bluuebunny/crossref_metadata_2025 huggingface dataset:
Ocean channel modes
Authors: Ripa P. , Zavala‐Garay J.
Published: December 2003

There are two types of normal modes in a uniform channel: (1) Those defined at a constant along‐channel wavenumber k, for which the eigenvalues are the frequencies ωa (k) and the eigenmodes are Kelvin, Poincaré, and Rossby waves, and (2) Those defined at a fixed ω, for which the eigenvalues are ka (ω) and the eigenmodes are Kelvin and Poincaré/Rossby waves. The first set is useful in the initial value problem, whereas the second one is applied to the study of forced solutions and the wave‐scattering phenomena. Orthogonality conditions are derived for both types of normal modes and shown to be related to the existence of adjoint systems whose normal modes coincide with those of the direct system. The corresponding inner products are similar, but not exactly equal, to the energy density and flux, respectively. They do not define a metric, and, consequently, there is not a property of the wave field equal to the sum of the real nonnegative contributions of each mode (except in the first case and without friction). A numerical approximation that has the required orthogonality properties is presented and used to give examples of forced solutions, such as Taylor's problem with wind forcing and topography.
Influence of stratification on residual tidal currents in the Yellow Sea
Authors: Lee Sang‐Ho , Beardsley Robert C.
Published: December 2003

The M2 tide and residual current generation in the Yellow Sea are examined using the Blumberg and Mellor [1987] numerical coastal ocean circulation model. With no stratification the model tide and tidal currents show good agreement with the observed tide and tidal currents. Coherent residual currents are predicted around the southwest of Korea, flowing northward as a strong jet with a maximum speed of 8 cm s−1, and around Yangtze Bank, flowing southeastward as a weak broad current. With initial stratification, tidal mixing occurs in the bottom boundary layer, which leads to surface tidal fronts around Yangtze Bank and the southwest of Korea, and our model experiments reproduce successfully the basic water property structure in these regions. The resulting stratified tidal rectification intensifies the residual currents at the front and at the top of the bottom boundary layer over the sloping bottom, so that the vertical structure of the residual currents is modified. The strong residual currents in the top of the bottom mixed layer around Yangtze Bank support the interpretation of Kim et al. [1991] for the southeastward movement of Yellow Sea Bottom Cold Water in summer. The residual currents in the surface layer reach ∼40% of the mean currents observed by Beardsley et al. [1992] using satellite‐tracked drifters deployed in the Yellow Sea in summer. These results suggest that stratified tidal rectification is an important process in driving the summertime subtidal flow in the Yellow Sea.
Data assimilation model study of the Santa Barbara Channel circulation
Authors: Chen Chi‐Shao , Wang Dong‐Ping
Published: December 2003

The seasonal circulation in the Santa Barbara Channel (SBC) consists of a cyclonic eddy in the western basin and a poleward current along the northern coast. In this study the monthly mean circulation is simulated using a three‐dimensional primitive‐equation model with data assimilation included. The data are derived from the Santa Barbara Channel and Santa Maria Basin Coastal Circulation Study, covering an 8‐month period from January to August 1994. The 6‐hourly moored temperature observations in the upper 100 m water column are continuously assimilated into the model using a local correction method, although salinity and wind forcing are not explicitly included. Comparisons with the observations indicate that the model results generate realistic flow and temperature structures inside the SBC. The assimilation model faithfully reproduces the cyclonic eddy, but it fails to predict the currents at the exits of the channel. The issue of the data need is addressed through a sensitivity study.
Dissolved inorganic carbon pool dynamics in northern Gerlache Strait, Antarctica
Authors: Carrillo Christopher J. , Karl David M.
Published: December 2003

The Research on Antarctic Coastal Ecosystem Rates (RACER) program was designed to study interactions between biological and physical processes in regions west of the Antarctic Peninsula. The field sampling effort was conducted from 1986 to 1992 and consisted of four cruises; three during separate austral spring‐summer seasons (RACER I, II and III) and one austral winter cruise (RACER IV). Extensive phytoplankton blooms were documented at several stations in northern Gerlache Strait during the three austral spring‐summer seasons. The growth and accumulation of photoautotrophic microorganisms resulted in the net removal of >100 μmol kg−1 of salinity normalized dissolved inorganic carbon (DIC). An upper estimate for net ecosystem production of 0.15 mol C m−2 d−1 was calculated from measured rates of DIC removal at one station (designated station A) that was occupied extensively during all four RACER cruises. Independent measurements of total gross microbial production, based on DNA synthesis rates, ranged from 0.3 to 1.0 mol C m−2 d−1 at the height of the bloom. The net accumulation of phytoplankton carbon and removal of DIC from the system resulted in an estimated mean air‐to‐sea flux of 6 mmol C m−2 d−1, a negligible fraction of the net removal of carbon by organic matter production. A characteristic feature of each austral spring‐summer cruise was the high degree of spatial and temporal heterogeneity in the inorganic carbon system parameters. The exact cause(s) of this variability is not known. As a result of spatial heterogeneity, carbon fluxes were not well constrained, and regional carbon budget closure was problematic.
A simple model for interannual sandbar behavior
Authors: Plant N. G. , Holman R. A. , Freilich M. H. , Birkemeier W. A.
Published: December 2003

