bluuebunny/crossref_metadata_2025 huggingface dataset:
Optical closure in an ultraturbid lake
Authors: Kirk John T. O. , Oliver Rod L.
Published: February 2004

A study has been carried out to determine whether a previously derived relationship for an apparent optical property (the vertical attenuation coefficient for downward irradiance, Kd) of a water body as a function of inherent optical properties (absorption and scattering coefficients, a and b) of the water, applies to a surface water body at the extreme high‐scattering end of the range of naturally occurring optical character, namely an ultraturbid lake in Australia. It was found that scattering by the very small clay particles responsible for the high turbidity of the water showed a very strong wavelength dependence in accordance with b ∝ (1/λ)226. When this wavelength dependency was taken into account, both in the laboratory determination of absorption coefficients, and in the calculation of attenuation of irradiance in the water body as a function of wavelength, then good agreement between the Kd(PAR), where PAR is photosynthetically available radiation, derived by calculation from measured a and b values and that obtained by measurement in the lake was found. Optical closure was thus achieved.
Surface heat flux parameterizations and the variability of thermohaline circulation
Authors: Cai Wenju , Godfrey Stuart J.
Published: February 2004

We describe a series of experiments using the Geophysical Fluid Dynamics Laboratory (GFDL) modular ocean model in an idealized Atlantic Ocean forced by a fixed freshwater flux and by two different heat flux parameterization schemes. Both of the parameterizations use a bulk formula: one assumes an atmosphere of infinite heat capacity and the other assumes an atmosphere of no heat capacity. These experiments show that the behavior, the stability, and the response to external perturbations of the oceanic thermohaline circulation depend heavily upon the way in which the heat flux is parameterized and upon the associated thermal capacity of the atmosphere to which the ocean is coupled. With an infinite heat capacity atmosphere the ocean's thermohaline circulation is very sensitive to perturbations in freshwater flux. In contrast, with the alternative scheme the ocean's thermohaline circulation is stable. The fundamental difference between the two schemes is that the sea surface temperature (SST) in the first case is not allowed to respond adequately to changes in poleward heat transport, while in the latter it is free to evolve. When the SST is curtailed, the oceanic negative feedback mechanism, which stablizes the thermohaline circulation, is disabled, and a positive feedback mechanism dominates. The above result highlights the importance of using an appropriate thermal forcing. It is also shown that the cause for multiple equilibria in the GFDL fully coupled model and in ocean‐only models under the scheme assuming infinite heat capacity is different and that multiple equilibria are likely under a no heat capacity atmosphere. Under the scheme assuming an infinite heat capacity atmosphere, interdecadal variability that is thermally driven by a mismatch between local heat storage rate and oceanic heat transport cannot survive. Under the scheme assuming a no heat capacity atmosphere, however, an SST anomaly associated with the variability is able to develop, and regular oscillations persist. Although all the above results suggest that, of the two schemes, the one assuming a no heat capacity atmosphere is a more realistic one for studies of the ocean's climate, fully coupled atmosphere‐ocean models are needed to properly investigate the stability and sensitivity of the thermohaline circulation.
A simple model of the Atlantic thermohaline circulation: Internal and forced variability with paleoclimatological implications
Authors: Sakai K. , Peltier W. R.
Published: February 2004

We describe the development and application of a simple two‐dimensional model of the thermohaline circulation in the Atlantic basin. The nondimensional parameters of the model consist of thermal and haline Rayleigh numbers, Lewis number, and ratios of horizontal and vertical diffusion coefficients for heat, salt, and momentum. Under mixed boundary conditions and with a convective adjustment scheme applied to represent the subgrid scale convective downwelling that drives the process of deepwater formation, the model support two modes of oscillatory behavior. One of these is on a timescale that ranges from subdecadal to decadal and appears as noisy fluctuations driven by the convective adjustment scheme. The second physical mode has a characteristic timescale of the order of centuries when the net influx of freshwater into the basin is near zero, a mode of variation which is similar to that found previously in full three‐dimensional ocean general circulation model analyses of the modern circulation. We demonstrate, however, that the timescale of this physical mode is strongly dependent on the net freshwater forcing: the weaker this forcing, the shorter the timescale, the stronger the forcing, the longer the timescale. This mode therefore provides a possible explanation both for the century timescale variations observed in the modern climate record and for the millennium timescale Dansgaard‐Oeschger oscillations that were characteristic of the late glacial climate state. The response of the model to freshwater anomalies similar to those known to have developed during the late glacial state of the ice age and during deglaciation seems to provide a straightforward explanation of the climatic variations recently revealed in δ18O time series for ice cores drilled at Summit, Greenland. This is especially true of the experiments that we have performed in an effort to better understand the Younger‐Dryas type climatic fluctuations that occurred during the late glacial and the last deglaciation periods. Our experiments strongly support the hypothesis that the Atlantic thermohaline circulation played a very important role in these intense events.
A conceptual model of the impact of flaws on the stress state of sea ice
Authors: Lewis James K.
Published: February 2004

