bluuebunny/crossref_metadata_2025 huggingface dataset:
A two‐dimensional model of runaway electron beams driven by quasi‐electrostatic thundercloud fields
Authors: Lehtinen N. G. , Bell T. F. , Pasko V. P. , Inan U. S.
Published: February 2004

Intense, transient quasi‐electrostatic (QE) fields, which exist above thunderclouds following a positive cloud‐to‐ground lightning discharge, can produce an upward travelling runaway electron (REL) beam. A new two‐dimensional (2D) REL‐QE model is developed, expanding the previously reported 1D model [Bell et al., 1995] and incorporating the QE [Pasko et al., 1997a] and the recently developed electrostatic heating (ESH) [Pasko et al., 1997b] models. The new model gives the lateral electron distribution in the beam and allows us to determine the ionospheric effects and the optical luminosities resulting from the simultaneous action of the QE fields on the ambient electrons and the runaway electrons. The model is self‐consistent and includes the changes in space charge and conductivity due to the REL. Optical emissions and γ‐ray emissions [Lehtinen et al., 1996a] are calculated and compared to experimental observations of Sprites and terrestrial γ‐ray flashes (TGF). It is shown that the structure of the electric field and the optical emissions can be significantly affected by the REL.
Evidence for an S‐velocity discontinuity in the lowermost mantle beneath the South Eastern Pacific Basin
Authors: Olivieri M. , Pino N. A. , Morelli A.
Published: February 2004

We used tangential component SH waveforms with S, ScS, sS and sScS arrivals of Chilean earthquakes recorded by four seismographic stations located in Antarctica to study the radial velocity structure at the core‐mantle boundary (CMB) beneath the South Eastern Pacific basin. The selected station‐event pairs give information on two distinct, close, patches of D″ beneath the Bellingshausen Sea. In one of these regions an anomalous arrival— interpreted as a reflection at the top of D″ (SdS phase)—is seen between S and ScS, together with early arrivals of ScS. This observation is well fit by adding to global reference model SP6 a reflector 315 km above the CMB, marking a 3% S‐velocity increase. The new velocity model, usb2, fits both SdS and ScS travel times and amplitudes. Available data do not provide clear evidence to confirm or refute the presence of a similar velocity discontinuity in the second patch.
Ozone, column ClO, and PSC measurements made at the NDSC Eureka Observatory (80°N, 86°W) during the spring of 1997
Authors: Donovan D. P. , Fast H. , Makino Y. , Bird J. C. , Carswell A. I. , Davies J. , Duck T. J. , Kaminski J. W. , McElroy C. T. , Mittermeiter R. L. , Pal S. R. , Savastiouk V. , Velkov D. , Whiteway J. A.
Published: February 2004

During winter/spring 96/97 ozone levels over the Eureka NDSC observatory (80°N,86°W) were measured using a lidar, sondes, and a Brewer spectrophotometer. Column ClO measurements were also made using an FTIR system. Measurements show that lower stratospheric ozone mixing ratios decreased rapidly between mid‐February and late‐March though the ozone mixing ratio losses appear to have been less than for the 95/96 season. Elevated column amounts of ClO were found to be present over Eureka until late March.
Meteorology of the polar vortex: Spring 1997
Authors: Coy Lawrence , Nash Eric R. , Newman Paul A.
Published: February 2004

The 1996–1997 northern hemisphere spring polar vortex was very strong, cold, and symmetric, somewhat similar to those found in the Antarctic spring vortex. The vortex did not form until late in December and was very symmetric from February into late April. The spring vortex was characterized by record low temperatures, record low ozone amounts as measured from the Total Ozone Mapping Spectrometer (TOMS) instruments, and a wide band of strong winds in the lower stratosphere. Spring wave activity was greatly reduced, with 100 hPa February–March eddy heat fluxes that were lower by a factor of 2 from any previously observed values over the last 18 years.
Titan under a red giant sun: A new kind of “habitable” moon
Authors: Lorenz Ralph D. , Lunine Jonathan I. , McKay Christopher P.
Published: February 2004

We explore the response of Titan's surface and massive atmosphere to the change in solar spectrum and intensity as the sun evolves into a red giant. Titan's surface temperature is insensitive to insolation increases as the haze‐laden atmosphere “puffs up” and blocks more sunlight. However, we find a window of several hundred Myr exists, roughly 6 Gyr from now, when liquid water‐ammonia can form oceans on the surface and react with the abundant organic compounds there. The window opens due to a drop in haze production as the ultraviolet flux from the reddening sun plummets. The duration of such a window exceeds the time necessary for life to have begun on Earth. Similar environments, with ∼200K water‐ammonia oceans warmed by methane greenhouses under red stars, are an alternative to the ∼300K water‐CO2 environments considered the classic “habitable” planet.
Numerical Estimation of the effective conductivity of heterogeneous media with a 2D cellular automaton fluid
Authors: Küntz M. , Mareschal J. C. , Lavallée P.
Published: February 2004

