bluuebunny/crossref_metadata_2025 huggingface dataset:
Variation in effective elastic plate thickness of the East Africa lithosphere
Authors: Tessema A. , Antoine L. A. G.
Published: May 2003

We use new and existing Bouguer gravity data to characterize the long‐term flexural rigidity of the lithosphere beneath East Africa. Four tectonic provinces are characterized by distinct effective elastic plate thicknesses. These are the Tanzanian Craton, the Proterozoic Mobile Belts, the Paleogene rift basins in Sudan and Kenya, and the Cenozoic East African Rift System. Beneath the Tanzanian craton the elastic plate thickness is ∼75 km, while the Pan African Mobile Belts and the Ubendian Belts are ∼55 and ∼65 km, respectively. The Paleogene rift basins exhibit an elastic plate thickness varying between 40 and 45 km. The effective elastic plate thickness within the faulted East African Rift System ranges from 14 km in the north to 33 km in the south. The minimum is found beneath the active rift of the Afar depression. The north to south along‐rift axis variation in effective elastic plate thickness can be attributed to contrasts in the mechanical strength of the lithosphere. This is consistent with the north‐south decrease in extensional velocity and the northward decrease in the focal depths of the earthquakes, reflecting a northward thinning in the brittle layer of the crust. The maximum elastic plate thickness is found beneath the Tanzanian craton, while the surrounding Mobile Belts have relatively moderate elastic plate thicknesses. The high effective elastic plate thickness of the Tanzanian craton suggests that this part of the lithosphere is rheologically competent compared to the surrounding Mobile Belts. The craton effectively resists deformation, even though it is located within a broad zone of an east‐west extensional tectonic regime. Consequently, it has altered the direction of the rift propagation into the warmer and weaker lithosphere of the surrounding Mobile Belts.
System‐dependent center‐of‐mass correction for spherical geodetic satellites
Authors: Otsubo Toshimichi , Appleby Graham M.
Published: May 2003

The temporal spread of optical pulse signals due to reflection from multiple onboard reflectors is now a critical problem in satellite laser ranging. The full rate residual profile of single‐photon laser ranging can be used to model the response function of geodetic satellites, resolving the uncertainty of far‐field diffraction. We constructed the response function model for three types of laser ranging targets already in Earth orbit, the LAGEOS, AJISAI, and ETALON satellites. The center‐of‐mass correction depends on the ranging system and observation policy at terrestrial stations and varies about 1 cm for LAGEOS and 5 cm for AJISAI and ETALON.
Phase relation and mineral chemistry in pyrolite to 2200°C under the lower mantle pressures and implications for dynamics of mantle plumes
Authors: Nishiyama Norimasa , Yagi Takehiko
Published: May 2003

Phase relations in pyrolite were studied at pressures of 24 and 30 GPa and temperatures up to 2200°C. At 24 GPa and 1600°C, an assemblage of Mg‐perovskite, magnesiowüstite, Ca‐perovskite, and garnet was observed, with the proportion of garnet increasing continuously with temperature. At 30 GPa, only the two perovskites and magnesiowüstite were observed, and no significant changes in mineral proportions with temperature were observed. Iron was almost equally partitioned between Mg‐perovskite and magnesiowüstite at temperatures below about 1800°C at 24 and 30 GPa. With increasing temperature, however, iron was concentrated more in magnesiowüstite at both of these pressures. The decrease of Al content in Mg‐perovskite with temperature, which is caused by an increase in the proportion of garnet, is likely to be responsible for the iron partitioning behavior at 24 GPa, while the minor elements (Na, Al, and Cr) in magnesiowüstite may play an important role in Fe partitioning at 30 GPa. The density difference between ascending hot plumes (ΔT = 200 and 400°C) and the surrounding mantle was calculated using the present experimental data. The density difference (ΔT = 400°C) at 30 GPa (806‐km depth) is 0.06 g/cm3 but increases to 0.22 g/cm3 at 24 GPa (670‐km depth). The large increase in the density difference that occurs in a hot plume at the uppermost part of the lower mantle originates from an increase of the proportion of garnet and may be a trigger for the plume's passage through the boundary between the lower mantle and the mantle transition zone.
Law for the distance between successive earthquakes
Authors: Abe Sumiyoshi , Suzuki Norikazu
Published: May 2003

Discovery of a new law for the three‐dimensional spatial distance between the foci of successive earthquakes is reported. Analyzing the seismic data taken between 1984 and 2001 in southern Californian, it is found that the cumulative distribution of the distances follows the modified Zipf‐Mandelbrot law, showing complexity of geometry of the events.
Field measurements of flow fields and sediment transport above mobile bed forms
Authors: Williams Jon J. , Bell Paul S. , Thorne Peter D.
Published: May 2003

