bluuebunny/crossref_metadata_2025 huggingface dataset:
Narrow barotropic currents impinging on an isolated seamount
Authors: Zhang Xiuzhang , McGuinness Daniel S. , Boyer Don L.
Published: February 2004

Laboratory and numerical experiments concerned with the interaction of homogeneous narrow currents (jets) and tall, isolated topographic features in the presence of background rotation were conducted. The laboratory jets (in an azimuthal direction) were produced in a rotating circular test cell using a circularly symmetric, free surface, source sink technique. By regulating the Rossby and Ekman numbers of the jets, the narrow currents so created can be either circularly symmetric (stable) or meandering (unstable). A theoretical analysis for a point sink at the test cell center is advanced to predict the strength and radial distribution of the azimuthal current in the absence of bottom topography. Laboratory observations are found to be in good agreement with the model. The interaction of stable currents with isolated topography was investigated. The system parameters are the Rossby and Ekman numbers, the normalized topographic width and height, the width of the current, and the position of the streamwise centerline. A numerical model was developed which employed a vertically averaged vorticity equation driven by Ekman layer forcing along the free surface and dissipated by Ekman pumping along the lower bounding surface. The model is supported by laboratory observations. Only stable currents are considered for the numerical calculations. Currents which pass either to the right or left of the obstacle center as well as those which impinge directly are considered. For all cases, anticyclonic motions are found atop the obstacle. In the numerical experiments the maximum values of cyclonic and anticyclonic vorticity above the topography are found to be monotonically increasing functions of the Rossby number, with other parameters fixed. For cases in which the current is to the right of the topographic feature, closed cyclonic vortices are formed in the left topography lee. The normalized area of these cyclones increases with increasing Rossby number, for the range of R0 investigated. For all cases the transport stream function lines in the right topography lee are deflected relatively more than the corresponding streamlines in cases for which the upstream flow is uniform and rectilinear (i.e., a broad current having no relative upstream vorticity).
Variability in equatorial Pacific sea surface topography during the verification phase of the TOPEX/POSEIDON mission
Authors: Busalacchi Antonio J. , McPhaden Michael J. , Picaut Joel
Published: February 2004

As part of the verification phase of the TOPEX/POSEIDON mission, 10‐day gridded fields of altimeter data derived from TOPEX geophysical data records are compared with 10‐day gridded fields of dynamic height derived from more than 60 moorings of the Tropical Ocean and Global Atmosphere‐Tropical Atmosphere Ocean (TOGA‐TAO) array in the equatorial Pacific Ocean. Access to TAO data in real time permits the first 500 days of the TOPEX/POSEIDON mission to be placed in the context of complementary, in situ measurements of surface winds, sea surface temperatures, and upper ocean thermal structure, as well as the time history of these variables prior to launch. Analysis of the space‐time structure in the TOPEX and TAO surface topography data indicates sea level variability primarily due to equatorial Kelvin wave activity generated by intense wind bursts west of the date line in association with the 1991–1993 El Niño. Cross correlations between the two data sets are generally >0.7, with RMS differences <4 cm. However, for reasons not fully understood, correlations drop to <0.5 in certain regions off the equator in the eastern Pacific, and RMS differences can be >5 cm north of the equator in the central and eastern Pacific.
An error covariance model for sea surface topography and velocity derived from TOPEX/POSEIDON Altimetry
Authors: Tsaoussi Lucia S. , Koblinsky Chester J.
Published: February 2004

In order to facilitate the use of satellite altimeter‐derived sea surface topography and velocity in oceanographic models, a methodology is presented for deriving the total error covariance and its geographic distribution from TOPEX/POSEIDON measurements. The model is formulated using a parametric model fit to the altimeter range observations. The topography and velocity are modeled with spherical harmonic expansions whose coefficients are found through optimal adjustment to the altimeter range residuals using Bayesian statistics. All other parameters, including the orbit, geoid, surface models, and range corrections are provided as unadjusted parameters. The maximum likelihood estimates and errors are derived from the probability density function of the altimeter range residuals conditioned with a priori information. Estimates of model errors for the unadjusted parameters are obtained from the TOPEX/POSEIDON postlaunch verification results and the error covariances for the orbit and the geoid, except for the ocean tides. The error in the ocean tides is modeled, first, as the difference between two global tide models and, second, as the correction to the present tide model, the correction derived from TOPEX/POSEIDON data. A formal error covariance propagation scheme is used to derive the total error. Our global total error estimate for the TOPEX/POSEIDON topography relative to the geoid for one 10‐day period is found to be 11 cm RMS. When the error in the geoid is removed, thereby providing an estimate of the time dependent error, the uncertainty in the topography is 3.5 cm RMS. This level of accuracy is consistent with direct comparisons of TOPEX/POSEIDON altimeter heights with tide gauge measurements at 28 stations. In addition, the error correlation length scales are derived globally in both east‐west and north‐south directions, which should prove useful for data assimilation. The largest error correlation length scales are found in the tropics. Errors in the velocity field are smallest in midlatitude regions; they are less than 2 cm/s over most of the Southern Ocean. For both variables the largest errors are caused by uncertainty in the geoid. More accurate representations of the geoid await a dedicated geopotential satellite mission. Substantial improvements in the accuracy of ocean tide models are expected in the very near future from research with TOPEX/POSEIDON data. Consequently, estimates of the time dependent sea surface topography from TOPEX/POSEIDON could achieve a total uncertainty of less than 3 cm RMS. This level of accuracy from a spaceborne sensor represents a remarkable engineering achievement and a major contribution to ocean science.
Calibration of TOPEX/POSEIDON at Platform Harvest
Authors: Christensen E. J. , Haines B. J. , Keihm S. J. , Morris C. S. , Norman R. A. , Purcell G. H. , Williams B. G. , Wilson B. D. , Born G. H. , Parke M. E. , Gill S. K. , Shum C. K. , Tapley B. D. , Kolenkiewicz R. , Nerem R. S.
Published: February 2004

