bluuebunny/crossref_metadata_2025 huggingface dataset:
A long‐lived eddy in the Iceland Basin 1998
Authors: Read J. F. , Pollard R. T.
Published: February 2004

A survey was made of a large (100 km) eddy in the Iceland Basin in June 1998. In situ observations from a shallow, undulating conductivity‐temperature‐depth profiler and vessel‐mounted acoustic Doppler current profiler were used to generate sea surface height along two TOPEX/Poseidon tracks. Comparisons were made with the altimetric measurements to arrive at an estimate of the sea surface height for 6 years of TOPEX/Poseidon altimeter data. The 3 month track of a drifting buoy was used to estimate the location of the center of the eddy. Comparison with the altimetric sea surface height showed that the position of the eddy correlated well with an elevated sea surface signal of 25–30 cm. Having identified this signal as a characteristic signature of eddies, inspection of the 6 year record revealed other similar features whose presence and longevity varied considerably. The presence or absence of eddies appeared to link with the large‐scale variability of the subpolar gyre: when the gyre expanded (e.g., 1993–1994), the North Atlantic Current tended to follow a southerly route and eddies in the Iceland Basin were few and weak, especially compared to periods when the gyre contracted (e.g., 1992–1993 and 1996–1998) and the North Atlantic Current turned through the Iceland Basin: then saltier water from the eastern North Atlantic extended westward into the Iceland Basin and into large, long‐lived eddies.
Mobile‐bed effects in oscillatory sheet flow
Authors: Dohmen‐Janssen C. Marjolein , Hassan Wael N. , Ribberink Jan S.
Published: February 2004

Field observations often show a considerable variation in mean grain size along the coastal profile. Under high waves in shallow water, bed ripples are washed out, and sheet flow becomes the dominant transport mode: large amounts of sand are transported in a thin layer close to the bed, i.e., the sheet flow layer. This paper focuses on grain size influences on transport processes in oscillatory sheet flow. Experiments were carried out in the Large Oscillating Water Tunnel (LOWT) of Delft Hydraulics, in which near‐bed orbital velocities in combination with a net current can be simulated at full scale. Three uniform sands with different mean grain size were used. It was found that in contradiction to expressions found in literature, both the erosion depth and the sheet flow layer thickness are larger for fine sand (D50 = 0.13 mm) than for coarser sand (D50 ≥ 0.21 mm). Measured time‐averaged velocity and concentration profiles indicate that the presence of a sheet flow layer leads to an increased flow resistance and to damping of turbulence and that these effects are stronger for a thicker sheet flow layer (i.e., for fine sand). These mobile‐bed effects are analyzed further by comparing the measurements with the results of a one‐dimensional vertical advection‐diffusion boundary layer model. Simulating the mobile‐bed effects in the model by introducing an increased roughness height and a reduced turbulent eddy viscosity showed that the roughness height is of the order of the sheet flow layer thickness and that turbulence damping increases for a decreasing grain size.
West Florida shelf response to local wind forcing: April 1998
Authors: Weisberg Robert H. , Li Zhenjiang , Muller‐Karger Frank
Published: February 2004

We compare west Florida shelf velocity and sea level data with a model simulation for April 1998. Responses for three upwelling and three downwelling favorable wind events are documented. Along‐shelf jets accompanied by oppositely directed upper and lower layer across‐shelf flows (with connecting vertical velocity) comprise the fully three‐dimensional inner shelf responses, which are sensitive to stratification. With an initial density field representative of April 1998 the model simulates velocity and sea level variations in general agreement with the observations, whereas substantial mismatches occur without stratification. Despite the winds being the primary motive agent for the inner shelf the stratification dependence requires that model density fields be maintained through a combination of adequate initial conditions; surface, offshore, and land‐derived buoyancy inputs; and data assimilation. Dynamical analyses define the inner shelf as the region where the surface and bottom boundary layers are important in the momentum balance. Kinematically, this is where surface Ekman layer divergence, fed by the bottom Ekman layer convergence, or conversely, sets up the across‐shelf pressure gradient. Stratification causes a response asymmetry wherein the offshore scale and magnitude of the upwelling responses are larger than those for the downwelling responses. This asymmetry is attributed to thermal wind effects across the bottom Ekman layer. Buoyancy torque by isopycnals bending into the bottom adds constructively (destructively) with planetary vorticity tilting under upwelling (downwelling) favorable winds, and this may have important implications for nutrients and other material property distributions on the shelf.
Far‐field simulation of tsunami propagation in the Pacific Ocean: Impact on the Marquesas Islands (French Polynesia)
Authors: Hébert Hélène , Heinrich Philippe , Schindelé François , Piatanesi Alessio
Published: February 2004

