large horizontal surface velocity shears in the ocean obtained from images of refracting swell and in situ moored current data aerial photographs showing strong refraction of swell near the northern california coastline were obtained on september 8 1982 the possibility that this refraction is due to a large horizontal current shear is discussed a maximum shear value of the order of 10 3 s 1 with the velocity directed along a linear slick like front is calculated from the wave data in the photo in situ current data obtained at 90 m depth at a nearby midshelf site on two surface moorings separated by 390 m also show occasional periods of large horizontal velocity shear on january 4 1982 a maximum horizontal shear of order 10 3 s 1 was observed for several hours these separate observations taken together indicate that large horizontal surface velocity shears of order 10 3 s 1 occur over the northern california continental shelf and that large surface current shear can be usefully detected and measured by high flying high resolution sensors

remote synoptic surface current measurements by gravity waves a method and its test in a small body of water abstract synoptic surface current data in a lagoon have been obtained utilizing a new approach that evolved from the kinematics of wave current interaction surface current at a position was determined from wavelength and direction of two monochromatic wavetrains with known frequency these wave data were required only at the position of current determination the data collected by aerial photography were processed optically by two dimensional fourier transforms possible extension to surface flow measurements in the open ocean is discussed

antarctic sea ice variability during 1958 1999 a simulation with a global ice ocean model 1 a hindcast simulation is carried out with a global ice ocean model driven by the national centers for environmental prediction ncep national center for atmospheric research ncar reanalysis daily surface air temperatures and winds in order to document the variability of the antarctic sea ice cover during 1958 1999 the model consists of an ocean general circulation model coupled to a thermodynamic dynamic sea ice model with viscous plastic rheology its horizontal resolution is of 1 5degrees x 1 5degrees both the mean state and variability of the antarctic ice pack over the satellite observing period are reasonably well reproduced by the model in particular from november 1978 to september 1998 the model produces an overall increasing trend in ice area defined as the total area of ice covered ocean of 11 400 2300 km 2 yr 1 which falls within the range of current uncertainty the model annual mean area averaged ice thickness displays no significant trend over 1958 1999 in contrast a retreat of the pack is simulated during the second half of the 1970s and the beginning of the 1980s leading to a loss of ice cover of roughly 0 5 x 10 6 km 2 between 1958 1976 and 1982 1999 the marked weakening of the antarctic semiannual oscillation that occurred since the mid to late 1970s seems to be partially responsible for this behavior part of the modeled decline might however be spurious and due to the 1979 change in the observing systems utilized in the ncep ncar reanalysis the simulation also reveals a pronounced decadal variability 8 10 years in both ice area and thickness locally the changes in ice thickness can reach 1 m during a particular cycle

a double vortex along the california coast a double vortex was observed at the entrance to the santa barbara channel california with sequential ir imagery from the noaa 7 satellite during may 1984 while the wind blew strongly to the southeast the double vortex moved westward with a velocity greater than that estimated for the mutual interaction of two idealized vortices the effect of the double vortex on incoming pacific swell is calculated by numerical integration of the ray equations this calculation shows areas of focusing of the rays near the entrance to the channel which suggests elevated wave energy there with reduced wave energy nearby the pattern of wave refraction by mesoscale features such as a double vortex can help detect them and estimate their velocity distribution from remotely sensed surface wave imagery the similarities and differences between refraction patterns produced by double vortices and by eddies is outlined the double vortex and its generation by an episodic or tidal flow is investigated using kashiwai s 1984 approach an example of the related generation of a single vortex in the alboran sea by tidal flow in the strait of gibraltar is briefly discussed

synthetic aperture radar observations of currents colocated with slicks during low wind conditions our synthetic aperture radar sar imagery of the ocean contains examples of strong perturbations in the geometry of persistent turbulent ship wakes at the intersection of wakes with dark slicklike features the perturbations in the wakes are presumably caused by currents and are invariably colocated with slicklike submesoscale features the current dynamics influencing the geometry of turbulent wakes are hypothesized to contribute to the efficient accretion of a biotic and abiotic microlayer in a localized surface zone which is observed as a dark slick in our sar imagery time displaced sar imagery allows us to track the time evolution of the turbulent wake perturbations to determine the differential surface current responsible for the perturbations and to estimate the associated shear in addition the time displaced imagery also allows us to observe the temporal stability of the slicklike features

