Showing papers on "Swell published in 1992"

Wave climate of the Sydney region, an energetic and highly variable ocean wave regime

Abstract: Sydney is located in the south-west Pacific at 34°s and receives waves generated in the southern Coral and Tasman Seas. Waves are generated by five meteorological systems: tropical cyclones, east-coast cyclones, mid-latitude cyclones, zonal anticyclonic highs and local summer seabreezes. An examination of 20 years of Sydney wave data reveals that each source has a characteristic seasonality, location and spectrum, permitting the wave climate to be decomposed accordingly. Although at first glance the annual wave climate is both energetic and highly variable, a distinct seasonality is present, with the months of February-March and June experiencing the largest average monthly wave heights. Although moderate waves dominate the climate, extreme waves (>4 m) and/or low swell may occur in any month.

Assimilation of altimeter data in a global third-generation wave model

Abstract: We investigated the effect of the assimilation of altimeter satellite data in the third-generation ocean wave model WAM. We used a sequential method, where analyzed significant wave height fields are created by optimum interpolation, and the analyzed values are then used to construct the analyzed wave spectrum. The method provides also an estimate of the surface stress showing the possibility of using the analysis of the wave spectrum to derive an analyzed surface stress field. In a first set of numerical experiments, the data, provided by the Seasat altimeter, have been assimilated in the WAM model for 1½ days. The comparison between model results and satellite data during the continuation of the run shows a positive and persistent impact of the assimilation. In a second set of numerical experiments, Geosat altimeter data were assimilated for 10 days and the resulting analysis was compared with buoy data. Although the assimilation improves the model results, it is not capable of compensating the differences between model and buoys. Some failures are clearly derived from the absence in the satellite data of the high-wave events that were reported by the buoys. Other failures may be the consequence of an excessive swell attenuation in the WAM model, which compromises the effect of a previous correction. In fact, the comparison of WAM model results with altimeter data suggests that there is a tendency of the model to overevaluate initially the wind sea, and successively to overestimate the decay of the wave energy, when the waves leave the area of the storm.

Effects of Numerics on the Physics in a Third-Generation Wind-Wave Model

Abstract: Numerical errors in third-generation ocean wave models can result in a misinterpretation of the physics in the model. Using idealized situations, it is shown that numerical errors significantly influence the initial growth, the response of wave fields to turning winds, the scaling behavior of model results with wind speed, and the propagation of swell. Furthermore, the numerics may influence the dynamic interaction between wind sea and swell. Surprisingly, fetch-limited model behavior is hardly influenced by numerical errors in wave propagation. Simple modifications of the numerics are presented to reduce or eliminate such errors. The impact of numerical improvements for realistic conditions is illustrated by performing hindcasts for the Atlantic basin and for a smaller region off the east coast of the United States.

Observations of infragravity waves

Abstract: Infragravity-wave (periods of one-half to a few minutes) energy levels observed for about 1 year in 8-m water depth in the Pacific and in 8- and 13-m depths in the Atlantic are highly correlated with energy in the swell-frequency band (7- to 20-s periods), suggesting the infragravity waves were generated locally by the swell. The amplification of infragravity-wave energy between 13- and 8-m depth (separated by 1 km in the cross shore) is about 2, indicating that the observed infragravity motions are dominated by free waves, not by group-forced bound waves, which in theory are amplified by an order of magnitude in energy between the two locations. However, bound waves are more important for the relatively few cases with very energetic swell, when the observed amplification between 13- and 8-m depth of infragravity-wave energy was sometimes 3 times greater than expected for free waves. Bispectra are consistent with increased coupling between infragravity waves and groups of swell and sea for high-energy incident waves.