A 2D cellular automaton fluid is used to estimate the effective transport properties of macroscopically homogeneous and isotropic two‐phase mixtures. The method is applied to the determination of the electrical conductivity of two‐phases agregates. The effective conductivity is calculated as a function of the relative concentration and for different values of the conductivity ratio between the constituting phases. Numerical results agree well with theoretical bounds in the case of alternating layers parallel or perpendicular to the direction of flow. For random isotropic distribution of the inclusion phase, the conductivity estimated by the cellular automaton falls between the theoretical Hashin‐Shtrikman (H‐S) bounds. The results presented are very close to experimental data, such as conductivity measurements on Al‐Bi mixtures.
Earthquake near field and W phase observations at teleseismic distances
Authors: Cummins Phil R.
Published: February 2004

Near field phases can be observed on individual broadband seismograms at teleseismic distances from great earthquakes. Even at distances of 20°–30°, such phases can have amplitudes of several tenths of a mm. While previous studies have identified near field phases in vertical component data, I show that such phases can be much more easily observed using the transverse component. Although observed effects agree well with complete waveform calculations using point moment tensor sources, they do not satisfy some of the assumptions commonly made in seismic data analysis. It is therefore suggested that care be taken in the analysis of very broadband seismograms which may contain these effects. Recent W phase observations are shown to be a case in point.
Analytical results on MHD intermediate shocks
Authors: Myong R. S.
Published: February 2004

The admissibility of MHD intermediate shocks has been a matter of much debate over the years. Though the legitimacy of such shocks was shown in a recent series of investigations by Wu, rigorous analytical proof of the results was considered being unsettled. In this paper, as a step towards developing such proof, we present a theory of MHD shocks. On the basis of global analysis, we develop a shock admissibility condition. Using this theory, we explain how intermediate shocks and compound waves are generated.
The relationship between total ozone and spectral UV irradiance from Brewer observations and its use for derivation of total ozone from UV measurements
Authors: Fioletov Vitali E. , Kerr James B. , Wardle David I.
Published: February 2004

Spectral UV irradiance measurements made by Brewer spectrophotometers at 7 stations over periods from 4 to 7 years together with Dobson and Brewer direct sun total ozone measurements have been used to establish a statistical relationship between total ozone, solar zenith angle, and UV irradiance. Using the relationship it is possible to estimate daily mean total ozone from routine UV measurements with a standard error of about 3%. Also the spectral irradiance for wavelengths between 300 and 325 nm can be estimated with error of about 25% at 300 nm and lower at longer wavelengths. The relationship is helpful for quality control of UV data. It can also be used to estimate total ozone at sites where spectral UV irradiance is measured and no direct ozone observations are made.
Geoid and topographic swells over temperature‐dependent thermal plumes in spherical‐axisymmetric geometry
Authors: King Scott D.
Published: February 2004

Geoid and topographic profiles over constant‐viscosity thermal plumes in spherical‐axisymmetric geometry at moderate Rayleigh numbers are compared with results from previous studies in other geometries. Plumes in spherical‐axisymmetric models produce larger geoid anomalies than plumes in either Cartesian or cylindrical‐axisymmetric geometries; however, the topographic anomalies over the plumes are smaller than the corresponding anomalies in a cylindrical‐axisymmetric geometry but larger than those in a Cartesian geometry. For plumes in a temperature‐dependent viscosity fluid, the geoid and topographic profiles are reduced by a factor of two when compared to a constant viscosity fluid with the same background viscosity profile. In this case, the radially‐averaged temperature profile around the plume creates a low‐viscosity ‘layer’ beneath the lithosphere.
The high‐latitude solar wind near sunspot Maximum
Authors: Wang Y.‐M. , Sheeley N. R.
Published: February 2004

We use an empirical relation between solar wind speed and coronal flux‐tube expansion to predict what Ulysses might have seen had it flown over the solar poles during 1989–1991 instead of 1994–1996. The wind speed patterns, derived from solar magnetograph data, show the following characteristics: (1) high‐speed streams having recurrence rates of 28–29 days and originating from midlatitude extensions of the polar coronal holes dominate the rising phase of the sunspot cycle (1987–1989); (2) the persistent high‐speed polar wind disappears and low‐speed wind is found at all latitudes during 1989–1990; (3) very fast, episodic “polar jets” are generated as active region fields surge to the poles at the time of polar field reversal (1990–1991). The wind speed patterns that Ulysses encounters during its second polar orbit are expected to show the same general characteristics.
observations of high—frequency scattered energy associated with the core PhasePKKP
Authors: Earle Paul S. , Shearer Peter M.
Published: February 2004

Stacks of global seismic network data reveal emergent high—frequency (∼1 Hz) energy extending back from the PKKPc caustic at 72° to a distance of about 60°. The arrival's emergent onset, long duration (∼150 s), apparent slowness, and proximity to the core phase PKKP indicate a deep scattering origin. However, the amplitude and timing of the observations are difficult to explain with simple models of PKKP scattering.
First lidar observations of mesospheric hydroxyl
Authors: Brinksma E. J. , Meijer Y. J. , McDermid I. S. , Cageao R. P. , Bergwerff J. B. , Swart D. P. J. , Ubachs W. , Matthews W. A. , Hogervorst W. , Hovenier J. W.
Published: February 2004

