bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of rotation on vertical mixing and associated turbulence in stratified fluids
Authors: Kantha L. H. , Rosati A. , Galperin B.
Published: February 2008

Combined effects of stratification and rotation on vertical mixing and the characteristics of associated small‐scale turbulence are explored using second‐moment closure methodology; the rotational terms in the equations for Reynolds stresses and turbulent heat fluxes are retained, not ignored as in earlier works. Semianalytical results valid for arbitrary values of rotation and stratification are derived by further invoking the local equilibrium limit of closure. Two cases are considered: nonzero vertical rotation and nonzero meridional rotation; the latter case is of more general interest in geophysics because of its potential application to equatorial mixed layers. In both cases the influence of rotation on mixing coefficients and Monin‐Obukhov constant flux layer similarity relations is investigated for arbitrary values of rotation and stratification. In both cases, turbulent mixing coefficients assume tensorial properties. However, meridional rotation appears to have a stronger influence on vertical mixing and turbulence characteristics than does vertical rotation. These results, along with perturbation expansions for weak rotation, suggest that for geophysical flows, in most cases, the direct effect of rotation on vertical turbulent mixing itself is but a small correction, a few tens of percent at best. It is seldom large, although it might not be negligible in some particular cases. Nevertheless, the study of rotational effects on small‐scale turbulence provides a fascinating insight into the direct impact of rotation on the characteristics of small‐scale turbulence and mixing in stratified fluids; the results are also of interest in other fields such as engineering.
Atmospheric turbulence structure in the vicinity of an oceanic front
Authors: Singh Khalsa Siri Jodha , Greenhut Gary K.
Published: February 2008

Fast response data taken aboard the National Oceanic and Atmospheric Administration WP‐3D aircraft are used to determine the structure of atmospheric boundary layer turbulence on either side of a well‐developed sea surface temperature front southwest of Bermuda. The data were taken on February 17, 1986, as part of the Frontal Air‐Sea Interaction Experiment (FASINEX). A broad region of low‐humidity air extending from 15 km to 35 km south of the front is probably due to the presence of a frontally induced secondary circulation. Evidence for a secondary flow is found in both the time series of atmospheric variables and the statistics obtained from conditionally sampled updrafts and downdrafts in the transect across the SST front. Larger sea‐air temperature and humidity differences on the warm (south) side of the front give rise to surface layer sensible and latent heat and buoyancy fluxes that are larger than those on the cold side. Turbulence structure appears to be influenced as much by the presence of strong wind shear at the top of the boundary layer as by differing conditions at the surface on either side of the front. A larger rate of entrainment on the warm side of the front is indicated by the greater influence of low‐momentum air from the overlying shear layer on updrafts in the upper part of the mixed layer, as well as the more frequent overturning of cool/moist updrafts and warm/dry downdrafts. It is conjectured that the larger entrainment rate is due to the interaction between the inversion layer and more energetic updrafts produced by greater surface forcing on the warm side of the front.
The heat budget of the equatorial western Pacific surface mixed layer
Authors: Godfrey J. S. , Lindstrom E. J.
Published: February 2008

The turbulent heat flux through the equatorial western Pacific surface mixed layer is estimated, using conductivity‐temperature‐depth and Doppler profiler data obtained in January 1986 and the assumption that a useful approximation to the heat flux can be obtained with the Pacanowski‐Philander eddy diffusivity. Fluxes appeared to be an order of magnitude less along the equator than in the central Pacific because the only layer of very low Richardson number (and hence large eddy diffusivity) lay in the isothermal but salt‐stratified “barrier layer.” The top of the thermocline, just below the barrier layer, coincided with the core of the (subsurface) South Equatorial Current; hence current shears, eddy diffusivities, and heat fluxes were small in this region. Away from the equator, heat flux estimates were of the order of 3 W m−2. Use of the recent Peters et al. diffusivities reduces these estimates still further. In combination with previous analyses of heat advection in the equatorial western Pacific heat pool, our results indicate that the net surface heat flux entering the region is near zero. There must therefore be systematic errors in several recent estimates of this net heat flux in the equatorial western Pacific, which range between 20 and 80 W m−2.
Coastal wind in the transition from turbulence to mesoscale
Authors: Champagne‐Philippe Michèle
Published: February 2008