Bubble clouds and the dynamics of the upper ocean

Abstract: This is a review of turbulence in the upper ocean close to the sea surface, particularly of the information that has been obtained from sonar observations of bubble clouds produced by breaking wind waves. These clouds provide tracers of the turbulent motions and are important, especially at high wind speeds, in the process of air-sea gas transfer. the observations of bubble clouds are here related to other measurements of turbulence, particularly to direct measurements of currents and temperatures in lakes or at sea, and to laboratory studies. Some novel observations of bubble clouds and breaking waves, their frequency and relation to Langmuir circulation, are presented. There is now emerging a pattern of clues that point to the dominance of breaking surface gravity waves as a source of turbulence to a depth below the surface of 0.04 to 0.2 times the wavelength of the dominant breaking waves, although the effect of swell has yet to be clarified. the relative depth appears to increase with increasing values of W,1/c, where W10 is the wind speed and c the phase speed of the dominant waves. Below this region the generation of turbulence may be dominated by shear-stress or convection. Here, turbulence is generally similar to that in the atmospheric boundary layer. There are, however, coherent motions on the scale of the mixing layer that persist for periods of a few minutes to an hour or so, to which the transport of a large part of the momentum and heat fluxes can be attributed, and which strongly affect the dispersion of buoyant particles or bubbles. These motions deserve special study to establish their contribution to heat and momentum transport, and hence to determine if, or when, they should be specifically represented in models of the upper ocean devised, for example, to describe the dispersion of passive and non-passive tracers or the air-sea transfer of gases.

Bound infragravity waves

Abstract: Model predictions of bound (i.e., nonlinearly forced by and coupled to wave groups) infragravity wave energy are compared with about 2 years of observations in 8- to 13-m depths at Imperial Beach, California, and Barbers Point, Hawaii. Frequency-directional spectra of free waves at sea and swell frequencies, estimated with a small array of four pressure sensors, are used to predict the bound wave spectra below 0.04 Hz. The predicted total bound wave energy is always less than the observed infragravity energy, and the underprediction increases with increasing water depth and especially with decreasing swell energy. At most half, and usually much less, of the observed infragravity energy is bound. Bound wave spectra are also predicted with data from a single wave gage in 183-m depth at Point Conception, California, and the assumption of unidirectional sea and swell. Even with energetic swell, less than 10% of the total observed infragravity energy in 183-m depth is bound. Free waves, either leaky or edge waves, are more energetic than bound waves at both the shallow and deep sites. The low level of infragravity energy observed in 183-m depth compared with 8- to 13-m depths, with similarly moderate sea and swell energy, suggests that leaky (and very high-mode edge) waves contribute less than 10% of the infragravity energy in 8–13 m. Most of the free infragravity energy in shallow water is refractively trapped and does not reach deep water.

Morphology and tectonics of the Mid-Atlantic Ridge (25°–27°30′S) from Sea Beam and magnetic data

Abstract: High-resolution Sea Beam bathymetry and sea surface magnetic data have been collected between 25°S and 27°30′S on the Mid-Atlantic Ridge. Analysis of the morphology and structure of the ridge axis and flanking terrain as well as the results of a three-dimensional inversion of sea surface magnetic data are presented. This 227-km-long portion of the Mid-Atlantic Ridge is offset 42 km by the right-lateral Rio Grande Transform at 25°40′S and 9.5 km by the left-lateral nontransform Moore Discontinuity at 26°30′S. A third nontransform discontinuity, the Midway Discontinuity, exists near 26°12′S and is defined by a small displacement of the axial magnetization distribution and the disruption in the along-strike continuity of ridge-parallel terrain elements. Although the detailed survey presented in this paper extends out to only 5- to 7-m.y.-old crust, a regional compilation of magnetic data from this area by Cande et al. (1988) indicates that the relative positions and dimensions of the spreading cells have characterized this part of the Mid-Atlantic Ridge for at least the past 55 m.y. The offsets dimensions and geometries, however, have changed markedly because of prolonged differential asymmetric spreading between segments. Between and within the accretionary units that are spreading at uniform rates (35 mm/yr total opening rate), the morphology of the ridge axis varies dramatically, from a well-defined rift valley (20–25 km wide, 1–1.5 km deep) to an axial swell that rises 1500 m above the adjacent valley floor. The axial magnetic structure is also highly variable along-strike. The inversion solution shows a series of generally linear, short-wavelength (<10 km) magnetization highs within the central Brunhes anomaly over each of the four segments in the survey area. These anomalies are interpreted to be the central anomaly magnetization high that is observed over the neovolcanic zones of many of the world's mid-ocean spreading centers. The distinct axial magnetization highs can extend out to 2-m.y.-old crust at ridge-discontinuity intersections, suggesting that in some instances they may also be the product temporal variations in the geochemistry of the extrusive rocks, e.g., highly fractionated and strongly magnetized basalts enriched in iron and titanium. The magnetic inversion shows that the central anomaly is locally attenuated at the minimum depth points of three of the four spreading segments in the survey area. Both inversion and forward modeling of magnetic data show that topography alone cannot produce the anomalously low values observed. In the case of an axial swell centered at 26°S, attenuation of the central anomaly is attributed to the effective thinning of the source layer by demagnetization effects of a crustal thermal anomaly and/or a zone of substantial alteration. Taken together, the bathymetric data and magnetic modeling document strong along-axis gradients in crustal and upper mantle properties.

