bluuebunny/crossref_metadata_2025 huggingface dataset:
Joint inversion of shear wave travel time residuals and geoid and depth anomalies for long‐wavelength variations in upper mantle temperature and composition along the Mid‐Atlantic Ridge
Authors: Sheehan Anne F. , Solomon Sean C.
Published: February 2008

We report measurements of SS‐S differential travel time residuals for nearly 500 paths crossing the northern Mid‐Atlantic Ridge. Differential travel times of such phases as SS and S with identical source and receiver have the advantage that residuals are likely to be dominated by contributions from the upper mantle near the surface bounce point of the reflected phase (SS). Under this assumption, differential SS‐S travel time residuals are mapped at the SS bounce points as a means of delineating lateral variations in mantle structure. After removing the signature of lithosphere age, we find evidence for variations in SS‐S residuals along the ridge at wavelengths of 1000–7000 km. These travel time anomalies correlate qualitatively with along‐axis variations in bathymetry and geoid height. We formulate a joint inversion of travel time residual, geoid height, and bathymetry under the assumption that all arise from variations in upper mantle temperature or bulk composition (parameterized in terms of Mg #). The inversion employs geoid and topography kernels which depend on the mantle viscosity structure. Inversion for thermal perturbations alone provides good fits to travel time and geoid data. The fit to topography, which is likely dominated by unmodeled crustal thickness variations, is not as good. The inversions for temperature favor the presence of a thin low‐viscosity layer in the upper mantle and temperature perturbations concentrated at depths less than 300 km. Compositional variations alone are unable to match the travel time and geoid or bathymetry data simultaneously. A joint inversion for temperature and composition provides good fits to both geoid and travel time anomalies. Temperature variations are ±50 K and compositional variations are ±0.5–3% Mg # for models with the temperature variations uniformly distributed over the uppermost 300 km and the compositional variations either distributed uniformly over the same interval or concentrated at shallower depths. The magnitudes of these variations are consistent with the chemistry and geothermometry of dredged peridotites along the Mid‐Atlantic Ridge.
Evolution of hydrothermal activity on the Juan de Fuca Ridge: Observations, mineral ages, and Ra isotope ratios
Authors: Stakes Debra , Moore Willard S.
Published: February 2008

We discuss observations of hydrothermal activity from sites along the Juan de Fuca Ridge and report the ages and radium isotope ratios of samples from eight sites collected on a series of 11 Alvin dives in September 1988. This was a pilot program to assess whether uranium and thorium series isotopes could provide age information on the duration of active venting. The ages are based on the ingrowth of 228Th from 228Ra in both active and inactive chimneys and chimney talus composed of barite, anhydrite, chalcopyrite, wurtzite, and pyrite. Samples were obtained from (from north to south) the Fairycastle Site (northern Endeavour Ridge), the Endeavour Hydrothermal Site, and the southern Juan de Fuca Sites (named vent 3, vent 1, and Plume areas). Samples from active chimneys (20) yielded 228Th/228Ra ages in the range 0.2 to 1.2 years. We recorded no ages in the range 1.2 to 12 years for inactive chimneys or rubble. Our tentative conclusions are that the individual chimneys form rapidly and that the presently active chimney structures are <2 years old. In the Fairycastle Site there is at least one episode of hydrothermal activity recorded in the rubble which occurred at least 20 years earlier. Large sulfide edifices in the Endeavour Hydrothermal Site probably record a minimum of 20 years of discharge for each particular structure. Chimney structures in the southern Juan de Fuca Ridge may be older; they show no evidence of significant recent growth except for an outer barite‐rich layer. The results of this study suggest that hydrothermal chimneys of diverse mineralogy can be dated by measuring the ingrowth of 228Th from 228Ra decay. The technique is most accurate for chimneys that are less than 10 years old. The technique, in conjunction with detailed petrological analyses, could be used to assess the growth rates of individual chimney walls. Variations in the 228Ra/226Ra activity ratios among the three areas reflect either differences in fluid residence time, depth of generation of hydrothermal fluids, or interaction with basaltic rocks previously altered at low temperatures.
Contemporary, Holocene, and Quaternary deformation of the Asal Rift, Djibouti: Implications for the mechanics of slow spreading ridges
Authors: Stein Ross S. , Briole Pierre , Ruegg Jean‐Claude , Tapponnier Paul , Gasse Francoise
Published: February 2008

