bluuebunny/crossref_metadata_2025 huggingface dataset:
Possible evidence of gravity wave coupling into the mid‐latitude F region ionosphere during the SEEK Campaign
Authors: Taylor M. J. , Jahn J.‐M. , Fukao S. , Saito A.
Published: February 2004

On five of eight observation nights during the 1996 SEEK (Sporadic E Experiment over Kyushu) campaign, Japan, unusual “wave‐like” structures were imaged in the 630 nm thermospheric nightglow emission. Four of these events were observed to travel towards the southwest, providing new evidence in support of recent theories describing the coupling of medium‐scale gravity waves into the mid‐latitude F region ionosphere. Available ionosonde data and the visual characteristics of the wave structures indicate no association with the occurrence of mid‐latitude spread F or F region upwellings. Instead, the data support the novel concept of feedback from the ionosphere into the gravity wave, via the Perkins instability, to enhance its visibility.
Measurements of ice nucleating aerosols during SUCCESS
Authors: Rogers David C. , DeMott Paul J. , Kreidenweis Sonia M. , Chen Yalei
Published: February 2004

Instruments on NASA's DC‐8 Airborne Laboratory were used to obtain measurements of heterogeneous ice nucleating aerosols (IN) and CN (condensation nuclei, particles >0.012 µm diameter) in background and aircraft‐affected air during the April–May 1996 SUCCESS project (Subsonic Aircraft: Contrail and Cloud Effects Special Study). IN were measured with a continuous flow diffusion chamber over a range of processing conditions from about −15 to −40°C and from ice saturation to ∼15% water supersaturation. IN concentrations ranged from <0.1 to ∼500 ℓ−1, being generally greater at colder temperatures and higher supersaturations. Within limited sample periods, IN concentration related strongly to sampling temperature and supersaturation, but overall, there was wide variability. IN and CN concentrations did not generally correlate. During penetrations of aircraft exhaust plumes, CN exhibited a very strong response, but there was no strong evidence that exhaust is a significant IN source for the temperature and supersaturation conditions of our measurements.
Volcanic eruption velocities measured with a micro radar
Authors: Hort M. , Seyfried R.
Published: February 2004

A novel technique to quantitatively assess volcanic eruption dynamics is presented. We report on measurements of volcanic eruption velocities with a modified micro radar Doppler anemometer on Stromboli volcano, Italy. The mean vertical eruption velocity recorded during a four day period in late 1996 was around 10 m/s which is somewhat on the small side for strombolian eruptions. However, the activity of Stromboli volcano during this period was very weak explaining the relatively small velocities. The technique used is much better in resolution than older techniques based on static photos and allows a continuous monitoring of the volcanic activity. In addition to the velocity information recorded by the Doppler Spectrum it also contains information on the density of the ejected material and the size distribution of the particles in the volcanic jet.
Growth of fluoroform (CHF3, HFC‐23) in the background atmosphere
Authors: Oram D. E. , Sturges W. T. , Penkett S. A. , McCulloch A. , Fraser P. J.
Published: February 2004

There is growing concern over the emission and accumulation of very long‐lived fluorinated trace gases in the atmosphere, due to their large global warming potentials (GWPs). Unlike CFCs and other ozone‐depleting, chlorinated and brominated chemicals, consumption of these fluorinated compounds is not controlled by the Montreal Protocol or any other international agreement. Of all the known and potential trace ‘greenhouse’ gases, the two with the highest GWPs are sulfur hexafluoride (SF6) and fluoroform (CHF3, HFC‐23). Whereas several studies have reported the detection and accumulation of SF6 in the atmosphere, the presence of HFC‐23 has remained unreported. We have found that present‐day HFC‐23 concentrations (c. 11 pptv in late 1995) exceed those of SF6 by a factor of three. Concentrations have steadily increased in the atmosphere since at least 1978, and are continuing to do so at a present rate of 5% per year. Furthermore, HFC‐23 appears to be long‐lived in the atmosphere, with a stratospheric lifetime of at least 1000 years, and a modelled tropospheric lifetime of 230 years. In terms of global warming, the cumulative emissions of HFC‐23 up to, and including, 1995 are equivalent to 1.6 billion tonnes of CO2.
An interhemispheric comparison of ozone mini‐hole climatologies
Authors: James Paul M.
Published: February 2004

An ozone mini‐hole is a synoptic‐scale area of strongly reduced column total ozone, which undergoes a growth‐decay cycle in association with tropospheric weather systems. A climatology of mini‐hole occurrence over mid‐latitudes of the southern hemisphere is constructed, based on 13 years of daily satellite measurements from the Nimbus‐7 TOMS instrument, version 7 data. A computer‐based analytical method for searching for synoptic‐scale minima in the ozone field is employed to build up a catalogue of mini‐hole events, recording their daily locations and intensities. The resulting statistics are compared to equivalent, recently published findings for the northern hemisphere, constructed using identical procedures. Interhemispheric differences in the spatial and seasonal properties of mini‐holes are assessed. It is found that the great Antarctic ozone hole has a significant impact on mini‐hole occurrence during the southern spring since it provides an extra source of depleted ozone not generally available over the northern hemisphere. Apart from this effect, however, mini‐holes are shown to be less frequent than over the northern hemisphere.
Three dimensional models of Martian mantle convection with phase transitions
Authors: Breuer Doris , Yuen Dave A. , Spohn Tilman , Zhang Shuxia
Published: February 2004

