bluuebunny/crossref_metadata_2025 huggingface dataset:
Modeled acoustic propagation through an ice edge eddy in the East Greenland Sea Marginal Ice Zone
Authors: Mellberg Leonard E. , Johannessen Ola M. , Connors Donald N. , Botseas George , Browning David
Published: February 2008

A detailed modeling study of the relationship between the various oceanographic features of a cyclonic eddy located at the ice edge off the coast of northeastern Greenland and the eddy's effects on the propagation of acoustic energy is presented. The effects of the eddy on acoustic propagation as a function of location, depth, and frequency of the acoustic source and depth of the receiver for nominal frequencies of 50 Hz and 1 kHz are discussed. Significant differences sometimes greater than 20 dB in the acoustic field were observed for variations in any of the source and receiver parameters. The sound speed structure of the eddy makes propagation loss and acoustic modes very dependent on direction and location. A significant finding is that the interior environment of the eddy can generate a frequency dependence on propagation loss so that 1 kHz sustains a loss as much as 20 dB smaller than does 50 Hz, i.e., dispersion phenomenon. A similar propagation loss anomaly was observed and appears to be due to a combination of marginal ice zone (MIZ) fine structure and interference patterns created by reflections from the sea surface. Such an anomaly can exist in the MIZ with or without the eddy environment. Both of these frequency anomalies are a function of source and receiver depth. Low‐frequency (50 Hz) energy in the presence of the eddy can experience stronger downward refraction and thereby possibly suffer greater bottom interaction than high frequencies (1 kHz). As compared to the noneddy environment, propagation in the presence of the eddy, for any of the source and receiver depths studied, can result in changes in the levels and distribution of energy in the acoustic field by more than 10 dB as well as changes in the mode of propagation of some rays (e.g., refracted/surface‐reflected to refracted/refracted).
Northward flow in the Bering and Chukchi seas
Authors: Overland J. E. , Roach A. T.
Published: February 2008

The ocean circulation on the Bering and Chukchi sea shelves is investigated using a barotropic numerical model. In the presence of the winter seasonal wind stress from the northeast, observed northward transport through Bering Strait of 0.6 Sv for the 8‐month winter season 1981–1982 can be driven by a sea level difference of 0.4 m between the Pacific and Arctic oceans. This sea level difference between basins is in agreement with hydrographic estimates. In the absence of wind stress a 0.4‐m sea level difference drives a northward transport through Bering Strait of 1.1 Sv. Maximum transport through Bering Strait is geostrophically limited, not frictionally or inertially limited. Major qualitative circulation features of the region are evident in the simulations. Westward intensification of the northward transport across the Bering Sea shelf is confirmed, with the major transport occurring in the relatively deep Gulf of Anadyr. In the absence of wind, Anadyr Strait contributes 72% of the northward transport through Bering Strait. With northeasterly winds there is a net clockwise circulation around Saint Lawrence Island. Within the 20‐m isobath, Norton and Kotzebue sounds are little influenced by the general circulation. Northward transport in the Chukchi Sea bifurcates in the vicinity of Point Hope. The predominant branch is directed toward the northwest in a broad canyon feature south of Herald Shoal, and the balance becomes the Alaskan Coastal Current between Cape Lisburne and Point Barrow.
Validation of Geosat altimeter‐derived wind speeds and significant wave heights using buoy data
Authors: Dobson Ella , Monaldo Frank , Goldhirsh Julius , Wilkerson John
Published: February 2008

This paper presents a comparison of Geosat radar altimeter‐derived wind speeds and significant wave heights with those measured by buoys in the National Data Buoy Center (NDBC) network. Measurements from a subset of 43 buoys moored in coastal regions and deep ocean, including the North Pacific, North Atlantic, and Gulf of Mexico, were examined. Data from other buoys within the NDBC network were not included because of their proximity to land. Seven months of altimeter comparisons were obtained during the Geosat geodetic mission for the periods May–August and October–December 1985. Only Geosat data within 150 km of buoy locations were accessed; this resulted in 1166 wind speed and significant wave height pairs. An error analysis was performed to better understand the differences between altimeter and buoy‐derived results and to establish consistency between the two sets of measurements. The following error sources were identified and quantified in terms of their impact on wind speed and significant wave height uncertainties: (1) inaccuracies due to buoy instrumentation, (2) errors resulting from temporal and spatial separations, (3) errors arising from averaging of the buoy data in time and averaging of altimeter data over its footprint, and (4) altimeter instrumentation errors. Four algorithms relating altimeter radar cross section to ocean surface wind speed were investigated. The Brown algorithm (Brown et al., 1981), and the smoothed form of the algorithm of Brown et al. (Goldhirsh and Dobson, 1985) were found to give the smallest rms errors. These were 1.7 m/s and less for a maximum altimeter track‐buoy separation of 50 km for most cases considered. This uncertainty agreed with the error obtained from the error analysis. Significant wave heights measured by the altimeter were found to be in agreement with buoy‐derived values to within 0.5 m rms. This value was also found to be in agreement with that derived from the error analysis. The measurement goals for Geosat were 1.8 m/s rms for wind speeds from 1 m/s to 18 m/s and 0.5 m rms for significant wave height or 10%, whichever is greater (Frain et al., 1985). These goals were met for the range of wind speeds compared.
Vertical heat fluxes through the Beaufort Sea thermohaline staircase
Authors: Padman Laurie , Dillon Thomas M.
Published: February 2008

