modification of a linear bar trough system by a standing edge wave abstract detailed fluid motions and bathymetric changes in the lacustrine nearshore zone at wymbolwood beach georgian bay canada were monitored during storms to test theoretical postulates about the importance of low frequency wave motions to nearshore circulation of water and sediments eighteen continuous resistance wave staffs and two electromagnetic current meters were deployed along one shore normal and two shore parallel instrument arrays over a period of two months during one intense storm a mode 3 standing edge wave at 0 035 hz was the predominant low frequency motion prior to the storm the bathymetry was characterized by linear bar trough systems and during the storm these features migrated onshore and became rhythmic in response to the length scales and relatively fixed locations of nodal and antinodal lines of the standing edge wave however a well defined crescentic bar as predicted by the model under the condition of zero net sediment flux divergence did not evolve because the hydrodynamic forcing ceased before an equilibrium condition could be attained nevertheless the location of scour and accretion features demonstrated the tendency towards such a crescentic form the offshore and alongshore irregularities of the post storm bathymetry are not readily explained by alternative hydrodynamic interactions

magnetization modeling in the north and equatorial atlantic ocean using magsat data we have analysed the shorter wavelength component of the magnetic field observed by magsat over the north and equatorial atlantic ocean with the idea of trying to determine the possible sources of this component of the field in most cases inversion techniques produce both normally and reversely magnetized sources and we show that this is not appropriate for the possible sources in the oceanic crust which at the wavelengths observed by magsat need to be entirely normally magnetized or with a magnetization of zero in order to do this we have applied an annihilator to the solutions produced by our inversion technique we show how the annihilator that we use is convenient for matching up magnetization solutions for areas that have been inverted separately one problem with the inversion technique is that the best spacing of the sources can only be found by trial and error we show that much of the final variation in source spacing is produced by a latitudinal effect which seems to require sources spaced further apart the lower the latitude this may be due to contamination of the anomalous field by noise which might be expected to be worse at lower latitudes we have also examined the possibility that much of the magnetic source in the oceanic crust is due to induced magnetization the values of susceptibility necessary are much higher than those measured directly on samples taken from the ocean basins remanent magnetization is a more likely candidate but the values especially after application of the annihilator are still large compared with observed values only considerable thickening of the crust combined with remanent magnetization can explain the high intensities of magnetization that we have modeled viscous magnetization is a possible source for the high intensities of magnetization of our model one problem with an explanation of the field from crustal sources is that in many cases there is little correlation with the geological history of the region an alternative is that noncrustal sources contribute to the anomalous magnetic field derived from magsat data this field derived by subtracting degrees of harmonic 1 through 13 from a spherical harmonic model may be contaminated with core field signals from leakage of high amplitude short wavelength anomalies into longer wavelengths and by external fields

field observations of swash zone infragravity motions and beach cusp evolution 1 fluid flows consistent with low mode edge waves were evident in video observations of swash motions during a field experiment in which beach cusps developed on an initially smooth beach as beach cusps grew energy lying along low mode dispersion curves increased the most energetic edge wave propagation direction changed from upcoast to downcoast as the orientation of the cusp horns rotated these observations suggest a coupling between morphodynamics and hydrodynamics and are evidence that beach cusp evolution might control low mode edge wave dynamics

interpretation of satellite elevation magnetic anomalies the magnetic field satellite magsat obtained total field data over almost the whole of the earth we have studied these data in two different ways the first way was to use spherical harmonic coefficients it is often believed that these coefficients can be separated into those originating within the core of the earth and those from the crust at about degree 13 because of the change in slope of the power spectrum using coefficients above degree 13 in two different representations we have shown that there is little latitudinal variation of the source function and that the continent to ocean source function ratio is almost unity we suggest that these unlikely observations are in reality due to the possibility that much of the signal carried in coefficients above degree 13 may originate within the core of the earth if this is true it is then possible to keep the energy external to the core of the field generated within the core at a constant even though the dipole field has varied in strength by a factor of 3 over the past 104 years we have also produced models of crustal magnetization using the 2 2 average anomalous field and have shown how it is possible to obtain a physically meaningful susceptibility model in which magnetizations are always positive this is done by using an annihilator which is a magnetization distribution within a spherical shell which produces no external field the use of an annihilator is also appropriate for many problems connected with oceanic basins since present day oceanic crust is everywhere less than 180 ma the average magnetization seen at the altitude of a satellite is either zero for crust which is produced during times of rapid field reversal or positive for crust produced during times of constant normal polarity some results from continental and oceanic areas are discussed in the light of applying an annihilator