We have employed a three‐dimensional compressible convection model to study the dynamics of phase transitions in the Martian mantle. A large core model with two exothermic phase transitions, the olivine to β‐spinel and the β‐ to γ‐spinel transition, and a small core model including also the endothermic spinel to perovskite transition have been considered. The two exothermic transitions create ‘thermal barriers’ for small upwellings due to the latent heat consumption from the phase change. Upwelling plumes lose part or all of their buoyancy, which causes the formation of one stable area full of plumes. This tendency for the merging of plumes increases with internal heating. This type of convective planform is consistent with the relatively few large volcanic centers. The presence of a 175 km thick perovskite layer above the core‐mantle boundary (CMB) yields a similar flow pattern, albeit with an even smaller number of plumes. However, the excess temperatures of the plumes and the mantle flow velocities in the lower mantle are smaller than those found in models without perovskite layer. The phase transitions cause an increase of temperature near the CMB, which prevents the lower mantle and the core from extensive cooling. A model with a perovskite layer decreasing in thickness with time can account for a peak in volcanic and magnetic activity early in the Martian history.
Evidence for long‐term cooling of the upper atmosphere in ionosonde data
Authors: Ulich Thomas , Turunen Esa
Published: February 2004

While several model estimates predict cooling of the upper atmosphere as a result of increasing concentrations of greenhouse gases, direct observational evidence of such a trend is scarce and partly susceptible, because the relevant data series do not cover sufficiently long time periods. We study the long‐term data set from the ionosonde station at Sodankylä (67.4°N, 26.7°E), which has been operated during almost 4 solar cycles. We find a close to linear decrease in the altitude of the F2 layer peak during the last 39 years, when the effect of solar cycle variations is removed from the data. This local trend is qualitatively consistent with the model predictions of a cooling of the lower thermosphere.
Compressibility of hydrous wadsleyite (β‐phase) in Mg2SiO4 by high pressure X‐ray diffraction
Authors: Yusa Hitoshi , Inoue Toru
Published: February 2004

Isothermal compressibility of hydrous wadsleyite has been measured for the first time by using in‐situ X‐ray diffraction in a diamond anvil apparatus at pressures up to 8.5 GPa of a sample which was synthesized at 15.5 GPa and 1300°C under hydrous conditions. The hydrous wadsleyite sample included 2.5 ± 0.3 wt% of H2O as determined by secondary ion mass spectroscopy. The cell parameters of the present hydrous wadsleyite are fit to the orthorhombic symmetry within error. The linear compressibilities along a‐ and c‐axes of the present hydrous wadsleyite are very similar to those of the anhydrous wadsleyite, but the liner compressibility along the b‐axis of the present hydrous wadsleyite is ∼15% larger than that of the anhydrous wadsleyite. Consequently, the calculated bulk modulus (K0) is 155 ± 2 GPa with the pressure derivative (K′0) fixed to 4.3. This value is 5∼11% smaller than that of anhydrous wadsleyite. On the basis of the above result, the seismic discontinuity around 400 km is discussed.
Remote CO, COS, CO2, SO2, HCl detection and temperature estimation of volcanic gas
Authors: Mori Toshiya , Notsu Kenji
Published: February 2004

Volcanic gas from an inaccessible fumarole at Aso volcano, Japan, was recently measured with a Fourier Transform InfraRed (FT‐IR) spectral radiometer. Absorption features of CO, COS, CO2, SO2, and HCl were identified in the volcanic plume spectra. This is the first time the five volcanic gas components are detected simultaneously by a remote technique. Based on the measured CO/CO2 ratio of 2.6 × 10−3, the equilibrium temperature of the volcanic gas is estimated to be 750° ± 120°C.
Geodetic observations of interseismic strain segmentation at the Sumatra Subduction Zone
Authors: Prawirodirdjo L. , Bocl Y. , McCaffrey R. , Genrich J. , Calais E. , Stevens C. , Puntodewo S. S. O. , Subarya C. , Rais J. , Zwick P. , Fauzi R. McCaffrey
Published: February 2004

Deformation above the Sumatra subduction zone, revealed by Global Positioning System (GPS) geodetic surveys, shows nearly complete coupling of the forearc to the subducting plate south of 0.5°S and half as much to the north. The abrupt change in plate coupling coincides with the boundary between the rupture zones of the 1833 and 1861 (Mw>8) thrust earthquakes. The rupture boundary appears as an abrupt change in strain accumulation well into the interseismic cycle, suggesting that seismic segmentation is controlled by properties of the plate interface that persist through more than one earthquake cycle. Structural evidence indicates that differences in basal shear stress may be related to elevated pore pressure in the north.
MLS observations of Arctic ozone loss in 1996–97
Authors: Manney G. L. , Froidevaux L. , Santee M. L. , Zurek R. W. , Waters J. W.
Published: February 2004

