seamount topography by least squares inversion of altimetric geoid heights and shipborne profiles of bathymetry and or gravity anomalies summary altimetric measurements of sea surface heights have long been used to locate uncharted marine features newly available data sets geosat gm ers 1 and topex poseidon now allow the topography of submarine volcanoes to be accurately restored and a method based upon inverse modelling is presented in this paper this method allows a joint assimilation of altimetric data and shipborne profiles of bathymetry and or gravity anomalies when available furthermore uncertainties can be computed together with the topography which constitutes a noticeable improvement since these parameters are still missing in the usual bathymetric data bases tests are performed using simulated data in order to characterize the errors which may or must occur with the modelled topography of a previously unknown volcano in particular the errors due to data uncertainties and the lack of precise knowledge of sensitive parameters that are used in the modelling these errors can be maintained under the 100 m level rms for altimetric data with a measurement noise of 5 cm rms or less which is achieved for geosat ers 1 and topex poseidon altimeters an example of inversion of seasat data is presented in the case of a seamount in french polynesia

sea surface height determination in the arctic ocean from ers altimetry 1 accurate sea surface height measurements have been extracted from ers altimeter data in sea ice covered regions for the first time the data have been used to construct a mean sea surface of the arctic ocean between the latitudes of 60 degreesn and 81 5 degreesn based on 4 years of ers 2 data an rms value for the crossover differences of mean sea surface profiles of 4 2 cm was observed in the ice covered canada basin compared with 3 8 cm in the ice free greenland iceland norwegian seas comparisons are made with an existing global mean sea surface osumss95 highlighting significant differences between the two surfaces in permanently ice covered seas in addition we present the first altimeter derived sea surface height variability map of the arctic ocean comparisons with a high resolution coupled ocean sea ice general circulation model reveal a good qualitative agreement in the spatial distribution of variability quantitatively we found that the observed variability was on average a factor of 3 4 greater than model predictions

three dimensional estimation of elastic thickness under the louisville ridge a three dimensional approach to estimating elastic thickness is presented which uses dense satellite altimetry and sparse ship bathymetry this technique is applied to the louisville ridge system to study the tectonic history of the region the inversion is performed as both a first order approximation and a nonlinear relationship between gravity and topography based on parker s 1973 equation while the higher order effect on the gravity anomaly is nearly zero for most of the region the magnitude is significant over the summits of the ridge nevertheless the inclusion of the nonlinear terms has only a minor influence on the elastic thickness estimate within each region lowering the value by 1 2 km compared with the linear result the incorrect assumption of two dimensionality for circular features exhibits a marked effect on the gravitational anomaly resulting in false sidelobe structure of nearly 20 mgal for large seamounts our elastic thickness estimates are compared with the contradictory values obtained in previous studies by cazenave and dominh 1984 and watts et al 1988 we find an increasing elastic thickness along the chain from southeast to northwest with a discontinuity along the wishbone scarp the jump in elastic thickness values northwest of the scarp appears to be an indication of an age discontinuity caused by an extinct spreading center north of the ridge

seasat altimetry the north atlantic geoid and evaluation by shipborne subsatellite profiles a geological evaluation of geoid anomalies over the north atlantic is presented with emphasis on short 40 300 km wavelength undulations mapped by seasat radar altimetry long 3000 km and intermediate 300 3000 km wavelength undulations constrain subcrustal processes and are briefly reviewed portions of three seasat revolutions revs totaling 9300 km were followed by research ship which measured gravity magnetics topography and for a 800 km segment also seismic reflection data to form a ground truth data set for comparison to the altimetry the intermediate wavelength geoid the residual upon removal of a 14 14 long wavelength surface correlates with plate age slope 10 cm m y over cenozoic crust and major fracture zones appear as geoid steps a 3 5 to 5 m high over the bermuda rise may reflect thermal rejuvenation created within the plate 40 50 ma short wavelength geoid anomalies typically 10 50 cm in the open ocean were analyzed by high pass 200 or 400 km cutoff along track filtering and smoothing the vertical deflection computed from the seasat geoid compares well with one based on seismic reflection shipborne free air gravity except for a higher frequency cutoff correlates closely with geoid anomalies except near seamounts the principal contribution to geoid anomalies is fracture zone topography all major and many minor fracture zones fz s appear as geoid lineations particularly between the 15 20 and kane fz from the ridge axis to anomaly 34 the wsw trend of fz s between anomaly 21 and 34 is clearly expressed in the altimetry as is the smooth geoid over mid cretaceous smooth basement formed by fast spreading crustal ribbons 50 150 km wide of higher or lower geoid parallel fz trends some geoid lineations are oblique to both isochron and fz trends east of the puerto rico trench such anomalies may reflect compressional deformation within atlantic lithosphere

