bluuebunny/crossref_metadata_2025 huggingface dataset:
Geochemical constraints on initial and final depths of melting beneath mid‐ocean ridges
Authors: Shen Yang , Forsyth Donald W.
Published: February 2004

The comparison of the composition and volume of mid‐ocean ridge basalts to predicted values inferred from experimental and theoretical studies is one of the primary tools for mapping thermal structure and the geometry of melting beneath mid‐ocean ridges. Several indicators of either the pressure or degree of melting are commonly employed: Na2O concentration in melts is roughly inversely proportional to the extent of melting; (Srn/Yb)N is affected by the proportion of melt that is generated in the garnet stability field (deeper than 60 km); and FeO content increases with increasing pressure of melting. Crustal thickness also depends on the geometry and extent of melting. K2O/TiO2 is used as an indicator of the enrichment of the mantle source. For normal ridges, we find that regional averages of Fe(8) (FeO normalized to 8 wt % MgO to correct for fractionation) are strongly dependent on enrichment. Thus if Fe(8) is used to infer to the pressure of melting, correction for the effects of compositional heterogeneity of the source is needed. Even with the correction for heterogeneity, however, it is difficult to correct for all fractionation effects accurately enough to use FeO as a depth indicator except in the most extreme cases. Mantle heterogeneity also affects Na(8) and (Sm/Yb)N, but the correction for enrichment by linear regression of the Na(8) and (Sm/Yb)N to a common reference value of K2O/TiO2 does not change the relationship between (Sm/Yb)N and Na(8) that is used to estimate depth and degree of melting. In simple melting models, the composition and crustal thickness depend on the rate of pressure release melting per kilometer of uplift; the initial depth of onset of melting; the final depth of termination of melting; the nature of melt equilibration, i.e., batch or fractional melting; and the geometry of upwelling, i.e., passive or dynamic flow. We find that melting beneath normal ridges commences in a narrow depth range in the spinel‐garnet transition zone (about 60–70 km in depth), suggesting a much smaller variation in potential temperature beneath normal ridges (<60°K) than previously suggested (250°K, Klein and Langmuir, 1987; Langmuir et al., 1992). The difference is due primarily to the component of Fe(8) variation caused by mantle heterogeneity in the global data set and to difficulties in correcting FeO for fractionation. The initial depth of melting beneath hotspots is greater than beneath normal ridges, indicating significantly higher temperatures. Instead of melting continuing to the base of the crust beneath normal ridges, it ceases at a variety of depths beneath the crust. Variations in final depth of melting may be due to cooling to the surface, effects of transform faults, and the local, relative importance of dynamic upwelling.
Compositional variations of basaltic glasses from the Mid‐Cayman Rise spreading center
Authors: Elthon Don , Ross D. Kent , Meen James K.
Published: February 2004

Basaltic glasses from the Mid‐Cayman Rise (MCR) spreading center have a significant range of compositions, from 7.7% to 5.4% MgO. Well‐defined trends in the major element and trace element contents of these glasses are consistent with their development by up to 50% crystallization of parental magmas with only a small range of compositional variability. Calculated liquid lines of descent and 1 atm phase equilibria studies of primitive MCR glasses, as well as petrographic relations, indicate that olivine and plagioclase (±spinel) crystallize from MCR liquids at low pressures until the residual liquid has <6.3% MgO at which point clinopyroxene crystallizes along with olivine and plagioclase. The CaO versus MgO variations in MCR glasses, however, clearly indicate that clinopyroxene + olivine + plagioclase crystallization has occurred throughout this suite, not just at the most evolved end. The onset of clino‐pyroxene saturation at ∼7.7% MgO in the MCR glasses, compared to at 6.3% in 1 atm experimental studies, is attributed to crystallization at ∼6 kbar of pressure where the most primitive MCR glasses are expected to be cosaturated with olivine, plagioclase, and clino‐pyroxene. The major element and trace element variations in the MCR glasses are consistent with the extraction of olivine gabbro cumulates similar in composition to the relatively simple MCR gabbroic rocks. Although there is a significant range in the compositions of potential primary magmas for the MCR glasses, the suite could have been produced by the accumulation of all individual melt fractions produced by 10–12% fractional melting of undepleted mantle or 3–6% melting of previously depleted mantle.
Upper crustal tomographic images of the Amiata‐Vulsini geothermal region, central Italy
Authors: Chiarabba C. , Amato A. , Fiordelisi A.
Published: February 2004