Ground‐based lidars have been used to detect and identify ground‐state (v″=0) hydroxyl radicals (OH) in the mesosphere between about 75 and 85 km altitude. These lidars operate near 308 nm and OH is observed through laser‐induced‐fluorescence on the A²Σ+−X²Π(0,0) band. The results expose a valuable global set of nighttime OH observations, since existing long‐term lidar data at several NDSC sites contain the (serendipitous) OH information. Results of lidar observations are presented from two mid‐latitude sites, one in each hemisphere: Table Mountain (34°N), California, and Lauder (45°S), New Zealand. They show observations of a geometrically thin (∼3 km) nocturnal layer of OH near 80 km. For the Table Mountain observations, the derived values for the OH density at 80 km typically are 2 ‐ 4×105cm−3 which is in accordance with model predictions [Dodd et al., 1994]. The temporal behavior of the mesospheric OH signal, following sunset, that was found, supports previous model predictions [Allen et al., 1984] in a qualitative fashion.
Petrology of Baby Bare and Mama Bare lavas
Authors: Karsten J. L. , Becker N. , Mottl M. J. , Wheat C. G.
Published: February 2004

The Three Bares are small volcanoes that host the only‐known hydrothermal vents active in oceanic crust older than 2 Ma. Major element data for Baby and Mama Bare lavas indicate formation by low extents of partial melting of a depleted mantle source, with limited cooling and crystallization prior to eruption. These lavas are compositionally unlike all lavas produced at the adjacent spreading center, but resemble a subset of lavas from small seamounts nearby. We conclude that the Three Bares represent off‐axis volcanism rather than normal crustal accretion. Such off‐axis volcanism increases basement roughness locally, and thus plays an important role in hydrothermal circulation in sedimented ridge flank settings.
Spectral attenuation of track‐line noise
Authors: Woo Kim Jeong , Kim Jeong‐Hee , von Frese Ralph R. B. , Roman Daniel R. , Jezek Kenneth C.
Published: February 2004

Satellite, airborne, and marine geophysical surveys usually sample features better along‐track than across‐track. In maps, this aliasing together with errors in along‐track data processing produce a systematic “washboard” effect across the survey lines known as track‐line noise. In regions covered by two line surveys at different azimuths, the combined data generate maps with this noise orthogonal to both survey azimuths. Statistical efforts to minimize random noise will only be partially effective in attenuating systematic track‐line noise. We present a method that exploits the spectral differences resulting from the different survey azimuths to reduce this systematic noise. In each survey, only two of the spectral quadrants are normally contaminated. Combining the four least contaminated quadrants from both surveys generates a map of minimal track‐line noise. This method is used to improve the recovery of geoid undulations from satellite altimetry of the Barents Sea in the Arctic.
A tracer study of the formation, dispersal, and renewal of Levantine Intermediate Water
Authors: Stratford K. , Williams R. G.
Published: February 2004

The formation and dispersal of Levantine Intermediate Water (LIW) are examined using a series of tracer experiments based on an eddy‐permitting general circulation model of the Mediterranean. LIW is formed in the region of the Rhodes gyre and disperses via both the time‐mean and time‐varying circulation. Tracer released in the Rhodes gyre is transferred westward by the time‐mean circulation but also southward and eastward by the time‐varying circulation. Off‐line tracer experiments suggest that the time‐varying circulation exhibits both advective and diffusive components, where the advective component opposes downgradient transfer in some regions. The rate of invasion of tracer into the main thermocline suggests that the renewal timescale is 10–20 years for LIW and of order 100 years for Eastern Mediterranean Deep Water. Assuming advection dominates the transfer of tracer, the implied annual subduction rate is 0.6±0.1 Sv in the Rhodes gyre and 0.2±0.1 Sv in the Adriatic Sea. The LIW mode is found to become cooler and fresher as a result of mixing with the Adriatic outflow, while its potential vorticity appears well conserved.
Bowen ratio estimates in return flow over the Gulf of Mexico
Authors: Lewis J. M. , Martin W. J. , Guinasso N. L.
Published: February 2004

The Bowen ratio (and associated turbulent fluxes at the ocean‐air interface) has been estimated over the Gulf of Mexico's continental shelf. These estimates are time‐averaged values during the outflow phase of a particular return flow event. The study uses standard buoy data and oceanic soundings from the Louisiana‐Texas Shelf Physical Oceanography Program (LATEX). Since the buoys are not equipped with water vapor sensors, the latent heat flux is determined indirectly. The calculation is made possible by combining the buoy data with a one‐dimensional oceanic slab model and climatological estimates of net radiation. The application also requires a locally isothermal ocean. Estimates have been made for the return flow event of November 3–12, 1992, which exhibited maximum ocean‐air temperature differences of ∼10°C. During a 2 day period when the ocean‐air differences were near their maximum and when mixing extended to the seafloor beneath the buoy sites, the Bowen ratio is found to be 0.38±0.06 (0.35±0.08) at buoy sites at depths of 14 (24) m. The corresponding estimates of sensible and latent heat flux are 115±7 (115±7) and 302±48 (329±75) W m−2, respectively. These values compare favorably with results from other field programs that used bulk aerodynamic methods to estimate turbulent fluxes in cold air outflow. The methodology appears to be generally applicable to return flow events in the Gulf of Mexico.
The CO2 concentration of air trapped in Greenland Ice Sheet Project 2 ice formed during periods of rapid climate change
Authors: Smith H. J. , Wahlen M. , Mastroianni D. , Taylor K. , Mayewski P.
Published: February 2004