We present estimates for the mean bias of the TOPEX/POSEIDON NASA altimeter (ALT) and the Centre National d'Etudes Spatiales altimeter (SSALT) using in situ data gathered at Platform Harvest during the first 36 cycles of the mission. Data for 21 overflights of the ALT and six overflights of the SSALT have been analyzed. The analysis includes an independent assessment of in situ measurements of sea level, the radial component of the orbit, wet tropospheric path delay, and ionospheric path delay. (The sign convention used in this paper is such that, to correct the geophysical data record values for sea level, add the bias algebraically. Unless otherwise stated, the uncertainty in a given parameter is depicted by ±σx, where σx is the sample standard deviation of x about the mean.) Tide gauges at Harvest provide estimates of sea level with an uncertainty of ±1.5 cm. The uncertainty in the radial component of the orbit is estimated to be ±1.3 cm. In situ measurements of tropospheric path delay at Harvest compare to within ±1.3 cm of the TOPEX/POSEIDON microwave radiometer, and in situ measurements of the ionospheric path delay compare to within −0.4±0.7 cm of the dual‐frequency ALT and 1.1±0.6 cm of Doppler orbitography and radiopositioning integrated by satellite. We obtain mean bias estimates of −14.5±2.9 cm for the ALT and +0.9±3.1 cm for the SSALT (where the uncertainties are based on the standard deviation of the estimated mean
, which is derived from sample statistics and estimates for errors that cannot be observed). These results are consistent with independent estimates for the relative bias between the two altimeters. A linear regression applied to the complete set of data shows that there is a discernable secular trend in the time series for the ALT bias estimates. A preliminary analysis of data obtained through cycle 48 suggests that the apparent secular drift may be the result of a poorly sampled annual signal.
Precision orbit determination for TOPEX/POSEIDON
Authors: Tapley B. D. , Ries J. C. , Davis G. W. , Eanes R. J. , Schutz B. E. , Shum C. K. , Watkins M. M. , Marshall J. A. , Nerem R. S. , Putney B. H. , Klosko S. M. , Luthcke S. B. , Pavlis D. , Williamson R. G. , Zelensky N. P.
Published: February 2004

The TOPEX/POSEIDON mission objective requires that the radial position of the spacecraft be determined with an accuracy better than 13 cm RMS (root mean square). This stringent requirement is an order of magnitude below the accuracy achieved for any altimeter mission prior to the definition of the TOPEX/POSEIDON mission. To satisfy this objective, the TOPEX Precision Orbit Determination (POD) Team was established as a joint effort between the NASA Goddard Space Flight Center and the University of Texas at Austin, with collaboration from the University of Colorado and the Jet Propulsion Laboratory. During the prelaunch development and the postlaunch verification phases, the POD team improved, calibrated, and validated the precision orbit determination computer software systems. The accomplishments include (1) increased accuracy of the gravity and surface force models and (2) improved performance of both the laser ranging and Doppler tracking systems. The result of these efforts led to orbit accuracies for TOPEX/POSEIDON which are significantly better than the original mission requirement. Tests based on data fits, covariance analysis, and orbit comparisons indicate that the radial component of the TOPEX/POSEIDON spacecraft is determined, relative to the Earth's mass center, with an RMS error in the range of 3 to 4 cm RMS. This orbit accuracy, together with the near continuous dual‐frequency altimetry from this mission, provides the means to determine the ocean's dynamic topography with an unprecedented accuracy.
On the origins of the North Brazil Current
Authors: da Silveira Ilson C. A. , de Miranda Luiz B. , Brown Wendell S.
Published: February 2004