wave propagation in the marginal ice zone model predictions and comparisons with buoy and synthetic aperture radar data in this paper the ocean wave dispersion relation and viscous attenuation by a sea ice cover are studied for waves propagating into the marginal ice zone miz the derivation of the dispersion relation and the viscous attenuation by an ice sheet are discussed for waves under flexure and pack compression in the miz the flexure effect is important for short waves for a fixed wave period the changes in wavelength and group velocity as a function of ice thickness are significant in turn the exponential wave attenuation rate shows a rollover at short wave periods whereby the rapid increase in wave attenuation rate with decreasing wave period slows down or even turns into a decrease the labrador ice margin experiment limex conducted on the miz off the east coast of newfoundland canada in march 1987 provides us with aircraft synthetic aperture radar sar imagery wave buoy and ice property data on the basis of the wave number spectrum from the sar data and the concurrent wave frequency spectrum from the ocean buoy data and accelerometer data on the ice during limex 87 the dispersion relation has been estimated and compared with a model wave energy attenuation rates are estimated from sar data and the ice motion package data which were deployed at the ice edge and into the ice pack and compared with the model the model data comparisons are reasonably good for the ice conditions observed during limex 87 some previously reported data of wave attenuation in the miz are revisited for model comparison

swash oscillations on a natural beach run up swash oscillations were measured on a gently sloping beach face for a variety of incident wave conditions run up energy spectra at wind wave frequencies show an 3 dependence and energy levels that are independent of incident wave height this suggests saturation in contrast run up energy at surf beat periods increase approximately linearly with increasing incident wave energy thus in the inner surf zone where wave breaking limits the energy at wind wave frequencies the principal manifestation of large incident wind waves is energetic surf beat

magnetic exploration of ocean crust for craters of impact origin model results magnetic data sets over deep ocean areas may contain clues to the existence of craters formed by the impact of an extraterrestrial body with the earth s ocean crust to aid in the magnetic exploration of the ocean crust for oceanic impact craters basic but effective computations from an impact model are studied from an aeromagnetic point of view the main assumption of the analysis is that a sufficiently large impact can excavate large volumes of magnetized basalt vaporize basalt and raise basalt to temperatures above the curie temperature approximately 500 c to alter the preimpact magnetization of the ocean floor and result in a magnetic anomaly being associated with an oceanic impact crater in the absence of an existing theory on the influence of impacts on ocean crustal magnetization the representation of a crater on the ocean floor by a simple potential provides apparently for the first time quantitative estimates of the crater s magnetic anomaly along a horizontal surface numerical results

boundary layer upper ocean and ice observations in the greenland sea marginal ice zone during the 1984 marginal ice zone experiment a coordinated ocean boundary layer and mixed layer experiment was carried out in concert with a complete heat and mass balance study in the summer marginal ice zone miz of the greenland sea the measurements were made at two drifting ice stations and included observations of ocean boundary layer turbulence profiles of water temperature salinity and velocity ice ablation ice concentration solar radiation and spectral albedos ocean conditions were found to be extremely variable the ice and water velocities were heavily influenced by tidal and inertial motions with amplitudes from 6 to 12 cm s 1 in the course of the drift a 30 km eddy was crossed wherein ice and water velocities exceeded 15 cm s 1 the northern extension of the east greenland polar front was encountered and a northward flow of atlantic water into the arctic basin was observed below the eastern edge of the front but the drift station never moved quite far enough west to be caught in the southward transport of arctic surface water heat from the ocean was a major factor in ice melt and when the ice was over atlantic water it was the dominant factor the events that had the greatest impact on the ice were two outbreaks during which off ice winds blew the ice across warm ice edge ocean fronts once south of these fronts and over surface water as warm as 1 c the ice bottom melted at rates up to 100 kg m 2 d 1 the rapid changes in stratification associated with the outbreaks had a dramatic impact on the drag and heat transfer coefficients the drag coefficient relative to the square of the relative water velocity at 30 m ranged from 0 002 to 0 02 and averaged 0 006 the heat transfer coefficient relative to the friction velocity u and the mixed layer temperature elevation above freezing ranged from 0 0025 to 0 009 and averaged 0 0038 in agreement with the idea that stratification inhibits turbulent exchange the heat transfer coefficient decreased when stratification increased in fact the observed melt rates can be simulated well if molecular heat and salt transfer through a thin near surface laminar boundary layer are accounted for as well as transfer through the turbulent outer boundary layer also in agreement with turbulent boundary layer theory the drag coefficient decreased when stratification increased slightly above neutral however for the large increases in surface stratification the drag coefficient increased this was due to the generation of internal gravity waves by ice bottom roughness because the surface exchange coefficients are so sensitive to surface stratification which is in turn highly variable in the miz accurate simulations of ice motion and ice melt will require realistic estimates of stratification and a good understanding of the relation between the ice ocean interactions and upper ocean conditions

