bluuebunny/crossref_metadata_2025 huggingface dataset:
Secondary currents in a curved, stratified, estuarine channel
Authors: Lacy Jessica R. , Monismith Stephen G.
Published: February 2004

This paper presents a study of secondary circulation in a curved stratified channel in northern San Francisco Bay over a 12.5‐hour tidal cycle. Secondary currents were strong at times (varying by up to 35 cm/s from top to bottom) but relatively transient, as the balance between centrifugal and lateral baroclinic forcing changed over time. The short travel time around the bend did not allow a steady state balance to develop between centrifugal and lateral baroclinic forcing. During the flood tide the confluence of two streams with different velocities produced a strong lateral gradient in streamwise velocity. As a result, lateral advection was a significant term in the streamwise momentum balance, having the same order of magnitude as the barotropic and baroclinic pressure gradients, and the frictional terms. During the first part of the ebb, secondary currents were induced by lateral baroclinic forcing. The direction of the secondary circulation reversed later in the ebb, as the baroclinic forcing became weaker than the centrifugal acceleration. The gradient Richardson number showed that stratification was stable over most of the tidal cycle, decreasing the importance of friction and allowing secondary currents to persist.
Linear transition ripple migration and wave orbital velocity skewness: Observations
Authors: Crawford Anna M. , Hay Alex E.
Published: February 2004

Field observations were made in 3–4 m water depth of linear transition ripple geometry and migration using a high‐resolution laser‐video bed profiling system and acoustic scanning sensors during both the growth and decay phases of an autumn storm event. Linear transition ripples are long‐crested, low‐steepness bedforms of the anorbital ripple type and were observed to occur here at relatively high wave energies just below the flatbed threshold, with wavelengths of 8.5±0.5 cm and heights of 0.3±0.1 cm. The maximum observed migration rate was 0.7 cm/min. Migration was offshore during storm growth and onshore during storm decay. The observed ripple migration velocities were highly correlated (r2 > 0.7) with nearbed wave orbital velocity skewness in both cross‐shore directions. During storm growth the incident wave spectrum was bimodal and the orbital velocity skewness was negative. During storm decay the wave spectrum was unimodal and the velocity skewness was positive. Bispectral analysis shows that the main contribution to negative velocity skewness during storm growth was due to a difference interaction between the two principal (sea and swell) components of the bimodal velocity spectrum. Positive skewness during storm decay was due to self‐self interaction of the narrowband residual swell. The negative velocity skewness observed during storm growth is consistent with prediction by a two‐frequency second‐order wave theory.
On the parameterization of mixing in three‐dimensional Baltic Sea models
Authors: Meier H. E. Markus
Published: February 2004

As mixing plays a dominant role for the physics of an estuary like the Baltic Sea (seasonal heat storage, mixing in channels, deepwater mixing), different mixing parameterizations for use in three‐dimensional (3‐D) Baltic Sea models are discussed. Within the Swedish regional climate modeling program, SWECLIM, a 3‐D coupled ice‐ocean model for the Baltic Sea has been coupled with an improved version of the two‐equation k‐ε turbulence model using a corrected dissipation term, flux boundary conditions to include the effect of a turbulence enhanced layer due to breaking surface gravity waves, and a parameterization for breaking internal waves. Results of multiyear simulations are compared with observations. The seasonal thermocline (the main focus of this paper) is simulated satisfactory. During the stagnation period between 1983 and 1993, simulated salinity in the lower layer of the Baltic Sea decreases as observed. Unsolved problems of the k‐ε model are discussed. To replace the controversial equation for dissipation, the performance of a hierarchy of k models has been tested and compared with the k‐ε model. In addition, it is shown that the results of the 1‐D turbulence submodel depend very much on the dimensionality of the hydrodynamic model. Using the same turbulence parameterization, vertical velocity shear and density gradients are simulated differently in 1‐D column models compared to 3‐D ocean circulation models. Finally, the impact of two mixing parameterizations on Baltic Sea climate is discussed.
Variability of the Denmark Strait overflow: A numerical study
Authors: Shi Xiao Bing , Røed Lars Petter , Hackett Bruce
Published: February 2004

Mesoscale variability in the Denmark Strait overflow is investigated using a version of the Miami Isopycnic Coordinate Ocean Model that includes a recently developed intermittent and vigorous, turbulent and diffusive diapycnal mixing scheme. Earlier idealized modeling work on the subject is further extended in that a realistic replica of the shelf‐slope topography and irregular coastline geometry of the Denmark Strait area is also employed. Compared with earlier studies, our experiments reveal that (1) the introduction of the new diapycnal scheme and the true shelf‐slope topography do not affect the formation of the intense cyclonic eddies south of the sill and along the continental slope and (2) the new diapycnal scheme has a significant effect on the bottom plume, in that the distribution, volume transport and properties of the bottom plume appear to be more realistic and more in line with what is observed. The first conclusion adds support to the vortex‐stretching mechanism suggested in earlier studies as the cause for the formation of the cyclonic eddies. This mechanism is therefore a robust feature and provides an explanation for the observed variability in the Denmark Strait overflow. The second conclusion underscores the importance of including realistic parameterizations of diapycnal mixing in isopycnal coordinate ocean models. Specifically, the results show that the water that descends along the bottom south of the sill becomes lighter and is more voluminous compared to experiments without the new mixing scheme. Even when including the mixing scheme, the bottom plume separates into distinct boluses of enhanced thickness. Moreover, these boluses are also associated with water of anomalously high density, as well as with the cyclonic eddies in the upper water column.
The influence of sea ice physics on simulations of climate change
Authors: Holland Marika M. , Bitz Cecilia M. , Weaver A. J.
Published: February 2004