Field measurements of hydrodynamic conditions, bed forms, and suspended sediments, obtained using a suite of approximately colocated sensors above a mobile bed composed of coarse sand (D50 = 1.28 mm) in a strong tidal current with superimposed waves are used to investigate the mean and turbulent flow fields, bed forms, and the resuspension and transport of sediments in suspension and as bed load. While there is no evidence for flow separation, properties of the local turbulent flow field are found to vary spatially in response to the bed morphology. Time‐averaged suspended sediment concentration profiles, c‐profiles, are found to be spatially variable with higher concentrations observed in the trough regions between bed form crests. Well‐known expressions used to predict c‐profiles are found to agree well with measured time‐averaged c‐profiles. The instantaneous suspended sediment concentration field (SSCF) is found to comprise a series of semiperiodic resuspension events interspersed by relatively quiescent periods. Using a modified quadrant analysis scheme, these events are shown to be correlated closely with times when current and wave‐induced flows combine and exceed the threshold for suspension. The effectiveness of a given combined wave‐current flow event in resuspending bed sediments is found to depend upon its location relative to bed morphology. It is considered that local bed response may be influenced by spatial differences in grain size and compaction brought about by the processes of bed load transport over the bed forms. The total mass of sediment in suspension is found to be less than the potential transport capacity of the flow suggesting strongly that the supply of fine sediments is source limited. While measured bed form geometry agrees well with predicted dimensions for ripples, grain size alone suggests only dunes should be present. These were not detected. Bed load transport, Qb, calculated from measured bed form migration speeds and the measured suspended sediment flux, Qs, was found to agree favorably with empirical predictions.
Validation of SeaWiFS ocean color satellite data using a moored databuoy
Authors: Pinkerton Matthew H. , Lavender Samantha J. , Aiken James
Published: May 2003

The Plymouth Marine Bio‐Optical Databuoy (PlyMBODy) was deployed in the western English Channel to measure Lwn over an extended period of time in order to vicariously validate ocean color data from the NASA Sea‐Viewing Wide Field‐of‐View Sensor (SeaWiFS). Several experiments have shown that the buoy had radiometric accuracy similar to standard sea‐going optical profiling equipment (about ±8%). There were 15 match‐ups of good quality SeaWiFS and PlyMBODy data during 10 months of deployment between May 1997 and September 1998. Differences between PlyMBODy measurements of Lwn and SeaWiFS data (processed using SeaDAS version 4.1) were found to be spectrally variable and sometimes large. The differences were small in the blue‐green and green bands (SeaWiFS bands 3–5, 490, 510, and 555 nm) where the biases were <3% and the mean absolute differences were ∼20%. Bands 1, 2, and 6 (412, 443, and 670 nm) were significantly underestimated by SeaWiFS: the average differences were 55%, 19%, and 58%, respectively. The mean absolute differences were also much larger in these bands (30–77%). SeaWiFS radiometric measurements were generally sufficiently accurate to estimate chlorophyll‐a concentration using OC4v4 to within the mission target of ±35% but with a high bias of ∼8%, though the performance of the algorithm itself was not tested. Bias and scatter of SeaWiFS Lwn measurements relative to PlyMBODy measurements were much poorer when SeaDAS version 3.3p5, rather than version 4.1, was used. The upgrade from SeaDAS 4.0p3 to 4.1 made little difference to the agreement.
Tidal flow field in a small basin
Authors: Fagherazzi Sergio , Wiberg Patricia L. , Howard Alan D.
Published: May 2003

The tidal flow field in a basin of small dimensions with respect to the tidal wavelength is calculated. Under these conditions, the tide becomes a standing wave oscillating synchronously (with a flat water surface) over the whole basin. The shallow water equations can thus be strongly simplified, expressing the discharge vector field in terms of a potential function and a stream function. The potential function can be independently solved with the continuity equation, and is responsible for the total water balance in the basin. Moreover, the flow field derived from the potential function is shown to represent the tidal motion in a deep basin with flat bottom. Departures from this situation are treated with a stream function, that is, a correction for the potential function solution, and is solved through the vorticity equation. The stream function accounts for the nonlinear inertial terms and the friction in the shallow water equations, as well as bottom topography. In basins where channels incise within shallow tidal flats, the solution demonstrates that friction redistributes momentum, increasing the flow in the channels and decreasing it on the flats. The model is tested in San Diego Bay, California, with satisfactory results.
Satellite and hydrographic observations of eddy‐induced shelf‐slope exchange in the northwestern Gulf of Alaska
Authors: Okkonen S. R. , Weingartner T. J. , Danielson S. L. , Musgrave D. L. , Schmidt G. M.
Published: May 2003

Satellite and hydrographic observations of oceanographic conditions in the northwestern Gulf of Alaska indicate that anticyclonic eddies propagating adjacent to the continental shelf alter the structure of the shelf break front and, in doing so, influence the shelf‐slope exchange of biota and water mass properties. Eddies typically form in the northern Gulf of Alaska during fall and winter and propagate southwestward within an ∼200 km wide corridor along and above the continental slope. Eddy activity within this corridor diminishes in the downstream direction. The trajectories of faster propagating (>4 km/day) eddies tend to lie closer to the shelf break than do the trajectories of slower propagating (∼1.5 km/day) eddies. The interaction between azimuthal eddy currents and the shelf break frontal jet (1) establishes an upwelling zone, associated with the leading flank of the eddy, that strengthens cross‐slope gradients, (2) weakens cross‐slope gradients and promotes shelf‐slope exchange where the eddy is adjacent to the shelf, and (3) reestablishes a weak upwelling zone associated with the trailing flank of the eddy.
Tropical Pacific baroclinic mode contribution and associated long waves for the 1994–1999 period from an assimilation experiment with altimetric data
Authors: Dewitte Boris , Illig Séréna , Parent Laurent , duPenhoat Yves , Gourdeau Lionel , Verron Jacques
Published: May 2003