The Microwave Limb Sounder (MLS) observed ozone (O3) loss in the Arctic vortex beginning in January 1997 at 585 K (∼25 hPa) and in February 1997 at 465 K (∼50 hPa). Minimum vortex‐averaged O3 mixing ratios observed in 1997 were higher than those in 1996, which were the lowest ever recorded by MLS. The vertical extent of O3 loss and maximum local O3 decreases were larger, but the decrease filled the vortex less completely, in 1997 than in 1996. Unusually low high‐latitude column O3 above 100 hPa in April 1997 resulted mainly from dynamical effects of the unusually persistent lower stratospheric vortex and winter‐like temperature patterns. Column O3 above 100 hPa averaged in comparable regions of the vortex showed a stronger decreasing trend in 1996–97 than in 1995–96, consistent with the larger vertical extent and maximum local values of lower stratospheric O3 loss. Chemical O3 loss resulted in an ∼10% observed decrease in column O3 between late January and early April 1997.
The effects of the conversion of nitrogen oxides in aircraft exhaust plumes in global models
Authors: Meijer E. W. , van Velthoven P. F. J. , Wauben W. M. F. , Beck J. P. , Velders G. J. M.
Published: February 2004

A parametrization is needed in global models to account for the sub‐grid chemical processes taking place in the plume of an aircraft, since these processes can cause the conversion of a considerable amount of the emitted NOx to reservoir species, such as HNO3. For this purpose, the chemical conversions of nitrogen oxides in the plume of an aircraft were investigated with a newly developed model. The calculated fractions of different nitrogen compounds formed within 24 hours in the exhaust plumes, differentiated for the global domain and season, were used to modify the original aircraft NOx emissions from the ANCAT emission inventory to emissions of various nitrogen compounds and we applied these to the global Chemistry Transport Model KNMI (CTMK). The results obtained imply that neglect of aircraft plume processes in global modeling leads to an overestimation of the NOx and O3 perturbations. Compared with a CTMK calculation with unmodified aircraft NOx emissions, the NOx perturbations in the North Atlantic flight corridor (NAFC) decreased by 15%–55%, due to conversions in the plumes. The resulting O3 perturbation decreased by 15%–25%.
Coriolis influences on the interchange instability
Authors: Pontius Duane H.
Published: February 2004

Previous treatments of rotationally driven interchange motion assume rigid corotation and incorporate centrifugal force into an effective gravity. When the ionospheric acceleration time is short, this approximation is acceptable, but in remote regions of the Jovian magnetosphere, the constraining influence of the ionosphere is increasingly ineffective. Conservation of angular momentum then governs local dynamics, and the Coriolis force is an essential part of the perturbation analysis. If the ionosphere is absent, the result agrees with earlier studies of magnetized Couette flow. If the ionosphere is merely very ineffective, the interchange criterion is not changed, but the instability growth rate is significantly reduced from earlier calculations.
Upper ocean velocities in the Beaufort Gyre
Authors: Plueddemann A. J. , Krishfield R. , Takizawa T. , Hatakeyama K. , Honjo S.
Published: February 2004

A 23 month time series of upper ocean velocities in the Beaufort Gyre was obtained from an autonomous drifting buoy equipped with an Acoustic Doppler Current Profiler. The buoy was deployed in the Beaufort Sea and drifted over the Canada Basin, Chukchi Shelf, and Chukchi Plateau. Time‐evolving spectral analysis was used to examine velocity variability in three frequency bands: an eddy band (2–5 day periods), a diurnal band, and a semidiurnal band. The eddy band and the semidiurnal band dominated the horizontal kinetic energy. Although eddy energy was clearly confined to the Beaufort Sea and Canada basin, eddies were found further to the south and west than in previous observations. The semidiurnal band was dominated by near‐inertial motions with amplitude that varied seasonally (largest in late summer and smallest in late winter). Diurnal tides were weak (<1 cm s−1) except over a shallow portion of the Chukchi Shelf.
Observation of the seismic nucleation phase in the Ridgecrest, California, Earthquake sequence
Authors: Ellsworth William L. , Beroza Gregory C.
Published: February 2004

Near‐source observations of five M 3.8–5.2 earthquakes near Ridgecrest, California are consistent with the presence of a seismic nucleation phase. These earthquakes start abruptly, but then slow or stop before rapidly growing again toward their maximum rate of moment release. Deconvolution of instrument and path effects by empirical Green's functions demonstrates that the initial complexity at the start of the earthquake is a source effect. The rapid growth of the P‐wave arrival at the start of the seismic nucleation phase supports the conclusion of Mori and Kanamori [1996] that these earthquakes begin without a magnitude‐scaled slow initial phase of the type observed by Iio [1992, 1995].
Reflection and transmission of Alfvén waves at the auroral acceleration region
Authors: Vogt J. , Haerendel G.
Published: February 2004

Alfvén waves transfer field‐aligned currents, momentum and energy from the source region in the equatorial magnetosphere along Earth's magnetic field lines to the polar ionosphere and play therefore an important role in auroral electrodynamics. The parallel potential drop of some kV in the auroral acceleration region (AAR) at altitudes of some 1000 km has considerable effect on the propagation of these waves. The purpose of this report is to investigate the reflection and transmission properties of the AAR in the frame of a thin sheet approximation. Combining key relations of auroral electrodynamics with the requirement of current continuity results in an easy‐to‐handle physical description of the problem. The model provides an opportunity to study different acceleration mechanisms. It turns out that the reflection properties of the auroral zone depends strongly on the perpendicular scale length of the incident wave.
On the accuracy of HF radar surface current measurements: Intercomparisons with ship‐based sensors
Authors: Chapman R. D. , Shay L. K. , Graber H. C. , Edson J. B. , Karachintsev A. , Trump C. L. , Ross D. B.
Published: February 2004