The CO2 content of air occluded in Greenland Ice Sheet Project 2 (GISP2) ice formed over two separate intervals of rapidly changing climate, centered at approximately 46 and 63 kyr B. P., is as much as 90 ppm more during warm periods (interstadials) than during cold periods (stadials). These CO2 variations are superimposed on changes in annual layer thickness and δ18O of the ice and do not show the 200‐ to 700‐year offsets which would be expected for concurrent variations in the atmosphere and the ice. The CO2 concentrations during the stadials are similar to the atmospheric values recorded by Antarctic ice of the same age, so processes occurring in the ice after bubble enclosure must be enriching the air trapped in GISP2 ice formed during the interstadials. This conclusion is supported by Ca content and electrical conductivity measurements of the ice, which show that adequate carbonate is present to produce these enrichments and that CO2 content is high only when the electrical conductivity (a proxy for H+ concentration) is high. High‐resolution mapping of one 4‐cm section of ice shows a 200‐ppm increase in the CO2 content of the trapped air, from approximately 275 to 475 ppm. Analyses of the total inorganic carbon of ice from both the LGM and Holocene show that most of the Ca in the ice is from CaCO3 and that the δ13CO2 approaches that of soil and marine carbonates. These results show that the CO2 record preserved in ice can be altered by in situ decarbonation reactions and that only ice containing either abundant carbonate or essentially no carbonate contains a reliable record of paleoatmospheric CO2.
CO2 record between 40 and 8 kyr B.P. from the Greenland Ice Core Project ice core
Authors: Anklin M. , Schwander J. , Stauffer B. , Tschumi J. , Fuchs A. , Barnola J. M. , Raynaud D.
Published: February 2004

CO2 ice‐core records show an increase in the atmospheric concentration of 80–100 parts per million by volume (ppmv) from the last glacial maximum (LGM) to the early Holocene. We present CO2 measurements performed on an ice core from central Greenland, drilled during the Greenland Ice Core Project (GRIP). This CO2 profile from GRIP confirms the most prominent CO2 increase from the LGM, with a mean concentration of 200 ppmv, to the early Holocene with concentrations between 290 and 310 ppmv. Some structures of the new CO2 record are similar to those previously obtained from the Dye 3 ice core (Greenland), which indicated a dilemma between Greenland and Antarctic CO2 records [Oeschger et al., 1988]. Both Greenland cores show high CO2 values for rather mild climatic periods during the last glaciation, whereas CO2 records from Antarctica do not show such high CO2 variations during the glaciation and, furthermore, the CO2 values in the early Holocene are about 20–30 ppmv higher in the GRIP record than in Antarctic records. There is some evidence that the difference could be due to chemical reactions between impurities in the ice leading to an increase of the CO2 concentration under certain conditions. If in situ processes can change the CO2 concentration in the air bubbles, the question arises about how reliably do CO2 records from ice cores reflect the atmospheric composition at the time of ice formation. The discrepancies between the CO2 profiles from Greenland and Antarctica can be explained by in situ production of excess CO2 due to interactions between carbonate and acidic species. Since the carbonate concentration in Antarctic ice is much lower than in Greenland ice, CO2 records from Antarctica are much less affected by such in situ‐produced CO2.
Altimetric assimilation in a Mediterranean general circulation model
Authors: Drakopoulos P. G. , Haines K. , Wu P.
Published: February 2004

Identical twin experiments are performed with a realistic Mediterranean general circulation model, assimilating surface pressure at the rigid lid (simulating altimeter data) in order to reproduce the three‐dimensional circulation and water column structure. The first twin pair is forced with monthly varying wind stress and surface buoyancy, and the assimilation method of Cooper and Haines [1996] is used. Water columns are displaced vertically by an amount calculated to ensure that the surface pressure change required at assimilation time is reduced to zero at the seabed. The assimilation is successful with T, S and velocities having an error reduction of about 40% after 1 year, assimilating surface pressure every 15 days. A modification to this assimilation method is introduced in which a change to bottom pressure is calculated based on a balance between relative vorticity and stretching in the vertically displaced water column. A second identical twin experiment pair is forced with daily varying wind stress (from the European Centre for Medium‐Range Weather Forecasts in 1992–1993) but still monthly varying surface buoyancy forcing. The variability in surface pressure is much greater when daily winds are used, with much of the high‐frequency current signal being a barotropic response (with a strong signal at the seafloor). Applying the correct daily winds without any surface pressure assimilation can reduce the T, S and velocity errors by about 50% after a year, suggesting that much of the additional barotropic variability is deterministic. The additional assimilation of surface pressure every 20 days, now with the bottom pressure updated, leads to a further 30% error reduction. A further twin experiment with daily winds, in which the barotropic mode is allowed to converge first, before surface pressure assimilation begins, shows that a bottom pressure update during assimilation may be unnecessary if the correct wind stresses are known.
Vertical fluxes of carbon, nitrogen, and phosphorus in the North Pacific Subtropical Gyre near Hawaii
Authors: Christian James R. , Lewis Marlon R. , Karl David M.
Published: February 2004

