bluuebunny/crossref_metadata_2025 huggingface dataset:
Cassini and Wind stereoscopic observations of Jovian nonthermal radio emissions: Measurement of beam widths
Authors: Kaiser M. L. , Zarka P. , Kurth W. S. , Hospodarsky G. B. , Gurnett D. A.
Published: February 2004

During two intervals in 1999, simultaneous observations of Jupiter's decametric and hectometric radio emissions were made with the Cassini radio and plasma wave instrument (RPWS) and the radio and plasma wave instrument (WAVES) on the Wind spacecraft in Earth orbit. During January the Jovian longitude difference between the two spacecraft was about 5°, whereas for the August–September Earth flyby of Cassini, the angle ranged from 0° to about 2.5° (the Jovicentric latitudinal difference was <0.3° during both intervals). With these separations the instantaneous widths of the walls of the hollow conical radiation beams of some of the decametric arcs were measured by cross correlating dynamic spectra. The results suggest that the typical width is approximately 1.5° ±0.5°. The conical beams seem to move at Io's revolution rate for Io‐controlled arcs. Additionally, some of the nonarc hectometric wavelength emissions show some properties of both wide and very narrow beam widths.
Type II flare continuum in the corona and solar wind
Authors: Leblanc Y. , Dulk G. A. , Cairns I. H. , Bougeret J.‐L.
Published: February 2004

A solar radio type II/type IV event with exceptionally low frequency flare continuum radiation was observed on May 2, 1998 with the Wind spacecraft. This flare continuum, associated with the type II burst (FCII), descended to 7.5 MHz (2.5–3 solar radii), the lowest frequency ever observed for this type of emission. It lasted for >2 hours at 13.8 MHz. Simultaneous observations were made with ground‐based radiospectrographs, and with the Extreme Ultraviolet Imaging Telescope (EIT) and Large Angle and Spectrometric Coronagraph (LASCO) telescopes. The radio event consists of a group of intense type III bursts observed from 1000 MHz down to 0.03 MHz, the plasma frequency at 1 AU. The type II burst was recorded from 45 MHz down to 0.4 MHz, and an interplanetary shock was observed at 1 AU on May 4 at 0500 UT. The type II shock commenced within a few minutes of the flash phase of the flare and of the liftoff time of a coronal mass ejection (CME) observed by EIT and LASCO. The derived speeds of the type II shock, the CME in the plane of the sky, and the shock from the Sun to 1 AU are all ≈ 1000 km s−1. After estimating the liftoff time and radial speed of the CME front, we find that the type II shock and flare continuum were in the wake of the CME. This event shows evidence of acceleration of electrons in the corona out to 3 AS for ≳2 hours. Theoretical implications on the generation of the flare continuum radiation and its relation to the observed brightness temperature are considered. The source model of type II‐flare continuum of Robinson [1985], in which electrons are accelerated by the shock wave traversing CME expanding loops, is discussed in view of these observations.
Comprehensive particle and field observations of magnetic storms at different local times from the CRRES spacecraft
Authors: Korth A. , Friedel R. H. W. , Mouikis C. G. , Fennell J. F. , Wygant J. R. , Korth H.
Published: February 2004

The response of ring current intensification to three magnetic storms sampled at dawn, midnight, and dusk is investigated. We use a comprehensive set of data from the CRRES satellite, using plasma, energetic particle (ion composition), electric field, and magnetic field data, which is ideal for investigating the interrelationship between the ring current strength as measured by Dst, the particle (current carriers) in the outer radiation belt, their effects on the global magnetic field, and the convection effects caused by large dawn‐dusk electric fields. This yields a comprehensive and self‐consistent picture of storm time radiation belt formation based entirely on data. At all local times investigated strong, stretching (flattening) of the magnetic field down to L < 4 is observed during the storm main phase, showing that this field line stretching is not limited to midnight. Ring current ions above 100 keV are shown to form a partial ring current during the main phase as they are only sampled at dawn during the recovery phase when the electric field vanishes. Comparing this feature to Kp‐dependent models of the proton Alfvén layer shows that dawn is only accessible to these ions after the main phase. Ionospheric origin ions (O+) follow dynamics very similar to those of H+, indicating a source in the plasma sheet. Solar wind ions (He++) are controlled by their solar wind source and have immediate access during the main phase. He+, which is generated in the ionosphere as well as by charge exchange, has behavior similar to that of O+ and H+. In contrast to the current view, plasma sheet ions in the energy range from 5 to 28 keV contribute significantly in the energy density of the ring current during the storm main phase.
Self‐organized branching of magnetotail current systems near the percolation threshold
Authors: Milovanov Alexander V. , Zelenyi Lev M. , Zimbardo Gaetano , Veltri Pierluigi
Published: February 2004