The islands in the Marquesas archipelago are most exposed to far‐field tsunamis in French Polynesia. Four recent earthquakes located at the Pacific rim (Kurile 1994, Chile 1995, Mexico 1995 and Peru 1996 earthquakes) generated trans‐Pacific tsunamis and caused contrasting inundations in several inhabited Marquesian bays. The aim of this study is to better understand the amplification phenomena observed in the Marquesas Islands and to improve the risk assessment. We present here numerical simulations of these four tsunamis by means of a finite difference model solving the equations of tsunami propagation at different scales. The water heights computed in the bays are in excellent agreement with the available observations and thus validate our numerical method. These numerical results allow us to determine (1) which propagation azimuths are dangerous for the whole Marquesas archipelago and (2) the behavior of each studied bay (amplitude, frequency) in response to tsunami waves. We observe that the seismogenic subduction zone along South America is likely to define one of the most dangerous tsunami generation areas for the Marquesas archipelago, owing to the source geometries and to the trapping of the tsunami energy by bathymetric features in the southern Pacific Ocean (fracture zones, seamount chains).
Internal tide dissipation, mixing, and vertical nitrate flux at the shelf edge of NE New Zealand
Authors: Sharples Jonathan , Moore C. Mark , Abraham Edward R.
Published: February 2004

An internal tide on the NE shelf of New Zealand was observed with a combination of moored temperature loggers and current meters and vertical profiling with a microstructure probe. Internal tide energy flux across the shelf edge was calculated to be ∼400 W m−1, with considerable variability driven by the passage of a storm through the region. Energy associated with the internal tide was significantly greater than the energy of the barotropic tide or of inertial shear. Dissipation of the internal tide calculated from the energy loss between two of the moorings was estimated to be 15±10 mW m−2. The associated vertical eddy diffusivity was (4 ± 3) × 10−4 m2 s−1. The microstructure observations indicated internal tide‐driven vertical diffusivities at the nitracline of ∼7×10−4 m2 s−1. The observations of vertical eddy diffusivities are combined with measurements of the vertical nitrate gradient to suggest that mixing driven by the internal tide is the dominant mechanism for driving diapycnal nutrient supply during summer. The calculated flux of about 12 mmol N m−2 d−1 into the photic zone is suggested to drive significant new subsurface production throughout the summer, amounting to a possible contribution to annual new production on the shelf of 100 g C m−2.
Use of data assimilation via linear low‐order models for the initialization of El Niño‐Southern Oscillation predictions
Authors: Cañizares Rafael , Kaplan Alexey , Cane Mark A. , Chen Dake , Zebiak Stephen E.
Published: February 2004

The utility of a Kalman filter (KF) for initialization of an intermediate nonlinear coupled model for El Niño‐Southern Oscillation prediction is studied via an approximation of the nonlinear coupled model by a system of seasonally dependent linear models. The low‐dimensional nature of such an approximation allows one to determine a sequence of “perfect” initial states that start a trajectory segment best fitting the observed data. Defining these perfect initial conditions as “true” states of the model, we compute a priori parameters of the KF and test its ability to produce an estimate of the “truth” superior to the less theoretically sound estimates. We find that in this application such a KF does not produce an estimate outperforming a pure observational projection as an initial condition for the coupled model forecast. The violation of standard KF assumptions on temporal whiteness of observational errors and system noise is identified as the reason for this failure.
A numerical study of the seasonal variability in the circulation off central Chile
Authors: Leth Ole , Shaffer Gary
Published: February 2004

The Princeton Ocean Model is used to study the coastal ocean circulation off central Chile (between 30° and 45°S) and its seasonal transitions in response to monthly mean climatological wind stress and surface heat flux. The model reproduces many of the dominant circulation features, including an equatorward coastal current, coastal upwelling, and a poleward undercurrent. To get the correct seasonal variability in the undercurrent, it was necessary to prescribe the inflow of the undercurrent on the northern model boundary. Meanders and eddies form throughout the simulation period seaward of the coastal current in response to baroclinic instability of the flow. The flow associated with the meanders shows upwelling in the shoreward flow and downwelling in the seaward flow. This upwelling/downwelling pattern is explained by conservation of relative vorticity in the flow. The upwelling velocity associated with the meanders corresponds to a vertical migration of 15–30 m over a typical timescale of the meanders of 1–2 months. It is argued that this upwelling may be important for maintaining observed high phytoplankton biomass and primary production seaward of the coastal zone directly affected by wind‐driven upwelling.
Improving the utility of ocean circulation models through adjustment of the momentum balance
Authors: Sheng Jinyu , Greatbatch Richard J. , Wright Daniel G.
Published: February 2004