The hypothesized “biological pump” mechanism for removing carbon from the euphotic zone to the deep ocean requires different rates of recycling of C, N, and P in sinking particles. At Station ALOHA in the subtropical North Pacific Ocean (22° 45′N, 158°W), the C/N and C/P ratios in sinking particles increase with depth, while the ratios in the upward mixing flux decrease. An exponential model of the sinking flux as a function of depth is used, which permits remineralization length and time scales (more properly called solubilization scales) to be estimated. Time scales (2–5 days) are consistent with microbiological decomposition, and length scales are greater for C (368 m) than for N and P (249 and 248 m), consistent with the existence of a biological pump for carbon. Assuming that the system is nutrient limited (total upward and downward fluxes of N or P are equal) permits us to estimate vertical eddy diffusivity (Kz) and the net export of carbon from the system. Values of Kz range from 5×10−6 to 4×10−5 m2 s−1 and are consistent with independent estimates for similar environments. We estimate that approximately 20% of the air‐to‐sea carbon flux at Station ALOHA and a similar fraction of new carbon production are exported to the deep ocean (σθ>27) by particle sinking. Eddy diffusivities are lower and net carbon export greater if phosphorus is assumed to be the controlling element. This has implications for the role of dinitrogen fixation in the subtropical North Pacific.
Particle transport by nonbreaking, solitary internal waves
Authors: Lamb Kevin G.
Published: February 2004

The horizontal transport of particles by solitary internal waves is investigated using a fully nonlinear numerical model. The effect of an additional constant velocity Ud due to another physical mechanism such as surface wind drift or swimming motion in the case of live organisms is also considered. Weakly nonlinear theory is used to derive approximate analytic expressions for the transport distance of surface particles. The theoretical results are compared with transport distances obtained using the numerical model. The numerical model is also used to determine the vertical dependence of the horizontal transport. Two stratifications are considered. For one, with the strength of the stratification increasing monotonically toward the surface, the agreement between the theoretical predictions and the model results is excellent right up to the breaking amplitude. For the second, with a pycnocline between weakly stratified upper and lower layers, the nonlinear waves in the fully nonlinear numerical simulations are much wider than those predicted by weakly nonlinear theory. As a consequence, weakly nonlinear theory significantly underestimates the transport distance. It is found that significant particle transport occurs only when the waves are near the breaking amplitude, when they are very long, or when Ud plus the wave's surface current is comparable to the wave's propagation speed.
Mesoscale thermohaline variability in the Eastern Gotland Basin following the 1993 major Baltic inflow
Authors: Zhurbas V. M. , Paka V. T.
Published: February 2004

High horizontal resolution data from a “tow‐yo” conductivity‐temperature‐depth profiling system are analyzed to examine the mesoscale and fine scale response of the Gotland Basin to the 1993 major Baltic inflow. The permanent halocline in the basin is characterized by intensive thermohaline intrusions. A well‐defined front of the intrusive region was found to be propagating north from the Stolpe Channel to the Gotland Deep with a speed of 2 cm s−1 or more. A substantial horizontal intermittency of intrusion intensity, related to mesoscale eddies, was observed behind the front. A strongly nonlinear cyclonic eddy, undetectable on the sea surface, was found in the halocline. The cyclone contributed to the transport of salty water after the inflow. The combined effect of the mesoscale eddies and thermohaline intrusions is shown to be one of the mechanisms responsible for the deep water ventilation by major Baltic inflows.
Altimeter‐derived variability of surface velocities in the California Current System: 1. Evaluation of TOPEX altimeter velocity resolution
Authors: Strub P. Ted , Chereskin Teresa K. , Niiler Pearn P. , James Corinne , Levine Murray D.
Published: February 2004

In this paper, we evaluate the temporal and horizontal resolution of geostrophic surface velocities calculated from TOPEX satellite altimeter heights. Moored velocities (from vector‐averaging current meters and an acoustic Doppler current profiler) at depths below the Ekman layer are used to estimate the temporal evolution and accuracy of altimeter geostrophic surface velocities at a point. Surface temperature gradients from satellite fields are used to determine the altimeter's horizontal resolution of features in the velocity field. The results indicate that the altimeter resolves horizontal scales of 50–80 km in the along‐track direction. The rms differences between the altimeter and current meters are 7–8 cm s−1, much of which comes from small‐scale variability in the oceanic currents. We estimate the error in the altimeter velocities to have an rms magnitude of 3–5 cm s−1 or less. Uncertainties in the eddy momentum fluxes at crossovers are more difficult to evaluate and may be affected by aliasing of fluctuations with frequencies higher than the altimeter's Nyquist frequency of 0.05 cycles d−1, as indicated by spectra from subsampled current meter data. The eddy statistics that are in best agreement are the velocity variances, eddy kinetic energy and the major axis of the variance ellipses. Spatial averaging of the current meter velocities produces greater agreement with all altimeter statistics and increases our confidence that the altimeter's momentum fluxes and the orientation of its variance ellipses (the statistics differing the most with single moorings) represent well the statistics of spatially averaged currents (scales of 50–100 km) in the ocean. Besides evaluating altimeter performance, the study reveals several properties of the circulation in the California Current System: (1) velocity components are not isotropic but are polarized, strongly so at some locations, (2) there are instances of strong and persistent small‐scale variability in the velocity, and (3) the energetic region of the California Current is isolated and surrounded by a region of lower energy starting 500–700 km offshore. This suggests that the source of the high eddy energy within 500 km of the coast is the seasonal jet that develops each spring and moves offshore to the central region of the California Current, rather than a deep‐ocean eddy field approaching the coast from farther offshore.
Ocean carbon transport in a box‐diffusion versus a general circulation model
Authors: Joos Fortunat , Orr James C. , Siegenthaler Ulrich
Published: February 2004