Recent theoretical advances have lead to a rheology for sea ice and an increased understanding of how stresses are generated in an ice floe by thermally induced strain rates. One of the underlying assumptions of the ice rheology is that floes in general (order of 300 m or greater in diameter) are too large to bend or twist in response to vertical or horizontal variations in strain rates. In essence, there are no nonzero bending moments. A numerical model based on this rheology has been used to produce good predictions of observed ice stresses and times of ice fracturing (Lewis, 1994; Lewis et al., 1994). On the basis of these results this paper presents a conceptual model of how existing flaws in an ice floe can impact the stress state of the ice while still assuming no bending or twisting of the floe. A modified ice rheology is developed which quantifies the extent of existing cracks in the ice and how they impact the overall strain rate within a floe.
Ocean tides in the continental margin off the Pacific Northwest Shelf
Authors: Mofjeld H. O. , González F. I. , Eble M. C. , Newman J. C.
Published: February 2004

A comparison of bottom pressure observations and global tide models shows that the regional distributions of the diurnal and semidiurnal tides off the Oregon and Washington shelves are determined mainly by the large‐scale amphidromic systems in the North Pacific Ocean. These cause the tidal amplitudes to increase toward the northeast and the phase lags to increase toward the northwest. Within 200–500 km of the shelfbreak, the curving continental boundary causes the cophase lines to be closely packed near Cape Mendocino and more widely spread in the bight to the north. Farther seaward, the M2 tide is consistent with a Kelvin wave propagating along the British Columbia boundary (315°T), while the K1 tide decays offshore more slowly than expected from Kelvin wave theory. A long‐term (4.2‐year) bottom pressure series at Axial Seamount (45°58′N, 130°00′W; depth 1540 m) yields a nongravitational S′2 tide that is consistent with a large‐scale barotropic tide extending seaward from the coast. At lower frequencies, the observed Mf amplitude matches recent global models, while the Mf phase lag is marginally earlier than model phases. Nearby Mf currents are bottom‐trapped and consistent in phase with quasi‐geostrophic dynamics; their magnitudes near the bottom are much larger than barotropic Mf currents observed farther west in the North Pacific. The models reproduce the distributions of the diurnal and semidiurnal harmonic constants but with broad‐scale offsets in the amplitudes and phase lags. These offsets cause errors (standard deviation = 3.0 cm) in predicted time series of the diurnal and semidiurnal tides that fluctuate on fortnightly to interannual timescales. The ≤29% seasonal variations of the errors are largest in winter and are modulated by 4–5‐year fluctuations associated with the nodal cycle. In situ measurements can be used to remove these regional offsets. This would improve the temporal and spatial resolution of nontidal sea level estimates that are based on satellite altimeter data.
Modal decomposition of deep ocean circulation models: Comparison with reduced‐gravity models
Authors: Furue Ryo , Nakajima Kensuke , Ishikawa Ichiro
Published: February 2004

The results of two simple multilevel numerical experiments for the deep ocean circulation are decomposed into vertical modes, and a comparison is made between the decomposed flow fields and the results of corresponding reduced‐gravity models. Although the basic equations of the multilevel model are fully nonlinear, it is a good approximation that first‐mode flow fields of the multilevel model are the linear responses of a reduced‐gravity model to a concentrated source and an almost uniformly distributed sink. In the multilevel model forced by a surface differential cooling the vertical advection of density anomaly in the region where the cooling is strongest acts as the major mass source for the first baroclinic mode. The convective adjustment, on the other hand, has only a minor contribution to the source for lower modes. The interior broad, uniform sink for the first mode, which is an important element in the Stommel‐Arons model, is provided by the vertical diffusion of the basic density stratification. For such high modes as to constitute the stacked jets along the equator, nonlinear forcing at lower latitudes cannot be neglected.
A comparison of the nonlinear frictional characteristics of two‐dimensional and three‐dimensional models of a shallow tidal embayment
Authors: Grenier R. R. , Luettich R. A. , Westerink J. J.
Published: February 2004

The nonlinear frictional behavior of two‐dimensional (2‐D) and three‐dimensional (3‐D) models are compared in this study of tides in the Bight of Abaco. The shallow depths and the existence of an extensive set of tidal elevation data (five astronomical and two overtide constituents at 25 stations) from Filloux and Snyder (1979) offer an excellent opportunity to compare the effects of different frictional formulations. In addition, previous modeling efforts in the bight have consistently overpredicted the M6 and generally overdamped the O1, K1, and S2 tides. The results indicate that although the 2‐D and 3‐D models may be calibrated to produce nearly identical responses for the dominant M2 tide, there are systematic differences in the responses of the primary overrides. These differences are explained using analytical expansions of the friction terms and are shown to be due to differences in the terms that are nonlinear in velocity and in water level. The investigation concludes that the overgeneration of M6 and the overdamping of secondary astronomical tides will occur in 3‐D models as well as 2‐D models. Although several causes for these problems were considered, improvement in these constituents could be achieved only by modifying the standard quadratic friction or flow‐dependent eddy viscosity relations to reduce the nonlinear frictional effect relative to the linear frictional effect. The required modifications suggest the presence of a constant background velocity, residual turbulence field, or possibly the need for a more advanced frictional closure.
Circulation and transport of water along the western Weddell Sea margin
Authors: Muench Robin D. , Gordon Arnold L.
Published: February 2004