Microstructure profiles of temperature, conductivity, and velocity shear during the Arctic Internal Wave Experiment (AIWEX) in March–April 1985 in the Beaufort Sea are used to investigate the thermodynamic processes in a diffusive thermohaline staircase. The staircase occurs between depths of about 320 and 430 m, above the core of the relatively warm, salty Atlantic water, where the mean temperature and salinity are increasing with depth. Individual isothermal layers can be tracked for at least several hours, suggesting a horizontal length scale of several hundred meters or more, assuming a typical relative velocity of 0.01 m s−1 at this time. Over the depth range 320–430 m the mean (average over several steps) density ratio 〈Rρ〉 = β〈Sz〉/α〈Tz〉 varies between 4 and 6, while the typical temperature difference between layers decreases from 0.012° to 0.004°C. The mean thickness of the layers also varies, from 1 m at 320 m depth to 2 m at 430 m. The relationship proposed by Kelley (1984), relating layer height to 〈N2〉, 〈Rρ〉, and molecular properties of the fluid, overestimates the mean layer thickness by about a factor of 2. The variability of staircase characteristics suggests that oceanic staircases may rarely, if ever, be steady state, but in general be slowly evolving from previous perturbations. Heat fluxes estimated from laboratory‐based flux laws, involving Rρ and ΔT, are in the range 0.02 <FH < 0.1 W m−2, which is in agreement with the molecular heat fluxes through the maximum interfacial temperature gradients. There are no interfaces where the kinetic energy dissipation rate (averaged over 0.5 m) exceeds the lower limit for diapycnal mixing, 24.5νN2.
Near‐bottom cross‐shelf heat flux along central Florida's Atlantic shelf break: Winter months
Authors: Smith Ned P.
Published: February 2008

Near‐bottom current and temperature data from a 154‐day period in winter 1984–1985 are combined to investigate cross‐shelf heat fluxes along central Florida's Atlantic coast between 27° and 30°N. Temperature and current time series recorded at the shelf break exhibit considerable variability over time scales of the order of 1–2 weeks. Low‐frequency temperature fluctuations, superimposed onto a seasonal cooling of approximately 0.4°C/week, decrease from south to north. Cross‐shelf motion results in little net displacement at latitudes 27° and 30°N but results in considerable net shoreward displacement at latitude 28°N. Cross‐shelf heat flux is shoreward at all study sites, but values decrease approximately linearly from south to north. The study shows that a shoreward flux of relatively cool water can continue through winter months in the vicinity of Cape Canaveral. Compared with summer up welling in the same area, the characteristic time scale is considerably shorter because the heat flux appears to be related to frontal eddies embedded in the cyclonic shear zone of the Florida Current rather than to seasonal variations in volume transport combined with a westward shift of the axis of the Florida Current.
Subtidal volume exchange through the Chesapeake and Delaware Canal
Authors: Wong Kuo‐Chuin
Published: February 2008

The Chesapeake and Delaware Canal is a man‐made waterway connecting the head of the Chesapeake Bay and the upper reaches of the Delaware estuary. We found significant subtidal variability along the canal during a 1‐month survey in 1975. There were significant differences between subtidal sea level fluctuations at the two ends of the canal, reflecting the different response characteristics of the two adjoining estuaries to atmospheric forcing. The resulting hydraulic head along the canal forced energetic subtidal volume flux through the canal which was of the same order of magnitude as the Delaware River discharge during the spring freshet period.
Effects of the 1984 El Niño on the summer phytoplankton of a Baja California upwelling zone
Authors: Torres‐Moye Guillermo , Alvarez‐Borrego Saúl
Published: February 2008

During the summer of 1984, nannoplankton dominated the phytoplankton assemblages of the coastal upwelling zone off San Quintin bay (30°24′N, 116°00′W), Baja California, Mexico. Upwelling was masked by the warming of surface and near‐surface waters as a product of El Niño influence. Diatom and dinoflagellate abundances were between 1 and 2 orders of magnitude lower than those reported for the summers of 1977 and 1979. A 21‐day time series shows that assimilation numbers of surface phytoplankton had a range of 2.0 to 5.4 mg C mg chlorophyll a−1 h−1. These values are lower than those reported for coastal waters of the California Current for 1969, 1970, 1977, and 1979 and are similar to values reported for 1983. In 1984, surface chlorophyll a concentrations and primary productivity were an order of magnitude lower than values reported for 1977 and 1979.
An alluvial record of El Niño events from northern coastal Peru
Authors: Wells Lisa E.
Published: February 2008