An Ocean General Circulation Model (OGCM) of the tropical Pacific in which combined TOPEX/Poseidon and ERS sea level anomalies are assimilated over January 1994 through July 1999, is used to investigate equatorial wave characteristics during the intense 1997–1999 El Niño‐La Niña event. Near the equator, the linear vertical modes are estimated at each grid point of the OGCM simulation with and without assimilation. Consistently with an increase of the vertical gradient within the thermocline and a rise of the thermocline depth in the eastern basin, the assimilation results in an increased contribution of the higher‐order baroclinic modes in the eastern basin and a decreased contribution of the first baroclinic mode in the western Pacific for the zonal current variability. For pressure, the first baroclinic mode contribution is reduced whereas the higher‐order modes contribution is weakly impacted. Kelvin and first‐meridional Rossby waves are then derived for the first two more energetic baroclinic modes in the simulation with assimilation. Kelvin waves of both modes constructively contribute to the strong warming observed in 1997, with the first (second) baroclinic mode being more energetic than the second (first) baroclinic mode in the early (mature) stage of the warming. Kelvin waves of both modes reflect as first meridional Rossby waves at the eastern boundary (reflection efficiency of ∼95%) and contribute to push back the warm pool westward. The reversal of the warming is apparently initiated by the second baroclinic mode contribution which controls the position of the 28°C isotherm in the surface layer in the far eastern Pacific from January 1998. At the western boundary, reflections of Rossby waves take place for both modes with an estimated total efficiency of ∼50% at 165°E. This suggests that, in our model, the delayed oscillator theory is not applicable for explaining the reversal from warm to cold conditions in 1998 while the zonal advective feedback was at work. More generally, the study suggests that it is necessary to take into account the vertical structure of the ocean when interpreting altimetric data, which can be done through an assimilation experiment.
Annual cycles of sea level and sea surface temperature in the western Mediterranean Sea
Authors: Bouzinac Catherine , Font Jordi , Johannessen Johnny
Published: May 2003

In the western Mediterranean Sea, sea level variations are the result of seasonal steric effect, atmospheric forcing and mesoscale circulation activity, particularly in the Alboran and Algerian basins, as well as water mass budget variations including flux variations through the Straits of Gibraltar and Sicily. However, the partition of the magnitude of the sea level variations associated with these different components remains a challenging problem. A better determination of the steric effect component will allow a better knowledge of the other annual components. The aim of this study is to improve the estimation and understanding of the annual sea level variations in the different basins of the western Mediterranean Sea, using sea level anomalies (SLA) derived from TOPEX/POSEIDON and ERS‐1/2 altimeters and sea surface temperatures (SST) derived from NOAA/AVHRR infrared imagery, obtained between 1992 and 1998. The first mode of empirical orthogonal function (EOF) analysis on SLA is annual and represents 63% of the total variance. The first mode of EOF analysis on SST anomalies is also annual and represents 98% of the total variance. The lag between the annual variations of SLA and SST is about 40 days. From these EOF results and a parameterisation of the mixed layer, the seasonal dynamic height and the steric effect are estimated and removed from the SLA. The remaining signal is used to examine the annual cycles of the sea level due to water mass budget variations, mesoscale oceanic variability and atmospheric forcing.
Profiles of dimethylsulphoniopropionate (DMSP), algal pigments, nutrients, and salinity in the fast ice of Prydz Bay, Antarctica
Authors: Trevena Anne J. , Jones Graham B. , Wright Simon W. , van den Enden Rick L.
Published: May 2003

Total dimethylsulphoniopropionate (DMSPt), chlorophylla(Chla), and algal marker pigments were measured in 12 fast ice cores collected from Prydz Bay, eastern Antarctica (68°–69°S, 77°–79°E) in October 1997 and November 1998. Patterns of DMSPt distribution through the ice were similar on spatial scales of meters to tens of kilometers within ice sheets grouped according to growth history. This reflects the association of DMSP in fast ice with autotrophic biomass distribution, which is intrinsically linked with ice growth and differed between the ice sheets. The 12 fast ice cores were divided into three groups on the basis of ice thickness and year. Concentrations of DMSPt ranged widely from 9 to 1478 nM with marked peaks occurring within each core. Mean DMSPt concentrations were higher (200 nM) in the medium first‐year ice (0.7–1.2 m) than in the thick (>1.2 m) first‐year ice (90 nM), mainly because of a local surface algal assemblage that may be atypical. The fast ice algal assemblages in surface, interior, and bottom ice were dominated by diatoms (Fucoxanthin:Chlaconcentrations >80%). Dinoflagellates and haptophytes were generally small and variable components of the assemblages (Peridinin:Chla2–11% and 19′‐hexanoyloxyfucoxanthin:Chla2–4%, respectively). Our data support the important contribution of diatoms to DMSP production in sea ice. Nutrient (nitrate, silicate, phosphate) concentrations were measured for one group of cores. Silicate and Chlaconcentrations were significantly correlated (r = 0.30, P < 0.02, Pearson), implying that silicate availability may have regulated algal growth. The Si:P:N ratio in interior ice (27:1:10) was different to that in surface and bottom ice (46:1:23). We have summarized DMSP data reported from six Antarctic sea ice studies to investigate whether comparisons within the growing database need to consider differences in sea ice type, thickness, location, or season. Although concentrations from individual samples ranged over 4 orders of magnitude (<1 to >1000 nM,n= 410), the mean DMSP concentrations during spring/summer were within the range of 107–322 nM, with an overall mean of 178 nM. Mean DMSP concentrations in Antarctic sea ice appear to be comparable between studies and across the Antarctic sea ice zone. We estimate that the Antarctic sea ice zone may contain up to 9 Mmole sulphur as DMSP.
On the interaction between internal tides and wind‐induced near‐inertial currents at the shelf edge
Authors: Davies Alan M. , Xing Jiuxing
Published: May 2003