Ocean current, temperature, and salinity data obtained from the western Weddell Sea during the austral winter 1992 U.S.‐Russian drifting ice station experiment Ice Station Weddell 1 (ISW‐1) are used to describe water circulation and transport. Surface‐to‐bottom baroclinic currents were computed by applying the geostrophic approximation to derived density data. These were corrected using current measurements obtained from drifting current meter arrays, and the resulting total currents were vertically integrated to obtain volume transports. Transport was found to be northward in the region, which encompassed the western boundary current of the cyclonic Weddell Sea gyre. This northward transport increased from south to north by more than a factor of 2, from about 12×106m3 s−1 in the southwestern Weddell to about 28×106m3s−1 farther north. The increase in northward transport was compensated for by westward flow from the interior of the gyre into the western boundary region. About 5–6×106m3s−1 of the northward transport was contained in a 300–500 m thick bottom layer of cold water. This layer, whose transport increased by about 1×106 m3 s−1 from south to north, was identifiable by its water mass characteristics as Weddell Sea Bottom Water originating on the southwestern and western shelf regions. Its north flowing volume was consistent with past estimates of a 1.5–2×106 m3 s−1 production rate coupled with a 300–400% transport increase due to entrainment during downslope flow from the shelves to the deep basin. The maximum (northernmost) northward transport, less the bottom water transport, is consistent with previous estimates for wind‐driven transport in the Weddell Gyre provided that bottom friction and the sea ice influence on wind forcing are taken into consideration.
Temporal variation of meandering intensity and domain‐wide lateral oscillations of the Gulf Stream
Authors: Lee Tong , Cornillon Peter
Published: February 2004

The path of the Gulf Stream exhibits two modes of variability: wavelike spatial meanders associated with instability processes and large‐scale lateral shifts of the path presumably due to atmospheric forcing. The objectives of this study are to examine the temporal variation of the intensity of spatial meandering in the stream, to characterize large‐scale lateral oscillations in the stream's path, and to study the correlation between these two dynamically distinct modes of variability. The data used for this analysis are path displacements of the Gulf Stream between 75° and 60°W obtained from AVHRR‐derived (Advanced Very High Resolution Radiometer) infrared images for the period April 1982 through December 1989. Meandering intensity, measured by the spatial root‐mean‐square displacement of the stream path, displays a 9‐month dominant periodicity which is persistent through the study period. The 9‐month fluctuation in meandering intensity may be related to the interaction of Rossby waves with the stream. Interannual variation of meandering intensity is also found to be significant, with meandering being much more intense during 1985 than it was in 1987. Annual variation, however, is weak and not well‐defined. The spatially averaged position of the stream, which reflects nonmeandering large‐scale lateral oscillations of the stream path, is dominated by an annual cycle. On average, the mean position is farthest north in November and farthest south in April. The first empirical orthogonal function mode of the space‐time path displacements represents lateral oscillations that are in‐phase over the entire study domain. Interannual oscillations are also observed and are found to be weaker than the annual oscillation. The eigenvalue of the first mode indicates that about 21.5% of the total space‐time variability of the stream path can be attributed to domain‐wide lateral oscillations. The correlation between meandering intensity and domain‐wide lateral oscillations is very weak.
Branching of Atlantic Water within the Greenland‐Spitsbergen Passage: An estimate of recirculation
Authors: Manley T. O.
Published: February 2004

Atlantic Water (AW) supplies the largest source of heat, mass, and salt to the Arctic Ocean via Fram Strait (Greenland‐Spitsbergen Passage), yet it represents only a fraction of the Atlantic Water that resides in the Greenland, Iceland, Norwegian, and Barents Seas. This is a result of both the branching of the central core of AW along its northward flow and the modification of its T‐S signature through air‐sea‐ice interactions and internal mixing. This paper addresses the quantitative analysis of the three dominant Atlantic Water cores within Fram Strait and north of 76°N using an 11‐year (1977 to 1987) hydrographic database. Spatial variations of heat, volume, and salt along its flow path of some 600 km showed that the major core of Atlantic Water that directly enters the Arctic Ocean (Svalbard branch) did not extend past 20°E. Of the 9719 km3 of Atlantic Water existing within the region, one third resided within the Svalbard branch; the remainder, 22% and 45%, were held within the Return Atlantic Current and the Yermak branches, respectively. Restricting the analysis to a southern limit of 79°N effectively removed the Return Atlantic Current and showed a nearly equal split between the two remaining branches. Work completed by Bourke et al. (1988) indicated that the Yermak branch is largely recirculated to the south; if true, this analysis supports Rudels' (1987) model estimate of a 50% recirculation of AW within this region.
Infragravity velocity profiles in the surf zone
Authors: Putrevu Uday , Svendsen Ib A.
Published: February 2004

We present a theoretical solution for the vertical structure of the velocity profiles in infragravity waves. The solution predicts that while the velocity without local forcing of the infragravity waves does not vary significantly with the vertical coordinate, the velocity with local forcing of the infragravity waves has a substantial vertical structure.
Warm Weddell Deep Water west of Maud Rise
Authors: Gordon Arnold L. , Huber Bruce A.
Published: February 2004

