bluuebunny/crossref_metadata_2025 huggingface dataset:
Shuttle Imaging Radar B (SIR‐B) Weddell Sea ice observations: A comparison of SIR‐B and scanning multichannel microwave radiometer ice concentrations
Authors: Martin Seelye , Holt Benjamin , Cavalieri Donald J. , Squire Vernon
Published: February 2008

The October 1984 Shuttle Imaging Radar B (SIR‐B) flight made three radar passes over the Weddell Sea ice, providing the first high‐resolution look at the Weddell marginal ice zone properties. Using these data, this paper discusses the effect of ocean waves on the radar return at the ice edge and compares ice concentrations derived from the SIR‐B with coincident concentrations from the Nimbus 7 scanning multichannel microwave radiometer (SMMR). The comparison of the SIR and SMMR concentrations is possible because SIR cross‐track width and the diameter of the SMMR 37‐GHz integrated field‐of‐view are both about 30 km. The SIR ice concentrations are computed in two ways: first, using a training area classification scheme at the Jet Propulsion Laboratory (JPL); second, using a manual classification method at the Scott Polar Research Institute. The SMMR ice concentrations are calculated using the Goddard Space Flight Center algorithm. At the ice edge, where there were no coincident SMMR data and where ice bands predominated to yield an ice concentration of the order of 10% or less, comparison of the two different analysis techniques on the same images showed that, for the JPL technique to avoid classifying some of the open water as ice, two classes of open water must be defined. These two classes accounted for the rougher ocean surface upwind of the bands and the smoother downwind surface. In the ice interior, comparison of the coincident SIR and SMMR ice concentrations shows that for concentrations greater than 40%, which was the smallest concentration jointly observed, the mean difference between the two data sets for 12 points is 2% and the standard deviation is 7%.
A coupled dynamic‐thermodynamic model of an ice‐ocean system in the marginal ice zone
Authors: Häkkinen Sirpa
Published: February 2008

Wind‐driven ice‐ocean processes in the marginal ice zone are studied when thermodynamics is included into a coupled ice‐ocean model. The ocean is considered to be a two‐layer ocean, where a “fresh” and cold (freezing) mixed layer is lying above the stagnant, warm, and saline lower layer. The essential part of the thermodynamics is the entrainment between the mixed layer and the lower layer which is parameterized using the frictional velocity at the ocean surface, a Kraus‐Turner type parameterization. It is shown that upwelling at the ice edge, which is generated by the difference in the air‐ice and air‐water surface stresses, will give rise to a strong entrainment by bringing the pycnocline nearer to the surface. Furthermore, the entrainment is negligible outside the areas affected by the ice edge upwelling. When cooling at the top is included, the heat and salt exchange are further enhanced in the upwelling areas because the entrainment depends inversely on the density difference between the layers. New ice formation occurs in the area which is not affected by ice edge upwelling (that is, no entrainment of the warm lower‐layer fluid, so the mixed layer temperature stays at the freezing point). The ice growth will give rise to a large salt flux to the upper layer, but the subsequent salinity change is not as large as that due to the entrainment of the lower‐layer fluid. The mixed layer in the areas affected by the ice edge upwelling is only slightly less saline than the lower layer. It is suggested that these high‐salinity mixed layer regions, with a scale of a few Rossby radii of deformation (∼ 10 km), will overturn due to cooling, possibly as a plume, and may this contribute to the formation of deep water.
Eddy formation in unsteady flows
Authors: Black K. P. , Gay S. L.
Published: February 2008

Unsteadiness of flow at M2‐S2 tidal frequencies is demonstrated to be a dominant factor forming eddies in natural, shallow‐water and continental shelf environments. Numerical hydrodynamic modelling and field measurements show that during the later stages of the half tidal cycle in a low‐friction environment, currents in the lee of an obstruction accelerate with supporting sea gradients opposite to the adjacent, inertial free stream, which is still decelerating. These opposed currents appear as a phase eddy and ideally can endure for 3 hours in M2‐S2 reversing currents. Case studies are cited, and numerical hydrodynamic modeling is applied to examine the formation of these circulation patterns.
Wind‐driven meridional eddy heat flux in the Gulf of Guinea
Authors: Houghton Robert W. , Colin Christian
Published: February 2008

Seasonal Response of the Equatorial Atlantic/Programme Français Océan et Climat dans l'Atlantique Equatorial data for 1984 are analyzed to estimate heat fluxes in the interior of the Gulf of Guinea between 1°N and 4°N. In this region during May–July the mixed layer, roughly 35 m deep, cools uniformly by nearly 4°C in 3 months representing a mean rate of heat loss of approximately 70 W/m2. Climatic heat budget calculations by Hastenrath and Lamb (1978) show a surface flux heat loss of 30 W/m2 during this time. This suggests that although surface flux makes a significant contribution to the mixed layer cooling, additional divergent heat flux is required to balance the local heat budget. All advective and diffusive terms are estimated to be small except for the meridional eddy heat flux v′T′. The cospectrum of v and T has a peak near 15 days. At this frequency, v is coherent with the meridional wind. The thermocline displacements forced by this meridional wind are nearly antisymmetric about the equator. The frequency, meridional length scale, and asymmetry of these fluctuations suggest that a second baroclinic Rossby‐gravity wave is being locally forced. Thus in the interior of the Gulf of Guinea the eddy flux driven by the fortnightly wind fluctuations makes a significant contribution to the cooling of the mixed layer during the seasonal upwelling. This is in contrast to the central and western equatorial Atlantic Ocean, where a meridional heat flux is produced by 25 to 30 day fluctuations generated by current instabilities.
Longitudinal variations in tropical tropopause properties in relation to tropical convection and El Niño‐Southern Oscillation events
Authors: Gage K. S. , Reid G. C.
Published: February 2008