Overbank flood deposits of northern coastal Peru provide the potential for the development of a late Quaternary chronology of El Niño events. Alluvial deposits from the 1982–1983 El Niño event are the basis for establishing a type El Niño deposit. Sedimentary structures suggesting depositional processes range from sheet flows to debris flows, with sheet flood deposits being the most common. The 1982–1983 deposits are characterized by a 50‐ to 100‐cm‐ thick basal gravel, overlain by a 10‐ to 100‐cm‐thick sand bed, grading into a 1‐ to 10‐cm‐thick silty sand bed and capped by a very thin layer of silt or clay. The surface of the deposit commonly displays the original shear flow lines crosscut by postdepositional mud cracks and footprints (human and animal). Stacked sequences of flood deposits are present in Pleistocene and Holocene alluvial fill, suggesting that El Niño type events likely occurred throughout the late Quaternary. A relative chronology of the deposits is developed based on terrace and soil stratigraphy and on the degree of preservation of surficial features. A minimum of 15 El Niño events occurred during the Holocene; a minimum of 21 events occurred during the late Pleistocene. Timing of the Holocene events is bracketed by isochrons derived from the archaeologic stratigraphy. Corrected radiocarbon ages from included detrital wood provide the following absolute dates for El Niño events: 1720 ± 60 A.D., 1460 ± 20 A.D., 1380 ± 140 A.D. (error overlaps with the A.D. 1460 event; these may represent a single event), and 1230 ± 60 B.C.
Photographic measurements of bubble populations from breaking wind waves at sea
Authors: Walsh A. L. , Mulhearn P. J.
Published: February 2008

A photographic technique was used to measure bubble size spectra at depths of 0.5 – 2.0 m in wind speeds of 2–14 m s−1. Sampled bubble populations were found to be highly variable with time. The bubble density was found to be a strong function of wind speed, W, varying as W3.3 and few bubbles were found below 6 m s−1. For wind speeds between 6 m s−1 and 14 m s−1 the total air volume entrained by bubbles increased as W4.9. The state of development of the wave height spectrum also appeared to influence the bubble density. Bubble size spectra varied with radius, r, on average, as r−4. The more limited oceanic results of previous workers are compatible with these findings.
Bottom currents and fluviomarine sedimentation on the Mississippi Prodelta Shelf: February‐May 1984
Authors: Adams Charles E. , Swift Donald J. P. , Coleman James M.
Published: February 2008

Surface winds and currents, temperature, and water transparency at heights of 1 and 5 m above bottom in 60 m of water were measured during the period from February 23 to May 2, 1984, at a site on the Mississippi delta. The objective was to better understand the influence of near‐bottom flow on the character of sedimentation on the prodelta shelf. Data analysis showed currents to be parabathic, with periodicities of 5 and 20 days. Flow showed a strong east‐west polarization, with net flow 0.4 cm/s onshore at the lower level and 0.5 cm/s offshore at the upper. With easterly flow, water temperature declined (indicating upwelling), and current veering between 1 and 5 m was 12°–15° in the sense of a bottom Ekman layer. With westerly flow, water temperature increased (indicating downwelling), and veering was of the same magnitude but in the sense of a surface Ekman layer. Flow and suspended sediment data suggest that during the period of study, deposition of sediment from the turbid plume of the Mississippi River was continuous. Calculations of sediment‐settling velocities indicate that the surface plume of turbid river water transported, in suspension, particles with hydraulic equivalent diameters ranging from clay to fine‐sand size. Computations of friction velocity indicate that here on the outer prodelta shelf, the observed bottom currents were not competent to erode bottom sediment. Short‐period (1–3 days) fluctuations in mean bottom turbidity may be due to variations in the supply of suspended sediment as the surface plume moves laterally in response to the wind and, in at least some cases, to a vertical redistribution of sediment in suspension by currents too weak to initiate resuspension. These fluctuations are probably responsible for the silty and sand laminae (generally a millimeter thick or less) observed in bottom cores. The thickness of the annual flood deposit may approach 1 m in the vicinity of the plume.
Anchor ice, seabed freezing, and sediment dynamics in shallow Arctic Seas
Authors: Reimnitz Erk , Kempema E. W. , Barnes P. W.
Published: February 2008

Diving investigations confirm previous circumstantial evidence of seafloor freezing and anchor ice accretion during freeze‐up storms in the Alaskan Beaufort Sea. These related bottom types were found to be continuous from shore to 2‐m depth and spotty to 4.5‐m depth. Spotty anchor ice occurred as pillow‐shaped crystal aggregates on buried slabs of frozen sand surrounded by unfrozen sand. Considerations of required conditions for ice bonding and anchor ice growth allows regional extrapolation and suggests the possibility of anchor ice growth out to 20‐m depth, the estimated maximum depth of supercooling during fall storms. Anchor ice and seabed freezing apparently do not develop during a calm freeze‐up. Because of the abrupt growth of anchor ice during a freezing storm and its release soon after formation of a surface ice cover, this ice type has not been documented before. The concretelike nature of frozen bottom, where present, should prohibit sediment transport by any conceivable wave or current regime during the freezing storm. But elsewhere, particularly where the bonded crust is broken by grounded ice, anchor ice lifts coarse material off the bottom and incorporates it into the ice canopy, thereby leading to significant ice rafting of shallow shelf sediment and likely sediment loss to the deep sea.
A relationship between heat transfer to sea ice and temperature‐salinity properties of Arctic ocean waters
Authors: Moore R. M. , Wallace D. W. R.
Published: February 2008