The analysis of the hydrographic data obtained during the “Nordeste III” (NEIII) survey by the R/V Almirante Saldanha (Brazilian Navy) between May 4 and July 12, 1986, is reported in this work. Distributions of geostrophic speeds and transports (relative to 1000 m) reveal the North Brazil Current (NBC) and other major features of the tropical Atlantic circulation west of 30°30′W between 4°S and 10°30′S. The well‐developed NBC appears in all analyzed crossisobath sections bordering the Brazilian continental slope between 10°S and 5°S, transporting an average of 21 Sv. The velocity core of the NBC is found to be centered at about 100–200 m, instead of at the surface, as previously thought. The central branch of the South Equatorial Current (CSEC) enters the study region at about 5°S–6°S and separates into two portions. The northern portion of the CSEC transports about 15 Sv northwestward, and north of 5°S, it coalesces with the NBC to form a 300‐km‐wide current that transports more than 36 Sv equatorward. The southern branch of the CSEC transports 14 Sv southwestward until about 9°30′S, where it makes a cyclonic turn to the north and merges with northward flow from the southernmost branch of South Equatorial Current (SSEC). The high‐salinity signature of waters from the SSEC appears in the NBC, indicating that it is formed to the south of the study area. By contrast, historical ship drift and Lagrangian drifter observations suggest that the NBC and the Brazil Current are formed at the bifurcation of the CSEC. This apparent contradiction is explained by the surface Ekman drift which obscures the geostrophic flow patterns.
Comparison of mesoscale meanders and eddies simulated by quasi‐geostrophic and primitive equation models: A case study
Authors: Wood Richard A. , Ikeda Motoyoshi
Published: February 2004

A comparison is made between meanders and eddies simulated by a two‐layer, quasi‐geostrophic numerical model and those simulated by a Bryan‐Cox type primitive equation model at various values of the Rossby number. The model geometry and parameter settings are based on a previous study of the Norwegian Coastal Current by Ikeda et al. (1989). The aim is to identify the dynamical effects of increasing the Rossby number and to assess the performance of the quasi‐gestrophic (QG) model when applied to meandering currents which are outside its formal range of validity. The growth rates of meanders in the primitive equation (PE) model are particularly sensitive to the vertical resolution used; this sensitivity is strongest at short wavelengths, and suggests that very fine vertical resolution may be needed in the Bryan‐Cox model for accurate simulation of meander growth. The QG model simulates the phase speed of meanders in the PE model well but overestimates the growth rate at moderate Rossby number (by about 25% when the Rossby number is 0.69). As the Rossby number is increased, the asymmetry of the primitive equation vorticity equation comes increasingly into play, and at finite meander amplitude this results in small, intense cyclones and large, weak anticyclones. This effect, which is absent in the quasi‐geostrophic model, can lead to significant changes in the potential vorticity field (and hence, by implication, in the concentration fields of other tracers). The principal trends in the solutions as the Rossby number is increased are as follows: (1) the disturbance becomes less energetic (in a dimensionless sense) at all stages of its life cycle; (2) the effects of the disturbance become increasingly trapped near the position of the original frontal jet; and (3) cross‐frontal transport, particularly offshore transport of coastal water, is inhibited as coastal water is wrapped around the intense cyclones. Our results provide guidance in interpreting the results of Ikeda et al. and those of other eddy models.
Modeling of the Greenland, Iceland, and Norwegian Seas with a coupled sea ice—mixed layer—isopycnal ocean model
Authors: Aukrust T. , Oberhuber J. M.
Published: February 2004

A coupled sea ice—mixed layer—isopycnal ocean model is used to simulate the circulation in the Greenland, Iceland, and Norwegian (GIN) Seas. The model domain consists of the North Atlantic, including the GIN Sea and the Arctic Ocean. The Atlantic Ocean is bounded meridionally at about 10°S, and in the Arctic the Bering Strait is closed. In order to obtain sufficient resolution of the current system in the GIN Sea, the horizontal resolution increases from 2°×2° near the equator and in the Pacific sector of the Arctic to less than 30 km×30 km in the GIN Sea. As initial conditions, observed annual mean temperature and salinity are used. The surface‐forcing fields are determined from monthly mean atmospheric quantities. The surface salinity is prescribed (with a weak Newtonian relaxation coupling) as an annual mean. Except for the deep ocean, the GIN Sea adjusts to its own physics after a few years. Longer timescales are introduced owing to the interactions with the North Atlantic and the Arctic Oceans. The salinity in the GIN Sea has become slightly lower than the initial data. The general convection depth is consistent with observations during the 1980s. Ice fluxes and the fluxes in the upper ocean are consistent with previous estimates. The outflow of intermediate and deep water into the North Atlantic is underestimated. However, the model reproduces the main features of the ice‐ocean circulation in the GIN Sea and the adjacent basins.
Simulated interannual variability of the Greenland Sea deep water formation and its connection to surface forcing
Authors: Häkkinen Sirpa
Published: February 2004

A fully prognostic Arctic ice‐ocean model is used to study the interannual variability of deepwater formation in the Greenland Sea gyre based on the simulations for the Arctic ice‐ocean system for the period 1955 and 1960–1985. The model uses monthly climatology for thermodynamic forcing components (such as air temperature and cloudiness), together with constant annual net precipitation and river runoff. The daily wind forcing is derived from analyzed sea level air pressures from the National Center for Atmospheric Research. In summary, the model shows that the occurrence of deep convection in the Greenland Sea gyre is controlled by the extensive Fram Strait ice export and/or local wind conditions in the Greenland Sea. In the latter case the weakening of the local wind curl allows the Polar Front to move eastward. The movement of the Polar Front causes adverse ice conditions, often together with much larger than normal ice export from the Arctic, such as in 1968, which can block convection in the gyre. The density difference between upper and lower layers is investigated as an indication of water mass formation through convection, occurring as strong diffusion in the model. The model‐simulated density difference between the average top 100 m and deep levels reveals that the period 1960–1985 had only a few distinct years with weak stratification, and, especially, the model predicts no deep convection since the mid‐1970s. The common factor for the years of the weakest stratification is extreme wintertime wind events. Deep convection is also evident as a decrease of the model‐predicted heat content of the upper 2000 m which can, to a high degree, be explained by local heat loss. The salt content changes in the Greenland gyre are independent of the heat content changes and are determined by the advection of fresh or salty anomalies, either from the Arctic or from the frontal movement. The model indicates that the surface salinity flux favors freshening, even in wintertime, so that brine rejection is less important in buoyancy loss than heat loss in leading to deep convection. The model shows that the system has a memory; deep convection in one year will precondition the following year.
Application of the adjoint technique to processing of a standard section data set: World Ocean Circulation Experiment section S4 along 67°S in the Pacific Ocean
Authors: Nechaev D. A. , Yaremchuk M. I.
Published: February 2004