This paper focuses on the longitudinal variations in tropopause properties evident in the historical record of tropical Pacific radiosonde data. Over 30 years of radiosonde temperature soundings from tropical Pacific stations (Ponape, Yap, Guam, Majuro, Koror, and Eniwetok) have been analyzed to yield tropopause height and tropopause potential temperature. Both tropopause height and tropopause potential temperature show a clear year‐to‐year variation in response to changes in tropical convection associated with El Niño‐Southern Oscillation (ENSO) events. In particular, the tropopause potential temperature appears to be very sensitive to the longitude of major convective activity. As the locus of convective activity moves eastward during an El Niño, there is a concurrent change in the longitudinal gradient of tropopause potential temperature. During an El Niño year the tropopause potential temperature is systematically warmer at Koror as compared to Majuro. This is the reverse of the normal tropopause potential temperature difference between these two stations. The difference in the tropopause potential temperature between these two stations forms an ENSO index with interannual variations well correlated with the conventional ENSO index formed by the Tahiti‐Darwin surface pressure.
The torque and flux balances in the upper equatorial ocean
Authors: Kraus Eric B.
Published: February 2008

Local vertical fluxes of energy and momentum are parameterized by a method that involves a spectrum of differently sized eddies. Simulations based on this method indicate that the diurnal changes in the surface heat flux should affect the dynamics of the upper equatorial ocean to a considerable extent. In particular, they predict the existence of significant diurnal changes in the surface drift velocity. A lack of balance between the torques exerted by the wind stress, the zonal buoyancy gradient, and zonal advection suggests that meridional eddy transports and their variation with depth must play an essential role in the local dynamics.
Response to El Niño signals of the epiplanktonic copepod populations in the eastern tropical Pacific
Authors: Dessier A. , Donguy J. R.
Published: February 2008

Interannual variability in composition and abundance of the surface copepod populations in the eastern equatorial Pacific was examined in relation to environmental properties (sea surface temperature, salinity, chlorophyll content, and vertical thermal structure) between 1979 and 1984. This period is characterized by the 1982–1983 El Niño‐Southern Oscillation (ENSO) episode, inferring eastward propagation along the equator of several climatic and oceanographic parameters following two phases which have been distinctly observed in the eastern Pacific. During the first phase, November 1982, the thermocline deepening induces a strong drop in primary production and consequently a decrease in abundance of the total copepods in the equatorial area until the end of 1984. During the second phase (second quarter of 1983), a new areal distribution of copepod populations occurs, connected to the southwestward extension of the low‐salinity surface waters from the Gulf of Panama. During the whole ENSO event, the seasonal signal is missing.
Flow around islands in Rupert Bay: An investigation of the bottom friction effect
Authors: Ingram R. Grant , Chu Vincent H.
Published: February 2008

Aerial photographs of transverse shear flows around islands in Rupert Bay, a shallow and turbid estuary in northern Quebec, were studied for the effect of bottom friction. The island wake depends on the stability of the transverse shear layers developed along the two sides of the island. Two basic patterns of wake flows were observed. A vortex street wake was formed when transverse shear was large, as in flows around small islands. On the other hand, for small transverse shear, as in flows around large islands, the shear layers were stabilized by bottom friction and, in that case, a clear water wake of low turbulence and low sediment concentration was observed. Laboratory demonstration of the bottom friction influence on the turbulent wake in shallow water was made using a shallow water table. A similarity solution was obtained for the wake flow in the far‐field region, and numerical calculations were made of the recirculating flow in the wake bubble. A wake stability parameter was introduced for wake classification and for correlation of field and laboratory data. In Rupert Bay, vortex street wakes were observed in nine out of 27 events when the wake stability parameter was less than a critical value. This result is consistent with the laboratory and numerical simulations.
Observed and simulated kinematic properties of Loop Current rings
Authors: Kirwan A. D. , Lewis J. K. , Indest A. W. , Reinersman P. , Quintero I.
Published: February 2008