A pool of relatively warm (>1.0°C) Weddell Deep Water (WDW) immediately west of Maud Rise appears to be a quasi‐stationary feature. The “warm pool” is derived from the flow of warm WDW around the flanks of Maud Rise. An austral spring 1989 expedition of the R/V Akademik Fedorov obtained detailed measurements of the warm pool. It displays the general regional relationship of temperature maximum (t‐max) warmth to shallowness of the pycnocline, as well as mixed layer oxygen concentration well below full saturation. Depression of oxygen values is a product of injection of WDW into the mixed layer during the ice‐covered winter period. Associated with this transfer are high vertical fluxes of heat and salt which limit the thickness of the sea ice cover. In the winter the atmosphere is sufficiently cold to remove the WDW heat without massive sea ice melting, though the regional ice thickness is restricted. In the spring the atmospheric conditions cannot remove the ocean heat, and ice melting ensues before the atmospheric heat budget alone can account for the ice melt. This is clearly seen in the warm pool as very low mixed layer oxygen, which is a reflection of high WDW entrainment and vertical heat flux and is normally associated with high mixed layer salinity. Here it is coupled instead with reduced salinity, a result of approximately 0.5 m of ice melt. The Fedorov data confirm the role of oceanic heat flux in early removal of Southern Ocean sea ice at the end of winter, in that the region of highest WDW entertainment is associated with greater amounts of melt water.
Longitudinal vortices beneath breaking waves
Authors: Nepf H. M. , Cowen E. A. , Kimmel S. J. , Monismith S. G.
Published: February 2004

The formation of longitudinal vortices has been observed in a wavy channel flow and appears to be linked to spilling breaking and/or to vertical vorticity generated by a wave instability at the wave maker. Both conditions were present when the wave slope, ak exceeded 0.25. The wave instability produced velocity jets beneath and just downstream of the plunger that could provide the initial perturbation for the CL2 instability mechanism (Faller and Caponi, 1978). The breaker activity could also contribute to the CL2 production mechanism by eliminating the negative, stabilizing shear observed within the wave maker wake and by providing seed perturbations to the vorticity field. As the cells evolved downstream, they were maintained through interaction with the bottom boundary layer. When the vortices were present, both vertical mixing and turbulent kinetic energy were enhanced. Despite some differences in scale these results suggest that Langmuir circulation may produce similar changes in the mixed layer.
Effect of alongshore nonuniformities on longshore current predictions
Authors: Putrevu Uday , Oltman‐Shay Joan , Svendsen Ib A.
Published: February 2004

Longshore currents have been primarily modeled by assuming alongshore uniformity. In this paper we revisit the problem of nonuniform longshore currents to refocus attention on the importance of accounting for alongshore nonuniformity in longshore current predictions and to provide an analytical estimate of the importance of this nonuniformity. We derive a semianalytical solution for the longshore currents that allows for weak alongshore variations in the bottom topography. The solution shows that the alongshore pressure gradient induced by alongshore variations in the bottom topography could contribute substantially to the forcing for the longshore currents. An example calculation shows that the longshore current could deviate by up to ±30% from the mean for a ±10% deviation of the bottom topography. Thus we suggest that in many practical cases it is important to include the alongshore pressure gradient to accurately model longshore currents.
Forcing of anomalous sea surface temperature evolution in the tropical Atlantic during Pacific warm events
Authors: Curtis Scott , Hastenrath Stefan
Published: February 2004

The seasonal evolution of anomalous interhemispheric sea surface temperature (SST) gradients in the tropical Atlantic from January to April is studied by composites of the 10 warmest (warm) and 10 coldest (cold) Januaries during 1948–1993 in the equatorial Pacific using Comprehensive Ocean‐Atmosphere Data Set ship observations. In the warm as compared with the cold years, an anomalous weakening of the northward SST gradient develops, mainly due to anomalous warming in the tropical North Atlantic. This stems from the combination of three forcings all related to the weakened North Atlantic trade winds during Pacific warm events. Most important are the reduced latent heat flux in much of the tropical North Atlantic and anomalous downwelling equatorward of 20°N, with a further contribution from increased net radiation resulting from the reduced cloudiness due to the diminished convergence in the downstream portion of the North Atlantic trades. In response to the development of warm anomalies in the tropical North Atlantic during January–March, the cross‐equatorial northward winds accelerate to April, and this leads south of the equator not only to Ekman downwelling, enhanced divergence, reduced cloudiness, and increased net radiation, but also to enhanced wind speed and evaporation. The result is a modest anomalous warming in the western tropical South Atlantic. Increased air temperature over the entire basin, presumably due to atmospheric advection from the Pacific, has little effect on the Atlantic SST pattern. The anomalous interhemispheric SST gradient, controlled primarily by the warm anomaly in the North Atlantic, has a pivotal role to play in steering the late boreal winter atmospheric circulation in the tropical Atlantic sector and thus regional climate anomalies.
Infragravity energy modulation by tides
Authors: Okihiro Michele , Guza R. T.
Published: February 2004

