bluuebunny/crossref_metadata_2025 huggingface dataset:
Observations of turbulence in a tidal beam and across a coastal ridge
Authors: Lien R.‐C. , Gregg M. C.
Published: February 2004

During a microstructure survey off California in Monterey Bay, we found a midwater beam of strong turbulence emanating from the shelf break along the ray path of the semidiurnal M2internal tide. Within the 50‐m‐thick beam the turbulence kinetic energy dissipation rate ε exceeded 10−6W kg−1, and the diapycnal eddy diffusivityKρwas >0.01 m2s−1. The beam extended 4 km off the shelf break. Several factors suggest that this beam of strong turbulence resulted from the breaking of semidiurnal internal tides: the beam appeared to originate from the shelf break, which is a potential generation site for semidiurnal internal tides; the beam closely followed the ray path of the semidiurnal internal tide; the average ε off the shelf break varied by a factor of 100 with a semidiurnal tidal periodicity; the isopycnal displacement confirmed the presence of semidiurnal internal tides. Processes associated with the breaking of internal tides are intermittent and sporadic. At the same location we also observed equally intense turbulence in a ∼100‐m‐thick layer of stratified water across the ridge of a sea fan. This layer of strong turbulence was separated from the bottom and was clearly not generated by bottom friction. Although less well resolved in time, the strong turbulence above the bottom seemed to vary with the semidiurnal tide and existed at the lee of the ridge, where the isopycnal surface dipped and rebounded in a pattern resembling that of internal hydraulic jumps. On the basis of the behavior of the density field, we believe that the deep mixing was most likely produced by the across‐ridge current of internal tides. The breaking of internal tides at middepth, where the Richardson number is close to the critical value, is likely due to shear instability. The presence of the coastal ridge provides an alternative pathway for converting energy from internal tides to turbulence via internal hydraulics. Multiplying the average ε in the midwater beam by the length of the global coastline gives 31 GW, only a small fraction of the estimated 360 GW dissipated globally by M2internal tides. Our observations suggest that either most internal tides are generated away from shelf breaks or most internal tides generated at shelf breaks propagate away from their generation sites, rather than dissipate locally, and eventually contribute to pelagic mixing.
Optical properties of waters in the Australasian sector of the Southern Ocean
Authors: Clementson Lesley A. , Parslow John S. , Turnbull Alison R. , McKenzie Donald C. , Rathbone Christopher E.
Published: February 2004

During March 1998 we studied in situ bio‐optical parameters along a north‐south transect (142?°E) between 42° and 55?°S. Surface chorophyll a (chla) reflected mixed layer chlaconcentrations and showed a general decrease with increasing latitude. Changes in chlaconcentration often coincided with physical boundaries, and differences in fluorescence yield and photoadaption by the phytoplankton were observed north and south of the Subantarctic Front. In this region chromophoric dissolved organic matter (CDOM) absorption, generally exceeded phytoplankton pigment absorption at 443 nm. Satellite‐derived chla, using the Sea‐viewing Wide Field‐of‐view Sensor (SeaWiFS) OC4 algorithm, generally underestimated the in situ chlaconcentration except in areas of low chla(<0.15 mg m−3) where the SeaWiFS algorithm was found to overestimate in situ chla.
Water depth and surface current retrievals from airborne optical measurements of surface gravity wave dispersion
Authors: Dugan J. P. , Piotrowski C. C. , Williams J. Z.
Published: February 2004

Visible images of nearshore ocean waves obtained from an aircraft have been utilized to estimate the surface currents and water depth below the waves. A digital framing camera was mounted in a motion‐stabilized turret and used to obtain temporal sequences of high‐quality optical images of shoaling ocean waves. Data on the position and attitude of the camera/turret were used to map the image data to a rectilinear coordinate system at the level of the surface, effectively separating the spatial and temporal modulations due to the waves. The resulting three‐dimensional (3‐D) space‐time data sets were Fourier transformed to obtain frequency‐wave number spectra of these modulations. These spectra contain information on the propagation characteristics of the waves, such as their wavelengths and frequencies, and their directions and speeds of propagation. The water depth and current vector have been estimated by choosing these parameters so that a “best” fit is obtained between the theoretical dispersion relation for linear gravity waves and these 3‐D wave spectra. Image data sets were acquired during the Shoaling Waves Experiment (SHOWEX) along the quasi‐linear coastline in the vicinity of the Army Corps of Engineers' Field Research Facility (FRF) near Duck on the North Carolina Outer Banks. Summary wave parameters and bathymetry and current retrievals are typically within 10% of contemporaneous in situ measurements, though outliers occur.
An eddy‐permitting model of the Atlantic circulation: Evaluating open boundary conditions
Authors: Treguier A. M. , Barnier B. , de Miranda A. P. , Molines J. M. , Grima N. , Imbard M. , Madec G. , Messager C. , Reynaud T. , Michel S.
Published: February 2004