In this study, we examine the influence of ice dynamics and the sub‐grid‐scale ice thickness distribution on present‐day and climate change simulations in a global coupled ice‐ocean‐atmosphere model of intermediate complexity. Under present‐day conditions the sea‐ice physics modifies both the annual mean and seasonal variation of ice‐ocean‐atmosphere conditions. In models with motionless sea ice, the ice volume increases and undergoes a smaller seasonal cycle. Resolving the ice thickness distribution also increases the ice thickness, but enhances the seasonal cycle. The response of the system to increased atmospheric CO2 forcing is also dependent on the sea ice physics. Simulating ice dynamics and the ice thickness distribution enhances the ice area response. However, the ice volume response is diminished when ice dynamics is included and enhanced when the ice thickness distribution is resolved. The oceanic thermohaline circulation and regional air temperature response to global warming is also dependent on the sea ice parameterizations. Additional simulations were performed to quantify the influence of the albedo feedback mechanism on climate change simulations. When the albedo feedback was inactive, amplified warming was still present (although reduced) at high latitudes due to the poleward retreat of the ice cover and larger ocean‐atmosphere heat exchange. In these simulations, the albedo feedback accounted for 17% of the global air temperature increase and over 30% of the Northern Hemisphere ice area and volume decrease.
Effects of river inputs into the Bay of Bengal
Authors: Howden Stephan D. , Murtugudde Raghu
Published: February 2004

The effect of river runoff in the Bay of Bengal is examined using a reduced gravity primitive equation ocean model coupled to an atmospheric boundary layer model. Model simulations are carried out by including river discharges as surface freshwater forcing at the mouths of the rivers. To assess the effect of river inputs on the dynamics and thermodynamics of the tropical Indian Ocean, parallel simulations are carried out by neglecting the river inputs. Additionally, another set of parallel runs without penetrative radiation loss through the mixed layer is carried out. The freshwater flux due to rivers results in lower salinities and shallower mixed layers, as expected. However, the influence of this additional freshwater flux into the bay is rather counterintuitive. With the inclusion of river discharges more heat is absorbed by the ocean, but sea surface temperatures are slightly cooler in the bay because of enhanced entrainment cooling of the shallower mixed layer, enhanced penetrative radiation, and an enhanced effect of latent heat loss on the temperature tendency. This is despite the greater latent heat loss when river input is neglected. Conversely, neglect of penetrative radiation results in a shallower but slightly warmer mixed layer with river input. River input and penetrative radiation each affect the mixed layer depths, the salinity and temperature structure, and currents in the Bay of Bengal, but they have a small effect on SST. Annual SST, averaged over the Bay of Bengal, is only 0.1°C colder with river input. Neglecting penetrative radiation in the river run results in an increase of only 0.2°C for the annual SST. The lack of persistence of a barrier layer in the bay helps regulate SST even in the presence of enhanced buoyancy forcing due to river input. Averaged over the bay, a barrier layer forms as mixed layer detrainment occurs, and the thermocline deepens just after the southwest monsoon and the northeast monsoon. The barrier layer is short‐lived in each case it is eroded by mixing. The effect of riverine input in the bay is not confined to the surface waters. A pool of cold anomaly (−1°C) and fresher waters is centered near 100 m depth in the bay with riverine input. This cold pool beneath the mixed layer allows entrainment cooling of the mixed layer to be more effective, even though mass entrainment is lower relative to the case neglecting river input. The more diffuse thermocline in the bay is consistent with enhanced vertical mixing despite the large positive buoyancy forcing.
Deep ocean influence on upper ocean baroclinic instability
Authors: Olascoaga M. J.
Published: February 2004

Reduced gravity models, namely, those having an active layer of fluid floating on top of a motionless one, have been largely used to study the upper ocean. This approximation is formally valid when the total ocean depth tends to infinity. The effect of a finite ocean depth on the upper ocean baroclinic instability is examined here using a quasi‐geostrophic three‐layer model and comparing the results with those obtained using a reduced gravity quasi‐geostrophic two‐layer model or 2.5‐layer model. The basic state is a zonal current with uniform velocities within each layer. The ratio ε between the sum of the upper two layers mean depths and the lower layer mean depth is the relevant new parameter of the problem. Even for very small values of ε, important differences between the 2.5‐layer (ε = 0) and the three‐layer (ε >0) model are found. As ε increases, the region of Arnold stable states decreases. For certain basic states, new normal mode instability branches are found, whose growth rates increase with ε. An asymptotic expansion in ε is made in order to shed some light on the transition regime between both models. This allows one to interpret the new instabilities as a consequence of the resonant interplay between the stable modes in the 2.5‐layer model and a short Rossby wave in the deep layer. The growth rates of the new instability branches are O(ε1/3) and O(ε1/2) and cannot be neglected even for reasonably small values of ε.
Correlations between surface chlorophyll and sea surface height in the tropical Pacific during the 1997–1999 El Niño‐Southern Oscillation event
Authors: Wilson Cara , Adamec David
Published: February 2004