We present a variational data assimilation scheme designed to recover the steady state of an ocean region along a conductivity‐temperature‐depth section track. Oceanic velocity and tracer fields are assumed to satisfy simple stationary dynamical constraints, which include the equation of state of seawater, a dynamical constraint on vertical velocity, stationary tracer transfer equations, geostrophic relations for horizontal velocities, and the kinematic constraint, imposed on the net transport across the section plane. Cousin data assimilation experiments with the steady state geostrophic component of the fine resolution antarctic model have shown that in addition to the absolute cross‐section velocity component and along‐section gradients of properties, the algorithm is capable of reconstructing the cross‐section gradients of temperature, salinity, and along‐section velocity component. The algorithm can also process an arbitrary set of passive tracer fields measured at the section. These fields are coupled statistically with active tracers and between each other via the depth dependent in situ covariance matrix. Error analysis of the assimilated data set taken at 67°S in the southern Pacific has shown that the major state vector components were statistically reliable.
Assessment of the TOPEX altimeter performance using waveform retracking
Authors: Rodríguez Ernesto , Martin Jan M.
Published: February 2004

To assess the accuracy of the TOPEX altimeter data, we have reprocessed the raw altimeter waveform data using more sophisticated algorithms than those implemented in the altimeter hardware. We discuss systematic contamination of the waveform which we have observed and its effect on very long wavelength errors. We conclude that these systematic errors are responsible for a very long wavelength error whose peak‐to‐peak magnitude for the Ku band altimeter is of the order of 1 cm. We also examine the ability of retracked data to reduce the repeat pass variance and correct for significant wave height (SWH) and acceleration dependent errors. We find that the ground postprocessing contains SWH dependent biases which depend on the altimeter fine height correction.
Upper layer circulation in the western tropical North Atlantic Ocean during August 1989
Authors: Wilson W. Douglas , Johns E. , Molinari R. L.
Published: February 2004

Shipboard acoustic Doppler current profiler velocity measurements and profiles of temperature, salinity, and dissolved oxygen measurements are used to map the flow field above the σθ = 26.8 isopycnal (approximately the upper 300 m of the water column) in the North Brazil Current (NBC) retroflection region (0° to 14°N, 60° to 40°W) during August of 1989. The water column is divided into a near‐surface, upper thermocline layer (above σθ = 24.5) and a main to subthermocline layer (σθ = 24.5 to σ0 = 26.8). In the upper layer the eastward flowing North Equatorial Countercurrent (NECC) is composed of 16×106 m3/s of NBC transport that has retroflected from the coast between 6°N and 8°N and 8×106 m3/s of North Equatorial Current (NEC) transport returning eastward. An anticyclonic eddy with a 10×106 m3/s transport lies northwest of the NBC retroflection. No throughflow is observed along the boundary, and only a small portion of the observed NEC transport (1.5×106 m3/s) enters the Caribbean Sea. In the lower layer the NBC transports 8×106 m3/s into the subthermocline North Equatorial Undercurrent (NEUC). In addition, 15×106 m3/s joins the NEUC from the north. At 44°W the subsurface core of the NEUC lies south of the near‐surface core of the NECC. Nearly half of the NEUC transport is made up of a mixed water type with salinity‐oxygen (S‐O2) characteristics intermediate to the characteristics of the original component transports. There is no evidence of continuous NBC flow into the Caribbean Sea in the lower level. The closed eddy to the north of the retroflection, however, contains water masses with South Atlantic S‐O2 properties.
Geographical representation of radial orbit perturbations due to ocean tides: Implications for satellite altimetry
Authors: Bettadpur Srinivas V. , Eanes Richard J.
Published: February 2004