Infragravity waves, motions with frequencies immediately below wind‐generated sea and swell, are believed to be radiated seaward from the surf zone, where they are excited by nonlinear interactions between waves in the sea‐swell frequency band. We describe tidal modulation of sea surface elevation variance at infragravity frequencies observed in 8–30 m water depth, a few kilometers from shore at several California sites. Infragravity spectral levels vary by as much as a factor of 10 between high and low tide, possibly because the surf zone width and beach face slope vary significantly with tidal level on the usually concave‐shaped beaches onshore of the observations. At a fixed sensor and with approximately constant sea‐swell energy, the observed infragravity energy is lowest at low tide.
A mechanism for the formation and maintenance of shore‐oblique sand ridges on storm‐dominated shelves
Authors: Trowbridge J. H.
Published: February 2004

A remarkable set of shore‐oblique sand ridges occupies the inner portion of the storm‐dominated Middle Atlantic and South Atlantic Bights, off the east coast of the United States of America. A mechanism for present‐day formation and growth of these distinctive geological features follows from a linearized analysis of the stability of alongshore‐periodic topographical perturbations, based on a depth‐averaged fluid dynamical model with a simple representation of sediment transport. The analysis indicates exponential growth of shore‐oblique features as a result of offshore deflection of storm‐driven alongshore flows at ridge crests, which leads to a convergence of sediment flux because the effective carrying capacity decreases with increasing distance offshore. The analysis indicates a set of conditions required for ridge growth. Model solutions for topography under conditions favorable for ridge growth are strikingly similar to the observed sand ridges.
Preconditioning the Greenland Sea for deep convection: Ice formation and ice drift
Authors: Visbeck Martin , Fischer Jürgen , Schott Friedrich
Published: February 2004

The role of sea ice in preconditioning the mixed layers of the central Greenland Sea for deep convection is investigated, with particular emphasis on the formation of the “Nordbukta.” The opening of the ice free bay in late January 1989 indicated that the upper layer was well preconditioned for deep convection which reached down to 1500 m depth in March 1989. We propose that the ice free bay occurred due to diminishing new ice formation without extensive ice melt. A key process is wind‐driven ice drift to the southwest, as observed by upward looking acoustic Doppler current profilers, which will alter the upper ocean freshwater budget when an ice volume gradient along the ice‐drift direction exists. We investigated the importance and effects of such an ice‐drift‐induced freshwater loss on upper ocean properties using an ice‐ocean mixed‐layer model. Observed temperature and salt profiles from December 1988 served as initial conditions, and the model was integrated over the winter season. Given the one‐dimensional physics and climatological surface fluxes, the model was not able to produce a reasonable ice and mixed‐layer evolution. However, allowing ice drift to reduce the local ice thickness improved the ice‐ocean model performance dramatically. An average ice export of 5–8 mm d−1 was needed to be consistent with the observed evolution of mixed‐layer properties and ice cover. Using the same fluxes and ice export, but initial conditions from the “Is Odden” region, yielded ice cover throughout the winter over a shallow mixed layer, both of which are consistent with the observations from the Odden region.
Interocean exchange and meridional mass and heat fluxes in the South Atlantic
Authors: Boddem J. , Schlitzer R.
Published: February 2004

The meridional transport of mass and heat in the South Atlantic as well as the interocean exchange with the South Pacific and Indian Ocean is examined using a model based on the adjoint method. Large amounts of historical temperature (θ) and salinity (s) data are assimilated and an optimal model circulation is searched to satisfy two main objectives: (1) the vertical shear of the model flows is consistent with geostrophic shear estimates, and (2) the model flows correctly reproduce the measured distributions of θ and s. Information on baroclinic flows contained in the historical θ/s distributions is incorporated in the model circulation via constraint 1, and the unknown reference velocities are chosen in a way such that the resulting absolute flow velocities (horizontal and vertical) yield realistic θ and s fields (constraint 2). The model is mass, heat, and salt conserving and has realistic topography. It is shown that realistic θ and s model distributions can be obtained with flows that are consistent with geostrophy. The model‐derived reference velocities (2500 dbar) show southward flow of North Atlantic Deep Water (NADW) along the South American coast and show eastward flow along the path of the Antarctic Circumpolar Current (ACC). Satisfactory θ/s simulations are obtained for a variety of model flows that differ considerably with respect to strength of the meridional overturning cell, magnitude of meridional heat transport and inflow rate of Indian Ocean Central (thermocline) Water (IOCW) into the South Atlantic. A strong correlation is found between meridional heat transport, strength of the meridional cell and inflow from the Indian Ocean. For all the model states that were found to be consistent with hydrographie data, the northward transport above the NADW layer is dominated by intermediate water with only relatively small contributions of warm surface water. The maximal acceptable meridional heat transport across 30°S amounts to 0.35 PW. Inflow (shallow and intermediate) from the Indian Ocean into the South Atlantic in the range of 4–7 Sv can be accommodated by the model, but larger values have to be rejected. All acceptable model solutions show net heat gain of the South Atlantic from the atmosphere and a net export of warm water into the Indian Ocean.
Trapped, near‐inertial waves and enhanced chlorophyll distributions
Authors: Granata T. , Wiggert J. , Dickey T.
Published: February 2004

