bluuebunny/crossref_metadata_2025 huggingface dataset:
The Magnetospheric Sash and the Cross‐Tail S
Authors: White Willard W. , Siscoe George L. , Erickson Gary M. , Kaymaz Zerefsan , Maynard Nelson C. , Siebert Keith D. , Sonnerup Bengt U. Ö , Weimer Daniel R.
Published: February 2004

As revealed in MHD simulation, the magnetospheric sash is a band of weak magnetic field that, for the usual case in which the IMF is approximately perpendicular to the geomagnetic dipole, runs tailward along the high‐latitude magnetopause flanks from one dayside cusp to the other, closing via the cross‐tail neutral sheet. On the magnetopause flanks, it contains the magnetic separator line, at which all three topological types of field lines meet. Seen in a cross‐sectional plane through the near‐Earth tail, the magnetospheric sash takes the form of the cross‐tail S, a weak‐field feature comprised of the tail neutral sheet with diagonally symmetric extensions along the magnetopause flanks connecting it to the separator line. The cross‐tail S is evident in the MHD results and in cross‐sectional maps based on IMP 8 data. The magnetopause expression of the sash is latent in prior works that described the geometry of antiparallel fields across the magnetopause and the consequent cancellation of the fields within the magnetopause layer. The sash picture bears a strong resemblance to antiparallel merging geometry.
Oscillations of magnetospheric boundaries driven by IMF rotations
Authors: Laakso H. , Fairfield D. H. , Collier M. R. , Opgenoorth H. , Phan T.‐D. , Sibeck D. G. , Giles B. L. , Singer H. J. , Lepping R. P. , Lin R. P. , Mozer F. S. , Pfaff R. F. , Tsuruda K. , Wygant J. R.
Published: February 2004

The arrival of the magnetic cloud on January 10, 1997, 01:05 UT, was preceded by a 12‐hour interval of increased fluctuations in the IMF orientation, which drove boundary oscillations in the magnetosphere. The GEOTAIL spacecraft, initially located near the dawnside magnetopause, experienced multiple crossings of the magnetopause; at the geosynchronous distance, GOES 9 observed weak perturbations associated with these oscillations. During this time, the POLAR spacecraft encountered the dense high‐latitude boundary layer, followed by several rapid encounters with the tenuous lobe. The boundary between the mantle and the lobe was observed to be thin as the crossings lasted only 30 seconds. The source of these boundary oscillations is magnetosheath pressure variations produced by rapid IMF directional changes.
ELF radiation produced by electrical currents in sprites
Authors: Cummer S. A. , Inan U. S. , Bell T. F. , Barrington‐Leigh C. P.
Published: February 2004

Measurements of ELF‐radiating currents associated with sprite‐producing lightning discharges exhibit a second current peak simultaneous in time with sprite luminosity, suggesting that the observed ELF radiation is produced by intense electrical currents flowing in the body of the sprite.
An MHD model for driven high m field line resonances
Authors: Mann Ian R.
Published: February 2004

Radar observations of field line resonances (FLRs) show two categories of waves with large or small azimuthal wavenumbers (m). Low m resonances are relatively well understood theoretically, however high m waves appear to be continually driven and display characteristics which can only be understood by considering the effects of the driver. We discuss an heuristic model driving term suggested by [Southwood, 1974], and recently adopted by [Fenrich and Samson, 1997] in an attempt to explain their observations. We show how their model introduces non‐causal (unphysical) features which are not good approximations to reality. We offer a possible alternative heuristic MHD model which can better explain the observational features of high m FLRs. The new model formulation may generate modal solutions with the observed equatorwards phase motion, growth in time, and radially oscillatory characteristics.
Influence of volcanic sulfur dioxide on spectral UV irradiance as measured by Brewer Spectrophotometers
Authors: Fioletov Vitali E. , Griffioen Erik , Kerr James B. , Wardle David I. , Uchino Osamu
Published: February 2004

Spectra of UV irradiance measured by Brewer spectrophotometers at 13 stations in Japan and Canada have been examined to determine the effect of absorption by sulfur dioxide (SO2). A simple algorithm to estimate the total column amount of SO2 from global UV irradiance measurements has been developed. The algorithm which overestimates SO2 amounts at low altitudes is useful for identifying large SO2 absorption and for establishing upper limits of the SO2 values. Except at one station, the overall occurrence of days with SO2 amounts greater than 10 Dobson Units (DU) was about 0.22%. The exception was the Kagoshima observatory located 10 km from the volcano, Sakurajima. Volcanic activity resulted in frequent observation of column SO2 amounts of more than 20 DU over Kagoshima and in extreme cases up to 100 DU. The reduction in the long‐term erythemally weighted UV irradiation due to absorption by SO2 is between one and two percent at Kagoshima and negligible at the twelve other stations.
Asymmetric radiation of seismic waves from an Atoll – Nuclear tests in French Polynesia
Authors: Weber Michael , Wicks Charles W. , Krüger Frank , Jahnke Gunnar , Schlittenhardt Jörg
Published: February 2004