We have compared vertical transport of temperature, anthropogenic CO2, natural radiocarbon (14C), and bomb 14C in a global box‐diffusion model (B‐D) and a three‐dimensional (3‐D) ocean general circulation model from the Geophysical Fluid Dynamics Laboratory. Our main objectives were (1) to test the eddy diffusion parameterization of large‐scale vertical transport in ocean box models and (2) to assess the utility of bomb‐produced and natural 14C observations to validate ocean models used to estimate anthropogenic CO2 uptake. Prom the 3‐D model's distributions and fluxes of natural 14C, bomb 14C, and anthropogenic CO2, we have calculated apparent diffusivities (Kap) vertically over the global ocean that range mostly between 4000 and 8000m2 yr−1. These Kap agree quantitatively with diffusivities found by fitting B‐D models to observed distributions of natural and bomb 14C We then used these sets of Kap in different runs of a global B‐D model. Results from all B‐D models runs matched to within 13% those from the 3‐D model for global uptake of anthropogenic CO2 and bomb‐14C penetration depth. Although Kap from 3‐D simulations for bomb 14C vary with time, those from 3‐D runs for anthropogenic CO2 are essentially constant. Still, we found nearly the same results with the B‐D model when Kap from 3‐D bomb 14C simulations are approximated as time invariant. The best agreement (within 3%) between 3‐D CO2 simulations and B‐D model runs was found when applying Kap derived from bomb 14C in the surface and from natural 14C in the deep. Agreement was worse when using Kap from 3‐D simulations for anthropogenic CO2itself, mostly because in this case deeper Kap could only be extrapolated from higher surface values. We have found it appropriate to study global oceanic uptake of anthropogenic CO2 with B‐D model and to validate anthropogenic carbon uptake models using natural and bomb 14C observations. For bomb 14C in the 3‐D model, convective transport was most important during 1955–1964 while atmospheric levels were rising; afterward, atmospheric levels drop, and advective overturning dominates as for natural 14C. Thus 14C seems less than ideal to validate the convective scheme of general circulation models.
On the intermediate and deep water flows in the South Atlantic Ocean
Authors: Larqué Lionel , Maamaatuaiahutapu Keitapu , Garçon Véronique
Published: February 2004

A multiparameter analysis is applied on zonal and meridional hydrographic sections obtained for the South Atlantic Ventilation Experiment (SAVE) to determine the spreading and mixing of water masses in the South Atlantic Ocean, focusing our interest on the large‐scale flow of intermediate and deep waters. The method utilizes all information from the hydrographic data set including temperature, salinity, dissolved oxygen, and nutrient fields. Mixing proportions are quantified and plotted along the eight sections considered. Results show no evidence of a primary route of Antarctic Intermediate Water along the western boundary of the South Atlantic. In the eastern basin the eastward extension of the Upper North Atlantic Deep Water (NADW) in the Guinea Basin following the cyclonic subequatorial gyre is confirmed. In the Angola Basin a weak but thick NADW core layer is observed in conjunction with very little presence of Lower Circumpolar Deep Water (LCDW). High LCDW concentrations in Cape Basin are indicative of the communication of this basin to cold water sources in the south. The method is sensitive enough to detect for instance the presence of the Congo River Plume in the Angola Basin or the influence of the Weddell Sea Deep Water in the vicinity of the Romanche and Chain Fracture Zones in the equatorial region. In conjunction with the multiparameter analyses along SAVE sections, an analysis of components of the residual vector R indicates a middepth minimum in the RN/P utilization ratio. Both a suitable explanation for the minimum and the potential consequences for the multiparameter analyses of South Atlantic water mass circulation are still to be found.
Adaptation of a fast optimal interpolation algorithm to the mapping of oceanographic data
Authors: Menemenlis Dimitris , Fieguth Paul , Wunsch Carl , Willsky Alan
Published: February 2004

A fast, recently developed, multiscale optimal interpolation algorithm has been adapted to the mapping of hydrographic and other oceanographic data. This algorithm produces solution and error estimates which are consistent with those obtained from exact least squares methods, but at a small fraction of the computational cost. Problems whose solution would be completely impractical using exact least squares, that is, problems with tens or hundreds of thousands of measurements and estimation grid points, can easily be solved on a small workstation using the multiscale algorithm. In contrast to methods previously proposed for solving large least squares problems, our approach provides estimation error statistics while permitting long‐range correlations, using all measurements, and permitting arbitrary measurement locations. The multiscale algorithm itself, published elsewhere, is not the focus of this paper. However, the algorithm requires statistical models having a very particular multiscale structure; it is the development of a class of multiscale statistical models, appropriate for oceanographic mapping problems, with which we concern ourselves in this paper. The approach is illustrated by mapping temperature in the northeastern Pacific. The number of hydrographic stations is kept deliberately small to show that multiscale and exact least squares results are comparable. A portion of the data were not used in the analysis; these data serve to test the multiscale estimates. A major advantage of the present approach is the ability to repeat the estimation procedure a large number of times for sensitivity studies, parameter estimation, and model testing. We have made available by anonymous Ftp a set of MATLAB‐callable routines which implement the multiscale algorithm and the statistical models developed in this paper.
Air‐sea fluxes retrieved from special sensor microwave imager data
Authors: Chou Shu‐Hsien , Shie Chung‐Lin , Atlas Robert M. , Ardizzone Joe
Published: February 2004