on geoid heights and flexure of the lithosphere at seamounts the sea surface height has now been mapped to an accuracy of better than 1 m by using radar altimeters on board orbiting satellites the major influence on the mean sea surface height is the marine geoid which is an equipotential surface we have carried out preliminary studies of how oceanic volcanoes which rise above the ocean floor as isolated seamounts and oceanic islands or linear ridges contribute to the marine geoid simple one and two dimensional models have been constructed in which it is assumed that the oceanic lithosphere responds to volcanic loads as a thin elastic plate overlying a weak fluid substratum previous studies based on gravity and bathymetry data and uplift subsidence patterns show that the effective flexural rigidity of oceanic lithosphere and the equivalent elastic thickness te increase with the age of the lithosphere at the time of loading the models predict that isolated seamounts emplaced on relatively young lithosphere on or near a mid ocean ridge crest will be associated with relatively low amplitude geoid anomalies about 0 4 0 5 m km of height while seamounts formed on relatively old lithosphere on ridge flanks will be associated with much higher amplitude anomalies 1 4 1 5 m km studies of the seasat altimetric geoid prepared by nasa s jet propulsion laboratory support these model predictions geoid amplitudes are relatively low over the mid pacific mountains and line islands which formed on or near a mid ocean ridge crest and relatively high over the magellan seamounts and wake guyots which formed off ridge direct modeling of the altimetric geoid over these features is complicated however by the wide spacing of the satellite tracks which can exceed 100 km and poor bathymetric control beneath individual satellite tracks in regions where multibeam bathymetric surveys are available models can be constructed that fit the altimetric geoid to better than 1 m studies of geoid anomalies over the emperor seamount chain for example suggest that these seamounts formed on 20 30 ma old oceanic lithosphere while anomalies over the louisville ridge and the hawaiian ridge suggest that these seamounts formed on lithosphere older than about 80 ma this indicates that geoid anomalies over bathymetrie features depend not only on the crustal structure prior to loading and the overall shape and density of the features but also on their tectonic setting we examine the implications of these results to studies which attempt 1 to separate temporal variations of the sea surface due to oceanographic effects 2 to predict seamounts directly from satellite derived gravity and geoid data and 3 to isolate the geoid effects of deep processes occurring beneath the lithosphere such as those due to mantle convection

predicting bathymetry from geosat erm and shipborne profiles in the south atlantic ocean abstract portions of two geosat erm altimeter tracks and corresponding suborbital shipboard gravity and bathymetry profiles in the south atlantic ocean were analyzed one across the walvis ridge about 1100 km long and the other in the brazil basin about 2300 km long together these profiles sample those types of sea floor topography which dominate the gravity signature at wavelengths of 20 to 300 km the walvis ridge is a massive aseismic ridge and the brazil basin profile crosses both an old seamount emplaced at the time the crust was young and a very young mid plate volcano both profiles cross fracture zones after the gravity and bathymetry profiles were split into subprofiles various cross spectral characteristics could be determined by fft techniques analysis showed that observed admittance is not well constrained by either an airy type or flexural compensation models across the walvis ridge but those over the brazil basin can be readily explained by an airy type model with a mean crustal thickness of about 20 km a theoretical filter was then designed based on a priori geological knowledge and used to predict bathymetry from the high passed gravity geoid anomalies not surprisingly the predicted bathymetry shows more detailed and correct short wavelength 20 300 km features than those predicted from the historical data base as represented e g by the dbdb5 gridded bathymetric model for areas where historical shipboard bathymetry measurements are widely spaced longer than about 10 km for single beam data but where some regional geologic information is available such as the relative ages of mid plate volcanoes and crust bathymetry predicted from altimetric data can be used to upgrade regional bathymetrie data bases on which regional geologic geophysical understanding depends