It is demonstrated that the characteristic temperature‐salinity relationship shown by thermocline waters of the Arctic Ocean can be reproduced using a simple model based on transfer of heat directly from these waters to sea ice. From this we deduce that there is direct sensible heat flux from waters of Atlantic origin to the ice cover, such interaction could play a major role in determining the physical properties of Arctic water masses.
Tracking fronts in solutions of the shallow‐water equations
Authors: Bennett Andrew F. , Cummins Patrick F.
Published: February 2008

A front‐tracking algorithm of Chern et al. (1986) is tested on the shallow‐water equations, using the Parrett and Cullen (1984) and Williams and Hori (1970) initial state, consisting of smooth finite amplitude waves depending on one space dimension alone. At high resolution the solution is almost indistinguishable from that obtained with the Glimm algorithm. The latter is known to converge to the true frontal solution, but is 20 times less efficient at the same resolution. The solutions obtained using the front‐tracking algorithm at 8 times coarser resolution are quite acceptable, indicating a very substantial gain in efficiency, which encourages application in realistic ocean models possessing two or three space dimensions.
Winter observations of iceberg frequencies and sizes in the south Atlantic Ocean
Authors: Wadhams Peter
Published: February 2008

The numbers and apparent sizes of icebergs in the South Atlantic Ocean in midwinter were measured by radar and visually from F. S. Polarstern during the 1986 Winter Weddell Sea Project cruise. Results show that in a heavy sea (sea state 7–8), icebergs have to be at least 115 m in diameter to be detected at all and that detectability falls off severely for all bergs at ranges exceeding 8 n. mi. (15 km); that most bergs had diameters of less than 1 km with a preferred size of 400–500 m; and that a high density of icebergs in the latitude band 53°–56°S at longitude 19°–30°W contrasted with a virtual absence of bergs in the same latitude band at longitude 1°–9°E. The latter effect is ascribed to melt and wave–induced deterioration causing the disappearance of this iceberg population between the two sets of longitudes.
The drift of icebergs under wind action
Authors: Crépon M. , Houssais M. N. , Guily B. Saint
Published: February 2008

The steady motion of an iceberg under wind action is studied in the presence of a mixed layer. Analytical solutions are obtained when the iceberg depth is less than or much larger than the mixed layer depth. A few numerical examples are given. Our results show that winds of medium or low strength have a limited effect on the motion of deep icebergs extending below the thermocline. Furthermore, the larger the horizontal scale of the iceberg the smaller the wind drift. Hence for winds of less than 10 m s−1the trajectory of deep icebergs more than 1 km long is shown to be representative of geostrophic currents. The effect of a possible “lift” force is also estimated.
Fourier transform filtering: A cautionary note
Authors: Forbes A. M. G.
Published: February 2008

Many different filters exist to remove unwanted signals from time series of ocean currents and sea levels, and in this paper we examine two: a tidal harmonic “filter” and a discrete Fourier transform (DFT) filter. DFT filters are particularly easy to use if working in the frequency domain, but as with all filters it is necessary to understand the effects of a DFT filter before it can be used with complete confidence. A number of sample time series are used, some artificial and some real, to test DFT filters. It is shown that “ringing” in the retransformed time series can be minimized by careful choice of the filter bandwidth and the amount of tapering of the sides of the filter.
On the sources, formation, and circulation of Red Sea deep water
Authors: Cember Richard P.
Published: February 2008

Red Sea Deep Water is renewed from sources in the far north of the Red Sea. Upwelling of deep water throughout the basin maintains the sharp pycnocline with its base near the depth of the sill at Bab el Mandeb. Before now, deep water renewal was thought to occur mainly by means of convective deep water formation in the north which renews the bottom water and gives rise to a southward‐flowing bottom current. Carbon‐14 and 3He data from the GEOSECS Indian Ocean Expedition lead to the identification of an additional, volumetrically larger, mode of northern deep water formation in which new deep water is injected immediately beneath the pycnocline. This mode of deep water formation (to be called the isopycnal mode) also gives rise to a southward‐flowing current, in this case at the top of the deep water mass. GEOSECS data also demonstrate the existence at intermediate depths in the deep water of a return flow to the north. The surface history in the nuclear era of the Δ14C in the deep water source region was reconstructed by direct radiocarbon measurements on annual growth bands of corals. Calculation then shows that new deep water forms in the convective mode at a rate of 0.05±0.01 Sverdrup (Sv) (expressed as if the flow were continuous) and is due principally to processes occurring in the mouth of the Gulf of Suez. The injection beneath the pycnocline of new deep water formed in the isopycnal mode is 0.11±0.02 Sv. This new deep water appears to originate mainly in the extreme open northern Red Sea. The Gulf of Aqaba makes a modest but significant contribution to both modes. The total deep water renewal rate is about 0.16 Sv, yielding a bulk deep water residence time of about 36 years. Oxygen consumption in the deep water shows approximately exponential decrease of consumption rate with depth. As expected on geological and geophysical grounds, hydrographically significant fluxes of heat and salt from the sea floor into the Red Sea deep water are discernible in the hydrography. Helium‐3 flux from the sea floor into the deep water is about 69 1 STP/year.
The separation of the Yucatan Current from the Campeche Bank and the intrusion of the Loop Current into the Gulf of Mexico
Authors: Molinari Robert L. , Morrison John
Published: February 2008