As part of the French CLIPPER project, an eddy permitting model of the Atlantic circulation has been run for 22 years. The domain has open boundaries at Drake passage and at 30°E, from Africa to Antarctica. The simulated mean circulation, as well as the eddy activity, is satisfactory for a 1/3° model resolution, and the meridional heat transport at 30°S is within the range estimated from observations. We use the “mixed” open boundary algorithm of Barnier et al. [1998], which has both a radiation condition and a relaxation to climatology. The climatological boundary forcing strongly constrains the solution in the whole domain. The model heat balance adjusts through the surface (heat flux retroaction term) more than the open boundaries. The radiation phase velocities calculated within the algorithm are analyzed. This shows, quite surprisingly, that both the eastern and western boundaries have a similar behavior, regardless of the preferred directions for advection (mainly eastward) and wave propagation (mainly westward). Our results confirm that open boundary algorithms behave differently according to the dynamics of the region considered. The passive boundary condition that Penduff et al. [2000] applied successfully in the north eastern Atlantic does not work in the present South Atlantic model. We emphasize the need for a careful prescription of the climatology at the open boundary, for which a new approach based on synoptic sections is implemented.
Nonlinear evolution of ocean internal solitons propagating along an inhomogeneous thermocline
Authors: Zheng Quanan , Klemas Vic , Yan Xiao‐Hai , Pan Jiayi
Published: February 2004

Measurements of the upper ocean thermal structure show that on the continental shelves the thermocline depth may shoal or deepen generally depending on the bottom topography. Thermocline shoaling and deepening cause changes in the phase speeds of internal waves as described by linear wave theories. On the other hand, the ocean area where internal waves have variable phase speeds may be treated as a dynamically inhomogeneous medium. In this case, theories of nonlinear dispersive wave propagation in inhomogeneous media developed by Tappert and Zabusky [1971] may stand. We used these theories to analyze the evolution of ocean internal solitary waves passing over a seamount in the Gulf of Aden. The results indicate that a surprisingly sharp recess of an internal solitary wave packet, imaged by the space shuttle Discovery, is a signature of spatial phase delay caused by thermocline shoaling over the seamount. Soliton fission due to thermocline shoaling was also observed in the imagery. The observed number of transmitted solitons over the seamount agrees with theoretical predictions. Relative soliton amplitudes measured from the imagery also agree qualitatively with predictions.
A global quasi‐biennial wave in surface temperature and pressure and its decadal modulation from 1900 to 1994
Authors: White Warren B. , Allan Rob J.
Published: February 2004

Zonal wave number frequency spectra of surface temperature (ST) and sea level pressure (SLP) anomalies extending around the global tropical ocean at 10°N from 1950 to 1997 display significant peaks for 2.0 and 2.4 year periods, dominated by eastward propagating zonal wave numbers of global scale. Applying complex principal component analysis to quasi‐biennial ST, SLP, and surface wind (SW) anomalies for 25 years from 1973 to 1997 separates this global quasi‐biennial wave (GBW) from corresponding global standing modes, with patterns and evolution of the former similar to those observed in the global El Niño‐Southern Oscillation wave [White and Cayan, 2000] but centered on the equatorial waveguide and taking only 3 to 4 years (instead of 4 to 6 years) to transit the global tropical ocean at 0.30 to 0.40 m s−1. We find the GBW contributing to the quasi‐biennial signal in Niño‐3 ST and SLP indices with amplitude twice that of global standing mode. In the GBW the ocean forces the atmosphere with SW anomalies directed toward warm sea surface temperature anomalies, while the atmosphere forces the ocean with SW‐induced Kelvin waves on the equator and SW‐induced sensible‐plus‐latent heat flux and/or vertical mixing anomalies elsewhere, both advancing ST anomalies eastward. Examining the GBW from 1900 to 1994 finds its amplitude modulated by the decadal signal of ∼10 year periodicity, robust (weak) when tropical temperature gradients and trade winds were stronger (weaker) than normal.
Bubble entrainment by breaking waves and their influence on optical scattering in the upper ocean
Authors: Terrill Eric J. , Melville W. Kendall , Stramski Dariusz
Published: February 2004

Breaking waves at the ocean's surface inject bubbles and turbulence into the water column. During periods of rough weather the scales of wave breaking will increase with increasing sea states and result in mixing of the surface waters and the turbulent transport of bubbles to depth. Depending on their concentrations and size distribution, the entrained bubbles can significantly change the optical properties of water, introducing potentially significant errors in retrieval of remotely sensed hyperspectral data products. In this paper, the effects of bubbles on optical scattering in the upper ocean are investigated through optical scattering calculations based on field measurements of bubble populations. The field measurements were obtained offshore Point Conception, California, in June 1997, using an acoustical technique which measured the bubble size distribution at 2 Hz from a surface buoy designed to follow the longer waves. The effects of the bubbles on the bulk optical scattering and backscattering coefficients, b and bb, respectively, are determined by using the acoustically measured size distributions, and size‐dependent scattering efficiencies based on Mie scattering calculations. Time series of the bubble distributions measured in rough conditions (wind speed, U10 = 15 m/s, significant wave height, H1/3 = 3.2 m) suggest that the bubble contribution to light scattering is highly variable near the ocean surface, with values spanning roughly 5 decades over time periods of O(10) minutes. Bubble size distributions measured at a 0.7‐m depth indicate that the optical effects of the bubbles on bb, and hence the remote sensing reflectance, will be significant at bubble void fractions above 10−6 and that the bubble contribution to total bb will exceed values of 10−2 m−1 inside bubble clouds.
Direct covariance air‐sea CO2 fluxes
Authors: McGillis W. R. , Edson J. B. , Hare J. E. , Fairall C. W.
Published: February 2004