We propose a new method to improve the utility of three‐dimensional ocean circulation models. The method uses climatological temperature and salinity data to adjust the momentum balance of the model, while leaving the tracer equations fully prognostic and unconstrained. The adjustment is accomplished by replacing density in the hydrostatic equation by a linear combination of model‐computed and climatological density. The procedure is equivalent to adding a forcing term to the horizontal momentum equation through a modification of the model's horizontal pressure gradient term. The forcing term modifies the model‐computed velocity field, which, in turn, affects the model‐computed temperature and salinity fields through the advection term (there is no adjustment of the tracer equations carried by the model). Assuming the linear combination coefficient to be invariant in time and space, we suggest a statistical approach to estimating its optimal value. We apply this “semiprognostic” method to the northwest Atlantic. A primitive equation circulation model is initialized with January climatological temperature and salinity and is forced by monthly mean Comprehensive Ocean‐Atmosphere Data Set surface wind stress and heat flux, by restoration of the surface salinity to monthly mean climatology, and by flows through the open boundaries. Both the model‐computed tracer and velocity fields produced using the semiprognostic method show significant improvement over those produced by a purely prognostic calculation; drift of the tracer and velocity fields away from climatology is greatly reduced. Further, convective mixing is explicitly represented, thus improving the utility of results over those obtained from pure diagnostic calculations. The velocity fields obtained with the new approach are somewhat more realistic than those obtained from pure diagnostic calculations. The method reproduces many well‐known circulation features in the region, including the Labrador Current, the Gulf Stream, and the North Atlantic Current. More significantly, the method reproduces reasonably well the seasonal evolution of temperature and salinity in the region despite the fact that the model's tracer fields are not constrained directly by the new method. This result suggests that the semiprognostic approach will be useful for examining the evolution of tracers that are not easily determined by observations.
Modeling circulation and thermal structure in Lake Michigan: Annual cycle and interannual variability
Authors: Beletsky Dmitry , Schwab David J.
Published: February 2004

A three‐dimensional primitive equation numerical model was applied to Lake Michigan for the periods 1982–1983 and 1994–1995 to study seasonal and interannual variability of lake‐wide circulation and thermal structure in the lake. The model was able to reproduce all of the basic features of the thermal structure in Lake Michigan: spring thermal bar, full stratification, deepening of the thermocline during the fall cooling, and finally, an overturn in the late fall. Large‐scale circulation patterns tend to be cyclonic (counterclockwise), with cyclonic circulation within each subbasin. The largest currents and maximum cyclonic vorticity occur in the fall and winter when temperature gradients are low but wind stresses are strongest. The smallest currents and minimum cyclonic vorticity occur in spring and summer when temperature gradients are strong but wind stresses are weakest. All these facts are in agreement with observations. The main shortcoming of the model was that it tended to predict a more diffuse thermocline than was indicated by observations and explained only up to half of the variance observed in horizontal currents at timescales shorter than a day.
Surface current variability east of Okinawa Island obtained from remotely sensed and in situ observational data
Authors: Hisaki Yukiharu , Tokeshi Tsutomu , Fujiie Wataru , Sato Kenji , Fujii Satoshi
Published: February 2004

We investigated ocean surface current variability in the region east of Okinawa Island in the spring of 1998 using in situ and remotely sensed data including surface velocity estimates inferred from HF ocean radars. Most previous studies utilizing HF ocean radars were limited to shallow continental shelves. In contrast, most of the observation area in this study was in the open ocean. During the observation period the HF ocean radars sampled a region where anticyclonic and cyclonic mesoscale eddies were adjacent to each other. The HF‐radar‐derived ocean currents agree with those measured by a current meter. The surface currents were highly variable in both time and space and were affected by the eddy field in the offshore region. The current field was related to the near‐surface water temperature. For example, a local temperature rise was often associated with northeastward flows. The surface currents in regions shallower than about 1000 m had a significant correlation with local wind forcing. Our HF ocean radars often detected a strongly convergent zone, which was different in character from those described in previous studies in that it was in the deep ocean. From other data such as National Oceanic and Atmospheric Administration advanced very high resolution radiometer imagery, we concluded that the convergent zone was due to mesoscale eddy fronts.
The upper ocean heat content of the western equatorial Pacific: Processes controlling its change during the Tropical Ocean‐Global Atmosphere Coupled Ocean‐Atmosphere Response Experiment
Authors: Richards Kelvin J. , Inall Mark E.
Published: February 2004

The processes controlling the upper ocean heat content in the western equatorial Pacific are examined using data collected during the Tropical Ocean‐Global Atmosphere Coupled Ocean‐Atmosphere Response Experiment. The work centers on a number of meridional sections taken along 156°E from 5°S to 5°N undertaken from December 1992 to February 1993. We compare and contrast the conditions north and south of the equator. The net change in the depth‐averaged temperature (here taken over the top 40 m of the ocean) over the entire sampling period is found to be very similar over the observed range of latitude; the change being a decrease of approximately 0.5°C. However, there are periods lasting a few tens of days when there are large differences (by as much as 1°C) in the change to the depth‐averaged temperature north and south of the equator. By examining the heat balance, these changes are found to be brought about by a combination of the differing surface heat fluxes and ocean currents in the two subregions. The surface heat flux over the region is estimated using an empirical formula based on winds from the Tropical Ocean Atmosphere array and cloud albedo measurements from the GMS satellite. Intense meridional flows in the ocean in response to high‐frequency wind events play a crucial role in the heat balance both north and south of the equator. The timing of such flows varies with latitude. The redistribution of heat caused by the ocean flow produces changes to the meridional gradient of sea surface temperature. These changes in temperature gradient have the potential of feeding back to the atmosphere.
Swash zone sediment suspension and transport and the importance of bore‐generated turbulence
Authors: Puleo J. A. , Beach R. A. , Holman R. A. , Allen J. S.
Published: February 2004