In analogy to the geographical representation of the zeroth‐order radial orbit perturbations due to the static geopotential, similar relationships have been derived for radial orbit perturbations due to the ocean tides. At each location these perturbations are seen to be coherent with the tide height variations. The study of this singularity is of obvious importance to the estimation of ocean tides from satellite altimeter data. We derive analytical expressions for the sensitivity of altimeter derived ocean tide models to the ocean tide force model induced errors in the orbits of the altimeter satellite. In particular, we focus on characterizing and quantifying the nonresonant tidal orbit perturbations, which cannot be adjusted into the empirical accelerations or radial perturbation adjustments commonly used during orbit determination and in altimeter data processing. As an illustration of the utility of this technique, we study the differences between a TOPEX/POSEIDON‐derived ocean tide model and the Cartwright and Ray 1991 Geosat model. This analysis shows that nearly 60 percent of the variance of this difference for M2can be explained by the Geosat radial orbit error due to the omission of coefficients from the GEM‐T2 background ocean tide model. For O1, K1, S2, and K2the orbital effects account for approximately 10 to 40 percent of the variances of these differences. The utility of this technique to an assessment of the ocean tide induced errors in the TOPEX/POSEIDON‐derived tide models is also discussed.
Geosat‐derived sea level and surface current anomalies in the equatorial Pacific during the 1986–1989 El Niño and La Niña
Authors: Delcroix Thierry , Boulanger Jean‐Philippe , Masia François , Menkes Christophe
Published: February 2004

Equatorial wave dynamics are essential in most oceanic models for reproducing aspects of the El Niño‐Southern Oscillation (ENSO) phenomenon. In this paper, observational evidence of first baroclinic equatorial Kelvin and first symmetric meridional Rossby waves is found in Geosat‐derived sea level anomalies (SLA) and surface zonal current anomalies (ZCA) in the equatorial Pacific ocean during the 1986–1987 El Niño and the ensuing 1988–1989 La Niña. This was made possible after extensive quality control and specific processing of the recently improved Geosat geophysical data records pertaining to the 17‐day Exact Repeat Mission. In particular, the processing was made so that the Geosat‐derived ZCA best fit near‐surface zonal currents from three equatorial moorings at 165°E, 140°W, and 110°W. The Geosatderived SLA and ZCA are decomposed into first baroclinic equatorial Kelvin and gravest Rossby modes. The emphasis is then put on the chronology of the ZCA at the equator where the currents are the most energetic and where Kelvin and first symmetric Rossby waves explain most of the variance in ZCA, in similar proportion. The 1986–1987 El Niño is mostly characterized by a strong downwelling Kelvin wave in December 1986, a series of downwelling Kelvin waves in March–October 1987, and a strong upwelling Rossby wave in March–September 1987. These waves are consistent with wind forcing, and all give rise to notable eastward ZCA for almost an entire year. During the El Niño‐La Niña transition period there is the occurrence of two downwelling Rossby waves originating from the eastern Pacific at times of favorable wind forcing. These Rossby waves switch the basin‐wide ZCA from eastward to westward, terminating the warm event. Then, the 1988–1989 La Niña shows the generation of a series of upwelling Kelvin waves which are clearly identified in March/April and September/October 1988. These upwelling Kelvin waves are apparently related to the enhancement of the trade wind in the western half of the basin. These waves maintain the basin‐wide westward ZCA for more than a year, although the ZCA is slightly reduced by a front of upwelling Rossby waves in phase with the normal seasonal cycle. The Kelvin wave forcing and sign of Kelvin wave contribution versus Rossby wave contribution to ZCA in the western Pacific are discussed, and we conclude that neither pleads in favor of the delayed action oscillator mechanism.
Eastward jets over diverging isobaths, with applications to the Gulf Stream past the Grand Banks
Authors: Shi Chuan , Chao Shenn‐Yu
Published: February 2004

A three‐dimensional primitive‐equation model is employed to investigate how an initially eastward jet responds to isobaths that diverge eastward. This problem is investigated using first a barotropic model and then a baroclinic model. In the barotropic regime, the diverging isobaths force an initially supercritical jet to widen and become subcritical after a hydraulic jump. Increasing eastward advection of vorticity moves the diverging point eastward and narrows the jet locally but widens the jet farther downstream. The incoming jet axis is fixed at a preferred latitude due to the steep meridional bottom slope upstream. In the baroclinic regime, the eastward divergence of the jet becomes much more pronounced. This strong divergence leads to the bifurcation of the jet downstream. The northern branch flows northeastward and the main branch continues eastward. The former is a western boundary current along the northeastward continental slope. The bifurcation of baroclinic jets is caused by hydraulic jumps reinforced by instabilities of the baroclinic current system. Among possible applications, the model is particularly relevant to the bifurcation of the Gulf Stream to the south of the Grand Banks, where the essentially zonal isobaths begin to bifurcate eastward.
On treatments of fetch and stability sensitivity in large‐area estimates of sensible heat flux over sea ice
Authors: Maslanik J. A. , Key J.
Published: February 2004