Because the frequency and character of rifting events along mid‐ocean ridges are largely unknown, how the repetition of such events gives rise to rift structures is unexplored. The Asal rift in the Afar depression of Djibouti, Africa, provides the world's best subaerial analogue for young slow spreading mid‐ocean ridges. Seismic, geodetic, and field observations of a seismovolcanic event in 1978 at Asal yield estimates of the fault and dike locations, geometry, displacement, and volume of basalt extruded in a rifting event. A 6–9 kyr‐old lake shore highstand at Asal has been warped downward by 70 m, providing a Holocene measure of the vertical deformation across the rift. The rift topography furnishes an older datum, which we infer to be 34±6 kyr old using the Holocene deformation rate. We find that faults throughout the rift valley are active; Holocene slip rates diminish beyond 4 km from the rift axis; late Quaternary rates decrease beyond 6–7 km. The Holocene slip rates are used to estimate repeat times by taking the displacement on the faults which slipped in 1978 as characteristic; we find tectonic events on individual faults recur every 200– 300 years. Half the rift faults slipped together in the 1978 event. If this is typical, then groups of faults are activated every 100–150 years. We suggest that half the events take place in the rift axis accompanied by volcanic extrusion; the remainder occur peripheral to the neovolcanic zone and involve fault slip only, both events having a repeat time of 200–300 years. Given the 10 km width of the rift and its 16 mm yr−1 spreading rate, the mean age of the material in the rift should be ∼350 kyr, an order of magnitude older than the inferred age of the formation of the rift topography. The subsidence rate of the rift axis during the past 35 kyr is 8–9 mm yr−1, with the rate of infilling by volcanic extrusion <1 mm yr−1. The resulting net subsidence rate, about equal to the half‐spreading rate of the rift, could not be sustained for 300 kyr without significant infilling by lavas. Thus both observations suggest that the long‐term vertical deformation in the rift has not been steady state. Instead, we suggest that there is a rifting/filling cycle at Asal, with the most recent filling episode ending ∼35 kyr.
Shock wave equation of state of muscovite
Authors: Sekine Toshimori , Rubin Allan M. , Ahrens Thomas J.
Published: February 2008

Shock wave data to provide an equation of state of muscovite (initial density: 2.835 g/cm3) were determined up to a pressure of 141 GPa. The shock velocity (Us) versus particle velocity (Up) data are fit with a single linear relationship: Us=4.62(±0.12) +1.27(±0.04)Up (km/s). Third‐order Birch‐Murnaghan equation of state parameters (isentropic bulk modulus and isentropic pressure derivative of bulk modulus) are Kos=52±4GPa and K'os=3.2±0.3. The pressure‐temperature relation along the Hugoniot suggests that muscovite may dehydrate to KAlSi3O8 (hollandite), corundum, and water, with a small volume change, above 80 GPa. Thermodynamic calculations of the equilibrium pressure for the dehydration yields a significantly lower value. Observed unloading paths from shock pressures up to about 80 GPa are steeper in a density‐pressure plane than the Hugoniot and become shallower with increasing shock pressure above that pressure. The changing slope may indicate that devolatilization occurs during unloading above 80 GPa. The present equation of state data for muscovite are compared with results of previously reported recovery experiments.
Anomalously deep crustal earthquakes in the Ventura Basin, southern California
Authors: Bryant A. Steven , Jones Lucile M.
Published: February 2008

Earthquakes at depths of 20–30 km occur beneath the Ventura Basin in southern California. The epicentral distribution of deep seismicity outlines an east trending ellipse which corresponds closely with the mapped Santa Clara Syncline, the main structural element of the Ventura basin with 15 km thickness of sediments. Clear Pn and Pg phases are seen for even the deepest earthquakes which demonstrate that these earthquakes are occurring within the crust. Travel time curves from shallow and deep earthquakes in the western Transverse Ranges require that the depth to the Moho under the Ventura basin must be depressed by 7–10 km relative to the surrounding area, indicating crustal thickening of 20–35%. Other researchers have shown that the Ventura basin has the lowest heat flow in western California. Thus, low heat flow, exceptionally thick (15 km) sediments, rapid shortening of the crust, the deep earthquakes, and a depressed Moho all coincide within a small area, strongly suggesting a common cause for all five phenomena. The confinement of the deep earthquakes and depressed Moho to the Ventura region suggests that the rapid shortening of the Ventura basin is anomalous and not representative of the rate of shortening across all of the western Transverse Ranges.
Analytical models for flow through obstructed domains
Authors: Piggott Andrew R. , Elsworth Derek
Published: February 2008

The effects of low‐ or zero‐conductivity inclusions on flow through two‐ and three‐dimensional domains are represented using an analogy to percolation theory. Here, the term flow refers to the analogous processes of fluid, electrical current, and heat conduction. Mapping functions are derived from renormalization group models as a solution to the percolation analogy. These functions relate the extent to which a domain is obstructed to the probability that the domain is conductive and to the bulk conductivity of the domain. Stochastic methods are used to define a relation between the proportion of a domain which is isolated from flow and the proportion which is obstructed. A description of the influence of the scale of the obstructions, relative to that of the domain, is inherent in the formulation. Data obtained from numerical and electrical analogues are used to verify the mapping functions and assess the limitations of the representation.
Gravity fields of the southern ocean from Geosat Data
Authors: McAdoo David C. , Marks Karen M.
Published: February 2008