Time and length scales of beach variability have been quantified using 16 years of beach surveys sampled at the Army Corps of Engineers' Field Research Facility, located on the U.S. Atlantic coast. Between 50% and 90% of the bathymetric variability at this site was explained by alongshore‐uniform response over the approximately 1 km alongshore span of the surveys. Although the incident wave height variance was dominated by frequencies at or higher than 1 cycle/yr, more than 80% of the bathymetric variance at all cross‐shore locations was explained by frequencies <1 cycle/yr. Interannual cycles consisting of sandbar formation, migration, and decay contributed to the low‐frequency variability. The observed behavior can be explained by a simple, heuristic model. The model assumes that bars migrate toward a wave height dependent equilibrium position. This position was shown to coincide with the wave “breakpoint.” Additionally, the rate of bar response is taken to be variable and was empirically determined to be proportional to the wave height cubed. The net effect of a variable response rate is to shift the expected long‐term mean sandbar position offshore, toward the equilibrium position associated with the largest waves. The model explained up to 80% of the observed bar position time series variance and up to 70% of the variance of bar crest velocity time series, which were extracted from three different sandbars. Characteristic bar response times (related to the inverse of the response rate) were found to be long relative to the characteristic timescale of the forcing (1 year in our case). As a result, transient response (i.e., bar position far from equilibrium) tended to persist for many cycles of the forcing. Transient bar behavior appears in the observations when bars formed near the shoreline or when outer bars decayed and inner bars faced a changed wave climate. While the present model is able to explain the evolution of these transients, it does not contain a mechanism for their introduction.
Regional analysis of the inverted barometer effect over the global ocean using TOPEX/POSEIDON data and model results
Authors: Ponte Rui M. , Gaspar Philippe
Published: December 2003

Crossover sea level differences from almost 5 years of TOPEX/POSEIDON data are regressed against corresponding differences in atmospheric pressure (−Δpa), and the regression coefficient (α) is examined for deviations from the value of ∼1 cm/mbar expected under a pure inverted barometer (IB) signal. Only crossovers within each 10‐day repeat cycle are used. We focus on variability at the shortest periods where non‐IB response is more likely. Results indicate a marked tendency to have α < 1 cm/mbar, with values in the general range of 0.8–1 cm/mbar in middle and high latitudes and 0.4–0.8 cm/mbar in the tropics. Effects of errors in Δpa and altimeter data seem small to explain all of the observed IB deviations, which imply then positive correlations between Δpa and dynamic sea level Δη′ (sea level adjusted for an IB signal). A simple constant‐density ocean model is used to help interpret the regression results. Only effects of winds and pa on Δη′ are considered. Model‐based α estimates agree qualitatively with the data estimates. On the basis of the model results, wind‐driven Δη′ signals contribute importantly to the spatial variability of α observed at middle and high latitudes. This is particularly evident in the Southern Ocean, where strongest wind‐driven effects on α coincide with regions of anomalous ambient potential vorticity gradients and different vorticity dynamics. In contrast, the observed decrease in α toward low latitudes is due to a true dynamic response to pa, mostly at periods shorter than 10 days. The stronger non‐IB signals suggest a response closer to resonance and may be due to the richer resonance spectrum in the tropics. The non‐IB signals are largely remotely driven and therefore weakly correlated with local pa. Thus regression results tend to underestimate the importance of the nonisostatic response in the tropics but also at higher latitudes.
Waves and currents over a fixed rippled bed: 3. Bottom and apparent roughness for spectral waves and currents
Authors: Mathisen Paul Peter , Madsen Ole Secher
Published: December 2003

This paper is the third in a series of three that presents the results of experiments designed to verify the use of a single bottom roughness length scale for waves and currents over a rough bed. While the first two papers concentrated on the bottom roughnesses experienced by monochromatic wave and current boundary layer flows, this paper presents the results of additional experiments that investigate the use of an equivalent wave representation to extend these results to spectral wave and current boundary layer flows. Spectral waves, simulated by five components, and currents were generated in a 20‐m‐long wave flume with a fixed rippled bottom. Attenuation due to bottom friction is determined from total attenuation measurements for individual wave components by removing the effects of sidewall dissipation and wave‐wave interactions. These attenuation estimates are used to establish representative friction factors, which are used in conjunction with an existing eddy viscosity model to determine bottom roughnesses. The bottom roughnesses experienced by spectral waves (in the presence and absence of a current) match the bottom roughnesses for monochromatic waves. When these experimentally determined bottom roughnesses are used in conjunction with the eddy viscosity model, predictions of attenuation for individual wave components closely match measurements. When the wave boundary layer thickness is defined to be the height at which the predicted velocity deficit in the wave boundary layer is within 5% of the free stream velocity, excellent agreement is obtained between predicted and measured velocity profiles for currents in the presence of codirectional waves. Therefore these experiments show that a single bottom roughness, when used in conjunction with an equivalent wave representation, adequately characterizes both monochromatic and spectral wave‐current boundary layer flows over a fixed rippled bed.
Spatially averaged flow over a wavy boundary revisited
Authors: McLean S. R. , Wolfe S. R. , Nelson J. M.
Published: December 2003