High‐frequency (HF) radar systems can provide periodic, two‐dimensional, vector current estimates over an area approaching 1000 km2. As the use of these HF systems has gained wider acceptance, a number of attempts have been made to estimate the accuracy of such systems. However, comparisons of HF radar current estimates with in situ sensors are difficult to interpret since HF systems measure currents averaged over an area of ∼1 km2and to a depth of only ∼50 cm while in situ sensors measure currents at a point and somewhat greater depths (∼1 to 10 m). Previous studies of the accuracy of HF radar technology have thus attributed the differences observed between HF radar and in situ sensors to an unknown combination of vertical shear, horizontal inhomogeneity, in situ instrument errors, and HF radar system errors. This study examines the accuracy of HF radar current measurements using data from the 1993 High Resolution Remote Sensing Experiment, conducted off Cape Hatteras, North Carolina. Data from four shipborne in situ current meters are compared with data from an Ocean Surface Current Radar (OSCR), a commercial current‐measuring radar. We attempt to discern the predominant sources of error in these data by using multiple simultaneous measurements from different sensors and by examining the variation of observed current differences as a function of location. The results suggest an upper bound on the accuracy of the OSCR‐derived radial currents of 7 to 8 cm/s.
Marginal ice zone rheology: Comparison of results from continuum‐plastic models and discrete‐particle simulations
Authors: Gutfraind Ricardo , Savage Stuart B.
Published: February 2004

Computer simulations of ice behaviour in the marginal ice zone (MIZ) are described. Two cases were studied, a constant velocity wind blowing at certain angle with respect to the coast in what can be considered a simplified MIZ problem, and a vortex‐wind force field. The viscous‐plastic approach of Hibler has been used to model the ice rheology. We study the effects of adopting two different rheologies: cavitating fluid and Mohr‐Coulomb. The introduction of shear strength effects through a Mohr‐Coulomb yield criterion plays an important role in determining ice drift velocities in the MIZ. The effects that ice floe collisions can have in determining the ice rheological behavior have been tested by using discrete‐particle simulations. In order to do so, simulations for the MIZ were performed using different mean floe sizes, as well as different MIZ widths and wind conditions. The predictions of the discrete‐particle simulations are in general agreement with the results obtained by using the Mohr‐Coulomb rheology in the viscous‐plastic model. The continuum models were solved by using a fully Lagrangian method known as smoothed particle hydrodynamics (SPH). The method has the advantage of the Lagrangian techniques in avoiding problems of numerical diffusion. An additional advantage of SPH in comparison with classical Lagrangian approaches is that SPH does not need grids, and this makes it well suited to handle problems of large deformations.
The effect of possible Taylor columns on the summer ice retreat in the Chukchi Sea
Authors: Martin Seelye , Drucker Robert
Published: February 2004

The Chukchi Sea topography consists of a broad, flat, 50‐m‐deep plain, with two prominent shoals, Herald and Hanna, which have horizontal dimensions of about 100 km and rise to depths of about 30 m. Herald Shoal in particular is a prominent, isolated seamount which, because of the warm water flux in summer through Bering Strait, has a warm 0.1 m s−1 flow incident upon it. Examination of active and passive microwave imagery of the region for 1992–1994 shows that as the general ice cover recedes, the ice remains preferentially over Herald Shoal for 3–4 weeks after the surrounding ice has melted. A scale analysis suggests that the cause of the ice persistence is the formation of a Taylor column over the shoal, which traps cold water and ice above it. A similar trapping of ice occurs over Hanna Shoal and on its eastern slope, but the response is complicated by a very different topography and the northward flow down Barrow Canyon.
The climate signal in the stable isotopes of snow from Summit, Greenland: Results of comparisons with modern climate observations
Authors: White J. W. C. , Barlow L. K. , Fisher D. , Grootes P. , Jouzel J. , Johnsen S. J. , Stuiver M. , Clausen H.
Published: February 2004

Recent efforts to link the isotopic composition of snow in Greenland with meteorological and climatic parameters have indicated that relatively local information such as observed annual temperatures from coastal Greenland sites, as well as more synoptic scale features such as the North Atlantic Oscillation (NAO) and the temperature seesaw between Jakobshaven, Greenland, and Oslo, Norway, are significantly correlated with δ18O and δD values from the past few hundred years measured in ice cores. In this study we review those efforts and then use a new record of isotope values from the Greenland Ice Sheet Project 2 and Greenland Ice Core Project sites at Summit, Greenland, to compare with meteorological and climatic parameters. This new record consists of six individual annually resolved isotopic records which have been average to produce a Summit stacked isotope record. The stacked record is significantly correlated with local Greenland temperatures over the past century (r=0.471), as well as a number of other records including temperatures and pressures from specific locations as well as temperature and pressure patterns such as the temperature seesaw and the North Atlantic Oscillation. A multiple linear regression of the stacked isotope record with a number of meteorological and climatic parameters in the North Atlantic region reveals that five variables contribute significantly to the variance in the isotope record: winter NAO, solar irradiance (as recorded by sunspot numbers), average Greenland coastal temperature, sea surface temperature in the moisture source region for Summit (30°–20°N), and the annual temperature seesaw between Jakobshaven and Oslo. Combined, these variables yield a correlation coefficient of r=0.71, explaining half of the variance in the stacked isotope record.
A new method to determine the mean sea surface dynamic topography from satellite altimeter observations
Authors: Feron R. C. V. , De Ruijter W. P. M. , van Leeuwen P. J.
Published: February 2004