Direct covariance air‐sea CO2 flux measurements over the open ocean are reported. These measurements were performed during June 1998 in the North Atlantic within a significant CO2 sink. These direct estimates are in general agreement with the traditional geochemical isotope constraints. The covariance, or eddy correlation, technique directly measures the air‐sea CO2 flux over hour timescales by correlating the fluctuations of CO2 with the turbulent vertical velocity fluctuations in the atmospheric surface layer. These measurements quantify the transfer of CO2 between the atmosphere and ocean over a range of wind speeds and improve the understanding of the environmental factors controlling the flux. The relatively large flux of CO2 in the study region, together with improved analytical techniques, facilitated the measurements. The half‐hour mean wind speeds varied from 0.9 to 16.3 m s−1 over the month‐long experiment. The mean pCO2 during the study period was −85.8±16.0 μatm, and the mean covariance CO2 flux was estimated at 4.6 mol m−2 yr−1. The average observed wind speed was 7.7 m s−1. This is in close agreement with 3.9 mol m−2 yr−1, the approximate CO2 flux based on 14C parameterizations at this wind speed. At high winds, where the relationship between gas physical properties, surface processes, and air‐sea gas exchange is still elusive, direct CO2 flux measurements are crucial. The measurements for winds in excess of 11 m s−1 show a general enhancement of gas transfer velocity over previous indirect measurements, and it is believed that this enhancement can be explained by the fact that the indirect methods cannot discriminate surface process variability such as atmospheric stability, upper ocean mixing, wave age, wave breaking, or surface films.
Inertial oscillations in the Korea Strait
Authors: Jacobs G. A. , Book J. W. , Perkins H. T. , Teague W. J.
Published: February 2004

Inertial oscillations (IO) are examined in the Korea Strait based on measurements from 13 acoustic Doppler current profilers covering the time period May 1999 through March 2000. Strong IO responses to wind stress occur during summer. A simple linear model predicts that winter wind stress is expected to generate inertial responses of the same order of magnitude as those in summer. However, the observed winter IO response is much weaker than predicted. During summer, the currents within the mixed layer and below the mixed layer are of comparable amplitude but in opposite directions. The depth at which the currents reverse directions varies throughout the year as the mixed layer deepens from about 40 m during summer to the bottom of the water column in November. During winter, the velocity structure is more uniform in depth with currents in the same direction throughout the water column. One possible explanation for these phenomena is related to the combined effect of the strait boundaries and the strong summer stratification. The stratification prevents the wind stress momentum flux from mixing downward below the thermocline and thus allows the development of a bottom current separate from the surface current. Such a velocity structure is necessary to satisfy the no‐flow condition through the land boundaries.
Sand suspension, storage, advection, and settling in surf and swash zones
Authors: Kobayashi Nobuhisa , Johnson Bradley D.
Published: February 2004

A time‐dependent cross‐shore sediment transport model in the surf and swash zones on beaches is developed to predict both beach accretion and erosion under the assumptions of alongshore uniformity and normally incident waves. The model is based on the depth‐integrated sediment continuity equation, which includes sediment suspension by turbulence generated by wave breaking and bottom friction, sediment storage in the entire water column, sediment advection by waves and wave‐induced return current, and sediment settling on the movable bottom. The hydrodynamic input required for this sediment transport model is predicted using the finite‐amplitude shallow‐water equations including bottom friction. The developed model is compared with three large‐scale laboratory tests with accretional, neutral (little), and erosional beach profile changes under regular waves. The model predicts sediment suspension under the steep front of breaking waves and due to bottom friction in the swash zone. The computed depth‐averaged sediment concentration does not respond to local sediment suspension instantaneously because of the sediment storage and advection. The mean sediment concentration becomes large in comparison to the oscillatory concentration with the decrease of the normalized sediment fall velocity. The net cross‐shore sediment transport rate is shown to be the small difference between the onshore transport rate due to the positively correlated oscillatory components of the suspended sediment volume per unit area and the horizontal sediment velocity and the offshore transport rate due to the product of the mean suspended sediment volume and the mean horizontal sediment velocity. Relatedly, the net accretion or erosion rate of the movable bottom is determined by the small difference between the mean sediment settling rate and the mean suspension rate caused by wave breaking and bottom friction. The present computation is limited to the initial beach profile change, but the numerical model is capable of predicting the accretional, erosional, and neutral profile changes.
Creation of residual flows in a partially stratified estuary
Authors: Stacey Mark T. , Burau Jon R. , Monismith Stephen G.
Published: February 2004