Vertical profiles of streamwise velocity measured over bed forms are commonly used to deduce boundary shear stress for the purpose of estimating sediment transport. These profiles may be derived locally or from some sort of spatial average. Arguments for using the latter procedure are based on the assumption that spatial averaging of the momentum equation effectively removes local accelerations from the problem. Using analogies based on steady, uniform flows, it has been argued that the spatially averaged velocity profiles are approximately logarithmic and can be used to infer values of boundary shear stress. This technique of using logarithmic profiles is investigated using detailed laboratory measurements of flow structure and boundary shear stress over fixed two‐dimensional bed forms. Spatial averages over the length of the bed form of mean velocity measurements at constant distances from the mean bed elevation yield vertical profiles that are highly logarithmic even though the effect of the bottom topography is observed throughout the water column. However, logarithmic fits of these averaged profiles do not yield accurate estimates of the measured total boundary shear stress.
Flow convergence and stability at a tidal estuarine front: Acoustic Doppler current observations
Authors: Brubaker John M. , Simpson John H.
Published: December 2003

Characteristics of the flow field in an estuarine frontal zone have been investigated in a field study in the lower James River estuary. Underway sampling with an acoustic Doppler current profiler (ADCP) on repeated transects across the front provided information on the structure of the flow field near the front and its evolution in time. As this tidal intrusion front advanced up the estuary during the flooding tide, prominent and consistent features in the velocity field included a localized zone of convergent flow beneath the visible surface line and a stratified shear layer just upriver of the front. Within the shear layer between the buoyant surface water and the faster, higher‐salinity undercurrent, gradient Richardson number estimates suggest that the flow was at or near the threshold for shear instability. Another shear‐type gradient in the flow field, the across‐front variation of the along‐front velocity component, strengthened over a sequence of transects, with intensity increasing toward the surface. Tracking of the front was then interrupted when the identifying line of foam and accumulated material on the surface, previously sharp and well defined, broke up and dispersed to such an extent that the visible signature of the front was lost temporarily. A visible frontal expression later reappeared, and propagation upriver continued. Lower bound estimates of downwelling flow in the frontal zone were determined by continuity considerations.
Ring‐slope interactions and the formation of the western boundary current in the Gulf of Mexico
Authors: Vidal Víctor M. V. , Vidal Francisco V. , Meza Eustorgio , Portilla Josué , Zambrano Lorenzo , Jaimes Benjamín
Published: December 2003

Hydrographic data from the Gulf of Mexico (gulf) provide evidence that a western boundary current was set up by the interaction of an anticyclonic Loop Current (LC) ring with the continental margin of the western gulf during March‐August 1985. The March 1985 geostrophic circulation reveals a remnant anticyclonic ring colliding with the slope. During this collision, two cyclonic rings were shed as the anticyclone transferred vorticity to the surrounding slope water. During July‐August 1985, the ring triad weakened and evolved into a ∼900‐km‐long, north flowing, along‐slope, western boundary current and cyclonic‐anticyclonic ring pairs distributed throughout the central and western gulf. This western boundary current attained maximum northward flow speeds of 25 cm s−1 and an 8.3‐Sv mass transport between 94°–96°W at 25°N. Our March‐August 1985 observations reveal that the residence time and decay period of LC anticyclones in the western gulf may exceed 150 days. Within this time period the western gulf's cyclonic‐anticyclonic vorticity field decayed ∼50%. Thus the western boundary current's evolutionary period, from its gestation to its absolute decay, is estimated to be of the order of 300 days. Although the presence of a western boundary current in the gulf has been attributed to the annual wind stress curl cycle [Sturges, 1993], our analyses of the western gulf March and July‐August 1985 ring‐driven geostrophic circulation and corresponding (January, February and May, June 1985) monthly mean synoptic wind stress curl distributions reveal that these constitute competing forcing mechanisms for the gulf's regional circulation. However, when very strong local forcing such as large eddies are present, the wind‐driven background circulation is overwhelmed by such eddy forcing.
Hydrodynamics induced by wind waves in a vegetation field
Authors: Méndez Fernando J. , Losada Iñigo J. , Losada Miguel A.
Published: December 2003

The wave‐induced kinematics and dynamics of a submerged or emerged vegetation field is analyzed. Using potential flow and an eigenfunction expansion, the problem is solved considering regular as well as irregular incident waves. The model takes into account the vegetation motion and solves for the complete wave system on the vegetation field and in its vicinity. The model is validated against experimental laboratory data obtained by other authors, showing a much better agreement than previous theoretical models. In this paper the model is used to evaluate wave height evolution (damping), vegetation and fluid motion, and forces and moments on the vegetation. Furthermore, the inclusion of irregular waves provides force and moment distributions on the vegetation field depending on the wave climate statistics.
A note on hydrodynamic instabilities and horizontal circulation in the surf zone
Authors: Falqués A. , Montoto A. , Vila D.
Published: December 2003