A method is presented to calculate mean sea surface dynamic topography from satellite altimeter observations of its temporal variability. Time averaging of a simplified version of the quasi‐geostrophic potential vorticity equation for the upper ocean layer results in a differential equation for the averaged relative vorticity in which the mean divergence of the eddy vorticity fluxes acts as a source or sink. The essential part is that these eddy fluxes can be determined from the altimeter observations. Consequently, no parameterisations appear in the averaged vorticity equation. From the average vorticity field, surface geostrophic velocities and related mean dynamic sea surface topography can then simply be derived. The usefulness of the method is established using “perfect” data, namely numerical output from the United Kingdom Fine Resolution Antarctic Model. The method appears applicable to areas of the ocean with strong enough mesoscale variability such as the major western boundary currents and their extensions and to frontal regions of the Antarctic Circumpolar Current. Quite realistic results are presented for active regions of the world ocean. Results are compared with hydrographic observations for the major western boundary current extensions of the Southern Ocean. An important application is to combine the newly derived averaged flow field with the observed eddy field to derive the total time‐varying geostrophic surface velocity field. As a striking example this is applied to the Agulhas Current retroflection, where the repeated shedding of large rings can now be synoptically reconstructed as a continuous process.
Inferring meridional mass and heat transports of the Indian Ocean by fitting a general circulation model to climatological data
Authors: Lee Tong , Marotzke Jochem
Published: February 2004

The meridional overturning and heat transport of the Indian Ocean are studied by fitting the steady state dynamics of a general circulation model (GCM) to climatological annual mean temperatures, salinities, and surface forcings using the GCM and its adjoint. By estimating target temperatures and salinities near the artificially closed side boundaries as part of the optimization procedure, a steady solution that is consistent with climatological data within limits of observational errors is found. The resultant meridional overturning is vigorous (14 Sv; 1 Sv=106 m3/s) only in the upper 1000 m. The estimated deep inflow entering the Indian Ocean from the south is weak. Requiring a large net northward inflow at depth does not result in strong interior upwelling but leads to unrealistically large baroclinic mass exchange and implied vertical mixing near the Indonesian throughflow region. The shallow overturning is the main carrier of the southward heat transport, which has a maximum of 0.8 PW (1 PW=1015 W) near 150S. Wind forcing plays a key role in driving the estimated overturning of the Indian Ocean. This result is in disagreement with previous interpretations about the central role of surface heat flux in driving a vigorous deep overturning of the Indian Ocean.
On the dynamics of the Lakshadweep high and low in the southeastern Arabian Sea
Authors: Shankar D. , Shetye S. R.
Published: February 2004

Hydrography and altimetry show that a ”high” in surface topography forms off southwest India in January (midnortheast monsoon) and that a ”low” forms during the southwest monsoon; the high and low, called Lakshadweep high and low after the island chain in the vicinity of where they form, propagate westward, extending across the southern Arabian Sea a few months after genesis. We investigate the dynamics of the high and low with an analytic model and with numerical simulations using a dynamical reduced‐gravity model for the north Indian Ocean. We conclude that the high and low do not owe their existence to nonlinearity. They are a consequence of westward propagating Rossby waves radiated by Kelvin waves propagating poleward along the western margin of the Indian subcontinent. Most important for the annual cycle of the high and low are the annual and semiannual Kelvin waves off southwest India; these appear to be forced primarily by alongshore winds in the Bay of Bengal and by winds in the equatorial Indian Ocean. Other Kelvin waves, provided their period is greater than about 40 days, can contribute to the high and low; these Kelvin waves are expected to introduce intra‐annual and interannual variability in the annual cycle. Below the critical period of 40 days, energy is trapped at the coast. The analysis provides a mechanism for the early onset of upwelling off southwest India, which is known to begin in February, well before the onset of the upwelling‐favorable southwest monsoon winds.
The modulation of radar backscatter by long ocean waves: A quadratically nonlinear process?
Authors: Schmidt Anke , Bao Mingquan
Published: February 2004