Two rings, shed by the Loop Current in 1980 and 1982, were observed for several months by satellite‐tracked drifters to migrate across the Gulf of Mexico. The drifter path data have been inverted to obtain estimates of the paths of the centers of the two rings, ring shape, and the swirl velocities. Three drifters were deployed in the 1980 ring, and the analysis of that data set establishes the variability of the above kinematic estimates for one ring. A comparison of the analysis of data from both rings provides some idea on inter‐ring variability. Both rings impacted the Mexican continental slope at about 22.8°N, 95.5°W. After a brief adjustment period, both rings reestablished and maintained a vortex character for several months in the slope region while migrating slowly to the north. The paths of the centers of the two rings along the slope are virtually identical. The same analysis routine was applied to some simulated drifter data obtained from the Hurlburt and Thompson (1980) Gulf of Mexico primitive equation model. In the midgulf, the agreement between the observed rings and the simulated ring is good, although the former showed stronger interaction with the continental slope topography and/or circulation than was seen in the latter. Along the slope, the model ring kinematic characteristics were in extraordinary agreement with the observations.
Variability in Gulf Stream surface‐subsurface frontal separation: The unimportance of Ekman advection
Authors: Horton Charles W. , Horsley LaVerne E.
Published: February 2008

Simultaneous observations of the Gulf Stream surface and subsurface (15°C at 200 m) fronts have shown their relative positions to be quite variable. Cross‐frontal Ekman advection of the surface front by the local wind stress is a possible major source of this variability. We investigate this possibility and show that on the average and for a particular set of observations, only a small fraction of the observed variability in the Gulf Stream surface‐subsurface frontal separation is due to advection of the surface front by the local wind stress. As an alternative, it is suggested that most of the observed variability in frontal separation may be due to unstable meandering of the Gulf Stream surface front.
Diagnostic study of the Fram Strait marginal ice zone during summer from 1983 and 1984 Marginal Ice Zone Experiment Lagrangian observations
Authors: Gascard Jean‐Claude , Kergomard Claude , Jeannin Pierre‐François , Fily Michel
Published: February 2008

From 1984 Marginal Ice Zone Experiment observations in Fram Strait, three main results concerning large scale, mesoscale, and summer marginal ice zone (MIZ) features are presented and discussed in this paper. Regarding the large scale, the main conclusion is that the East Greenland Current (EGC) is a narrow, stable current, in contradiction with past results presenting the EGC as a broad and unstable current. In the mesoscale, the West Spitsbergen Current (WSC) and the Norwegian Current are the main generators for eddies in Fram Strait. These eddies propagate northwestward, preferentially following topographic features like fracture zones, which are quite abundant in this region. In our opinion, these WSC eddies represent an important part of the Atlantic water recirculation in Fram Strait. When approaching the east Greenland continental slope, they interact with the EGC and they form well‐developed eddies. These ice edge eddies associated with ice edge tongues are quite recognizable with remote sensing techniques. The summer marginal ice zone in Fram Strait is clearly characterized by sea ice concentrations from 1/10 to 3/10 and floe sizes limited to 1 km or less. There is a well‐defined limit which separates the pack ice from the MIZ where no big ice floes are ever recorded. The breaking action of waves and swell propagating into the ice are likely to be the only mechanisms explaining such a drastic filtering effect. According to their amplitudes and directions, winds and eddies act in the MIZ in shrinking or stretching this zone.
Island wakes in deep and shallow water
Authors: Tomczak Matthias
Published: February 2008

Observations of island wakes from the continental shelf have been used by various authors to suggest that the Reynolds number is not an appropriate parameter for wake behaviour in shallow water. It is shown here that wake behaviour both in deep and shallow water is well described by the Reynolds number if it is derived correctly, by taking into account frictional effects in horizontal as well as vertical boundary layers.
Global patterns of ocean transparency: Implications for the new production of the open ocean
Authors: Lewis Marlon R. , Kuring Norman , Yentsch Charles
Published: February 2008

Climatological mean fields of ocean transparency derived from Secchi depth measurements held by the National Oceanographic Data Center are presented. Clearly evident are the major coastal upwelling regions off Peru, northwest and south Africa as well as the coastally enriched areas globally. The central gyre regions in both North Atlantic and North Pacific waters are separated by a sharp discontinuity in water clarity from the more northern waters; the change in Secchi depth across this subtropical front is equivalent to a difference of ∼1 mg chlorophyll a m−3 over meridional distances of 1000 km. Sufficient data exist in the western Pacific to demonstrate that the front migrates approximately 1000 km seasonally from a northern extreme at ∼28° N in September–November, to a southern extreme at 18° N in March–May. A simple explanation for this feature, which is also clearly evident in recent satellite ocean color images of the the northern ocean basins, is offered in terms of the effective vertical ‘upwelling’ and resulting new production associated with seasonal variations in deep convective mixing.
An experiment assimilating sea surface temperatures into a one‐dimensional model of the mixed layer
Authors: Mohr D. L. , French J. P. , Warn‐Varnas A. C.
Published: February 2008