Data collected in the eastern Gulf of Mexico during 1974, 1975, and 1976 show that the penetration of the Loop Current into the gulf is strongly correlated with the location of the Yucatan Current on the Campeche Bank. The Loop does not penetrate far into the gulf when the Yucatan Current separates from the bank in the vicinity of the Catoche Tongue (i.e., the eastern Campeche Bank). Deep Loop penetrations are correlated with separations farther west on the bank. The angle of the Yucatan Current at separation is also correlated with the location of separation (i.e., smaller angles relative to due east are correlated with separations from farther east on the bank). Thus small angles at separation are correlated with shallow intrusions of the Loop. Historical temperature data collected in the eastern gulf are reviewed and support these correlations. Simple conservation of potential vorticity considerations can explain the correlation between the angle at separation and the penetration of the Loop.
Introduction
Authors: Hughes B. A. , Gasparovic R. F.
Published: February 2008

The synthetic aperture radar (SAR) flown on the Seasat satellite in 1978 yielded a rich collection of radar imagery exhibiting surface expressions from a variety of oceanic phenomena. These images have stimulated substantial research efforts to develop a quantitative understanding of the process by which surface waves, bathymetric features, internal waves, fronts, eddies, and ship wakes modify microwave energy back‐scattered from the sea surface and manifest their presence as intensity variations in SAR images.

Conceptually, the process can be subdivided into three generic problem areas: (1) hydrodynamics, which concentrates on describing how the phenomenon of interest alters the surface wave roughness characteristics; (2) microwave backscattering, which addresses the manner in which surface roughness variations are encoded in electromagnetic energy reflected from the sea surface; and (3) the SAR image formation process, which accounts for the effects of imaging geometry and scatterer motion on the coherence properties of the backscattered radiation.
Comparison of Joint Canada‐U.S. Ocean Wave Investigation Project synthetic aperture radar data with internal wave observations and modeling results
Authors: Shuchman R. A. , Lyzenga D. R. , Lake B. M. , Hughes B. A. , Gasparovic R. F. , Kasischke E. S.
Published: February 2008

Comparisons are made of L band and X band SAR images with in situ measurements of surface currents and surface wave spectral perturbations for two of the Joint Canada‐U.S. Ocean Wave Investigation Project internal wave data sets. Predicted and observed perturbations show agreement to within approximately a factor of 2 for L band, while the agreement at X band is much less satisfactory. Comparison of predicted microwave backscatter at X band with near‐surface radar measurements indicates large discrepancies even when surface tilt and specular reflection effects are included in the model. Synthetic aperture radar (SAR) X band modulations compare favorably with near‐surface radar measurements for range‐traveling internal waves, but for other directions, the SAR modulations are smaller than the near‐surface radar measurements. Simultaneous X and L band SAR images show comparable internal wave modulations.
Contrast ratios of internal waves in synthetic aperture radar imagery: A comparison of SAR Internal Wave Signature Experiment observations with theory
Authors: Kasischke Eric S. , Lyzenga David R. , Shuchman Robert A. , Wackerman Christopher C.
Published: February 2008

Techniques were developed to extract radar scattering coefficient (σ0) signatures associated with the internal wave patterns observed in the SAR Internal Wave Signature Experiment (SARSEX) data set. The SARSEX experiment was conducted in the New York Bight region of the Atlantic Ocean in August‐September 1984. A theoretical model was exercised which uses environmental conditions and radar imaging parameters to calculate the changes in the synthetic aperture radar (SAR) image intensity due to internal waves. For conditions present during SARSEX, this model predicts that the modulations should be higher at L band than at X band, the modulations should decrease at both frequencies as incidence angle increases, range‐traveling waves should have higher modulations than azimuth‐traveling waves, and azimuth‐traveling internal waves should have broader signatures than range‐traveling waves. The analysis of the SAR‐observed internal wave patterns showed that the X band internal wave modulations were comparable in magnitude to those found at L band, the range‐traveling waves had higher modulations than azimuth‐traveling waves, and the expected decrease in modulation with increasing incidence angle was present at times for the range‐traveling cases but not present in the azimuth‐traveling cases. Finally, comparisons showed that the SAR‐observed modulations were still somewhat underpredicted by the theoretical model, especially for the azimuth‐traveling internal wave case.
An analytic representation of the synthetic aperture radar image spectrum for ocean waves
Authors: Lyzenga David R.
Published: February 2008

An analytic expression is derived for the spectrum of a synthetic aperture radar image in terms of the surface reflectivity covariance function. It is shown that within the severe constraints imposed by the short integration time, the third (temporal) dimension of the spectrum can be extracted by varying the processor focus parameter. The results also illustrate the dependence of the image spectrum on the scene coherence time, as well as various nonlinear effects associated with the velocity bunching mechanism. Example calculations are presented for two cases corresponding to data sets collected during the Tower Ocean Wave and Radar Dependence Experiment.
Ocean/atmosphere heat fluxes within the Agulhas Retroflection region
Authors: Walker Nan D. , Mey Rosemary D.
Published: February 2008