Seismic records of nuclear tests detonated in the Mururoa Atoll in French Polynesia show large unpredicted arrivals 2.2 and 4.5 seconds (X1 and X2) after the P‐wave at the Australian Warramunga Array. These arrivals are not observed at the Canadian Yellowknife Array. X1 and X2 are also absent on Warramunga Array recordings of tests carried out at the Fangataufa Atoll situated 40 km SSE of Mururoa. Array analysis shows that X1 and X2 are produced within the source area. The layered crustal structure of the atoll, significant local inhomogeneities, and focusing effects due to the elongated shape and the steep flanks of the Mururoa Atoll are most likely responsible for X1 and X2. The form of Mururoa (28 ×10 km) and its East‐West orientation is due to its location on the Austral Fracture Zone (AFZ). The Fangataufa Atoll on the other hand is almost circular (10 km diameter) and is unaffected by the dynamics along the AFZ. Our observations demonstrate that complicated structures in the source area can significantly alter the wave field at teleseismic distances and produce a large magnitude (mb) bias. A better understanding of the exact cause of these unusual seismic observations will only become possible, if the coordinates of the tests and information on the detailed 3‐D structure of the atolls are released.
Can the direction of interstellar magnetic field be determined from the heliotail ENA flux?
Authors: Czechowski Andrzej , Grzedzielski Stanislaw
Published: February 2004

The effect of pressure of the (arbitrarily directed) interstellar magnetic field on the shape of the heliotail and consequently on space distribution of the pick‐up and anomalous cosmic ray ions convected into the heliotail region is estimated in a simple model based on a Newtonian approximation. The ion distribution in the heliotail produces (via transcharging on background gas) a flux of energetic neutral atoms which can be detected from the vicinity of the Earth. Directional dependence of this flux can be used to determine the direction and possibly strength of the interstellar field.
The 1997 Arctic Ozone depletion quantified from three‐dimensional model simulations
Authors: Lefèvre Franck , Figarol François , Carslaw Kenneth S. , Peter Thomas
Published: February 2004

Three‐dimensional simulations of total ozone are reported for the 1996–97 Arctic winter. The record low ozone values observed by satellite in late March are well reproduced by the chemistry‐transport model. The comparison between the chemically integrated ozone and a passive tracer with identical initialization allows us to discrimate chemical changes from variations due to dynamical processes. In addition to a substantial total ozone chemical loss (60 to 120 Dobson Units), the simulation reveals an dynamically‐induced reduction of ∼70 DU also responsible for the ozone minimum observed in the Arctic in late March 1997.
The 1997 Umbria‐Marche, Italy, Earthquake Sequence: A first look at the main shocks and aftershocks
Authors: Amato A. , Azzara R. , Chiarabba C. , Cimini G. B. , Cocco M. , Di Bona M. , Margheriti L. , Mazza S. , Mele F. , Selvaggi G. , Basili A. , Boschi E. , Courboulex F. , Deschamps A. , Gaffet S. , Bittarelli G. , Chiaraluce L. , Piccinini D. , Ripepe M.
Published: February 2004

A long sequence of earthquakes, six with magnitudes between 5 and 6, struck Central Italy starting on September 26, 1997, causing severe damages and loss of human lives. The seismogenic structure consists of a NW‐SE elongated fault zone extending for about 40 km. The focal mechanisms of the largest shocks reveal normal faulting with NE‐SW extension perpendicular to the trend of the Apennines, consistently with the Quaternary tectonic setting of the internal sector of the belt and with previous earthquakes in adjacent regions. Preliminary data on the main shocks and aftershocks show that extension in this region of the Apennines is accomplished by normal faults dipping at low angle (∼40°) to the southwest, and confined in the upper ∼8 km of the crust. These normal faults might have reactivated thrust planes of the Pliocene compressional tectonics. The aftershock distribution and the damage patterns also suggest that the three main shocks ruptured distinct 5 to 15 km‐long fault segments, adjacent and slightly offset from one another.
Flow shear across solar wind discontinuities: WIND observations
Authors: De Keyser J. , Roth M. , Söding A.
Published: February 2004

We examine the tangential magnetic field and velocity shears across directional discontinuities (DDs) with significant change in magnetic field intensity observed by WIND in slow and fast solar wind streams. The magnetic field rotation sense in fast wind DDs is that predicted by theory for outward propagating rotational discontinuities (RDs), but flow shear magnitude and orientation do not always satisfy RD theory. Alternatively, DDs with small normal magnetic field can be regarded as tangential discontinuities (TDs); the observed shears imply that the length scale over which the proton velocity distribution changes at the discontinuity can be both smaller or larger than that of the electron distribution. The slow wind includes a larger fraction of DDs that disagree with RD theory. It is shown that the flow shear orientations allowed in a TD provide a continuous transition between the opposite orientations for RDs propagating along or against the magnetic field direction.
Analysis of total cloud amount over China, 1951–1994
Authors: Kaiser Dale P.
Published: February 2004