crustal structure of the churchill superior boundary zone between 80 and 98 w longitude from magsat anomaly maps and stacked passes a modeling technique using spherical shell elements and equivalent dipole sources has been applied to magsat signatures at the churchill superior boundary in manitoba ontario and ungava a large satellite magnetic anomaly 12 nt amplitude on pogo and magsat maps near the churchill superior boundary was found to be related to the richmond gulf aulacogen the averaged crustal magnetization in the source region is 5 2 a m 1 stacking of the magnetic traces from magsat passes reveals a magnetic signature 10 nt amplitude at the churchill superior boundary in an area studied between 80 w and 98 w modeling suggests a steplike thickening of the crust on the churchill side of the boundary in a layer with a magnetization of 5 a m 1 signatures on aeromagnetic maps are also found in the source areas for both of these satellite anomalies

intermediate wavelength magnetic anomaly field of the north pacific and possible source distributions the objective of this study is to investigate the feasibility of extracting intermediate wavelength magnetic anomalies of wavelengths between 4000 and 400 km from total field data acquired in marine magnetic surveys the intention is to develop a second data set which may be used in conjunction with the satellite magnetic field to determine anomaly and source distributions we apply our technique to the recovery of intermediate wavelength total intensity anomalies over the north pacific from marine survey data the technique also provides a record of secular variation during the period 1960 to 1980 in areas of high track density the sea surface data can be used to refine the magsat data processing techniques and also provide further constraints on source models in areas of reduced sea surface track density the magsat field provides higher resolution the magsat data set also records vector field data as will be shown the filtered sea surface data set provides a higher resolution field than the magsat data due to proximity to lithospheric source bodies the sea surface data also provide an independent check of the magsat field since the two fields should be equivalent after upward continuation of the sea surface field we notice however that for the central pacific the upward continued sea surface field is approximately 2 times the amplitude of the magsat field the spatial correlation of the two anomaly fields is very strong

infragravity velocity profiles in the surf zone we present a theoretical solution for the vertical structure of the velocity profiles in infragravity waves the solution predicts that while the velocity without local forcing of the infragravity waves does not vary significantly with the vertical coordinate the velocity with local forcing of the infragravity waves has a substantial vertical structure

frequency dependent cross shore suspended sediment transport 1 a non barred shoreface abstract the local time varying and time averaged suspended sediment transports across a marine non barred shoreface were obtained from field measurements of the near bed velocity and sediment concentration vectors using electromagnetic current meters and optical backscatterance suspended solids sensors co spectral analyses of velocity and sediment concentration revealed the contributions of waves of different frequencies to the total transport the transport attributable to quasi steady currents was determined from the product of the time averaged cross shore velocity and sediment concentration estimates of the local time integrated sediment volume flux total and net and the associated erosion or accretion were determined using depth of activity rods suspended sediment transport was associated with 1 local wind forced waves 2 swell 3 low frequency waves group bound long waves and 4 offshore directed mean currents undertow these transport components varied spatially and temporally in response to changes in the wave height to water depth ratio the local net oscillatory transport rate at wind wave and swell wave frequencies was directed onshore predominantly and increased as water depth decreased in association with wave shoaling the local mean sediment transport rate was predominantly offshore and dominated the net transport rate wherever wave induced components were weak relative to the mean currents under breaking waves the suspended sediment transport contribution associated with wind waves and swell waves was reduced as a result of dissipation the net suspended sediment transport rate exhibited a distinct vertical structure reflecting the balance at each elevation between the opposing mean and oscillatory components of transport the local time averaged total and net sediment volume flux and resulting erosion and accretion patterns support the hypothesis of a near balance in sediment flux for the complete storm event

the satellite magnetic anomaly of ahaggar evidence for african plate motion the ahaggar volcanic province of north central africa is considered a region of excess heat flow hot spot and hence elevated curie isotherm using a modified version of the parker fft potential field representation magnetic signals were calculated at magsat altitudes for models in which the african plate is both fixed and moving the moving plate model extends the curie isotherm anomaly in the direction of plate motion and provides a satisfactory match to vertical component z anomaly data when the magnitude of plate velocity is 0 75 cm yr although the signal levels are marginal for the scalar component anomalies of this region the same model provides an adequate match to this data set and is clearly preferable to a fixed plate model