Correlations between Sea‐viewing Wide Field‐of‐view Sensor (SeaWiFS) surface chlorophyll and TOPEX sea surface height (SSH) are examined in the tropical Pacific (30°S‐30°N) using empirical orthogonal function (EOF) analysis both separately and jointly on the two fields. This analysis is done on data from September 1997, the start of data from the SeaWiFS satellite, through December 1999, a time period dominated by the El Niño‐Southern Oscillation. Four distinct biological responses are observed. The dominant response is a symmetric off‐equatorial chlorophyll increase during La Niña that extends between 2° and 18° latitude from the eastern Pacific to the date line. The chlorophyll mode is tightly correlated to SSH, suggesting that the chlorophyll increase is a result of the shoaling thermocline, which increases the surface nutrient supply. The better known equatorial decrease in chlorophyll during El Niño is seen in a separate EOF mode. Using acoustic Doppler current profiler data from the Tropical Atmosphere‐Ocean/Triton array, it is shown that the cessation of the El Niño equatorial chlorophyll minimum is tied to the recommencement of the iron‐rich Equatorial Undercurrent which occurs several months prior to the termination of the El Niño. There is an off‐equatorial bloom during the peak of the El Niño between 120°W‐180°W and 8°N‐15°N. This bloom occurs within the area covered by the previously mentioned La Niña bloom, but it is more localized, and its fluctuations appear correlated with changes in the North Equatorial Counter Current. The shoaling thermocline in the western warm pool during El Niño results in a chlorophyll bloom that extends from the Philippines to 155°E between 0°N and 15°N. This bloom terminates in unison with the end of the El Niño when elevated SSH is reestablished in the western basin.
Lee region of Gran Canaria
Authors: Barton Eric D. , Basterretxea Gotzon , Flament Pierre , Mitchelson‐Jacob E. Gay , Jones Bethan , Arístegui Javier , Herrera Felix
Published: February 2004

The mountainous Canary Islands present obstacles to the trade winds and to the Canary Current flowing equatorward past them. In situ observations of hydrographic properties and surface winds south of Gran Canaria, together with advanced very high resolution radiometer and synthetic aperture radar images during 2 weeks in summer 1995 are analyzed. A cyclonic eddy shed from the west of the island drifted southwestward at 5 cm s−1, while the southeast coast was approached by an upwelling filament originating off NW Africa. A wind lee region bounded by intense horizontal shear lines had a weak return islandward wind in its center. The lee formed a triangular, diurnally varying, warm water pool with two sea surface temperature maxima separated by lower temperatures below the return wind. Shallow temperature stratification occurred behind the island in contrast to the uniform surface mixed layer in exposed regions. Upwelling and downwelling of 10–20 m d−1 were indicated on the cyclonic and anticyclonic sides of the lee region. In the SAR images, lines of strong current shear along a temperature front between the cyclonic eddy and the upwelling filament were identifiable. However, the radar images were dominated by atmospheric phenomena, including mountain lee wave packets, windrows, and wind shear lines. Estimation of the wind field from the SAR backscatter intensity revealed complex structure and intensification on the edges of the warm lee.
Eddy‐induced mixed layer shallowing and mixed layer/thermocline exchange
Authors: Nurser A. J. G. , Zhang J. W.
Published: February 2004

The effects of eddies on the mixed layer and on the transfer of properties between the mixed layer and thermocline are studied in a high‐resolution model of an unstable front. A spring scenario is considered, with a mixed layer maintained against surface warming by wind mixing. Tilting of the buoyancy gradients by ageostrophic flow is found to shallow the mixed layer over the front, giving a mixed layer as shallow as 10 m in some places, compared with the equilibrium Monin‐Obukhov depth of ∼30 m away from the front. Variations of mixed layer depth with time and lateral advection lead to a subduction field that is substantially different from the vertical velocity field. Vertical velocities associated with the growing instabilities push tongues of a ventilation tracer, initially only present within the surface layer, down to depths of up to 100 m. After 24 days a quarter of this ventilation tracer has been deposited in the thermocline. Conversely, the vertical motions pull nutrients from the thermocline up into the mixed layer, giving enhanced surface values along the front. In the basin average the geostrophic eddies give vertical eddy diffusivities for a ventilation tracer of O(3 ‐ 10×10−4m2s−1).
Distribution and formation of the mesothermal structure (temperature inversions) in the North Pacific subarctic region
Authors: Ueno Hiromichi , Yasuda Ichiro
Published: February 2004