Bulk transfer coefficients estimated as a function of atmospheric stability and sea‐ice lead width are combined with Arctic meteorological observations and ice thickness data to calculate the sensitivity of turbulent flux estimates to changes in lead width, wind speed, air temperature, and ice thickness for a high‐concentration ice pack. These results are considered in terms of bulk transfer parameterizations that use a fixed transfer coefficient or that address atmospheric stability only. On the basis of the fetch‐sensitive parameterizations considered here, differences in lead width for widths up to about 200 m can exert a substantial influence on sensible heat transfer coefficients and heat flux from leads under typical Arctic conditions. Fluxes from an open water lead decrease by 34% if fetch increases from 10 m to 100 m. This effect is greatest for open water leads, decreases considerably as leads refreeze, and is negligible for ice thicker than about 0.3 m. If open or newly refrozen leads make up 2% of the ice cover, then an increase in mean fetch from 10 m to 100 m yields a decrease of about 2 W m−2 in areally averaged flux from the ice pack. Calculations using observed and theoretical lead width distributions suggest that parameterizing lead widths in a sea ice model can be done effectively using a single, representative lead width rather than requiring a full distribution of widths. When coupled to the lower atmospheric boundary layer using a bulk similarity theory model, this sensitivity of heat transfer to fetch results in substantially higher near‐surface air temperatures over narrow leads, with equilibrium air temperatures decreasing by about 50% as fetch increases from 10 to 100 m.
A necessary condition for the representativeness of island tidal measurements
Authors: Kagan Boris A. , Kivman Gennady A.
Published: February 2004

A condition for representativeness of island tidal measurements is found on the assumption that disturbances in the tidal field due to interaction of a tidal wave and a compact set of islands (an archipelago) must be small compared with undisturbed tidal elevations. The corresponding condition for a single island follows from the above as a particular case. It is shown that tidal characteristics belonging even to small islands may be nonrepresentative from the point of view of possibility of their spreading onto the deep ocean. This takes place specifically for islands situated within nodal zones and in the vicinity of amphidromic centers.
Baroclinic disturbances propagating westward in the Kuroshio Extension region as seen by a satellite altimeter and radiometers
Authors: Aoki Shigeru , Imawaki Shiro , Ichikawa Kaoru
Published: February 2004

The Geosat radar altimeter data during the first year of its exact repeat mission are analyzed to investigate the behavior of individual anomalies of sea surface dynamic topography (SSDT) and their statistical properties in the Kuroshio Extension region. The SSDT anomalies are compared with mesoscale anomalies of sea surface temperature (SST) derived from satellite radiometer data. Remarkable baroclinic anomalies are detected in the time series of both SSDT and SST, and their westward propagation is clearly traced. The pinching‐off of a Kuroshio Extension meander at 147°E is clearly seen as the coalescence of two cyclonic anomalies which had moved fairly constantly westward without changing their relative positions. Westward propagation is statistically dominant in the SSDT and SST anomaly field. Approximate agreement between SSDT and SST anomalies suggests the baroclinic nature of the anomalies. The westward phase speed of SSDT anomalies is faster than the theoretical phase speed of the baroclinic first‐mode long Rossby wave in the upstream region of the Kuroshio Extension. In the downstream region it is consistent with the theoretical value but is somewhat faster around 35°N. In the area between 170°E and 180°, SSDT anomalies have a structure like plane waves with major axis oriented NW‐SE in the northern part and NE‐SW in the southern part. The distribution of the Reynolds stress suggests the tendency of acceleration of the mean current by those anomalies. This strong acceleration tendency at 170°E‐180° may be related to bathymetry.
Bulk transfer coefficients and dissipation‐derived fluxes in low wind speed conditions over the western equatorial Pacific Ocean
Authors: Greenhut Gary K. , Khalsa Siri Jodha Singh
Published: February 2004

Bulk transfer coefficients are derived using latent heat flux, stress magnitude, and mean quantity data obtained aboard a National Oceanic and Atmospheric Administration research aircraft flying over the far western equatorial Pacific Ocean. At wind speeds less than about 4 m s−1, the bulk transfer coefficients increase rapidly with decreasing wind speed. The validity of the inertial dissipation method for deriving turbulent fluxes in low wind speed conditions is tested using the aircraft data. At wind speeds below 4 m s−1, dissipation‐derived fluxes tend to be smaller than the corresponding eddy correlation fluxes by about 10–20%.
Measurements of net and gross O2 production, dark O2 respiration, and 14C assimilation at the Marine Light‐Mixed Layers site (59°N, 21°W) in the northeast Atlantic Ocean
Authors: Langdon Christopher , Marra John , Knudson Carol
Published: February 2004

Rates of primary production and respiration were determined at a moderately eutrophic site (1–4 mg Chl a m−3) in the northeast Atlantic Ocean from in vitro changes in dissolved oxygen and 14C uptake. The oxygen rates were measured with new instruments called productivity autosamplers (PA's), which automate the process of water sampling, incubation, and O2 measurement. Measurements of net and gross production and respiration were obtained at two depths over an 8‐day period in May and a 7‐day period in August 1991. Carbon 14 uptake was measured in dawn‐to‐dusk and 24‐hour incubations at six depths during the same periods. The results indicate that gross O2 production and 14C uptake were tightly coupled; the ratio of gross O2 production to dawn‐to‐dusk 14C uptake averaged 1.4±0.1 (SE). Net autotrophic O2 production, estimated as gross O2 production minus a floristically determined chlorophyll a specific respiration rate times the chlorophyll a concentration, was compared for the first time with 14C uptake. These two processes were not tightly coupled; the ratio of net autotrophic O2 production to 14C uptake varied from 1.0 at 2 m to 0.2 at 15 m. Particulate organic carbon (POC) turnover rates (kpoc) in the mixed layer, estimated from day‐to‐day changes in the water column POC inventory, were compared with turnover rates estimated from the in vitro O2 and 14C incubations. The 24‐hour 14C‐based turnover rates (k14C) exhibited little relationship to in situ POC turnover rates. Turnover rate based on 24‐hour net community O2 production (kO2) converted to carbon using a molar ratio of O2 produced to CO2 assimilated (PQ) of 1.4 was a more useful indicator of kPOC.
An in situ experimental study of young sea ice formation on an Antarctic lead
Authors: Melnikov Igor
Published: February 2004