the seasat altimeter data and its accuracy assessment the seasat satellite launched on june 27 1978 carried a radar altimeter designed to measure the altitude of the satellite above the ocean surface the surface wave height and the ocean surface backscatter coefficient from which wind speed can be inferred postlaunch engineering assessment indicated that the seasat altimeter met the performance specification for 10 cm precision noise in the altitude measurements however to determine the accuracy of this measurement as well as the accuracy of the wave height and wind speed measurements a significant calibration validation and model development effort was required this summary describes the instrument atmospheric and geophysical effects which influence the radar altimeter measurement accuracies and the attendant correction models adopted for the altimeter geophysical data record also summarized are the activities of the seasat altimeter precision orbit determination experiment team directed towards the validation and improvement of these models as well as the investigations required to assess the accuracy of the altimeter measurement and the computed satellite altitude ephemeris finally an accuracy assessment is made for the various altimeter measurement corrections which are included on the altimeter geophysical data record

thermal and mechanical properties of the cape verde rise the cape verde rise represents an extreme case of the process of oceanic midplate volcanism the regional depth and geoid anomalies relative to the plate model are the largest observed peaking at 2 4 km and 12m respectively the late tertiary volcanoes rest on mesozoic aged seafloor indicating that the lithosphere was about 130 m y old when loaded depth and geoid anomalies in the cape verde region are examined using both forward filtering and admittance techniques to determine the thermomechanical properties of the swell the effective elastic thickness for the plate supporting the volcanoes is 28 4 km and the compensation depth for the broad swell is 69 10 km these two results combined with heat flow observations from the rise are consistent with a model for reheating of the lithosphere beneath cape verde such that the average thermal gradient is increased by 35 while there is no doubt that the cape verde lithosphere has been reheated it is unlikely that the entire depth anomaly is explained by thermal anomalies confined to the lithosphere a linear programming inversion of the geophysical data places an upper bound of 1 7 km on the portion of the depth anomaly due to lithospheric reheating there are no permissible thermal models fitting all of the geophysical data that do not require either dynamic support from or elevated temperature within the asthenosphere

seafloor tectonic fabric from satellite altimetry abstract ocean floor structures with horizontal scales of 10 to a few hundred kilometers and vertical scales of 100 m or more generate sea surface gravity anomalies observable with satellite altimetry prior to 1990 altimeter data resolved only tectonic lineaments some seamounts and some aspects of mid ocean ridge structure new altimeter data available since mid 1995 resolve 10 km scale structures over nearly all the world s oceans these data are the basis of new global bathymetric maps and have been interpreted as exhibiting complexities in the sea floor spreading process including ridge jumps propagating rifts and variations in magma supply this chapter reviews the satellite altimetry technique and its resolution of tectonic structures gives examples of intriguing tectonic phenomena and shows that structures as small as abyssal hills are partially resolved a new result obtained here is that the amplitude of the fine scale 10 80 km roughness of old ocean floor is spreading rate dependent i

new global seamount census from altimetry derived gravity data summary recent revisions to the satellite derived vertical gravity gradient vgg data reveal more detail of the ocean bottom and have allowed us to develop a non linear inversion method to detect seamounts in vgg data we approximate vgg anomalies over seamounts as sums of individual partially overlapping elliptical polynomial functions which allows us to form a non linear inverse problem by fitting the polynomial model to the observations model parameters for a potential seamount include geographical location peak vgg amplitude major and minor axes of the elliptical base and the azimuth of the major axis the non linear inversion is very sensitive to the initial values for the location and amplitude hence they are constrained by the centre and amplitude of the uppermost contours obtained with a 1 eotvos contour interval with these initial conditions from contouring we execute a step wise and fully automated inversion and obtain optimal model estimates for potential seamounts these are statistically evaluated for significance using the akaike information criterion and f tests a logarithmic barrier technique is applied to ensure positivity of all seamount amplitudes after automatic and manual inspections of the model parameters we estimate actual heights and basal ellipses of the inspected potential seamounts directly from the predicted bathymetry grid in this study we find globally 24 643 potential seamounts h 0 1 km that are located away from continental margins 8458 potential seamounts are taller than 1 km although our global estimate is significantly lower than predictions from previous studies a first order reconciliation of the size frequency statistics obtained from those studies reveals that the previous counts are systematically overestimated because of the ambiguity of gravity signals due to small seamounts of h 1 km and the overlap with abyssal hills we estimate the global seamount census to lie in the 40 000 55 000 range the seamount data from this study are accessible from http www soest hawaii edu pt smts