The creation of residual flows in estuaries is examined using acoustic Doppler current profiler data sets from northern San Francisco Bay. The data sets are analyzed using principal component analysis to examine the temporal variability of the flows which create the residual circulation. It is seen that in this periodically and partially stratified estuary the residual flows are created through a series of pulses with strong variability at the 24‐hour timescale, through the interaction of shear, stratification and mixing. This interaction is captured through the use of a dimensionless number, the horizontal Richardson number (Rix), which is developed to examine the local balance between the stratifying and destratifying forces at the tidal timescale. It is seen that Rix is a valuable parameter in predicting the onset of the residual‐creating events, with a threshold value of ≈3 on ebb tides. This critical value is argued to be a threshold, above which the stratification and shear flow create a feedback effect, each further intensifying the other. This feedback results in a highly variable exchange flow which creates the estuarine residual in intermittent pulses rather than as a steady flow. Although typically attributed to baroclinic forcing, an argument is made that these pulses of residual‐creating exchange flow could be created by barotropic forcing in the presence of variable stratification which is asymmetric between flood and ebb tides. This result poses a great challenge for turbulence modeling, as the timing and magnitude of stratification and shear must be correctly simulated on the tidal timescale in order to reproduce the effects seen in the data sets presented.
How iceberg calving and grounding change the circulation and hydrography in the Filchner Ice Shelf‐Ocean System
Authors: Grosfeld K. , Schröder M. , Fahrbach E. , Gerdes R. , Mackensen A.
Published: February 2004

The formation of bottom water in the southern Weddell Sea is strongly influenced by the flow of Ice Shelf Water (ISW) out of the Filchner‐Ronne Ice Shelf cavity. The breakout of three giant icebergs in 1986 and their grounding on the shallow Berkner Bank modified the circulation and water mass formation in the Filchner Trough and the adjacent sea areas. Hydrographic measurements along the Filchner Ice Shelf front, carried out with RV Polarstern in 1995, show significant changes in the water mass characteristics and flow patterns in the Filchner Trough in comparison to measurements from the early 1980s. Changes in the trough will affect the flow over the sill to the deep Weddell Abyssal Plain. We combine a three‐dimensional ocean circulation model with conductivity‐temperature‐depth and stable isotope measurements to investigate the details of the circulation in front of and beneath the Filchner Ice Shelf. We assess the impact of stranded icebergs and a more southerly ice shelf front position caused by a 1986 iceberg calving event on the circulation and observed water mass properties. Results indicate variations of the flow pattern in the Filchner Trough and on Berkner Bank, where High‐Salinity Shelf Water, the feedstock for ISW, is produced. The calving and grounding impacts illustrate the sensitivity of the ice shelf‐ocean system to perturbations in local bathymetric settings.
Does the Yellow Sea Warm Current really exist as a persistent mean flow?
Authors: Lie Heung‐Jae , Cho Cheol‐Ho , Lee Jae‐Hak , Lee Seok , Tang Yuxiang , Zou Emei
Published: February 2004

Does saline water of Kuroshio origin intrude into the Yellow Sea interior by a persistent mean current through the Yellow Sea Warm Current or by an intermittently generated wind‐driven current? This very fundamental question is discussed by analyzing comprehensive conductivity‐temperature‐depth data collected over the entire southern Yellow Sea basin and by analyzing satellite‐tracked drifter trajectories collected in 1996 and 1997. In the southwestern Yellow Sea, relatively saline water (33.0–34.0 psu) persists year‐round, implying a northwestward intrusion of Cheju Warm Current water (CWCW). This water is a mixture of saline CWCW, with salinity higher than 34.0 psu, and fresh Yellow Sea waters, with salinity <33.0 psu. During the winter monsoon the mixed water distribution is tongue‐shaped, apparently intruding from the CWCW area. At the same time a strong thermohaline front with a horizontal form similar to the Greek letter Γ forms across the southern entrance of the central Yellow Sea trough and separates the mixed water area from the CWCW. During the summer monsoon the CWCW intrusion is weak, and the mixed water that intruded in the previous winter monsoon remains inside the trough as its salinity decreases. In winter, occasional collapses of part of the front on the western side of the trough are observed, presumably in response to an imbalance between northwest currents generated by strong northerly wind bursts and southeast, tide‐induced residual currents. Saline water in the frontal zone may then flush into the southwestern Yellow Sea through the collapsed area of the front. We conclude that the YSWC is not a persistent mean current. We also conclude that CWCW may be transported to the southwestern Yellow Sea in winter by the intermittently generated northwest currents on the western slope of the trough, which override the tide‐induced residual currents that flow to the southeast in this part of the basin.
Modeling the alongshore current on barred beaches
Authors: Ruessink B. G. , Miles J. R. , Feddersen F. , Guza R. T. , Elgar Steve
Published: February 2004

Mean alongshore currents observed on two barred beaches are compared with predictions based on the one‐dimensional, time‐ and depth‐averaged alongshore momentum balance between forcing (by breaking waves, wind, and 10–100 km scale alongshore surface slopes), bottom stress, and lateral mixing. The observations span 500 hours at Egmond, Netherlands, and 1000 hours at Duck, North Carolina, and include a wide range of conditions with maximum mean currents of 1.4 m/s. Including rollers in the wave forcing results in improved predictions of the observed alongshore‐current structure by shifting the predicted velocity maxima shoreward and increasing the velocity in the bar trough compared with model predictions without rollers. For these data, wave forcing balances the bottom stress within the surfzone, with the other terms of secondary importance. The good agreement between observations and predictions implies that the one‐dimensional assumption holds for the range of conditions examined, despite the presence of small alongshore bathymetric nonuniformities. With stronger bathymetric variations the model skill deteriorates, particularly in the bar trough, consistent with earlier modeling and laboratory studies.
Sediment resuspension over a continental shelf during Hurricanes Edouard and Hortense
Authors: Chang G. C. , Dickey T. D. , Williams A. J.
Published: February 2004