Sea surface temperatures (SSTs) from ship observations are assimilated into the Mellor‐Yamada level 2 one‐dimensional model of the mixed layer using a nonlinear version of the Kalman filter, similar to the extended Kalman filter. The model is initialized from expendable bathythermograph (XBT) temperature profiles at a weather ship location and is driven using the ship's observed meteorological forcing. As the SSTs arrive, usually every 3 hours, they are blended with the model results to produce an analysis. At the end of 3 and 5 days, the analyses are compared with another XBT profile at the same location. The data assimilation improves on the model forecasts by giving greater accuracy in the 20 to 25 m near the surface, which is a large portion of the very shallow mixed layer in these data. No improvement was found below the mixed layer.
Specular point scattering contribution to the mean synthetic aperture radar image of the ocean surface
Authors: Winebrenner Dale P. , Hasselmann Klaus
Published: February 2008

In general, the return signal scattered from the ocean surface used to form synthetic aperture radar (SAR) images contains contributions from at least two scattering mechanisms. In addition to resonant Bragg‐type scattering, specular point scattering becomes important as the angle of incidence becomes small ( ≲ 20°). In this paper we include the specular point rough surface scattering mechanism in a model for the mean SAR image of the ocean surface and examine the effects of this scattering mechanism theoretically. We find that the complete mean SAR intensity image consists of a sum of images due to specular point scattering and Bragg‐type resonant scattering. Because surface specular points have a short coherence time and move with considerable velocities, the contribution to the mean image due to these scatterers is of low azimuthal resolution and is displaced from the actual sea surface, typically by several SAR resolution cells. The bandwidth of this image can easily exceed the bandwidth of a typical SAR processor, leading to a loss of mean image intensity. The local backscatter cross‐section modulation is strong and nonlinear in the slope of the longwave field in the SAR range direction. At small incidence angles, this causes the specular point return from wave slopes tipped toward the SAR to become much brighter than the Bragg‐scattering return. Taken together, these effects are capable of producing azimuthally oriented streaks in SAR images, such as have been observed by Seasat. We present numerical estimates of coherence time, azimuthal displacement, cross‐section modulation, etc., computed using the parameters of the Seasat and shuttle imaging radar‐B SARs as well as typical parameters for an airborne X band SAR.
Effect of suspended sediment on the heating of Lake Banyoles
Authors: Casamitjana X. , Roget E. , Jou D. , Llebot J. E.
Published: February 2008

Lake Banyoles, of karstic origin, is continually heated by the incoming groundwater. This water enters the lake at certain places which are basin‐shaped, keeping sediment in suspension. The temperature of suspended sediment is 2°–3°C higher than that of hypolimnetic water in summer, and 8°–10°C higher in winter. Because of this the cooling of the lake is measurably lessened. In this work a model to calculate the heat flux from underground is presented where both diffusive and advective heat fluxes are considered. The degree of turbulence of the water at the interface of suspended sediment is also evaluated. Finally, this model is applied over a 1‐year cycle.
Comparison of Nimbus 7 scanning multichannel microwave radiometer radiance and derived sea ice concentrations with Landsat imagery for the north water area of Baffin Bay
Authors: Steffen Konrad , Maslanik James A.
Published: February 2008

Landsat imagery for the northern Baffin Bay (May to June, 1981) is used to compare ice concentration estimates derived from Nimbus 7 scanning multichannel microwave radiometer (SMMR) data. Periods of premelt, onset of melt and melt are identified based on changes in brightness temperature and on gridded air temperature data from the European Center for Mid‐Range Weather Forecasting. Differences between multispectral SMMR‐ and Landsat‐derived total ice concentrations are within 3.5% for premelt and onset of melt periods. During melt, differences increase to about 10%. Differences for individual SMMR footprints are greater and contribute to a mean correlation coefficient of 0.53 between SMMR‐ and Landsat‐derived ice concentrations. The SMMR data yield slightly lower estimates of total ice concentration, and for individual SMMR footprints they tend to yield lower estimates at high concentrations and higher estimates at low concentrations relative to the Landsat interpretation. Although statistically different, the mean concentrations derived using the Landsat and SMMR data were separated by less than 10% in virtually all cases. Possible reasons for these discrepancies are discussed. The brightness temperatures for compact pack ice compared with fast ice are on the average 10 K higher at vertical polarization and 15 K higher at horizontal polarization for 18 and 37 GHz during the premelt period. Classification of ice types such as first‐year ice, young ice, and nilas by means of polarization ratios seems feasible for large homogeneous areas of a single ice type.
Conceptual models of El Niño and the Southern Oscillation
Authors: Vallis G. K.
Published: February 2008