semi diurnal variation in beachface aggradation and degradation abstract sequences of morphologic change on the beachface at south beach wollongong n s w were monitored for several semi diurnal spring tide cycles the observations were made to establish temporal and spatial variation in the beachface response to tide swash and groundwater interactions it was anticipated that the sequences would differ under high and low wave energy conditions and on different parts of the same beach where the foreshore morphology varied markedly to test these propositions two dimensional beachface responses to tide swash and groundwater level variation have been compared for two beachface profiles one transecting the horn of a giant cusp and the other the embayment of the same cusp the beach survey results were specified in two ways 1 by examining the changes in sand surface elevation at each survey point along the profiles and 2 by transforming the survey data with a moving axis system whose origin followed the intersection of the sea level with the beachface over the tide cycle in the first instance the sequences of beach change along each profile were similar to those described from elsewhere although the profiles were undergoing different secular displacements during the course of the survey the salient was eroded and the embayment infilled the net changes were determined by the dynamic balance of erosion versus deposition occurring with the flood and ebb of each tide cycle application of the moving axis technique identified switching between erosional and depositional states in the swash zone on the salient the swash zone changes were controlled by the tide and groundwater fluctuations beachface degradation began with the tide rise and terminated shortly after mid ebb tide maximum degradation occurred when the beachface was most saturated approximately 1 to 2 h after high tide in both high and low energy conditions the sequence of cut and fill was similar to that described in earlier work tidal migration of the sea level and water table effluent line across the beachface established the location of the zones of erosion and deposition above the effluent line an unsaturated beach condition enhanced deposition below the line the beachface was saturated and erosional conditions prevailed the dynamic balance given as the amount of erosion versus deposition over each tide cycle was associated with the different levels of wave energy and sediment supply

the partition of air ice ocean momentum exchange as a function of ice concentration floe size and draft a coupled sea ice ocean model is used to examine the relative magnitudes of terms in the sea ice momentum equation for ice concentrations less than 100 this numerical model for steady state horizontally homogeneous flow includes two terms that have previously been neglected in such quantitative models these are oceanic form drag and wave radiation pressure both of which are lateral forces that act on the sides of floes the former is generally decelerative while the latter is accelerative the model accepts as input variables average ice draft areal concentration and floe diameter it is found that form drag can be greater than or of the same order as skin friction drag for small thick floes at low concentrations an exception to this rule occurs when the effect of wakes between floes reduces the local form drag the higher net drag that generally exists at low concentrations implies lower ice floe speeds wave radiation pressure is negligible in all cases although this is sensitive to our choice of fetch parameterization ocean drag is often parameterized in numerical models of sea ice as a skin friction term alone with a constant drag coefficient between ice and prescribed geostrophic current velocities results from the present model are plotted in terms of such a drag coefficient which is a function of concentration floe size and draft this could be useful in future work where floe size data become regularly available for instance from radar satellite imagery

rhythmic beach cusp formation a conceptual synthesis abstract observations of a sandy beach at santa barbara california during february 1980 documented the sequence of beach cusp formation replacement by a second set of rhythmic cusps obliterating the first set and finally complete planation of all rhythmic cusps during a major storm although neither cusp building event was observed during its initiation the wave and tide conditions during formation were well documented by in situ instrumentation based on these field observations and those by other investigators a coherent conceptual theory of beach cusp formation is proposed including formative maintenance and destructive phases this theory proposes rhythmic cusp formation during relative stillstands of sea level of sufficient duration to permit initial perturbation of beach geometry regularly spaced cusps acquire their initial spacing from subharmonic edge waves such that spacing derives from the incident wave period growth of the initial perturbation continues until cusps reach a limiting value not necessarily with continuation of edge wave motion the cusp spacing persists until a change in wave conditions either destroys the cusps or establishes a radically different spacing and finally that large storms will destroy the rhythmic cusp field leaving either a planar beach or a beach with arhythmic cusp like features

spatial resolution and repeatability of magsat crustal anomaly data over the indian ocean preliminary results from analysis of individual magsat tracks in the eastern indian ocean demonstrate the resolution capability and repeatability of these data spectrum analysis indicates that 250 km is the absolute lower limit for resolving crustal magnetic anomalies in magsat along track data spectral coherence analysis of closely spaced tracks shows that magnetic anomaly features with wavelengths greater than 700 km are highly repeatable from track to track

magnetic anomalies at satellite altitude over continent ocean boundaries abstract a pronounced difference in vertically integrated magnetization produces strong magnetic anomalies at satellite altitude over continent ocean boundaries however these anomalies are seldom seen in satellite anomaly maps our simulation of the technique normally used to extract anomaly fields from satellite observations of the geomagnetic field suggests that power leaks from short wavelength crustal anomalies into the longer wavelengths this phenomenon distorts the anomaly field and adds power to the lower degrees in the anomaly field power spectrum we will also show that some of the few magnetic anomalies observed in satellite anomaly maps over continent ocean boundaries in the atlantic ocean could be caused by thermal destruction of magnetic minerals rather than by a pronounced difference in vertically integrated magnetization the very low permeability of sediments deposited along continental margins basically precludes convective heat transfer in the sediments the low thermal conductivity of these sediments results in the underlying lithosphere having a high temperature due to the temperature gradient within the sediment blanket our calculations predict that thermal destruction should occur along continental margins where several kilometres of sediments have accumulated in order to produce magnetic anomalies similar to magsat anomalies we assume that the upper mantle is magnetized