We computed the three‐dimensional P wave velocity structure of the upper crust beneath the adjacent Amiata and Vulsini Quaternary volcanoes (central Italy), inverting local earthquake arrival times. We used 676 selected earthquakes out of more than 3000 (in the M range 1.5–4.0) located between 1977 and 1992 by two contiguous microseismic networks operated by Ente Nazionale Energia Elettrica to monitor three geothermal areas (namely, Latera, Torre Alfina, and Mount Amiata). The resolving capability of the data set has been verified both with a priori synthetic tests and an a posteriori analysis of the resolution matrix. We find a 40×25 × 7 (h) km3 crustal volume where we can resolve details as small as 2 to 4 km. Our analysis is favored by numerous geological and geophysical data collected for geothermal exploration. These independent data give us specific constraints for both choosing a starting model and interpreting tomographic results. We find high‐velocity anomalies in the upper 3 km of the crust beneath Latera, Torre Alfina, and Mount Amiata geothermal fields, related to uplifted limestone units. The lateral extension of limestone units is defined by the three‐dimensional velocity pattern. A negative anomaly clearly identifies the Radicofani Graben, a Plio‐Quaternary depression filled with low velocity clayey sediments, between 1 and 3 km depth. A sharp gradient between high velocities (∼6.9 km/s) in the northern region and lower velocities in the southern one is recovered at 7 km depth. This feature may indicate the transition between the Tuscan province uplifted metamorphic crust and thickened carbonate series of the Latium province.
Micromechanics of rock friction: 3. Quantitative modeling of base friction
Authors: Wang Weibin , Scholz C. H.
Published: February 2004

A micromechanical model for friction of brittle elastic solids is introduced and applied to rock friction. It is assumed that contacts are characterized by an adhesive shear strength, a material property that is the only free variable in the formulation. The net contact state of rough surfaces is calculated from elastic contact theory based on the measured topography of the surfaces. The model is tested by comparing it with friction measured with the same surfaces. Good agreement between theory and experiment was obtained from the initial application of shear stress to the onset of steady‐state friction. The early stages of friction were predicted by the model based on the initial surface topographies and including the contributions of both normal and oblique contacts. Slip hardening was successfully modeled by including the evolution of the surface topography and contact state due to wear.
Rupture planes of the Gazli earthquakes deduced from local stress tensor calculation and geodetic data inversion: Geotectonic implications
Authors: Amorèse D. , Grasso J.‐R.
Published: February 2004

Three major earthquakes (with Ms ≥ 7) were clustered in space and time (1976–1984) near Gazli, Uzbekistan, without reported evidence for fault scarps. To constrain the kinematics of the seismic sequence, we invert both focal mechanism solutions of the mainshocks and major aftershocks and vertical surface displacements associated with the three major shocks. For seven M > 4 earthquakes that occurred in the 1976–1984 period, we find that all the focal mechanisms agree with a single stress tensor. One of the mechanical implications of this mathematical solution is the choice of the seismic fault plane. The 1976 earthquake fracture planes are well constrained by the stress tensor to be striking E‐W and ENE‐WSW and dipping to the south and the NNW respectively. From stress tensor inversion alone, it is impossible to determine if the fracture plane of the 1984 event dips to the SE or to the NW. Coseismic vertical surface displacements allow us to infer possible fault plane orientations of the 1976 and 1984 events. The best fit to the surface displacement for the 1976 earthquakes is obtained from the geometries that are derived from the homogeneous stress tensor inversion. The inversion of vertical surface displacements for the 1984 event constrains the associated fault plane to correspond to a NE‐WS striking and SE dipping plane. The inversions of complementary geophysical data argue for the complexity of the orientations of the faults activated by the 1976–1984 seismic sequence. The 1984 earthquake fault plane is well orientated to allow failure to be activated by a horizontal σ1 under a Mohr‐Coulomb failure criterion. Under the same failure criterion and stress conditions, unlike the 1984 fracture plane, the 1976 fault planes are misorientated and likely to correspond to preexisting discontinuities. Concerning the principle stress orientations, the Gazli region seems to be under control of the regional tectonic setting. From a regional point of view, the WNW‐ESE strike of σ1 and the NNE‐SSW strike of σ2 deduced from this study for the Kyzylkum Desert can be explained at once by the interaction of the regional plate boundary motions.
Crustal structure of northern Juan de Fuca Plate from multichannel reflection data
Authors: Hasselgren Elizabeth O. , Clowes Ron M.
Published: February 2004

The northern Juan de Fuca plate forms the basement of a young (<8 Ma) ocean basin off western Canada. Two subparallel multichannel seismic reflection lines, totaling 230 km in length and trending obliquely to the spreading direction, were recorded across the basin and provide high‐quality images of crustal structure. The processing sequence applied to the data includes a prestack inside‐trace mute of common midpoint (CMP) gathers to reduce noise levels on the deep data, CMP stack, poststack dip filtering, f‐k migration, and band‐pass filtering. Coherency‐filtered stacks are helpful in tracing weaker reflectors. The stacked sections reveal a horizontally layered sedimentary sequence overlying a rugged and prominent basement reflector which dips slightly landward. Topography of the basement was formed at or near the spreading ridge; there is no evidence for subsequent deformation. A strong, fairly continuous reflection from the base of the crust (oceanic Moho) at 2.0–2.2 s two‐way travel time below the basement surface generally mimics the basement topography and shows the characteristic multicyclic waveform seen in other surveys. Estimates of reflection coefficients for the Moho at several locations indicate that, in general, the crust‐mantle transition is unlikely to occur as a discrete first‐order interface. Although much of the crust appears acoustically transparent, a number of discontinuous intracrustal reflectors are observed over 40 km at the eastern end of the northern line; some of these are interpreted to arise from the oceanic layer 2/3 and layer 3A/3B boundaries. The noncontinuous nature of these reflectors is an indication of the complexity of the processes producing intracrustal reflectivity and the lateral variability of crustal formation. However, the interpreted crustal thickness varies by only about 10% away from the distal ends of spreading segments, indicating that while the process of crustal formation may be locally variable, it produces a broadly uniform result.
Fracture characterization from vertical seismic profiling data
Authors: Cicerone Robert D. , Toksöz M. Nafi
Published: February 2004