A study of swash zone sediment transport was conducted at Gleneden Beach, Oregon during February 25–28, 1994. The data collected included suspended sediment concentration (SSC), sea surface elevation, and velocity (initially 4 and 8 cm above the bed) at three cross‐shore locations within the swash zone spanning high tides. Ensemble averages of 6, 9, and 12 s duration swash events showed that the uprush suspension was high, concentrated in the leading edge, and nearly vertically uniform above the lower 1–2 cm of the water column. Shortly after the sensors were inundated by run‐up, the sediment rapidly settled out of the water. During flow reversal the SSC was small but increased again in the backwash. Backwash vertical profiles were markedly different from uprush profiles with much of the suspension being confined to very near the bed where strong vertical gradients in SSC existed. These marked differences show that the backwash is not simply the reverse of the uprush, implying significant differences in the underlying fluid dynamics and sediment transport mechanisms. Backwash sediment suspension increased with flow duration. However, ensemble‐averaged SSC profiles of varying duration showed that the backwash concentrations were not consistent at the same temporal phases, which suggests that water depth, in addition to flow duration, may be a controlling factor. Strong cross‐shore gradients in SSC suggest that bore‐derived turbulence may affect local sediment transport. Specifically, our data show this bore‐generated turbulence (turbulent kinetic energy) directly influences local sediment suspension, hence, standard bed shear (Bagnold‐type) sediment transport models may no longer be valid in the vicinity of the bore. In the vicinity of the bore a higher correlation between bore‐generated turbulence and suspended sediment transport was found than between a Bagnold‐type formulation and suspended sediment transport.
Convergence of lateral flow along a coastal plain estuary
Authors: Valle‐Levinson Arnoldo , Li Chunyan , Wong Kuo‐Chuin , Lwiza Kamazima M. M.
Published: February 2004

A set of velocity profiles obtained in the James River estuary with an acoustic Doppler current profiler was used in combination with the results of an analytic tidal model to depict the appearance of surface lateral flow convergences (∂v/∂y) during both flood and ebb stages of the tidal cycle. The bathymetry of the estuary was characterized by a main channel and a secondary channel separated by relatively narrow shoals. Lateral surface flow convergences appeared over the edges of the channels and were produced by the phase lag of the flow in the channel relative to the shoals. Flood convergences developed in the late tidal stages and ebb convergences appeared soon after maximum currents. Most of these convergences caused fronts in the density field and flotsam lines that also appeared over the edges of the channel and that lasted <2 hours. The transverse flows associated with the convergences were mostly in the same direction throughout the water column. In fact, the vertically averaged flow produced the same convergence patterns as those near the surface. The analytic tidal model reproduced well the timing and location of the convergences as observed in the James River. Model results with different bathymetry emulated the results in other estuaries, e.g., axial convergence in an estuary with a channel in the middle. This work showed that the strength of lateral convergences along the estuary was proportional to the tidal amplitude and the channel steepness. It also suggested that the convergences were produced mainly by the tidal flow interacting with the channel‐shoal bathymetry, i.e., that they did not require the presence of density gradients. However, the analytic model underestimated the magnitude of the convergences and did not account for vertical circulations associated with fronts. The formation of fronts resulted from the interaction of the tidal flow with the bathymetry and the density field.
Sea surface velocities from sea surface temperature image sequences: 1. Method and validation using primitive equation model output
Authors: Vigan Xavier , Provost Christine , Bleck Rainer , Courtier Philippe
Published: February 2004

An inverse variational finite element model is developed for the purpose of estimating ocean surface velocity fields from sequences of temperature fields. The cross‐isotherm component of the velocity is controlled by a mixed layer integrated formulation of the heat balance. The aperture problem imposes additional constraints on the flow field: we control the divergence and the vorticity of the horizontal velocity. The method is then applied to sequences of sea surface temperature (SST) fields from a fine‐mesh numerical simulation over the Brazil‐Malvinas Confluence region. The difference between the actual velocity and the SST inverted velocity is 11% rms in magnitude and 17° rms in direction. These differences are analyzed; divergence and vorticity are computed. The hypothesis of neglecting the source terms made in the formulation of the mixed layer integrated heat balance is verified. The sensitivity of the solution to the influence of the constraints is examined. Perturbations are performed yielding fields of ellipses of covariance. We obtain on average 15% uncertainties in magnitude and 25° in direction of the velocity. Differences between the actual and SST inverted velocities fall into those covariance ellipses, except over regions where temperature does not change from one image to the other.
Laboratory measurement of bottom shear stress on a movable bed
Authors: Rankin Kelly L. , Hires Richard I.
Published: February 2004