During the second survey of the Travaux d'Océanographie Spatiale: Capteurs actifs dans l'Atlantique Nord‐Est (TOSCANE T) experiment (February 14 to April 17, 1985), seven wind masts were operated on the shore of the “Baie d'Audierne.” Distances between them ranged from 1.5 to 13.7 km, and the data were sampled at 3 s. An important portion of the data was recorded under severe weather conditions. Results from 27 cases of wind blowing from the sea, which corresponded to synoptically stationary wind regimes, show that for both horizontal components the spectral energy in the transition region between mesoscale and Kolmogorov turbulence takes the shape of a well‐marked dip when weather types are stable or slightly unstable. But, in more convective cases the dip disappears and the transition region becomes almost horizontal; spectral energy density follows an n−1 law (where n is equal to frequency) until the Kolmogorov region is reached. Coherences and cross correlations between masts show that in the 6‐s to 1‐hour period range, only mesoscale fluctuations are coherent. Turbulent fluctuations are not correlated for the separation distances of the masts. Under synoptically steady or slightly unstable conditions, such single‐point measurements could reliably be time‐averaged for use in satellite wind sensor calibration. In more convective conditions, especially for the ubiquitous open mesoscale cells found over mid‐latitude oceans in cold air advections, interpretation problems might occur because mesoscale events, as time‐averaged from coastal masts, buoys, or ships, could be different from those spatially integrated in the footprint of a satellite sensor. In these cases, some relationship must be used to relate single‐point averaging times to the area illuminated by the satellite. To do so, Taylor's hypothesis is commonly extended to the mesoscale; but, the present data show that such an extension cannot be made under usual actual conditions because of the structure of the wind profile in the atmospheric layers concerned.
Tidally generated residual currents in a sea level canal or tidal strait with constant breadth and depth
Authors: Wong Kuo‐Chuin
Published: February 2008

This study extends the analysis of Ianniello (1977) in deriving the two‐dimensional (x‐z) analytical solutions for the tidally induced residual currents in narrow sea level canals connecting two large adjoining bodies of water. The sea level canal is forced by tides from both open ends, and residual Eulerian and Lagrangian currents are induced in the canal by nonlinear interactions of the first‐order tides. It was found that the residual currents are strongly influenced by the tidal amplitude and phase differences between the two open boundaries. Depending on the type of boundary tidal forcing, the residual Eulerian velocity 〈u2〉 may flow in either direction and produce two‐layer structures in the vertical. The Stokes velocity 〈us〉 may work either in concert or in opposition with 〈u2〉, so the residual Lagrangian velocity 〈ulag〉 may be greater than 〈u2〉. Given a particular set of boundary forcing conditions, the depth‐averaged residual Lagrangian velocity 〈ulag〉d is a nonzero constant, so residual mass transport between the adjoining water bodies through the canal is permitted. The most effective way of producing vigorous residual currents through the canal is for the tidal forcing at one open boundary to have a higher amplitude and a phase lead relative to the tidal forcing from the opposite end.
Properties of thermal staircases off the northeast coast of South America, spring and fall 1985
Authors: Boyd Janice D.
Published: February 2008

In spring and fall 1985, air expendable bathythermograph surveys of a large area off the northeast coast of South America revealed an extensive field of subthermocline thermal staircases which is probably the site of vigorous salt fingering. During both seasons the staircases were shallowest in the south (180–360 m) and deepest in the northwest (420–650 m), occurring on the average between the 8° and 13°C isotherms. Typically, each staircase consisted of 10 well‐mixed layers with average thickness of 16 m and average interlayer temperature change of 0.52°. Thicker layers seemed to be associated with larger temperature changes. No seasonal variation in these parameters was observed. The interfaces separating the layers had a modal thickness of 2 m, with interfaces in the spring tending to be thinner than in the fall. The region bounded by 10°–14°N, 52°–57°W tended to have staircases with more layers, thicker layers, and larger interlayer temperature changes. The sharp northeastern staircase field boundary occurs at the confluence of the Subtropical Underwater and the Antarctic Intermediate Water, suggesting that the staircases may be formed in this region and then extend south and west to form the rest of the field.
Measuring winds from underneath the ocean surface by upward looking acoustic Doppler current profilers
Authors: Schott Friedrich
Published: February 2008

Upward looking acoustic Doppler current profilers were deployed in the Gulf of Lions, in the western Mediterranean for winter convection studies during early 1987. The near‐surface Doppler data, which are dominated by sidelobe echo returns from the surface and which are therefore usually discarded for the analysis of subsurface current profiles, were found to contain information useful for determining surface winds. The horizontal Doppler directions were highly correlated with wind direction, measured by a research vessel in the vicinity. Wind speed, while not correlated with the size of the Doppler shifts, was significantly correlated with the acoustic backscatter energy. At 150 kHz frequency and 20° main beam angle the wind speed exponent was determined as 1.32; and at 75 kHz and approximately 24° sidelobe reception angle it was 0.76. At near‐vertical incidence, where more and more energy is scattered sideways with increasing wind speed, the exponent was found to be negative. The wind data were too limited to determine whether a saturation limit of the wind dependence existed. While Bragg scattering seems to be primarily responsible for the surface Doppler shifts, their magnitude cannot be explained by the Bragg effect alone; the modulation of the Bragg scattering ripple waves by longer waves of the surface wave spectrum also appears to be important.
Wave transformation over coral reefs
Authors: Young Ian R.
Published: February 2008

Ocean wave attenuation on coral reefs is discussed using data obtained from a preliminary field experiment and from the Seasat altimeter. Marked attenuation of the waves is observed, the rate being consistent with existing theories of bottom friction and wave breaking decay. In addition, there is a significant broadening of the spectrum during propagation across reefs. Three‐dimensional effects, such as refraction and defraction, can also lead to substantial wave height reduction for significant distances adjacent to coral reefs. As a result, a matrix of such reefs provides significantly more wave attenuation than may initially be expected.
Structure and evolution of warm core eddies in the eastern Mediterranean Levantine Basin
Authors: Brenner Stephen
Published: February 2008