observation of wave refraction at an ice edge by synthetic aperture radar in this note the refraction of waves at the ice edge is studied by using aircraft synthetic aperture radar sar penetration of a dominant swell from open ocean into the ice cover was observed by sar during the labrador ice margin experiment limex conducted on the marginal ice zone miz off the east coast of newfoundland canada in march 1987 at an ice edge with a large curvature the dominant swell component disappeared locally in the sar imagery six subscenes of waves in the miz from the sar image have been processed revealing total reflection refraction and energy reduction of the ocean waves by the ice cover the observed variations of wave spectra from sar near the ice edge are consistent with the model prediction of wave refraction at the ice edge due to the change of wave dispersion relation in ice developed by liu and mollo christensen 1988

observations of wind waves in the gulf stream frontal zone experimental results of a study of wind waves in the gulf stream frontal zone are presented measurements of two dimensional radar spectra of surface waves and surface current were performed during ship crossings of the gulf stream under different wind conditions waves reflected from the gulf stream were observed on the windward side for an along current wind these effects resulted in a widening of the spectrum angular distribution and a deviation of the mean wave direction from that of the wind trapped waves were observed under against current winds depending on wave age the trapped waves were either locally generated developing waves or entered into the jet downstream nearly fully developed waves trapped waves significantly increased the wave energy locally the experimental data are interpreted in terms of wave kinematics and the equation of spectrum evolution in the relaxation approximation

effects of the gulf stream on ocean waves in the present study a third generation numerical wave model is used to study effects of a straight gulf stream and a gulf stream ring on ocean waves in swell and storm conditions the model accounts for all relevant processes of propagation generation and dissipation of the waves including current effects without imposing a priori restraints on the spectral development of the waves the dominating mechanism affecting the waves appears to be current induced refraction even though the short crestedness of the incoming waves tends to mask its effects also in swell conditions depending on wind and wave conditions refraction may trap locally generated waves in the straight gulf stream or it may reflect wave energy back to the open ocean in the gulf stream ring refraction induces a considerable variation in significant wave height and short crestedness but it hardly affects the mean wave direction in storm conditions the processes of generation and dissipation are considerably enhanced in countercurrent situations and reduced following current situations

wave refraction in an ocean front side looking airborne radar near cape hatteras north carolina recorded images of ocean waves wind streaks and intermediate water bodies within the gulf stream front in the enclosed waters of pamlico sound the radar images showed good accuracy in defining directions and distinguished waves from wind streaks at overlapping frequencies in the spectrum in passing through the cyclonic shear in the gulf stream front waves from both sides turned to their left and so specified their directions of propagation differences between spectra obtained across the front are in accord with current refraction theory an inverse wave refraction calculation furnished current shear values from which horizontal profiles of water velocity were compiled the total calculated velocity change of 1 5 m s is similar to previous measurements a secondary front near shore contained a velocity difference of 0 25 m s velocity changes across the gulf stream front were correlated with temperature changes the radar there showed bands of high reflectivity at the edges of intermediate water bodies whose temperature differences implied very small velocity steps increased roughness at the intermediate water boundaries was attributed to current interaction with waves

a case study of wave current interaction near an oceanic front high resolution directional wave spectrum data were obtained from two nasa airborne radars during the frontal air sea interaction experiment fasinex in february 1986 the observations show a significant change in the wave number spectrum across the front on the basis of surveys from a towed sensor and on satellite imagery the front location and current field are estimated a numerical model is developed for the wave current interaction and is used to model the wave refraction across the frontal current a parametric study is performed to demonstrate the effects of current meandering the main consequence of meandering is the formation of caustics and shadow zone regions in which the wave energy is significantly enhanced or reduced spectral simulation along the aircraft track reveals a reduction of more than 60 in wave energy in the shadow zone this is consistent with the observations

the shear convergence and thermohaline structure of a front abstract with the objective of measuring convergence directly to confirm previous observations of frontal subduction a seaward upwelling jet off central california was studied using satellite infrared images hydrographic sections ship drift and clusters of surface drifters the cyclonic front of the jet was sharper than 1 km resulting in a shear several times larger than f a cross frontal convergence of 7 cm s 1 over the width of the front equivalent to 0 8f was visible as a 20 m wide accumulation of debris the sharpness of the front lasted at least for a day away from the cyclonic front the divergence of the flow was small and the shear was less than 0 6f thermohaline layers originating at the front were interleaving along isopycnals suggesting water subduction it is proposed that the asymetry between anticyclonic and cyclonic sides of the jet and the strong convergence at the cyclonicfront resulted from a frictionally driven ageostrophic secondary circulation superimposed on the geostr