Numerical calculations are performed at the shelf edge to examine the role of nonlinear processes in transferring energy from the M2 tide‐ and wind‐induced currents close to the inertial frequency f into a wave at the sum of their frequencies, termed the fM2 frequency. A numerical model of the Hebrides shelf edge (represented by a cross section), initially with idealized topography and subsequently with realistic topography is used in these calculations. Results show that in the near‐coastal ocean, currents at the fM2 frequency are primarily due to coupling between wind‐induced inertial oscillations and the M2 internal tide. A major source is associated with vertical shear in the inertial oscillations and the vertical velocity due to the internal tide. A secondary source is due to the nonlinear momentum advection term. In the case in which eddy viscosity is computed from a turbulence energy model, shear across the thermocline is larger than when a constant viscosity is used. The reduction in shear with a constant viscosity reduces the role of the nonlinear term involving vertical shear, and hence the magnitude of the fM2 current in the region of the thermocline. Increasing the wind stress leads to a deeper thermocline, and hence the location of maximum fM2 current in the water column. Changing the vertical stratification influences the intensity of the inertial oscillations in the surface layer and the distribution of the internal tide, and hence changes the pattern of fM2 currents. Results with realistic topography confirm the major conclusions.
Analysis of surface wind and roughness length evolution with fetch using a combination of airborne lidar and radar measurements
Authors: Flamant Cyrille , Pelon Jacques , Hauser Danièle , Quentin Céline , Drennan William M. , Gohin Francis , Chapron Bertrand , Gourrion Jrome
Published: May 2003

A combination of surface wind speed (SWS) and sea state variables, derived from quasi‐simultaneous airborne lidar and radar measurements, made in the framework of the Flux, État de mer et Télédétection en Condition de fetcH variable (FETCH) experiment, is used to analyze the evolution of surface roughness length, neutral drag coefficient, and friction velocity coefficient with fetch in the first hundred kilometers offshore over the Gulf of Lion, Western Mediterranean. The study focuses on the Tramontane/Mistral event documented in the afternoon of 24 March 1998. Particular attention is given to SWS derived from nadir lidar measurements. The SWS retrieval methodology developed and validated for open ocean conditions byFlamant et al.[1998]has been modified to account for the specificity of the coastal Mediterranean environment (complex mixture of continental and maritime aerosol; turbid, productive waters). The lidar‐derived SWS evolution with fetch observed on 24 March 1998 in the afternoon was validated against in situ and remote sensing measurements made from a buoy, a ship, as well as from the spaceborne altimeter TOPEX. The spatial variability in SWS observed with the airborne lidar was controlled by the structure of the wake regions downstream of the Massif Central and the Maritime Alps, delimiting the longitudinal extension of the Mistral, and was influenced by swell resulting from the action of a steady northeasterly flow coming from the Ligurian Sea in connection with intense Alpine lee cyclogenesis. These findings were supported by the other measurements. It is further shown that, based on a formulation of the dimensionless roughness dependance with wave age, airborne lidar and radar measurements can be combined to provide insight into the evolution with fetch of roughness length, neutral drag coefficient, and friction velocity. Four distinct sea state regimes over a distance of 100 km could be identified from the remotely sensed variables obtained with this novel approach in connection with atmospheric forcing. The dependance of lidar/radar derived drag coefficient with lidar‐derived SWS for the four regimes was found to be remarkably consistent with the relationship derived from the buoy measurements. Finally, lidar/radar derived friction velocities were found in good agreement with the buoy and in situ aircraft measurements.
A new formula for determining the atmospheric longwave flux at the ocean surface at mid‐high latitudes
Authors: Josey S. A. , Pascal R. W. , Taylor P. K. , Yelland M. J.
Published: May 2003