Two recent cruises of the R/V Shikmona provided detailed coverage of a large portion of the Levantine Basin of the eastern Mediterranean. The first cruise was conducted in October 1985, corresponding to late summer, and the second was in March 1986, corresponding to the following winter. An objective analysis of the combined conductivity‐temperature‐depth and expendable bathythermograph data from these cruises reveals a variety of warm core and cold core mesoscale eddies during both seasons. Some of these features are unique to each cruise while others appear to retain their identities from summer to winter. Of the latter, there are two intense anticyclonic eddies. One of these appears to have formed off the coast of Egypt as a meander of the North African current. The other is a rather persistent quasi‐stationary feature to the southeast of Cyprus which has been seen near this location for over 3 years. Lack of adequate satellite images for the period and region covered led us to use water mass properties to identify and track the development of these two eddies. Below the layer of seasonal influence it was found that each eddy had a unique signature in its TS diagram which changed very little over the 6 months covered by the two cruises and the period between them. The water mass properties also provide additional evidence as to the possible formation regions for these two eddies.
The partition of air‐ice‐ocean momentum exchange as a function of ice concentration, floe size, and draft
Authors: Steele Michael , Morison James H. , Untersteiner Norbert
Published: February 2008

A coupled sea ice‐ocean model is used to examine the relative magnitudes of terms in the sea ice momentum equation, for ice concentrations less than 100%. This numerical model for steady state, horizontally homogeneous flow includes two terms that have previously been neglected in such quantitative models. These are oceanic form drag and wave radiation pressure, both of which are lateral forces that act on the sides of floes. The former is generally decelerative, while the latter is accelerative. The model accepts as input variables average ice draft, areal concentration, and floe diameter. It is found that form drag can be greater than or of the same order as skin friction drag for small, thick floes at low concentrations. An exception to this rule occurs when the effect of wakes between floes reduces the local form drag. The higher net drag that generally exists at low concentrations implies lower ice floe speeds. Wave radiation pressure is negligible in all cases, although this is sensitive to our choice of fetch parameterization. Ocean drag is often parameterized in numerical models of sea ice as a skin friction term alone, with a constant drag coefficient between ice and (prescribed) geostrophic current velocities. Results from the present model are plotted in terms of such a drag coefficient, which is a function of concentration, floe size, and draft. This could be useful in future work where floe size data become regularly available, for instance, from radar satellite imagery.
The seasonal variation of water mass content in the western Mediterranean and its relationship with the inflows through the straits of Gibraltar and Sicily
Authors: Manzella Giuseppe M. R. , La Violette Paul E.
Published: February 2008

An analysis of western Mediterranean current meter and historical hydrographic data suggests a direct relation in the seasonal inflows of Levantine Intermediate Water (LIW) through the Strait of Sicily and Atlantic Water (AW) through the Strait of Gibraltar. The analysis indicates that the seasonal extremes of the two inflows are phased 90° apart. Thus LIW and AW alternate in accumulating in the western Mediterranean, the minimum LIW volume occurring during February‐June and the maximum during July‐December, with the extreme AW volumes reversed during these periods. It is suggested that during the period of maximum LIW volume, the ratio of the denser LIW to the lighter AW in the water column increases and, due to the resulting denser water column, the sea level of the western Mediterranean is lowered. The depressed sea level in turn causes an increase in the AW inflow through the Strait of Gibraltar. The seasonal AW inflow changes generated by these events are inferred to be primarily responsible for the year‐round variability of the western Mediterranean surface layer circulation.
Effects of islands in the Geosat footprint
Authors: Brooks Ronald L. , Lockwood Dennis W. , Hancock David W.
Published: February 2008

The presence of land within a radar altimeter's footprint may contaminate the altimeter's tracking performance and perturb the nadir oceanographic signal. An altimeter tracker tends to move towards the brighter, more specular land reflections in the radar return waveforms. Researchers may unintentionally include tracking data over small mid‐ocean islands in their data sets. If their goal is to obtain sea surface height (SSH) information at the subdecimeter level, such inclusion is inappropriate because islands can locally affect SSH by more than ±4 m. Altimeter signal strengths and wave height‐related measurements are also adversely affected. The tainted data should be edited. However, if editing criteria near island areas are too conservative, useful geoid, tide, current, sea state, and storm surge information will be excluded. A study has been performed which examines the effects of small mid‐ocean islands on the Geosat tracker. The surface return waveforms, as well as the altimeter measurements, have been analyzed. Open ocean waveforms have been compared with waveforms containing land reflections; they differ perceptibly. Computations could be performed, based on a combination of waveform data, altimeter signal strength, and sea surface heights, to detect contaminated SSH values for subsequent editing.
Verification of Geosat sea surface topography in the Gulf Stream extension with surface drifting buoys and hydrographic measurements
Authors: Willebrand J. , Käse R. H. , Stammer D. , Hinrichsen H.‐H. , Krauss W.
Published: February 2008