The southern section of the Agulhas western boundary current system exhibits unique characteristics as regards ocean/atmosphere heat flux processes. The Agulhas Retroflection region's high heat flux core from 37°S to 41°S, 16°E to 22°E does not demonstrate a distinct annual cycle of turbulent heat fluxes (latent and sensible) as is characteristic of its northern hemisphere counterparts. Rather, a weak semiannual heat flux cycle is found with maximum average losses during winter and summer (200 and 211 W/m2 ) and minimum losses during spring and autumn (185 and 162 W/m2 ). Upstream where the Agulhas Current is closer to land, winter heat losses exceed those of summer, but the differences are small. This behavior contrasts with that encountered at the poleward ends of northern hemisphere western boundary currents where winter heat fluxes are several times those of summer. The main reason for this difference is persistent westerly and southwesterly wind flow over the Agulhas Retroflection region throughout the year which ensures that cold, unsaturated maritime air repeatedly forces loss of heat from the ocean's surface. Spatial heat flux gradients associated with the Agulhas‐Subtropical Convergence surface temperature front are more pronounced in summer than in winter, indicating that cyclogenesis locally may be less seasonally dependent than in the northern hemisphere situation. Average oceanic cooling rates in the core region of the Retroflection, based on net heat flux calculations and a mixed surface layer of 75 m, range from 1.35°C/month during winter to 0.25°C/month during summer. Interannual variability in ocean/atmosphere heat fluxes within the Agulhas Retroflection region often exceeds the variability illustrated by the annual cycle. West of the Agulhas Retroflection core region, interannual sea surface temperature (SST) anomalies are more influential in the generation of heat flux anomalies by virtue of their large temporal variability. This high SST variability is primarily attributed to interannual changes in flux of Agulhas Current water into the southeast Atlantic Ocean. Oceanic heat loss within this warm water zone is an important modifying influence to both ocean and atmosphere, thus meriting further research.
Probability function of breaking‐limited surface elevation
Authors: Tung C. C. , Huang N. E. , Yuan Y. , Long S. R.
Published: February 2008

The effect of wave breaking on the probability function of surface elevation is examined. The surface elevation limited by wave breaking ζb(t) is first related to the original wave elevation ζ(t) and its second derivative ¨ζ(t). An approximate, second‐order, nonlinear, non‐Gaussian model for ζ(t) of arbitrary but moderate bandwidth is presented, and an expression for the probability density function ƒb( ) of ζb(t) is derived. The results show clearly that the effect of wave breaking on the probability density function of surface elevation is to introduce a secondary hump on the positive side of the probability density function, a phenomenon also observed in wind wave tank experiments.
Vertical fluxes of nitrate associated with salt fingers in the world's oceans
Authors: Hamilton James M. , Lewis Marlon R. , Ruddick Barry R.
Published: February 2008

The primary means by which the oceans serve as a sink for atmospheric carbon dioxide is through the vertical flux of sinking organic carbon. This organic material is derived from primary photosynthetic production in the upper ocean. Its rate of loss to the deep sea, the “new production”, is rigorously constrained by the upward flux of the limiting nutrient, nitrate. New production estimates have been made by using velocity microstructure measurements to infer nitrate fluxes, where it is assumed that turbulent production (mixing) balances viscous energy dissipation. However, estimates based on a salt finger convection model lead to nitrate fluxes which are as much as an order of magnitude larger than a turbulent mixing model predicts and agree much better with fluxes inferred from biological uptake measurements and tracer studies. Heat flux estimates are also higher, but by less than a factor of 3. Observations of the “scaled dissipation ratio” Γ, a quantity that is based on both velocity and temperature microstructure measurements, may provide a means of distinguishing between the two mixing hypotheses. Where both turbulent production and salt fingering contribute to the mixing this distinction is more difficult, but by using measurements of Γ the potential error in the vertical flux estimate due to an improper choice of mixing models is greatly reduced.
On the variability of Southern Ocean front locations between southern Brazil and the Antarctic Peninsula
Authors: Ikeda Yoshimine , Siedler Gerold , Zwierz Marek
Published: February 2008

A 4‐year expendable bathythermograph data set (1984–1987) from the area between southern Brazil and the Antarctic Peninsula provides information on the interannual variability of front locations. Two boundaries of subtropical water at different depths are identified north and south of the Brazil Current‐Falkland (Malvinas) Current confluence zone. The northern Subtropical Front is displaced over a large part of the Argentine Basin from one observational period to the other. The shallow southern Subtropical Front appears fixed to the Falkland Escarpment. The Polar Front and Subantarctic Front locations do not vary much, except for one case where a cold core eddy in the Polar Frontal Zone causes a large northward displacement of the Subantarctic Front.
On the origin of the Azores Current
Authors: Klein Birgit , Siedler Gerold
Published: February 2008