The accuracy of two empirical formulae used in recent climatological studies to estimate the atmospheric longwave flux at the ocean surface from ship meteorological reports has been evaluated using research cruise measurements from the northeast Atlantic. The measurements were obtained with a pyrgeometer and corrected for differential heating of the pyrgeometer dome and shortwave transmission through the dome. The formulae tested were from Clark et al. [1974] and Bignami et al. [1995]; neither was capable of providing consistently reliable estimates of the longwave flux. Clark overestimated the mean measured longwave of 341.1 Wm−2 by 11.7 Wm−2, while Bignami underestimated by 12.1 Wm−2. A new formula is developed that expresses the effects of cloud cover and other parameters on the longwave through an adjustment to the measured air temperature. The air temperature is adjusted by the amount necessary to obtain the effective temperature of a blackbody with a radiative flux equivalent to that from the atmosphere. A simple parameterization of the adjustment in terms of the total cloud amount gives longwave estimates that have an improved mean bias error with respect to the measurements of −1.3 Wm−2. The new formula is still biased under overcast, low cloud base conditions. However, by including a dependence on dew point depression in the formula, this bias is resolved, and the mean error reduced to 0.2 Wm−2. The new formula has been tested using measurements made on two subsequent cruises and found to agree to within 2 Wm−2 in the mean at middle‐high latitudes.
Variational inverse parameter estimation in a cohesive sediment transport model: An adjoint approach
Authors: Yang Zhaoqing , Hamrick John M.
Published: May 2003

Parameter estimation in the sediment deposition and resuspension process is an important issue in numerical modeling of suspended sediment transport. The sediment settling velocity and resuspension rate are two critical parameters controlling the sediment exchange process between the water column and sediment bed. In this paper a variational inverse data assimilation scheme for estimation of the sediment settling velocity and resuspension rate is developed and tested with a three‐dimensional cohesive sediment transport model. The sediment settling velocity and resuspension rate are treated as poorly known parameters in the model and are estimated by the variational inverse scheme using an adjoint approach. A limited‐memory quasi‐Newtonian conjugate gradient algorithm is used in the minimization process. The variational inverse model is tested in the James River estuary in Virginia by identical twin experiments. Numerical experimental results show that variational inverse data assimilation is a useful tool for retrieving poorly known parameters in a cohesive sediment transport model.
Effect of grain size sorting on the formation of shoreface‐connected sand ridges
Authors: Walgreen M. , de Swart H. E. , Calvete D.
Published: May 2003

Field data of shoreface‐connected ridges show persistent spatial variations of mean grain size over the bed forms. In the shore‐normal direction, the profiles of bottom topography and mean grain size are approximately 90° out of phase. To investigate the mechanisms responsible for the observed grain size distribution and the influence of sediment sorting on the temporal and spatial characteristics of shoreface‐connected ridges, a model is developed and analyzed. A linear stability analysis of an alongshore uniform basic state (describing a storm‐driven flow on a microtidal inner shelf) with respect to small bottom perturbations is carried out. The transport of nonuniform sediment is described by formulations for both bed load and suspended load, both of which account for dynamic hiding effects. A one‐layer model for the bed evolution and a bottom friction term, which depends on the grain size, are used. The initial formation of the ridges is studied for a bimodal sediment mixture. The results of the model indicate that the phase shift between bed topography and mean grain size for shoreface‐connected ridges is due to the selective transport via suspended load of grains with different sizes. A net stabilizing effect on the growth of bed forms and enhanced migration are predicted, caused by the bimodal character of the sediment. The wavelengths of the bed forms are only slightly affected. Including a tidal current or a grain size dependent formulation for the bottom friction has no effect on the results. A physical explanation for the model results is also given.
Warm water pathways, transports, and transformations in the northwestern North Atlantic and their modification by cold air outbreaks
Authors: Luo Yiyong , Zhang Huai‐Min , Prater Mark D. , Rothstein Lewis M.
Published: May 2003

This paper presents a numerical study of the warm water pathways, transports, and water mass transformation in the Newfoundland Basin region and an investigation of the rectification effects of a series of cold air outbreaks (CAOs) on the above processes. An initial “mean state” simulation was compared with observations and showed good agreement. Then, repeated CAO events were explicitly included in the surface fluxes and illustrated the following rectification effects. The thermal regime of the entire baroclinic layer was impacted, as the thermocline deepened and the temperature anomaly was noticeable down to ∼500 m. Different mechanisms were responsible for the propagation of the temperature anomaly at different stages. During the onset of the CAO, vertical diffusion propagated the temperature anomaly downward near the surface, then vertical advection further propagated the anomaly downward between CAO events to about 500 m depth. With CAOs, the North Atlantic Current carried more warm water, and its pathway in the Northwest Corner shifted towards the southeast. The volume‐averaged mean and eddy kinetic energy increased by 30% and 20%, respectively, and the water mass transformation rate in the Newfoundland basin was doubled. The Gulf Stream carried more heat, but heat transport to the eastern basin decreased owing to the increase in heat release to the atmosphere.
On the efficiency of statistical assimilation techniques in the presence of model and data error
Authors: Killworth Peter D. , Li Jian‐Guo , Smeed David
Published: May 2003