We present a few simple models which are intended to encapsulate some of the basic mechanisms of the El Niño/Southern Oscillation phenomenon. We consider one‐ and two‐dimensional, continuous and low order models, with and without external stochastic forcing. In the low order models, even in the absence of stochastic forcing, chaos and aperiodic ENSO events can occur. This behavior is, however, rather sensitive to the choice of parameters and to the precise difference formulation used. Some of the detailed behavior of very low order models can also be unrealistic. However, the presence of multiple solutions is robust and insensitive to the differencing assumptions. One notable result of these models is that El Niño events partially phase‐locked to the seasonal cycle can be produced both by by variations in trade‐wind intensity and by imposed annual cycles in the temperature forcing. A continuous model which reduces to a chaotic low‐order model in the limit of very coarse finite differencing is presented. Multiple, analytically derivable solutions exist which, however, are stable to infinitesimal perturbations. Larger perturbations or stochastic forcing can cause irregular oscillations between the two stationary states and El Niño like events, even in the absence of equatorial waves. Adding gravity waves produces somewhat more oscillatory behavior. Sustained oscillations leading to El Niño‐like events are easy to produce with the addition of a seasonal cycle and some random noise. El Nino events are then found as occasional amplifications of the seasonal cycle. The difference between El Niño “events” and irregular amplifications of the seasonal cycle is then rather arbitrary. A common feature in all models is an oscillation between two equilibria due to an instability, either linear or to finite size perturbations, of the coupled ocean atmosphere system. The robustness of this suggests it may be a feature of the real system. The timing within events is governed by the detailed dynamics allowed in the particular model, but oscillatory behavior is readily obtained simply by allowing sufficiently strong, but not too strong, coupling between model atmosphere and ocean, plus perhaps some noise.
Correlation function study for sea ice
Authors: Lin F. C. , Kong J. A. , Shin R. T. , Gow A. J. , Arcone S. A.
Published: February 2008

For active and passive microwave remote sensing of sea ice, a correlation function of exponential form is extracted from the photograph of a horizontal thin section taken from a sample of artificially grown saline ice that closely resembled Arctic congelation sea ice. It is found that the extracted correlation lengths are consistent with the published average size of brine pockets. With the application of strong fluctuation theory and the bilocal approximation, the effective permittivity tensor is derived in the low‐frequency limit for an unbounded uniaxial random medium with two‐phase mixtures. Using the extracted correlation lengths, the effective permittivity tensor is computed as a function of fractional volume of brine inclusions and compared with in situ measurements at 4.8 and 9.5 GHz.
Large‐scale patchiness due to an annual plankton cycle
Authors: Bennett Andrew F. , Denman Kenneth L.
Published: February 2008

Attempts are now underway to estimate primary productivity on global scales from a combination of satellite color imagery and manned or unmanned platforms. Regardless of the observing network, productivity over basin scales will be estimated from plankton biomass, estimated in turn from composite (or averaged) satellite images. We show mathematically that the interaction of two‐dimensional horizontal turbulent diffusion with an annual cycle in plankton growth induces an intrinsic large‐scale patchiness in the resulting plankton biomass distributions. We assume, as typical scales, a root‐mean‐square horizontal turbulent velocity of 0.05 m s−1 and a turbulent mixing length of 50 km. The corresponding length scale for the resulting patchiness in the annual cycle is of the order of 700 km, or at least 10 times the mixing length. Such patchiness can occur even when the growth rate is spatially uncorrelated; in fact, we show that small‐scale grazing by zooplankton could actually enhance the patchiness at a length scale of 700 km. The existence of the patchiness is not contingent upon assuming a given, finite breadth for the region of fertile water, unlike the earlier “KISS” models, nor upon spatial forcing of any type, but depends only on the existence of an annual cycle in primary productivity. The implication for numerical modeling is obvious. The most plausible physical requirement might be a turbulent quasi‐geostrophic motion field at or approaching statistical stationarity, but the first priority for the biological component embedded in the modeled motion field ought to be a model of the onset of the spring bloom and the subsequent development of a seasonal cycle.
Temperature and velocity structure of the Gulf Stream northeast of Cape Hatteras: Modes of variability
Authors: Manning James P. , Watts D. Randolph
Published: February 2008

Thirty‐five expendable bathythermograph sections and 10 Pegasus velocity sections taken approximately normal to the Gulf Stream northeast of Cape Hatteras were used to examine the variability of the current's structure. The regridded profiles were superimposed in a stream coordinate system such that the temperature variance associated with the superposition was minimized. The resulting variance field was examined using empirical orthogonal functions. The temperature variability is heavily weighted by seasonal warming; hence the following results exclude temperatures from the upper 200 m. The first mode (48% of the variance) is due to a deepening of the thermocline relative to the mean on the offshore side of the Gulf Stream and a shoaling on the inshore edge, associated with a change of geostrophic velocity and baroclinic transport. The amplitudes of this “transport mode” indicate a slight downstream increase in baroclinic transport (∼2% per 100 km) and a seasonal modulation (5 Sverdrups rms) that peaks in May‐June. The second temperature mode below 200 m (13–21% of the variance) is associated with a change in horizontal shear, a “vorticity mode,” and is positively correlated with measured path curvature and layer thickness. The directly observed Pegasus velocity sections were combined with corresponding temperature sections to compute their joint empirical modes. The dominant combined mode has a thermocline structure change as in the transport mode that is coupled with an increase in alongstream velocity and, for levels above the main thermocline, an inflow from both sides toward the center. Both temperature and velocity data indicate that high transport in the Gulf Stream is associated with lateral displacement offshore of its mean position.
Substructure of oceanic salt finger interfaces
Authors: Marmorino G. O.
Published: February 2008