This paper advocates an application of the fractal topology formalism to an analysis of the magnetotail current systems. Our main attention is concentrated on the magnetotail regions with the considerably stretched background magnetic field; such conditions can be the case in the distant tail, as well as in the near‐Earth tail at the late substorm growth phase. Structural properties of the current system at the self‐organized (quasi)stationary states are analyzed in relationship with the universal fractal geometry of the percolating fractal sets near the critical threshold. We found that the violation of the criticality character leads to the topological phase transition in the system; this transition has features of a structural catastrophe and is associated with the magnetospheric substorm onset. A comparison with various observational studies is given, and the possible directions of the future research are outlined.
Particle acceleration and coronal mass ejection driven shocks: A theoretical model
Authors: Zank G. P. , Rice W. K. M. , Wu C. C.
Published: February 2004

There is increasing evidence to suggest that energetic particles observed in “gradual” solar energetic particle (SEP) events are accelerated at shock waves driven out of the corona by coronal mass ejections. Energetic particle abundances suggest too that SEPs are accelerated from in situ solar wind or coronal plasma rather than from high‐temperature flare material. A dynamical time‐dependent model of particle acceleration at a propagating, evolving interplanetary shock is presented here. The theoretical model includes the determination of the particle injection energy (injection here refers to the injection of particles into the diffusive shock acceleration mechanism), the maximum energy of particles accelerated at the shock, energetic particle spectra at all spatial and temporal locations, and the dynamical distribution of particles that escape upstream and downstream from the evolving shock complex. As the shock evolves, energetic particles are trapped downstream of the shock and diffuse slowly away. In the immediate vicinity of the shock, broken power law spectra are predicted for the energetic particle distribution function. The escaping distribution consists primarily of very energetic particles initially with a very hard power law spectrum (harder than that at the shock itself) with a rollover at lower energies. As the shock propagates further into the solar wind, the escaping ion distribution fills in at lower energies, and the overall spectrum remains hard. Downstream of the shock, the shape of the accelerated particle spectrum evolves from a convex, broken power law shape near the shock to a concave spectrum far downstream of the shock. Intensity profiles for particles of different energies are computed, and the relation between arrival times, maximum predicted energies, and shock propagation characteristics are described. These results are of particular importance in the context of predictive space weather studies.
Background reduction for quiet time particle fluxes aboard the Solar and Heliospheric Observatory
Authors: Valtonen E. , Kecskeméty K. , Kunow H. , Müller‐Mellin R. , Torsti J.
Published: February 2004

Two versions of a method are presented to determine the background of energetic particle telescopes during low‐flux periods at low energies. Traditional procedures first evaluate pulse heights of particle sensors working in coincidence then select certain areas of the ΔE/Δx versus E plane, where E is energy, and finally, accept all events falling on those areas as real particles. Such methods, however, often fail during low‐flux conditions, as the background may become comparable to the genuine particle flux. The alternative methods suggested here analyze the shape of the distribution near the track of genuine particles and provide statistical estimation of its parameters to separate the background from the real particle flux. Results are presented for protons and helium during quiet activity periods of the recent solar minimum using data from energetic particle telescopes Energetic and Relativistic Nuclei and Electron (ERNE) Low Energy Detector (LED) and Comprehensive Suprathermal and Energetic Particle Analyzer (COSTEP) Electron Proton Helium Instrument (EPHIN) aboard the Solar and Heliospheric Observatory (SOHO) spacecraft. SOHO is a project of international collaboration between the European Space Agency and NASA.
A transmission line analogy for the development of equatorial ionospheric bubbles
Authors: Bhattacharyya Archana , Burke William J.
Published: February 2004

The Pedersen conductivity of the conjugate E regions couples to the equatorial F region through geomagnetic field lines and plays an important role in the development of equatorial spread F bubbles. Earlier work has suggested that the coupling between the E and F regions is effected through field‐aligned currents (FACs). However, these currents have not yet been explicitly introduced into theoretical models. This paper considers oppositely propagating Alfvén waves which are launched by equatorial F region perturbations as carriers of FACs and transverse polarization currents. A transmission line analogy is drawn, with the E region loads at the two ends and the generator in the equatorial F region. The currents which flow through the E regions depend on the plasma density of the propagation medium in which the transmission line is immersed. We conclude that whereas small angles between the solar terminator and the magnetic meridian favor the growth of equatorial bubbles, an increase in the plasma density of the propagation medium and a higher altitude of the equatorial F layer allow greater relaxation of the restriction imposed by the E region conductivities on the growth of equatorial bubbles.
Timing of substorm signatures during the November 24, 1996, Geospace Environment Modeling event
Authors: Lyons L. R. , McPherron R. L. , Zesta E. , Reeves G. D. , Sigwarth J. B. , Frank L. A.
Published: February 2004