Statistically based assimilation methods make use of error statistics. When these statistics apply to data, some aspects of them are known. However, model error statistics are at best poorly known. The variability of these error fields causes the strength of the assimilated signal to vary. This (spatial) variability, plus diffusion, can cause the assimilated signal to rapidly disappear, decreasing the impact of the data. Our demonstration uses a statistical method [from Oschlies and Willebrand, 1996], which assimilates sea surface height using vertical regressions. An idealized Gaussian depression of sea surface height about 10° in diameter was assimilated, using both the original statistical scheme and a scheme with a horizontally uniform regression coefficient, to test the effects of variability in model errors. Results indicated that the uniform scheme is more efficient in assimilation of sea surface height than the original scheme (in that assimilated information is lost less rapidly because of small‐scale mixing) for the simplified Gaussian problem, but predictive skill was not improved when assimilating over the full North Atlantic. The influence of interpolated satellite observation errors caused by limitations of satellite track coverage was also assessed, using a chessboard sea surface depression that would be induced from the (statistically) optimal combination of model and data. Artificial eddies on the scale of the satellite track intervals are induced by including the satellite errors in the ocean model. These eddies dissipate rapidly in the presence of mixing, taking with them much of the assimilated signal. We suggest a possible solution.
Remotely forced nearshore upwelling in Southern California
Authors: Pringle James M. , Riser Kristin
Published: May 2003

Alongshore winds in Baja California strongly influence nearshore temperatures hundreds of kilometers to the north at Point Loma, San Diego, California, on timescales of a week to a year. The time lag between wind and temperature is consistent with first mode coastal trapped wave phase speed. The nearshore cross‐shelf circulation forced by the coastal trapped waves is, at least much of the year, oppositely directed at the surface and bottom. No relation is found between the winds and temperature for periods greater than a year. It is argued that similar results may be found elsewhere in the Southern California Bight. The relationship between stratification and bottom temperature varies over the 1.3 years of data, but for much of the time, warmer bottom waters are associated with even warmer surface waters and thus stronger stratification. The effects of the remotely forced cross‐shelf exchange on coastal pollution, nutrient dynamics, and larval transport are briefly discussed.
A model study of the circulation in the Pearl River Estuary (PRE) and its adjacent coastal waters: 1. Simulations and comparison with observations
Authors: Wong L. A. , Chen J. C. , Xue H. , Dong L. X. , Su J. L. , Heinke G.
Published: May 2003

The Princeton Ocean Model is used to study the circulation features in the Pearl River Estuary and their responses to tide, river discharge, wind, and heat flux in the winter dry and summer wet seasons. The model has an orthogonal curvilinear grid in the horizontal plane with variable spacing from 0.5 km in the estuary to 1 km on the shelf and 15 sigma levels in the vertical direction. The initial conditions and the subtidal open boundary forcing are obtained from an associated larger‐scale model of the northern South China Sea. Buoyancy forcing uses the climatological monthly heat fluxes and river discharges, and both the climatological monthly wind and the realistic wind are used in the sensitivity experiments. The tidal forcing is represented by sinusoidal functions with the observed amplitudes and phases. In this paper, the simulated tide is first examined. The simulated seasonal distributions of the salinity, as well as the temporal variations of the salinity and velocity over a tidal cycle are described and then compared with the in situ survey data from July 1999 and January 2000. The model successfully reproduces the main hydrodynamic processes, such as the stratification, mixing, frontal dynamics, summer upwelling, two‐layer gravitational circulation, etc., and the distributions of hydrodynamic parameters in the Pearl River Estuary and coastal waters for both the winter and the summer season.
A model study of the circulation in the Pearl River Estuary (PRE) and its adjacent coastal waters: 2. Sensitivity experiments
Authors: Wong L. A. , Chen J. C. , Xue H. , Dong L. X. , Guan W. B. , Su J. L.
Published: May 2003

The Princeton Ocean Model is used to study the circulation in the Pear River Estuary (PRE) and the adjacent coastal waters in the winter and summer seasons. Wong et al. [2003] compares the simulation results with the in situ measurements collected during the Pearl River Estuary Pollution Project (PREPP). In this paper, sensitivity experiments are carried out to examine the plume and the associated frontal dynamics in response to seasonal discharges and monsoon winds. During the winter, convergence between the seaward spreading plume water and the saline coastal water sets up a salinity front that aligns from the northeast to the southwest inside the PRE. During the summer the plume water fills the PRE at the surface and spreads eastward in the coastal waters in response to the prevailing southwesterly monsoon. The overall alignment of the plume is from the northwest to the southeast. The subsurface front is similar to that in the winter and summer except that the summer front is closer to the mouth and the winter front closer to the head of the estuary. Inside the PRE, bottom flows are always toward the head of the estuary, attributed to the density gradient associated with the plume front. In contrast, bottom flows in the shelf change from offshore in winter to onshore in summer, reflecting respectively the wintertime downwelling and summertime upwelling. Wind also plays an essential role in controlling the plume at the surface. An easterly wind drives the plume westward regardless winter or summer. The eastward spreading of the plume during the summer can be attributed to the southerly component of the wind. On the other hand, the surface area of the plume is positively proportional to the amount of discharge.
Subinertial flows and transports in Cozumel Channel
Authors: Chávez Gabriela , Candela Julio , Ochoa José
Published: May 2003