Radar imaging of ocean scenes requires knowledge of the modulation of the radar backscatter by long ocean waves. By using the conventional composite surface theory, the modulation of the radar backscatter can be related to a single parameter of the ocean wave field, such as slope or orbital velocity of the long waves. Usually, this relationship is assumed to be linear and described by a linear modulation transfer function (MTF). Consequently, most investigations of the modulation of the ocean radar backscatter concentrate on the calculation of this linear MTF. However, the coherence between the variations of the backscattered radar power and those of the orbital velocity associated with the long waves is generally low at C and X bands. In this paper we make the hypothesis that the low coherence is due to a nonlinear relationship between the radar backscatter and the long ocean waves (rather than to the neglect of other possible modulation sources). In order to quantify this nonlinearity, we estimate linear and quadratic MTFs from time series of the backscattered radar power and the Doppler shift of the backscattered radar signal. The latter is directly related to the orbital velocity of the long waves. The data were acquired by a scatterometer mounted on a sea‐based platform in the North Sea. We define the coherence as the fraction of the output spectrum that can be related to the input spectrum by a measured transfer function, which can be either linear or linear and quadratic. We find that the coherence between the backscattered radar power and the Doppler shift can be increased by up to 0.14 by using linear and quadratic MTFs instead of a linear one. However, at X band the coherence never becomes larger than 0.5. Thus we conclude that the modulation process cannot be described fully by a quadratically nonlinear system which relates the radar backscatter to the orbital velocity of the long waves. Therefore a higher order of nonlinearity has to be considered.
Secondary circulation in a highly stratified estuary
Authors: Chant Robert J. , Wilson Robert E.
Published: February 2004

Observations from the Hudson River estuary are presented to discuss the interaction between secondary flows and stratification in the vicinity of a headland during highly stratified conditions. Analysis of the cross‐stream momentum balance suggests that centrifugal accelerations are in near balance with the cross‐stream baroclincity. Results emphasize that the interaction between stratification and secondary flows can set up a cross‐stream internal seiche. This results in a reduction and subsequent reversal of the secondary flows downstream of a headland. Simple scaling arguments suggest that the spin‐down time for tidally driven eddies is related to stratification.
Long‐term trends of temperature, salinity, density, and transient tracers in the central Greenland Sea
Authors: Bönisch Gerhard , Blindheim Johan , Bullister John L. , Schlosser Peter , Wallace Douglas W. R.
Published: February 2004

We present long‐term observations of temperature, salinity, tritium/3He, chlorofluorocarbon‐11 (CFC 11), and chlorofluorocarbon‐12 (CFC 12) for the central Greenland Gyre. The time series span the periods between 1952 and 1994 (temperature), 1981 and 1994 (salinity), 1972 and 1994 (tritium/3He), and 1982 and 1994 (CFCs). The correlation between hydrographic and transient tracer data indicates that low temperatures in the deep water in the early 1950s and between 1960 and 1980 reflect periods of higher deep water formation rates whereas periods of increasing temperatures in the late 1950s and between 1980 and 1994 are related to low deep water formation rates. However, the transient tracer observations obtained in the 1980s and early 1990s indicate that even during periods of low deep water formation, some water from the upper water column contributed to deep water formation between 1980 and 1994. In 1994, the deep water reached temperatures and salinities of −1.149 °C and 34.899, respectively, and no longer fits most of the classical definitions of Greenland Sea Deep Water (−1.29°C< Θ < −1.0°C, 34.88<S<34.90). The temperature increase in the water column between 200 and 2000 m depth between 1980 and 1994 corresponds to an average heating rate of about 5 Wm−2 over this period, resulting in a decrease in density. The 13‐year warming could be balanced by intensive cooling in two winters. The surface salinity steadily increased from 34.50 in 1991 to 34.85 in 1994.
Mooring observations of the thermal structure, salinity, and currents in the SE Bering Sea basin
Authors: Cokelet E. D. , Stabeno P. J.
Published: February 2004

Utilizing recently developed deep‐sea mooring technology, we present the first interannual time series measurements of the ocean temperature, salinity, and velocity fields in the Bering Sea basin. These were made during spring and summer of 1992 and 1993 and late winter to summer of 1994, in 2195 m of water. Results show a weak background flow of ∼5 cm/s with semidiurnal tides superimposed. Anticyclonic (clockwise) eddies pass by sporadically, inducing currents up to ∼80 cm/s and depressing the isopycnals and isotherms up to ∼200 m. The thermal structure in the upper 400 m shows a deep temperature maximum from warm Alaskan Stream inflows and a minimum above due to winter cooling. A sea surface cooling event was observed in the late winter of 1994, with cold water penetrating to ∼120‐m depth. Spring warming begins in late April to early May of each year, marked by the arrival of the 3.75°C isotherm. The local wind plays little role in forcing ocean currents at the site except at the inertial frequency.
Coastal circulations driven by river outflow in a variable‐density 1½‐layer model
Authors: McCreary Julian P. , Zhang Shuliang , Shetye Satish R.
Published: February 2004