Waves with near‐inertial frequencies were observed along a front associated with a large mesoscale feature in the Sargasso Sea during the late summer of 1987. High subsurface chlorophyll concentrations occurred on the edge of this front, coincident with the wave packets. The amplitude of the waves increased with time, and kinetic energy propagated downward, reducing 20‐m Richardson numbers in the thermocline to 1 or less. Chlorophyll levels were episodic, showing no periodicity coincident with wave dynamics. However, on two occasions, chlorophyll concentration increased from <0.5 to >1 mg Chl m−3, several hours after the waves penetrated the thermocline. It was hypothesized that mixing associated with shear instabilities stimulated new production. A diffusivity model combined with nutrient data produced a phytoplankton bloom that accounted for only one of the maxima. The other increase in chlorophyll may have been the result of horizontal advection of the wave packets near the front.
Annual period equatorial waves in the Pacific Ocean
Authors: Minobe Shoshiro , Takeuchi Kensuke
Published: February 2004

This paper analyzes behavior of annual period equatorial waves and their role in oceanic variations, namely, the sea level displacement, zonal current, and sea surface temperature (SST) along the equator in the Pacific. Using a linear model forced by observed wind stress, the amplitudes and phases of the annual equatorial waves are calculated. The model equatorial waves are compared with the waves decomposed from Levitus' (1982) climatological density data. The modeled and decomposed first‐meridional‐mode Rossby waves of the two gravest vertical modes indicate common features that those waves are forced in the eastern Pacific and propagate to the central Pacific. These first‐meridional‐mode Rossby waves play a dominant role in the model sea level displacement and cause the westward propagating feature in the model surface zonal current in the central Pacific. The analysis suggests that the propagation of the surface zonal current is significantly responsible for the well‐known westward propagation of annual SST variation in this region. The modeled and decomposed Kelvin waves are significantly different, but the model suggests that the role of the annual Kelvin waves in the equatorial oceanic variations is smaller than that of the first‐meridional‐mode Rossby waves.
A two‐dimensional tidal model for the Mediterranean Sea
Authors: Tsimplis M. N. , Proctor R. , Flather R. A.
Published: February 2004

The tidal propagation in the Mediterranean Sea is described through a high‐resolution, two‐dimensional hydrodynamic model forced by the equilibrium tide and the incoming tide at the Strait of Gibraltar. The four most significant constituents, M2, S2, K1, and O1, are included in the model. Good agreement with a set of 63 coastal gauges is achieved. The significance of the equilibrium tide and the forcing at the open boundary is investigated. The incoming wave from the Strait of Gibraltar is important in tuning the tides in the whole of the Mediterranean. For the north Aegean Sea the solution without the forcing at Gibraltar results in a doubling of the amplitudes of the semidiurnal tides. The estimated energy dissipation due to bottom friction is 8.8×108 W. The major area of dissipation is the Gulf of Gabes. Energy fluxes through the major straits are calculated.
Water mass characteristics and geostrophic circulation in the South Brazil Bight: Summer of 1991
Authors: Campos E. J. D. , Gonçalves J. E. , Ikeda Y.
Published: February 2004

A hydrographic survey on board the German ship FS Victor Hensen was carried out in Brazilian coastal waters between Santos (23°56'S) and Rio de Janeiro (22°54'S) from January 15 to January 22, 1991. Analyses of conductivity‐temperature‐depth (CTD) data collected during this cruise show that the geostrophic flow and water mass structure of the Brazil Current in that region have characteristics similar overall to those in the better observed Cabo Frio region. The uppermost 200 m of the water column is dominated by the warm, highly saline Tropical Water flowing predominantly to the southwest. This flow direction appears to persist to approximately 900 m, the maximum depth reached by the CTD casts. The water lying between 250 and 750 m has the characteristics of South Atlantic Central Water (SACW). Below 750 m, down to 900 m, the water characteristics are those of Antarctic Intermediate Water. The thermohaline structure and geostrophic calculations indicate the presence of a meandering pattern, with a trough that appears to be the early stage of formation of a cold‐core eddy over the upper slope region. This feature extends from the surface down to approximately 500 m depth and is apparently associated with the intrusion of SACW onto the continental shelf. The geostrophic computations (with respect to two reference levels, 750 and 900 dbar) yield maximum current values in the range 0.6–0.7 m s−1. The southwestward volume transport, averaged over the entire domain, was approximately 7.3 Sv with respect to 750 dbar and 8.8 Sv with respect to 900 dbar.
The global frequency‐wavenumber spectrum of oceanic variability estimated from TOPEX/POSEIDON altimetric measurements
Authors: Wunsch Carl , Stammer Detlef
Published: February 2004