investigation of plate boundaries in the eastern indian ocean using magsat data abstract plate boundaries are regions of significant tectonic activity in the area between the ninety east ridge and sumatra seismicity and bathymetry suggest the presence of differing plate boundaries and other tectonic features e g n s trending fracture zones and the southwest asian continental margin a magsat anomaly map was made of this area in order to study the magnetic signature of these major plate boundaries at satellite altitude forty 16 descending and 24 ascending low altitude 340 360 km magsat orbits were selected over this region 90 105 e 8 s 4 n these profiles were chosen to ensure the most noise free data a magnetic anomaly map was derived from these data using the technique of minimizing orbital cross over errors and was contoured at 0 5 nt the largest magnetic anomaly a positive 3 nt is located over the most conspicuous younger volcanic complex of the southeastern half of sumatra because these data are from the southern magnetic hemisphere i 25 a positive magsat anomaly implies an aggregate reverse remanent magnetization further northwest along sumatra where the volcanic rocks are less prevalent there are no distinct anomalies the ninety east ridge has a small anomaly 2 nt which has been modeled as indicating normal magnetization while several other positive bathymetrie features exhibit satellite altitude anomaly signatures however the sunda trench does not present a distinct magnetic anomaly in addition the existence of a diffuse plate boundary across the wharton basin was not supported by these data at an altitude of 350 km the presence of a continental oceanic magnetic edge effect was not detectable a much lower orbiting magnetometer bearing satellite will be required to delineate these smaller tectonic elements

the attenuation rates of ocean waves in the marginal ice zone during field operations in the greenland and bering seas in 1978 1979 and 1983 a number of experiments were carried out in which wave energy was measured along a line of stations running from the open sea deep into an icefield wave buoys in the water and accelerometer packages on floes were the instruments employed with airborne vertical photography to supply information on floe size distribution it was found that the decay of waves is exponential with a decay coefficient which generally increases with frequency except for a roll over at the highest frequencies the observations can be fitted reasonably well to a theory of one dimensional scattering

age dependent three dimensional magnetic modeling of the north pacific and north atlantic oceanic crust at intermediate wavelengths three dimensional magnetic modeling of the north atlantic and northeast pacific is performed at intermediate wavelengths using three models for the acquisition of a natural remanent magnetization it is shown that a remanent magnetization which is dependent on the crustal age is the dominant source for the intermediate wavelength pattern in both basins however a pure thermoremanent magnetization of layer 2 alone is insufficient to model the intensity and shape of the observed magnetic anomalies at satellite altitude we conclude that the best fitting magnetization model for both basins is a combination of a chemical remanent magnetization in the altered upper crust and a thermoviscous remanent magnetization of the slowly cooling lower crust and upper mantle the north pacific requires a bulk magnetization which is 50 higher than that of the north atlantic in order to fit the magsat field geological processes associated with a faster spreading rate such as a faster hydrothermal alteration and the growth of a thicker gabbro layer at the expense of a weakly magnetized sheeted dike layer are plausible explanations for the higher north pacific magnetization the lineated positive magnetic anomaly observed over the north atlantic spreading center is not well reproduced by our models this anomaly is likely due to a highly magnetized body along or in the vicinity of the spreading center this highly magnetic body could be an unstable serpentinized lens of crustal material younger than 20 m y which develops within the zone cooled by hydrothermal circulation

small scale features in the earth s magnetic field observed by magsat a spherical harmonic expansion to degree and order 29 is derived using a selected magnetically quiet sample of magsat data global maps representing the contribution due to terms of the expansion above n 13 at 400 km altitude are compared with previously published residual anomaly maps and shown to be similar even in polar regions an expansion with such a high degree and order displays all but the sharpest features seen by the satellite and gives a more consistent picture of the high order field structure at a constant altitude than do component maps derived independently the appendix is available with the entire article on microfiche order from american geophysical union 2000 florida avenue n w washington dc 20009 document b83 008 2 50 payment must accompany order

comparison of viscoelastic type models for ocean wave attenuation in ice covered seas continuum based models that describe the propagation of ocean waves in ice infested seas are considered where the surface ocean layer including ice floes brash ice etc is modeled by a homogeneous viscoelastic material which causes waves to attenuate as they travel through the medium three ice layer models are compared namely a viscoelastic fluid layer model currently being trialed in the spectral wave model wavewatch iii and two simpler viscoelastic thin beam models all three models are two dimensional a comparative analysis shows that one of the beam models provides similar predictions for wave attenuation and wavelength to the viscoelastic fluid model the three models are calibrated using wave attenuation data recently collected in the antarctic marginal ice zone as an example although agreement with the data is obtained with all three models several important issues related to the viscoelastic fluid model are identified that raise questions about its suitability to characterize wave attenuation in ice covered seas viscoelastic beam models appear to provide a more robust parameterization of the phenomenon being modeled but still remain questionable as a valid characterization of wave ice interactions generally