A variable‐density, 1½‐layer model is used to investigate the dynamics of the fresher‐water plumes generated by river outflow. Solutions are found in a north‐south channel, and the transport Mr and salinity Sr of the outflow are specified as boundary conditions along a 25 km‐wide segment of the western boundary. In most cases, the river water discharges into a preexisting, oceanic mixed layer with thickness H1. When Mr is sufficiently low, plumes remain coastally trapped. Immediately after the outflow is switched on, a coastal Kelvin wave is excited at the river mouth that establishes a southward current of oceanic water along the right‐hand coast. In contrast, all the river water first bends to the left as it exits the river mouth, and the resulting plume advances northward along the left‐hand coast. At the plume nose, some of the fresher water reverses direction, and this water, together with some oceanic water, flows southward on either side of the offshore density front between the fresher and salty waters. Two processes cause this upstream movement: geostrophic adjustment generates the southward frontal current, and Kelvin‐wave propagation from the nose thins the layer within the plume thereby establishing the northward, geostrophic, coastal jet. When Mr is sufficiently high, coastally trapped plumes no longer exist. If the Rossby number of the outflow is also large enough, the river water flows directly offshore and only a portion of it recirculates to form a northward propagating coastal plume. The angle at which the outflow leaves the river mouth becomes more southward as Mr and Sr increase and as H1 decreases. The strength of the northward propagating plume is weakened as H1 decreases and Sr increases. When H1=0 so that there is no ambient oceanic layer, there is no northward plume at all. Likewise, the plume is weakened when the model includes entrainment, a process that acts to prevent the layer thickness from thinning appreciably.
Spectral evolution of shoaling and breaking waves on a barred beach
Authors: Elgar Steve , Guza R. T. , Raubenheimer B. , Herbers T. H. C. , Gallagher Edith L.
Published: February 2004

Field observations and numerical model predictions are used to investigate the effects of nonlinear interactions, reflection, and dissipation on the evolution of surface gravity waves propagating across a barred beach. Nonlinear interactions resulted in a doubling of the number of wave crests when moderately energetic (about 0.8‐m significant wave height), narrowband swell propagated without breaking across an 80‐m‐wide, nearly flat (2‐m depth) section of beach between a small offshore sand bar and a steep (slope = 0.1) beach face, where the waves finally broke. These nonlinear energy transfers are accurately predicted by a model based on the nondissipative, unidirectional (i.e., reflection is neglected) Boussinesq equations. For a lower‐energy (wave height about 0.4 m) bimodal wave field, high‐frequency seas dissipated in the surf zone, but lower‐frequency swell partially reflected from the steep beach face, resulting in significant cross‐shore modulation of swell energy. The combined effects of reflection from the beach face and dissipation across the sand bar and near the shoreline are described well by a bore propagation model based on the nondispersive nonlinear shallow water equations. Boussinesq model predictions on the flat section (where dissipation is weak) are improved by decomposing the wave field into seaward and shoreward propagating components. In more energetic (wave heights greater than 1 m) conditions, reflection is negligible, and the region of significant dissipation can extend well seaward of the sand bar. Differences between observed decreases in spectral levels and Boussinesq model predictions of nonlinear energy transfers are used to infer the spectrum of breaking wave induced dissipation between adjacent measurement locations. The inferred dissipation rates typically increase with increasing frequency and are comparable in magnitude to the nonlinear energy transfer rates.
Paleosalinity and δ18O: A critical assessment
Authors: Rohling Eelco J. , Bigg Grant R.
Published: February 2004

In paleoceanography, a traditional application of oxygen isotope ratios in fossil carbonate from deep‐sea cores concerns the reconstruction of paleotemperatures. Recently, isotopic data have been increasingly employed in the reconstruction of paleosalinities as well. This application involves a number of basic assumptions, a critical review of which is presented here. As the calculation of paleosalinity from δ18O residuals assumes constant and linear surface water salinity (S): δ18O relationships, we investigate this basic assumption for areas with important and variable freshwater budget and/or sea‐ice influences. We show that it is particularly unwarranted to assume such temporally invariant linearity in temperate and subpolar seas because of the effects of freezing and melting. More critically, we argue that the determinant of many regions' S:δ18O is advection, not the local water balance, so that the basic δ18O of seawater in many regions of the world ocean would likely have changed in a manner somewhat different from that expected from the ice volume record. Although the differences may not be significant for the gross reconstruction of such features as past sea level, we believe they do potentially bias paleosalinity reconstructions, mostly toward high salinities. To fully exploit the potential of oxygen isotope records as quantitative paleosalinity indicators, both the spatial and temporal variabilities need to be constrained. Hence we recommend that (1) for any time slice of specific interest to paleoceanographic and paleoclimatological studies, a coupled ocean‐atmosphere‐isotope model, or at least an ocean‐isotope model driven by a paleoclimate‐isotope model, should be used to define the distribution of the base, mixing end‐member, δ18O of sea water; (2) much further regional‐scale research is needed on flux rates and isotopic compositions of the various terms in the freshwater budget, as well as on the variability in the various advective terms. The latter variability may be assessed from reconstructions of past seaice distributions and of volume fluxes/isotopic compositions of outflow from marginal seas and through sea straits/passages.
Surface air‐sea fluxes and upper ocean heat budget at 156°E, 4°S during the Tropical Ocean‐Global Atmosphere Coupled Ocean Atmosphere Response Experiment
Authors: Song Xilong , Friehe Carl A.
Published: February 2004