Two years of altimetric data from the TOPEX/POSEIDON spacecraft have been used to produce preliminary estimates of the space and time spectra of global variability for both sea surface height and slope. The results are expressed in terms of both degree variances from spherical harmonic expansions and in along‐track wavenumbers. Simple analytic approximations both in terms of piecewise power laws and Padé fractions are provided for comparison with independent measurements and for easy use of the results. A number of uses of such spectra exist, including the possibility of combining the altimetric data with other observations, predictions of spatial coherences, and the estimation of the accuracy of apparent secular trends in sea level.
Runout and fine‐sediment deposits of axisymmetric turbidity currents
Authors: Dade W. Brian , Huppert Herbert E.
Published: February 2004

We develop a model that describes the runout behavior and resulting deposit of a radially spreading, suspension‐driven gravity current on a surface of negligible slope. Our analysis considers the separate cases of constant‐volume and constant‐flux sources. It incorporates expressions for the conservation of volume, a Froude number condition at the current front, and the evolution of the driving suspension due to settling of particles to the underlying bed. The model captures the key features of a range of experimental observations. The analysis also provides important scaling relationships between the geometry of a deposit and the source conditions for the deposit‐forming flow, as well as explicit expressions for flow speed and deposit thickness as functions of radial distance from the source. Among the results of our study we find that, in the absence of information regarding flow history, the geometries of relatively well‐sorted deposits generated by flows with source conditions of constant volume or constant flux are virtually indistinguishable. The results of our analysis can be used by geologists in the interpretation of some geologically important gravity‐surge deposits. Using our analytical results, we consider three previously studied, radially symmetric turbidites of the Hispaniola‐Caicos basin in the western Atlantic Ocean. From gross geometry and grain size of the turbidites alone we estimate for the respective deposit‐forming events that upon entry into the basin the initial sediment concentrations were approximately 3% by volume and the total volumes were roughly between 30 km3 and 100 km3. Each of the suspension‐driven flows is inferred to have spread into the basin with a characteristic speed of 3–5 m s−1, and reached its ultimate runout length of about 60–75 km while laying down a deposit over a period of about 10–12 hours.
A comparison of coincidental time series of the ocean surface height by satellite altimeter, mooring, and inverted echo sounder
Authors: Katz Eli Joel , Busalacchi Antonio , Bushnell Mark , Gonzalez Frank , Gourdeau Lionel , McPhaden Michael , Picaut Joel
Published: February 2004

Altimetric measurements of sea surface height at two locations in the western tropical Pacific Ocean are compared to estimates of the dynamic sea surface height computed from cotemporal surface‐to‐bottom temperature/salinity measurements on moorings and acoustic travel time measured by bottom‐moored inverted echo sounders. The results show statistically high correlation between the in situ measurements at periods greater than 5 days and between the altimeter and in situ measurements at periods greater than 20 days. The rms difference between any two modes of observation is consistently between 2 and 3 cm.
Large‐scale circulation and its variability in the south Indian Ocean from TOPEX/POSEIDON altimetry
Authors: Park Young‐Hyang , Gambéroni Lucien
Published: February 2004

The first 18 months of altimeter data from the TOPEX/POSEIDON (T/P) mission were analyzed to study the large‐scale mean surface circulation and its variability in the south Indian Ocean. A T/P sea level time series corrected for ocean tides using the Texas model (Eanes, 1994) was first validated against in situ sea level measurements from a bottom pressure recorder near Amsterdam Island. T/P data are a little contaminated by a 60‐day tidal aliasing effect, very near the alias periods of M2 and S2 constituents. When this 60‐day effect is removed, the data agree well with the bottom pressure‐derived sea level within 2.0 cm rms difference. This indicates that only low‐frequency ocean signals of periods greater than 60 days can be safely studied from T/P data. A T/P‐derived mean dynamic topography relative to the Ohio State University 91A geoid (Rapp et al., 1991) was compared to the output of the fine resolution Antarctic model (Webb et al., 1991) and to historical hydrographic data. The altimetric solution shows excellent agreement with the numerical model solution and indicates an anticyclonic subtropical gyre north of the Antarctic Circumpolar Current and two cyclonic subpolar gyres south of this current, respectively, west and east of the Kerguelen Plateau. The western subpolar gyre corresponding to the eastern boundary of the Weddell Gyre appears to extend as far east as 60°E in the Weddell‐Enderby Basin. However, the hydrography‐derived dynamic topography does not reveal any clear evidence of these gyre features because of its inability to detect the barotropic component of currents. Finally, the T/P‐derived sea level variability reveals clearly the well‐known, strong current‐topography interaction in the Southern Ocean. There appear no significant interannual variations of the mean position and magnitude of the major current systems in the south Indian Ocean.
Numerical experiments on the effects of a meridional ridge on the transmission of energy by barotropic Rossby waves
Authors: Matano R. P.
Published: February 2004