A method has been developed to estimate daily surface fluxes of momentum and sensible and latent heat over the global oceans using a stability‐dependent bulk scheme. Daily fluxes are computed from daily values of special sensor microwave imager (SSM/I) surface winds, SSM/I surface humidity, National Centers for Environmental Prediction sea surface temperatures (SSTs), and European Centre for Medium‐Range Weather Forecasts (SSTs minus 2‐m temperatures). Daily surface specific humidity is estimated from the SSM/I water vapor for an atmospheric column and the lower 500 m of the planetary boundary layer, using the method of Chou et al. [1995] with two modifications for the extratropical oceans. The modified method is described using two simple equations. Gustiness parameterization for the weak winds and convective situations is found to have an insignificant impact on the air‐sea fluxes derived from the SSM/I data and hence is not included. The SSM/I‐radiosonde comparison (over the global oceans for the entire annual cycle of 1993) shows that for a 25‐km resolution the instantaneous SSM/I surface humidity has a root‐mean‐square (rms) difference of 1.83 g kg−1. Daily SSM/I latent heat fluxes (and wind stresses) agree well with the flux measurements over the western Pacific warm pool, with a bias of 6.2 W m−2 (0.0061 N m−2), an rms difference of 29.0 W m−2 (0.0187 N m−2), and a correlation of 0.83 (0.86). Monthly results of February and August 1993 show that the patterns and seasonal variabilities of the SSM/I surface humidity, latent, and sensible heat fluxes are generally in good agreement with those of the Comprehensive Ocean‐Atmosphere Data Set (COADS) and climatologies derived from ship measurements. The SSM/I sensible heat flux is generally within ±10 W m−2 of COADS. However, the SSM/I latent heat flux is generally larger, especially over the wintertime trade wind belts. The result is consistent with previous climatological studies in that the latent heat fluxes based on ship measurements are systematically underestimated.
Annual and interannual variation of precipitation over the tropical Indian Ocean
Authors: Ramesh Kumar M. R. , Prasad T. G.
Published: February 2004

The annual and interannual variability of precipitation over the tropical Indian Ocean is studied for the period June 1986 to December 1990 using the data retrieved from the Indian National Satellite (INSAT). The seasonal and annual rainfall over the Bay of Bengal was found to be about 2–3 times the Arabian Sea values. Harmonic analysis of the monthly mean rainfall showed that the annual wave has its largest amplitude in the northern Bay of Bengal, where the amplitude exceeds 250 mm/month, and the lowest amplitudes are found in the western Indian Ocean, especially off the Arabian and east African coasts. The INSAT and GOES Precipitation Index (GPI) rainfall estimates correlated reasonably well with the island rainfall data, with correlation coefficients of 0.83 and 0.78, respectively, whereas the special sensor microwave imager (SSMI) rainfall estimates had the lowest correlation (r=0.64) with the island rainfall data. A comparison of the mean annual estimates by the three methods showed that the GPI rainfall estimates were higher than the INSAT and SSMI estimates by 16% and 41%, respectively, for the Indian Ocean area. The INSAT, GPI, and SSMI rainfall estimates document significant variations (both annual and seasonal) for all the study areas, with the Indian Ocean area exhibiting maximum variability during the summer monsoon (June, July, and August) season.
Beryllium 10 in the Greenland Ice Core Project ice core at Summit, Greenland
Authors: Yiou F. , Raisbeck G. M. , Baumgartner S. , Beer J. , Hammer C. , Johnsen S. , Jouzel J. , Kubik P. W. , Lestringuez J. , Stiévenard M. , Suter M. , Yiou P.
Published: February 2004

Concentrations of the cosmogenic isotope 10Be have been measured in more than 1350 samples from the Greenland Ice Core Project (GRIP) ice core drilled at Summit, Greenland. Although a dust‐associated component of 10Be retained by 0.45 μm filters in some of the samples complicates the interpretations, the results confirm that the first‐order origin of 10Be concentration variations is changes in precipitation rate associated with different climate regimes. This effect is seen not only between glacial and interglacial periods, but also during the shorter “Dansgaard‐Oeschger” interstadials. By contrast, the 10Be data do not support the interpretation of rapidly varying accumulation (i.e., climate) during the last interglacial. They can, however, be used to help place limits on the origin of the ice in these events. After taking into account variable snow accumulation effects, variations in the 10Be flux are observed, probably caused by solar and geomagnetic modulation, but possibly also by primary cosmic ray variations. The most dramatic is a 10Be peak ∼40,000 years ago, similar to that found in the Vostok ice core, thus permitting a very precise correlation between climate records from Arctic and Antarctic ice cores. The 36Cl/10Be ratio (considering either “total” or only ice‐associated 10Be) shows significant variability over the whole core depth, thus confirming the difficulty in using this parameter for “dating” ice cores.
Modeling the thermodynamics of a sea ice thickness distribution: 1. Sensitivity to ice thickness resolution
Authors: Schramm J. L. , Holland M. M. , Curry J. A. , Ebert E. E.
Published: February 2004