The present comprehensive physical and optical measurements have captured sediment resuspension associated with two hurricanes, Edouard and Hortense, that passed over the Coastal Mixing and Optics study site in the fall of 1996. Sediment resuspension associated with Hurricane Edouard was forced by combined current and wave processes. Combined current‐wave bottom shear stresses exceeded 3.5 dyn cm−2, well above the shear stress for the observed resuspension, which was determined to be 0.8 dyn cm−2. Hortense sediment resuspension, however, was caused primarily by waves with little or no contribution from subtidal plus tidal currents; combined current‐wave bottom shear stress reached 2 dyn cm−2. Beam attenuation data reveal that bottom sediments were resuspended from ∼70 m depth to more than 30 m up into the water column during both hurricanes. The relaxation of sediments to pre‐Edouard conditions occurred at about the same time at all depths, indicating advection and transport by subtidal currents. Analyses of optical data reveal that the resuspended sediments consisted mainly of detritus and relict pigments from the ocean bottom.
Seasonal heat content changes in the western Mediterranean Sea as a means for evaluating surface heat flux formulations
Authors: Krahmann Gerd , Schott Friedrich , Send Uwe
Published: February 2004

The semienclosed western Mediterranean Sea has proven to be a useful location to evaluate surface heat flux estimates. In the past the directly measured average oceanic heat transport from the Atlantic into the Mediterranean Sea through the Strait of Gibraltar of ∼5.2±1.3 W m−2 has been compared to estimates of the average heat flux across the surface of the Mediterranean Sea. On long timescales both should closely balance each other. By using a monthly temperature climatology of the western Mediterranean Sea we offer the possibility to extend the comparison to the seasonal timescale. This gives additional information with which different surface heat flux data sets can be evaluated. The seasonal heat content changes of the western Mediterranean and the advective exchange of heat through the Straits of Gibraltar and Sicily are estimated on the basis of a new extensive hydrographic data set and of published values for the volume transports. To demonstrate the method, a limited number of surface heat flux data sets are compared with the oceanographically calculated counterpart. The comparison reveals that some heat fluxes do not only agree well for the long‐term averages but also for the seasonal timescale, whereas others show larger deviations. The remaining rms discrepancies of ±10.2 W m−2 for the best heat flux data set are smaller than the uncertainty of the oceanographic estimate and of a reasonable magnitude compared to the uncertainty of the long‐term average of ∼5 W m−2.
Cyclone surface pressure fields and frontogenesis from NASA scatterometer (NSCAT) winds
Authors: Zierden David F. , Bourassa Mark A. , O'Brien James J.
Published: February 2004

Two extratropical marine cyclones and their associated frontal features are examined by computing surface pressure fields from NASA scatterometer (NSCAT) winds. A variational method solves for a new surface pressure field by blending high‐resolution (25 km) relative vorticity computed along the satellite track with an initial geostrophic vorticity field. Employing this method with each successive pass of the satellite over the study area allows this surface pressure field to evolve as dictated by the relative vorticity patterns computed from NSCAT winds. The result is a high‐resolution surface pressure field that captures features such as fronts and low‐pressure centers in more detail than National Centers for Environmental Prediction (NCEP) reanalyses. While using the actual relative vorticity to adjust the geostrophic vorticity ignores the ageostrophy of surface winds, which can be significant in the vicinity of fronts and jet streaks, it is a necessary approximation given that the technique uses only surface data. The NSCAT surface pressure fields prove to be nearly as accurate as NCEP reanalyses when compared to ship and buoy observations, which is an encouraging result given that NCEP reanalyses incorporate a myriad of data sources and the NSCAT fields rely primarily on one source. In addition, the high‐resolution relative vorticity fields computed from NSCAT winds reveal the location of surface fronts in great detail. These fronts are verified using NCEP analyses, in situ data, and satellite imagery.
Propagation characteristics of extratropical planetary waves observed in the ATSR global sea surface temperature record
Authors: Hill Katherine L. , Robinson Ian S. , Cipollini Paolo
Published: February 2004

This paper examines the characteristics of planetary wave signatures that have been found in the Along Track Scanning Radiometer averaged sea surface temperature (ASST) record for 1991–1996. Longitude‐time plots for every latitude between 5° and 50°, north and south, reveal westward propagating wave‐like patterns at many locations, whose speed decreases with latitude like baroclinic Rossby waves. A two‐dimensional Radon transform method is used to measure the wave speed and its variation with location and time, which broadly matches the Rossby wave speeds predicted by the most recent theory and those measured by TOPEX altimetry, although there are some discrepancies. At low latitudes the thermally detected speeds are slower than expected, a possible consequence of sampling limitations. Wave signatures are clearest between 25° and 40°S, where the meridional temperature gradient is strongest. Here observed speeds are 20–30% greater than theoretical predictions. Planetary wave speed varies considerably with longitude. In general, it increases toward the west of ocean basins, and distinct differences between ocean basins are evident. The propagation characteristics of the waves appear to change abruptly at locations consistent with latitudinal variations in seafloor bathymetry, particularly midocean ridges. In addition, eastward propagating signatures are found in the Southern Ocean. The results demonstrate the value of the ASST data set as a tool for studying basin‐scale wave processes as a complement to the use of altimetry. By observing the thermal signature of Rossby waves the method has the potential to clarify their influence on air‐sea interaction processes and to contribute to climate modeling studies.
Laboratory study of wave bottom interaction in the bar formation on an erodible sloping bed
Authors: Dulou C. , Belzons M. , Rey V.
Published: February 2004