A shear plate was developed to obtain direct measurements of bottom shear stress under nonbreaking surface gravity waves on a movable sand bed. The experiments were conducted under regular wave conditions in a large wave tank where time histories of bottom shear stress and surface elevation were obtained from a shear plate and wave gauges. Measurements were made at scales approaching those encountered in the field. These data were obtained under steep, vortex ripples where form drag dominated over skin friction for all data presented. Wave friction factors were calculated from measurements and were compared with existing theories as well as with previous laboratory studies. Values of the wave friction factor obtained using the shear plate exhibited reasonable agreement with existing theory. Close agreement between values of the wave friction factor obtained from the shear plate and those measured by previous studies demonstrates that use of a shear plate is a reliable additional method for the measurement of bottom shear stress over a movable bed in the laboratory.
Vertical structure of tidal currents over the Camarinal Sill at the Strait of Gibraltar
Authors: Tsimplis M. N.
Published: February 2004

An acoustic Doppler current profiler (ADCP) is used to estimate the vertical structure of currents at the Camarinal Sill at the Strait of Gibraltar. The most important tidal components, namely, M2, S2, O1, K1, M4, M3, Msf and Mm are examined. The semidiurnal tides are the most energetic, with along‐strait speeds of up to 120 cm s−1 for M2 and 48 cm s−1 for S2. The diurnal tides have along‐strait speeds of 15–35 cm s−1. M4 increases from 4 cm s−1 in the upper layer to ∼15 cm s−1 below 220 m. The fortnightly component Msf reaches speeds in excess of 30 cm s−1 at the upper 120 m. The vertical structure of the tidal signal is complicated, differs between semidiurnal and diurnal tides, and varies between spring and neap tides. Plausible assumptions about the density profile and the bottom friction yield tidal components similar to those observed. The effects of friction are described by the use of an analytical constant eddy viscosity model and the effects of stratification by estimating the internal modes at each tidal frequency. The major part of the tidal signal is barotropic, with the internal tide modes and the frictional effects contributing <20 cm s−1 for the semidiurnal tides. Nevertheless, individual modes reach 50 cm s−1. The internal tides for the diurnal components are <5 cm s−1, with individual modes reaching 8 cm s−1. The vertical profile of Msf can be explained by the use of internal modes reaching amplitudes of 40 cm s−1. The fitting of the internal wave modes to the vertical profiles also explains the observed phase change with depth of the semimajor axis. The comparison with pressure difference across the strait is consistent with previous studies. This report is the first on the ADCP time series on the Camarinal Sill.
Indian Ocean warming of 1997–1998
Authors: Yu Lisan , Rienecker Michele M.
Published: February 2004

The Indian Ocean underwent substantial changes in 1997–1998. The observations show not only the appearance of a dipole mode in the tropical region but also a persistent basin‐scale warming. We present in this study an analysis of the basin‐scale sea surface temperature (SST) variations during 1997–1998 using satellite observations, in situ temperature measurements, and National Centers for Environmental Prediction reanalyses. We find that the Indian SST anomaly peaks occurred at two periods, i.e., November–December–January coinciding with the Niño3 peak and the following April–May–June, and were phase locked to the Indian Ocean seasonal cycle. The changes of SST in the equatorial ocean were related to a coupled interaction between the atmosphere and the ocean. Oceanic upwelling in the east and downwelling in the west played a major role in giving rise to the SST anomalies associated with the dipole mode structure. The upwelling off the coast of Sumatra elevated the regional thermocline by more than 80 m in December 1997. On the other hand, the changes of SST in the southern Indian Ocean were largely induced by the changes of local latent flux. During boreal fall‐winter of 1997 the southeasterly trades were displaced and abnormally prolonged in their northernmost equatorial position. This shifted the center of the trades toward the equator, weakened the winds in the central Southern Ocean, reduced the latent heat flux in the region, and subsequently induced a surface warming. The total change of the SST anomalies integrated over the two periods, July–December 1997 and January–May 1998, were explained well by the same period latent flux integral in both intensity and pattern. The cross‐basin upper ocean temperature sections show that the extratropical warming was rather uniformly distributed in the upper 60 m, further supporting the role of mixed‐layer processes in the warming.
Seasonal current simulations for the western continental margin of Vancouver Island
Authors: Foreman M. G. G. , Thomson R. E. , Smith C. L.
Published: February 2004

Two three‐dimensional finite element models, one diagnostic and one semiprognostic, are used to simulate summer and winter currents for the western continental margin of Vancouver Island, British Columbia. The calculations are forced (or nudged) with seasonal winds, climatological density fields, and elevation‐specified boundary conditions that have been adjusted via inversion to more accurately represent the California Undercurrent and estuarine flow in Juan de Puca Strait. Tides are included in the simulations in order to correctly represent turbulent mixing, bottom friction, and the contribution of tidal rectification. The seasonal model currents are shown to compare favorably with multiyear, low‐pass filtered current meter observations and to capture strong shears both vertically in Juan de Fuca Strait and horizontally and vertically across the continental shelf and slope. The tidal currents are more accurate than those computed with a previous three‐dimensional barotropic model.
Mixed layer depth variability and barrier layer formation over the North Pacific Ocean
Authors: Kara A. Birol , Rochford Peter A. , Hurlburt Harley E.
Published: February 2004