The setup/setdown of the mean water level induced by incident waves on a beach has been hypothesized to be unstable and, thereby, a source of horizontal circulation, rip currents, and rhythmic topography but a direct proof was lacking. This paper examines the dynamics of a small‐amplitude circulation coupled with the subsequent perturbation on the mean water level and the wave radiation stress for normal, regular wave incidence on a nonerodible and nonbarred beach. Two cases are considered. In the first one, the wave refraction by currents and depth variations is neglected, whereas in the second one, wave refraction is accounted for. In the first case, it is proved by analytical methods that there is no instability. It is shown that the coupling between flow, mean free surface, and wave radiation stress provides a restoring force that reinforces gravity in the surf zone, while in the shoaling zone, the coupling opposes gravity. This destabilizing effect in the shoaling zone is too small to overcome gravity force. Although this restoring/destabilizing force cannot produce instability, it causes a small correction on the classical long wave phase speed that becomes now anisotropic. In contrast, when wave refraction is included in the model, the motionless basic equilibrium becomes unstable and circulation cells tend to develop. It is argued that, even without wave refraction, if sediment transport and topographic evolution is accounted for, the situation can be reversed and the basic equilibrium can be unstable. Therefore rip currents in nonbarred beaches cannot be explained from an instability originated only by the setup/setdown unless wave refraction or topographic evolution is taken into account.
Spatial distribution of trends and seasonally in the hemispheric sea ice covers: 1978–1996
Authors: Gloersen P. , Parkinson C. L. , Cavalieri D. J. , Comiso J. C. , Zwally H. J.
Published: December 2003

We extend earlier analyses of a 8.8‐year sea ice data set that described the local seasonal variations and trends in each of the hemispheric sea ice covers to the recently merged 18.2‐year sea ice record from four satellite instruments. The seasonal cycle characteristics remain essentially the same as for the shorter time series, but the local trends are markedly different, in some cases reversing sign. The sign reversal reflects the lack of a consistent long‐term trend and could be the result of localized long‐term oscillations in the hemispheric sea ice covers. By combining the separate hemispheric sea ice records into a global one, we have shown that there are statistically significant net decreases in the sea ice coverage on a global scale. The change in the global sea ice extent is −0.01 ± 0.003 × 106 km2 per decade. The decrease in the areal coverage of the sea ice is only slightly smaller, so that the difference in the two, the ice‐free areas within the packs, has no statistically significant change.
Evaluation of regional numerical weather prediction model surface fields over the Middle Atlantic Bight
Authors: Baumgartner Mark F. , Anderson Steven P.
Published: December 2003

Coastal ocean models often rely on the surface fields from numerical weather prediction (NWP) models for realistic surface boundary conditions, but the errors in these fields are poorly understood. We evaluate the surface meteorological and flux fields provided by three of the regional NWP models in operation during 1996 and 1997 at the U.S. National Centers for Environmental Prediction (NCEP): the Eta‐48, Eta‐29, and Rapid Update Cycle (RUC‐1) models. These model fields are compared to in situ measurements made from an air‐sea interaction buoy deployed from July 1996 to June 1997 at a midshelf location in the Middle Atlantic Bight during the Coastal Mixing and Optics experiment. In addition, data from six National Data Buoy Center buoys are used to evaluate spatial errors in the model fields. The Eta‐29 and RUC‐1 models overestimate the net ocean‐to‐atmosphere heat flux by an average 83 and 74 W m−2, respectively, with notable errors in each of the individual heat flux components. The poorly resolved sea surface temperature fields used in the 1996–1997 regional NWP models lead to significant errors in the latent and sensible heat fluxes over the continental shelf and slope. Moreover, wind speeds are slightly overestimated in the Eta‐48 and Eta‐29 models while the RUC‐1 model underestimates them by more than 1 m s−1. All of the models have mean wind direction errors of 7° to 13° east of north. In light of these evaluations, considerations for improving the accuracy of the surface flux fields for use in future ocean modeling studies are discussed.
Intercomparison of the upper layer circulation of the western equatorial Atlantic Ocean: In situ and satellite data
Authors: Arnault S. , Bourlès B. , Gouriou Y. , Chuchla R.
Published: December 2003

Thanks to the agreement found between in situ measurements and TOPEX/POSEIDON data in the western tropical Atlantic Ocean, a realistic picture of the spatial and temporal variability over the 1992–1997 period is obtained. The sea level variability clearly emphasizes three ranges of variability. The intraseasonal variability is associated with propagating features north of the equator, consistent with a first baroclinic Rossby wave characteristic. The annual variability, which represents the largest part of the variability, describes the seasonal cycle of the North Equatorial Countercurrent; but, more interesting, is a clear year‐to‐year variability in the northernmost part of the area. The surface currents also reveal an intraseasonal tendency with a peak of energy at 62 days at 5°N. Upper layer volume transport across 38°W and between 3°N and 9°N shows a regular seasonal contrast mostly due to the 3°N–6°N region. Year‐to‐year variations are also clearly evidenced. The eastward transport loses about 35% of its strength in the second half of 1995 compared with 1994. This event is followed by a stronger than usual westward transport in early 1996, and in early 1997, the transport seems abnormally eastward. However, contrary to the seasonal cycle, this interannual variability occurs mainly in the 6°N–9°N band. Interhemispheric transport computed across 7°30′N, between 50°W and 35°W, is northward during the whole period 1992–1995. A seasonal cycle can be detected with maximum transport during boreal winter and minimum in spring. An interesting result is the increasing tendency of the northward transport from April 1993 until boreal fall 1995, when the maximum value for the period is reached.
Compression of floating ice fields
Authors: Hopkins Mark A. , Tuhkuri Jukka
Published: December 2003