Spectral Modeling of Sea States With Multiple Wave Systems

Abstract: The spectral models of individual wave systems have one peak and are described by theoretical models that have gained general acceptance. This work deals with sea states with more than one wave system, leading to spectral models with two or more peaks. Use is made of spectra derived from measurements off the Portuguese Coast and data is provided as to their probability of occurrence as well as about the dependance of the spectral parameters on the significant wave height and peak period. It is shown that wind dominated and swell dominated two-peaked spectra tend to occur in different areas of the scatter diagram. The spectral parameters of the two peaked spectra show little correlation with significant wave height and peak period.

Gravity‐Wave Observations Using an Array of Microbarographs In the Alearic Islands

Abstract: Time series of surface pressure from an array of microbarographs in the Balearic Islands (western Mediterranean) are used to obtain wavelength, phase speed and direction of propagation of gravity waves on the Balearic Islands. These pressure waves are commonly found during the summer and they force the large seal-level oscillations observed in the inlet of Ciutadella; a phenomenon locally known as ‘rissaga’. Four events are described here, each of which has a duration of between 30 and 48 hours. Radiosonde data from Palma de Mallorca are used to relate the waves to the vertical structure of the atmosphere. the various source mechanisms of these waves and also the possibility of the existence of a duct layer trapping the wave energy near the surface are discussed. the waves are found to have no significant variation of phase speed with frequency and so they are basically non-dispersive. This is an indicator pointing either to a local generation of gravity waves by wind-shear instability or to the existence of neutral trapped waves, in the lower troposphere.

The bottom boundary layer of the bay stem plains environment of lower Chesapeake bay

Abstract: Field observations of bottom boundary layer processes in the bay stem plains environment of lower Chesapeake Bay (depth 11–12 m) were made using a bottom mounted tripod supporting electromagnetic current meters and a pressure sensor. The Prandtl-von Karman equation, the inertial dissipation method, and the Grant and Madsen wave-current interaction model were applied to the field data to estimate time-varying bed stress and to evaluate the likely contributions of waves and currents to total shear stress and to skin friction. Biogenic bed micromorphology dominated bed roughness. A background signal of long period, low amplitude swell was present for most of the time. Although these oscillations were too weak to agitate the bottom sediments, interactions of the swell with tidal currents in the presence of a biogenically-roughened bottom, caused the hydraulic roughness height, z ' 0 and drag coefficient, C D , ‘sensed’ by the mean current to be significantly increased. In addition, internal waves with frequencies in the neighbourhood of the pycnoclinal Brunt-Vaisala frequency (period range approximately 60–100 s) appeared intermittently and caused near-bottom velocity fluctuations with amplitudes of over 10 cm s −1 .

Orientation to oceanic waves by green turtle hatchlings

Abstract: Summary Minutes after emerging from underground nests, hatchling green turtles (Chelonia my das L.) enter the sea and begin a migration towards the open ocean. To test the hypothesis that migrating hatchlings use wave cues to maintain their seaward headings, we released turtles offshore during unusual weather conditions when waves moved in atypical directions. Hatchlings swam into approaching waves in all experiments, even when doing so resulted in orientation back towards land. These data suggest that green turtle hatchlings normally maintain seaward headings early in the offshore migration by using wave propagation direction as an orientation cue. Because waves and swells reliably move towards shore in shallow coastal areas, swimming into waves usually results in movement towards the open sea. The physiological mechanisms that underlie wave detection by sea turtle hatchlings are not known. Calculations indicate that, at the depth at which hatchlings swim, accelerations produced beneath typical waves and swells along the Florida coast are sufficient to be detected by the vertebrate inner ear. We therefore hypothesize that hatchlings determine wave direction while under water by monitoring the sequence of horizontal and vertical accelerations that occur as waves pass above.

Extrudate swell of high density polyethylene. Part II: Time dependency and effects of cooling and sagging

Abstract: For two high density polyethylene resins, the isothermal time dependency of extrudate swell has been measured. Very minor differences in the large molecular weight part of the molecular weight distribution, hardly detectable with gel permeation chromatography and low angle laser light scattering techniques, dramatically influence the time dependency of extrudate swell as well as the maximum swell attainable. The presence of larger molecules in sample 802 than in 801 is reflected in a lower short time (after seconds) and a larger long time (after minutes) or maximum extrudate swell value. Extruding the polymers through a capillary die L:D = 30:2 mm into air at ambient temperature allows only the short time swelling behavior to be observed, because cooling and sagging of the strand.