seasat derived gravity over the musicians seamounts the two dimensional gravity field over the musicians seamount province in the pacific ocean has been derived from seasat altimetry geoid maps were produced by fitting a minimum curvature surface to the sea surface height measurements as a check on the quality of this interpolation method we also gridded the data using weighted grid point averages fourier transforms of both the geoid and geoid gradient were used to determine the gravity field we have compared gravity maps produced these different ways in order to identify the problems involved in pushing seasat data to the limits of its spatial resolution and accuracy our minimum curvature interpolation scheme was determined to be the more accurate and cost effective mapping method while gravity obtained by transforming the geoid produced more reliable gravity maps the bathymetry of this area was used to predict the gravity field by filtering the bathymetry under the assumption that it is regionally compensated by a thin elastic plate gravity fields predicted for a variety of effective elastic thicknesses were compared to the seasat derived gravity particularly in areas with good track coverage the derived gravity tends to favor a thin plate with an effective elastic thickness of 5 km though the east west ridges in the south display a smaller signal more consistent with airy compensation this variation may be indicative of early fracturing of the lithosphere in the south or it may be a manifestation of the age difference and early thermal structure across the murray fracture zone which separates the seamount province into northern and southern sections neighboring seamounts with different flexural signatures particularly in the south may indicate that volcanism occurred in the same location over an extended period of time

ionospheric correction for seasat altimeter height measurement the ionosphere effect on signal speed and hence on the seasat radar altimeter measurement is directly proportional to the columnar electron content of the ionosphere this paper contains a description of the faraday rotation technique used by the seasat project to measure the ionosphere and of the mapping scheme used to map the measurements to the spacecraft location in addition it describes the evaluation procedure used for validation and accuracy assessment

sea state related altitude errors in the seasat radar altimeter a significant waveheight dependent bias is identified in seasat measured surface heights which cannot be neglected when using seasat data at accuracies within 10 cm ten second averages of the data from 63 waveform sampling rates in the seasat radar altimeter were least squares fitted with a six parameter model the time origin parameter of which provides a measure of the position of the actual mean radar waveform relative to the sampling rate and therefore provides altitude measurement corrections attention is given to actual altimeter point target response function waveform sampler noise baseline waveform sampler gain calibration attitude angle and sea surface skewness as effects not accounted for in seasat altimeter measurements and standard data processing

three dimensional gravity inversion model of the deep crustal structure of the central drake passage shackleton fracture zone and west scotia ridge antarctica 1 gravity and bathymetric data collected by the spanish r v hesperides over the shackleton fracture zone sfz and the west scotia ridge wsr were used to invert for the three dimensional 3 d structure of the deep crust data from the global gravity grid and global seafloor topography ggsft were also employed to enlarge the cruise area the merged data were analyzed to determine 3 d deep structure by numerical inversion water layer contribution to the gravity anomaly was eliminated taking into account the bathymetry spectral analysis of the reduced data yielded mean crust mantle interface cmi depths of 10 5 1 2 km inversion of the regional anomaly gave a 3 d detailed geometry of the cmi which generally agrees with the 2 d models established along profiles where gravity data and multichannel seismic lines are available the wsr shows an asymmetrical structure with a reverse fault located southeastward to the central valley this fault was developed probably as a consequence of the nw se compressive deformations which occurred following the spreading the 3 d view shows that the sw end of the spreading axis was affected more intensely by the compression the sfz represents an active sinistral transpressive fault zone of the scotia antarctica plate boundary and shows crustal thickening related to bathymetric highs the crustal thinning detected at the intersection with the inactive wsr suggests a complex interaction between these two structures

inversion of geohalo aerogravimetry to infer ocean bottom topography application to the tyrrhenian ionian and adriatic seas