Three series of experimental results were obtained in situ during the U.S.‐Russian Ice Station Weddell 1 Expedition 1992 in the western Weddell Sea. Changes in salinity, silicate, and chlorophyll a concentrations were examined over sampling scales of hours, days, and months as microalgal populations grew in young sea ice. Sea ice growth rates were 0.38 cm h−1 for ice up to 9 cm (May 19–20), 0.13 cm h−1 for ice growth to 28 cm over the next 8 days, and 0.03 cm h−1 during 81 days of observations on ice 42–97 cm thick (March 18–June 7). It was shown that at the initial stage of ice formation, salt and nutrient accumulation occurred, then ice desalination intensified with increasing ice thickness. Brine within the ice showed a 12‐hour period of oscillatory motion during the first day of ice growth and a 1.5‐ to 2‐hour oscillation in the skeletal layer of 28‐cm ice. Small numbers of diatoms were entrapped from seawater during the initial ice formation. Their reproduction (in terms of chlorophyll a concentration) markedly increased after the third day of ice formation. The highest concentrations of chlorophyll a (100 to 1000 times higher than the underlying seawater) were recorded within the bottom, brown‐colored layer of all young ice cores studied during the 81‐day experiment. The species composition of ice algal populations (99 species) was more diverse and rich than observed for phytoplankton (18 species) in surface seawater. The temporal and spatial distribution of all parameters studied were controlled by meteorological factors and brine drainage mechanisms.
The ocean response to operational westerly wind bursts during the 1991–1992 El Niño
Authors: Kindle John C. , Phoebus Patricia A.
Published: February 2004

Numerical simulations of the remotely forced ocean response to westerly wind bursts prior to and during the 1991–1992 El Niño are examined; the models are forced by wind stress from the U.S. Navy's atmospheric global operational analysis/forecast system. The study focuses on (1) the relative response of the first and second internal modes to a single episode of westerly bursts; (2) the role of westerly bursts in producing the eastern Pacific sea level variations from October 1990 to February 1992; and (3) the relative importance of the remotely forced sea level responses generated by central and western Pacific wind anomalies. The simulations use the Naval Research Laboratory global multilayer formulation; the suite of experiments includes hydrodynamic simulations that use both one‐ and three‐active‐layer reduced gravity models as well as an experiment that also includes thermodynamic effects. The models are executed from January 1, 1990, to March 1, 1992, a period that includes 10 significant westerly wind bursts or burst clusters and the 1991–1992 El Niño event. The numerical experiments reveal an ability to accurately simulate the eastern Pacific sea level variations during this period. In response to a single burst the three‐layer hydrodynamic simulation reveals that the second internal mode Kelvin wave yields a sea level change at the eastern boundary that is approximately one third that of the first mode and a surface velocity signature that is equivalent to the first mode. During the onset of the El Niño event the inclusion of higher modes also produces a more realistic representation of the observed eastern boundary sea level signal. Furthermore, by comparing the model response to particular wind bursts with the observed sea level at Baltra, Galapagos, a value of 2.5–2.6 m/s is suggested as the most appropriate mean speed for the first internal mode Kelvin wave during the onset phase. The models reveal the following scenario for the onset of the 1991–1992 El Niño. The eastern boundary sea level exhibited three, distinct pulselike events superimposed on a general rise; the observed sea level pulses occurred in October–November, December–January, and February. The numerical simulations indicate that the pulses in December–January and February were the result of three powerful, westerly wind bursts in the western Pacific preceded by westerly anomalies in the central Pacific. The Kelvin wave pulse generated in the western Pacific by the wind burst in late December to early January was reinforced by a strong central Pacific westerly wind anomaly in mid‐January. These westerly wind events were associated with the development of western Pacific tropical cyclones near the equator in one or both hemispheres.
Horizontally integrated atmospheric heat flux from an Arctic lead
Authors: Glendening John W.
Published: February 2004

Vertical distribution of the horizontally integrated heat injected into the atmosphere by an Arctic lead is investigated, based upon large‐eddy simulations of its convective plume. The horizontally integrated heat flux varies exponentially with height except under conditions of relatively weak cross flow, when vertical advective transport becomes significant. Its vertical scale, which depends upon the lead size, surface heat flux, atmospheric stratification, and cross‐lead wind component, is approximately one fourth of the maximum plume height, indicating that the depth of largest heat transfer is significantly shallower than the depth of the plume. A parameterization is developed to incorporate this small‐scale heat injection into large‐scale models which cannot resolve individual leads. The amount of heat recaptured by the ice downwind of a lead is significant when cross‐lead flow is relatively strong and when the plume depth is shallow.
The effect of sea ice on the solar energy budget in the atmosphere‐sea ice‐ocean system: A model study
Authors: Jin Zhonghai , Stamnes Knut , Weeks W. F. , Tsay Si‐Chee
Published: February 2004