Meteorological and oceanographic observations were made from R/V Kexue #1 in the Tropical Ocean‐Global Atmosphere Coupled Ocean Atmosphere Response Experiment (TOGA COARE) for three cruise periods over the 4 month intensive observation period from November 1, 1992, to February 28, 1993. Measurements of mean atmospheric variables were made with the Integrated Sounding System with ancillary measurements of longwave radiation and near‐surface sea temperature. The TOGA COARE bulk formulas were used to estimate sensible and latent heat fluxes. The temperature and salinity structure to 300 m was measured with conductivity‐temperature‐depth (CTD) and horizontal currents to 130 m with an acoustic Doppler current profiler (ADCP). The daily mean heat budget was obtained from the CTD data, surface fluxes, and assumptions about solar radiative absorption with depth and turbulent mixing across the bottom of the layer. Horizontal advection was estimated as the residual of the budget. The results show variability typical of the tropics: low winds (LW) with maximum solar radiation input and increasing sea surface temperature (SST); the westerly wind burst (WWB) with large latent heat flux and decreasing SST; and moderate winds with intermittent squalls (MW). Within the estimated errors of the surface heating, the upper ocean heat budget is in local balance without advection for the LW period. For the WWB and MW periods, horizontal advection is important and large for the WWB and is confirmed by currents measured by the ADCP. The horizontal advection is more important in the heat balance in the deeper layer than that in the mixed layer. The column budget analysis of heat and fresh water indicates that cold, fresh water advected into the station occupied by R/V Kexue #1 during the WWB period.
Asian provenance of glacial dust (stage 2) in the Greenland Ice Sheet Project 2 Ice Core, Summit, Greenland
Authors: Biscaye P. E. , Grousset F. E. , Revel M. , Van der Gaast S. , Zielinski G. A. , Vaars A. , Kukla G.
Published: February 2004

Samples of dust from the Greenland Ice Sheet Project 2 (GISP2) ice core, Summit, Greenland, dated within marine isotope stage 2 (between 23,340 and 26,180 calendar years B.P.) around the time of the coldest, local, last glacial temperatures, have been analyzed to determine their provenance. To accomplish this, we have compared them with approximately Coeval aeolian sediments (mostly loesses) sampled in possible source areas (PSAs) from around the northern hemisphere. The <5‐μm grain‐size fraction of these samples was analyzed on the basis that it corresponds to the atmospheric dust component of that time and locale, which was sufficiently fine grained to be transported over long distances. On the basis of comparison of the clay mineralogy and Sr, Nd and Pb isotope composition with ice dust and PSAs and assuming that we have sampled the most important PSAs, we have determined that the probable source area of these GISP2 dusts was in eastern Asia. The dust was not derived from either the midcontinental United States or the Sahara, two more proximal areas that have been suggested as potential sources based on atmospheric circulation modeling. Except for a brief period during an interstadial, when dust transport was exceptionally low (for glacial times) and had a mineralogical composition indicative of a slightly more southern provenance, the source area of the dust did not change significantly during times of variably higher fluxes of dust with larger mean grain size or lower fluxes of dust with smaller mean grain size. This includes the high‐dust period that correlates with the Heinrich 2 period of major iceberg discharge into the North Atlantic. Variable wind strengths must therefore be invoked to account for these abrupt and significant changes in dust flux and grain size.
Continental biogenic species in the Greenland Ice Core Project ice core: Tracing back the biomass history of the North American continent
Authors: Fuhrer Katrin , Legrand Michel
Published: February 2004

Ammonium, nitrate, and organic acid records from the Greenland Ice Core Project deep ice core are discussed. All species have a continental biogenic source that is situated predominantly on the North American continent for species deposited in Summit, central Greenland. The record therefore can be used to trace back the biomass history of the North American continent. Difficulty in the interpretation of these records arises from their unknown transfer behavior in a more alkaline atmosphere, which characterizes glacial time periods compared to interglacial stadials. This may have implications not only for weak acids such as formate and acetate, but also possibly for the transport and incorporation of HNO3into aerosols, whereas ammonium is probably not affected by the alkalinity change of the atmosphere. Our approach is to compare samples with similar H+concentrations throughout the record. From the records we infer several significant fluctuations in the extent of the Laurentide Ice Sheet during the last glaciation. We find evidence against the occurrence of an extensive Younger Dryas event in North America.
Defining and quantifying microscale wave breaking with infrared imagery
Authors: Jessup A. T. , Zappa C. J. , Yeh Harry
Published: February 2004

Breaking without air entrainment of very short wind‐forced waves, or microscale wave breaking, is undoubtedly widespread over the oceans and may prove to be a significant mechanism for enhancing the transfer of heat and gas across the air‐sea interface. However, quantifying the effects of microscale wave breaking has been difficult because the phenomenon lacks the visible manifestation of whitecapping. In this brief report we present limited but promising laboratory measurements which show that microscale wave breaking associated with evolving wind waves disturbs the thermal boundary layer at the air‐water interface, producing signatures that can be detected with infrared imagery. Simultaneous video and infrared observations show that the infrared signature itself may serve as a practical means of defining and characterizing the microscale breaking process. The infrared imagery is used to quantify microscale breaking waves in terms of the frequency of occurrence and the areal coverage, which is substantial under the moderate wind speed conditions investigated. The results imply that ”bursting“ phenomena observed beneath laboratory wind waves are likely produced by microscale breaking waves but that not all microscale breaking waves produce bursts. Oceanic measurements show the ability to quantify microscale wave breaking in the field. Our results demonstrate that infrared techniques can provide the information necessary to quantify the breaking process for inclusion in models of air‐sea heat and gas fluxes, as well as unprecedented details on the origin and evolution of microscale wave breaking.