In vertical seismic profiling (VSP) data, large‐amplitude borehole Stoneley waves are observed at depths where fractures intersect the borehole. We present a model which predicts the amplitudes of these Stoneley waves as a function of certain parameters of the fractures, namely, the fracture aperture (width), the orientation, and the degree of stiffness and roughness of the fracture. The proposed mechanism for the generation of these borehole Stoneley waves is coupling of a guided mode, induced in the fracture by an incident plane wave (typically a P wave), to the borehole. The model expresses the borehole Stoneley wave amplitude, normalized by the amplitude of the direct P wave, as a function of frequency, in terms of the fracture parameters. The model is used as the basis for an inversion scheme, employing a nonlinear least squares algorithm to estimate the fracture parameters. The inversion is then applied to VSP data where borehole Stoneley waves are observed at depths where fractures are known to intersect the borehole. The results of the inversion indicate that the aperture and vertical component of the orientation (i.e., dip) of the fracture can be accurately estimated but the horizontal component of the orientation (i.e., strike) is not well resolved. These conclusions are based on comparisons with independent estimates of these parameters from flow tests and borehole televiewer measurements. In addition, stiffness and roughness of the fractures are important effects which must be considered in order to obtain realistic estimates for the other parameters, especially the fracture aperture.
Simulating iceberg calving with a percolation model
Authors: Bahr David B.
Published: February 2004

The physics behind the iceberg calving process is poorly understood, but by using a simple fracturing criterion based on the accumulation and coupling of many microcracks, the complicated mechanics of calving can be simulated by a set of percolation rules. Calving simulations with this simple percolation model give results which are consistent with limited observations of iceberg size distributions and terminus morphologies. For steady state scenarios, the model suggests that the calving process can be characterized by a small set of scaling exponents.
Carbon tetrachloride and chlorofluorocarbons in the South Atlantic Ocean, 19°S
Authors: Wallace Douglas W. R. , Beining Peter , Putzka Alfred
Published: February 2004

Exploratory measurements of a suite of anthropogenic halocarbon compounds (CCl4, CCl2FCClF2 (CFC‐113), CH3CCl3, CCl3F (CFC‐11)) were made using a new analytical technique on RV Meteor cruise 15 along 19°S (World Ocean Circulation Experiment (WOCE) Line A9)) in the Atlantic Ocean during February–March 1991. A separate analytical system was used to determine CCl2F2 (CFC‐12) and CCl3F (CFC‐11). A limited number of CFC‐113 profiles indicated that it was undetectable below 400–500 m. The CCl4 data indicate that the entire Brazil Basin contains readily measurable levels of CCl4 (>0.05 pmol kg−1), whereas the deep Angola Basin contains very low levels (≤0.02 pmol kg−1). Slightly higher levels were found close to the bottom in the deep Angola Basin: possibly an anthropogenic signature. In contrast, most of the deep Brazil Basin and all of the deep Angola Basin (>1000 m) had undetectable levels of CFC‐11, CFC‐12, and CFC‐113. Preindustrial levels of CCl4 in the atmosphere were therefore negligible (atmospheric mixing ratio <0.1 pptv). CCl4/CFC‐11 ratios are used to estimate apparent ages and dilution factors for the North Atlantic Deep Water and Antarctic Bottom Water. Whereas CCl4/CFC‐11/CFC‐12 levels are internally consistent in deep waters, suggesting near‐conservative behavior, there is evidence for very rapid removal of CCl4 in the thermocline. Removal rates suggest that in addition to neutral hydrolysis, some other loss pathway must be involved.
Consistency in long‐term observations of oceans and ice from space
Authors: Zabel I. H. H. , Jezek K. C.
Published: February 2004

Satellite remote sensing has provided powerful tools for constructing long‐term records of observations of many of Earth's geophysical processes. Geophysicists studying climage change would like to construct time series of data which are as long, accurate, and consistent as possible. While technological advances continue to improve the quality of observations from space, they also present consistency problems. The scanning multichannel microwave radiometer (SMMR), which operated from October 1978 to August 1987, and the special sensor microwave imager (SSMI), which overlapped with the SMMR for nearly 2 months in 1987, have provided microwave radiances which have been used to monitor long‐term changes in sea ice concentration and snow accumulation, among other things. Inspection has shown, however, that brightness temperatures from the two sensors over the same polar firn‐covered scene can differ by as much as 14 K. Calibration corrections for polar firn have been derived; in this paper, we present calculations addressing the physics of the small frequency and viewing angle differences between the SMMR and the SSMI on microwave emission from polar scenes containing free water. We focus here only on surface emission. We use a surface scattering model and study the effects of surface roughness, snow wetness, snow density, and young sea ice concentration. We also compare data from Antarctica with our modeled results. We find that for most scenes, surface roughness dominates over the effects of frequency and viewing angle differences, but that for open ocean long‐term time series, data must be calibrated and constructed at the geophysical product level rather than at the level of measured radiances.
Microwave backscatter from the sea: Modulation of received power and Doppler bandwidth by long waves
Authors: Keller W. C. , Plant W. J. , Petitt R. A. , Terray E. A.
Published: February 2004