bathymetric prediction from seasat altimeter data the linear response function technique is used to analyze two 1300 km tracks of seasat altimeter data and corresponding bathymetry in the musician seamounts region north of hawaii bathymetry and geoid height are highly correlated in the 50 to 300 km wavelength range a predictive filter is developed which can operate on seasat altimetry in poorly surveyed oceanic regions to indicate the presence of major bathymetrie anomalies modeling of the bathymetry geoid correlation in the musician region is attempted using the elastic plate model the flexural rigidity d of the plate is not well constrained by our data but appears to lie in the range 5 1021 n m 5 1022 n m at the time of loading since the musician seamounts and the crust on which they lie are both late cretaceous in age this value represents the effective flexural rigidity of very young lithosphere that was frozen in at the time of voicanism the modeling indicates that the general form of a predictive filter will strongly depend on various geologic parameters especially the effective flexural rigidity hence some a priori geologic constraints are necessary to estimate successfully the bathymetry from the altimeter data alternately if high quality bathymetry is available a crude estimate of the age of loading i e voicanism can be made from the altimeter data

oceanographic and geophysical applications of satellite altimetry since the last quadrennial report new satellite altimeter data have not been generally available nevertheless important progress has been made from analyses of geos 3 and seasat data the marine gravity field has been determined to an extraordinary resolution and accuracy and important solid earth geophysical interpretations made in the area of oceanography statistical estimates of the variability of dynamic topography have been published methods for incorporating altimeter data into numerical models have been developed and applied and tentative but interesting determinations of the mean circulation have been made in addition altimeter waveform analysis has made it possible to use altimeter data to determine elevations of major ice fields finally important new insights into the nature of satellite ephemeris error and its influence on altimetrically derived sea surface topography have been achieved

global gravity bathymetry and the distribution of submarine volcanism through space and time 1 the seafloor is characterized by numerous seamounts and oceanic islands which are mainly volcanic in origin relatively few of these features 0 1 however have been dated and so little is known about their tectonic setting one parameter that is sensitive to whether a seamount formed on near or far from a mid ocean ridge is the elastic thickness te which is a proxy for the long term strength of the lithosphere most previous studies are based on using the bathymetry to calculate the gravity anomaly for different values of te and then comparing the calculated and observed gravity anomaly the problem with such an approach is that bathymetry data are usually limited to single beam echo sounder data acquired along a ship track and these data are too sparse to define seamount shape we therefore use the satellite derived gravity anomaly to predict the bathymetry for different values of te by comparing the predicted bathymetry to actual shipboard soundings in the vicinity of each locality in the wessel global seamount database we have obtained 9758 te estimates from a wide range of submarine volcanic features in the pacific indian and atlantic oceans comparisons where there are previous estimates show that bathymetric prediction is a robust way to estimate te and its upper and lower bounds te at sites where there is both a sample and crustal age show considerable scatter however and there is no simple relationship between te and age nevertheless we are able to tentatively assign a tectonic setting to each te estimate the most striking results are in the pacific ocean where a broad swath of on ridge volcanism extends from the foundation seamounts and ducie island easter island ridge in the southeast across the equator to the shatsky and hess rises in the northwest interspersed among the on ridge volcanism are flank ridge and off ridge features the indian and atlantic oceans also show a mix of tectonic settings off ridge volcanism dominates in the eastern north atlantic and northeast indian oceans while flank ridge volcanism dominates the northeastern indian and western south atlantic oceans we have been unable to assign the flank ridge and off ridge estimates an age but the on ridge estimates generally reflect we believe the age of the underlying oceanic crust we estimate the volume of on ridge volcanism to be 1 1 106 km3 which implies a mean seamount addition rate of 0 007 km3 yr 1 rates appear to have varied through geological time reaching their peak during the late early cretaceous and then declining to the present day

bathymetry estimates in the southern oceans from seasat altimetry a mean sea surface height based on seasat altimeter data has been high pass filtered to reveal a pattern of geoid anomalies which are highly correlated with sea floor topography the technique is used to locate new bathymetric features in the poorly surveyed southern oceans