Field observations of orbital velocities and pressure in weakly nonlinear surface gravity waves

Abstract: Field measurements of wave orbital velocities and pressure, collected in the lower part of the water column in 7 m depth with a three-component acoustic Doppler current meter and a co-located pressure transducer, are compared to the second-order theory for weakly nonlinear surface gravity waves in arbitrary water depth (Hasselmann 1962) Pressure and velocity spectra and cross-spectra are in excellent agreement with (linear) free wave transfer functions, even at (and higher than) twice the spectral peak frequency where nonlinearities (forced secondary waves) are expected to be important Theoretical predictions show that although secondary waves sometimes contribute a significant fraction of the energy observed at double swell and sea frequencies, their effect on velocity-pressure transfer functions is small However, forced waves are more apparent in deviations from Gaussian statistics Good agreement between observed and predicted third-order statistics shows that Hasselmann's weakly nonlinear theory accurately describes the secondary pressure and orbital velocity (both horizontal and vertical components) field at double swell and sea frequencies, even for moderately large (0(01–02)) values of the nonlinear perturbation parameter Only with near-breaking swell and relatively strong nonlinearities (perturbation parameter ≈ 022), do the observed third-order statistics diverge significantly from Hasselmann's theory

The validity of the composite surface model and its applications to the modulation of radar backscatter

Abstract: The composite surface model is compared with the Kirchoff method and shown to be equivalent for radar backscatter at moderate incidence angles in most commonly occurring sea states. Based on this comparison, limits are determined for the validity of the composite surface model. The model is then utilized to study the modulation of the radar cross-section of the ocean surface by long surface waves and internal waves. It is shown that apart from the well understood tilt modulation and the direct hydrodynamic modulation of the short waves, there is also an additional indirect modulation of the radar cross-section due to the hydrodynamic modulation of the intermediate waves by the long waves. Computer simulations are used to investigae these modulation mechanisms for different sea states, incidence angles and radar frequencies. It is shown that this modulation is strongest at small incidence angles and high radar frequencies. The simulations show that internal waves may be observed by X-band and Ka-b...

Two-dimensional wave number spectra of short wind waves: results from wind-wave facilities and extrapolation to the ocean

Abstract: A detailed study of 2-D wave number spectra of short water surface waves is presented. Using a refraction-based optical technique either the along-wind or the cross-wind slope is visualized in image sectors of up to 30 X 40 cm 2 . The resolution of the images is high enough (down to 1/3 mm) to resolve even the smallest capillary waves. The measurements were performed in the wind/wave facility of the IMST (University of Marseille, France) at 5 through 29 m fetch, the Delft wind wave flume (The Netherlands) from 6 to 100 m fetch, and the 4 m-diameter circular wind/wave facility of the Institute for Environmental Physics at the University of Heidelberg (Germany). A first preliminary analysis of the data is given. The angular dispersion of the waves is most sensitively influenced by the geometry of the facility, especially the width of the water channel. Therefore, it is hardly possible to extrapolate the measured angular dispersion of the waves to the ocean. The unidirectional and along-wind wave number spectra, however, show clear trends which allow for an extrapolation to the ocean. At high fetches and wind speeds, the spectral densities for the wave height are proportional to (kappa) -3.5 well into the capillary wave region until a sharp and almost wind speed independent cutoff occurs at (kappa) approximately equals 1100 m -1 ((lambda) approximately equals 0.6 cm). The increase of the spectral densities with friction velocity depends both on wave number and fetch. While the spectral density for small gravity waves depends only weakly on the friction velocity, it increases strongly at higher wave numbers. Generally, this steepness is smaller at higher fetches.