Two continuous wave microwave systems were mounted on an elevator on the German Forschungsplattform Nordsee (FPN) during the Synthetic Aperture and X Band Ocean Nonlinearities (SAXON)‐FPN experiment. Here we report on measurements of long‐wave effects in the power received by these systems and in the Doppler bandwidths recorded from the systems. The two systems operated at X and Ka bands (10 and 35 GHz) and collected HH‐ and VV‐polarized backscatter signals simultaneously. The elevator system allowed us to vary the altitude of the microwave antennas above the sea surface from 7.5 to 27 m, always in the far field of the antennas. Most data were collected at a 45° incidence angle, which implied that the Ka band system illuminated areas from 0.4 to 6.0 m2 while the X band system viewed spots between 2.9 and 41.3 m2. We have attempted to characterize the modulation of both received power and Doppler bandwidth in terms of the standard modulation transfer function (MTF) concept. We find that our measurements of the MTF for received power agree well with previous measurements at X band but that the phase of the MTF for VV polarization is different from that previously reported for Ka band at a 60° incidence angle. We show how the amplitude and phase of the MTF as well as the coherence function vary with wind speed, long‐wave frequency, long‐wave propagation direction, and antenna height. In the case of the first three parameters our results agree with previous work except as noted above for Ka band. The dependence of the MTF on antenna height has not been investigated previously. Our results show that this dependence is small and well explained by the height variations induced as long waves propagate through the footprint on the surface. When we apply the MTF concept to the Doppler bandwidth, we find that the MTF of the bandwidth is small except at very low wind speeds. In view of the association of this bandwidth with velocities of subresolution‐scale, or intermediate‐scale, waves which are themselves modulated by the long waves, we find these small MTFs associated with Doppler bandwidths to be somewhat surprising. We present a simple model to attempt to draw inferences about the amplitude modulation of intermediate‐scale waves from our results. We also conclude from the small MTFs for Doppler bandwidths that the assumption of a constant correlation time in theories of synthetic aperture radar imagery of the ocean is well justified.
Detailed observations of a naturally occurring shear instability
Authors: Seim Harvey E. , Gregg Michael C.
Published: February 2004

Simultaneous profiles of microstructure, horizontal velocity, and acoustic backscatter allow one of the most complete descriptions of a naturally occurring shear instability to date. Shear increased rapidly after passing through a lateral constriction which formed a hydraulic control. A kilometer‐long set of 20‐m‐tall billows grew on a middepth density interface where the Richardson number fell below 0.25. The velocity interface thickened steadily after the billows formed, consistent with rapid momentum mixing across a shear layer with a Reynolds number of 3×106. The billows generated large density overturns and dissipation rates greater than 10−5 W kg−1, even within the first large overturn, indicating that these structures were fully turbulent early in their development. As the billows grew, a well‐mixed layer developed at the interface and survived as an actively turbulent layer for up to 6 buoyancy periods, 3 times longer than in laboratory studies at low Reynolds number. Variations in the mean density of the billows lead us to infer that the vertical offset of the velocity and density interfaces varied with time where the billows first formed. With data from the large overturns within the shear layer, we find ε/νN2 ≈ 3 × 104, an average root‐mean‐square overturn scale (
) of 2.6 m, and a buoyancy scale (Lb) of 2.7 m. Despite having sampled the billows at varying stages of their evolution, we find no indication that the ratio Lrms/Lb is ever significantly different than 1 for this shear instability.
Application of the GEM‐T2 gravity field to altimetric satellite orbit computation
Authors: Haines Bruce J. , Born George H. , Williamson Ronald G. , Koblinsky Chester J.
Published: February 2004

As part of a continuing effort to provide improved orbits for use with existing altimeter data, we have recomputed ephemerides for both the Seasat and Geosat Exact Repeat altimeter missions. The orbits were computed in a consistent fashion, using the Goddard Earth Model T2 gravity field along with available ground‐based tracking data. Such an approach allows direct comparisons of sea level between the two altimeter systems. Evaluation of the resulting ephemerides indicates that root‐mean‐square accuracies of 30–50 cm have been achieved for the radial component of the orbits for both satellites. An exception occurs for the last year of the Geosat Exact Repeat Mission, when the rms radial orbit accuracy degrades to the 1‐m level at times owing to the inability to adequately model the drag force arising from the increased solar activity.
Gulf Stream warm surges: Thermal anomalies resulting from cold core ring interactions
Authors: Gilman Craig , Rothstein Lewis
Published: February 2004