An excellent data set has been gathered for the November 24, 1996, Geospace Environment Modeling substorm interval that included an ∼95 min interval of strongly southward interplanetary magnetic field. There were two expansion phase onsets and a pseudobreakup during this period. For both onsets the classical signatures of onset in the auroral ionosphere (ground magnetometer signatures of electrojet formation, ground observations of Pi2 pulsations, and auroral brightening) all occurred within less than a minute of each other, indicating consistency between these onset indicators and giving reliable identification of onset times. On the other hand, low‐latitude positive bay observations that were most likely made near or within the longitude range of substorm onset did not provide accurate or consistent onset timing. Low‐latitude Pi2s identified both onsets, though one was delayed by l min from the auroral zone onset. Our most important and unexpected result is that particle injection at synchronous orbit was observed to initiate ∼2.5 min prior to both expansion onsets in the auroral ionosphere. We suggest that the early detection of substorm onset at synchronous orbit was at least in part due to the unusually low latitude (63° magnetic) of the auroral zone onsets, which maps closer to synchronous orbit than usual. Onsets more often occur at somewhat higher latitudes, which map to a few RE beyond synchronous orbit. As a result, particle injections at synchronous orbit are generally delayed with respect to the time of current wedge initiation within the plasma sheet. We do not currently know whether or not onset within the near‐Earth plasma sheet generally occurs ∼2.5 min prior to expansion onset in the auroral ionosphere. While we have no reason to believe that the timing we have observed is unique, other studies of low‐latitude substorm onsets will be needed to ascertain the generality of this result.
Geospace Environment Modeling (GEM) magnetic reconnection challenge: Resistive tearing, anisotropic pressure and Hall effects
Authors: Birn Joachim , Hesse Michael
Published: February 2004

The nonlinear evolution of resistive tearing is studied in various regimes, including one‐fluid isotropic MHD, anisotropic (gyrotropic) MHD with terms approximating the effects of anisotropy driven instabilities, and HaIl‐MHD. Both uniform resistivity and spatially localized resistivity are investigated. The initial state is a plane one‐dimensional current sheet with an initial perturbation representing a periodic structure with magnetic islands and x type neutral points, chosen for the “GEM magnetic reconnection challenge” [Birn et al., this issue]. In the absence of dissipation, within ideal MHD, the initially imposed x type configuration collapses into a thin current sheet of finite length, while the magnetic islands contract, conserving their magnetic flux. The current density in the thin sheet becomes significantly enhanced, so that microscopic dissipation mechanisms can be expected to be ignited, even if they were absent in the initial state. Finite resistivity enables reconnection, and the growth of magnetic islands as in particle simulations. A comparison of reconnection rates shows that for the chosen initial current sheet thickness, a localized resistivity, corresponding to a Lundquist number (magnetic Reynolds number) of order unity, is necessary to approximate the growth and the reconnection electric field in the particle simulations. Pressure anisotropy, governed by double adiabatic conservation laws (modified by Ohmic heating), leads to reduced growth rates. This reduction is drastic for uniform resistivity. Isotropizing terms, modeling the effects of anisotropy driven microinstabilities again destabilize. HaIl‐MHD simulations can reproduce the fast growth of the particle simulations. The various models show both similarities and dissimilarities in their spatial structure. For similar amounts of reconnected flux, the variation of the normal magnetic field and of the (ion) flow speed along the current sheet are similar, but electric field and current density show significant differences.
Hall magnetohydrodynamic reconnection: The Geospace Environment Modeling challenge
Authors: Ma Z. W. , Bhattacharjee A.
Published: February 2004

Numerical results are presented on the Geospace Environment Modeling (GEM) reconnection challenge (and its variants) from the Hall magnetohydrodynamics (MHD) code developed at the University of Iowa (UI). Resistivity provides the mechanism for breaking field lines in this study. It is shown that the peak reconnection rate in the quasi‐saturated regime is controlled dominantly by ions and has a weak dependence on the resistivity. The reconnection rate is close to those obtained from other particle‐in‐cell, hybrid, and Hall MHD codes. Some differences between the results from the UI Hall MHD code and other codes are discussed.
Alfvénic collisionless magnetic reconnection and the Hall term
Authors: Shay M. A. , Drake J. F. , Rogers B. N. , Denton R. E.
Published: February 2004

The Geospace Environment Modeling (GEM) Challenge Harris current sheet problem is simulated in 2 1/2 dimensions using full particle, hybrid, and Hall MHD simulations. The same gross reconnection rate is found in all of the simulations independent of the type of code used, as long as the Hall term is included. In addition, the reconnection rate is independent of the mechanism which breaks the frozen‐in flux condition, whether it is electron inertia or grid scale diffusion. The insensitivity to the mechanism which breaks the frozen‐in condition is a consequence of whistler waves, which control the plasma dynamics at the small scales where the ions become unmagnetized. The dispersive character of whistlers, in which the phase velocity increases with decreasing scale size, allows the flux of electrons flowing away from the dissipation region to remain finite even as the strength of the dissipation approaches zero. As a consequence, the throttling of the reconnection process as a result of the small scale size of the dissipation region, which occurs in the magnetohydrodynamic model, iio longer takes place. The important consequence is that the minimum physical model necessary to produce physically correct reconnection rates is a Hall MHD description which includes the Hall term in Ohm's law. A density depletion layer, which lies just downstream from the magnetic separatrix, is identified and linked to the strong in‐plane Hall currents which characterize kinetic models of magnetic reconnection.
The deuterium/proton ratio as a measure of anomalous hydrogen
Authors: Lopate C.
Published: February 2004