In August 1990, the U.S. Navy declassified all Geodetic Mission (GM) radar altimeter data acquired by the Geosat satellite over oceanic regions south of 60°S. We have used these GM data in conjunction with the unclassified, lower‐resolution Geosat Exact Repeat Mission (ERM) altimeter data to construct high‐resolution gravity fields on a 5‐km grid covering the annular region of the southern ocean, which lies between 60°S and 72°S and encircles Antarctica. During the GM a complete mapping of the marine geoid (between 72° and 72°N) was accomplished. The GM produced more densely spaced ground tracks (typically 2 or 3 km at 60°S) than those of either the ERM or Seasat. Consequently, we were able to use the GM data to map the marine gravity field at a higher resolution than was previously possible using satellite altimeter data. This paper describes the techniques we used to derive these gravity fields and image them. These techniques involve (1) computing along‐track sea surface height slopes, (2) gridding of these ascending and descending slopes, (3) converting the slopes to conventional deflections of the vertical, (4) transforming the deflections to gravity anomalies in the frequency domain, and (5) imaging. The resulting images of the marine gravity field reveal much that is new about the seafloor and the tectonic fabric of the southern ocean: a region which includes large expanses of seafloor that have never been surveyed by ships.
Thin, low‐velocity crust beneath the southern Yukon‐Tanana Terrane, east central Alaska: Results from Trans‐Alaska crustal transect refraction/wide‐angle reflection data
Authors: Beaudoin Bruce C. , Fuis Gary S. , Mooney Walter D. , Nokleberg Warren J. , Christensen Nikolas I.
Published: February 2008

A seismic refraction/wide‐angle reflection survey for the Trans‐Alaska Crustal Transect program reveals a thin, reflective crust beneath the southern Yukon‐Tanana terrane (YTT) in east central Alaska. These data are the first detailed refraction survey of the southern YTT and compose a 130‐km‐long reversed profile along the Alaska and Richardson highways. Results from this study indicate that low‐velocity (≤ 6.4 km/s) rocks extend to approximately 27 km in depth. Based on these low velocities and an average Poisson's ratio of 0.23 determined for depths of ≤27 km, an overall silicic composition is interpreted for this portion of the crust beneath the Yukon‐Tanana terrane. From approximately 8 to 27 km depth the crust exhibits an increase in reflectivity. This middle to lower crustal reflectivity is modeled as alternating high‐ and low‐velocity lamellae with an average velocity of 6.1 km/s at 10 km depth to an average velocity of 6.4 km/s at 27 km depth. Beneath these reflective, low‐velocity rocks a 3‐ to 5‐km‐thick, 7.0 km/s basal crustal layer produces a prominent reflection that extends to offsets of up to 280 km. The crust‐mantle boundary, modeled at an average depth of 30 km, produces a variable PmP reflection, which may indicate lateral heterogeneity of this boundary, and a weak and emergent Pn refraction with a velocity of 8.2 km/s. We interpret the crustal section as follows: the low‐velocity rocks of the southern YTT extend from the surface to depths of approximately 10 km; underthrust Mesozoic flysch of the Kahiltna terrane, rocks of the Gravina arc, and basement of the Wrangellia(?) terrane extend from 10 to 27 km depth; a 3‐ to 5‐km‐thick layer of mantle‐derived mafic rocks, relic oceanic crust, or Wrangellia(?) terrane lower crust extends from 27 to approximately 30 km depth; a tectonically young Moho beneath the southern YTT is found at an average depth of 30 km; and it is underlain by a mantle that may be relatively cool and/or olivine rich. In this interpretation, the Yukon‐Tanana terrane is a thin‐skinned terrane. Our results indicate that tectonic, and possibly magmatic, underplating has played a significant role in crustal growth for central Alaska.
Interannual variability of the spatial distribution of sea ice in the north polar region
Authors: Parkinson Claire L.
Published: February 2008

Passive microwave data from the Nimbus 5 and Nimbus 7 satellites have been used to determine baseline information on the interannual variability in monthly average sea ice distributions in the north polar region over the period 1973–1987. Monthly maps of the spatial range of ice distributions found for the 1973–1987 period show that, for instance, in February the greatest interannual variability occurs in the Sea of Okhotsk and in July particularly high interannual variability occurs in both Hudson Bay and the northern Barents Sea. In contrast, the portions of the ice edge with the least interannual variability in their February ice distributions lie in the southern Greenland Sea (59°–67°N) and immediately to the south and west of Svalbard. Plots of percent variability by month show that the marginal ice zones of the Sea of Okhotsk and the Bering Sea have their greatest ice distribution variability in winter and early spring, while the central Arctic has its greatest variability in late summer, and Hudson Bay has its greatest variability in July and November, during the ice decay and ice growth seasons, respectively. Since none of the regions showing high variability experienced a consistent (or even near‐consistent) trend toward either increasing or decreasing ice coverage, the demonstrated variability cannot easily be attributed to climate change. The maps and analysis presented here provide a 1973–1987 baseline against which future northern hemisphere sea ice distributions can be compared in the continuing attempt to analyze global climate and to determine whether or not significant climate change is occurring.
Concurrent high resolution bio‐optical and physical time series observations in the Sargasso Sea during the spring of 1987
Authors: Dickey T. , Marra J. , Granata T. , Langdon C. , Hamilton M. , Wiggert J. , Siegel D. , Bratkovich A.
Published: February 2008