Observations from thermal satellite imagery reveal a thermal anomaly pattern in the sea surface temperature (SST) field of the Gulf Stream downstream of Cape Hatteras which is a direct result of Gulf Stream‐eddy interactions upstream of Cape Hatteras. The general sequence of events observed in the images is as follows: (1) Detrainment of Gulf Stream surface water occurs as a result of an interaction with a cold core ring; (2) downstream of the interaction the Gulf Stream's SST decreases several degrees over a few days owing to the loss of the upstream surface flow; (3) a visible “jet” of warm surface water is advected down the major axis of the Gulf Stream after the stream‐ring interaction subsides and the detrainment diminishes. The jet maintains a 1–3°C temperature difference with the surrounding Gulf Stream water as it propagates downstream. This paired cooling/warming process is labeled a “warm surge event” because the sharp leading edge of the warm jet (i.e., the surge) is easily detectable in the satellite images, while the anomalous surface cooling preceding the passage of the warm jet is difficult to observe. Vertical profiles of temperature, downstream velocity, and cross‐stream velocity all indicate that the upper 100–150 m of the Gulf Stream are significantly altered during an event. Satellite and in situ data also indicate that just downstream of the interaction there is enhanced entrainment of Sargasso Sea surface waters into the Gulf Stream to replenish the detrained Gulf Stream water.
Interdecadal variability in an idealized model of the North Atlantic
Authors: Weaver Andrew J. , Aura Stella M. , Myers Paul G.
Published: February 2004

A coarse resolution model is developed to study the thermohaline circulation of the North Atlantic. This model is driven by the annual mean Hellerman and Rosenstein wind stress field, Levitus sea surface restoring temperatures, and Schmitt, Bogden, and Dorman freshwater flux fields (mixed boundary conditions) together with various parameterizations of Arctic freshwater export into the North Atlantic. The model simulations indicate the existence of self‐sustained, internal variability of the thermohaline circulation with a period of about 20 years. Associated with the variability is a large variation in the deep‐water formation rate in the Labrador Sea and hence the poleward heat transport in the North Atlantic. It is shown that the variability is insensitive to the freshwater flux and wind forcing used and that the timescale for this thermally driven convective/advective oscillation is set by the cooling time of the Labrador Sea. The variability is robust to various parameterizations of Arctic freshwater export but may be suppressed if there is a strong freshwater flux through the Canadian Archipelago (or equivalently, large precipitation) into the Labrador Sea. The importance of topography, although poorly resolved in this coarse resolution study, is addressed and the results are compared with a coupled atmosphere‐ocean simulation and observations taken over the North Atlantic.
The flux of overflow water through the Charlie‐Gibbs Fracture Zone
Authors: Saunders Peter M.
Published: February 2004

We report the results of a set of measurements made within the Charlie‐Gibbs Fracture Zone in 1988–1989. The principal data sources are year‐long current meter records, 16 in number, and 23 conductivity‐temperature‐depth stations. The aim was to determine the flux of overflow water passing westward out of the Iceland Basin in the eastern North Atlantic. Within the fracture zone, overflow water is defined either as having salinity greater than 34.94 at the present time or, more generally, as having a potential density σ0 > 27.8 kg/m3. In 1988–1989 the core salinity had a value equal to that reported in 1981 and 1983 but was fresher by about 0.02 than was found in 1964. The flux was estimated twice per day and was found to fluctuate strongly, with energy spread in a broad band of periods, ranging from 10 to 400 days. No annual signal could be detected. The mean value was 2.4±0.5 Sv, less than all previous estimates based on hydrography alone. It is suggested that hydrography may systematically overestimate transport because diffusion is neglected. Flux estimates from current meter arrays are rarely used in initializing numerical ocean models; consequently measurements, like those reported here, are an independent and important means for assessing the performance of such models. At the deepest levels, water enters the two transform valleys of the fracture zone from the west. In the northern valley the flow is periodically flushed out by energetic bursts of overflow water. A critical value of an inverse Froude number is shown to define the flushing threshold.
In situ measurements of capillary‐gravity wave spectra using a scanning laser slope gauge and microwave radars
Authors: Hara Tetsu , Bock Erik J. , Lyzenga David
Published: February 2004

Capillary‐gravity wave spectra are measured using a scanning laser slope gauge (SLSG), and simultaneously by X and K band Doppler radars off the Chemotaxis Dock at the Quissett campus of the Woods Hole Oceanographic Institution at Woods Hole, Massachusetts. Wave spectral densities estimated from the radar measurements using the Bragg theory agree with those measured using the SLSG at the Bragg wavenumber to within a few decibels, suggesting that Bragg scattering theory is valid for the conditions of this experiment. The observed degree of saturation of capillary‐gravity waves is in reasonable agreement with measurements by Jähne and Riemer (1990) obtained from measurements in a large wind‐wave flume at intermediate wind speeds, but our data indicate a higher degree of saturation at very low wind speeds. The rate at which the slope‐frequency spectrum falls off, however, is much lower in the field than in laboratories, even at moderate winds, suggesting long waves are responsible for a large Doppler shift of capillary‐gravity waves. Close examination of combined wavenumber‐frequency slope spectra also reveals significant smearing of the spectra in the frequency domain due to long waves. These observations confirm that spatial measurements (wavenumber spectra measurements) are essential for characterizing short capillary‐gravity waves, since this strong Doppler shift will dramatically change apparent frequency spectra.
Sea surface temperature estimation by application of microwave dual‐frequency data to infrared data
Authors: Takayama Yozo
Published: February 2004