The distribution and formation of mesothermal structure (temperature inversions) in the North Pacific subarctic region are investigated through analysis of climatological hydrographic data. It is suggested that the heat and salt that maintain the mesothermal water and thus the halocline in the density range of 26.7–27.2σθ are transported as a crossgyre flow from the transition domain just east of Japan, where the waters are influenced by the subtropical gyre water mass, to the eastern subarctic region. Along the transport route the isopycnal potential temperature and thus salinity are well conserved. In the western subarctic gyre, the Bering Sea, and the northern Gulf of Alaska, the temperature reaches its minimum at the surface in winter and the areal coverage agrees well with the distribution of the mesothermal structure. In the southeastern part of the zonally distributed mesothermal structure in the area of 170°E–150°W and 45°–50°N, where the winter sea surface temperature is higher than that in the deeper layer, dichothermal water is formed by subsurface intrusion of the low‐temperature and low‐salinity water that outcropped in the previous winter over the warm and saline water transported from the transition domain.
Sea ice circulation in the Laptev Sea and ice export to the Arctic Ocean: Results from satellite remote sensing and numerical modeling
Authors: Alexandrov Vitaly Y. , Martin Thomas , Kolatschek Josef , Eicken Hajo , Kreyscher Martin , Makshtas Alexandr P.
Published: February 2004

Sea ice circulation in the Laptev Sea and ice exchange with the Arctic Ocean have been studied based on remote sensing data and numerical modeling. Ice drift patterns for short‐ and long‐term periods were retrieved from successive Okean radar images and Special Sensor Microwave/Imager data for the winters 1987/1988 and 1994/1995. Seasonal and interannual variabilities of ice drift in the Laptev Sea and ice exchange with the Arctic Ocean during the period from 1979 to 1995 were studied with a large‐scale dynamic‐thermodynamic sea ice model. During an “average year,” sea ice was exported from the Laptev Sea through its northern and eastern boundaries, with maximum and minimum export occurring in February and August, respectively. The winter ice outflow from the Laptev Sea varied between 251,000 km2 (1984/1985) and 732,000 km2 (1988/1989) with the mean value of 483,000 km2. Sea ice was exported into the East Siberian Sea mostly in summers with the mean value of 69,000 km2. Out of the 17 investigated summers, 12 were characterized by sea ice import from the Arctic Ocean into the Laptev Sea through its northern boundary. Magnitude and direction of ice export from the Laptev Sea corresponded with the large‐scale Arctic Ocean drift patterns during periods of prevailing cyclonic or anticyclonic circulation. Based on a semiempirical method that has been validated with the large‐scale model and satellite data, ice exchange between the Laptev Sea and the Arctic Ocean during the period from 1936 to 1995 has been estimated as 309,000km2 with strong interannual variability and no significant trend apparent.
Loop Current rings and the deep circulation in the Gulf of Mexico
Authors: Welsh Susan Elizabeth , Inoue Masamichi
Published: February 2004

A detailed examination of the three‐dimensional circulation associated with Loop Current rings in the Gulf of Mexico has been made using the Modular Ocean Model with 15 levels in the vertical and ⅛° horizontal grid spacing. Monthly mean wind stress and a seasonal boundary condition on temperature and salinity fields are applied at the surface. The vertical shear and geostrophic transport of the Yucatan Current also vary seasonally. The upper layer seasonal circulation, and the formation, structure, and westward migration of Loop Current rings in the model compare favorably with observations. An anticyclone‐cyclone pair develops in the deep layer beneath the Loop Current as a ring is forming in the eastern Gulf. The model circulation beneath the Loop Current reverses from anticyclonic to cyclonic because the anticyclone leads the pair in the deep layer as the ring separates and migrates westward in the surface layer. The deep anticyclone‐cyclone pairs are guided by the bottom bathymetry as they migrate westward in tandem with the rings in the surface layer. The deep anticyclone decays more quickly than the cyclone in the western Gulf, and the cyclone dominates the deep circulation beneath the ring. Cyclonic flow in the deep Gulf of Mexico has been observed using current meters, inferred from hydrographic data, and produced in other numerical and theoretical models. Although the formation and westward migration of deep anticyclone‐cyclone pairs in conjunction with rings has not been directly observed, the results of this modeling study agree with other published reports on the deep circulation in the Gulf of Mexico.
Thermohaline variability in the upper ocean
Authors: Ferrari Raffaele , Rudnick Daniel L.
Published: February 2004

The main goal of this exploratory study is to determine how the temperature‐salinity relationship changes with horizontal length scale and depth in the ocean. Temperature and salinity were measured on a range of scales from 4 m to 1000 km, towing a SeaSoar along isobars and isopycnals in the subtropical gyre of the North Pacific, during the winter of 1997. The wavelet transform technique is used to compute the horizontal density ratio and thermohaline variability as a function of scale and location. Measurements along an isobar in the mixed layer show that the horizontal density ratio is 1 at all scales observed; that is, horizontal temperature and salinity gradients tend to cancel each other in their effect on density. Thermohaline variability at small scales is intermittent and clusters around large‐scale thermohaline anomalies. Below the base of the mixed layer, horizontal gradients of temperature are only partially opposed by salinity, and the density ratio is close to 2. In the thermocline the distribution of thermohaline variability is uniform along isobars but intermittent and colocated at different scales along isopycnals. Density‐compensated variability, ubiquitous in the mixed layer, is reduced along deeper isopycnals. Compensation of horizontal temperature and salinity gradients supports recent theoretical ideas that mixing in the winter mixed layer depends on horizontal density gradients.
Energetics of the surface circulation of the Japan/East Sea
Authors: Lee Dong‐Kyu , Niiler Pearn P. , Lee Sang‐Ryong , Kim Kuh , Lie Heung‐Jae
Published: February 2004