monitoring ice thickness in fram strait time series of ice thickness in fram strait are evaluated together with ice drift for estimation of the ice flux from the arctic ocean ice draft was measured from moored sonars every 4 min suggesting a nominal sampling interval of 30 m on the basis of these data the annual cycle of many statistics can be resolved the modal ice thickness shows a maximum of 3 15 m in may and a minimum in september of 2 43 m indicating an annual variation range of 0 72 m this compares with model results comprehensive drillings and most of the submarine observations during the last 2 3 decades indicating no significant long term ice thickness change this is also concurrent with the lack of significant temperature changes in the arctic ocean during the same period the mean ice thickness in the central core of the ice stream shows an annual amplitude of 1 m with a minimum in september of 2 25 m and a maximum in april may of 3 25 m the mean thickness of ice only open water excluded is 3 27 m and compares with 3 29 m which is the mean thickness of ice only from 17 submarine tracks in the arctic ocean 1960 1982 the mean annual ice export which is equal to the mean annual net production of ice in the arctic ocean is 2850 km 3 yr 1 for the period 1990 1996 this efflux shows significant variations from year to year with a minimum in 1990 1991 of 2050 km 3 yr 1 and a maximum in 1994 1995 of 4700 km 3 yr 1 this significant variation of nearly 130 is mainly caused by an increased atmospheric forcing and to a lesser degree by an increase in the annual mean ice thickness of 20 over the same period

wave current interaction near the gulf stream during the surface wave dynamics experiment this paper presents a case study on the wave current interaction near the local curvature of a gulf stream meander the wave data were obtained from in situ measurements by a pitch roll discus buoy during the surface wave dynamics experiment swade conducted off wallops island virginia from october 1990 to march 1991 owing to the advection of the gulf stream by the semidiurnal tide the discus buoy was alternately located outside and inside the gulf stream the directional wave measurements from the buoy show the changes in wave direction wave energy and directional spreading when waves encountered the current in the gulf stream meanders a wave refraction model using the ray tracing method with an estimated gulf stream velocity field and meandering condition was used to simulate wave refraction patterns and to estimate wave parameters at relative locations corresponding to buoy measurements the numerical simulation shows that a focusing zone of wave rays was formed near the boundary and behind the crest of a simulated gulf stream meander the focusing of wave rays causes changes in wave direction increases in wave energy and decreases in wave directional spreading which are in good agreement with the results from the buoy measurements

variability of fram strait ice flux and north atlantic oscillation we estimate the winter sea ice export through the fram strait using ice motion from satellite passive microwave data sea ice motion october may is obtained by tracking the displacement of common features in sequential 85 and 37 ghz brightness temperature fields the average winter area flux over the 18 year record 1978 1996 is 670 000 km2 7 of the area of the arctic ocean the winter area flux ranges from a minimum of 450 000 km2 in 1984 to a maximum of 906 000 km2 in 1995 the daily monthly and interannual variabilities of the ice area flux are high there is an upward trend in the ice area flux over the 18 year record the average winter volume flux over the winters of october 1990 through may 1995 is 1745 km3 ranging from a low of 1375 km3 in the 1990 flux to a high of 2791 km3 in 1994 the sea level pressure gradient across the fram strait explains more than 80 of the variance in the ice flux over the 18 year record we use the coefficients from the regression of the time series of area flux versus pressure gradient across the fram strait and ice thickness data to estimate the summer area and volume flux the average 12 month area flux and volume flux are 919 000 km2 and 2366 km3 we find a significant correlation r 0 86 between the area flux and positive phases of the north atlantic oscillation nao index over the months of december march correlation between our 5 years of volume flux estimates and the nao index gives r 0 56 during the high nao years a more intense icelandic low increases the gradient in the sea level pressure by almost 1 mbar across the fram strait thus increasing the atmospheric forcing on ice transport correlation is reduced during the negative nao years because of decreased dominance of this large scale atmospheric pattern on the sea level pressure gradient across the fram strait