A technique is proposed to estimate sea surface temperature (SST) using dual‐frequency microwave data (24 GHz and 31 GHz) to correct infrared (IR) window data for atmospheric absorption effects. In the ideal noise‐free environment the new method has the same accuracy in SST as the IR split‐window method. However, the method using the microwave channels has eight times more tolerance of instrumental noise than the IR split‐window method. Hence using the microwave channels yields better SST estimations in the presence of instrumental noise in the radiometers; even in the low noise level of the IR radiometer, the combined use of the dual‐frequency microwave data with the IR window data improves the accuracy in the estimated SST.
On the possibility of a ridge current along the Hawaiian Islands
Authors: Price James Michael , Van Woert Michael L. , Vitousek Martin
Published: February 2004

Four evenly spaced aircraft‐deployed expendable bathythermograph and concurrent satellite advanced very high resolution radiometer (AVHRR) surveys were made over an extended area during a 1‐year period to observe possible ridge currents along the Hawaiian Islands predicted or hypothesized by other investigators. None of the surveys found extended currents running along the ridge. Instead, fields of energetic eddies were found, and a curious cusp‐shaped sea surface temperature pattern was observed in two of the AVHRR fields. Apparent cyclonic currents around the island of Hawaii were also observed. We conclude that either ridge currents do not exist or that they exist but are intermittent (episodic) in nature or exist in a form more complicated than could be observed by this study.
Spatial and temporal distributions of acoustically estimated Zooplankton biomass near the Marine Light‐Mixed Layers station (59°30′N, 21°00′W) in the North Atlantic in May 1991
Authors: Batchelder H. P. , VanKeuren J. R. , Vaillancourt R. , Swift E.
Published: February 2004

Temporal and spatial acoustic backscatter estimates of Zooplankton biomass were made using an unmodified hull‐mounted 153‐kHz acoustic Doppler current profiler (ADCP) during the May 1991 Marine Light‐Mixed Layers (MLML) cruises to the North Atlantic. Relative backscatter from the ADCP was converted to Zooplankton biomass estimates using individual plankton taxa abundances and weights from Zooplankton samples collected during the cruises. There was a small but consistent diel pattern in the 20 to 250‐m depth‐integrated backscatter, with highest values during darkness. Removal of the diel signal with harmonic analysis revealed slightly higher Zooplankton biomass to the southwest and west of the mooring than to the northeast, in common with gradients in surface temperature and chlorophyll during the mapping cruise. Overall however, depth‐integrated Zooplankton biomass during the mapping cruise varied by only a factor of 2, comparable to what one observes in replicate plankton tows. The nightly 0 to 250‐m obliquely collected Zooplankton samples (May 16–24) indicated increasing densities (and biomasses) of probable Zooplankton scatterers (especially the copepod Calanus fintmarchicus ) during middle to late May, soon after the peak in the spring phytoplankton bloom. This increase in May was mirrored by a comparable increase in depth‐ integrated acoustic backscatler. The distribution of Zooplankton changed following two 50+ kn (1 kn = 1.85 km h−1 ) wind storms on May 19 and May 21; Zooplankton biomass was higher and extended much deeper in the water column at night following these strong mixing episodes. Before the storm events, the patterns of Zooplankton diel vertical redistribution were consistent from day to day. Diel patterns of Zooplankton variability measured using shipboard acoustics are qualitatively similar to patterns observed from an ADCP on the MLML mooring, presenting the possibility of calculating a nearly continuous seasonal measure of Zooplankton biomass from the mooring ADCP data.
Phytoplankton production in the Sargasso Sea as determined using optical mooring data
Authors: Waters K. J. , Smith R. C. , Marra J.
Published: February 2004

Optical measurements from an untended mooring provide high‐frequency observations of in‐water optical properties and permit the estimation of important biological parameters continuously as a function of time. A 9‐month time series, composed of three separate deployments, of optical data from the BIOWATT 1987 deep‐sea mooring located in the oligotrophic waters of the Sargasso Sea at 34°N, 70°W are presented. These data have been tested using several bio‐optical models for the purpose of providing a continuous estimate of phytoplankton productivity. The data are discussed in the context of contemporaneous shipboard observations and for future ocean color satellite observations. We present a continuous estimation of phytoplankton productivity for the 9‐month time series. Results from the first 70‐day deployment are emphasized to demonstrate the utility of optical observations as proxy measures of biological parameters, to present preliminary analysis, and to compare our bio‐optical observations with concurrent physical observations. The bio‐optical features show variation in response to physical forcings including diel variations of incident solar irradiance, episodic changes corresponding to wind forcing, variability caused by advective mesoscale eddy events in the vicinity of the mooring, and seasonal variability corresponding to changes in solar radiation, shoaling of the mixed layer depth, and succession of phytoplankton populations.
Precise Centre National d'Etudes Spatiales orbits for TOPEX/POSEIDON: Is reaching 2 cm still a challenge?
Authors: Nouël F. , Berthias J. P. , Deleuze M. , Guitart A. , Laudet P. , Piuzzi A. , Pradines D. , Valorge C. , Dejoie C. , Susini M. F. , Taburiau D.
Published: February 2004