The compression of ice fields made up of thin floes is central to the processes of ice jam formation in northern rivers, pressure ridge formation in northern seas, and the dynamics of ice fields in Arctic and Antarctic marginal seas. This work describes the results of computer simulations in which a floating layer of circular floes, confined in a rectangular channel, is compressed by a pusher plate moving at a constant speed. The accuracy of the simulations is assessed by comparison with a series of similar physical experiments performed in a refrigerated basin. Following this comparison, the computer model is used to perform an extensive series of simulations to explore the effect of variations in channel length and width, the ratio of floe diameter to thickness, floe on floe friction coefficients, and the distribution of floe diameters on the force required to compress the floes. The results show that reducing the aspect ratio of the floes or increasing the friction coefficient increases the force needed to compress the floes. Both changes increase the force by changing the dominant failure mechanism in the layer of floes from rafting to underturning. Increasing channel width reduced the compressive force (per unit channel width) by reducing the relative importance of frictional drag at the channel edges. Last, the results of a simulation using a distribution of floe diameters was indistinguishable from those of a simulation using floes with a single diameter equal to the average diameter of the distribution.
Linkages between salinity and brine channel distribution in young sea ice
Authors: Cottier Finlo , Eicken H. , Wadhams P.
Published: December 2003

A high‐resolution study of bulk salinity was undertaken on laboratory‐grown sea ice to determine the extent of spatial variability of salinity and whether this was associated with brine channel structures. Ice samples at two different stages of development were compared with respect to physical state, brine distribution, and brine channel structures. Cold, growing sea ice is shown to have steep bulk salinity gradients and a highly variable brine distribution that is closely linked to the location and morphology of brine channels. Areas of high bulk salinity are found to correspond directly to the positions of the brine channels, and these areas are surrounded by ice that shows significant brine depletion. It is observed that redistribution of the brine occurs as the ice cover goes through warming and melting phases. The redistribution is attributed to an increase in porosity and pore connectivity permitting migration of brine through the ice. The distribution of brine in “warm” ice is shown to be more homogeneous and independent of brine channel structures. A number of mechanisms exploiting the enhanced mobility of brine in warm sea ice are used to explain the changes in the salinity profiles and brine distribution.
Seasonal water mass distribution in the Indonesian throughflow entering the Indian Ocean
Authors: Coatanoan C. , Metzl N. , Fieux M. , Coste B.
Published: December 2003

A multiparametric approach is used to analyze the seasonal properties of water masses in the eastern Indian Ocean. The data were measured during two cruises of the Java Australia Dynamic Experiment (JADE) program carried out during two opposite seasons: August 1989 (SE monsoon) and February‐March 1992 (NW monsoon). These cruises took place at the end of a La Niña event and during an El Niño episode, respectively. Seven sources have been identified in the studied region for the 200–800 m layer: the Subtropical Indian Water, the Indian Central Water, the modified Antarctic Intermediate Water, the Indonesian Subsurface Water, the Indonesian Intermediate Water, the Arabian Sea‐Persian Gulf Water (AS‐PGW), and the Arabian Sea‐Red Sea Water (AS‐RSW). The selected tracers are potential temperature, salinity and oxygen with mass conservation and positive mixing coefficients as constraints. The analysis indicates the proportion of each water source along the Australia‐Bali section and into the Indonesian channels. Although no large changes are observed for Indonesian waters, significant seasonal variations are found for the southern and northern Indian Ocean water. During the NW monsoon, the contribution of the AS‐RSW increases at the entrance of the Indonesian archipelago whereas the contribution of the south Indian waters decreases in the northwest Australia basin. In a complementary study, nutrients are introduced into the multiparametric analysis in order to more clearly separate the signature of the north Indian waters (AS‐PGW, AS‐RSW) and to provide supplementary information on the biological history of the water masses, which is compared to large‐scale primary production estimates.
Cyclogenesis in the deep ocean beneath the Gulf Stream: 2. Dynamics
Authors: Savidge Dana K. , Bane John M.
Published: December 2003

