares procedure. The major advantages of this new approach over using a climatological T–S relationship are that seasonal‐to‐interannual variability is better taken into account and that the method combines in a consistent way different sources of information such as T and S profiles and sea surface height. The present results estimate the salinity along 165°E in the western Pacific Ocean for the 1993–1998 period and emphasize the importance of the salinity on sea level variability. The reverse problem of estimating the salinity variability along the water column from the satellite‐derived sea level and sea surface fields is also investigated. Finally, comparison with in situ salinity observations demonstrates the possibility of extracting useful information about the salinity variability from the TOPEX/Poseidon altimeter data.
Upper equatorial Pacific Ocean current and salinity variability during the 1996–1998 El Niño–La Niña cycle
Authors: Johnson Gregory C. , McPhaden Michael J. , Rowe G. Dail , McTaggart Kristene E.
Published: September 2002

The recent El Niño–La Niña cycle exhibited striking patterns of current and salinity variability in the upper equatorial Pacific Ocean. This evolution is described from mid‐1996 through 1998 using a remarkable data set of 35 meridional conductivity‐temperature‐depth (CTD)/acoustic Doppler current profiler (ADCP) sections along with buoy data. The sections, nominally from 8°S to 8°N between 165°E and 95°W, were occupied over the course of 27 months. A wide range of current variability was sampled with currents that appeared or disappeared some time during the cycle, including an equatorially trapped eastward surface current, the Equatorial Undercurrent, the northern branch of the South Equatorial Current, and the North Equatorial Countercurrent. Basin‐wide, interannual changes in upper ocean and pycnocline zonal transports were as large as 64±32×106 m3 s−1. Changes in the salinity structure included a deep and fresh mixed layer in the central equatorial Pacific that built up during the El Niño and was then disrupted by upwelled salty water with the onset of La Niña, a very fresh mixed layer observed in the eastern equatorial Pacific late in the El Niño, and a reduction in the strength of the meridional equatorial salinity gradient within the pycnocline to one third of the usual value during the El Niño. Finally, the zonal transports above the thermocline from 5°S to 5°N were well correlated with the rate of change of warm‐water volume west of the individual CTD/ADCP sections.
How can we improve a global ocean tide model at a regional scale? A test on the Yellow Sea and the East China Sea
Authors: Lefèvre F. , Le Provost C. , Lyard F. H.
Published: September 2002

A global ocean model has been developed within the context of TOPEX/Poseidon (T/P) on the basis of a finite element hydrodynamic modeling approach combined with data assimilation based on the representer method. The solution produced at global scale represents a spectacular improvement over what was available before the era of T/P. Typically, the mean discrepancy between the main tidal components and a hundred in situ tide gauges is 1.7 cm for M2 and 1 cm or less for the other components. However, the accuracies of this solution and of that produced by different authors within the T/P tide working group are all worse near coastlines and over continental shelves. This is the case for our finite element solution (FES) FES94.1 solution over the Yellow Sea and the East China Sea (YS‐ECS) where the discrepancy is 33 cm for the M2 tide and 9 cm for the K1 tide when compared to a set of 192 tide gauges distributed along the coastlines. This is due largely to the complex geometry of the basin and the limited knowledge of the bathymetry. The aim of this paper is to investigate how our FES model can be improved with a dedicated application focused on one of the most energetic coastal basins: the YS‐ECS (∼180 Gigawatts for M2, i.e. 8% of the global energy dissipated in the whole ocean). For this application, a finite grid was implemented with a resolution down to 5 km along the coasts and over the continental shelf break. Particular attention was paid to bathymetry by complementing the ETOP05 database with regional maps. Sensitivity tests to the tuning of the bottom friction and the nonlinear interactions between the diurnal and semidiurnal components allow us to investigate the impact of dissipation parameterization and to produce an optimal solution, without any data assimilation, for nine main components of the tidal spectrum (improvement by a factor of 2). M2 distance to the observations is now ∼17.5 cm (global variance of 92 cm), and K1 is 4 cm (global variance of 21 cm). As nonlinear components are significant over these basins, the two main quarterdiurnal components are calculated. The improvements are explained by five points: (1) the choice of a mixed friction, (2) a refinement of the mesh, (3) a choice of reliable boundary conditions, (4) a refined topography and, (5) the use of a specific friction coefficient 1.5×10−3. Surprisingly, the energy budget for the M2 component leads to a dissipation similar to the value estimated by the global FES94.1. This is a major result of this study, which leads us to the conclusion that when forcing a regional model with sea level boundary conditions along the open limits, the simulated velocity field adjusts to the tuning of friction coefficients in order to dissipate the same amount of energy made available at its open boundaries.
Seasonal variability of the Bay of Bengal circulation inferred from TOPEX/Poseidon altimetry
Authors: Eigenheer Andreas , Quadfasel Detlef
Published: September 2002