To achieve maximum benefit from the altimetric data collected by the French‐American TOPEX/POSEIDON spacecraft, radial orbit accuracy of 10 cm or better is required. This unprecedented requirement led the French Space Agency Centre National d'Etudes Spatiales (CNES) to develop a new high‐accuracy tracking system, Doppler orbitography and radiopositioning integrated by satellite (DORIS), and a new precision orbit production facility, the Service d'Orbitographie DORIS. A global effort produced new models and new orbit determination strategies. The result of these efforts has been assessed after 1 year of operation. The original goal has clearly been met, and the TOPEX/POSEIDON orbits produced by NASA and CNES agree to better than the 5 cm RMS level in the radial direction. At this level of accuracy, traditional techniques cannot correctly describe the actual orbit error, and some new procedures are proposed.
Seasonal variation of the three‐dimensional residual circulation on Georges Bank
Authors: Naimie Christopher E. , Loder John W. , Lynch Daniel R.
Published: February 2004

The seasonal variation of the low‐frequency circulation in the Georges Bank region is studied numerically by computing six bimonthly circulation fields subject to forcing from the barotropic M2 tide, mean baroclinic pressure gradients, and mean wind stresses. The model is three dimensional, diagnostic, and nonlinear, with quadratic vertical eddy viscosity that is stratification‐dependent. Tidal forcing is imposed at the boundary of the domain, and an extensive set of density and wind data is used to determine climatological mean forcings. The magnitude of the M2 tidally rectified around‐bank velocities and transports is sensitive to stratification influences on the eddy viscosity, with up to a 50% intensification relative to computations which assume no influences. The dependence occurs through both the local advective tidal stress terms and the large‐scale barotropic pressure field. The six bimonthly solutions (January–February, March–April, May–June, July–August, September–October, November–December) indicate important contributions from tidal rectification, baroclinic pressure gradients, and mean wind stress to the seasonal intensification of the Georges Bank gyre. Tidal rectification and baroclinicity are the dominant forcings, while wind stress generates opposing around‐bank flow and cross‐bank surface drift in winter. Overall, the solutions are in approximate agreement with observed Eulerian around‐bank currents and transports, although there are both local and bank‐wide discrepancies. The seasonal intensification of recirculation around Georges Bank is well produced by the model, with estimates of the recirculating transport increasing from under 0.02 Sv (January–February) to over 0.13 Sv (September–October).
Bio‐optical and physical variability in the subarctic North Atlantic Ocean during the spring of 1989
Authors: Dickey T. , Marra J. , Stramska M. , Langdon C. , Granata T. , Plueddemann A. , Weller R. , Yoder J.
Published: February 2004

A unique set of physical, bio‐optical, and meteorological observations were made from a mooring located in the open ocean south of Iceland (59°29.5′N, 20°49.8′W) from April 13 to June 12, 1989. The present measurements are apparently the first to resolve the rapid transition to springtime physical and biological conditions at such a high latitude site. Our data were collected with bio‐optical and physical moored systems every few minutes. The abrupt onset of springtime stratification was observed with the mixed layer shoaling from ∼550 m to ∼50 m in ∼5 days. During this period a major phytoplankton bloom occurred with a tenfold increase in near‐surface chlorophyll concentration in less than 3 weeks. Our statistical analysis indicates that the velocity shear in the upper layer is driven primarily by local wind stress. Mesoscale variability is also apparent from these and concurrent airborne oceanographic lidar observations. Our complementary modeling results suggest that the near‐surface layer may be reasonably well described by a one‐dimensional model and that the spring bloom was initiated during incipient near‐surface restratification.
Satellite investigations of a cyclonic gyre in the central Tyrrhenian Sea (western Mediterranean Sea)
Authors: Perilli Angelo , Rupolo Volfango , Salusti Ettore
Published: February 2004

Thermal satellite images related to surface temperature of the Tyrrhenian Sea (western Mediterranean Sea) show an elliptic cold patch, suggesting the presence of a wind‐induced cyclonic gyre with well‐defined meanders of comparatively constant wavelength. In order to gain insight into the dynamics of the gyre these data are compared with results of two classical linear models, that of Crépon and Richez on wind‐induced upwellings and that of Gill on currents due to a buoyancy discontinuity in rotating channels. The large‐scale gyre meanders are analyzed using theoretical information on baroclinic instability models. It results that the gyre is due to winds blowing eastward from the Mouths of Bonifacio and to the buoyancy due to a thermal front along 41°E. With this general procedure, from satellite images an estimate of the deep hydrologic quantities is obtained; this technique is of interest also for other similar systems.
Analysis of ERS 1 synthetic aperture radar data of frozen lakes in northern Montana and implications for climate studies
Authors: Hall Dorothy K. , Fagre Daniel B. , Klasner Fritz , Linebaugh Gregg , Liston Glen E.
Published: February 2004