Trends in Chinese total cloud amount were analyzed for the period 1951–1994. Cloud data were obtained from a database of 6‐hourly weather observations provided by the China Meteorological Administration (CMA) to the U.S. Department of Energy's Carbon Dioxide Information Analysis Center (CDIAC) through a bilateral research agreement. Seasonal and annual means of midday (1400 Beijing Time (BT)) and midnight (0200 BT) observations were computed for each of 196 stations and over 8 specific regions of China. Linear regression analysis was used to characterize seasonal and annual trends in total cloud amount from 1951–1994. Decreasing trends in both midday and midnight cloud amount are observed over much of China; most stations in central, eastern, and northeastern China show statistically significant decreases of 1–3% sky cover per decade. These decreases in cloud amount are especially interesting in light of recent temperature trends observed over China. Several studies have shown significant increasing trends in daily minimum temperatures over China since 1951, especially in the northeastern part of the country, precisely where the strongest decreasing trends in total cloud amount are observed. Increases in cloud amount have been offered as a possible explanation for increasing minimum temperatures in other parts of the world; however, in China it seems that some mechanism(s) other than increasing cloud amount must be considered for understanding the observed increase in minimum temperatures.
Transient tracers in a global OGCM: Source functions and simulated distributions
Authors: Heinze C. , Maier‐Reimer E. , Schlosser P.
Published: February 2004

The global oceanic distributions of the transient tracers tritium, tritiogenic 3He, and the chlorofluorocarbon CFC‐11 (CFC13) are modeled by using the velocity field of the Hamburg Large Scale Geostrophic (LSG) model. Source functions for these tracers are presented including the extension of the global tritium source function from 1975 to 1995. The tracer distributions are compared with observations. The sensitivity of the tritium and CFC‐11 simulations to the variation of adjustable parameters in the source functions is determined. The adjustable source function parameters cannot be varied to simultaneously achieve both a proper reproduction of the overall tracer inventories and the observed patterns of the tracer distributions. Further improvements are only possible through a change in the velocity field itself. Therefore anthropogenic tracers indeed provide a firm reality check for model ocean velocity fields. The model is used to simulate the. future evolution of transient tracer fields for different oceanic regions until the year 2050.
Hydrothermal helium plumes in the Pacific Ocean
Authors: Lupton John
Published: February 2004

Hydrothermal activity along the global mid‐ocean ridge system and at active seamounts introduces a 3He‐rich signal into the deep ocean basins, which can be used to trace patterns of ocean circulation and mixing. This hydrothermal helium signal is especially strong in the Pacific Ocean, where the spreading rate of the ridges is the greatest. In several areas of the Pacific the hydrothermal activity is of sufficient strength to produce intense helium plumes which clearly define the regional circulation. Among these are (1) a pair of helium plumes at 2500‐m depth, which extend westward from the East Pacific Rise at 10°N and 15°S into the interior of the Pacific basin; (2) a helium plume at 2000‐m depth extending southwest from sources on the Juan de Fuca Ridge in the northeast Pacific; and (3) a plume at 1100‐m depth emanating from Loihi Seamount at 20°N, which extends eastward from Hawaii to the coast of Mexico. The flow field implied by the helium distribution appears to be in reasonable agreement with Reid's [1997] steric height analysis for the Pacific.
A statistical comparison of wind speed, wave height, and wave period derived from satellite altimeters and ocean buoys in the Gulf of Mexico region
Authors: Hwang Paul A. , Teague William J. , Jacobs Gregg A. , Wang David W.
Published: February 2004

The capability of spaceborne altimeters to provide precise measurement of significant wave height and wind speed has been demonstrated repeatedly. It is shown in this paper that in addition to the significant wave height and wind speed, the wave period can be calculated from the semiempirical functions established from earlier wave research. The calculated characteristic wave period using the altimeter‐derived wind speed and wave height are found to be in excellent agreement with the peak wave period and average wave period from the ocean buoy measurements in the Gulf of Mexico. Also, with the long time series of collocated data set, it is possible to compare altimeter output of wind and wave parameters with ocean buoy measurements taking into consideration the spatial lags between the buoy locations and the altimeter footprints, and the temporal lags between the two sensor systems. It is found that when the spatial lags are less than 10 km, the RMS difference of the significant wave height is approximately 0.1 m, which is the digitization resolution of the output from both altimeter and ocean buoy. For the wind speed, the RMS difference approaches 1.2 m/s in the Gulf of Mexico using the empirical algorithms. The wind speed agreement is significantly improved to 0.8 m/s when the tilting effect on the altimeter cross section is accounted for. In contrast to the spatial lags, temporal lags of up to 1 hour do not appear to produce significant difference in the statistics of comparison based on this study.
The effect of bubble‐mediated gas transfer on purposeful dual‐gaseous tracer experiments
Authors: Asher William E. , Wanninkhof Rik
Published: February 2004