The subinertial flow of the Cozumel Channel is examined with nearly six months of current, subsurface pressure and meteorological data in order to (a) verify the validity of the geostrophic balance in Cozumel Channel and analyze the influence of other terms of the momentum equation in observed ageostrophic variations, and (b) estimate the transport through the Channel using a set of continuous current profile observations at the Channel's center and the cross section velocity maps obtained from shipboard ADCP observations. Over periods longer than a month, the momentum balance in the channel is mainly geostrophic, but on alternative periods of similar duration, intense ageostrophic fluctuations are found that have timescales of days to weeks and up to 1 m/s amplitudes. There is evidence that the advective nonlinear terms are the most probable cause of the lack of geostrophy over these periods. Other terms such as the local acceleration and the wind stress are found to contribute minimally in the ageostrophic fluctuations. A mean transport of approximately 5 Sv toward the Gulf of Mexico is estimated for the 5 months of measurements (December 1996 to May 1997), although with great variability: a minimum of 1.3 Sv around January, a maximum of 7.5 Sv by the beginning of May, and a standard deviation of 1.2 Sv. Neither surface currents nor the geostrophically deduced current is suitable for estimating the observed transport in the Channel.
Internal tides in the Northern Gulf of California
Authors: Filonov Anatoliy E. , Lavín M. F.
Published: May 2003

The characteristics of the internal tide in the Northern Gulf of California are described using data from two moored arrays of temperature and current sensors, one for summer and one for winter, located between Angel de la Guarda Island and the mainland. From the summer six‐sensor mooring it was found that: (1) the current fluctuations are dominated by the semidiurnal frequency band, while the quarterdiurnal frequency dominated the temperature fluctuations. (2) The baroclinic semidiurnal horizontal current fluctuations are aligned with the gulf axis, and have amplitudes of 10–15 cm s−1; the vertical displacements reached 4 m in this frequency band. (3) The vertical modal structure for the temperature and velocity oscillations was dominated by the first and third modes. (4) The energy of the semidiurnal internal tide is 45% of that of the barotropic tide. (5) Vertical wave number spectra showed slightly asymmetric peaks in the high wave number components, indicating that their downflowing energy is larger than that flowing upward. From the winter two‐sensor mooring, it was found that the vertical oscillations were mainly semidiurnal, with root mean square amplitudes of 7 m.
Numerical modeling of gravity‐driven sediment transport and deposition on an energetic continental shelf: Eel River, northern California
Authors: Scully M. E. , Friedrichs C. T. , Wright L. D.
Published: May 2003

A two‐dimensional numerical model was applied to predict large‐scale deposition by wave‐supported sediment gravity flows on the Eel River continental shelf for four consecutive flood seasons using measured bathymetry, waves and river forcing. The model assumes that sediment‐induced stratification maintains the near‐bed Richardson number at its critical value, which determines the sediment carrying capacity of the wave boundary layer. Deposition is predicted when the gravity‐driven flux of sediment exceeds the carrying capacity. The model predicted 26% of fine sediment discharged by the Eel River to be deposited on the midshelf with a magnitude and distribution largely consistent with field observations. Greatest deposition on the midshelf was predicted well north of the river mouth despite greater sediment input nearest the river mouth. Model results indicate that when the river delivers sufficient sediment to critically stratify the wave boundary layer, wave intensity and the bathymetry of the Eel shelf are the dominant factors controlling the observed pattern of deposition. Large wave energy caused the majority of fine sediment (65%) to escape the shelf as gravity‐driven flows. The greatest amount of sediment was predicted to leave the shelf from the region off‐shelf of the river mouth (including 11% into the Eel Canyon) where inshore sediment input was high and the concave downward bathymetry associated with the Eel River subaqueous delta prevents significant midshelf gravity‐driven deposition.
Highest natural bed forms
Authors: Giménez‐Curto Luis A. , Corniero Miguel A.
Published: May 2003

Fluid flow interacts with sedimentary beds forming waves of different kinds, which are of considerable practical importance since they influence significantly the near‐bed flow, both over and below the bed, sediment transport, and wave height attenuation. We focus here on steep bed forms capable of producing flow separation. In this case, the large‐scale vorticity generated in the phenomenon of separation rules the process of friction, which appears to be practically unaffected by sediment motion. Under the crests of the bed forms, the mean shear force due to friction is balanced by the force that bed forms exert on the flow via pressure, which can be calculated from the work of Giménez‐Curto and Corniero [2002]. Bed forms grow until they have a height such that friction at their troughs is negligible, thus ceasing the motion of fluid and sediment. This condition leads to a very simple expression for the limiting steepness, which compares favorably with existing observations on bed form geometry under steady open channel flow as well as under oscillatory flow.
Cabbeling and the density of the North Pacific Intermediate Water quantified by an inverse method
Authors: Yun Jae‐Yul , Talley Lynne D.
Published: May 2003