oceanic ridge crest processes oceanic ridge crest processes constitute a vigorous frontier of multidisciplinary research with global ramifications in the earth atmospheric and biologic sciences this deep ocean frontier is gradually opening in response to investigations which are elucidating the role of these processes in the lithosphere hydrosphere atmosphere and biosphere and the application of advanced instrumental and interpretive techniques which are increasing the quality and quantity of data these techniques include crustal drilling with borehole logging deep sea drilling project reports high resolution bathymetry sea beam etc side scan sonar seamarc i and ii gloria etc imagery with video and photography multichannel seismics determination of water column properties ctd with rosette sampler etc and submersible applications

marine tectonics processes at mid ocean ridges the use of multibeam echo sounders and various side looking sonar systems has greatly increased our knowledge and understanding of mid ocean ridge mor processes since the last iugg review on mid ocean ridge research macdonald 1983 the extensive use of swath mapping tools such as sea beam deep tow seamarc i and ii the application of multichannel seismic reflection techniques to the detection of axial magma chambers and the use of satellite data in the study of oceanic lithosphere have rejuvenated marine tectonics and have generated and stimulated a large number of important contributions this review will focus on what we feel were the major developments in marine tectonics with an emphasis on mid ocean ridges in the last four years certain topics such as marine seismicity and satellite studies are only briefly mentionned for reference and will be covered in greater detail in other reviews because of the exponential growth in the number of publications related to mors during 1982 1986 only a few references pertinent to each of the major research topics will be quoted in the text as examples but the reader will find an extensive bibliography at the end of the text references published prior to 1982 were added only where appropriate

evidence for small scale mantle convection from seasat altimeter data evidence for the early development of convective instability in the thermal boundary layer associated with cooling plates has been found from gravity anomalies and residual sea surface heights derived from seasat altimeter data subtle lineated patterns trending in the direction of plate motion in the hot spot reference frame are observed over the younger portions of the fast moving pacific and indo australian plates in particular for the east central pacific ocean the seasat derived data sets reveal lineations with 1 wavelengths in the range 150 500 km perhaps increasing with plate age and 2 peak to trough amplitudes of 5 20 mgal for gravity anomalies the lineated pattern over the pacific plate which first becomes discernable over seafloor 5 to 10 m y old west of the east pacific rise is clearly oblique to the trends of the prominent pacific farallon fracture zones onset of convective instability at such early ages can be understood if young oceanic lithosphere is underlain by a layer having a viscosity of about 1018 pa s 1019 p which is about 3 orders of magnitude less than mantle viscosity inferred from postglacial rebound studies recent numerical studies of convection in fluids with a temperature and pressuredependent viscosity support an early onset time for convective instability and low viscosity zones in the upper mantle beneath young lithosphere such low viscosity zones may correspond to similarly located regions of low shear wave velocities resolved through studies of surface waves

estimation of seamount compensation using satellite altimetry the degree of seamount compensation can be estimated from satellite radar altimetry and bathymetry data seamount signatures in the sea surface topography are detected located and extracted from seasat radar altimetry data using a matched filtering algorithm we have developed an efficient method for determining the two dimensional undulation signature of any axi symmetric seamount with an arbitrary degree of compensation given the seamount dimensions as determined from bathymetry we vary its degree of compensation until the model signature matches the signature extracted from seasat data we use this technique to map the relative degrees of local seamount compensation within a region we have applied this technique to 14 of the musicians seamounts two of these seamounts have been analyzed using two crossing tracks each and confirm the repeatability of the technique the results suggest the existence of a chain of low compensation seamounts within the highly compensated musicians independent verification is needed from other geophysical and geochemical analyses to support this hypothesis

altimetry derived gravity predictions of bathymetry by the gravity geologic method the gravity geologic method ggm was implemented for 2 by 2 bathymetric determinations in a 1 6 longitude by 1 0 latitude region centered on the eastern end of the shackleton fracture zone in the drake passage antarctica the ggm used the bouguer slab approximation to process satellite altimetry derived marine free air gravity anomalies and 6 548 local shipborne bathymetric sounding measurements from the korea ocean research and development institute to update the surrounding off track bathymetry the limitations of the bouguer slab for modeling the gravity effects of variable density rugged bathymetric relief at distances up to several kilometers were mitigated by establishing tuning densities that stabilized the ggm predictions tests using two thirds of the shipborne bathymetric measurements to estimate the remaining third indicated that the tuning densities minimized root mean square deviations to about 29 m the optimum ggm bathymetry model honoring all the ship observations correlated very well with widely available bathymetry models despite local differences that ranged up to a few kilometers the great analytical simplicity of ggm facilitates accurately and efficiently updating bathymetry as new gravity and bathymetric sounding data become available furthermore the availability of marine free air gravity anomaly data ensures that the ggm is more effective than simply extrapolating or interpolating ship bathymetry coverage into unmapped regions