Strong cyclones in the deep ocean beneath the Gulf Stream have been observed during the period June 13, 1988, to August 7, 1990, near 68°W, 37°N in data from the Synoptic Ocean Prediction (SYNOP) Experiment. These cyclones developed in association with the evolution of large amplitude, quasi‐stationary meander troughs in the Gulf Stream. It is likely that baroclinic instability is responsible for cyclone spin‐up. The dynamical analysis of these cyclones indicates the process is analogous with atmospheric cyclogenesis from the perspectives of divergence and conservation of potential vorticity, but not in terms of the density field evolution. Large positive vertical velocities in the thermocline over developing low pressure centers at 3500 m are consistent with convergence at depth and divergence in the upper ocean, and with stretching of the lower water column and shortening of the upper water column. The stretching of the lower water column accounts for the generation of positive relative vorticity there. However, the evolution of the density field in the oceanic case does not resemble the atmospheric case. In the atmosphere, density field adjustments in the air column above the low pressure center at the Earth's surface are in the correct sense to account for decreasing pressure there. In the ocean, density field adjustments in the water column fail to account for the developing low pressure centers, so sea surface height depressions must be responsible. These depressions must have an approximate magnitude of 0.5 m depth over a 250 km horizontal extent (the cyclone's diameter).
Cyclogenesis in the deep ocean beneath the Gulf Stream: 1. Description
Authors: Savidge Dana K. , Bane John M.
Published: December 2003

One of the primary scientific results of the Synoptic Ocean Prediction (SYNOP) observational program was the discovery of strong cyclones in the deep ocean beneath the large amplitude Gulf Stream meander troughs that routinely form at about 68°W. These strong well‐organized cyclones extend to at least 3500 m below the sea surface and are an important component of the overall dynamical variability of the Gulf Stream and adjacent deep waters. Typically, a small amplitude Gulf Stream meander “stalls” near 68°W and begins to amplify. As the amplitude of troughs in the Gulf Stream jet increases, the currents at 3500 m strengthen and turn, forming a cyclonic circulation pattern. During SYNOP, six well‐defined instances of meander trough amplification and deep cyclogenesis occurred. The cyclones were characterized by strong swirl speeds (up to 0.5 m s−1) and were long‐lived (typically lasting 6–9 weeks) frequent occurrences (present 35% of the time during the 25 month deployment period). The structure of the cyclones at 3500 m is characterized by increasing velocity from the cyclone center out to some radius of maximum velocity and decreasing velocity beyond that radius. This structure was robust over the lifetime of an event and from event to event. Cyclone radius and the radius to maximum velocity were consistently ≃130 km and ≃55 km, respectively. Evidence of cyclones at upper measurement levels and the low vertical shear values apparent in the deep water below the thermocline indicate that the cyclones extended throughout the entire water column; from the benthic boundary layer, through the thermocline, and to the ocean's surface.
Biological and physical signatures in the tropical and subtropical Atlantic
Authors: Signorini S. R. , Murtugudde R. G. , McClain C. R. , Christian J. R. , Picaut J. , Busalacchi A. J.
Published: December 2003

The variability of sea surface chlorophyll concentration in the tropical and subtropical Atlantic during the first year of Sea‐viewing Wide Field‐of‐view Sensor (SeaWiFS) imagery is examined. An Ocean General Circulation Model (OGCM) is used, along with TOPEX/Poseidon dynamic height observations and global gridded wind stress data sets, to explain the physical forcing of surface ocean color signals. Regions of high surface chlorophyll are strongly correlated with mesoscale and large‐scale physical processes such as the strong upwelling off the west coast of Africa, the relatively high oceanic production within the Guinea Dome region, and the generation and propagation of large anticyclonic eddies along the coast of South America, north of the equator. The major river outflows (Amazon, Orinoco, and Congo) have strong signatures with plumes of apparently high Chl a in excess of 10 mg m−3 near their deltas. The fall bloom in the eastern tropical Atlantic observed by the Coastal Zone Color Scanner (CZCS) was absent in 1997, whereas a bloom was observed in this region in July‐September 1998, which was not observed by the CZCS. We attribute these apparent anomalies to the projection of the 1997–1998 El Niño event into the tropical Atlantic basin; these signals are correlated with sea surface temperature anomalies known to be associated with ENSO. The SeaWiFS images show that there are seasonal blooms within the hydrographic provinces of the Guinea and Angola domes. These hydrographic provinces are characterized by the dynamic uplift of the thermocline at the North Equatorial Current southern boundary (Guinea Dome) and the Benguela Current eastern boundary (Angola Dome). Within these domes, the Ekman pumping and transport are significant due to the strong trade winds at the surface. The Ekman drift plays a major role in the spreading of surface blooms. The spreading of the oceanic bloom at 12°N, 30°W, the Congo River plume, and the areal extent of the upwelling blooms off the coast of Africa, parallels the strength and extent of the Ekman surface drift. Upwelling, when broadly defined to include large scale vertical excursions of the thermocline, explains virtually all of the surface chlorophyll observations in excess of 0.5 mg m−3, except in the river plumes.
Relationships between Antarctic krill (Euphausia superba) variability and westerly fluctuations and ozone depletion in the Antarctic Peninsula area
Authors: Naganobu Mikio , Kutsuwada Kunio , Sasai Yoshikazu , Taguchi Satoru , Siegel Volker
Published: December 2003