During the period of August 1987 to June 1998, 96 Argos‐tracked drifters traversed through the Japan/East Sea. The displacement data from these drifters were used to compute 1–3 day average current vectors, which described the circulation at 15 m depth. The drifters that passed through the Korea Strait via the western channel were caught by the East Korean Warm Current, and the drifters that passed through the eastern channel were caught by the Tsushima Current. Individual observations of the 3 day average current over 40 cm s−1 were made in the warm waters south of the subpolar front and along the coasts. There was a distinct decrease of eddy energy northward across the subpolar front. Conversion rates of eddy kinetic energy due to eddy mean flow interaction showed significant regions of both dissipation and production.
Melt ponds on sea ice in the Canadian Archipelago: 2. On the use of RADARSAT‐1 synthetic aperture radar for geophysical inversion
Authors: Yackel J. J. , Barber D. G.
Published: February 2004

Microwave scattering from a first‐year sea ice (FYI) melt ponded surface is examined using RADARSAT‐1 synthetic aperture radar (SAR) data collected during the 1997 Collaborative‐Interdisciplinary Cryospheric Experiment (C‐ICE'97) near Resolute Bay, Nunavut. This paper (1) investigates the utility of time series of microwave scattering to detect melt pond formation and (2) investigates approaches toward geophysically inverting information on the physical and radiative properties of this surface. We found melt pond formation to coincide with a sharp rise in the temporal evolution of the microwave scattering coefficient (σ°) over FYI. RADARSAT‐1 incidence angle and surface wind speed explained >90% of the variation in σ°. RADARSAT‐1 σ° was sensitive (R2 = 0.80) to the fractional coverage of melt ponds during windy conditions (∼ 5.3 m s−1). Spatial and temporal coincident measurements of RADARSAT‐1 σ° and the integrated shortwave albedo revealed a strong negative statistical correlation (R2 = 0.91) during windy conditions (∼ 5.3 m s−1). A weaker, but strong, negative relationship (R2 = 0.78) was observed for less windy conditions (∼ 3.2 m s−1), and a very weak positive relationship (R2 = 0.19) was found for low wind speed conditions (∼ 1.5 m s−1). All relationships were observed for melt pond fractions between 13 and 34%.
Spectral particulate attenuation and particle size distribution in the bottom boundary layer of a continental shelf
Authors: Boss Emmanuel , Pegau W. Scott , Gardner Wilford D. , Zaneveld J. Ronald V. , Barnard Andrew H. , Twardowski Michael S. , Chang G. C. , Dickey T. D.
Published: February 2004

Spectral attenuation and absorption coefficients of particulate matter and colocated hydrographic measurements were obtained in the Mid‐Atlantic Bight during the fall of 1996 and the spring of 1997 as part of the Coastal Mixing and Optics experiment. Within the bottom boundary layer (BBL) the magnitude of the beam attenuation decreased and its spectral shape became steeper with distance from the bottom. Concurrently, the slope of the particulate size distribution (PSD) was found to increase with distance from the bottom. Changes in the PSD shape and the magnitude of the beam attenuation as functions of distance from the bottom in the BBL are consistent with particle resuspension and settling in the BBL, two processes that are dependent on particle size and density. For particles of similar density, resuspension and settling would result in a flattening of the PSD and an increase in the beam attenuation toward the bottom. In both fall and spring the magnitude of the particle attenuation coefficient correlates with its spectral shape, with a flatter shape associated with higher values of the attenuation. This observation is consistent with idealized optical theory for polydispersed nonabsorbing spheres. According to this theory, changes in the steepness of the particle size distribution (particle concentration as a function of size) will be associated with changes in the steepness of the attenuation spectra as a function of wavelength; a flatter particle size distribution will be associated with a flatter attenuation spectrum. In addition, the observed ranges of the beam attenuation spectral slope and the PSD exponent are found to be consistent with this theory.
On the seasonal mixed layer simulated by a basin‐scale ocean model and the Mellor‐Yamada turbulence scheme
Authors: Ezer Tal
Published: February 2004

Seasonal changes and vertical mixing processes in the upper layers of the North Atlantic Ocean are simulated with a basin‐scale sigma coordinate ocean model that uses the Mellor‐Yamada turbulence closure scheme. The cause of insufficient surface mixing and a too shallow summertime thermocline, common problems of ocean models of this type, is investigated in detail by performing a series of sensitivity experiments with different surface forcing conditions and different turbulence parameterizations. A recent improvement in the parameterization of the dissipation term in the Mellor‐Yamada turbulence scheme, which has shown a significant improvement in one‐dimensional calculations, had a positive but relatively small influence on the three‐dimensional calculations. The results quantify the improvement in the model upper ocean thermal structure as surface forcing becomes more realistic from one experiment to another, for example, when monthly mean winds are replaced by 6 hour variable winds. The inclusion of shortwave radiation penetration is especially important to prevent overly shallow model mixed layers during the summer and seems to affect not only the surface layer but also the thermal structure of the upper 200 m of the ocean. The difficulty of evaluating turbulent mixing processes in three‐dimensional models due to errors in surface fluxes, spatial changes, and three‐dimensional effects, as shown here, points to the important role still left for one‐dimensional turbulence models in improving parameterizations used in three‐dimensional realistic models.
ADEOS observations of chlorophyll a concentration, sea surface temperature, and wind stress change in the equatorial Pacific during the 1997 El Niño onset
Authors: Murakami Hiroshi , Ishizaka Joji , Kawamura Hiroshi
Published: February 2004