For air‐water gas exchange across unbroken surfaces, the only gas‐dependent parameter affecting the transfer velocity is the molecular diffusivity of the transferring species. In contrast, bubble‐mediated transfer processes can cause the transfer velocity to depend on both molecular diffusivity and aqueous‐phase solubility. This can complicate the analysis of data from dual‐gaseous tracer gas transfer experiments. Bubble effects also complicate the estimation of transfer velocities for other gases from the transfer velocity calculated using the dual‐tracer data. Herein a method for incorporating the effects of bubble‐mediated gas transfer processes on the transfer velocity is presented. This new procedure is used to analyze the data from two recent dual‐tracer gas transfer experiments. Transfer velocities that include the effect of bubbles are calculated using the data from two previous oceanic dual‐gaseous tracer experiments. Comparing these transfer velocities with transfer velocities calculated by neglecting the effect of bubbles shows that bubble‐mediated transfer increased the transfer velocity of helium 3 by 5% at a wind speed of 10.6 m s−1. However, when using the transfer velocities for helium 3 to calculate transfer velocities for carbon dioxide under the same conditions, including the effect of bubbles decreases the transfer velocity of carbon dioxide by 18%. This shows that bubble‐mediated transfer does not have a large effect on the analysis of dual‐tracer data, but it is important in relating transfer velocities determined using helium 3 and sulfur hexafluoride to transfer velocities of more soluble gases at wind speeds above 10 m s−1.
How much deep water is formed in the Southern Ocean?
Authors: Broecker W. S. , Peacock S. L. , Walker S. , Weiss R. , Fahrbach E. , Schroeder M. , Mikolajewicz U. , Heinze C. , Key R. , Peng T.‐H. , Rubin S.
Published: February 2004

Three tracers are used to place constraints on the production rate of ventilated deep water in the Southern Ocean. The distribution of the water mass tracer PO4* (“phosphate star”) in the deep sea suggests that the amount of ventilated deep water produced in the Southern Ocean is equal to or greater than the outflow of North Atlantic Deep Water from the Atlantic. Radiocarbon distributions yield an export flux of water from the North Atlantic which has averaged about 15 Sv over the last several hundred years. CFC inventories are used as a direct indicator of the current production rate of ventilated deep water in the Southern Ocean. Although coverage is as yet sparse, it appears that the CFC inventory is not inconsistent with the deep water production rate required by the distributions of PO4* and radiocarbon. It has been widely accepted that the major part of the deep water production in the Southern Ocean takes place in the Weddell Sea. However, our estimate of the Southern Ocean ventilated deep water flux is in conflict with previous estimates of the flux of ventilated deep water from the Weddell Sea, which lie in the range 1–5 Sv. Possible reasons for this difference are discussed.
Comparing the performance of the Mellor‐Yamada and the κ‐ε two‐equation turbulence models
Authors: Burchard Hans , Petersen Ole , Rippeth Tom P.
Published: February 2004

The aim of this paper is to systematically compare κ‐ε and Mellor‐Yamada two‐equation turbulence models. Both models include prognostic equations for turbulent kinetic energy and a length scale related parameter which are used to calculate eddy viscosities and vertical diffusivities. The results from laboratory experiments, using mixed and stratified flows, are simulated in order to systematically compare and calibrate the models. It is shown that the Monin‐Obukhov similarity theory is well represented in both models. The models are used to simulate stratified tidal flow in the Irish Sea, and the results show that the κ‐ε models generally predict a larger phase lag between currents and turbulent dissipation, in the bottom boundary layer, than the Mellor‐Yamada models. The comparison between the model results and field measurements, of the rate of dissipation of turbulent kinetic energy, shows that both models require modification through the inclusion of an internal wave parameterization in order that they are able to correctly predict the observed levels of turbulent dissipation. As the main result, it is shown that the choice of the stability functions, which are used as proportionality factors for calculating the eddy viscosity and diffusivity, has a stronger influence on the performance of the turbulence model than does the choice of length scale related equation.
Simulation of the South Atlantic Ocean circulation and its seasonal variability
Authors: Gan Jianping , Mysak Lawrence A. , Straub David N.
Published: February 2004

The high‐resolution Princeton Ocean Model is used to simulate the circulation and seasonal variability of the South Atlantic Ocean. A diagnostic calculation, using the Levitus annual mean fields and forcing consisting of the climatological annual mean wind stress, produces a realistic steady circulation pattern. In particular, the meridional overturning cell shows equatorward flow in the surface and intermediate waters and poleward flow at depth. Associated with this circulation is a northward heat transport that reaches a maximum of 1 PW near 20°S. The model results are improved when the density field near the ocean bottom is allowed to deviate from the Levitus values. Using forcing consisting of observed monthly wind stress, heat flux and a prescribed seasonally varying Antarctic Circumpolar Current (ACC), a prognostic calculation is next carried out to study the seasonal variability in the South Atlantic Ocean. With a realistic heat flux, the model output for the seasonal thermal field compares well with that in the Levitus data. An analysis suggests that the seasonal cycle of the Brazil Current separation latitude, as well as the strength of the Falkland‐Malvinas Current depends more strongly on the prescribed seasonal variability in the strength of the ACC than on the seasonal variations in wind and buoyancy forcing in the South Atlantic.
Historical position of the North Atlantic Current
Authors: Kearns Edward J. , Rossby H. Thomas
Published: February 2004