The degree to which anomalous hydrogen exists in cosmic rays may have a profound effect on structures in the heliosphere, such as the termination shock. Since the source of the anomalous component is believed to be neutral gas in the local interstellar medium (LISM) and since there is little neutral deuterium in the LISM, the deuterium/proton ratio provides information about the fraction of observed hydrogen which could be anomalous. The Pioneer 10 data have been reanalyzed with an improved galactic deuterium spectrum. This report includes all the Pioneer 10 deuterium data from the University of Chicago instrument. Our results agree with recent reports that anomalous hydrogen is a significant component to the total hydrogen measurements in the outer heliosphere. These new results suggest that the anomalous hydrogen comprised about 60% during the solar minimum of 1987 and about 45% during the 1978 solar minimum, of the hydrogen flux near 70 MeV. The anomalous hydrogen disappeared by 1991, reappeared at Pioneer 10 near 64 AU in 1995, and persisted strongly since then.
Upper atmospheric observations at the Arecibo Observatory: Examples obtained using new capabilities
Authors: Isham B. , Tepley C. A. , Sulzer M. P. , Zhou Q. H. , Kelley M. C. , Friedman J. S. , González S. A.
Published: February 2004

The Arecibo Observatory will soon complete a major instrumental upgrade which will provide improved capabilities for observations of the upper atmosphere. As in the past, Arecibo capabilities center on 430‐MHz incoherent scatter radar (ISR) measurements of the ionosphere made locally or in conjunction with other national and international incoherent scatter facilities; the upgrade will add the capability for simultaneous two beam incoherent scatter observations and will extend plasma line measurements to ±15 MHz. Aeronomical studies may also be performed using a 46.8‐MHz coherent scatter radar, the feed of which is coaxial with the 430‐MHz line feed, the newly refurbished medium‐frequency (MF) radar facility, and a new digital ionosonde. The original airglow laboratory continues to house the Fabry‐Perot interferometers, Ebert‐Fastie spectrometer, and tilting filter photometers used for observations of mesospheric, thermospheric, and exospheric airglow, while a new laboratory provides a permanent home for the lidars used for resonance fluorescence observations of atomic metal layers in the mesopause region and for Doppler Rayleigh measurements of the upper stratosphere and lower mesosphere. Finally, the recently upgraded high‐frequency, high‐power transmitter facility has performed active aeronomical and plasma physical studies alone and in conjunction with other Arecibo and visitor‐supplied instruments. Observations performed as the upgrade nears completion have already provided a first look at what lies ahead in areas as diverse and fundamental as the structure and dynamics of the turbopause, the properties of upper atmospheric tides, the electrodynamics of the E and F regions, the dynamics of light ions in the topside, and the physics of plasma turbulence. Examples of these and other observations are presented, and the opportunities for future investigations are discussed.
The absolute and convective instability of the magnetospheric flanks
Authors: Wright Andrew N. , Mills Katharine J. , Ruderman Michael S. , Brevdo Leonid
Published: February 2004

Despite the existence of flank waveguide modes which are Kelvin‐Helmholtz unstable, the flanks of the terrestrial magnetosphere are observed to be remarkably stable and free of nonlinear disturbances. We suggest the explanation may be found in a more detailed stability analysis which shows that localized disturbances are convectively unstable in the Earth's rest frame. This means that as a wave packet grows and broadens, it also propagates at a sufficiently high speed so it is convected away leaving ultimately no disturbance at any fixed point in space (as t → ∞). We estimate that the magnetopause surface wave has an e‐folding length of the order of an Earth radius and soon becomes nonlinear, resulting in a magnetopause boundary layer [e.g., Manuel and Samson, 1993]. In contrast, the waveguide modes (which penetrate deep into the body of the magnetosphere) should grow by no more than a factor of about e as they propagate around the flanks to the tail. This also explains why theorists have had such success at modeling basic ULF waveguide processes with linear theory and why nonlinear waves in, or disruptions to, the body of the magnetospheric flanks are not observed: Wavepackets may grow by only a small amount as they propagate into the tail. Ultimately, they leave the flank undisturbed and with the appearance of stability, although they are actually convectively unstable.
Seismic volcanostratigraphy of large‐volume basaltic extrusive complexes on rifted margins
Authors: Planke Sverre , Symonds Philip A. , Alvestad Eivind , Skogseid Jakob
Published: February 2004