a bathymetry and altimetry profile across the southwest indian ridge crest at 31 s latitude abstract during march 1983 the m v marion dufresne obtained a bathymetry profile along more than 1500 km of a seasat altimeter sub track across the southwest indian ridge crest at 31 s latitude we have used these data to estimate the thermal and elastic thickness of the oceanic lithosphere in the vicinity of the ridge and to evaluate the performance of the seasat altimeter the thermal thickness appears to be in the range 72 109 km while the elastic thickness is about 2 5 km there is a good correlation between the geoid calculated from the bathymetry and its compensation and the observed geoid derived from seasat data the correlation is strongest for wavelengths greater than 45 km and poorest for shorter wavelengths we attribute the poor resolution at short wavelengths to instrument and oceanographic noise in the altimeter data combined with the reduced sensitivity of the geoid to bathymetric relief at very short wavelengths

global seafloor topography from a least squares inversion of altimetry based high resolution mean sea surface and shipboard soundings summary we present a new worldwide 1 16 1 16 map of seafloor topography computed from an iterative inversion combining altimetry measurements of the marine geoid and shipboard echo soundings from the global national geophysical data center ngdc database the input geoid data for this computation are derived from altimetry measurements of the sea surface height from the ers 1 geodetic mission and the geosat ers 1 and topex poseidon exact repeat missions to calculate the seafloor topography from sea surface height measurements it is necessary to take the regional isostatic compensation of the topographic load into account for that purpose we consider a model of elastic flexure of the oceanic lithosphere in which the elastic plate thickness increases with crustal age with the solution we provide an uncertainty map that reflects the uneven distribution of and errors in the data and uncertainties in the model parameters as well as the increase of the geopotential error with the deepening of the seafloor the bathymetric solution is compared with the smith sandwell s 1997 science 227 1956 1962 solution the root mean square difference between the two amounts to 350 m however the two maps agree within 150 m over 65 per cent of the ocean surface

oceanography from space active and passive spaceborne instruments that can observe the sea are discussed attention is given to satellite observations of ocean surface temperature and heating wind speed and direction ocean currents wave height ocean color and sea ice specific measurements now being made from space are described the accuracy of various instruments is considered and problems associated with the analysis of satellite data are examined it is concluded that the satellites and techniques used by different nations should be sufficiently standard that data from one satellite can be directly compared with data from another and that accurate calibration and overlap of satellite data are necessary to confirm the continuity and homogeneity of the data

amundsen sea bathymetry the benefits of using gravity data for bathymetric prediction bathymetric charts are essential for modeling oceanic processes yet in remote areas direct measurements of seafloor depth are often scarce it is possible to augment sparse depth soundings with dense satellite derived gravity data to provide additional bathymetric detail in regions devoid of sounding data we demonstrate this method by using marine gravity derived from the european remote sensing ers 1 satellite altimeter combined with depth soundings to form a bathymetric prediction of the amundsen sea west antarctica we estimate the root mean square error of depth estimates at unsurveyed locations in our solution to be 120 m we use a monte carlo method to assess the value of gravity as a bathymetric predictor in sparsely surveyed regions by comparing our solution to predictions formed from depth soundings alone when less than 11 of 10 km grid cells contain depth soundings inclusion of gravity data improves the depth accuracy of the solution by up to 17 as compared to a minimum curvature surface interpolation of the depth soundings alone when depth data are sparse our gravity derived prediction reveals additional short wavelength bathymetric features such as troughs on the continental shelf which are not resolved by interpolations of the depth soundings alone