A one‐dimensional ice thickness distribution model is presented to determine the minimum number of ice thicknesses necessary to resolve the area‐averaged annual cycles of ice thickness and turbulent fluxes. The baseline case includes 40 ice thickness categories; ice thickness and area, meltwater ponds, ice salinity and age, snow cover, and surface albedo evolve independently for each ice category. A ridging and ice export parameterization, and a coupled one‐dimensional ocean mixed layer model are also included. Sensitivity studies indicate that 16 ice thickness categories can accurately resolve the baseline annual cycles of area‐averaged ice thickness and the summertime turbulent fluxes in this model, provided that one third of the thickness categories represent ice thinner than 0.8 m. Resolving the distribution of ice in this thickness range is important in simulating the area‐averaged ice characteristics. Wintertime values of the turbulent fluxes differ from the baseline by up to 1 W m−2 for fewer than 40 ice thickness categories. The large difference in turbulent fluxes between open water and ice thicker than 1.6 m makes the area‐averaged value sensitive to the number of ice thickness categories, since this number can affect the categories that are merged, the categories that melt completely, ice that is ridged, and the resolution of the ice thickness distribution. This makes an accurate simulation of the baseline wintertime turbulent fluxes difficult. Model simulations with fewer than 40 categories provide a reasonable estimate of the baseline response to a surface longwave heat flux perturbation of greater than 5 W m−2. The annually area‐averaged ice thickness is within 10 cm. The model response to a heat flux perturbation of less than 5 W m−2 is similar for a wide range of ice thickness categories, since the thickness distribution of ice thinner than 1 m is not affected.
Primary aerosol (sea salt and soil dust) deposited in Greenland ice during the last climatic cycle: Comparison with east Antarctic records
Authors: De Angelis M. , Steffensen J. P. , Legrand M. , Clausen H. , Hammer C.
Published: February 2004

Ion chromatography data of Ca, Mg, Cl and Ca and Coulter® counter particle measurements are used to study the cycle of marine and continental primary aerosol reaching Greenland in relation to climatic changes over the last 150 kyr. A detailed comparison between Greenland (Dome Summit) and Antarctic (Vostok) records provides new insight on a potential link between northern and southern patterns. Ca is a good indicator of continental input and is mainly emitted as CaCO3. An attempt is made to estimate the contribution of aluminosilicates using the concentration of insoluble particles greater than 0.5 μm in diameter. The relative abundance of non‐sea‐salt Mg and Ca and of aluminosilicate shows that the calcium content of continental background aerosol over Greenland was much higher during the glacial age. The neutralization capacity of carbonaceous aerosol is estimated. The inverse relationship between δ18O and continental input as well as the response of this input to the rapid climatic variations that have occurred during the second part of the glacial age are discussed in terms of source and transport modification in relation to the presence and the extent of the great Laurentide ice cap. The corresponding Vostok profiles strongly suggest that some of the phenomena observed at high northern latitudes are of global concern. The marine component of Na (Nam) is a good tracer of sea‐salt aerosol. Similarly to continental input, it shows an inverse but more linear relationship with δ18O. The sensitivity of Greenland and Antarctic marine input to climate variations of small and large amplitude is compared, and a corresponding estimation is made for the aeolian contribution. The respective influence of atmospheric circulation and the water vapor cycle is discussed. The chlorine to marine sodium weight ratio increases with temperature from values very close to the bulk seawater ratio during the last glacial maximum (18–20 kyr B.P.) to values significantly higher during the Holocene and warm Eemian. The corresponding excess of chloride (HCl) is discussed in terms of atmospheric transport, taking into account the role of atmospheric acidity on sea‐salt fractionation processes. Owing to postdepositional phenomena, similar Vostok data must be considered cautiously. Nevertheless, aerosol fractionation seems to have been much more important over the Vostok site, except during glacial extrema.
Internal gravity waves in the upper eastern equatorial Pacific: Observations and numerical solutions
Authors: Mack Andrew P. , Hebert D.
Published: February 2004

On the basis of data from a towed thermistor chain collected near 140°W on the equator during April 1987, the zonal wavenumber and vertical structure of internal gravity waves were observed to vary significantly between wave events. Our hypothesis is that this variability is due to changes in the vertical structure of mean horizontal velocity and density. Assuming that the observed waves were the fastest growing modes for shear instability, we solve the Taylor‐Goldstein equation, using different analytical basic states, including a zonal and meridional flow, to simulate the different conditions during 4 nights of intense internal wave activity. We find that while the observed waves are of finite amplitude, linear shear instability is sufficient to explain the wavelength and vertical structure of vertical displacement for most of the waves. The fastest growing, unstable, mode‐one solutions have e‐folding growth times of less than 10 min. These solutions show wave phase speeds and vertical structures to be highly dependent upon the velocity structure of the uppermost 40 m. Near the base of the mixed layer at a flow inflection point the kinetic energy of the mean flow is extracted for wave growth. Wave vertical displacement is maximum near this inflection point. Zonal phase speeds range from −0.8 to −0.1 m/s. The propagation direction of waves with growth rates of 75% of the maximum growth rate can range from about 45° north to 45° south of the zonal direction. The vertical wave‐induced Reynolds stress divergence could explain a discrepancy in zonal momentum budgets of the upper 90 m of this region. Estimates of this stress divergence show that only about 100 days of intense internal wave activity is needed per year for these internal waves to explain estimated residuals of the mean zonal momentum budgets of this region at 50‐ to 100‐m depth.