Altimeter data from Geosat have been analyzed in the Gulf Stream extension area. Horizontal maps of the sea surface height anomaly relative to an annual mean for various 17‐day intervals were constructed using an objective mapping procedure. The mean sea level was approximated by the dynamic topography from climatological hydrographic data. Geostrophic surface velocities derived from the composite maps (mean plus anomaly) are significantly correlated with surface drifter velocities observed during an oceanographie experiment in the spring of 1987. The drifter velocities contain much energy on scales less than 100 km which are not resolved in the altimetric maps. It is shown that the composite sea surface height also agrees well with ground verification from hydrographic data along sections in a triangle between the Azores, Newfoundland, and Bermuda, except in regions of high mean gradients.
High‐accuracy, high‐resolution gravity profiles from 2 years of the Geosat Exact Repeat Mission
Authors: Sandwell David T. , McAdoo David C.
Published: February 2008

Satellite altimeter data from the first 44 repeat cycles (2 years) of the Geosat Exact Repeat Mission (Geosat ERM) were averaged to improve accuracy, resolution and coverage of the marine gravity field. Individual 17‐day repeat cycles (two points per second) were first edited and differentiated resulting in alongtrack vertical deflection (i.e., alongtrack gravity disturbance). To increase the signal to noise ratio, 44 of these cycles were then averaged to form a single, highly accurate vertical deflection profile. The largest contributions to the vertical deflection error is short‐wavelength altimeter noise and longer‐wavelength oceanographic variability; the combined noise level is typically 6 μrad. Both types of noise are reduced by averaging many repeat cycles. Over most ocean areas the uncertainly of the average profile is less than 1 μrad (0.206 arcsec) which corresponds to 1 mgal of alongtrack gravity disturbance. However, in areas of seasonal ice coverage, its uncertainty can exceed 5 μrad. To assess the resolution of individual and average Geosat gravity profiles, the cross‐spectral analysis technique was applied to repeat profiles. Individual Geosat repeat cycles are coherent (>0.5) for wavelengths greater than about 30 km and become increasingly incoherent at shorter wavelengths. This Emit of resolution is governed by the signal‐to‐noise ratio. Thus when many Geosat repeat profiles are averaged together, the resolution limit typically improves to about 20 km. Except in shallow water areas, further improvements in resolution will be increasingly difficult to achieve because the short‐wavelength components are attenuated by upward continuation from the seafloor to the sea surface. These results suggest that the marine gravity field can be completely mapped to an accuracy of 2 mgal and a half‐wavelength resolution of 12 km by a 4.5‐year satellite altimeter mapping mission.
Ice island detection and characterization with airborne synthetic aperture radar
Authors: Jeffries M. O. , Sackinger W. M.
Published: February 2008

A 1∶300,000 scale airborne synthetic aperture radar (SAR) image of an area of the Arctic Ocean adjacent to the Queen Elizabeth Islands, Canadian High Arctic, is examined to determine the number and characteristics of ice islands in the image and to assess the capability of airborne and satellite SAR to detect ice islands. Twelve ice islands have been identified, and their dimensions range from as large as 5.7 km by 8.7 km to as small as 0.15 km by 0.25 km. A significant SAR characteristic of the shelf ice portions of ice islands is a return with a ribbed texture of alternating lighter and darker grey tones resulting from the undulating shelf ice surfaces of the ice islands. The appearance of the ribbed texture varies according to the ice islands' orientation relative to the illumination direction and consequently the incidence angle. Some ice islands also include extensive areas of textureless dark tone attached to the shelf ice. The weak returns correspond to (1) multiyear landfast sea ice that was attached to the front of the Ward Hunt Ice Shelf at the time of calving and which has remained attached since then and (2) multiyear pack ice that has become attached and consolidated since the calving, indicating that ice islands can increase their area and mass significantly as they drift. Other SAR characteristics include straight, bright returns along the edges arising from sea ice pressure ridges or shelf ice cliffs. Other physical characteristics of ice islands that make them identifiable in SAR images include their greater size and shapes longer, thinner, and more angular than those of surrounding pack ice floes. It is evident that ice islands are easily discernible in SAR images and for the future SAR represents a promising technique to obtain a census of ice islands in the Arctic Ocean. However, any SAR‐based census probably will be conservative because ice islands smaller than 300–400 m across are likely to remain undetected, particularly in areas of heavy ice ridging which produces strong SAR clutter.
Forward scattering of electromagnetic energy from rough sea surfaces
Authors: Maurer Donald E.
Published: February 2008

A series of test flights was conducted over the ocean to collect forward‐scattered UHF signals composed of one component directly received from the transmitting aircraft and an indirect component reflected from the ocean. These two signal components, received at different angles, have different Doppler frequencies induced by the relative motion of transmitter and receiver. This paper proposes a differential Doppler processing technique for ocean remote sensing that can be used with this type of signal data base to study ocean scattering physics. To illustrate the use of this technique, characteristics of differential Doppler spectra obtained from preliminary analysis of the data are compared with simulations using a particular scattering model. However, the technique is independent of the model and therefore can be used to compare alternatives. The differential Doppler technique maps signal energy reflected from different regions of the ocean surface onto different resolution cells in the frequency domain. Thus is should be possible to study surface correlation features by comparing variations in the spectral responses at pairs of resolution cells. However, more needs to be done to further develop and validate this approach.
Estimating and removing sensor‐induced correlation from advanced very high resolution radiometer satellite data
Authors: Breaker Laurence C.
Published: February 2008