Measurements made with a towed thermistor chain in the thermohaline staircase east of Barbados show that the thick salt finger interfaces have horizontally coherent vertical fine structure. In an extreme form this interfacial substructure resembles a “mini‐staircase” with several thin, high‐gradient subinterfaces separating 2‐m‐thick sublayers which appear to be stirred by convective plumes. Signals from a simultaneously towed microstructure probe are consistent with previous models of salt fingers. Finger wavelength roughly follows the expected dT/dz−0.25 dependence; Cox number values associated with the fingers are most commonly between 10 and 20, independent of dT/dz; and finger amplitude increases roughly as dT/dz0.75, where dT/dz is the local vertical temperature gradient.
Observational evidence of salt lens formation in the Iberian Basin
Authors: Käse Rolf H. , Beckmann Aike , Hinrichsen Hans‐Harald
Published: February 2008

The quasi‐synoptic picture of the mesoscale eddy field off Portugal is presented on the basis of recent hydrographic measurements. The dynamic topography and the salinity field at middepths (900–1300 m) is shown to be dominated by several isolated meddy structures superimposed on a general east‐west gradient. Deduced from the actual dynamical situation, there is evidence for meddy generation in this region. Numerical process studies demonstrate the possibility of vortex formation by baroclinic instability of a northward middepth jet of Mediterranean Water and by fragmentation of larger‐scale Mediterranean Water blobs.
The behavior of double‐diffusive intrusion in a rotating system
Authors: Yoshida J. , Nagashima H. , Niino H.
Published: February 2008

The effects of Earth's rotation on the stability of a thermohaline front of finite width are studied by means of a linear theory. It is found that when the rotation is present, two different types of unstable modes are possible. When the front is narrow and a Rossby radius of deformation based on Ruddick and Turner's (1979) vertical scale is large in comparison with the width of the front, the fastest growing intrusion is nearly two dimensional (nonrotational mode), and its vertical scale is given by Ruddick and Turner's scale. When the Rossby radius becomes small, in addition to the nonrotational mode there appears another unstable mode (the rotational mode) which has a smaller vertical wave number than the nonrotational mode. With the introduction of rotation, the fastest growing mode has nonzero along‐frontal wave number; that is, the intrusion becomes tilted in the along‐frontal direction. When the Rossby radius of deformation is sufficiently small in comparison with the width of the front, transition from the nonrotational mode to the rotational one occurs. The transition from nonrotational to rotational mode becomes less pronounced when the width of the front is increased for fixed horizontal density‐compensating gradients of temperature and salinity. For a wide front the growth rate and vertical wave number for both modes becomes similar, which agrees with the results of previous studies for infinite fronts that rotation does not modify the behavior of the intrusion except for the occurrence of along‐frontal tilt.
The tritium:krypton‐85 age of Denmark Strait Overflow Water and Gibbs Fracture Zone Water just south of Denmark Strait
Authors: Smethie William M. , Swift James H.
Published: February 2008

The opposite time trends of the input of tritium and 85Kr to the surface ocean produce a tritium:85Kr ratio for surface water that is a strong function of time and this ratio was used to determine the age of Denmark Strait Overflow Water (DSOW) and Gibbs Fracture Zone Water (GFZW) just south of Denmark Strait. DSOW and GFZW were identified by their temperature, salinity, oxygen, and silica characteristics in a section of stations 460 km south of Denmark Strait taken during the Transient Tracers in the Ocean/North Atlantic Study expedition in 1981. DSOW was the densest water observed in the section and there were two types, a low salinity type and a slightly higher salinity, more dense type. Both types originated from Arctic Intermediate Water (AIW) behind the Greenland‐Iceland ridge. The tritium and 85Kr data reveal that the low salinity type resided behind the Greenland‐Iceland ridge for about 1 year before flowing into the Irminger Sea, compared to about 15 years for the higher salinity type. The volume transport of the low salinity type of DSOW was estimated to have a lower limit of 0.8 Sv. GFZW forms in the northeastern Atlantic from a mixture of water flowing out of the Norwegian Sea at about 900 m depth and the northeastern Atlantic water into which it flows. About 70% of the tritium and 85Kr burden of GFZW comes from northeastern Atlantic water and 30% from Norwegian Sea water. The age of GFZW just south of Denmark Strait relative to its formation in the northeastern Atlantic is 7.5+4/−6.5 years which corresponds to a mean current speed of 1.6+10.1/−0.6 cm/s.
Results from the Genesis of Atlantic Lows Experiment physical oceanographic studies: Introduction
Authors: Bane John M.
Published: February 2008

The Genesis of Atlantic Lows Experiment (GALE) was conducted during early 1986 to investigate the rapidly developing, extratropical winter cyclones that travel across eastern North America and out over the northwestern Atlantic Ocean. Until only recently, these storms have been difficult to predict accurately [Sanders, 1986a, b], and they are well known for the devastating weather they sometimes bring to the eastern seaboard [Bosart, 1981; Kocin and Uccellini, 1984]. Such storms frequently produce “crippling” ice, heavy snow, and gale force winds, and they may batter the east coast from the Carolinas northward, often causing property damage and loss of life. The strong winds that accompany such a storm, as well as the cold, dry, continental air that often flows out over the Atlantic during the cold air outbreak stage of the storm's passage, are responsible for some of the most vigorous air‐sea exchanges of momentum, heat, and moisture in this part of the world [Budyko, 1974; Bunker and Worthington, 1976].
Modeling the nutrient and carbon cycles of the North Atlantic: 2. New production, particle fluxes, CO2 gas exchange, and the role of organic nutrients
Authors: Schlitzer Reiner
Published: February 2008

