magnitude scales for very local earthquakes application for deception island volcano antarctica abstract different magnitude scales are calculated for a set of volcano tectonic earthquakes recorded in deception island volcano antarctica the data set includes earthquakes recorded during an intense seismic series that occurred in january february 1999 with hypocentral distances that range between 0 5 and 15 km this data set is enlarged to include some regional earthquakes with hypocentral distances up to 200 km the local magnitude scale m l fixed at a hypocentral distance of 17 km is used as the reference for the other magnitude scales studied in the present work m l is determined on a standard wood anderson simulated trace assuming a gain of 2080 maximum peak to peak amplitudes are measured on the vertical components of a short period sensor the m w scale is calculated in the vertical component both for p and s waves the attenuation correction of the ground motion displacement spectra is introduced using data from coda waves studied in the area the comparison between m l values and m w estimations indicates severe discrepancies between both values a magnitude duration scale is calibrated from the comparison between coda durations of the recorded events and their assigned local magnitude scales in order to investigate the causes of the discrepancy between the m l and m w values we analyze two possible error sources a wrong coda q value or the effects of the near surface attenuation that initially are not taken into account in the correction of the ground displacement spectra the analysis reveals that the main cause of this discrepancy is the effect of the near surface attenuation the near surface attenuation is also the cause of the determination of an anomalous spectral decay slope after the corner frequency and the determination of this corner frequency value this near surface attenuation represented by is estimated over the data set obtaining an average value of 0 025 with this value the m w scale is recalculated using an automatic algorithm the new m w values are more consistent with the m l values obtaining a relationship of m w 0 78 m l 0 02

observations of the vertical structure of the eddy field in the kuroshio extension normalized kinetic energy frequency distributions at a site in the kuroshio extension along 152 e are independent of depth as a good first approximation the lowest frequencies are strongly vertically coherent from 350 to 6000 m depths nominal none of the other vertical sets of time series previously examined possess these characteristics to such a degree coherence being typically much lower between the thermocline and abyssal depths with spectral shape changing significantly as well the vertical distributions of eddy kinetic energy at this kuroshio extension site and for the gulf stream along 55 w are very similar if a depth independent contribution of approximately 100 cm2 s 2 be added to the former the latter observation suggests the existence of a more energetic weakly depth dependent component for the eddy field in the north atlantic than for the north pacific at special locations in the vicinity of their mid latitude jets possibly a signature of distinct instability mechanisms

regional eddy vorticity transport and the equilibrium vorticity budgets of a numerical model ocean circulation abstract the dynamical balances of the mean flow of a numerical model ocean general circulation experiment are examined through evaluation of regional vorticity budgets the instantaneous flow is strongly time dependent and the effect of eddy terms in the mean budgets is of primary interest budgets have been computed over volumes ranging in size from less than that of a typical model eddy up to an entire wind driven gyre using time series of 5 and 10 year durations the statistical reliability of terms in the budgets varies significantly with the region size over regions the size of an eddy or smaller the reliability is often poor but over the selected larger regions it is satisfactory the final analysis regions are selected by requiring that each be identified clearly with some part of the mean flow and that cancellation of the locally dominant terms within each region be minimized whenever possible the primary mechanism for balancing the wind stress curl vorticity input in each half basin is found

the flux and mixing rates of antarctic bottom water within the north atlantic calculation was made of the northward flow of antarctic bottom water into the western north atlantic basin through a passageway approximately 300 km wide between the ceara rise and the mid atlantic ridge at about 4 n two moorings carrying four current meters each were placed in the region where currents were expected to be the greatest and were left for 360 days the current was not steady but exhibited large surges with approximately a 60 day time scale whose cause is unknown current meters in the fastest flowing region all exhibited a net northward flow it is estimated that approximately 0 8 106 m3 s 1 of water colder than 1 9 c potential temperature flowed on the average into the north atlantic with this number possibly being too small by 0 3 106 m3 s 1 or too big by 0 1 106 m3 s 1 depending upon estimates of the width of the region which the current meters sample however these numbers differ from geostrophic estimates of 1 98 106 m3 s 1 the disagreement coming almost entirely from the fact that the geostrophic estimates give a sizeable flux of water colder than 1 2 c while the current meters do not based upon the two different flux rates residence times and mixing coefficients are calculated for antarctic bottom water for each 0 1 c potential temperature interval from 1 0 c to 1 9 c finally some comments on the dynamics of the northward flow are made