Large‐volume extrusive basaltic constructions have distinct morphologies and seismic properties depending on the eruption and emplacement environments. The presence and amount of water is of main importance, while local rift basin configuration, erosion, and resedimentation determine the overall geometry of the volcanic constructions. We have developed the concept of seismic volcanostratigraphy, a subset of seismic stratigraphy, to analyze volcanic deposits imaged on seismic reflection data. The method places special focus on identification and mapping of seismic facies units and the volcanological interpretation of these units. Interpretation of seismic reflection data along the Atlantic and Western Australia rifted margins reveals six characteristic volcanic seismic facies units named (1) Landward Flows, (2) Lava Delta, (3) Inner Flows, (4) Inner Seaward Dipping Reflectors (Inner SDR), (5) Outer High, and (6) Outer SDR. These units are interpreted in terms of a five‐stage tectonomagmatic volcanic margin evolution model comprising (1) explosive volcanism in a wet sediment, broad basin setting, (2) subaerial effusive volcanism forming Gilbert‐type lava deltas along paleoshorelines, (3) subaerial effusive volcanism infilling a fairly narrow rift basin, (4) shallow marine explosive volcanism as the injection axis is submerged below sea level, and finally (5) deep marine sheet flow or pillow‐basalt volcanism. Further, erosion and resedimentation processes are particularly important during the shallow marine stages. Seismic volcanostratigraphy provides important constraints on rifted‐margin development, in particular, on the prevolcanic basin configuration, relative timing of tectonomagmatic events, total amount of volcanic rocks, location of paleoshorelines, and margin subsidence history. These parameters give key boundary conditions for understanding the processes forming volcanic margins and other large‐volume basaltic provinces.
Satellite‐measured chlorophyll and temperature variability off northern Chile during the 1996–1998 La Niña and El Niño
Authors: Thomas A. C. , Blanco J. L. , Carr M. E. , Strub P. T. , Osses J.
Published: February 2004

Time series of satellite measurements are used to describe patterns of surface temperature and chlorophyll associated with the 1996 cold La Niña phase and the 1997–1998 warm El Niño phase of the El Niño‐Southern Oscillation cycle in the upwelling region off northern Chile. Surface temperature data are available through the entire study period. Sea‐viewing Wide Field‐of‐view Sensor (SeaWiFS) data first became available in September 1997 during a relaxation in El Niño conditions identified by in situ hydrographic data. Over the time period of coincident satellite data, chlorophyll patterns closely track surface temperature patterns. Increases both in nearshore chlorophyll concentration and in cross‐shelf extension of elevated concentrations are associated with decreased coastal temperatures during both the relaxation in El Niño conditions in September‐November 1997 and the recovery from El Niño conditions after March 1998. Between these two periods during austral summer (December 1997 to March 1998) and maximum El Niño temperature anomalies, temperature patterns normally associated with upwelling were absent and chlorophyll concentrations were minimal. Cross‐shelf chlorophyll distributions appear to be modulated by surface temperature frontal zones and are positively correlated with a satellite‐derived upwelling index. Frontal zone patterns and the upwelling index in 1996 imply an austral summer nearshore chlorophyll maximum, consistent with SeaWiFS data from 1998–1999, after the El Niño. SeaWiFS retrievals in the data set used here are higher than in situ measurements by a factor of 2–4; however, consistency in the offset suggests relative patterns are valid.
Dense water production on the northern Okhotsk shelves: Comparison of ship‐based spring‐summer observations for 1996 and 1997 with satellite observations
Authors: Gladyshev Sergey , Martin Seelye , Riser Steve , Figurkin Alexander
Published: February 2004

For the northern Okhotsk Sea polynyas, five Russian CTD surveys taken during 1995 to 1997 are used to examine the evolution of the polynya dense water. The surveys show that consistent with other investigations, the largest potential densities are 26.99 σθ, and the densest water occurs in Sakhalin and Shelikhov Bays. The surveys also suggest that the Shelikhov water drains directly into the deep Okhotsk, while on the northern shelves, gravity currents transport the dense water west to Sakhalin Bay. For comparison, determination of the polynya sizes and ice production from satellite passive microwave and meteorological observations shows that polynyas occur on the northwest shelf (NWS) between Ayan and Okhotsk City, on the northern shelf between Okhotsk City and Magadan, and in Shelikhov Bay. In contrast, the observations show that Sakhalin Bay is a region of land fast ice with no polynyas, so that the dense water observed here cannot form locally. For all polynyas the satellite observations show that the NWS contributes 60 to 70% of the total ice production, and due to the warmer 1997 air temperatures, the 1996 production is about 1.5 times the 1997 value. An estimate of the ice production from the surveys shows a similar regional distribution and enhancement of the 1996 production, with the satellite and ship estimates in agreement within their error bars. Finally, analysis of the dense water outflow shows that the upper Okhotsk Sea Mode Water has a renewal time of about 4 years; the lower part, about 14 years.
Remote forcing at the Last Glacial Maximum in the tropical Pacific Ocean
Authors: Andreasen Dyke H. , Ravelo A. Christina , Broccoli Anthony J.
Published: February 2004