An assessment of the environmental processes influencing variability in the recruitment and density of Antarctic krill (Euphausia superba) is important, as variability in krill stocks affects the Antarctic marine ecosystem as a whole. We have assessed variability in krill recruitment and density with hypothesized environmental factors, including strength of westerly winds (westerlies) determined from sea level pressure differences across the Drake Passage, sea ice cover, and ozone depletion. We found a significant positive correlation between krill recruitment in the Antarctic Peninsula area and the strength of westerlies during 1982–1998. Years with strong westerlies during the austral summer season resulted in high krill recruitment in 1987/1988, 1990/1991, and 1994/1995, while the years of weak westerlies resulted in low krill recruitment in 1982/1983, 1988/1989, 1992/1993, and 1996/1997. The strength of westerlies was significantly related to recruitment of 1‐year‐old krill (r = 0.57) and 2‐year‐old krill (r = 0.69) with a level of significance of 5%. In addition, the strength of westerlies also had a strong correlation with chlorophyll a (r = 0.63) and sea ice cover with a 1‐year time lag (r = 0.67). The strength of westerlies is considered to be a key environmental factor. We also found significant correlations between krill density in the Antarctic Peninsula area and the Antarctic ozone depletion parameters during 1977–1997 (e.g., total ozone in October at Faraday/Vernadsky Station of r = 0.76 with a level of significance of 1%). We suspect that ozone depletion impacts directly and/or indirectly on the variability in krill density.
New chemical, bio‐optical and physical observations of upper ocean response to the passage of a mesoscale eddy off Bermuda
Authors: McNeil J. D. , Jannasch H. W. , Dickey T. , McGillicuddy D. , Brzezinski M. , Sakamoto C. M.
Published: December 2003

A mesoscale eddy advected across the Bermuda Testbed Mooring site over a 30‐day period centered on July 14, 1995. Temperature and current measurements along with biogeochemical measurements were used to characterize the biological response of the upper ocean associated with the introduction of nitrate into the euphotic layer due to the doming of isotherms associated with the eddy. Complementary shipboard data showed an anomalous water mass, which extended from a depth of ∼50 to 1000 m, manifesting as a cold surface expression and warm anomaly at depth. Although mesoscale eddies are frequently observed in the Sargasso Sea, the present observations are particularly unique because of the high‐temporal‐resolution measurements of the new instrumentation deployed on the mooring. Analyzers that measure nitrate plus nitrite were placed at depths of 80 and 200 m and bio‐optical sensors were located at depths of 20, 35, 45, 71, and 86 m. Peak nitrate values of nearly 3.0 μM at 80 m and chlorophyll a values of 1.4 mg m−3 at 71 m were observed, as well as a 25‐to 30‐meter shoaling of the 1% light level depth. A Doppler shift from the inertial period (22.8 hours) to 25.2 hours was observed in several time series records due to the movement of the eddy across the mooring. Inertial pumping brought cold, nutrient‐rich waters farther into the euphotic zone than would occur solely by isothermal lifting. Silicic acid was depleted to undetectable levels owing to the growth of diatoms within the eddy. The chlorophyll a values associated with the eddy appear to be the largest recorded during the 8 years of the ongoing U.S. JGOFS Bermuda Atlantic Time Series Study (BATS) program.
On the water masses and mean circulation of the South Atlantic Ocean
Authors: Stramma Lothar , England Matthew
Published: December 2003

We examine recent observations of water mass distribution and circulation schemes at different depths of the South Atlantic Ocean to propose a layered, qualitative representation of the mean distribution of flow in this region. This furthers the simple upper layer geostrophic flow estimates of Peterson and Stramma [1991]. In addition, we assess how well ocean general circulation models (GCMs) capture the overall structure of flow in the South Atlantic in this regard. The South Atlantic Central Water (SACW) is of South Atlantic origin in the subtropical gyre, while the SACW in the tropical region in part originates from the South Indian Ocean. The Antarctic Intermediate Water in the South Atlantic originates from a surface region of the circumpolar layer, especially in the northern Drake Passage and the Falkland Current loop, but also receives some water from the Indian Ocean. The subtropical South Atlantic above the North Atlantic Deep Water and north of the Antarctic Circumpolar Current (ACC) is dominated by the anticyclonic subtropical gyre. In the eastern tropical South Atlantic the cyclonic Angola Gyre exists, embedded in a large tropical cyclonic gyre. The equatorial part of the South Atlantic shows several depth‐dependent zonal current bands besides the Angola Gyre. Ocean GCMs have difficulty capturing this detailed zonal circulation structure, even at eddy‐permitting resolution. The northward extent of the subtropical gyre reduces with increasing depth, located near Brazil at 16°S in the near‐surface layer and at 26°S in the Antarctic Intermediate Water layer, while the tropical cyclonic gyre progresses southward. The southward shift of the northern part of the subtropical gyre is well resolved in global ocean GCMs. However, high horizontal resolution is required to capture the South Atlantic Current north of the ACC. The North Atlantic Deep Water in the South Atlantic progresses mainly southward in the Deep Western Boundary Current, but some water also moves southward at the eastern boundary.
Analysis of C and Ku band ocean backscatter measurements under low‐wind conditions
Authors: Carswell James R. , Donnelly William J. , McIntosh Robert E. , Donelan Mark A. , Vandemark Douglas C.
Published: December 2003