Lakes that freeze each winter are good indicators of regional climate change if key parameters, such as freeze‐up and breakup date and maximum ice thickness, are measured over a decade‐scale time frame. Synthetic aperture radar (SAR) satellite data have proven to be especially useful for measurement of climatologically significant parameters characteristic of frozen lakes. In this paper, five lakes in Glacier National Park, Montana, have been studied both in the field and using ERS 1 SAR data during the 1992–1993 winter. The lakes are characterized by clear ice, sometimes with tubular or rounded bubbles, and often with a layer of snow ice on top of the clear ice. They are also often snow covered. Freeze‐up is detected quite easily using ERS 1 SAR data as soon as a thin layer of ice forms. The effect of snow ice on the backscatter is thought to be significant but is, as yet, undetermined. On the five lakes studied, relative backscatter was found to increase with ice thickness until a maximum was reached in February. Breakup, an often ill‐defined occurrence, is difficult to detect because surface water causes the SAR signal to be absorbed, thus masking the ice below. Comparison of the bubble structure of thaw lakes in northern Alaska with lakes in northern Montana has shown that the ice structure is quite different, and this difference may contribute to differential SAR signature evolution in the lakes of the two areas.
Accuracy of current profile measurements: Effect of tropical and midlatitude internal waves
Authors: Send Uwe
Published: February 2004

The effect of midlatitude and tropical internal wave variability on current profile measurements is investigated and quantified to yield practical error estimates. First, a data set of Pegasus current profiles from the tropical Atlantic (6°S to 6°N) is analyzed for their rms down/up differences, which are compared with predictions from Garrett‐Munk type internal wave theory and with statistics derived from current meter moorings in the same region. The agreement in terms of amplitudes and vertical distribution proves that most of those differences are due to internal waves and not instrumental errors. Nonetheless, this is the noise of the measurements, if low‐frequency motions are sought, and the errors can thus be quantified using the same internal wave theories. At midlatitudes the error variance is the usual 44(N/3 cph) cm2/s2 with some latitude dependence, and the effect of averaging in the vertical or summing several profiles (e.g., up and down) is estimated. The same is done for equatorial situations, where construction of a crude equatorial frequency spectrum for internal waves yields 77(N/3 cph)cm2/s2 for the error variance. Again, error reduction due to averaging is estimated.
Application of a mixed layer model to the inner Newfoundland shelf
Authors: Mathieu T. , de Young B.
Published: February 2004

A mixed layer model is developed and applied based upon the model of Price, Weller, and Pinkel (Price et al., 1986). The new model includes diffusion, adds salinity to the calculation of density, and permits the assimilation of observed monthly mean salinity profiles. A simulation over a 2 year period on the inner part of the Newfoundland shelf shows that the new model performs well, much better than the previous model. The assimilation of salinity has relatively little effect, but the model results are sensitive to the parameterization of the vertical diffusivity. The best results are found for the case where the diffusivity is dependent upon N2; hence there is an indirect dependence on the salinity profile. The heat content of the model is simulated well except in late summer when there is too much heat in the model.
Transport, potential vorticity, and current/temperature structure across Northwest Providence and Santaren Channels and the Florida Current off Cay Sal Bank
Authors: Leaman Kevin D. , Vertes Peter S. , Atkinson Larry P. , Lee Thomas N. , Hamilton Peter , Waddell Evans
Published: February 2004

Currents and temperatures were measured using Pegasus current profilers across Northwest Providence and Santaren Channels and across the Florida Current off Cay Sal Bank during four cruises from November 1990 to September 1991. On average, Northwest Providence (1.2 Sv) and Santaren (1.8 Sv) contribute about 3 Sv to the total Florida Current transport farther north (e.g., 27°N). Partitioning of transport into temperature layers shows that about one‐half of this transport is of “18°C” water (17°C–19.5°C); this can account for all of the “excess” 18°C water observed in previous experiments. This excess is thought to be injected into the 18°C layer in its region of formation in the northwestern North Atlantic Ocean. Due to its large thickness, potential vorticities in this layer in its area of formation are very low. In our data, lowest potential vorticities in this layer are found on the northern end of Northwest Providence Channel and are comparable to those observed on the eastern side of the Florida Current at 27°N. On average a low‐potential‐vorticity 18°C layer was not found in the Florida Current off Cay Sal Bank.
Seasonal variability in the surface currents of the equatorial Pacific
Authors: Reverdin Gilles , Frankignoul Claude , Kestenare Elodie , McPhaden Michael J.
Published: February 2004

Buoy drifts and current meter records between January 1987 and April 1992 are used to investigate the seasonal variability of the equatorial Pacific Ocean currents at a depth of 15 m. The buoy drifts and current meter data are well correlated, and their differences are small, although slightly larger currents may be given by the buoy drifts. The seasonal cycle in the currents is analyzed between 20°N and 20°S on a 1°×5° grid using a function‐fitting algorithm which somewhat smoothes the zonal structure but retains the meridional structure. The analysis captures a large, zonally coherent seasonal variability of the currents within 15° of the equator, which significantly exceeds the estimated errors that originate from the limited sampling of the interannual, intraseasonal, and higher‐frequency fluctuations of the currents. Many features of the new climatology are shared with other analyses of the surface currents in the equatorial Pacific, particularly the timing of the seasonal cycle of the main currents. There are, however, differences in the current velocities that are illustrated by a comparison with the ship drift data, which are analyzed here with the same spatial resolution. The analysis of the ship drifts presents larger meridional scales which are probably the result of the spatial smoothing involved in estimating a ship drift. The ship drifts are noticeably downwind of the 15‐m currents. At the equator, they are also more westward than in the analysis of the 15‐m currents between November and March near the date line and in January and July in the eastern Pacific which at least partially results from differences in the climatic conditions sampled in the two data sets.