The path of the North Atlantic Current (NAC) within the Newfoundland Basin is determined from the historical hydrographic database (1910–1988). Horizontal probability distributions for dynamical and property front locations on specific volume anomaly surfaces are derived from an analysis of all available hydrographic stations in the basin. These probability distributions indicate where the fronts were most likely to have been observed in the past; they are observed to exhibit a pattern of stationary meanders, with troughs located over the southeast Newfoundland Rise, over the Newfoundland Seamounts, and to the east of the Flemish Cap. The width of the NAC's envelope varies from <150 km near the Mann Eddy (42°N, 44°W) to >300 km over the southeast Newfoundland Rise. Temporal subsamples of the historical data indicate that the path and meander patterns are persistent features for all seasons and for the four decades between 1950 and 1990.
Greenland Sea Odden sea ice feature: Intra‐annual and interannual variability
Authors: Shuchman Robert A. , Josberger Edward G. , Russel Catherine A. , Fischer Kenneth W. , Johannessen Ola M. , Johannessen Johnny , Gloersen Per
Published: February 2004

The “Odden” is a large sea ice feature that forms in the east Greenland Sea that may protrude eastward to 5°E from the main sea ice pack (at about 8°W) between 73° and 77°N. It generally forms at the beginning of the winter season and can cover 300,000 km2. Throughout the winter the outer edge of the Odden may advance and retreat by several hundred kilometers on timescales of a few days to weeks. Satellite passive microwave observations from 1978 through 1995 provide a continuous record of the spatial and temporal variations of this extremely dynamic phenomenon. Aircraft synthetic aperture radar, satellite passive microwave, and ship observations in the Odden show that the Odden consists of new ice types, rather than older ice types advected eastward from the main pack. The 17‐year record shows both strong interannual and intra‐annual variations in Odden extent and temporal behavior. For example, in 1983 the Odden was weak, in 1984 the Odden did not occur, and in 1985 the Odden returned late in the season. An analysis of the ice area and extent time series derived from the satellite passive microwave observations along with meteorological data from the International Arctic Buoy Program (IABP) determined the meteorological forcing associated with Odden growth, maintenance, and decay. The key meteorological parameters that are related to the rapid ice formation and decay associated with the Odden are, in order of importance, air temperature, wind speed, and wind direction. Oceanographic parameters must play an important role in controlling Odden formation, but it is not yet possible to quantify this role because of a lack of long‐term oceanographic observations.
A three‐dimensional ocean mesoscale simulation using data from the SEMAPHORE experiment: Mixed layer heat budget
Authors: Caniaux Guy , Planton Serge
Published: February 2004

A primitive equation model is used to simulate the mesoscale circulation associated with a portion of the Azores Front investigated during the intensive observation period (IOP) of the Structure des Echanges Mer‐Atmosphere, Proprietes des Heterogeneites Oceaniques: Recherche Experimentale (SEMAPHORE) experiment in fall 1993. The model is a mesoscale version of the ocean general circulation model (OGCM) developed at the Laboratoire d'Océanographie Dynamique et de Climatologie (LODYC) in Paris and includes open lateral boundaries, a 1.5‐level‐order turbulence closure scheme, and fine mesh resolution (0.11° for latitude and 0.09° for longitude). The atmospheric forcing is provided by satellite data for the solar and infrared fluxes and by analyzed (or reanalyzed for the wind) atmospheric data from the European Centre for Medium‐Range Weather Forecasts (ECMWF) forecast model. The extended data set collected during the IOP of SEMAPHORE enables a detailed initialization of the model, a coupling with the rest of the basin through time dependent open boundaries, and a model/data comparison for validation. The analysis of model outputs indicates that most features are in good agreement with independent available observations. The surface front evolution is subject to an intense deformation different from that of the deep front system, which evolves only weakly. An estimate of the upper layer heat budget is performed during the 22 days of the integration of the model. Each term of this budget is analyzed according to various atmospheric events that occurred during the experiment, such as the passage of a strong storm. This facilitates extended estimates of mixed layer or relevant surface processes beyond those which are obtainable directly from observations. Surface fluxes represent 54% of the heat loss in the mixed layer and 70% in the top 100‐m layer, while vertical transport at the mixed layer bottom accounts for 31% and three‐dimensional processes account for 14%.
Seasonal variability of flow instabilities in the Strait of Sicily
Authors: Onken Reiner , Sellschopp Jürgen
Published: February 2004