The nutrient and carbon cycles in the North Atlantic are studied by combining mapped nutrient station data with the annual mean hydrographic data of Levitus (1982) in the context of an inverse model. New production rates, Corg, CaCO3 and opal fluxes, and CO2 air‐sea gas exchange rates consistent with the nutrient distributions and with the circulation pattern evolving from the Levitus data are calculated. In a first model only the inorganic components phosphate, nitrate, and ΣCO2 are considered in the budgets for the nutrients. Model calculations show that the distributions of the inorganic nutrients are consistent with the Levitus circulation field and that model new production, particle fluxes, and CO2 gas exchange rates are compatible with independent estimates. The influence of dissolved organic nutrients on the oceanic nutrient and carbon cycles has been investigated with a series of models which include traditionally accepted values for the dissolved organic nutrients or recently reported data by Sugimura and Suzuki (1988). The model with traditionally accepted values for dissolved organic nutrients is feasible, and the results differ only slightly from the model with inorganic nutrients only. Models with data from Sugimura and Suzuki (1988) are infeasible. When concentrations of dissolved organic nutrients are derived from the correlation with apparent oxygen utilization (AOU), violations of model constraints are relatively small. Nevertheless, the correlation between dissolved organic nutrients and AOU found by Sugimura and Suzuki (1988) for the northwest Pacific seems not to hold in the North Atlantic because it would imply unreasonably small new production rates. More data for the dissolved organic nutrients must be collected before conclusive answers concerning their role in the oceanic nutrient cycles can be given.
Sediment transport processes in estuaries: An introduction
Authors: Perillo Gerardo M. E. , Lavelle J. William
Published: February 2008

Research on estuarine sediment transport processes has received increasing attention in recent years, attention related to concerns about water clarity, pollutant distribution and transport, dredge spoil disposal, creation and maintenance of channels and basins for navigational purposes, and shoreline erosion. Still, the geophysical community that addresses these concerns and the underlying fundamentals of sediment transport in an estuary is widely but relatively sparsely distributed around the world. The need to draw these researchers together to discuss ideas and outlooks led to the AGU Chapman Conference on Sediment Transport Processes in Estuaries that was held at the Universidad Nacional del Sur in Bahía Bianca, Argentina, from June 13 to June 17, 1988 [Perillo and Lavelle, 1988]. The meeting sought to provide a timely impetus to further progress in sediment transport research in estuaries, promote communication among researchers using different investigatory approaches, and develop collaborations among estuarine scientists in developed and developing nations.
The relationship between total precipitable water and surface‐level humidity over the sea surface: A further evaluation
Authors: Hsu S. A. , Blanchard B. W.
Published: February 2008

The fifth‐order polynomial regression relating total precipitable water (W) and surface‐level humidity (Q) over the sea surface, proposed by Liu (1986), has been further verified by instantaneous radiosonde ascents from 13 experiments conducted in geographically different regions ranging from the equatorial Pacific to the Norwegian Sea. When Q ranges from 3 to 21 g/kg, the composite root‐mean‐square‐error between Liu's formula and these 13 experiments is found to be approximately 1 g/kg. It is therefore suggested that as a first approximation, Liu's formula may be useful for operational weather analysis and forecasting using instantaneous data, inasmuch as Q can be determined with W measured routinely worldwide by satellites.
Estimation of the two‐dimensional ocean current shear field with a synthetic aperture radar
Authors: Barnett Tim P. , Kelley Francis , Holt Benjamin
Published: February 2008

A double inverse method to convert SAR data to estimates of current shear was developed theoretically. Problems of applying the technique to real SAR data were examined. The first step of the inversion process involved elimination of noise in the (
) wave spectra deduced from SAR images. The second step, estimation of current shear, required the existence of well defined swell and a favorable single of interaction between swell and ocean current. Application of the method in the Antarctic Circumpolar Current encountered sea conditions that both satisfied and violated these requirements. Where the wave‐current geometry was favorable, realistic surface current velocity fields were obtained, suggesting the presence of a substantial mesoscale eddy. Another attempt to apply the method in the Gulf Stream produced wave spectral estimates that showed no clear peak and hence produced no current estimates.
The modulation of the radar cross section of the ocean surface by an azimuthally traveling long gravity wave
Authors: West James C. , Moore Richard K. , Holtzman Julian C. , Gogineni S. P.
Published: February 2008