evaluation of indian coastal response an integrated approach abstract since its separation 130 ma from the mainland the indian subcontinent has experienced major evolutionary changes a comparison of the structure evolution and tectonic elements of the east and west coast reveals characteristic differences thus while the eastern coast shows significant anomalous behaviour in all geophysical observations in contrast the corresponding observations on the western coast are relatively devoid of such anomalies in this investigation available magsat gravity electromagnetic gds and heatflow data have been utilized to search for a possible coherent cause for these different anomalies and thereby some inferences are sought to relate these causes to the geodynamics of this part of the indian plate

spectral modeling of ice induced wave decay abstractthree dissipative two viscoelastic and one viscous ice models are implemented in the spectral wave model wavewatch iii to estimate the ice induced wave attenuation rate these models are

airborne remote sensing of ocean wave directional wavenumber spectra in the marginal ice zone interactions between surface waves and sea ice are thought to be an important but poorly understood physical process in the atmosphere ice ocean system in this work airborne scanning lidar was used to observe ocean waves propagating into the marginal ice zone miz these represent the first direct spatial measurements of the surface wave field in the polar miz data were compared against two attenuation models one based on viscous dissipation and one based on scattering both models were capable of reproducing the measured wave energy the observed wavenumber dependence of attenuation was found to be consistent with viscous processes while the spectral spreading of higher wavenumbers suggested a scattering mechanism both models reproduced a change in peak direction due to preferential directional filtering floe sizes were recorded using co located visible imagery and their distribution was found to be consistent with ice breakup by the wave field

magsat equivalent source anomalies over the southeastern united states implications for crustal magnetization the magsat crustal anomaly field depicts a previously unidentified long wavelength negative anomaly centered over southeastern georgia examination of magsat ascending and descending passes clearly identifies the anomalous region despite the high frequency noise present in the data using ancillary seismic electrical conductivity bouguer gravity and aeromagnetic data a preliminary model of crustal magnetization for the southern appalachian region is presented a lower crust characterized by a pervasive negative magnetization contrast extends from the new york alabama lineament southeast to the fall line in southern georgia and eastern alabama coincident with the brunswick terrane the model calls for lower crustal magnetization contrast of 2 4 a m northern georgia and the carolinas are modeled with contrasts of 1 5 a m large scale blocks in the upper crust which correspond to the blue ridge charlotte belt and carolina slate belt are modeled with magnetization contrasts of 1 2 a m 1 2 a m and 1 2 a m respectively the model accurately reproduces the amplitude of the observed low in the equivalent source magsat anomaly field calculated at 325 km altitude and is spatially consistent with the 400 km lowpass filtered aeromagnetic map of the region

wave attenuation by sea ice turbulence the dissipation of wave energy in the marginal ice zone is often attributed to wave scattering and the dissipative mechanisms associated with the ice layer in this study we present observations indicating that turbulence generated by the differential velocity between the sea ice cover and the orbital wave motion may be an important dissipative mechanism of wave energy through field measurements of under ice turbulence dissipation rates in pancake and frazil ice it is shown that turbulence induced wave attenuation coefficients are in agreement with observed wave attenuation in the marginal ice zone the results suggest that the turbulence induced attenuation rates can be parameterized by the characteristic wave properties and a coefficient the coefficient is determined by the ice layer properties

on time delays in the nearshore zone between onshore and longshore currents at incident wave frequencies observations of horizontal velocities just outside the surf zone on two natural beaches show unexpected time lags between the onshore offshore currents and the longshore currents at incident wave frequencies these time lags typically around 20 s appear to have been first noticed by katoh 1981 in data from a beach in japan a number of possible hypotheses to explain these time lags are examined of these propagation of secondary waves formed by reflection or diffraction at a point alongshore from the measurement site seems most able to explain the observations this suggests a significance for reflected diffracted waves some distance from breakwaters or headlands which has not previously been recognized in field studies however a number of questions remain that will only be resolved by further field data particularly from beaches far removed from end effects

waves long waves and nearshore morphology abstract recent field measurements on beaches of different slopes have established that wave motion at periods substantially longer than the incident waves dominates the velocity field close to the shore analysis of a number of extensive data sets shows that much of this long wave motion is in the form of progessive edge waves though forced wave motion standing edge waves and free waves propagating away from the shore may also contribute to the energy theoretically the drift velocities in bottom boundary layers due to edge waves show spatial patterns of convergence and divergence which may move sediment to form either regular crescentic or cuspate features when only one edge wave mode dominates or a bewildering array of bars bumps and holes when several phase locked modes exist together convincing field demonstration of the link between nearshore topography and edge waves only exists for the special case of small scale beach cusps on steep beaches formed by edge waves at the subharmonic twice the period of the incident waves at longer periods the link is proving more difficult to establish due to the longer time scales of topographic changes the interaction between pre existing topography and the water motion and the observation of broad banded edge wave motion which is not readily linked to topography with a well defined scale these ideas are however central to the study of nearshore processes as most of the plausible alternate hypotheses do not seem to lead to quantitative predictions clearly further theoretical and observational work is essential