In order to estimate the spatial covariance structure of sea surface temperature (SST) using advanced very high resolution radiometer (AVHRR) satellite data, autocorrelation induced by the sensor itself must be determined and then removed before the correlative properties of SST per se can be found. Sensor‐induced autocorrelation arises from (1) partially redundant sampling and (2) a nonideal impulse response. Simulation techniques are used to model the correlation structure produced by the AVHRR. Results indicate that an autocorrelation of 0.39 at lag 1 arises from the overlap of adjacent pixels in the alongscan direction. Autocorrelations of 0.51 (alongscan) and 0.43 (alongtrack) arise from the sensor impulse response at lag 1. The autocorrelation at lag 2 is still significant (0.07) due to the sensor impulse response in the alongscan direction. Overall, an autocorrelation of 0.46 occurs at lag 1 due to the combined effects of pixel overlap and the sensor impulse response, at nadir. Procedures for removing this sensor‐induced correlation are presented. Oceanographically, these results indicate, for example, that the gradients associated with sharp oceanic fronts will be suppressed. Application of the results to observed AVHRR satellite data indicate that correlation length scales for SST over one portion of the Gulf Stream are on the order of 20 km. The results also indicate that the effective spatial resolution of the AVHRR is roughly 1.25 km at nadir, somewhat lower than the commonly accepted value. Finally, it is concluded that the accuracy of the calculations is limited by (1) an incomplete specification of the AVHRR system modulation transfer function (MTF) and (2) the sine wave response function that is used to approximate the system MTF.
Vertical distributions of spray droplets near the sea surface: Influences of jet drop ejection and surface tearing
Authors: Wu Jin
Published: February 2008

Vertical distributions of droplet concentrations immediately above the sea surface were observed by de Leeuw to have a minimum at 0.5 m elevation under low winds and a maximum at elevations of 1–2 m under high winds. The minimum at low winds is suggested to be caused by the jet drop ejection combined with wave‐augmented airflows and wave breaking, while the maximum at high winds is associated with the surface tearing.
Multiyear sea ice in the Arctic: Model‐ and satellite‐derived
Authors: Walsh John E. , Zwally H. Jay
Published: February 2008

The behavior of sea ice is a strong function of ice strength which, in the central Arctic, depends on the relative fractions of multiyear and first‐year ice coverage. A dynamic‐thermodynamic sea ice model has been configured to include both multiyear and first‐year ice. The model has been run for a 7‐year period, 1979–1985, for which a satellite microwave record of multiyear sea ice coverage is available. The simulated concentrations of multiyear ice are slightly higher than the satellite‐derived concentrations, although the total coverage of simulated multiyear ice agrees well with the satellite‐derived coverage. Both the simulated and the satellite‐derived coverages in the early 1980s show large interannual variations that represent potentially large perturbations in the mass balance of Arctic pack ice. These fluctuations are generally consistent in the model and satellite data, and they are generally consistent with year‐to‐year differences in the large‐scale wind forcing. In at least some cases the wintertime distribution of multiyear ice is influenced by the pattern of ice drift as far back as the previous summer. The late December coverage of multiyear ice in the Alaskan region appears to foreshadow summer ice conditions in the same region. The short‐term response to an intense synoptic system is considerably larger in the satellite data than in the model results.
Entrainment and mixing of shelf/slope waters in the near‐surface Gulf Stream
Authors: Lillibridge J. L. , Hitchcock G. , Rossby T. , Lessard E. , Mork M. , Golmen L.
Published: February 2008

An interdisciplinary study of the entrainment of shelf and slope waters in the Gulf Stream front was undertaken in October 1985 northeast of Cape Hatteras. Fifteen hydrographic transects of the Gulf Stream front and of the shelf water intrusion known as Ford water were completed in 2 1/2 days with a towed undulating profiler, the SeaSoar, equipped with a conductivity‐temperature‐depth probe and a fluorometer. Upstream sections within 50 km of the shelf break show entrainment of surface and subsurface waters along the northern edge of the high‐velocity Gulf Stream. The low‐salinity core, first observed at 70 m, is subducted to >100 m. The subsurface Ford water is also at a maximum in chlorophyll, fluorescence, and dissolved oxygen and contains a distinct diatom assemblage of nearshore species. Productivity rates in the Ford water may be equivalent to those in slope waters. Expendable current profilers yield an estimated transport for subsurface shelf waters of 1 to 5×105 m3 s−1 and indicate that vertical shear at the depth of maximum static stability is typically 2×10−2 s−1. A bulk Richardson number is estimated over vertical scales of several meters by combining SeaSoar density profiles with velocity shear from concurrent expendable current profiler deployments. The minimum values are generally >1, and only infrequently are they at or below the 0.25 threshold for shear instability. The presence of double‐diffusive processes around the low‐salinity core of Ford water is indicated by elevated conductivity Cox numbers. The stability parameter “Turner angle” shows that low‐salinity Ford water and its associated T‐S property front are sites of double‐diffusive mixing, given general agreement between the distributions of Turner angle and Cox number. We conclude that double‐diffusive processes are more important than shear flow instability in governing cross‐isopycnal mixing. However, downstream transit times are so swift that no measurable change or decay occurs in the Ford water. This explains the occurrence of distinct shelf water phytoplankton species within the low‐salinity waters downstream of Cape Hatteras.
Drainage basin weathering and major element transport of two large Chinese rivers (Huanghe and Changjiang)
Authors: Zhang Jing , Huang Wei Wen , Liu Min Guang , Zhou Qing
Published: February 2008