some examples of detection of oceanic mesoscale eddies by the seasat synthetic aperture radar this note presents images of three dynamically different regions in the ocean to demonstrate the variety of mesoscale variabilities detected by the seasat synthetic aperture radar sar south of the grand banks of newfoundland a cold eddy is observed to form as the result of the southward intrusion of labrador sea water perhaps having led to the birth of a gulf stream extension ring off the northern coast of california features resulting from the offshore intrusions of cold upwelling water are observed two topographically generated eddies are detected near misteriosa bank on the cayman ridge in the northwestern caribbean comparisons are made with concurrent noaa 5 infrared images whenever the eddies have thermal signatures

influence of the exchanges between the atlantic and the arctic on sea ice volume variations during the period 1955 97 in order to investigate the causes of sea ice volume variations two simulations have been performed using two different sea ice ocean models driven by ncep ncar reanalysis data over the period 1955 97 in both models the largest changes in ice thickness occur in the central arctic the anomalous exchanges of sea ice air and water between the north atlantic and the arctic play a large role in explaining ice volume variations in particular the anomaly of oceanic heat transport between iceland and norway could alone explain a large part of the simulated ice volume changes the time evolution of the ice volume appears well correlated with the low frequency variations of the north atlantic oscillation nao nevertheless the link between nao and ice volume changes could operate through different mechanisms between the first and second half of the simulation the influence of the nao on the meridional heat transport in the north atlantic and in the nordic seas hold during the whole simulation besides the link between nao and fram strait ice export which is another important source of ice volume variability is valid only for the second half of the simulation

evidence for a recent change in the link between the north atlantic oscillation and arctic sea ice export evidence for a recent change in the link between the north atlantic oscillation nao and arctic sea ice export through fram strait during wintertime djfm is presented from the analysis of simulated arctic sea ice and observations for the period 1958 1997 whereas no correlation between the two time series is found from 1958 to 1977 r 0 1 the correlation increased significantly thereafter r 0 7 1978 1997 the increased coherency between the nao and arctic sea ice export through fram strait during the last two decades resulted from anomalous meridional wind components near fram strait which were associated with the more easterly position of the nao s centers of interannual variability compared to 1958 1977

simulation of the interannual variability of the wind driven arctic sea ice cover during 1958 1998 a thermodynamic dynamic sea ice model based on a granular material rheology developed by tremblay and mysak is used to study the interannual variability of the arctic sea ice cover during the 41 year period 1958 98 monthly wind stress forcing derived from the national centers for environmental prediction ncep reanalysis data is used to produce the year to year variations in the sea ice circulation and thickness we focus on analyzing the variability of the sea ice volume in the arctic basin and the subsequent changes in sea ice export into the greenland sea via fram strait the relative contributions of the fram strait sea ice thickness and velocity anomalies to the sea ice export anomalies are first investigated and the former is shown to be particularly important during several large export events the sea ice export anomalies for these events are next linked to prior sea ice volume anomalies in the arctic basin the origin and evolution of the sea ice volume anomalies are then related to the sea ice circulation and atmospheric forcing patterns in the arctic large sea ice export anomalies are generally preceded by large volume anomalies formed along the east siberian coast due to anomalous winds which occur when the arctic high is centered closer than usual to this coastal area when the center of this high relocates over the beaufort sea and the icelandic low extends far into the arctic basin the ice volume anomalies are transported to the fram strait region via the transpolar drift stream finally the link between the sea ice export and the north atlantic oscillation nao index is briefly discussed the overall results from this study show that the arctic basin and its ice volume anomalies must be considered in order to fully understand the export through fram strait

evidence for rapid thinning of sea ice in the western arctic ocean at the end of the 1980s examination of springtime ice drafts obtained from submarine profiles in a narrow band of the western arctic ocean from offshore alaska to 89 n indicates that the mean ice draft decreased 1 5 m between the mid 1980s and early 1990s no similar trend was evident in ice drafts near the north pole the 1980s drafts were composed largely of ice exceeding 3 5 m while the early 1990s drafts contained more ice in thinner categories the differences in drafts between the two periods appear to be related largely to ice dynamics effects associated with the presence and strength of the beaufort gyre which weakened considerably in the early 1990s