Analytical and numerical models are used to study the effects of a meridional ridge on the propagation of barotropic Rossby waves produced by distant wind stress forcing. The analytical model illustrates the qualitative aspects of the problem by solving a simplified form of the potential vorticity equation. The analytical results are complemented by numerical experiments using a shallow‐water model. In these experiments, waves are excited near the eastern boundary of the model. The effect that the presence of a meridional ridge has on the propagation of these waves is evaluated using spectral analysis. The numerical experiments show that the ridge acts as a low‐pass filter for westward propagating waves. In a 4000‐m deep ocean a ridge 500 km wide and 500 m high has little effect in preventing energy transmission by barotropic waves. The topographic effect increases sharply with the ridge's height so that a 2000‐m ridge almost isolates two neighboring subbasins. The numerical results indicate that for the range of geophysically relevant cases the ridge's width plays only a minor role.
Seasonal and interannual changes in the North Atlantic subpolar gyre from Geosat and TOPEX/POSEIDON altimetry
Authors: White Margaret A. , Heywood Karen J.
Published: February 2004

Sea surface slopes from the altimetric satellites Geosat and TOPEX/POSEIDON are used to calculate eddy kinetic energy of the North Atlantic subpolar gyre (40–65°N, 60°–5°W). Using two years of data from each satellite (December 1986 to December 1988 and October 1992 to September 1994, respectively), interannual differences in the North Atlantic Current (NAC) are revealed. In regions of strong currents the eddies are driven by baroclinic instability of the mean flow, are not seasonally varying, and may therefore be used as a surrogate for the mean flow itself. Wind stress curl fields for the same periods show that the northward and southward shifts in the current branches across the Mid‐Atlantic Ridge are related to interannual differences in the winter wind stress curl pattern when the zero in wind stress curl is well defined and wind stress is at a maximum. Outside the NAC, the eddies are driven primarily by wind stress, indicated by a significant seasonality. Time series of eddy kinetic energy and wind stress are generated for the 4 years, and the magnitude, phase, and significance of annual and semiannual signals are determined. Interannual changes in the timing of the maximum eddy kinetic energy are associated with that of the maximum wind stress, with each being shifted by a month or two later or earlier. Throughout much of the northern North Atlantic a significant annual signal in eddy kinetic energy is observed, lagging the wind stress by, on average, 6 weeks. A negligible lag is found in the region of the East Greenland Current and Irminger Sea, whereas the greatest lag, about 2 months, is found in the Labrador Sea.
Satellite altimeter derived geostrophic currents in the western tropical Pacific during 1992–1993 and their validation with drifting buoy trajectories
Authors: Yu Yunyue , Emery William J. , Leben Robert R.
Published: February 2004

TOPEX/POSEIDON satellite altimeter measurements from the western tropical Pacific (15°S–20°N, 125°E–165°W) are used to make monthly sea surface topography maps for the period October 1992 to October 1993. The altimeter data were processed using standard corrections to estimate along‐track sea surface height anomalies which were interpolated to monthly 1/10° resolution maps. To convert these surface height anomalies to absolute sea surface height, a 0/1000‐dbar mean seasonal dynamic height climatology was computed from the same input data used by Levitus (1982) to compute his dynamic height climatology, with less spatial smoothing. By adding the appropriate monthly altimeter heights to these seasonal means, we produce 12 approximately monthly sea surface topography maps from 36 cycles of altimeter data. The corresponding monthly geostrophic surface currents were compared with the tracks of nearly coincident satellite‐tracked drifting buoys and were found to agree well in most cases. The velocity components (zonal and meridional) were plotted separately with respect to the velocities estimated from the buoy positions, and correlations of these two data sets are very high. The seasonal variability of the surface geostrophic currents in this region is depicted by the monthly maps of sea surface height.
Empirical ocean tide models estimated from TOPEX/POSEIDON altimetry
Authors: Desai Shailen D. , Wahr John M.
Published: February 2004

Three empirical ocean tide models are determined from repeat cycles 10 to 78 of the TOPEX/POSEIDON (T/P) altimeter mission. The three models investigate the effects of the satellite orbit ephemeris on the ocean tides determined from T/P altimetry and the effect of extracting the free core nutation resonance in the definition of the diurnal ocean tide admittance. The altimetric data series use the Joint Gravity Model JGM‐2 geopotential orbit ephemeris and the preliminary JGM‐3 orbit ephemeris computed at the University of Texas, Center for Space Research. The altimetric data from the T/P mission are shown to have the precision necessary to estimate the monthly and fortnightly ocean tides in each bin. Inclusion of existing models of the ocean tides in the polar latitudes not sampled by the altimeter demonstrates the importance of these latitudes on spherical harmonic representations of the ocean tides. The ocean tides are first estimated in bins of size 2.834° in longitude by 1° in latitude and then smoothed to 1° by 1° grids within ±66° latitude. The orthotide response formalism of Groves and Reynolds (1975) is used to represent the diurnal and semidiurnal ocean tides, while a constant admittance is assumed across narrow bandwidths around each of the monthly (Mm), fortnightly (Mf), and termensual (Mt) tidal components. Comparisons of the T/P ocean tide models to tide gauge observations indicate their accuracies to be of the order of 2–3 cm. The T/P‐derived ocean tide models remove approximately 20 cm2 more of the T/P measured sea surface variance than the Cartwright and Ray (1991) tide model and show a 19 cm2 and 15 cm2 improvement over the Schwiderski (1980a, b) and Cartwright and Ray (1991) tide models, respectively, when compared to tide gauge estimates of the ocean tides.