Long‐term records show high concentrations of dissolved major elements in the Changjiang (Yangtze River) and the Huanghe (Yellow River). The Huanghe drainage basin is characterized by intense weathering and erosion of carbonates and evaporites, while weathering and erosion of carbonates and silicates over the drainage basin is the main source of major elements in the Changjiang. The rock/soil composition and tectonics of the drainage basin are more important than atmospheric precipitation in affecting major element composition of these two rivers. Compared with other large rivers of the world, the high suspended sediment and major element loads of the Changjiang and Huanghe reveal intense physical and chemical erosion over the Chinese continent.
Net surface heat flux over the north and south Atlantic in 1985–1986 from day 1 predictions of the European Center for Medium‐Range Weather Forecast
Authors: Barnier B. , Simonot J.‐Y.
Published: February 2008

Twelve months of data on surface heat fluxes (shortwave radiation, longwave radiation, latent heat and sensible heat, from August 1985 to July 1986), obtained from the European Center for Medium‐Range Weather Forecast (ECMWF) on a grid of 1.125° latitude by 1.125° longitude were analyzed. Comparison with older data from ECMWF (Simonot and Le Treut, 1987) indicates the occurrence of significant improvements in global and local distributions of the fluxes over oceans (mainly the latent and net heat fluxes), except for the shortwave radiative heat flux, which seems degraded. The global mean of the annual net air‐sea heat flux is not in equilibrium, but shows a relatively small loss of 5 W m−2 for the ocean (instead of a 40 W m−2 gain in an older ECMWF analysis). Meridional oceanic net heat transport is consistent with other climatological estimates (Hastenrath 1982) in the northern hemisphere, but less consistent with estimates from the southern hemisphere, Seasonal variability of the net heat flux at the air‐sea interface was studied over two limited mid‐latitude ocean basins: the North and the South Atlantic. Annual mean and low frequency rms are discussed. Fair agreement is found with climatological studies from Bunker (1976, 1988), Esbensen and Kushnir (1981), and Hsiung (1986), with discrepancies being large enough in the tropical South Atlantic to explain the deviation from climatology in the meridional oceanic heat transport in the southern hemisphere. Large‐scale variability of low‐frequency perturbation net heat flux was investigated by means of empirical orthogonal function (eof) analysis. For each ocean, the first eof accounts for a large amount of the total variance in the data (41% in the North Atlantic and 61% in the South Atlantic), and is characterized by basin‐sized oscillations with annual and semiannual cycles. Comparison of our data with a climatology of the Agulhas retroflection area (Walker and Mey, 1988) indicates the good quality of ECMWF analyses in regions where air‐sea exchanges are large; but large relative uncertainties remain among estimates in regions where fluxes are weak.
Focusing simulations of synthetic aperture radar ocean images
Authors: Hayt D. W. , Alpers W. , Brüning C. , DeWitt R. , Henyey F. , Kasilingam D. P. , Keller W. C. , Lyzenga D. R. , Plant W. J. , Schult R. L. , Shemdin O. H. , Wright J. A.
Published: February 2008

The SAR ocean images obtained in the Tower Ocean Wave and Radar Dependence Experiment (TOWARD) are carefully analyzed at different focus settings and compared with simulated results based on various theories for imaging surface waves. The agreement between the experimental data and all of the SAR simulations except one is found to be favorable. There is also surprisingly close agreement among the different theoretical models themselves, suggesting a closer fundamental similarity among the contending theories than was previously thought. It is shown that the width of the focusing curve has an inverse dependence on the SAR integration time. For the TOWARD conditions it is found that the image modulation due to azimuth‐traveling surface waves is greater than that due to range‐traveling waves.
On extracting tidal current profiles from vertically integrated two‐dimensional hydrodynamic models
Authors: Davies Alan M.
Published: February 2008

A numerically accurate and computationally efficient method is developed for extracting tidal current profiles from vertically integrated two‐dimensional models. The method can incorporate an arbitrary profile of eddy viscosity and determines tidal current profiles satisfying a no slip condition at the sea bed. Comparisons with observations and current profiles computed with full three‐dimensional models, confirm the accuracy of the method. Bottom boundary layer thicknesses determined from these current profiles clearly show regions of the European continental shelf where the bottom boundary layer extends to the sea surface. The boundary between these regions and other areas where the bottom layer is less than the water depth corresponds quite closely to the known position of tidal fronts.
Spatial scales of mesoscale variability in the North Atlantic as deduced from Geosat data
Authors: Le Traon P. Y. , Rouquet M. C. , Boissier C.
Published: February 2008