Seasonal variability in the isothermal and isopycnal surface mixed layers of the North Pacific Ocean is examined using the Naval Research Laboratory Ocean Mixed Layer Depth (NMLD) Climatology. A comparison with observations from 11 ocean weather stations in the northeast Pacific Ocean is performed that validates the NMLD climatology in this region. The general features of the isothermal layer depth (ILD) and mixed layer depth (MLD) obtained from these mixed layers are explained with wind stress, surface net heat flux, and freshwater flux climatologies, given guidance from a mixed layer model. Departures from a surface‐forced interpretation of turbulent mixing are found near the Kuroshio, where horizontal heat transport is important. The much deeper ILD in the northeast Pacific in winter and spring relative to the MLD reveals a 50 m “barrier layer” between the bottom of the MLD and the top of the thermocline. A detailed analysis shows this barrier layer extends over most of the North Pacific subpolar gyre. It forms when the seasonal thermocline is deepened in winter by surface cooling, such that salinity stratification due to evaporation minus precipitation less than zero (E ‐ P <0) becomes important in the formation of the MLD. A shallower halocline forms over the subpolar gyre than in other regions of the North Pacific because of precipitation dominating over evaporation in the annual mean. A mechanism for maintaining the shallow halocline is provided by upward vertical motion driven by positive wind stress curl in the presence of diapycnal mixing. Numerical models show this as part of a shallow meridional overturning cell.
Subtropical mode water variation in the northwestern part of the North Pacific subtropical gyre
Authors: Taneda Takeshi , Suga Toshio , Hanawa Kimio
Published: February 2004

Temporal variations of the North Pacific subtropical mode water (NPSTMW) are studied with the proper temporal resolution for describing seasonal variation using monthly hydrographic data taken near the Ogasawara Islands. NPSTMW is formed south of the Kuroshio and the Kuroshio Extension in late winter and advected to the Ogasawara site by the southwestward/westward return flow of the Kuroshio recirculation. While NPSTMW is seen at the Ogasawara site in almost all seasons, NPSTMW with a higher core temperature is advected to this site in April, and one with a colder core follows in and after June until January or February. Furthermore, the NPSTMW core temperature shows considerable interannual variation. This variation is very similar to that found in other regions away from the Ogasawara site, suggesting that it reflects the larger‐scale variation of NPSTMW and that this site would be suitable for monitoring the NPSTMW core temperature. A significant correlation between the NPSTMW core temperature and the monsoon index, which is defined as the difference in the sea level pressure between Nemuro, Japan, and Irkutsk, Russia, as an indicator of wintertime cooling, indicates that NPSTMW contains a memory of wintertime cooling in the preceding winter.
Melt ponds on sea ice in the Canadian Archipelago: 1. Variability in morphological and radiative properties
Authors: Yackel J. J. , Barber D. G. , Hanesiak J. M.
Published: February 2004

The morphological and radiative properties of melt ponds on first‐year sea ice (FYI) were investigated during the summer of 1997 in the Canadian Archipelago as part of the Collaborative‐Interdisciplinary Cryospheric Experiment (C‐ICE) near Resolute Bay, Nunavut. In this paper we (1) describe a classification technique used to identify surface cover types from airborne videography during the melt pond season, (2) use the classification results to examine the fractional coverage of surface types and morphological characteristics of melt ponds, and (3) provide an estimate of the integrated shortwave albedo of this surface. Cluster analysis on the videography data identified four distinct surface cover types during the summer melt season: snow, saturated snow, light‐colored melt ponds, and dark‐colored melt ponds. Melt pond coverage was found to be highly variable over our study area. We found that pond sizes tended to be twice as large in areas of high melt pond density compared to ponds in areas of low pond density owing to their interconnective nature. Video data also identified an elongated melt pond morphology pattern over the smoothest FYI within our study region. A derived estimate of the integrated shortwave albedo was strongly related to the fractional cover of snow on the surface (R2 = 0.86). An analysis of combined fractional coverage of light and dark melt ponds from two aerial survey dates (Julian Days 181 and 184) revealed an aerial increase in melt ponds of ∼10.3%.
Current variability under landfast sea ice in Lützow‐Holm Bay, Antarctica
Authors: Ohshima Kay I. , Kawamura Toshiyuki , Takizawa Takatoshi , Ushio Shuki , Miyakawa Takuya
Published: February 2004