The evolution of bio‐optical and physical properties of the upper layer of the open ocean has been examined at time scales from a few minutes to several months using recently developed multi‐variable moored systems (MVMS). Concurrent, colocated time series measurements of horizontal currents, temperature, photosynthetically available radiation, transmission of a beam of collimated light (660 nm), stimulated chlorophyll fluorescence, and dissolved oxygen concentration were made. The systems were located at eight depths in the upper 160 m of the Sargasso Sea (34°N, 70°W) and were deployed three times for a total of 9 months in 1987. The first deployment data presented here show considerably more variability than those of the latter two deployments because of the dynamic springtime shoaling of the mixed layer and the accompanying phytoplankton bloom and more mesoscale variability associated with cold core rings and warm outbreak waters associated with the Gulf Stream. These data are used to demonstrate the utility of the MVMS and indicate the importance of high‐frequency, long‐term sampling of bio‐optical and physical variables of the upper ocean for understanding and modeling dynamical changes in bio‐optical properties, primary production, and carbon fluxes of the upper ocean on time scales ranging from minutes to seasons to decades. Some phenomena observed with the systems include (1) diurnal variations in bio‐optical properties, (2) springtime stratification and rapid (∼2 days and less) episodic changes in the beam attenuation coefficient and in situ chlorophyll fluorescence, and (3) advective episodes associated with warm outbreaks of Gulf Stream waters and cold core Gulf Stream rings in the vicinity of the mooring.
A model of the formation of high‐salinity shelf water on polar continental shelves
Authors: Grumbine Robert W.
Published: February 2008

A model of the flow and salinity fields forced by sea‐surface salinity flux and wind stress curl is developed and used to examine the processes that create High‐Salinity Shelf Water (HSSW). The flow field is the sum of the baroclinic geostrophic flow driven by salinity variations and a barotropic geostrophic flow driven by wind stress curl. The salinity field is controlled by advection, convection, and sea surface salinity flux associated with sea ice formation. The model domain represents the Weddell Sea or Ross Sea continental shelf without topography. To examine the relative effects of wind stress and buoyancy forcing in HSSW production, the peak polynya freezing rate in the model is varied from 0.0 to 0.30 m d−1, and the Ekman pumping derived from the wind stress curl is varied independently from 0.0 to 1.8×10−6 m s−1. The Ekman pumping was seen to control the magnitude of the circulation, while the polynya freezing rate controlled the extent of salinization in the shelf water. The flux of HSSW increases linearly with increasing Ekman pumping above 0.3×10−6 m s−1. The flux of HSSW is linear with respect to the polynya freezing rate. The modelled flux of HSSW and the flux of derived Bottom Water for present estimates of the forcings (a peak freezing rate of 0.10 m d−1 and Ekman pumping of 0.2×10−6 m s−1) agree with with the fluxes inferred from physical and chemical observations in the deep Weddell Sea by oceanographic field programs. The modelled flux of Bottom Water for the Ross Sea also agrees with observations.
Suspension of sand due to wave groups
Authors: Hanes Daniel M.
Published: February 2008

Field observations of the suspension of sand near the seabed under shoaled waves in the nearshore region are presented and discussed with regard to the influence of wave groups. Sand is found to be suspended at both the incident wave frequency and at lower frequencies corresponding to wave groups. These limited observations indicate the likelihood that wave groups result in higher concentrations of suspended sand than randomly distributed waves of similar heights.
Sea‐to‐air transfer of surface‐active organic compounds by bursting bubbles
Authors: Tseng Ruo‐Shan , Viechnicki John T. , Skop Richard A. , Brown John W.
Published: February 2008

The bubble transport of natural surfactant materials from seawater to the air‐water interface and to the air is investigated experimentally by a radiotracer test system. We have studied the rates at which four lipid materials of differing molecular structure are scavenged from artificial seawater and ejected into the atmosphere by air bubbles of various size distributions at differing airflow rates and bubbling times. We have found that the bubble transport to the air for all materials studied is linearly proportional to the quantity of material transported to the surface microlayer, and the constant of proportionality appears to be independent of the type of material but is a function of the airflow rate and bubble size. Small bubbles were found generally to be more effective in transporting surface‐active organic compounds from seawater to the air.
Effects of stratification by suspended sediments on turbulent shear flows
Authors: Villaret Catherine , Trowbridge J. H.
Published: February 2008