mean ionospheric field correction for magsat data to obtain a meaningful anomaly map showing magnetic fields of crustal origin from satellite observations it is necessary to account adequately for and subtract the fields generated by the currents flowing in the ionosphere conventionally corrections are made separately for each path by empirical methods of fitting and subtracting linear quadratic or harmonic functions from the original data this procedure however lacks a sound physical basis we examined the field components after the main field and the magnetospheric field were subtracted from magsat data and found certain structures in the residual field the residual fields at 0600 and 1800 local times were separately averaged over longitude to obtain the effective contribution of ionospheric current system at these local times which is called the mean ionospheric field mif to obtain the crustal anomalies the mif was subtracted from the residual fields to account for the day to day variation in the ionospheric current system and for the difference in satellite altitude anomaly maps are constructed on 5 5 grid averages the dawn and dusk anomaly maps after the correction agree well with each other showing that the effect of the ionospheric field is well taken care of by the mif correction the amplitude of anomalies is large even at comparatively long wavelength and the maps successfully represent these crustal anomalies which have been greatly reduced in the previous maps

field observations of surf beat 1 progressive edge waves nineteen biaxial electromagnetic current meters have been used to determine the longshore and on offshore structure of currents at surf beat periods 1 4 min the sensors formed two linear arrays a long shore array within the surf zone and an on offshore array stretching from the shoreline to well beyond the breaker line analysis of the longshore current components yields a clear picture of progressive low mode edge waves with frequency wave number dispersion relations which are in remarkably good agreement with predictions some separation of edge wave modes is found with mode zero energy dominating in the frequency band 0 006 and 0 011 hz and mode one between 0 015 and 0 025 hz on offshore currents present a rather different picture which while not inconsistent with the longshore currents suggests that other sources of energy are also important to the on offshore currents these include standing edge waves probably formed by reflections at nearby scripps canyon and motions which are nonresonantly forced by incoming wave groups

dynamics and thermodynamics of the ice upper ocean system in the marginal ice zone of the greenland sea measurements were made of the ice upper ocean system in the marginal ice zone of the greenland sea during a 4 day storm in which wind blew the ice south across a frontal region coinciding with the ice edge that had existed when the storm began in response to wind stress of about 0 2 pa a turbulent boundary layer developed under the ice that exhibited marked ekman rotation in both mean velocity average surface speed about 0 17 m s 1 deflection angle about 33 and turbulent stress profiles typical reynolds stress about 0 1 pa ablation of the ice undersurface increased rapidly after crossing the surface temperature front and the observed melt rate corresponded with direct heat flux measurements in the oceanic boundary layer with maximum upward heat flux of about 200 w m 2 we discuss overall momentum and energy balances interpret observed boundary layer measurements with a numerical model and show that molecular effects are important for heat and mass transport at the hydraulically rough ice ocean interface we also develop a simple model for predicting ice melt from interfacial stress and temperature and salinity of the mixed layer

water drag coefficient of first year sea ice in may 1977 eddy flux measurements of momentum were made with a three component ultrasonic current meter in the oceanic boundary layer under a first year floe of fast sea ice in barrow strait n w t by using eddy correlation methods values of horizontal reynolds stress were computed from each of 20 data runs of about 1 hour duration during which the mean current was reasonably steady a least squares analysis in which water stress was assumed proportional to the square of the mean current yielded a value for the 1 m water drag coefficient of 1 32 10 3 with a standard deviation of 0 06 10 3 the diameter of the floe was greater than 1 km and its mean thickness was 128 cm with a standard deviation of 4 cm and a maximum deviation of 7 cm

intermediate wavelength magnetic anomalies over the central pacific we have extracted the intermediate wavelength magnetic anomaly field from the marine survey data of the central pacific in the band pass of 4000 400 km spherical harmonic degree 10 100 our technique minimizes the effects of external field sources secular variation and strike aliasing the objective of this paper is to demonstrate this capability compare the derived sea surface field to the equivalent magsat data set and demonstrate that anomalies observed in both fields are correctable to geologic features within the oceanic lithosphere as expected the sea surface field exhibits a strong spatial correlation to and a better resolution than the magsat field however the magsat field also displays a diminished resolution for wavelengths less than 1900 km when compared to the upward continued sea surface data two reasons are likely for this diminished resolution 1 differences in the resolution of the spherical harmonic reference fields and 2 an increased noise level in the magsat data at the shorter wavelengths other sources of error are also considered we also demonstrate that the remanent magnetization of the central pacific seamounts produces negative magnetic anomalies which are observed at satellite altitude other sources including the thickened crust of the oceanic plateaus and regional petrologic variations can be associated with intermediate wavelength anomalies observed in both the magsat and the sea surface fields