A coupled one‐dimensional multilayer and multistream radiative transfer model has been developed and applied to the study of radiative interactions in the atmosphere, sea ice, and ocean system. The consistent solution of the radiative transfer equation in this coupled system automatically takes into account the refraction and reflection at the air‐ice interface and allows flexibility in choice of stream numbers. The solar radiation spectrum (0.25 μm‐4.0 μm) is divided into 24 spectral bands to account adequately for gaseous absorption in the atmosphere. The effects of ice property changes, including salinity and density variations, as well as of melt ponds and snow cover variations over the ice on the solar energy distribution in the entire system have been studied quantitatively. The results show that for bare ice it is the scattering, determined by air bubbles and brine pockets, in just a few centimeters of the top layer of the ice that plays the most important role in the solar energy absorption and partitioning in the entire system. Ice thickness is important to the energy distribution only when the ice is thin, while the absorption in the atmosphere is not sensitive to ice thickness variations, nor is the total absorption in the entire system once the ice thickness exceeds about 70 cm. The presence of clouds moderates all the sensitivities of the absorptive amounts in each layer to the variations in the ice properties and ice thickness. Comparisons with observational spectral albedo values for two simple ice types are also presented.
Gulf Stream meanders in the South Atlantic Bight: 2. Momentum balances
Authors: Miller Jerry L. , Lee Thomas N.
Published: February 2004

Gulf Stream meanders and their associated frontal eddies have been numerically simulated using a model tuned to the southern portion of the South Atlantic Bight. A run which produced events most similar to those observed in nature was identified, and the dynamics of a particular meander event were examined in detail. A complex array of momentum balances are obtained, and they vary with position within the feature. While the along‐isobath flow is predominantly in geostrophic balance, several of the nonlinear advection terms in both horizontal momentum equations play important roles in the space‐time development of the flow. In particular, the vertical advection of horizontal momentum as well as frictional dissipation are now recognized as important contributors to the dynamical balances in the vicinity of the steep continental slope. It would appear that the semigeostrophic equations constitute the simplest description of this system.
Gulf Stream meanders in the South Atlantic Bight: 1. Scaling and energetics
Authors: Miller Jerry L. , Lee Thomas N.
Published: February 2004

As the Gulf Stream flows along the outer edge of the southeastern United States' continental shelf, it meanders on and offshore, creating frontal eddies which influence the exchange of water between the shelf and the adjacent Atlantic Ocean on weekly timescales. The dynamics controlling the development of such features in this region, the “South Atlantic Bight” or SAB, is the subject of this pair of papers. In the present paper we provide the background on the subject, summarize the length, time, and velocity scales associated with these features, and discuss meander energetics as simulated by a numerical model. The scaling indicates that significant ageostrophic dynamics may be crucial to the development of these flows and that the semigeostrophic equations may provide the simplest description of the system. These nonlinear dynamics may permit complex interactions among meander modes and with topography resulting in the observed behavior of the Gulf Stream in the SAB. The Princeton Ocean Model was tuned for conditions which prevail in the southern South Atlantic Bight, and idealized numerical simulations of Gulf Stream meanders and associated frontal eddies were performed. The system was found to be sensitive to both the amplitude and period of a small perturbation applied at the southern boundary. The energetics indicate that the development of these features is controlled by a mixture of both baroclinic and barotropic instabilities of the mean Gulf Stream flow. Tantalizing hints of a period‐doubling phenomenon are present in the energy conversion time series.
The freshwater budget and under‐ice spreading of Mackenzie River water in the Canadian Beaufort Sea based on salinity and 18O/16O measurements in water and ice
Authors: Macdonald Robie W. , Paton David W. , Carmack Eddy C. , Omstedt Anders
Published: February 2004

Observations of salinity and oxygen isotope composition (δ18O) were made for the Beaufort shelf‐Mackenzie estuary waters in September 1990, just prior to ice formation, and for both the water column and ice in April–May 1991, at the end of winter. These measurements are used to determine the apportioning of fresh water in the estuary between its two main sources, runoff and sea ice melt. Changes in disposition of water between seasons and amounts frozen into the growing ice sheet are also derived. Two domains are considered in order to construct a freshwater budget for the Mackenzie shelf, the nearshore within which landfast ice grows in winter and the outer shelf. Most of the winter inflow from the Mackenzie River appears to remain impounded as liquid under the ice within the landfast zone at the end of winter, and about 15% of it is incorporated into the landfast ice. Oxygen isotopes (δ18O) in ice cores collected from across the shelf record the progress beneath the ice of new Mackenzie inflow as it invades the nearshore throughout winter. Rates of spreading are about 0.2 cm s−1 away from the coast and 1.3 cm s−1 along the coast. As this inflow spreads across the shelf, it progressively shuts off convection driven by brine production at locations within the landfast ice. Salinity and δ18O in the offshore water column suggest that about 3 m of sea ice was formed in the outer shelf domain. Since both brine and newly formed sea ice can be advected off the shelf, a complete budget for brine or sea ice production cannot be established without first measuring the advection of one of these two components.