The circulation in the interior of the Bay of Bengal and of its western boundary current, the East Indian Coastal Current, is inferred from historical ship drift data and from TOPEX/Poseidon altimeter data. The boundary current shows a strong seasonal variability with reversals twice per year that lead the reversal of the local monsoon wind field by several months. On the basis of model simulations it has been suggested that this unusual behavior can be explained by the influence of remotely forced planetary waves. Our data analysis confirms and refines this view by showing the role of topography in the northern bay. We also give an estimate of the relative importance of the different contributions.
Character and dynamics of the Red Sea and Persian Gulf outflows
Authors: Bower Amy S. , Hunt Heather D. , Price James F.
Published: September 2002

Historical hydrographic data and a numerical plume model are used to investigate the initial transformation, dynamics, and spreading pathways of Red Sea and Persian Gulf outflow waters where they enter the Indian Ocean. The annual mean transport of these outflows is relatively small (<0.4 Sv), but they have a major impact on the hydrographic properties of the Indian Ocean at the thermocline level because of their high salinity. They are different from other outflows in that they flow over very shallow sills (depth < 200 m) into a highly stratified upper ocean environment and they are located at relatively low latitudes (12°N and 26°N). Furthermore, the Red Sea outflow exhibits strong seasonal variability in transport. The four main results of this study are as follows. First, on the basis of observed temperature‐salinity (T‐S) characteristics of the outflow source and product waters we estimate that the Red Sea and Persian Gulf outflows are diluted by factors of ∼2.5 and 4, respectively, as they descend from sill depth to their depth of neutral buoyancy. The high‐dilution factor for the Persian Gulf outflow results from the combined effects of large initial density difference between the outflow source water and oceanic water and low outflow transport. Second, the combination of low latitude and low outflow transport (and associated low outflow thickness) results in Ekman numbers for both outflows that are O(1). This indicates that they should be thought of as frictional density currents modified by rotation rather than geostrophic density currents modified by friction. Third, different mixing histories along the two channels that direct Red Sea outflow water into the open ocean result in product waters with significantly different densities, which probably contributes to the multilayered structure of the Red Sea product waters. In both outflows, seasonal variations in source water and oceanic properties have some effect on the T‐S of the product waters, but they have only a minor impact on equilibrium depth. Fourth, product waters from both outflows are advected away from the sill region in narrow boundary currents, at least during part of the year. At other times, the product water appears more in isolated patches.
Modeling the evolution of the Arctic mixed layer during the fall 1997 Surface Heat Budget of the Arctic Ocean (SHEBA) Project using measurements of 7Be
Authors: Kadko David
Published: September 2002

During the Surface Heat Budget of the Arctic Ocean (SHEBA) project in October 1997 the first measurements of 7Be ever made within the Arctic Ocean were used to reconstruct the evolution of the mixed layer over the previous season and confirmed that the reservoir of heat beneath the fall mixed layer was emplaced in the summer, rather than input cumulatively over several seasons or advected in from distant sources. As suggested by McPhee et al. [1998], several times as much heat was emplaced at SHEBA than at Arctic Ice Dynamics Joint Experiment 20 years earlier. A likely mechanism for this would be substantially greater lead opening coupled with the positive feedback loop between increased open water, increased heat absorption, and further ice melting. In addition, a rate of net primary production ≥13 gC m−2 for the preceding spring‐summer period was derived using this mixed layer history with an oxygen profile from the fall.