Statistical Study on the Local Equilibrium between Wind and Wind Waves by Using Data from Ocean Data Buoy Stations

Abstract: The local equilibrium between the wind and wind waves, which is defined by a range of the coefficient of the 3/2-power law between the non-dimensional significant wave height and period, is statistically investigated by using wind and wave data obtained at four ocean data buoy stations in the seas near Japan. The friction velocity is calculated from the wind speed measured at one height together with the significant wave period by using formulas of the wave dependent drag coefficient proposed by Tobaet al. (1990). The data for small waves or for weak winds indicate that the waves do not satisfy the criterion for the local equilibrium, because they may be affected by changing winds or remotely generated swells. In the seas near Japan, the data which satisfy the local equilibrium are about 6% through a year. Otherwise swells are dominant in most situations. Changing winds also cause deviations from the local equilibrium. The degree of satisfaction of the local equilibrium can be classified by ranges of the significant wave height. As the significant wave height exceeds 4 m, the local equilibrium is more frequently satisfied.

Wind-Wave Nonlinearity Observed at the Sea Floor. Part II: Wavenumbers and Third-Order Statistics

Abstract: This is Part 2 of a study of nonlinear effects on natural wind-generated surface gravity waves in 13-m depth, 30 km offshore of Virginia. At the sea floor in this depth, free surface gravity waves are only weakly attenuated at sea and swell frequencies (0.05–0.30 Hz) but are very strongly attenuated at frequencies higher than about 0.35 Hz. Hence, above 0.35 Hz, relatively long wavelength forced waves, excited by nonlinear interactions between directionally opposing free wind waves, are exposed at the sea floor. An array of pressure transducers at middepth was used to estimate the frequency-directional spectrum of (free) primary sea and swell waves, and the associated (forced) secondary pressure fluctuations were measured with an array on the sea floor. In Part 1, it was shown that forced-wave energy levels at the sea floor increase sharply in response to directionally opposing wind waves, in agreement with weakly nonlinear theory. In Part 2, wavelengths, propagation directions, and non-Gaussian ...

Wind and Wind Waves by Using Data from Ocean Data Buoy Stations

Abstract: (Received 14 October 1991; in revised form 29 November 1991; accepted 3 December 1991) The local equilibrium between the wind and wind waves, which is defined by a range of the coefficient of the 3/2-power law between the non-dimensional significant wave height and period, is statistically investigated by using wind and wave data obtained at four ocean data buoy stations in the seas near Japan. The friction velocity is calculated from the wind speed measured at one height together with the significant wave period by using formulas of the wave dependent drag coefficient proposed by Toba et al. (1990). The data for small waves or for weak winds indicate that the waves do not satisfy the criterion for the local equilibrium, because they may be affected by changing winds or remotely generated swells. In the seas near Japan, the data which satisfy the local equilibrium are about 6% through a year. Otherwise swells are dominant in most situations. Changing winds also cause deviations from the local equilibrium. The degree of satisfaction of the local equilibrium can be classified by ranges of the significant wave height. As the significant wave height exceeds 4 m, the local equilibrium is more frequently satisfied.

Water-level oscillations in esperance harbour

Abstract: Measurements of water‐surface elevation in a harbor were analyzed to determine the forcing mechanisms causing the excessive motion of moored ships. The spectral density of water‐surface elevation measured at four locations is presented by a time‐frequency distribution. A numerical model based on a modified mild slope equation approximation of wave refraction, diffraction, and reflection in and around the harbor was employed to simulate the transformation of waves entering the harbor. The performance of the model was validated against the field data by comparing predicted and observed wave heights at field measurement locations within the harbor; deviations of predicted ratios from those observed are discussed. Ship motions recorded at berth during the field study were analyzed in context of the concurrent wave spectra in order to isolate the dominant forcing mechanisms causing the moored ship response. The groupiness of incident swell waves was analyzed to investigate the origin of long‐period (>40sec) wa...

Data assimilation with a coupled wind-wave model

Abstract: A first attempt to assimilate observations into a coupled wind-wave model system is presented. A special feature of the wind model is that the drag coefficient depends on the wave age, reflecting the dependence of the air flow on the wave state. Usually, both wind and wave data are assimilated. The assimilation is carried out by means of the adjoint technique. This coupled wind-wave data assimilation not only corrects successfully both model fields, but their dynamical connection is retained. Furthermore, the assimilation of only wave data results in enough information on the atmospheric state at the sea surface that it can correct both wave and wind fields as well. Moreover, there is still some impact of the assimilation during the forecast period. This persistence is found to be dependent on the typical relaxation time scale of the wind model, and also to be related to the swell decay time scale due to wave breaking. The computations reveal the importance of both the coupling and the coupled wind-wave data assimilation scheme that has been developed.