We present results of a Last Glacial Maximum (LGM) wind stress sensitivity experiment using a high‐resolution ocean general circulation model of the tropical Pacific Ocean. LGM wind stress, used to drive the ocean model, was generated using an atmospheric general circulation model simulation forced by LGM boundary conditions as part of the Paleoclimate Modeling Intercomparison Project (PMIP) [Broccoli, 2000]. LGM wind stress anomalies were large in the western half of the basin, yet there was a significant hydrographic response in the eastern half. This ocean model experiment hind casts changes that are in close agreement with paleoceanographic data from the entire region, even without the explicit modeling of the air‐sea interactions. Data and model both predict that the annual average thermocline tilt across the basin was enhanced. Data and model are consistent with a stronger equatorial undercurrent which shoaled to the west of where it does today, and stronger advection of water from the Peru Current into the east equatorial Pacific and across the equator. Paleoproductivity and sea surface temperature (SST) data are interpreted in light of the modeling results, indicating that paleoproductivity changes were related to wind‐forced dynamical changes resulting from LGM boundary conditions, while SST changes were related to independent, possibly radiative, forcing. Overall, our results imply that much of the dynamic response of the tropical Pacific during the LGM can be explained by wind field changes resulting from global LGM boundary conditions.
Revisiting the South Pacific subtropical circulation: A synthesis of World Ocean Circulation Experiment observations along 32°S
Authors: Wijffels Susan E. , Toole John M. , Davis Russ
Published: February 2004

Hydrographic data about latitudes 30°–32°S in the Pacific Ocean are presented and used together with direct velocity observations from floats and moorings to investigate the spatial distribution of meridional flow and its associated transports. An initial velocity scheme is developed with reference to the water property distributions that is subsequently adjusted to meet constraints on net mass and dissolved silica transports. The resulting circulation is found to be sensitive to a priori scaling assumptions and differs in detail from past estimates of the flow in this region. Additionally constraining the flow with directly measured time averaged estimates of the velocity from floats and a current meter array significantly reduces the uncertainty in the derived flow field and improves consistency between the velocity and the property distributions. The derived circulation includes a nearly closed bottom‐to‐deep‐water overturning circulation of 16–20 Sv, below a subtropical gyre flow of thermocline and surface waters consisting of nearly distinct Tasman Sea (∼30 Sv) and basin interior (∼10 Sv) circulations and (a prescribed) ∼10±5 Sv of net northward flow to supply the Indonesian and Bering Strait throughflows. On the basis of this circulation scheme, the heat flux divergence for the Pacific north of the P6 line is estimated to be 0.75±0.56 pW (ocean gaining heat from the atmosphere) and the net freshwater divergence is estimated to be 0.1±0.1 Sv (ocean gains fresh water via air‐sea exchange and runoff). The former is highly dependent on the strength of the Indonesian throughflow; the latter also depends on the intensity of the lateral subtropical gyre circulation, which is constrained by the float data. Combined with estimates from the Indian and Atlantic Oceans, we find the subtropical Southern Hemisphere oceans transport 0.46±0.38 PW poleward, less than half the poleward heat transport achieved by the subtropical oceans in the Northern Hemisphere.
A sea ice climate switch mechanism for the 100‐kyr glacial cycles
Authors: Gildor Hezi , Tziperman Eli
Published: February 2004

A box model of the ocean‐atmosphere‐sea ice‐land ice climate system is used to study a novel mechanism for self‐sustained oscillations of the climate system on a time scale of 100,000 years, without external forcing. The oscillation in land ice volume has the familiar sawtooth shape of climate proxy records. The most novel aspect of the climate oscillations analyzed here is the crucial role played by the sea ice. The sea ice acts as a “switch” of the climate system, switching it from a growing land glaciers mode to a retreating land glaciers mode and shaping the oscillation's sawtooth structure. A simple explanation of the 100‐kyr timescale is formulated on the basis of the mechanism seen in the model. Finally, rapid sea ice changes such as those seen in our model, and their drastic effects on the climate system, may provide an explanation to some of the rapid climate changes observed to be a part of the variability at all timescales in the paleorecord.
Annual cycle of the Brazil‐Malvinas confluence region in the National Center for Atmospheric Research Climate System Model
Authors: Wainer Ilana , Gent Peter , Goni Gustavo
Published: February 2004

The objective of this study is to compare the mean and seasonal variability of the circulation in the southwest Atlantic with observations. The results used in the comparison are from the last 200 years of a 300 year control integration of the Climate System Model (CSM). The area of study includes the confluence region between the subtropical and subpolar waters represented by the Brazil and Malvinas Currents. The seasonal variation of transport and its relationship to changes in the wind stress forcing and in the sea surface temperature are examined and compared to available oceanographic observations. This study shows that a coarse resolution climate model, such as the CSM, can successfully reproduce major characteristics of the Brazil‐Malvinas confluence seasonally, although the mesoscale features involving recirculation and meander dynamics are not resolved. The CSM transport values in the region of 38°S are consistent with hydrographically derived values. The transport of the CSM Brazil Current is higher during austral summer and smaller during austral winter. Conversely, the Malvinas Current transport is weaker during austral summer and stronger during austral winter. This is also consistent with observations. The CSM seasonal cycle in transport associated with both the Brazil and Malvinas Currents and its meridional displacement is closely linked to the seasonal variations in the local wind stress curl. However, the displacement is much smaller in the model than in observations. The CSM results show that the latitudinal displacement of the 24°C and 17°C at the South American coast beween austral summer and winter is 20° and 12°, respectively. This is very similar to the displacement seen in observations.
Seasonal warming of Narragansett Bay and Rhode Island Sound in 1997: Advanced very high resolution radiometer sea surface temperature and in situ measurements
Authors: Fox Mary Frances , Kester Dana R. , Andrews James E. , Magnuson Andrea , Zoski Cynthia G.
Published: February 2004