Sediments suspended in turbulent flows of water over plane beds are known to influence the structure of the flows by which they are carried. Past attempts to model this effect have been based almost exclusively on a theoretical framework in which the dense solid particles stratify the flow and have an influence analogous to that of a downward heat flux in the stably stratified atmospheric surface layer. We compare results from a model based on this theoretical framework with laboratory measurements of ensemble‐averaged velocity and ensemble‐averaged particle concentration, obtained by previous investigators, in order to test the applicability of the theoretical approach to dilute suspensions of sand in turbulent flows of water. We find that the stratification effect can be observed qualitatively in measured velocity profiles, except in one series of experiments in which systematic measurement errors may have obscured the effect. Estimates of model constants based on measured velocity profiles are, overall, roughly consistent with expectations based on measurements in thermally stratified flows, although there is substantial variability in individual profiles. Some of this variability is explainable as a consequence of a parameter range in which stratification effects are too weak to be detected accurately, other variability is explainable as a consequence of approximations in the model, and still other variability is possibly a consequence of weak dependence of model constants on sediment grain size, which was not expected from the theoretical development. The stratification effect is not observed in individual particle concentration profiles but must instead be observed in experiments in which the flow conditions and particle properties are held fixed while the particle load is increased.
Phytoplankton pigment patterns and wind forcing off central California
Authors: Abbott Mark R. , Barksdale Brett
Published: February 2008

We use a 4‐year time series of high‐resolution coastal zone color scanner imagery to study mesoscale variability in phytoplankton pigment (as a surrogate for biomass) distributions off central California during the spring‐summer upwelling season. We use empirical orthogonal functions to decompose the time series of spatial images into its dominant modes of variability. Similarly, we analyze wind fields derived from the Fleet Numerical Oceanography Center pressure fields. The results from these analyses are used to investigate the coupling between wind forcing of the upper ocean and phytoplankton distributions on mesoscales. Prior to the spring transition, wind stress is dominated by strong northward conditions, and pigment is low and relatively uniform throughout the California Current domain. The first strong southward wind event soon results in a bloom of phytoplankton that is also uniformly distributed offshore. Nearshore values fluctuate relatively little in response to variations in wind forcing. After the spring transition, the wind becomes more steady (southward) and the fluctuations are dominated by changes in the strength of the curl. During southward events, the curl tends to become more positive. Depending on the fluctuations and strength of these curl episodes, filaments begin to form at particular locations. The overall pigment concentration in the California Current drops as a larger fraction of the pigment is distributed within the filaments. The filaments also tend to become shorter during this period, which starts soon after the spring transition and usually reaches its maximum intensity in early summer. Winds (and curl) become much weaker in the mid‐July to August period, and the filaments become much less prominent in terms of concentration and their length increases. Isolated wind events occasionally reinvigorate the filaments. A final bloom in the fall occasionally appears with spatial structure similar to the spring bloom. We conclude that wind forcing, in particular the curl of the wind stress, plays an important role in the distribution of phytoplankton pigment in the California Current. Although the underlying dynamics, especially of the filaments, may be dominated by processes other than forcing by wind stress curl (such as eddies or topographically induced instabilities), it appears that curl may force the variability of the filaments and hence the pigment patterns.
A comparison of techniques for tide removal from ship‐mounted acoustic Doppler measurements along the southwest coast of Vancouver Island
Authors: Foreman M. G. G. , Freeland H. J.
Published: February 2008

Two approaches are examined for the removal of tidal currents from acoustic Doppler current profiler (henceforth ADCP) measurements. The first uses predictions from a numerical model (Foreman and Walters, 1990) whose domain includes the measurement region. Although the numerical model is barotropic, adjustments are made to correct for baroclinicity in accordance with historical observations from stationary current meters in the region. The second approach (originally proposed by Candela et al. (1990)) performs a least squares polynomial or biharmonic spline fit to the ADCP data in order to determine the spatially varying amplitudes and phases of major tidal constituents and the magnitude of the residual flow. Both tide removal techniques are applied to ADCP data taken during a 3‐day cruise off the southwest coast of Vancouver Island, and their relative effectiveness is compared. Some of the detided currents are seen to compare well with geopotential surfaces and clearly show the Juan de Fuca Eddy (Freeland and Denman, 1982). A discussion of the results and limitations of each technique is also presented.
Variability of internal tides in the Laurentian Channel
Authors: Wang Jia , Ingram R. Grant , Mysak Lawrence A.
Published: February 2008