The Azores Current, south of the Azores Archipelago, is part of the subtropical North Atlantic gyre. Using an international hydrographic data set, we analyze mean and seasonal geostrophic transport fields in the upper 800 m of the ocean in order to determine the origin of the Azores Current in the western basin and seasonal changes in the related flow. Geostrophic currents are obtained by using the method applied by Stramma (1984) in the eastern basin. The Azores Current is found to originate in the area of the Southwest Newfoundland Rise (Figure 10). In winter an almost uniform current connects this region of origin with the Azores Current, while a branching into two current bands is observed in summer, with the southern band forming a marked cyclonic loop. Within the upper 800 m, all of the transport in the northern band and about 70% of the transport in the southern band recirculates in the eastern basin. Additionally, expendable bathythermograph data from the Azores Current region indicate an increase of eddy potential energy from winter to summer.
Sea surface topographic variability near the New England Seamounts: An intercomparison among in situ observations, numerical simulations, and Geosat altimetry from the regional energetics experiment
Authors: Hallock Zachariah R. , Mitchell Jim L. , Thompson J. Dana
Published: February 2008

Intercomparisons are made among three sets of results for two regions of the Gulf Stream, upstream and downstream of the New England Seamount Chain (NESC): observational records from 12 inverted echo sounders with pressure gauges (IES/PGs), concurrent Geosat altimeter crossover point differences, and results from a primitive equation model of the region. Standard deviations of sea surface topography show generally good agreement for the three data sources. No significant difference between the upstream and downstream regions is observed in the actual data. IES/PG records and Geosat topographic variability have values of about 30 cm on both sides of the NESC, although model results show a discrepancy in the eastern region. Barotropic fluctuations account for about 30% of the total surface topographic variability and are only partially correlated with baroclinic fluctuations.
Evolution of a warm‐core ring in the Gulf of Mexico: Lagrangian observations
Authors: Lewis James K. , Kirwan A. D. , Forristall George Z.
Published: February 2008

During 1985 and 1986, a Gulf of Mexico ring shed by the Loop Current was observed to migrate toward the western Gulf of Mexico. This movement across the gulf was well documented by observations that included drifter data within and outside the ring, sea surface temperature at weekly intervals, expendable bathythermograph surveys at various times, one major hydrographic cruise when the ring was in the northwestern gulf, and currents from moorings over which the ring passed. The drifter data were used to infer the movement of the ring center as well as the eccentricity and orientation of the major axes. The data from the drifters bridge the gaps between detailed surveys to the extent that a daily history of the position and shape of the ring can be constructed. The synthesis of these diverse but complimentary data sources provides a detailed description of how the ring interacted with the bathymetry of the northern Gulf of Mexico as well as with previously and subsequently shed rings.
Scaling turbulent dissipation in the thermocline
Authors: Gregg M. C.
Published: February 2008

By comparing observations from six diverse sites in the mid‐latitude thermocline, we find that, to within a factor of 2,
, where 〈εIW〉 is the average dissipation rate attributable to internal waves; N0 = 0.0052 s−1 is a reference buoyancy frequency; S10 is the observed shear having vertical wavelengths greater than 10 m; and SGM is the corresponding shear in the Garrett and Munk spectrum of internal waves. The functional form agrees with estimates by McComas and Müller and by Henyey, Wright, and Flatté of the rate of energy transfer within the internal wave spectrum, provided the energy density of the internal waves is treated as a variable instead of one of the constant parameters. Following Garrett and Munk, we assume that
, where EIW is the observed energy density and EGM is the energy density used by Garrett and Munk. The magnitude of εIW is twice that of Henyey et al. and one third that of McComas and Müller. Thus the observations agree with predictions sufficiently well to suggest that (1) a first‐order understanding of the link between internal waves and turbulence has been achieved, although Henyey et al. made some ad hoc assumptions and Garrett and Munk's model does not match important features in the internal wave spectrum reported by Pinkel, and (2) the simplest way to obtain average dissipation rates over large space and time scales is to measure
. Even though the observations were taken at latitudes of 42°−11.5°, the variability in the Coriolis parameter ƒ was too limited for a conclusive test of the ƒ dependence also predicted for 〈εIW〉 by the wave‐wave interaction models. An exception to the scaling occurs east of Barbados in the thermohaline staircase that is apparently formed and maintained by salt fingers. Although ε in the staircase is very low compared with rates at mid‐latitude sites, it is 8 times larger than predicted for ε due only to internal waves.
Observations of wind‐forced deep ocean currents in the North Pacific
Authors: Koblinsky C. J. , Niiler P. P. , Schmitz W. J.
Published: February 2008

This study examines the atmospheric response of the deep ocean using 200 instrument years of moored current measurements from across the breadth of the mid‐latitude North Pacific. We have found evidence of a seasonal modulation in the ocean eddy kinetic energy beneath the thermocline at several locations north of 35°N. This modulation is often in phase with the local atmospheric forcing function. At frequencies between 0.1 and 0.01 cpd, correlations between the local wind stress curl and the deep currents have been found. For a few locations the local forced response is parallel to the potential vorticity gradient (∇f/H) and consistent in amplitude with a local topographic Sverdrup balance. Nonlocal forcing in the form of flow along isolines of potential vorticity has been estimated, and it is comparable to the observed flow at some locations, but the modeled flow is not correlated with the observations. Throughout the mid‐latitude North Pacific the bottom slope tends to enhance the β effect. This suggests that topography narrows the available bandwidth for forced barotropic Rossby waves, facilitating a quasi‐steady topographic Sverdrup response over most of the basin. However, the enhanced β effect reduces the magnitude of the ocean's response to wind forcing. Nonlocally forced Sverdrup currents (flow along isolines of f/H) are the dominant component, but are only of the order of 1 cm s−1. Locally forced Sverdrup currents (flow across isolines of f/H) are typically of the order of 0.1 cm s−1 rms and are not commonly measured. Hence these wind‐forced variations are a background level signal in the mid‐latitude eddy kinetic energy. In a few isolated areas the bottom slope reduces the β effect. In these regions the local Sverdrup response is amplified and has been observed.
Hurricane‐generated currents on the outer continental shelf: 2. Model sensitivity studies
Authors: Cooper Cortis , Thompson J. Dana
Published: February 2008