The eastern and western Mediterranean Seas are connected by the Strait of Sicily. The mean flow conditions comprise a near‐surface flow of Atlantic Water (AW) into the eastern Mediterranean and an outflow of Levantine Intermediate Water (LIW) below. We present results of a numerical model investigating the stability of currents in the Strait. The model is nonlinear and uses a hybrid (quasi‐isopycnic) vertical coordinate. The model domain is a periodic channel with horizontal dimensions of 400 × 200 km and different bathymetries. As initial conditions, we use seasonal climatological density fields varying only in cross‐channel direction. External forcing is not applied. Combining three different bathymetries with both summer and winter climatological data, we show the results of six model integrations. In all cases the flow is baroclinically unstable, the most unstable waves being 100 and 133 km wavelength in summer and 67 km in winter. A cross‐channel parabolic bottom profile instead of a flat bottom stabilizes the flow in the north (on the Sicilian side of the Strait) but causes further destabilization in the south (on the African side). Further modification of the bathymetry in terms of shallow shelf sea areas to the south of Sicily leads to a deflection of the summer flow in both the AW and the LIW range and to the generation of stationary eddies. In agreement with observations the final kinetic and eddy kinetic energies of the model are higher in winter than in summer in all model runs. From an inspection of satellite images, evidence was found for unstable waves of similar wavelength as predicted by the model. Records from moored current meters support enhanced flow instability on the African side of the Strait. The position and circulation sense of the stationary eddies in the AW range are found to be consistent with climatology and previously known patches of low sea surface temperature.
Subtropical mode water in the Northeast Pacific Basin
Authors: Hautala Susan L. , Roemmich Dean H.
Published: February 2004

A new type of mode water in the upper thermocline of the eastern subtropical North Pacific is identified and examined using data from World Ocean Circulation Experiment high‐resolution repeat expendable bathythermograph (XBT) section PX37 and archives of historical XBT data. This water mass (labeled Eastern Subtropical Mode Water) is characterized by a subsurface potential vorticity minimum and is located east of Hawaii (Northeast Pacific Basin) in a density range of 24–25.4 σθ. It is a distinct water mass from the classical subtropical mode water (STMW) of the western Pacific. Eastern STMW is formed as a relatively deep late‐winter mixed layer, associated with the subtropical/subpolar water mass boundary near 25°–30°N, 135°–140°W, and is capped and subducted into the permanent thermocline. Along a section between San Francisco and Honolulu, Eastern STMW production is seen in every year for which there is adequate data. In this section the volume of Eastern STMW formed each winter and the temperature of the potential vorticity minimum are similar during the periods 1970–1979 and 1991–1997.
Estimation of the sea state bias in radar altimeter measurements of sea level: Results from a new nonparametric method
Authors: Gaspar Philippe , Florens Jean‐Pierre
Published: February 2004

The sea state bias (SSB) affecting altimetric measurements of the sea surface height (SSH) is classically estimated using empirical parametric models. The model parameters are determined to minimize the variance of the SSH differences at crossover points or along collinear tracks. It is shown here that so‐calibrated models are not true least squares approximations of the SSB. Simulations indicate that the difference from a true least squares solution is typically a few millimeters to a few centimeters, varying with sea state. This difference proves to be the result of the inevitably imperfect specification of the model's parametric form, which corrupts the calibration process when performed on SSH differences rather than directly on SSH measurements. To avoid specification of a parametric form, we present a new nonparametric version of the SSB estimation problem and propose a general solution based on the statistical technique of kernel smoothing. This solution is then used to obtain the first fully nonparametric estimate of the TOPEX altimeter SSB as a function of both the wind speed and the wave height. The obtained estimate, though based on a limited data set, proves to be better than those obtained with the standard parametric SSB models featured in the TOPEX‐POSEIDON Geophysical Data Records. The improvement is most significant in middle‐ and high‐latitude oceans.
The architecture of an anisotropic elastic‐plastic sea ice mechanics constitutive law
Authors: Coon Max D. , Knoke Gerald S. , Echert Douglas C. , Pritchard Robert S.
Published: February 2004

The architecture for a new large‐scale anisotropic constitutive law intended for use in a sea ice dynamics model is presented here. This architecture accounts directly for refrozen lead systems in the pack ice strength (with an anisotropic failure surface) and in the ice thickness distribution (with an oriented thickness distribution). The lower limit (5 km) of the model resolution is controlled by the fracture spacing of old, thicker ice and the maximum lead width. The upper limit of the model resolution (100 km) is controlled by curvature in the lead directions and variations in the lead width. These, in turn, are controlled by the variations in internal ice stress due to driving forces (winds and currents), which set the time resolution. This architecture features abrupt changes in the failure surface and the associated flow rule that cannot be averaged over a time step. In addition, the principal stress normal to a new lead must be zero as it opens. This model has subscale simulations that allow for the inclusion of phenomena such as ridging, rafting, buckling, and fracture on the behavior of the ice. With this new ice constitutive law, it is possible to test directly the ice failure strength, plastic flow rule, and ice thickness distribution. The data most useful for this testing come from ice stress and position buoys together with synthetic aperture radar deformation data. Some data comparisons have already been made.
Alongshore momentum balances in the nearshore
Authors: Feddersen Falk , Guza R. T. , Elgar Steve , Herbers T. H. C.
Published: February 2004