The atmosphere‐ocean system in the equatorial Pacific transited to El Niño conditions from the spring to summer in 1997. The ADEOS comprehensively captured the onset of 1997–1998 El Niño through the combined observations of surface wind stress vector (WS), sea surface temperature (SST), and concentration of the photosynthetic pigment chlorophyll a (CHL). Responses of SST and CHL to WS in the El Niño onset showed regional characteristics in the four areas of the equatorial Pacific. In the western equatorial Pacific, CHL increased and SST decreased intermittently because of vertical mixing and coastal upwelling around the islands caused by local WS bursts. In the central parts, CHL decreased by 40% and SST increased by 2°C corresponding to the zonal movement of warm pool water, which seems to be controlled by the WS field during the El Niño onset. In the eastern parts, WS decreased by about half and SST increased by 4°C; however, CHL decreased only a little because of regulated processes of nutrients. In the eastern coastal area, CHL decreased and SST increased according to deepening of the thermocline. The thermocline depth seemed to be modulated by a dynamic balance with the basin‐scale WS field and arrivals of Kelvin waves, which were caused by westerly WS bursts in the western equatorial Pacific and traveled in the thermocline to the eastern coastal area in the El Niño onset.
Yellow and East China Seas response to winds and currents
Authors: Jacobs G. A. , Hur H. B. , Riedlinger S. K.
Published: February 2004

The influences of the Kuroshio Current (KUC), Taiwan Warm Current (TWC), and surface wind stress on the Yellow and East China Seas (YES) are examined using tracers in a Princeton Ocean Model. Two experiments are performed: one with wind stress and one without. Seasonal variations in the inflow and outflow of the TWC and Tsushima Current are specified to examine the effects of the current transports. Two separate tracers are inserted into each model experiment to track the pattern of KUC and TWC waters into the YES. The two main areas of KUC and TWC water movement into the Yellow Sea are the southern entrance to the Yellow Sea trough and the Yangtze Relict River valley. Results indicate that KUC and TWC waters advect into the Yellow Sea regardless of wind stress. However, in the Yellow Sea during winter the wind stress increases KUC and TWC concentration at 20 m. The wind stress also produces short time period events that horizontally advect water masses and spatially smooth seasonally averaged water mass concentrations. The bottom Ekman layer appears to be one of the mechanisms driving the northward bottom flow across the East China Sea shelf. The bottom friction layer is stronger in summer when the TWC velocity is high. The bottom friction layer draws KUC water across the bottom of the continental shelf into the Yangtze Relict River valley and generates upwelling along the Chinese coast.
Vertical structure and transport on a transect across the North Atlantic Current near 42°N: Time series and mean
Authors: Meinen Christopher S. , Watts D. Randolph
Published: February 2004

A new method is presented for combining historical hydrography with the acoustic travel time measurements from an inverted echo sounder (IES) to obtain time series of full water column profiles of temperature T and specific volume anomaly. Hydrography is used to demonstrate that profiles constructed from the IES measurement account for more than 90% of the T and δ variance through the main thermocline range. Horizontal gradients between neighboring pairs of instruments, when combined with measurements from bottom pressure sensors and deep current meters, provide accurate estimates of absolute geostrophic velocity profiles. A section of IESs and current meters across the North Atlantic Current (NAC) along World Ocean Circulation Experiment line ACM6 near 42.5°N is analyzed using these methods, and the Eulerian mean temperature and absolute velocity sections are calculated. The mean transport for the combined NAC and northward flow within the inshore portion of the Mann Eddy is 146±13 (× 106 m3 s−1). The temporal standard deviation of the northward transport is 41×106 m3 s−1, arising not only from changes in the transport of the NAC but also from lateral shifts of the currents and eddies captured by the fixed span of moorings. Assumption of a level of no motion at the bottom and at 2000 dbar underestimates the true absolute transport by 33 or 60%, respectively. Historical measurements indicate that 50–60×106 m3 s−1 circulates within the Mann Eddy adjacent to the North Atlantic Current, thus 86–96×106 m3 s−1 appears to be throughput at the ACM6 line. Because the throughput observed on the ACM6 line considerably exceeds the combined historical estimates of northward and eastward outflow from the Newfoundland Basin of ≈ 30×106 m3 s−1, these measurements suggest a substantial (≈ 50 × 106 m3 s−1) recirculation or southward flow within the Newfoundland Basin in addition to the historical recirculation within the Mann Eddy.
Physical oceanographic conditions to the northwest of the sub‐Antarctic Island of South Georgia
Authors: Brandon Mark A. , Murphy E. J. , Trathan P. N. , Bone D. G.
Published: February 2004