The interaction between partially standing or almost progressive surface gravity waves and erodible beds is investigated in a small wave tank under nonbreaking wave conditions. The bed is initially flat and gently sloping, and the incident weakly nonlinear wave is either a regular wave (similar to a Stokes wave), or a regular wave superimposed on a free wave generated by the wave maker (bichromatic or bimodal wave). Accurate measurements of both the bed profile and the wave envelope are performed, showing a permanent coupled evolution of the wave field and of the bed shape. Sandbar formation is observed in the ultimate bed profile which reproduces the spatial modulations of the envelope of the first harmonic of the wave (e.g., the fundamental component). In the case of a pure regular wave, the interbar spacing is equal to half the local surface wavelength which corresponds to a first‐order Bragg resonance with the local wavenumber of the first harmonic satisfying the dispersion relation for small waves. In the case of a bimodal wave the preceding case is perturbed by a free wave of the same frequency and similar amplitude as the second harmonic of the regular wave. Then, nonlinear interactions between the first harmonic and the free wave occur, leading to sum and difference frequencies. The difference interaction modifies the amplitude and spatial modulation of the first harmonic. The Bragg resonance mechanism based on the first harmonic in the bar formation is then modified. However, in this case, the bed profile is also the replica of the envelope of the first harmonic of the wave: the perturbation induced in the first harmonic is also present in the bed shape. For partially standing bimodal waves, the Fourier analysis of the bed profile shows a second spatial modulation with the local wavenumber of the free wave. For almost progressive bimodal waves, a third modulation is found with the local wavenumber of the observed beating between the free wave and the second harmonic component of the wave.

These modulations of the envelope of the first harmonic are recovered analytically through a partially standing bimodal wave modeling. This study confirms that the bar formation under nonbreaking wave conditions is controlled by the envelope of the first harmonic of the wave even if perturbed by a free wave.
Pathways of the North Atlantic Current from surface drifters and subsurface floats
Authors: Carr Mary‐Elena , Rossby H. Thomas
Published: February 2004

The North Atlantic east of the Grand Banks presents a thermal front from 40° to 50°N between the warm North Atlantic Current (NAC) and the cold Labrador Current. Here we use the trajectories of 84 isopycnal RAFOS floats deployed on the 27.2 and 27.5 σT surface in 1993–1995 and 218 surface drifters deployed by the International Ice Patrol and the Institut für Meereskunde (Kiel, Germany) between 1978 and 1993 to understand the pathways of the NAC. The mean flow pattern, kinetic energy, directional stability, and most probable path through 1° by 1° boxes are derived for each of the three surfaces. Speed and kinetic energy decrease with depth, although directional stability is comparable at all levels. The axis of the NAC, which follows the 4000 m isobath along the western boundary, is described by mean kinetic energy (MKE) values surpassing 100 cm2 s−2 at the surface or 50 cm2 s−2 on the float surfaces and correspond to regions with directional stabilities in excess of 60%. Maximum values of eddy kinetic energy are found just offshore of the maximum MKE associated with the NAC main pathway and decrease rapidly to the east. Two cyclonic meanders are identified at 44°N, 45°W near the Newfoundland Seamounts and at 46°N, 42°W near Flemish Cap. The main difference between the patterns derived for the surface drifters and floats was the greater eastward extent of the cyclonic trough at 44°N and intensity of the Mann Eddy (42°N, 44°W) during the float sampling period (1993–1995).
Tidal asymmetry in an estuarine pycnocline: 2. Transport
Authors: Cudaback Cynthia N. , Jay David A.
Published: February 2004

Flood currents in shallow estuaries are driven by an along‐channel barotropic and baroclinic pressure gradient that increases monotonically toward the bottom, while friction retards near‐bottom currents. Therefore, in many estuaries there is a middepth maximum in flood currents. We explore this phenomenon using a simple three‐layer model in which each layer has vertically uniform currents and constant density. In this model the middle layer is of intermediate density and grows by shear‐induced entrainment from the other two layers. This very simple model produces a middepth maximum in flood currents and simulates observed currents in the Columbia River entrance channel within about 10%. There is good qualitative agreement between model salinity transport and observed transport. The model pycnocline rises and falls tidally, in phase with the observed pycnocline, although pycnocline depth and thickness are better simulated using results from a two‐layer model [Cudaback and Jay, 2000].
Introduction to special section: World Ocean Circulation Experiment: Southern Ocean results
Authors: King Brian A.
Published: February 2004