Current measurements were made under landfast sea ice in Lützow‐Holm Bay, Antarctica, in 1991. In spite of no direct wind forcing and negligible thermohaline forcing, currents with a magnitude of 0.1–0.3 m s−1 were observed. It is suggested that current variability strongly depends on the extent of the landfast ice on the eastward continental shelf. Before the fast ice edge extends to the shelf break the current at Ongul Strait, located near the continental coast, is highly coherent with the wind with a time lag of 1–1.5 days and somewhat coherent with the adjusted sea level with no time lag at periods of 6–8 days. After the fast ice edge is stably located along the shelf break the current is not at all coherent with the wind or sea level. We infer that the variability at periods of 6–8 days is mainly due to the first‐mode shelf wave excited by the wind stress over the eastward continental shelf. The direction of the prevailing current at Ongul Strait changed drastically from southward (the same direction of the wind) to northward in May, although the prevailing wind direction did not change. This change in timing coincides with the rapid development period of the fast ice extent.
Effect of landfast sea ice on coastal currents driven by the wind
Authors: Ohshima Kay I.
Published: February 2004

The present study examines the effects of landfast sea ice on the response of a homogeneous coastal ocean to wind forcings under the longwave approximation. For a fluctuating wind, continental shelf waves are induced by an infinite stress curl due to the existense of the ice edge as well as by the torque due to the topographic change. The amplitude and sign of each shelf wave mode excited by the forcing depend strongly on the ice edge location. The alongshore velocity field, which can be represented as the sum of all the shelf wave modes, has a maximum in amplitude at the ice edge with a strong lateral shear, which is a result of the infinite stress curl. The amplitudes of alongshore velocity and sea level near the coast are greatly reduced as the ice edge advances. The response to a steady wind is also examined, and it is found that even for the same wind, the velocity distributions depend strongly on the ice edge location. For instance, the current direction near the coast is the same as that of the wind stress in the case of no fast ice, whereas it is opposite in the case where the ice edge is located at the shelf break. The present theory is applied to the continental shelf off Enderby Land, Antarctica, and it gives the possible explanation for the observed features that the current variability, its coherence with the wind, and the prevailing current direction depend much on the extent of landfast sea ice.
Circulation pathways and transports of the Southern Ocean in the vicinity of the Southwest Indian Ridge
Authors: Pollard R. T. , Read J. F.
Published: February 2004

Circulation pathways and transports of the Antarctic Circumpolar Current (ACC) and Agulhas Return Current (ARC) around the complex bathymetry of the Southwest Indian Ridge and Del Caño Rise have been established from two hydrographic surveys and two‐year long moorings. After crossing the Southwest Indian Ridge in a 110 Sv (1 Sv = 106 m3s−1) flow concentrated at 48°S, the ACC fragments before being reconcentrated into several branches by the bathymetry. In particular 30–40 Sv turns north around the eastern flank of the Del Caño Rise, turns west as part of anticyclonic flow round the Del Caño Rise finally returning eastward as part of the Crozet Front. Eddy variability from both satellite data and moorings is low over the bathymetry of the Del Caño Rise, so the strong currents found between the Del Caño Rise and the Crozet Plateau are believed to be permanent features. This largely barotropic, time‐independent transport is the major pathway for the ACC to enter the Crozet Basin. The anticyclonic flow also transfers water from the Mozambique Basin across the Southwest Indian Ridge to the south side of the Del Caño Rise. Given the evidence for fragmentation of the ACC where it crosses the Southwest Indian Ridge and the concentration of several jets into the Crozet Front from the ACC, Subtropical Front and Agulhas Return Current, it is misleading to relate standard frontal definitions such as the Polar Front and Subantarctic Front to the major transport pathways.
Investigation of a self‐organization model for beach cusp formation and development
Authors: Coco G. , Huntley D. A. , O'Hare T. J.
Published: February 2004

A recent numerical investigation of “self‐organization” [Werner and Fink, 1993] suggests that the feedback process between currents and sediment response can result in “self‐organized” patterns and can be used to predict beach cusp formation and spacing. A similar model based on self‐organization is tested here in order to understand the processes occurring during beach cusp formation and development, to evaluate the sensitivity toward the parameters used, and to examine how the model might relate to field observations. Results obtained confirm the validity of the self‐organization approach and its capacity to predict beach cusp spacing, with values in fair agreement with the available field measurements, with most of the input parameters primarily affecting the rate of the process rather than the final spacing. However, changes in the random seed and runs for large numbers of swash cycles reveal a dynamical system with significant unpredictable behavior. Cusp spacing tends to change with time, and cusp regularity shows large long‐term variations. Cusps are found to be accretionary in the swash zone, and in agreement with most observations, mean flows are horn divergent over developed topography. Simulations over nonplanar slopes characterized by the presence of preexisting nonrhythmic or cuspate features have been performed. Results indicate that preexisting large‐amplitude cusps are destroyed if their spacing is substantially different from that expected under self‐organization and that the final spacing is consistent with that predicted by the model for an equivalent plane beach. These findings support the hypothesis that self‐organization is a robust mechanism for beach cusp formation.
Wintertime heat flux to the underside of East Antarctic pack ice
Authors: Lytle V. I. , Massom R. , Bindoff N. , Worby A. , Allison I.
Published: February 2004