A new technique is used to calculate the modulation of the radar cross section of the ocean surface by a long gravity wave having a single narrow‐band spectral peak. Spectral analysis of the measured wave height time series is used to determine the position on the dominant wave illuminated during each radar sample. When we average the samples of the signal power received from the same phase position (but during different cycles) on the long wave, the phase‐interference (Rayleigh) fading is greatly reduced. The result is a measurement of the average radar cross section of the ocean surface as a function of the position on the long wave. The technique was applied to experimental runs where the radar beam was parallel to the crest of the dominant wave (cross‐wave look direction). Significant modulation was observed at both C and X bands, although there was no tilt modulation. A scenario where hydrodynamic and aerodynamic effects give rise to the modulation is discussed. “Sea spikes” occurred primarily near the front face of the long wave, with some also appearing immediately after the crest has passed, supporting the theory that they are often associated with the breaking of ripple waves riding near the top of the long wave.
Periodic resuspension in Baltimore canyon by focusing of internal waves
Authors: Gardner Wilford D.
Published: February 2008

High concentrations of particulate matter are frequently reported in submarine canyons. This study demonstrates that in Baltimore submarine canyon, elevated concentrations result from periodic resuspension events. The sloping bottom and V‐shaped topography of submarine canyons could accelerate tidal flows in the canyon axis sufficiently to cause resuspension, which occurs predominantly at tidal frequencies. However, current meter and transmissometer data show that resuspension events are often associated with a borelike intrusion of cold water moving upslope near the canyon head; an event that is consistent with the focusing of internal wave energy toward the canyon head at tidal frequencies. Hydrographic conditions for the focusing of energy are most favorable in late winter and early spring when stratification in the canyon is diminished; direct observations reveal that the most pronounced resuspension events occur during those seasons. Bottom stresses generated in the canyon are sufficient to resuspend sediment along the axis at least between 200 and 800 m, with little resuspension occurring deeper along the axis, on the canyon walls, adjacent slope, or shelf. During a resuspension event, the sediment‐laden water is forced upcanyon, then sloshes back down the canyon until it reaches a layer of higher‐density water, detaches from the canyon floor, and moves seaward along a density surface. This is a means of transferring sediment and associated pollutants intercepted at the canyon head to deeper water, across the shelf‐slope front. Over geologic time, frequent resuspension and seaward transport may aid in the headward erosion of submarine canyons, or at least inhibit their infilling during interglacial times.
The origin and characteristics of the Algerian Current
Authors: Arnone Robert A. , Wiesenburg Denis A. , Saunders Kim D.
Published: February 2008

Satellite and ship data collected between May 1 and 22, 1986, show that the Algerian Current originates in the Almeria‐Oran Front and remains within 30 km of the Algerian coast for over 300 km, between Oran and Algiers. The differences between the water masses of the current and the resident regional waters are clearly observed in the chlorophyll concentrations and optical characteristics, thus permitting the resulting patterns to be used as tracers of the circulation. During this field period, the Algerian Current begins as an extension of the Almeria‐Oran Front, which is found at the western periphery of the Eastern Alboran Gyre. This major oceanographic salinity front (ΔS > 1/km) delineates the boundary of the Algerian Current, which has average surface velocities of 40 cm/s and maximum speeds reaching 80 cm/s. The current, which has a volume transport between 0.4 and 0.5 Sv, is confined to the upper 150 m and is in geostrophic balance. A sequence of coastal zone color scanner (CZCS) and advanced very high resolution radiometer (AVHRR) satellite imagery showing the rapidly evolving surface circulation features is augmented with in situ ship data of current and water mass characteristics. Rapid undulations and frontal movements of the current were observed during the study period; however, the flow remained unbroken between 1° and 3°E. Current instability increased eastward as the undulations were sharper and the current appeared to depart from the coast near Algiers.
Chlorophyll a and respiratory electron transport system activity in microplankton from the surface waters of the western Mediterranean
Authors: Martinez Rosa , Arnone Robert A. , Velasquez Zoila
Published: February 2008

The distribution of microplankton biomass and the metabolism of surface waters in the western Mediterranean are shown to be strongly associated with the circulation. Chlorophyll a and respiratory electron transport system (ETS) activity of microplankton were measured in surface waters during the Western Mediterranean Circulation Experiment in May and June 1986. Biological signatures, defined by satellite and surface ship measurements, defined complex circulation patterns of surface waters between the Alboran Basin and the Straits of Sicily. Close correlation was found between the distribution of in situ measured surface chlorophyll, ETS activity, ETS/Chl ratio (chlorophyll‐specific ETS activity), salinity, and temperature with satellite‐derived chlorophyll and surface temperature. High ETS/Chl ratios were found located in frontal regions of the Algerian Current and encircling a large anticyclonic eddy. Statistical correlations were performed on in situ and synoptic satellite data. Results show positive correlation between chlorophyll and ETS activity and ETS/Chl ratio and temperature. Excellent correlations were found between in situ chlorophyll and chlorophyll derived from the coastal zone color scanner. Improved statistical correlations were observed by separation of the surface waters based on their salinity into Modified Atlantic Water (MAW) (S < 37) and Mediterranean surface waters (S > 37). MAW with higher chlorophyll and metabolic activity is shown to characterize the circulation of the Algerian Current. The differences in the statistical correlations of these water masses indicate that regional circulation significantly influences the biological functions of microplankton populations.