The warming of Narragansett Bay and the offshore waters of Rhode Island Sound (RIS) and Buzzards Bay in the spring and early summer of 1997 was studied using in situ time series data and remotely sensed advanced very high resolution radiometer sea surface temperature (SST) satellite images. High‐resolution SST images of the New England area were expanded to highlight Narragansett Bay and RIS. To validate this procedure, the remotely sensed data were compared to in situ data at the NOAA buoy in Buzzards Bay and at a spar buoy in mid‐Narragansett Bay. The standard error (1.3°C) observed at the buoy in Narragansett Bay was slightly higher than that observed at the buoy in Buzzards Bay (1.0°C). A transect line down Narragansett Bay and into RIS and another across the entrance of Narragansett Bay and Buzzards Bay were extracted from the 47 images. A thermal front was observed at the mouth of the bay with the bay being warmer in the summer and cooler in the winter than the sound. Two areas of cold water were identified in the RIS transect: a cold water plume at the tip of Long Island and a second area near the Elizabeth Islands. We believe that both were caused by vertical mixing. There were three sources of in situ time series data to compare with the SST: (1) a spar buoy with sensors in the surface and bottom waters located near the middle of the Bay, (2) observations from a shore site near the mouth of the Bay, and (3) a National Oceanic and Atmospheric Administration buoy at the mouth of Buzzards Bay. Using the spar buoy data, we were able to calculate the vertical density gradient, and we found that salinity was more important than temperature in controlling the density structure at this site. Time series temperature data from the surface water in Buzzards Bay were almost identical to those observed in the bottom waters of Narragansett Bay, indicating that bottom water in the bay originates as surface water in RIS. Using a cooling event in the surface waters at the end of July, a transport time of ∼4 days was calculated for the offshore surface waters to reach the bottom at the mid‐bay location.
Topographically controlled circulation and mixing in a lake
Authors: Bowyer Peter A.
Published: February 2004

It is well known that the wind‐driven circulation in a wide lake may be controlled by its topography. Wind stress acting on the smaller mass of the water column in the shallows results in downwind flow there and return flow in the deeper water. In lakes with large areas of shallow water this circulation may cause rapid mixing in weakly stratified conditions. In more strongly stratified conditions, topographically induced horizontal wind driven circulation may influence the vertical stratification of the lake. Lough Mask is about 10 km long and has a maximum width of about 8 km. The lake may be divided into shallow eastern and deeper western portions with mean depths of 10 and 35 m respectively. In late Autumn, water at depth in the deep part of the lake moves upwind at speeds of up to 50 cm s−1 in response to axial winds of up to 15 m s−1. At the same location a weaker upwind flow is observed in the near surface water. This return flow is topographically driven. Temperature differences between near surface and near bottom meters were small (<0.5°C) and inversely correlated with the deep current. In a cool, windy summer, temperature time series in the deep part of the lake indicate a deep (>20 m) surface mixed layer and a cooler layer at depth whose temperature fluctuates in response to wind stress along the axis of the lake. Modelling results, supported by drogue measurements, indicate that the return flow associated with axial winds in these summer conditions is confined to the surface mixed layer. If this layer is thin, shear between the surface layer and the underlying water can be sufficient to cause mixing and a thickening of the surface layer until the Richardson number is ∼1. Thus the vertical temperature structure of the deep part of the lake can be influenced by flows driven by the lake topography.

In the shallow part of the lake, there is evidence of thermally driven motion in periods of calm weather in summer.
Eddy energy and shelf interactions in the Gulf of Mexico
Authors: Ohlmann J. Carter , Niiler P. Peter , Fox Chad A. , Leben Robert R.
Published: February 2004