This special section of Journal of Geophysical Research contains a collection of papers on the Southern Ocean, presenting results that have emerged during the late phase of the World Ocean Circulation Experiment (WOCE). Previous special sections have included papers with Pacific results (World Ocean Circulation Experiment (WOCE), Journal of Geophysical Research, 103, 12,897–13,092, 1998) and South Atlantic results (WOCE South Atlantic Results, Journal of Geophysical Research, 104, 20,859–21,226, 1999). As before, this special section was stimulated by a WOCE basin workshop: the WOCE Southern Ocean workshop was held at the Antarctic CRC in Hobart, Tasmania, in July 1997. However, the correspondence between results presented at the workshop and papers that were subsequently submitted to the section is not as close as for the other workshops.
Influences of diurnal and intraseasonal forcing on mixed‐layer and biological variability in the central Arabian Sea
Authors: McCreary Julian P. , Kohler Kevin E. , Hood Raleigh R. , Smith Sharon , Kindle John , Fischer Albert S. , Weller Robert A.
Published: February 2004

A three‐dimensional, physical‐biological model of the Indian Ocean is used to study the influences of diurnal and intraseasonal forcing on mixed‐layer and biological variability in the central Arabian Sea, where a mooring was deployed and maintained from October 1994 to October 1995 by the Woods Hole Oceanographic Institution Upper Ocean Processes group. The physical model consists of four active layers overlying an inert deep ocean, namely, a surface mixed layer of thickness h1, diurnal thermocline layer, seasonal thermocline, and main thermocline. The biological model consists of a set of advective‐diffusive equations in each layer that determine nitrogen concentrations in four compartments: nutrients, phytoplankton, zooplankton, and detritus. Both monthly climatological and “daily” fields are used to force solutions, the latter being a blend of daily‐averaged fields measured at the mooring site and other products that include intraseasonal forcing. Diurnal forcing is included by allowing the incoming solar radiation to have a daily cycle. In solutions forced by climatological fields, h1 thickens steadily throughout both monsoons. When h1 detrains at their ends, short‐lived, intense blooms develop (the model's spring and fall blooms) owing to the increase in depth‐averaged light intensity sensed by the phytoplankton in layer 1. In solutions forced by daily fields, h1 thins in a series of events associated with monsoon break periods. As a result, the spring and fall blooms are split into a series of detrainment blooms, broadening them considerably. Diurnal forcing alters the mixed‐layer and biological responses, among other things, by lengthening the time that h1 is thick during the northeast monsoon, by strengthening the spring and fall blooms and delaying them by 3 weeks, and by intensifying phytoplankton levels during intermonsoon periods. Solutions are compared with the mixed‐layer thickness, phytoplankton biomass, and phytoplankton production fields estimated from mooring observations. The solution driven by daily fields with diurnal forcing reproduces the observed fields most faithfully.
Particles in the bottom boundary layer: Concentration and size dynamics through events
Authors: Agrawal Y. C. , Traykovski P.
Published: February 2004

The dynamics of particles is examined using two laser diffraction instruments, LISST‐100, mounted on a tripod. These sensors measured optical transmission and the area and volume distribution of particles. Particle size and concentration data from these instruments are examined together with the mean stress estimates obtained from a nearby tripod using a Benthic Acoustic Stress Sensors array velocimeter. The following three significant results are presented: (1) the size distribution of particles varies widely in response to forcing conditions; (2) in a surprise observation during Hurricane Edouard, despite a decrease in optical transmission that would ordinarily be interpreted as an increasing sediment load, a dramatic decrease in suspended sizes revealed an actual reduction in suspended volume; and (3) there is a positive correlation between median particle size in the current boundary layer and mean current stress during combined wave current storm events. This final result indicates that after the initial breakup of flocculates during a storm event the size dynamics of the boundary layer is no longer controlled by aggregation/disaggregation processes but is controlled by a balance between gravitational settling and upward diffusion.
Interplanetary shock interaction with the heliospheric current sheet and its associated structures
Authors: Hu Y. Q. , Jia X. Z.
Published: February 2004

The interplanetary medium is characterized by a heliospheric current sheet (HCS), a heliospheric plasma sheet (HPS) in which the HCS is embedded, a slow wind astride the HCS and HPS, and a fast wind at high latitudes. In terms of perpendicular shock approximation, a simplified one‐dimensional analysis is presented on the interaction between interplanetary shocks and the HCS, HPS, and the slow wind. It is shown that the HCS alone is transparent for the shock transmission and thus does not affect the shock propagation at all, whereas the HPS is too thin and the density ratio between the HPS and its surroundings is too low to produce appreciable effect on the shock transmission. On the other hand, the slow wind astride the HCS and HPS does exert a significant influence on the shock which is weakened in strength after its transmission, and the shock speed is significantly reduced both during and after the transmission. Therefore we conclude that it is not the HCS and HPS but the slow wind associated with them that affects the interplanetary shock propagation. Although the interaction of the HCS and HPS on interplanetary shocks is negligible, both HCS and HPS are distorted in shape and internal structure under the action of the shock. These conclusions are demonstrated by 2.5‐D MHD numerical simulation examples.
Voyager and Pioneer spacecraft measurements of cosmic ray intensities in the outer heliosphere: Toward a new paradigm for understanding the global solar modulation process: 1. Minimum solar modulation (1987 and 1997)
Authors: Webber W. R. , Lockwood J. A.
Published: February 2004