autonomous underwater vehicle measurements of surface wave decay and directional spectra in the marginal sea ice zone in march 2003 several autonomous underwater vehicle auv missions were carried out under sea ice in the western bellingshausen sea data from the upward looking acoustic doppler current profiler adcp on the autosub auv indicate a strongly oscillating horizontal velocity of the ice due to ocean swell swell period height direction and directional spread have been computed every 800 m from the ice edge to 10 km inward for three missions exponential period dependent attenuation of waves propagating through sea ice was observed mean period increased with distance from the ice edge the wave field refracted during propagation the directional wave spread does not seem to relate to distance from the ice edge although higher frequencies tended to be more spread if suitably deployed an ordinary adcp may be used with this technique to study both scalar and directional properties of waves in open or ice covered water

sea ice floes dissipate the energy of steep ocean waves a laboratory experimental model of an incident ocean wave interacting with an ice floe is used to validate the canonical solitary floe version of contemporary theoretical models of wave attenuation in the ice covered ocean amplitudes of waves transmitted by the floe are presented as functions of incident wave steepness for different incident wavelengths the model is shown to predict the transmitted amplitudes accurately for low incident steepness but to overpredict the amplitudes by an increasing amount as the incident wave becomes steeper the proportion of incident wave energy dissipated by the floe in the experiments is shown to correlate with the agreement between the theoretical model and the experimental data thus implying that wave floe interactions increasingly dissipate wave energy as the incident wave becomes steeper

setup and swash on a natural beach wave setup and swash statistics were calculated from 154 runup time series measured on a moderately steep beach under incident waves varying from 0 4 to 4 0 m significant wave height when scaled by the incident wave height setup swash height and total runup the sum of setup and half the swash height were found to vary linearly with the surf zone similarity parameter 0 h0 l0 1 2 the foreshore slope appeared the appropriate value for the calculation of 0 although the setup data showed some influence of an offshore bar at low tide for low irribaren numbers the swash height in the incident frequency band becomes saturated while for high irribaren numbers no such signs of saturations were seen thus the infragravity band appears to become dominant in the swash below some value of 0 for these data that value is approximately 1 75 although there is considerable scatter associated with that estimate

generation of internal waves by sea ice observations in the greenland sea marginal ice zone suggest that when strong stratification occurs near the surface momentum flux into the internal wave field of the upper ocean can be a major component of the force balance that governs ice drift since energy is radiated away by the internal waves turbulent mixing and heat flux are also affected we investigate the downward flux of momentum and energy by internal waves in an ice covered ocean with an idealized model comprising a mixed layer overlying a deep uniformly stratified layer a buoyancy jump separates the mixed layer from the pycnocline drag from a single wave number component of the under ice roughness spectrum is derived as the product of a drag coefficient based on ice speed under ice roughness wave number and amplitude and uniform stratification to the ice ocean interface times an attenuation factor that depends additionally on mixed layer depth and the buoyancy jump at the base of the mixed layer the model is extended to a plausible spectrum of under ice roughness by integration over wave number space internal wave effects are parameterized in terms of the drag coefficient from the integrated spectrum and incorporated into an upper ocean turbulence model model calculations show that plausible values for the peak wave number of the idealized spectrum and roughness variance can account for 1 decreased ice velocity relative to the wind 2 decreased heat exchange coefficient between the ice and ocean and 3 lack of mixed layer deepening all of which were observed during the last week of the 1984 marginal ice zone experiment

estimating the crustal power spectrum from vector magsat data the earth s magnetic field can be subdivided into core and crustal components and we seek to characterize the crustal part through its spatial power spectrum rl we process vector magsat data to isolate the crustal field and then invert power spectral densities of flight local components along track for rl following o brien et al 1999 our model designated lppc is accurate up to approximately spherical harmonic degree 45 900 km this is the resolution limit of our data and suggests that global crustal anomaly maps constructed from vector magsat data should not contain features with wavelengths less than 900 km we find continental power spectra to be greater than oceanic ones and attribute this to the relative thicknesses of continental and oceanic crust

band limited global scalar magnetic anomaly map of the earth derived from magsat data the magnetometer satellite magsat data are separated into two distinct sets the dawn and the dusk sets the two sets are analyzed independently because of the dawn dusk asymmetry of the external magnetic field components of the data two scalar magnetic anomaly maps the dawn and the dusk maps are derived using the spherical harmonic expressions of the respective data set the significant correlation of the two maps in the harmonic range of 18 41 indicates the repeatability of the magsat data in these harmonics the shortest repeatable wavelength is about 920 km a band limited global scalar magnetic anomaly map is derived at about 410 km altitude by arithmetic averaging of the dawn and the dusk maps the map is then downward continued to about 10 km altitude the downward continued magnetic anomalies are relatively more localized the main features of the anomalies are as follows 1 continents have relatively stronger magnetic anomalies than oceans especially the younger oceans 2 many circum pacific subduction zones have magnetic signatures and 3 there is no overall correlation between anomalies and major shields