Data from three sequential stations of thermistor chains and current meters along the main axis of the lower St. Lawrence estuary during the summer‐fall period in 1978 were analyzed to help understand generation and variability of the internal tide. The Lagrangian (or direct tracing) method has been developed to compute the displacements of internal tides, which can overcome limitations of the Eulerian method caused by the very small vertical temperature gradients. Internal tides of semidiurnal frequency were observed, but diurnal tides were not, in both the time domain and frequency band. The surface diurnal tide behaves like a standing wave rather than a progressive one. Due to the nonlinear interaction of the internal tide, generated locally near the sill, with topography, quarter‐diurnal and higher frequency oscillations were also generated. The displacement of internal tides can reach 80, 40 and 30 m during spring tides, respectively, at station 1, and at two downstream stations 3 and 5. The internal tides propagate seaward with attenuation downstream, which mainly results from changing vertical stratification conditions, but may also be partly attributed to wave breaking and turbulent mixing in the upper layer and bottom and lateral friction. The amplitude ratio of S2/ M2 is smaller (0.2) for the baroclinic current than for the barotropic current (0.24) at station 1 but becomes larger at stations 3 and 5. This implies the presence of the narrower response frequency band of the baroclinic tide to the barotropic forcing near the generation site. The progressive semidiurnal and quarter‐diurnal internal tides were of the Poincare type, which implies a three‐dimensional structure. The “nutrient pumping” due to the internal tides was confirmed to occur only near the generation region of the internal tide. This convergence zone of higher primary productivity is considered to be equivalent to the “rotor clouds” in atmospheric mountain waves.
Mean flow and variability in the Kuroshio Extension from Geosat altimetry data
Authors: Qiu Bo , Kelly Kathryn A. , Joyce Terrence M.
Published: February 2008

Using altimeter data from the Geosat Exact Repeat Mission (ERM), we investigated the mean flow and temporal and spatial variations of the Kuroshio Extension in the region of 140°–180°E and 30°–40°N. Mean surface height profiles were estimated along individual tracks by assuming the velocity profile of the Kuroshio Extension to be Gaussian‐shaped and by successively fitting this synthetic current's height profile to the residual height data. Using the mean profiles from ascending and descending tracks, we derived the mean surface height field by an inverse method and obtained the absolute surface height fields for the first 2.5 years of the Geosat ERM. Both the mean and the instantaneous height fields thus derived compared well with the available hydrographic data and the SST patterns from the NOAA satellites. The mean surface height difference across the Kuroshio Extension attains its maximum around 146°E between the two quasi‐stationary meanders, and its decrease thereafter is mainly due to large‐scale recirculations on the southern side of the Kuroshio Extension. The ratio of the eddy kinetic energy over the mean kinetic energy has a nearly constant value of 1.5–2.0 along the Kuroshio Extension path. Propagation of mesoscale fluctuations in the height fields is generally westward except for the upstream region of the Kuroshio Extension. Effects of deep mean flow and baroclinic shear are found to be important in explaining the observed propagation speeds. In the upstream region of 141°E and 154°E, annual variations in the surface height difference across the Kuroshio Extension(δh) have a September maximum with an average amplitude of 0.2m. For large‐scale interannual fluctuations, anomalies in δh are found to be significantly correlated with those of the current axis positions: a larger surface height difference corresponds to a more northerly position of the Kuroshio Extension. The interannual changes in δh are possibly related to the 86/87 ENSO event in the low‐latitude Pacific Ocean.
The variance of the normalized radar cross section of the sea
Authors: Plant William J.
Published: February 2008

The dependence of the variance of the normalized radar cross section of the sea,
, on illuminated area and integration time is investigated. We show that the standard expression for this dependence cannot account for the correlation of power received from different points on the surface which is exhibited by microwave backscatter from the sea. An alternate expression is derived which does account for these correlations. We show that knowledge of the ocean wave spectrum and modulation transfer function along with the determination of three fundamental parameters, facet variance, the velocity spread of a facet, and correlation area, are necessary to specify the dependence of cross section variance on illuminated area and integration time. The correlation time is not a fundamental parameter but depends on illuminated area. An expression for that dependence is derived here. Issues of system noise and the relationship of received power variance to cross section variance are also addressed. We relate these calculations to the critical problem of determining confidence intervals for
measurements at high spatial resolution.
Wave‐induced advective transport below a rippled water‐sediment interface
Authors: Shum K. T.
Published: February 2008

The exchange between the water column and the sediment bed and the transport inside the permeable sediment layer are important processes in the cycles of chemical elements. In this paper we examine quantitatively the effects of modulations in the profile of the water‐sediment interface on this exchange and on the advective transport inside the sediment bed. The flow field inside a sediment layer bounded between spatially periodic ripples on top and an impermeable bottom surface is modeled using Darcy's law. The forcing is due to progressive gravity waves in the water above. The results of two different models for the pressure variation imposed on the upper boundary are compared. The two pressure profiles are derived from potential flow theory and from a numerical solution to the Navier‐Stokes equations for the oscillatory flow over a rippled bed. From an analytic solution to the two‐dimensional model, the trajectories of pore water particles immediately below the ripple profile are found to be quite different from the simple elliptical pattern found below a flat bed. The shapes of these trajectories can be quite complicated and vary considerably both along the length of the ripple and over the depth of the sediment layer close to its surface. The total exchange across the water‐sediment interface, averaged over one wave period, is significantly higher across a rippled interface than across a flat bed. This difference increases with increasing ripple slope and the strength of the wave motion, and it decreases with increasing thickness of the sediment layer relative to the length of the gravity wave. Since rippled bed forms are commonly found in coastal waters, the increase in the total exchange across a rippled water‐sediment boundary can enhance the exchange of solutes due to “wave pumping.” Immediately below the water‐sediment interface, circulation cells with net advective transport over a wave period are found. Such net advection patterns can lead to spatial (in the horizontal direction) inhomogeneities of the vertical concentration (or temperature) profiles if the overlying water column and/or the sediment bed act(s) as source or sink. This gives a plausible physical mechanism to explain the spatial variations in vertical concentration profiles found in field measurements.
Introduction to the WEC88 cruise: An investigation into why the equator is not greener
Authors: Barber Richard T.
Published: February 2008