Two years of Geosat data were used to systematically investigate the mesoscale variability and spatial scales of the North Atlantic. We first calculated sea surface height variability and eddy kinetic energy. These compare well with the eddy kinetic energy obtained with surface drifters. Then the spatial scales of mesoscale variability were characterized in 10° (latitude)×10° (longitude) areas by their wave number spectra and autocorrelation functions. Comparison with existing in situ data shows good agreement, and it is believed that the total error budget is globally below that of the oceanic signal. The wave number spectra show significant differences as a function of latitude and longitude, and these can be related to different types of forcing (instability of a mean current or fluctuating wind). Scales typically decrease from west to east and south to north. Simple proportionality with respect to the first internal Rossby radii does not apply everywhere.
Oceanic heat content variability in the tropical Pacific during the 1982–1983 El Niño
Authors: Springer Scott R. , Mcphaden Michael J. , Busalacchi Antonio J.
Published: February 2008

Anomalous heat transport and storage during the 1982–1983 El Niño are investigated using a linear, multimode model forced by observed winds. Heat transport is decomposed into symmetric (about the equator) and antisymmetric components. The former was dominated by anomalous northward Ekman transport which represented an enhancement of the usual seasonal cycle. The latter involved both Ekman and geostrophic transports. Near‐equatorial wind anomalies forced Kelvin and Rossby waves usually associated with El Niño; together these waves set up antisymmetric, geostrophic transport which tended to oppose direct Ekman transport. Because the opposition was imperfect, there was net heat convergence which caused variations in heat content in bands of latitude centered on the equator. Within a fairly narrow band (±5°) heat content was anomalously high preceding El Niño and was depleted following the event. Equatorial heat content anomalies were largely compensated by opposing anomalies in low latitudes of the extraequatorial ocean so that variability over broader bands of latitude about the equator was relatively small. A sampling study employing the model suggests that observational evidence for a heat content variations over the region ±15° is an artifact arising from inadequate spatial resolution offered by the sea level measurement network.
The North Brazil Current retroflection: Seasonal structure and eddy variability
Authors: Johns William E. , Lee Thomas N. , Schott Friedrich A. , Zantopp Rainer J. , Evans Robert H.
Published: February 2008

We report results from a 1‐year (September 1987 to September 1988) moored current meter array spanning the continental margin off French Guiana near 8° N in the western tropical Atlantic. Current profiles were recorded at three sites: at the shelf break, over the mid‐continental slope, and at the base of the continental rise. Upper level mean currents showed a northwestward flowing North Brazil Current (NBC) and offshore retroflection of this flow into the North Equatorial Countercurrent from late summer through about January. Generally weak upper level mean flows were observed during the spring (February–June). Persistent northwestward mean flow was observed at 900 m depth over the continental slope, indicating northward transport of Antarctic Intermediate Waters in a subsurface boundary flow at speeds of 10–15 cm s−1. Deep currents over the continental rise showed a strong southeastward Deep Western Boundary Current (DWBC) extending from 2500 m to the bottom, with mean core speeds of nearly 30 cm s−1 at 4300 m depth. Transport estimates based on these data and a few geostrophic sections suggest a DWBC transport of 20–40 × 106 m3 s−1 at this location. Low‐frequency current fluctuations were dominated by a well‐defined 40‐ to 60‐day oscillation with peak‐to‐peak meridional velocity amplitudes of > 1 m s−1 during the fall. Analysis of historical coastal zone color scanner imagery suggests that these oscillations are related to quasi‐periodic generation and subsequent westward movement of ≈ 400 km diameter eddies from the NBC retroflection. These results contrast sharply with earlier indications of a quasi‐permanent “Demerara Eddy” in this region, and suggest that this commonly observed feature is in fact a transient phenomenon associated with the time‐dependent behavior of the NBC retroflection.
The δ18O of dissolved O2 in seawater: A unique tracer of circulation and respiration in the deep sea
Authors: Bender Michael L.
Published: February 2008

The isotopic composition of dissolved O2 in seawater, expressed as the δ18O of O2, is unique among the bioactive tracers of the aphotic zone in that it is not linearly related to oxygen utilization via the stoichiometry of organic matter decomposition. In fact, δ18O of O2 depends on the history of water mixing and O2 consumption in the sample studied (Craig and Kroopnick, 1970; Kroopnick and Craig, 1976). For this reason, the variation of δ18O of O2 with O2 concentration depends on regional circulation patterns and oxygen utilization rates. The δ18O of O2 can be used to chartacterize these processes by decoupling their effects. As an example of this assertion, we interpret the covariation between the concentration of O2 and its isotopic composition in the Pacific Ocean as reported by Kroopnick (1987), using four simple representations of seawater mixing and respiration. Kroopnick's data are in general accord with an elementary model of isopycnal mixing represented by diffusive exchange and oxygen utilization in the ocean's interior, coupled with atmospheric equilibrium at the point where the isopycnals outcrop at the sea surface. This specific result illustrates the general point that δ18O of O2 in seawater can serve as an important constraint on more extensive and sophisticated physical models used to estimate rates of oxygen utilization in the deep sea.
Sea ice concentrations in the Canada Basin during 1988: Comparisons with other years and evidence of multiple forcing mechanisms
Authors: Serreze Mark C. , Maslanik James A. , Preller Ruth H. , Barry Roger G.
Published: February 2008