Sea surface height anomaly data from satellite are continuously available for the entire Gulf of Mexico. Surface current velocities derived from these remotely sensed data are compared with surface velocities from drifting buoys. The comparison shows that satellite altimetry does an excellent job resolving gulf eddies over the shelf rise (depths between ∼200 and 2000 m) if the proper length scale is used. Correlations between altimeter‐ and drifter‐derived velocities are statistically significant (r>0.5) when the surface slope is computed over 125 km, indicating that remotely sensed sea surface height anomaly data can be used to aid the understanding of circulation over the shelf rise. Velocity variance over the shelf rise from the altimetry data shows regions of pronounced eddy energy south of the Mississippi outflow, south of the Texas‐Louisiana shelf, and in the northwest and northeast corners of the gulf. These are the same locations where surface drifters are most likely to cross the shelf rise, suggesting gulf eddies promote cross‐shore flows. This is clearly exemplified with both warm and cold eddies. Finally, the contribution of gulf eddies and wind stress to changes in the mean circulation are compared. Results indicate that the eddy‐generated vorticity flux to the mean flow is greater than the contribution from the surface wind stress curl, especially in the region of the Loop current and along the shelf rise base in the western gulf. Future modeling efforts must not neglect the role of eddies in driving gulf circulation over the shelf rise.
Transport of mass, heat, salt, and nutrients in the southern California Current System: Annual cycle and interannual variability
Authors: Bograd Steven J. , Chereskin Teresa K. , Roemmich Dean
Published: February 2004

Net fluxes of mass, heat, salt, nutrients, oxygen, and chlorophyll into a control volume within the southern California Current System (CCS) were computed from data collected on 55 cruises over a 14 year period (1984–1997). This analysis builds on an earlier work [Roemmich, 1989] by using an additional 39 cruises over 10 years, allowing for reliable estimates of the temporal variability in the fluxes on seasonal and interannual timescales and a reduction in the corresponding error budgets. A close balance was found between geostrophic convergence and Ekman divergence for the 14 year, seasonal, and interannual cruise subsets using three different wind products. Wind data taken concomitantly with the hydrographic sampling provided the best balance and hence the best flux estimates. The southern CCS was found to be a region with higher evaporation over precipitation and net heat gain by the ocean from the atmosphere (86 W m−2 in the 14 year mean) in all seasons. Significant variability in both the Ekman and geostrophic transports and the net property fluxes was found to be related to low‐frequency (interpentadal and El Niño‐Southern Oscillation timescale) changes in the dominant wind and circulation patterns in the CCS. Variability in primary productivity, estimated from the derived nutrient fluxes, accompanied the environmental changes. Application of this model to the ongoing data collection will further reduce the error bars on the fluxes and will allow for continued monitoring of changes in the physical and biological structure of the southern CCS.
A probabilistic approach to scatterometer model function verification
Authors: Shankaranarayanan Kartik , Donelan Mark A.
Published: February 2004

The scatterometer backscatter model function has a threshold wind speed, below which there is no backscatter return from the ocean surface. Owing to the variability associated with the wind at the scatterometer footprint resolution, the sharp cutoff in the model backscatter response is not observed in Ku band spaceborne scatterometer data. A merged data set is generated by temporal and spatial collocation of the National Center for Environmental Prediction (NCEP) 10‐m neutral winds with individual National Aeronautics and Space Administration Scatterometer (NSCAT) backscatter measurements from level 1.7. A model is set up to compare the observed and expected probability distributions of σ0 in an NCEP grid resolution cell (1.875°×1.904°), after incorporating the variabilities associated with the wind and the backscatter measurements. The differences in the model responses arrived at using the NSCAT 1 and the new threshold model function (TMF) are presented. The TMF model does well in fitting the observed σ0 distributions for all incidence angles, antenna azimuths, and both polarizations over the wind speed range considered (1–10 m s−1). A comparison of the modes of the model‐predicted and observed distributions revealed the model function cutoff behavior at the threshold wind speed. The model also predicts increasing percentage occurrences of negative σ0 with decreasing wind speeds and increasing incidence angles, in line with the observations. The TMF model is corrected for the superthreshold winds to yield a better fit to the observed probability distributions.
Standing edge waves on a pocket beach
Authors: Özkan‐Haller H. Tuba , Vidal César , Losada Iñigo J. , Medina Raúl , Losada Miguel A.
Published: February 2004

Field measurements of edge waves obtained on a narrow pocket beach are described. The beach (named Usgo) is located on the north coast of Spain immediately to the west of the city of Santander and is exposed to the Atlantic Ocean. The edge wave field on this beach is analyzed utilizing data from a longshore array of current meters located in about 3 m of water depth. Frequency spectra of longshore velocities display several significant peaks, suggesting that the edge wave field on this pocket beach consists of several dominant modes. This is in contrast to observations on open coastal beaches [e.g., Oltman‐Shay and Guza, 1987], which show the existence of a broad‐banded edge wave field. We isolate four frequency peaks and examine the longshore variation of the motions at these frequencies and find convincing evidence of a nodal structure. Utilizing the measured bathymetry, we compute the cross‐shore profiles and theoretical wavelengths of edge waves at the dominant frequencies using a numerical solution of the shallow water equations by Howd et al. [1992]. We find that edge waves at these frequencies are theoretically confined between the headlands since their amplitudes decay to insignificant levels within a cross‐shore distance that is less than the offshore extent of the headlands. In addition, assuming perfect sidewall reflections, the theoretical nodal locations closely correspond to those inferred from the measurements. We conclude that discrete standing edge waves resulting from resonances related to the longshore width of the beach are present on Usgo Beach.