This issue of the Journal of Geophysical Research contains seven papers that describe results from a cruise of R/V Wecoma, which worked in the equatorial Pacific in February and March 1988. The name of the cruise was WEC88. The research plan for this cruise was based on the evolving understanding of the regulation of biological productivity in equatorial waters. When the eastern and central equatorial Pacific was originally found to be richer than waters to the north and south, the first question oceanographers asked was, “Why is the tropical region so rich?” Early work established that a zonal band straddling the equator and extending across the entire Pacific basin has enhanced nutrient concentrations, increased phytoplankton productivity, and increased abundance of Zooplankton and fish [Sette, 1955].
Suppression of wind‐generated ripples by natural films: A laboratory study
Authors: Tang Shih , Wu Jin
Published: February 2008

The suppression of wind‐generated ripples by natural films was investigated in an oval‐shaped wind‐wave tank filled with seawater. Experiments were performed over the water surface with and without natural films; the former were produced through aging and the latter was assured through overflowing. Time series of the water surface slope were obtained with a computer‐interfaced matrix camera system to determine the slope spectra. The inception of waves by wind was much delayed over the slick surface, while at higher winds there were insignificant differences between the mean square slopes over clean and slick surfaces. At low winds, suppression of ripples by natural films was observed from the intrinsic wave frequency of about 7 Hz and up, with the maximum suppression at about 19 Hz. No obvious spectral dip was observed to account for the Marangoni effect observed by others over artificial slick surfaces.
Bubble flux and marine aerosol spectra under various wind velocities
Authors: Wu Jin
Published: February 2008

A complete size spectrum of bubbles in the near‐surface ocean is first deduced from available measurements. Combining this with the functional variation of bubble population with wind velocity, rates of bubbles arriving at the sea surface, expressed in the form of flux spectra, are obtained under various wind velocities. In the meantime, production of film and jet drops by artificially generated bubbles at the surface of seawater is parameterized. Spectra of marine aerosols under various wind velocities are finally obtained from the field‐determined bubble flux spectrum and the laboratory‐determined aerosol production function. The estimated spectra compare favorably with oceanic measurements.
Drifter observations of a cold filament off Point Arena, California, in July 1988
Authors: Swenson Mark S. , Niiler Pearn P. , Brink Kenneth H. , Abbott Mark R.
Published: February 2008

We use the position fixes and temperature readings from 56 Tristar‐II mixed‐layer drifters, in conjunction with advanced very high resolution radiometer images, to provide a description of the mesoscale variability of the flows associated with cold‐water filaments in the California Current system off northern California in July 1988. We find that the northern, offshore temperature front of the filament is convergent and is closely associated with the core of a high‐speed (>100 cm/s) jet that has a broader spatial scale than the cold‐water filament. Mesoscale features are observed to be related to all developments of the cold‐water filament on a variety of time scales (1–30 days). In one well‐observed feature, estimates of vertical velocity (w) based on a vorticity budget are −20 m/d over an area of 150 km2 and are associated with a gain of vorticity. Estimates of w based on a local heat budget following a drifter are about 3 times larger over an area of 60 km2. These intense downwelling features are also places where horizontal stirring occurs.
Energy transfers across an internal wave‐vortical mode spectrum
Authors: Ramsden Dave , Holloway Greg
Published: February 2008

Nonlinear interactions among internal gravity waves are investigated by direct numerical experiments. Cases which solve the full Navier‐Stokes equations (three dimensional, 3D) are compared to cases in which variability is suppressed in one direction (two dimensional, 2D). It is found that the 3D and 2D simulations exhibit certain similarities. This result may seem surprising, remains in part unexplained, and also is encouraging with respect to the utility of application of 2D simulations. We find both in 3D and in 2D that the transfer of kinetic energy (KE) from large to small scales is less efficient than the transfer of potential energy (PE). The imbalance between these transfers leads to a characteristic buoyancy flux spectrum which is negative (KE to PE) at large scales and positive (PE to KE) at small scales. Integrated over all scales, the buoyancy flux is very small for a wide range of flow regimes. However, results concerning buoyancy flux are sensitive to assumptions about the manner of energetic forcing.
The remote sensing of ocean primary productivity: Use of a new data compilation to test satellite algorithms
Authors: Balch William , Evans Robert , Brown Jim , Feldman Gene , McClain Charles , Esaias Wayne
Published: February 2008