North Pacific Intermediate Water (NPIW), defined as the main salinity minimum in the subtropical North Pacific, at a density of 26.7–26.8σθ, is denser than the winter surface water in the Oyashio which is the source of the salinity minimum. We showed previously that cabbeling and double diffusion during mixing between the Oyashio water and more saline Kuroshio water can account for the density increase from the surface source water to the salinity minimum. An inverse method is employed herein to quantify the effect of cabbeling, using CTD data from the western North Pacific. The difference between proportional mixing between parcels of Oyashio and Kuroshio waters and mixing along isopycnals is exploited to compute the convergence of water into density layers. The diapycnal transport convergence associated with cabbeling into the NPIW density layer is estimated to be 0.56 Sv for an assumed turnover time of 1 year in the region between 142°E and 152°E. Diapycnal transport convergences in the regions 152°E–165°E, 165°E–175°W, and 175°W–136°W are similarly estimated by assuming longer turnover times. We estimate that the total diapycnal transport convergence into the NPIW density layer may be up to 2.3 Sv in the entire NPIW region.
Moored observations in the northern Gulf of California: A strong bottom current
Authors: López Manuel , García Joaquín
Published: May 2003

Measurements made during 1997–1998 in the northern Gulf of California show a persistent current flowing close to the bottom over a sill located at a depth of 340 m. The current flows at an average speed of 27 cm/s, 17 m above the bottom, toward the head of the gulf (NW). The sill is located near the Baja California coast and connects a relatively shallow basin (440 m) to the SE with a deeper one (800 m) to the NW. The current was recorded from November to August, and an estimate of the mean transport from November to March gives about 0.1 Sv (1 Sv = 106 m3 s−1). An independent transport estimation based on the salinity budget gave essentially the same value. A lower bound for the residence time of the water in the northernmost gulf is estimated to be 1.5 years. At depths above 260 m, mean currents were much more variable and weaker, but they veered abruptly toward the NW closer to the bottom. Another mooring, placed over the shallower continental shelf (100 m deep) on the mainland side, recorded weak mean currents toward the head of the gulf. Low‐frequency current fluctuations show significant correlation between both sides of the gulf. During the 9‐month observation period, the deep current fluctuations over the sill are in opposition to the currents over the shelf, which is consistent with a compensating mechanism for the exchange. Nearby coastal bottom pressure (CBP) is correlated with current fluctuations at both sides of the gulf, as well as with remote sea level at Manzanillo, well outside of the gulf. This latter correlation could be related to the El Niño signal penetrating to the northern gulf. Correlation between CBP and currents on the Baja California side is consistent with a barotropic, geostrophic response. Around 200 m, the temperature increases from November to January, and this warming event is accompained with an increase in salinity. These features are consistent with an event of water mass formation in the northern gulf. Near‐bottom currents and temperature are highly correlated, such that an increasing inflow is associated with cooler waters. The bottom current plays an important role in the exchange of the northern gulf with the Pacific Ocean.
Test of self‐organization in beach cusp formation
Authors: Coco Giovanni , Burnet T. K. , Werner B. T. , Elgar Steve
Published: May 2003

Field observations of swash flow patterns and morphology change are consistent with the hypothesis that beach cusps form by self‐organization, wherein positive feedback between swash flow and developing morphology causes initial development of the pattern and negative feedback owing to circulation of flow within beach cusp bays causes pattern stabilization. The self‐organization hypothesis is tested using measurements from three experiments on a barrier island beach in North Carolina. Beach cusps developed after the beach was smoothed by a storm and after existing beach cusps were smoothed by a bulldozer. Swash front motions were recorded on video during daylight hours, and morphology was measured by surveying at 3–4 hour intervals. Three signatures of self‐organization were observed in all experiments. First, time lags between swash front motions in beach cusp bays and horns increase with increasing relief, representing the effect of morphology on flow. Second, differential erosion between bays and horns initially increases with increasing time lag, representing the effect of flow on morphology change because positive feedback causes growth of beach cusps. Third, after initial growth, differential erosion decreases with increasing time lag, representing the onset of negative feedback that stabilizes beach cusps. A numerical model based on self‐organization, initialized with measured morphology and alongshore‐uniform distributions of initial velocities and positions of the swash front at the beginning of a swash cycle, reproduces the measurements, except for parts of one experiment, where limited surveys and a significant low‐frequency component to swash motions might have caused errors in model initialization.
Parameterizations of the ocean skin effect and implications for satellite‐based measurement of sea‐surface temperature
Authors: Horrocks Lisa A. , Candy Brett , Nightingale Tim J. , Saunders Roger W. , O'Carroll Anne , Harris Andrew R.
Published: May 2003

Satellite‐based retrievals of sea‐surface temperature (SST) hold great potential for augmenting and improving existing global climate analyses. However, optimal blending of satellite data with in situ measurements of SST requires an accurate estimate of the temperature difference between radiometric skin and bulk water sampled below the skin. Contemporaneous shipborne radiometer measurements and in situ measurements of SST were obtained during a cruise in the western Pacific and used to define the nighttime skin‐bulk temperature difference. A strong trend of increasing temperature difference with decreasing wind speed was observed. Three published parameterizations of the skin effect have been tested against the ship observations, with excellent agreement achieved by one of these. Good results were also obtained when this scheme was forced by heat fluxes from the Met Office numerical weather prediction global analysis collocated with the observations in space and time. The implementation of a skin effect model in a global satellite data processing system was tested using collocated observations of SST from buoys (bulk) and along‐track scanning radiometer 2 (skin) during 1995–1997. This is a vital step toward integrating accurate satellite retrievals of SST with traditional in situ data sources for climate variability studies.