The one‐dimensional, time‐averaged (over many wave periods) along‐shore momentum balance between forcing by wind and breaking waves and the bottom stress is examined with field observations spanning a wide range of conditions on a barred beach. Near‐bottom horizontal currents were measured for 2 months at 15 locations along a cross‐shore transect extending 750 m from the shoreline to 8‐m water depth. The hourly averaged bottom stress was estimated from observed currents using a quadratic drag law. The wave radiation stress was estimated in 8‐m depth from an array of pressure sensors, and the wind stress was estimated from an anemometer at the seaward end of a nearby pier. The combined wind and wave forcing integrated over the entire cross‐shore transect is balanced by the integrated bottom stress. The wind stress contributes about one third of the forcing over the transect. Analysis of the momentum balances in different cross‐shore regions shows that in the surf zone, wave forcing is much larger than wind forcing and that the bottom drag coefficient is larger in the surf zone than farther seaward, consistent with earlier studies.
COASTWATCH'95: ERS 1/2 SAR detection of natural film on the ocean surface
Authors: Espedal H. A. , Johannessen O. M. , Johannessen J. A. , Dano E. , Lyzenga D. R. , Knulst J. C.
Published: February 2004

Ocean mesoscale phenomena such as eddies and current convergence zones can often be seen in synthetic aperture radar (SAR) images due to characteristic patterns caused by natural film induced damping of the waves. Such films have also been found to exert a significant effect on air‐sea gas exchange, which may be important for the global scale climate system. Satellite SAR may prove very useful to quantify the extent of natural film. To investigate the composition of these films and their effect on radar return, we compared samples of the sea surface with ERS 1/2 SAR images of coastal ocean areas during the COASTWATCH'95 experiment. Simultaneous observations were made with a shipmounted C band dual‐polarized Doppler radar, and surface drifters were deployed to investigate the surface current variations in the vicinity of different slicks (areas where the short surface waves sensed by a radar are damped). One confirmed case of natural film was thus verified to be caused by a convergence zone. The study also showed that the films investigated during COASTWATCH'95 were generally less concentrated and originated from marine organisms, compared to the films with terrestrial influences found in a previous experiment in a fjord [Espedal et al., 1996]. The dependence of the existence of the films on wind speed is also investigated, and an estimate of natural film distribution during the experiment period is given, using a total of 71 ERS 1/2 SAR images collected over the same coastal area under a variety of wind conditions. Up to 40% natural film coverage was found for 2.5 m/s wind speeds, while already at 5–10 m/s all SAR imagery had under 5% film coverage.
Skin layer recovery of free‐surface wakes: Relationship to surface renewal and dependence on heat flux and background turbulence
Authors: Zappa C. J. , Jessup A. T. , Yeh Harry
Published: February 2004

The thermal signatures of free‐surface wakes observed in the open ocean show that the recovery of the cool skin layer is related to the degree of surface mixing and to ambient environmental conditions. Wakes produced by two surface‐piercing cables of O(10−2 m) in diameter are analyzed using infrared imagery. Under low‐wind‐speed conditions when the swell and surface current were aligned, the wakes exhibited distinctive patchlike features of O(1 m) in diameter that were generated by the passage of individual waves. The time t* required by the skin layer to recover from these disturbances is compared to the surface‐renewal timescale τ used in heat and gas flux models. At low wind speeds, t* is comparable to τ, but at moderate wind speeds the agreement is poor. The spatial and temporal variations in the skin temperature of these wakes are related to a wave Reynolds number used to characterize the strength of the disturbance due to the waves. The recovery process is characterized in terms of the restoring internal energy flux Jr which is proportional to both the initial thickness and the thermal recovery rate of the skin layer and was found to be directly related to the strength of the surface disruption. Comparison of the wake results with laboratory and other field measurements of breaking waves implies that Jr is also a strong function of the net heat flux and background turbulence, which relate directly to the existing environmental conditions such as wind stress and sea state. Our results demonstrate that Jr may vary by several orders of magnitude, depending on the environmental conditions.
Equatorial subthermocline currents across the Pacific
Authors: Firing Eric , Wijffels Susan E. , Hacker Peter
Published: February 2004

Recent sections crossing the equator at nine longitudes from 146°E to 86°W show zonal currents below 400 m similar to those found in a 16‐month mean section on 159°W from the Pacific Equatorial Ocean Dynamics (PEQUOD) program in 1982–1983. Most of the new measurements were made with a lowered acoustic Doppler current profiler. Eastward current extrema (North and South Intermediate Countercurrents) are found in all sections about 2° from the equator in the depth range 500–1500 m, with the possible exceptions of 146°E, where topography complicates the picture south of the equator, and 150°E, where the sections extend only from 1°S to 2°N. Poleward of the intermediate countercurrents, westward flow is found near 3° from the equator in most of the sections. On the equator, from 250–500 m depth, a westward Equatorial Intermediate Current is found in many but not all of the sections. Similarly, it is present in the PEQUOD mean but not in all synoptic sections. Below the intermediate currents and countercurrents, an eastward current near 3000 m was found at and south of the equator in the PEQUOD mean and in the new sections from 150°E to 110°W. Near 4000 m there is westward flow south of the equator in the PEQUOD mean and near the equator in the new sections from 179°E to 135°W. Geostrophic currents calculated from an 8‐year mean hydrographic section at 165°E also resemble the PEQUOD mean. Although not conclusive, the evidence presented here indicates that these currents are basin‐scale components of the general circulation, perhaps involving vigorous horizontal recirculation in a set of basin‐wide elongated gyres within a few degrees of the equator.
Lateral mixing and the North Atlantic Tracer Release Experiment: Observations and numerical simulations of Lagrangian particles and a passive tracer
Authors: Sundermeyer Miles A. , Price James F.
Published: February 2004