During summer 1988, a large area of reduced ice concentration developed within the Arctic pack ice of the Canada Basin. Data from the special sensor microwave imager (SSM/I) and visible‐band imagery show that by early September, this feature extended north of 80°N, with SSM/I‐derived ice concentrations of 40–80% covering an area approximately 1.0×106 km2. Drifting buoys, surface pressure fields, output from the Polar Ice Prediction System (PIPS) sea ice model and other meteorological data are used to examine processes responsible for development of the reduced ice concentrations. While the model indicates that ice divergence in summer offers a partial explanation, (1) anomalously warm atmospheric conditions, (2) generally clear skies in June and July, (3) extensive fracturing of the pack ice in spring and, perhaps, (4) anomalous advection of oceanic heat may all have played contributing roles. However, the second and third of these effects may occur in most years. Coupled with additional observations suggestive of recurring bathymetrically induced effects on lateral transport of ocean heat, we conclude that although the extent and magnitude of the concentration reductions during 1988 are unusual, these recurring factors tend to predispose the pack ice in the Canada Basin to decay.
A Gulf Stream‐derived pycnocline intrusion on the Middle Atlantic Bight shelf
Authors: Gawarkiewicz Glen , McCarthy Robert K. , Barton Kenneth , Masse Ann K. , Church Thomas M.
Published: February 2008

Saline intrusions from the upper slope onto the outer shelf are frequently observed at the pycnocline along the shelfbreak front in the Middle Atlantic Bight during the summer. A brief cruise was conducted in July, 1986 between Baltimore and Washington Canyons to examine along‐shelf variability of pycnocline salinity intrusions. A particularly saline intrusion of 35.8 Practical Salinity Units (PSU) was observed between 20 and 40 m in a water depth of 70 to 80 m. The along‐shelf extent was at least 40 km. The cooler, sub‐pycnocline outer shelf water was displaced 15 km shoreward of the shelfbreak. A Gulf Stream filament was present in the slope region prior to the hydrographic sampling, but was not visible in thermal imagery during the hydrographic sampling. Temperature‐salinity characteristics of the intrusion suggest that it was a mixture of Gulf Stream water and slope water, possibly from the filament. The shoreward penetration of saline water was most pronounced at the pycnocline and penetrated the shelfbreak front, with salinities as high as 35.0 PSU reaching as far shoreward as the 35 m isobath. These pycnocline intrusions may be an important mechanism for the transport of Gulf Stream‐derived water onto the shelf during the summer. The presence of filaments or other Gulf Stream‐derived water on the upper slope may account for some of the along‐front variability of the pycnocline salinity maximum that has previously been observed.
Mid‐stratospheric circulations in the Southern Hemisphere from super pressure balloon trajectories
Authors: Olivero John J. , Shaw A. W. , Williamson P. R. , Megill L. R.
Published: February 2008

Three instrumented super pressure balloons were flown throughout the southern hemisphere at 18 mbar, over a period of 5 months. The balloon trajectories, reconstructed from NIMBUS 6 satellite location data, have proved to be valuable indicators of mid‐stratospheric structure and dynamics in the Lagrangian frame of reference. Several examples of high zonal and meridional winds are reported. Also shown in the analysis is a feature encountered between 30°S–40°S during a 2 week period. The feature appears to be a breakdown of large‐scale zonal flow and its replacement by weak cyclonic (stationary) eddies. During the observational period the balloon motions document the spring reversal (of zonal winds and meridional temperature gradient). This is accomplished by the movement of a planetary scale anticyclonic cell, which itself established the boundaries between the easterlies and the westerlies on a day to day basis. Mean zonal wind speed analysis from balloon positions were obtained covering the area 15°S–65°S. The results presented here are in substantial agreement with those of Hartmann (1977).
CZCS data analysis in turbid coastal water
Authors: Viollier M. , Sturm B.
Published: February 2008

Different spectral signatures of coastal waters are presented and analysed with respect to possible improvements of the CZCS algorithms. When adjusted to the prevailing oceanographic conditions, algorithms can be improved in different ways: by adjustment of the coefficients in the ratio algorithms, by use of the amplitude of reflectance, or by adjustment of the method for the aerosol correction. It is shown that the combined use of ratio and amplitude of reflectance in a sediment algorithm can be used to distinguish between offshore upwellings and turbid near shore coastal water. The processing of CZCS scenes illustrates the importance of ocean color imagery in various aspects of oceanography: fundamental biology and transport of coastal pollutants.