A numerical model described and verified in part 1 of this two‐part series (Cooper and Thompson, this issue) is applied to study the sensitivity of hurricane‐generated currents on the outer shelf and slope. Numerical experiments are performed in a simple basin with a straight shelf. The sensitivity of the response to changes in storm parameters, direction of storm approach, and topography is quantified. Response is measured in terms of the mixed‐layer velocity and depth at sites along the storm track. Results reveal the most important factors are (in decreasing order) wind speed, storm translation speed, direction of storm approach, asymmetry in the wind field, entrainment parameterization, and advection at slower storm translation speeds. Response is largely insensitive (less than 10%) to radius of maximum wind, shelf and slope configuration, bottom friction, atmospheric pressure gradients, and further reductions in the model grid size. For a storm approaching cross shelf, the response is primarily baroclinic (greater than 90%) and only weakly dependent (less than 10%) on the water depth at the site.
Seasonal differences in the current and temperature variability over the northern California Shelf during the Coastal Ocean Dynamics Experiment
Authors: Lentz Steven J. , Chapman David C.
Published: February 2008

A long‐term moored array was maintained over the northern California continental shelf from April 1981 to April 1983 as part of the Coastal Ocean Dynamics Experiment to determine the seasonal differences in the current and water temperature characteristics. The array consisted of two midshelf moorings and one upper slope mooring from April 1981 through July 1982 and one midshelf mooring from August 1982 to April 1983. The two summers were characterized by southeastward mean wind stresses (upwelling favorable), cold water, isotherms sloping upward toward the coast, and a sheared equatorward mean shelf current. The intervening fall and winter were characterized by a weak mean wind stress, relatively warm shelf water, level isotherms, and a more barotropic poleward mean shelf current. In contrast to the differences in the characteristics of the means many of the characteristics of the current and temperature variability are the same for the summer and winter. The vertical structure of the largest current empirical orthogonal functions (EOFs) for each of the shelf moorings, which account for more than 75% of the subtidal variance, do not vary with season. The current is vertically sheared, oriented 15°–20°clockwise relative to local isobaths near the surface and along isobaths near the bottom. Currents at the two shelf moorings (alongshore separation 29–37 km) are correlated with each other and with the local wind stress, and these relationships do not exhibit a seasonal variation. Denser arrays of wind and current sensors during the two summers indicated that there were strong, persistent variations in the wind field over relatively small scales (tens of kilometers) with corresponding variations in the shelf current field during the summer of 1981 but not during the summer of 1982. Over the slope, the largest EOFs account for 56–72% of the current variance and have a vertical structure which is relatively uniform with depth during the summer, fall, and winter of 1981 but is strongly sheared in the upper 100 m during the summer of 1982. Current observations over the slope are not correlated with either the shelf currents or the local wind stress during any season. In contrast, the temperature variability, as described by EOFs, does exhibit a seasonal variation in vertical structure, with the variability concentrated in the upper 20–30 m in summer and more uniform with depth in winter. The near‐surface temperature variability over the slope is correlated with the shelf temperature variability. Estimates of the cross‐shelf heat flux due to the wind‐driven Ekman transport accounts for a large part of the shelf temperature variability. Observations from the midshelf mooring maintained through the fall and winter of 1982–1983 showed evidence of the 1982–1983 El Niño with persistent onshore and poleward currents and water temperatures typically above 12°C.
Remote sensing of ocean surface winds with the special sensor microwave/imager
Authors: Goodberlet M. A. , Swift C. T. , Wilkerson J. C.
Published: February 2008

As part of a subcontract with the manufacturer of the Defense Meteorological Space Program (DMSP) special sensor microwave/imager (SSM/I), an operational wind speed algorithm was developed by Environmental Research and Technology, Inc. (ERT). The ERT algorithm is based on the “D‐matrix” approach, which seeks a linear relationship between measured SSM/I brightness temperatures and environmental parameters. D‐matrix performance was validated by comparing algorithm derived wind speeds with near‐simultaneous and colocated measurements made by offshore ocean buoys maintained by the National Oceanic and Atmospheric Administration. The DMSP accuracy requirement of ±2 m/s for wind speed predictions in the range of 3 m/s to 25 m/s was not obtainable with the original version of the D‐matrix, which had severe bias and scaling problems. Revisions to the algorithm made at the University of Massachusetts caused it to perform within specifications. Other topics include error budget modeling, alternate wind speed algorithms, and D‐matrix performance with one or more inoperative SSM/I channels.