Using IMP, Voyager, and Pioneer spacecraft data, we have derived radial intensity profiles of cosmic rays in the heliosphere out to ∼70 AU as fractions of the expected interstellar intensities for five particle energies/species during the minimum solar modulation periods of 1987 and 1997. These radial profiles exhibit a markedly different behavior in the two periods of opposite solar magnetic polarity. In the negative polarity period (1987) the radial profiles for the highest‐rigidity particles extrapolate to the interstellar (IS) intensity at approximately the estimated location of heliospheric termination shock (TS) at 85–90 AU. The lower‐rigidity particle intensities extrapolate to only 0.45–0.60 of the IS intensity at this same location, indicating that a large part of the residual solar modulation for these particles at this time takes place at or beyond the location of the TS. In the 1997 positive polarity time period the fraction of the overall modulation that is occurring between the Earth and the TS is now much smaller, and since the intensity at the Earth is the same in both cycles, the consequence is that the extrapolated radial intensities at the TS location are now only 0.50 (at the highest rigidities) to 0.17 (at the lowest rigidities) of the respective IS intensities. Thus, at this time of minimum modulation most of the residual modulation apparently is taking place near to or beyond the TS. Also, the ratio of the fraction of the total residual modulation that is taking place between the Earth and the TS and that taking place beyond the TS changes dramatically between positive and negative modulation cycles. Viewed in this new way, contemporary modulation models including drifts, when they are assumed to occur within the TS, do not provide a satisfactory quantitative explanation of the modulation that is taking place at the minimum of solar activity. These models do not predict the large amount of modulation taking place near or beyond the termination shock. This outer heliospheric modulation dominates the overall residual modulation at low energies and is more effective in positive solar magnetic polarity periods. A new paradigm seems to be needed to interpret this large modulation that is taking place in the outer heliosphere.
Simulations of the magnetosphere for zero interplanetary magnetic field: The ground state
Authors: Sonnerup Bengt U. Ö. , Siebert Keith D. , White Willard W. , Weimer Daniel R. , Maynard Nelson C. , Schoendorf Jacqueline A. , Wilson Gordon R. , Siscoe George L. , Erickson Gary M.
Published: February 2004

A global MHD simulation code, the Integrated Space Weather Prediction Model, is used to examine the steady state properties of the magnetosphere for zero interplanetary magnetic field. In this “ground state” of the system, reconnection at the magnetopause is absent. Topics reported here include (1) qualitative description of global magnetic field, plasma flow, and current systems (Chapman‐Ferraro, geotail, Region 1 and Region 2 currents) (2) quantitative parametric studies of shock jump conditions, magnetopause and shock standoff distance, polar cap voltage, and total Region 1 current for different solar wind speeds and ionospheric Pedersen conductances; and (3) quantitative analysis of the low‐latitude boundary layer (LLBL) and its coupling to the ionosphere. The central part of the geomagnetic tail is found to be very long, extending beyond the downstream end of the simulation box at X = −300 RE. Along each flank a “wing‐like” region containing closed, albeit strongly stretched, field lines is present. Each such region contains a narrow convection cell, consisting of the tailward flowing LLBL and an adjoining narrow channel of sunward return flow. These cells are the result of viscous‐like interaction along the magnetospheric flanks, with an effective kinematic viscosity, entirely of numerical origin, estimated to be ν = 1.8 × 108m2 s−1. Except in certain regions near the magnetopause, the magnetosheath flow is steady and laminar while the internal motion in the tail displays turbulent vortical motion in the plasma sheet. Plasma transport in the tail occurs as a result of this turbulence, and substantial turbulent plasma entry across the equatorial magnetopause is seen in the region −10 RE < X < 0 RE behind the torus of dipolar field lines. The polar cap potential ΔϕPC is 29.9 ± 1.4 kV for VSW = 400 km s−1 and ∑P = 6 mho, which is in reasonable agreement with results inferred from satellite observations. About half of ΔϕPC can be attributed to the LLBLs with the remainder coming from a dawn‐to‐dusk potential drop along the dayside magnetopause, caused by nonlinearly switched resistivity, added explicitly to the MHD equations, and/or by numerical diffusion. The magnetospheric voltage‐current relation at VSW = 400 km s−1 has a constant negative slope with an open circuit voltage of ΔϕPC = 38.5kV. The total Region 1 current (into the northern dawn hemisphere) is 0. 66 MA (at VSW = 400 km s−1 and ∑P = 6 mho). It maximizes at about 2. 83 MA during short‐circuit conditions (∑P = ∞; ΔϕPC = 0).
HiTIES: A High Throughput Imaging Echelle Spectrogragh for ground‐based visible airglow and auroral studies
Authors: Chakrabarti S. , Pallamraju D. , Baumgardner J. , Vaillancourt J.
Published: February 2004

We describe an imaging spectrograph with high spectral resolution (0.3 Å) that uses an echelle grating without the conventional cross disperser. The spectrograph covers the entire visible region. However, only 5 – 20 wavelength regions can be observed simultaneously. The total wavelength coverage is limited by the number of available pixels on the detector and for a 1024 × 1024 pixels detector is ∼ 100 – 300 Å. Order separation is achieved by a mosaic of interference filters located at the image plane.