Ocean physics is a key aspect of the operation of island ecosystems, yet the hydrodynamics is sometimes poorly understood. A high‐resolution oceanographic survey to the north of South Georgia identified two water masses. These are South Georgia Shelf Water (SGSW) and Antarctic Zone Water (AZW). At the surface, SGSW is both colder and fresher than AZW; at the near‐surface temperature minimum these differences are reversed. There are two causes: SGSW is close to the island and has an additional contribution to the heat and salt balance from increased precipitation and island runoff, and second, AZW has advected from higher latitudes. Both of these mechanisms imply SGSW is retained around the island. Between these water masses is a variable width frontal region that has a mixture of both water masses. In general terms, the mean current of the AZW is to the west. In the SGSW the general flow also appears to be to the west, but at reduced magnitude; there is evidence of a stronger westerly current close to the shore. In both water masses there are easterly surface currents most likely driven by the prevailing wind. An approximation for the limit of SGSW is that it is constrained to water depths <500 m. A significant deviation is caused by an anticyclonic flow around a prominent bank that can be associated with significant upwelling. Finally, we comment on the ecological significance of the system observed.
Modeling stratified wave and current bottom boundary layers on the continental shelf
Authors: Styles Richard , Glenn Scott M.
Published: February 2004

The Glenn and Grant [1987] continental shelf bottom boundary layer model for the flow and suspended sediment concentration profiles in the constant stress layer above a noncohesive movable sediment bed has been updated. The Reynolds fluxes for sediment mass and fluid momentum are closed using a continuous, time‐invariant linear eddy viscosity modified by a continuous stability parameter to represent the influence of suspended sediment‐induced stratification throughout the constant stress region. Glenn and Grant [1987] use a less realistic discontinuous eddy viscosity and neglect the stratification correction in the wave boundary layer. For typical model parameters the two models produce currents above the wave boundary layer that are in better agreement than the suspended sediment concentrations. Within the wave boundary layer the differences are much greater for both the current and the sediment concentration. This leads to significant differences in the sediment transport throughout the constant stress layer. Sensitivities of the updated model were examined on the basis of observed wave and current data acquired during storms on the inner continental shelf. Comparisons between the stratified and neutral versions of the updated model indicate that the stratified version produces a total depth‐integrated sediment transport that can be 2 orders of magnitude less than, and time‐averaged shear velocities that can be nearly half of, that predicted by the neutral version. Sensitivities to grain size distributions indicate that even a small amount of finer sediment can stratify the storm‐driven flows. Sensitivities to closure constants within the range of reported values also produce up to an order of magnitude variation in sediment transport, illustrating the need for dedicated field experiments to refine further estimates of these parameters.
Data assimilation by an intermediate coupled ocean‐atmosphere model: Application to the 1997–1998 El Niño
Authors: Lee Tong , Boulanger Jean‐Philippe , Foo Alex , Fu Lee‐Lueng , Giering Ralf
Published: February 2004

Sea surface temperature, sea level, and pseudo wind stress anomaly data from late 1996 to early 1998 are assimilated into an intermediate coupled model of the Tropical Pacific. Model data consistency is examined. Impact of the assimilation on forecast is evaluated. The ocean component of the coupled model consists of a shallow water model with two baroclinic modes, an Ekman shear layer, and a mixed layer temperature equation. The atmospheric model is a statistical one (based on dominant covariance of historical surface temperature and pseudo wind stress anomaly data). The adjoint method is used to fit the coupled model to 6 months of data by optimally adjusting the initial state and model parameters. A forecast is performed using the end state of an assimilation experiment as initial conditions and using parameters estimated during the assimilation period. Thus the model state during the assimilation and that during the forecast belong to the same model trajectory in different periods. Such an initialization procedure is useful in avoiding initial shock during forecast due to inconsistency of an initial state with the coupled model physics. As a result of optimal adjustments of initial state and parameters, the model is able to reproduce observed interannual variability of sea surface temperature and sea level reasonably well. The averaged residual model data misfits over various 6 month periods are 0.5°C and 5 cm, respectively. The model has a limited skill in reproducing much of the off‐equatorial wind anomalies. The residual model data misfit in pseudo wind stress anomaly is larger than 10 m2 s−2. Forecasts initialized from the assimilation product are overall more realistic than those simply initialized from wind‐forced ocean states. Consistent improvement due to optimal initialization is found for sea surface temperature and sea level anomalies in the central‐eastern Pacific and zonal pseudo wind stress anomaly in the central Pacific, both in terms of root‐mean‐squared deviation from and correlation with the data. The adjustments of parameters in addition to initial state in a coupled context is found to be important to improving the model data consistency during the assimilation and the forecast. In particular, the estimated drag and damping coefficients properly regulate the relative strength of forcing and damping of the ocean state so as to fit the three types of observations during the assimilation (initialization) period, which facilitates the development of a large‐amplitude warming event during the forecast. The study demonstrates the utility of oceanic and atmospheric data to estimate initial state and model parameters in a coupled context, which is useful to the evaluation, improvement, and initialization of El Niño‐Southern Oscillation forecast models.
Chlorofluorocarbon‐derived formation rates of the deep and bottom waters of the Weddell Sea
Authors: Meredith Michael P. , Watson Andrew J. , Van Scoy Kim A. , Haine Thomas W. N.
Published: February 2004