an empirical determination of the effects of sea state bias on seasat altimetry a simple linear empirical model has been developed to allow the user of seasat altimetry data to correct the altitude measurements for sea state bias effects this bias is the result of two separate effects one relating to instrumental effects and the other relating to a bias in the electromagnetic mean sea level relative to the geometric mean sea level the electromagnetic bias is caused by the fact that the troughs of the ocean waves reflect the altimeter signal more strongly than the crests thus causing the altimeter determined mean sea level to shift toward the wave troughs an additional but independent instrument related bias is introduced because height corrections applied in the ground processor are not sufficient to compensate for simplifying assumptions made for the processor aboard seasat this altitude bias also is predominately wave height dependent and directed toward the wave trough the sum of these two effects is referred to as sea state bias after applying appropriate corrections to the altimetry data an empirical model for the sea state bias is obtained by differencing significant wave height and height measurements from coincident ground tracks ignoring small temporal variations in surface topography the height differences on a given track should be zero consequently any residual height difference is attributed to sea state bias the height differences are minimized by solving for the coefficient of a linear relationship between height differences and wave height differences that minimize the height differences it is concluded that for the altimeter data released by the jet propulsion laboratory 7 of the value of significant wave height should be subtracted from the seasat height measurements to correct for sea state bias this correction reduced biases in the seasat data by more than 50 in the 36 cases examined here

the world ocean circulation experiment the international oceanographic community is preparing for an ambitious five year experiment starting in 1990 it will provide the first comprehensive global survey of physical properties of the oceans the resulting data set will be used to test computer models of the ocean circulation needed for predicting decadal climate change the results will also benefit research in marine chemistry biology and geology this review looks at the climatalogicalproblems that motivated the project and discusses new observing techniques from space and in situ which make it timely to embark on this major step in physical exploration of the ocean

meanders and eddies from topographic transformation of coastal trapped waves abstractthis paper describes how topographic variations can transform a small amplitude linear coastal trapped wave ctw into a nonlinear wave or an eddy train the dispersion relation for ctws depends on the slope of the shelf provided the cross shelf slope varies sufficiently slowly along the shelf the local structure of the ctw adapts to the local geometry and the wave transformation can be analyzed by the wentzel kramers brillouin jeffreys wkbj method two regions of parameter space are straightforward adiabatic transmission where at the incident wave frequency a long wave exists everywhere along the shelf and short wave reflection where somewhere on the shelf no long wave exists at the incident frequency but the stratification is sufficiently weak that a short reflected wave can coexist with the incident wave this paper gives the solutions for these two cases but concentrates on a third parameter regime which includes all sufficiently strongly stratified flows where neither of these

estimating the compensation depth of the hawaiian swell with linear filters broad areas of anomalously shallow seafloor surround regions of active or recently active hot spot volcanism the cause of these midplate swells has been modeled both as elevated temperatures in a convecting layer and as thermal expansion within the conducting portion of the lithosphere according to the former explanation we would expect the low density material compensating the swell to lie beneath the conducting lid whereas the latter model allows compensation at shallower depths we present a technique based on the application of linear filters for determining the compensation depth of midplate swells given observations of topography and geoid or gravity anomalies our method does not require an a priori estimate of swell height or width nor does it assume complete spatial or spectral separation between the swell topography and the signal from the hot spot volcanoes which are compensated by flexure of the oceanic moho rather we exploit the difference in the predicted sign and amplitude of the moho deflection beneath elevated topography produced by loading above versus loading below the elastic plate in order to separate the two effects an application of these linear filters to residual topography and geoid data from the region surrounding the hawaiian hot spot yields a compensation depth of 70 10 km within the lower lithosphere

assessing the transferability and robustness of an enhanced temperature index glacier melt model we investigate the transferability of an enhanced temperature index melt model that was developed and tested on haut glacier d arolla switzerland in the 2001 season the model s empirical parameters temperature factor tf and shortwave radiation factor srf are recalibrated for 1 other locations on haut glacier d arolla 2 subperiods of distinct meteorological conditions 3 different years on haut glacier d arolla and 4 other glaciers in different years the model parameters are optimized against simulations of an energy balance model validated against ablation observations results are compared with those obtained with the original parameters the model works very well when applied to other sites seasons and glaciers with the exception of overcast conditions differences are due to underestimation of high melt rates the parameter values are associated with the prevailing energy balance conditions showing that high srf are obtained on clear sky days whereas higher tf are typical of locations where glacier winds prevail and turbulent fluxes are high we also provide a range of parameters clearly associated with the site s location and its meteorological characteristics that could help to assign parameter values to sites where few data are available