arctic warming evolution and spreading of the 1990s warm event in the nordic seas and the arctic ocean 1 observations in the arctic ocean revealed changes in oceanic temperature salinity and ice cover of the 1990s as compared with earlier data with a numerical model we favorably reproduce the development and subsequent propagation of temperature anomalies in water of atlantic origin in the 1980s and 1990s these propagated into the arctic ocean via the barents sea and the fram strait two warm anomalies entered the arctic ocean through these passages while the first smaller anomaly only warmed up the western eurasian basin the second large anomaly spread far into the eastern eurasian basin and across the lomonossov ridge into the western arctic basins intensified boundary currents during the high nao state in the first half of the 1990s significantly influenced the amplitude and speed of propagation of the temperature anomalies inside the arctic ocean in contrast to the notion of a continuous warming process during the 1990s our model results suggest the warming of the atlantic layer in the arctic ocean occurred in the form of events the event with the largest anomalous heat input during the modeled period entered the arctic between 1989 and 1994 it is possible to trace back the additional heat input into the arctic to an increased volume inflow via the faroer scotland passage and reduced heat loss to the atmosphere in the early 1990s after a weaker warm inflow in the second half of the 1990s the most recent observations and the model results point to a recurring warm anomaly in the inflow from 1999 onward

wave current interaction study in the gulf of alaska for detection of eddies by synthetic aperture radar high resolution ers i synthetic aperture radar sar images are used to detect a mesoscale eddy such features limit dispersal of pollock larvae and therefore likely influence recruitment of fish in the gulf of alaska during high sea states and high winds the direct surface signature of the eddy was not clearly visible but the wave refraction in the eddy area was observed the rays of the wave field are traced out directly from the sar image the ray pattern gives information on the refraction pattern and on the relative variation of the wave energy along a ray through wave current interaction these observations are simulated by a ray tracing model which incorporates a surface current field associated with the eddy the numerical results of the model show that the waves are refracted and diverge in the eddy field with energy density decreasing the model data comparison for each ray shows the model predictions are in good agreement with the sar data

the great salinity anomaly in the northern north atlantic 1968 1982 the widespread freshening of the upper 500 800m layer of the northern north atlantic which this paper describes represents one of the most persistent and extreme variations in global ocean climate yet observed in this century though a range of explanations have been advanced to explain this event including in situ changes in the surface moisture flux this paper describes the great salinity anomaly as largely though not entirely an advective event traceable around the atlantic subpolar gyre for over 14 years from its origins north of iceland in the mid to late 1960s until its return to the greenland sea in 1981 1982 the overall propagation speed around this subpolar gyre is estimated at about 3cm s 1 of the total salt deficit associated with the anomaly as it passed south along the labrador coast in 1971 1973 about 72 109 tonnes a deficit equivalent to about two thirds of this figure 47 109 tonnes ultimately passed through the faroe shetland channel to the barents sea arctic ocean and greenland sea during the mid 1970s possible effects on deep water formation and on the representativeness of historical section data are discussed

the open boundary condition in the united kingdom fine resolution antarctic model abstract the united kingdom fine resolution antartic model fram project it a community program to study the southern ocean central to this is an eddy resolving three dimensional primitive equation ocean general circulation model the open boundary condition at the northern boundary is described here the boundary condition is based on that of stevens 1990

sediment concentration and vertical mixing under breaking waves abstract field measurements of near bed sediment concentrations and sediment diffusivity in the breaker zone and in the inner surf zone of natural beaches were obtained using stacks of optical and fiber optical backscatter sensors under conditions with strongly plunging breakers as well as with spilling surf bores the field data suggest that significant differences exist between these wave types in terms of near bed sediment concentration levels and vertical mixing of sediment from the bed upward into the water column when plotting mean sediment concentration profiles in log c z space profiles are upward concave for spilling surf bores and sediment diffusivity increases approximately linearly upward from the bed for plunging waves however mean sediment concentration profiles are linear in a log c z plot indicating vertically constant sediment diffusivity this suggests that under surf bores sediment suspension from the bed occurs through diffusion processes while vertical mixing is mainly a convective process under plunging waves under surf bores sediment is largely confined to the lower layers of the water column while it is lifted to significantly higher elevations under plunging breakers

estimation of the two dimensional ocean current shear field with a synthetic aperture radar a double inverse method to convert sar data to estimates of current shear was developed theoretically problems of applying the technique to real sar data were examined the first step of the inversion process involved elimination of noise in the x vector k vector wave spectra deduced from sar images the second step estimation of current shear required the existence of well defined swell and a favorable single of interaction between swell and ocean current application of the method in the antarctic circumpolar current encountered sea conditions that both satisfied and violated these requirements where the wave current geometry was favorable realistic surface current velocity fields were obtained suggesting the presence of a substantial mesoscale eddy another attempt to apply the method in the gulf stream produced wave spectral estimates that showed no clear peak and hence produced no current estimates