In the sea ice zone, there is a delicate balance between the heat loss from the surface of the snow‐covered sea ice and the heat supplied to the underside of the ice by the deep ocean. The difference between these two heat fluxes determines the amount of ice growth or melt. In global atmospheric models the ocean heat flux is often prescribed and has been shown to be an important parameter in determining how thick sea ice will grow thermodynamically. Although this ocean heat flux is a critical component in climate models, there are relatively few direct measurements in the Antarctic sea ice region. In this study we use measurements of the temperature gradient through the sea ice and of ice growth during a field experiment to estimate the ocean heat flux in the East Antarctic region around 140°E, 65°S during August 1995. We find an average ocean heat flux of 13.0–14.5 W m−2. This is more than twice the winter values reported in the western Weddell Sea under multiyear sea ice but similar to the 19 W m−2 that has been estimated for the entire Weddell Gyre. During August 1995 the air temperature showed large variations, ranging from near freezing to −30°C. This resulted in the thicker ice floes (50–60 cm) alternating between a no‐growth or melting condition and an active ice growth at the base. However, it is likely that the thicker (>60 cm), ridged portions of the floes were continually melting.
A mechanism for the generation of wave‐driven rhythmic patterns in the surf zone
Authors: Falqués A. , Coco G. , Huntley D. A.
Published: February 2004

The coupling between topographic irregularities and wave‐driven mean water motion in the surf zone is examined. This coupling occurs because the topographic perturbations produce excess gradients in the wave radiation stress that cause a steady circulation. This circulation, in turn, creates a sediment transport pattern that can reinforce the bottom disturbance and may thereby lead to the growth of large‐scale bed forms. To investigate this coupling mechanism, the linearized stability problem with an originally plane sloping beach and normal wave incidence is solved in two different cases. First, the breaking line is considered to be fixed, and second, the perturbations in water depth that produce a displacement of the breaker line are accounted for. The first case shows that the basic topography can be unstable with respect to two different modes: a giant cusp pattern with shore‐attached transverse bars that extend across the whole surf zone and a crescentic pattern with alternate shoals and pools at both sides of the breaking line showing a mirroring effect. In the second case, the varying breaker line may have a strong influence on the circulation. This is clear for the giant cusp topography whose growth is totally inhibited. In contrast, the morphology and the growth of the crescentic pattern remains almost unchanged.
Internal solitary waves in a structured thermocline with implications for resuspension and the formation of thin particle‐laden layers
Authors: Wang B. J. , Bogucki D. J. , Redekopp L. G.
Published: February 2004

A structured thermocline model is employed to elucidate the role of environmental factors in the characteristics of internal solitary waves, including the related implications for wave‐induced resuspension and the formation of thin layers with enhanced concentration of passive particles. The model considers varying relative strengths between a thin seasonal thermocline and a broad permanent thermocline together with a sheared current in the upper mixed layer. The influence of these environmental factors on the stimulus of the benthic boundary layer under the footprint of solitary waves is assessed, particularly the potential for inducing boundary layer separation and lowering the near‐bottom Richardson number. Also, the flow field associated with an internal solitary wave is shown not only to transport particles horizontally, but to create a tendency for passive particles located in the mixed layer to migrate vertically and concentrate in a thin layer. This latter effect is especially pronounced for particles that are heavier than the host fluid and have particle Reynolds numbers of order unity or greater. Although specific calculations for bubbles have not been included here, the net vertical displacement for light particles following the passage of a solitary wave may be quite large.
Field observations of radar backscatter from the ocean surface under low wind speed conditions
Authors: Moller Delwyn , Mourad Pierre D. , Frasier Stephen J.
Published: February 2004

Unique simultaneous field observations of wind speed and images of vertically polarized radar backscatter from the ocean surface at low grazing angles during low wind and weakly stable stratification show interesting similarities and an important difference, depending on whether the average wind is falling or rising. When the wind speed begins to fall, there is a delay before the area‐averaged radar backscatter begins to fall. When the wind speed begins to rise, the backscatter rises immediately. This observation points to hysteresis in the relationship between radar backscatter and wind speed such that more wind is necessary to generate radar backscatter of a given intensity than is necessary to maintain it. This likely arises from differences between the generation of wind‐driven gravity capillary waves and the maintenance of gravity capillary waves and/or microbores. Also, saturation was observed in the radar backscatter at wind speeds between 2.1 and 2.8 m s−1 for both rising and falling winds. This saturation may be related to the energetics of the centimeter‐scale gravity capillary waves and/or microbores that cause the backscatter. During rising winds the backscatter may saturate because fine‐scale features of the ocean surface reach a maximum amplitude well before wind‐derived energy is transferred upscale to larger waves, whose tilting effect would ultimately produce the familiar increase in radar backscatter with increasing wind. During falling winds the backscatter may saturate because of residual momentum in the surface wave field. Finally, for both rising and falling winds, radar backscatter and wind speed are, in general, correlated above a wind speed of 1.7 m s−1, while below this value they are not. This observation of the critical wind speed necessary to generate or maintain wind‐related backscatter is consistent with results from a recent field observation and with existing laboratory‐based observations.