Airborne ocean backscatter measurements at C and Ku band wavelengths obtained in low to moderate‐wind conditions are presented. The differences between the low‐wind backscatter data and the CMOD4 and SASS‐II models are reported. The measurements show that the upwind/crosswind backscatter ratio is greater than predicted. These large upwind/crosswind backscatter ratios are attributed to a rapid decrease in the crosswind backscatter at low winds. Qualitative agreement with the composite surface model proposed by Donelan and Pierson suggests the rapid decrease in the crosswind backscatter may be caused by viscous dampening of the Bragg‐resonant capillary‐gravity waves. We show that for larger antenna footprints typical of satellite‐based scatterometers, the variability in the observed wind field smooths out the backscatter response such that the rapid decrease in the crosswind direction is not observed.
Onset of the Somali Jet in the Arabian Sea during June 1997
Authors: Halpern David , Woiceshyn Peter M.
Published: December 2003

The National Aeronautics and Space Administration scatterometer surface wind vectors are used to describe the rapid onset of the Somali Jet throughout the Arabian Sea. In June 1997 the time of Somali Jet onset varied over the Arabian Sea, with June 17–18 the average time. The Somali Jet appeared first in the western Arabian Sea, expanded over 2 weeks to encompass the Arabian Sea, and produced a threefold increase in surface wind convergence in the eastern Arabian Sea. The onset time of the 12 m s−1 isotach preceded by 3–4 days the onset of monsoon rainfall in Goa. For Somali Jet wind speeds above 10 m s−1 the 2 day 1° × 1° sea surface temperature decreased 0.5°C per 1 m s−1 increase in collocated wind speed. The Somali Jet created a north‐south distribution of Ekman pumping and suction in the central Arabian Sea to enhance the eastward surface current by 0.1 m s−1. The Somali Jet doubled the southward Ekman transport across the southern boundary of the Arabian Sea. In June 1997, when the most intense El Niño episode of the century was in its onset phase, the southward Ekman transport across the southern boundary of the Arabian Sea was one half that observed since 1992.
A generalized Korteweg‐de Vries model of internal tide transformation in the coastal zone
Authors: Holloway Peter E. , Pelinovsky Efim , Talipova Tatjana
Published: December 2003

A nonlinear model is developed, based on the rotated‐modified extended Korteweg‐de Vries (reKdV) equation, of the evolution of an initially sinusoidal long wave in the coastal zone, representing an internal tide, into nonlinear waves including internal solitary waves. The coefficients of the basic equation are calculated using observed conditions for the north west shelf (NWS) of Australia. The roles of both quadratic and cubic nonlinearity, the Earth's rotation, and frictional dissipation are discussed. The combined action of nonlinearity and rotation leads to a number of intersting features in the wave form including solitons of both polarities, “thick” solitons, and sharp waves with steep fronts. It is shown that rotation is important for modelling the evolution of the internal tide, even for the relatively low latitude on the NWS of 20°S. Rotation increases the phase speed of the long internal tide, reduces the number of internal solitary waves that form from a long wave, and changes the form of the waves. The effects of nonlinearity on the vertical modal structure of the internal waves are also discussed. Results of numerical simulations are compared with current and temperature observations of the internal wave field on the NWS which show many of the features produced by the generalized KdV model.
Fluxes of heat and tracers in “hot smoker” plumes
Authors: McDougall Trevor J. , Speer Kevin G.
Published: December 2003

Integral plume theory is used to demonstrate how tracer data from the vicinity of a “hot smoker” plume may be used to infer the heat flux emanating from the source of the plume.
Observations of a pulsed buoyancy current downstream of Chesapeake Bay
Authors: Rennie Sarah E. , Largier John L. , Lentz Steven J.
Published: December 2003

A plume of low‐salinity water was observed along the North Carolina coast 100 km south of the mouth of Chesapeake Bay during the Coastal Ocean Processes Pilot field program conducted from August through October 1994. The presence of the plume was episodic, occurring every 3 to 8 days. The timing was related to wind patterns, which influence both the delivery of estuarine discharge to the shelf and the plume's passage down the coast. When not affected by local winds, the low‐salinity water was confined to within 7–9 km of the coast and was about 8 m deep. Downwelling winds narrow and deepen the plume, whereas upwelling winds cause it to thin and spread offshore, eventually detaching from the coast. The low‐salinity plume propagated along the coast at speeds comparable to linear internal wave phase speed, except when strong downwelling wind conditions caused the plume to be in contact with the bottom. The observed propagation speeds are faster than those predicted by previous numerical modeling efforts. The plumes slowed with distance from the source, as mixing with ambient shelf water reduced the density contrast. This buoyancy source was balanced by an alongshore current with a southward velocity of 0.3 to 0.7 m s−1, bounded by a region of high horizontal velocity shear at the offshore salinity front. The currents observed in the nose of the plume are consistent with properties of an internal gravity current under rotation.