Modelling of laterally trapped surface waves with application to Rayleigh waves in the Hawaiian swell

Abstract: SUMMARY A method for synthetizing surface wave seismograms in anelastic 2-D structures is presented. It is based on the local mode coupling method, extended here to allow for propagation in any direction, and especially in the symmetry direction of the structure. This implies including both homogeneous and inhomogeneous waves in the wavefield representation. The complex wavenumbers of inhomogeneous waves require that a dual space of modes and a new bi-orthonormality relation between modes are defined. Combinations of local modes which are not singular for modes having an horizontal turning point in the 2-D structure are used as a basis for wavefield decomposition. This allows calculation of the propagation and coupling characteristics of homogeneous and inhomogeneous waves with a single set of equations. The mode coupling matrices are then combined with source terms and boundary conditions to yield synthetic seismograms with a procedure similar to the one used in the reflectivity method in 1-D structures. The method is applied to model Rayleigh wave fundamental modes propagating along the Hawaiian chain, in order to refine the interpretation of phase velocities measured for this area by LCvCque (1991). The a priori reheated and thinner lithosphere under the chain acts as lateral low-velocity waveguide where Rayleigh waves are trapped. We show that this waveguide has three lateral free modes in the period range 20 to 150 s. Synthetic seismograms for different source-receiver configurations are presented. The influence of the source-type, epicentral distance and channel width on the waveguide's amplification power is examined in the frequency domain. Apparent phase velocities between two stations situated in the middle of the channel are calculated, and the influence of the outer parts of the waveguide on these velocities is analysed. It is found that the velocities measured by LCvCque are probably influenced at 20 to 30 per cent by the normal lithosphere outside the reheated channel.

Laboratory studies on the surface drift current induced by wind and swell

Abstract: Systematic measurements of the surface drift current, the wind profile over the water surface and the wave spectra have been made for (i) pure wind-waves, (ii) a co-existing system of wind-waves and swell propagating against the wind, and (iii) a co-existing system of wind-waves and swell propagating in the direction of the wind. The surface drift current is gradually intensified by the swell propagating against the wind when the swell steepness increases. The maximum increase of the surface drift velocity caused by the opposing swell is about 46% of the surface drift velocity for pure wind-waves at the same wind speed. Such a phenomenon was not observed when the swell was propagating in the direction of the wind.

Developments in ocean wave measurement by HF radar

Abstract: Describes improvements that have been made to a method of inversion of the HF radar Doppler spectrum to provide a measurement of the ocean wave directional spectrum. The ability to make such measurements over the whole HF band using the same inversion method is demonstrated. Measurements obtained in storm conditions, in fetch-limited conditions and in mixed swell, wind-sea conditions are presented. The accuracy of these measurements is discussed.

Swell versus saturation for compacted clay

Abstract: The purpose of this study is to investigate the relationship between swell and degree of saturation for a compacted clay. Laboratory tests indicate that the percent swell increases in direct proportion to the degree of saturation at the end of the test. The linear relationship does not intercept the initial compacted degree of saturation. The coefficient of swell can be affected by the compacted effort. The dry, hard clods are broken down by the higher compactive effort. This reduction in interclod voids probably causes the significantly different coefficient of swell. Swell tests compacted to low, dry densities have a final saturation degree below 90%, even after two cycles of secondary swell. This indicates the presence of entrapped air. The main factor governing entrapped air for this soil appears to be the initial compacted void ratio. By itself, the degree of saturation does not seem to reliably indicate future potential for swell. One problem is the presence of entrapped air, which can result in a s...

Estimation of the Sea-State Bias in Radar Altimeter Geosat Data from Examination of Frontal Systems

Abstract: The sea-state bias is the deviation of the sea surface height as seen by a satellite altimeter due to differential scattering by the crest and troughs of waves and to oversimplification of the radar waveform algorithms that calculate this height. Here the sea-state bias is estimated by automatically analyzing with a least-squares technique the along-track deviation of sea surface in the case of fronts of significant wave height (SWH). Twenty-two thousand profiles of 1500-km length of Geosat altimeter data are analyzed. Since a very strict selection was applied, only nine clear front situations are found. Among these five correspond to atmospheric frontal situations as suggested by the altimetric wind profiles. The sea-state bias is of 3.6% ±0.6% of SWH (2σ error). The four other fronts are interpreted as swell fronts: they present smaller biases of the order of 1.0%±1.6% of SWH. These results are in agreement with experimental studies and justify the calculation of sea-state bias as a function of...