We tested global pigment and primary productivity algorithms based on a new data compilation of over 12,000 stations occupied mostly in the northern hemisphere, from the late 1950's to 1988. The results showed high variability of the fraction of total pigment contributed by chlorophyll a (ρ), which is required for subsequent predictions of primary productivity. Two models, which predict pigment concentration normalized to attenuation length or euphotic depth, were checked against 2,800 vertical profiles of pigments (chlorophyll a, phaeopigment and total pigment). Phaeopigments consistently showed maxima at about one optical depth below the chlorophyll maxima. We also checked the global Coastal Zone Color Scanner (CZCS; daily 20km resolution) archive for data coincident with the sea truth data. A regression of satellite‐derived pigment versus ship‐derived pigment had a coefficient of determination (r2) of 0.40 (n=731 stations). The satellite underestimated the true pigment concentration in mesotrophic and oligotrophic waters (< 1 mg pigment m−3) and overestimated the pigment concentration in eutrophic waters (> 1 mg pigment m‐3). The error in the satellite estimate showed no trends with time between 1978 and 1985. In general the variability of the satellite retrievals increased with pigment concentration. Several productivity algorithms were tested which utilize information on the photoadaptive parameters, biomass and optical parameters for predicting integral production. The most reliable algorithm which explained 67% of the variance in integral production for 1676 stations suggested that future success in deriving primary productivity from remotely sensed data will rely on accurate retrievals of “living” biomass from satellite data, as well as the prediction of at least one photoadaptive parameter such as maximum photosynthesis.
Radiance calibrations for advanced very high resolution radiometer infrared channels
Authors: Steyn‐Ross D. A. , Steyn‐Ross Moira L. , Clift S.
Published: February 2008

We examine in detail the calibration procedures for the advanced very high resolution radiometer (AVHRR) infrared channels. The AVHRR literature states that the linear mapping used to convert radiometric count to scene radiance is satisfactory for channel 3, but is only approximately correct for channels 4 and 5 since these channels have a slight curvature in their respective radiance‐versus‐count response functions, so a nonlinearity correction (NLC) is applied. Traditionally, these adjustments have been tabulated as brightness temperature corrections; we find that when expressed as radiance corrections, the NLCs can be written in a particularly simple form which depends only on the temperature of the internal calibration target. This new formulation eliminates the need to interpolate within the tables of temperature NLCs and permits the correction to be applied prior to conversion to equivalent brightness temperature, rather than after. Examination of the calibration records for 48 NOAA 11 passes sampled over a full year revealed several significant features: (1) For low Sun elevation angles relative to the satellite, sunlight can apparently leak into the optical path of the AVHRR, distorting the in‐flight calibration data for channel 3, producing errors in apparent scene temperature as large as 2°C. (2) There is a clear demarcation between the calibration slopes for day and night operation of the satellite. (3) The thermal step that occurs when the spacecraft crosses the night‐day boundary into full sunlight leads to transient calibration errors which decay to zero with a time constant of about 40 s. At the peak of the transient, the apparent scene temperature can be depressed by over 1°C in channel 3, and by up to 0.4°C in channels 4 and 5. Quality control measures and techniques for compensating for calibration problems are discussed.
A note on the seasonal variability of the South Java Current
Authors: Quadfasel Detlef , Cresswell George R.
Published: February 2008

The seasonal cycle of the South Java Current is explored using current measurements with satellite‐tracked drifters and climatological compilations of ship drifts. It is shown that the changing monsoon winds and the variations of the freshwater flux from the Indonesian archipelago are responsible for the annual cycle of the flow, while the actual reversals between the seasons are strongly influenced by remote forcing through equatorial and coastal long waves from the central Indian Ocean.
The 26‐day oscillation observed in the satellite sea surface temperature measurements in the equatorial western Indian Ocean
Authors: Tsai Pedro T. H. , O'Brien James J. , Luther Mark E.
Published: February 2008

A 26‐day oscillation in sea surface temperature (SST) data is observed in the western Indian Ocean, from 52° to 60°E and in the vicinity of the equator. The SST data used in this study are obtained from the NOAA 9 satellite and are for the years 1987 and 1988. This fluctuation of SST at a period near 26 days is found to be antisymmetric about the equator and is trapped within the equatorial waveguide (equator ±6°). The variance associated with this oscillation has a maximum located at about 3° latitude; furthermore, the variance decreases at a faster rate toward the equator than poleward. These characteristics are consistent with the latitudinal structure for the mixed Rossby‐gravity (or Yanai) waves as predicted from linear wave theory. The temporal variation of this 26‐day oscillation is most energetic during the summer season (July to September), with maximum values of 0.4°C and 0.8°C found during August of 1987 and 1988 respectively. This observation agrees with the temporal variation of Yanai waves inferred from drifting buoy observations and numerical studies of the Indian Ocean. Thus we conclude that the Yanai wave is responsible for the 26‐day fluctuation observed in the SST data in this region.