The spreading and mixing of deep and bottom waters from the Weddell Sea (originating with potential temperature <0°C) are investigated through analysis of new and historical CFC measurements. We investigate the component that enters the southern Indian Ocean with a simple one‐dimensional advection‐diffusion model of the deep boundary current. This model does not require a condition of mixing solely with tracer‐free water to be imposed. As boundary conditions, we derive time histories for the CFC concentrations of newly formed Weddell Sea deep and bottom waters. A range of best fit velocities is obtained by determining which model velocities produce model tracer concentrations closest to measured concentrations. From this, we derive a formation rate of 3.2±1.5 Sv for the waters originating with temperatures lower than 0°C that subsequently enter the southern Indian Ocean. This compares to an upper limit for these waters of 5.7 Sv derived from the CFC‐11/CFC‐12 ratio. Whilst the former value is dependent on the highly idealized concepts of the model used, the latter is a more robust upper limit. Using the CFC‐11/CFC‐12 ratio measured at Vema Channel during the World Ocean Circulation Experiment cruise A23, an upper limit of 0.9 Sv is derived for the formation rate of waters colder than 0°C that enter the South Atlantic. Overall, an upper limit of 6.6 Sv is derived for the rate of formation of the deep and bottom waters of the Weddell Sea. A better representation is obtained by combining the upper limit to the South Atlantic component with the model‐derived southern Indian Ocean estimate; we thus estimate the formation rate of the deep and bottom waters in the Weddell Sea to be 3.7±1.6 Sv. Whilst the range obtained is large and the assumptions implicit in the calculations are potentially significant, we believe this more realistically represents the true formation rate than estimates derived using an imposed condition of tracer‐free mixing. The range can be narrowed in future as more data become available.
Modal response of a deep stratified lake: western Lake Constance
Authors: Boehrer Bertram
Published: February 2004

The orthogonality relation of the internal wave equation has been derived and has been included in normalizing the particular modes. Under the assumption that observed currents in western Lake Constance all result from basin‐scale internal waves, they can be decomposed into contributions of the particular modes. In all cases of a response to a wind stress, at first the velocity profiles indicate a quite pure first‐mode wave. After strong winds, a second‐mode wave contributes remarkably and consistently to the velocity profile later during the response. During the zero transition of the first mode, the observed velocity profile can be attributed mainly to a second‐mode wave.
Dynamics of the North Brazil Current retroflection region from the Western Tropical Atlantic Experiment observations
Authors: da Silveira Ilson C. A. , Brown Wendell S. , Flierl Glenn R.
Published: February 2004

Hydrographic and velocity observations of the North Brazil Current (NBC) retroflection region during the 1990–1991 Western Tropical Atlantic Experiment (WESTRAX) are examined with the intent of extracting dynamical information about the NBC eddy shedding. A comparison is performed between the depth structure of empirical orthogonal functions and the dynamical normal modes of the NBC retroflection region on a beta plane centered on 5°N. The barotropic and first two baroclinic modes account for about 75% of the vertical structure of the NBC flow. Thus, a three‐layer model of the NBC region is suitable for dynamical studies of the NBC retroflection. In terms of flow structure the upper layer represents the retroflection of the surface layers of the NBC to feed the North Equatorial Countercurrent. The middle layer represents the separating subthermocline waters of the NBC which feed the North Equatorial Undercurrent. The third layer represents a weak meandering flow that may be thought of as the Deep Western Boundary Current signature in the three‐layer ocean. In terms of PV, a well‐defined front separates the NBC waters from the North Equatorial Current in the upper layer. Both upper and middle layers present closed PV contours associated with the eddy in the process of pinching off from the retroflecting NBC. The NBC separation region, although equatorial, complies reasonably well with the basic quasigeostrophic (QG) assumptions. Therefore QG methods are applied to investigate the NBC meander growth. By isolating the effect of PV anomalies in each of the layers on each of the layers, baroclinic growth is verified to occur during the NBC eddy shedding.
Topographic control of rotating deep water flow through the combination of a sill and a horizontal constriction
Authors: Laanearu Janek , Lundberg Peter
Published: February 2004

Many subsurface passages connecting the deep parts of the oceans have a topography such that the location of the most pronounced horizontal constriction does not coincide with that of the sill. When applying rotating hydraulic theory to describe the deep water flow through channels of this type, a problematic situation emerges since the standard analysis assumes a well‐defined controlling section. The present study demonstrates how the associated critical flow problem arising from the hydraulic model may be resolved on the basis of functional‐theoretical analysis. The methodology is applied to the deep water flow into the Baltic proper through the Bornholm Channel, where multiple critical cross sections were found. These, as well as the associated critical transport predictions, are discussed, and it is found that only one of them serves de facto as a control with unidirectional flow.