Mixing and stirring of Lagrangian particles and a passive tracer were studied by comparison of float and tracer observations from the North Atlantic Tracer Release Experiment (NATRE). Statistics computed from the NATRE floats were found to be similar to those estimated by Ledwell et al. [this issue] from the tracer dispersion. Mean velocities computed from the floats were
cm/s for the (zonal, meridional) components, and large‐scale effective eddy diffusivities were (κe11, κe22) = (1.5±0.7, 0.7±0.4) × 103 m2/s. The NATRE observations were used to evaluate theoretical models of tracer and particle dispersal. The tracer dispersion observed by Ledwell et al. [this issue] was consistent with an exponential growth phase for about the first 6 months and a linear growth at larger times. A numerical model of mesoscale turbulence that was calibrated with float statistics also showed an exponential growth phase of tracer and a reduced growth for longer times. Numerical results further show that Garrett's [1983] theory, relating the effective small‐scale diffusivity to the rms strain rate and tracer streak width, requires a scale factor of 2 when the observed growth rate of streak length is used as a measure of the strain rate. This scale factor will be different for different measures of the strain rate and may also be affected by temporal and spatial variations in the mesoscale strain field.
Structure of the marine atmospheric boundary layer over an oceanic thermal front: SEMAPHORE experiment
Authors: Kwon B. H. , Bénech B. , Lambert D. , Durand P. , Druilhet A. , Giordani H. , Planton S.
Published: February 2004

The Structure des Echanges Mer‐Atmosphere, Proprietes des Heterogeneites Oceaniques: Recherche Experimentale (SEMAPHORE) experiment, the third phase of which took place between October 4 and November 17, 1993, was conducted over the oceanic Azores Current located in the Azores basin and mainly marked at the surface by a thermal front due to the gradient of the sea surface temperature (SST) of about 1° to 2°C per 100 km. The evolution of the marine atmospheric boundary layer (MABL) over the SST front was studied with two aircraft and a ship in different meteorological conditions. For each case, the influence of the incoming air direction with respect to the orientation of the oceanic front was taken into account. During the campaign, advanced very high resolution radiometer pictures did not show any relation between the SST field and the cloud cover. The MABL was systematically thicker on the warm side than on the cold side. The mean MABL structure described from aircraft data collected in a vertical plane crossing the oceanic front was characterized by (1) an atmospheric horizontal gradient of 1° to 2°C per 100 km in the whole depth of the mixed layer and (2) an increase of the wind intensity from the cold to the warm side when the synoptic wind blew from the cold side. The surface sensible heat (latent heat) flux always increased from the cold to the warm sector owing to the increase of the wind and of the temperature (specific humidity) difference between the surface and the air. Turbulence increased from the cold to the warm side in conjunction with the MABL thickening, but the normalized profiles presented the same structure, regardless of the position over the SST front. In agreement with the Action de Recherche Programme te Petite Echelle and Grande Echelle model, the mean temperature and momentum budgets were highly influenced by the horizontal temperature gradient. In particular, the strong ageostrophic influence in the MABL above the SST front seems linked with the secondary circulation due to the SST front.
Chemical signals from submarine fluid advection onto the continental shelf
Authors: Moore Willard S. , Shaw Timothy J.
Published: February 2004

We report unambiguous signals of subsurface fluid input on the continental shelf 40–80 km from shore in water depths of 20–45 m. The site of this input is near the continental shelf break off the South Carolina coast between 32°–33°N and 78.5°–79.5°W. Repeated transects over this area reveal enrichments of 226Ra, 228Ra, and Ba by factors of 3–4 above ambient ocean concentrations. The short‐lived 223Ra and 224Ra are enriched by over an order of magnitude. The Ra and Ba enrichments are not caused by up welling of deep water as no such 226Ra or Ba concentrations occur in the upper 2000 m of the Atlantic Ocean. Sediments in the region are largely relic sand and do not offer a viable source for 226Ra, 228Ra, and Ba enrichments. We conclude that the source is the discharge of submarine fluids enriched in 226Ra, 228Ra, and Ba. The salinity of water in the Ra‐Ba‐enriched zone is diluted by 0.2–0.6‰ relative to water at the same depth farther offshore. Pronounced salinity and density anomalies characterize one site of active discharge. Estimates of 226Ra and Ba concentrations in the fluids and the replacement time of water in the enriched zone suggest the fluid flux is similar to the summertime flow of major rivers in the area. The region of Ra‐Ba enrichment is also characterized by a strong fluorescence signal, indicating high concentrations of chlorophyll in the water. It is probable that the leaking fluids supply nutrients that stimulate productivity in the bottom waters which are within the photic zone. Because the fluorescence signal is restricted to depths greater than 20 m, it is unlikely that the signal could be recognized by satellite or aircraft sensors.