mid latitude mesoscale variability neither homogeneous quasi geostrophic turbulence nor the effect of eddies on the interior ocean circulation remains the major focus of mesoscale research as was the case for much of the preceding 20 years following identification of the phenomena concentration on basin wide geographical exploration of eddy properties was rather well established by 1978 over the last few years a promising zero order description and rationalization of the mesoscale field has become available that is eddy energy is largest near strong flows intensification not only in western boundary currents but along their mid latitude open ocean extension roughly as indicated by gyre scale numerical models where eddies develop as a result of instability processes in the mid latitude jet and recirculation regimes and then propagate into the ocean interior our point of view is that mesoscale eddies drive recirculations but do not play a crucial role in the dynamics of the mean flow in the ocean interior in the north atlantic for example the idea is that the downstream increase in transport of the gulf stream is eddy driven but not the flow through the florida straits in addition the fluctuating response to forcing by variable winds may now be widely perceived as forming the horizontally homogeneous background signal in subtropical gyres the time dependent field associated with thermohaline flows at abyssal depths also has a special character

gulf stream trajectories measured with free drifting buoys abstract during 1975 78 35 free drifting buoys measured surface currents in the gulf stream region the buoy trajectories trace numerous paths of the stream and show that the stream is strongly influenced by the new england seamounts this influence is manifested as 1 a quasi permanent 100 km southeastward deflection of the stream and the frequent occurrence of a ring meander over the seamounts 2 large amplitude meanders beginning at the seamounts and extending eastward and 3 small 20 km diameter eddies which appear to be generated locally by individual seamounts a chart of the mean temperature field at a depth of 450 m agrees with several of the patterns seen in the buoy trajectories west of the seamounts the mean path of the gulf stream is eastward over the seamounts the path turns sharply northeastward and the isotherms in the stream abruptly diverge

on gulf stream variability and meanders over the blake plateau at 30 n current and temperature variability of the gulf stream at 30 sup 0 n was observed with a subsurface array of current meter moorings from august 1980 to october 1981 energetic current fluctuations in the 2 14 day period band accounted for 45 70 of the total observed variability flow perturbations in the cyclonic shear zone tended to have a cyclonic sense of rotation and were 180 sup 0 out of phase from anticyclonic flow perturbations within the anticyclonic shear region these fluctuations appear to result from northward propagating wave like meanders of the gulf stream axis the fluctuations do not appear to be directly related to local wind forcing observations of near bottom flow on the blake plateau east of the gulf stream showed periods of prolonged southward currents lasting up to 40 days with speeds in excess of 30 cm s sup 1 the mean flow was also southward due to these strong events the source of the southward flow is unknown it may be connected to cold core eddies that are not detectable in the surface with remote sensing or possibly a remnant of the southward undercurrent that has been observed beneath the gulf stream off cape hatteas and on the deep slope of the blake escarpment

the three dimensional circulation near the eastern north pacific subtropical front abstract the purpose of our study is to describe and compute the large scale three dimensional circulation near the subtropical front in the eastern north pacific along 31 n this was accomplished through the use of four extensive hydrographic surveys historical wind stress data and also the movement of surface drifters our results indicate that in wintertime surface water sinks on the north side of the front and rises on its south side during the summer however the subtropical salty surface water overflows the frontal area to the north potential vorticity and heat are best conserved in a vertical flow pattern where the annual mean ekman convergence sinks to a depth of 300 m and water upwells throughout the main thermocline the computed horizontal flow below 700 m amounts to less than 0 6 cm s 1 both strength and direction depend greatly on the treatment of noise within the data set and also on the conservation statement that is specified in addition to geostrophic and hydrostatic dynamics a q

low frequency eddy kinetic energy spectrum in the deep western north pacific the frequency spectrum for the low frequency eddy kinetic energy is estimated from a long term current meter record obtained in a deep layer in the western north pacific the eddy field is characterized by three time scales an annual scale with zonal dominance of eddy motions a temporal mesoscale with meridional dominance and a monthly scale with horizontal isotropy about two thirds of the eddy kinetic energy is contained in the temporal mesoscale

characteristics of surface current flow inferred from a global ocean current data set a seasonal global ocean current data set ocds digitized on a 5 sup 0 grid from long term mean shipdrift derived currents from pilot charts is presented and described annual zonal means of v component currents show subtropical convergence zones which moved closest to the equator during the respective winters in each hemisphere net annual v component surface flow at the equator is northward zonally average u component currents have greatest seasonal variance in the tropics with strongest westward currents in the winter hemisphere an ensemble of ocean currents measured by buoys and current meters compares favorably with ocds data in spite of widely varying time and space scales the ocds currents and directly measured currents are about twice as large as computed geostrophic currents an analysis of equatorial pacific currents suggests that dynamic topography and sea level change indicative of the geostrophic flow component cannot be relied on solely to infer absolute strength of surface currents which include a strong ekman component comparison of ocds v component currents and meridional transports predicted by ekman theory shows agreement in the sign of transports in the midlatitudes and tropics in both hemispheres ekman depths required to scale ocds v component currents to computed ekman transports are reasonable at most latitudes withmore layer depths deepening closer to the equator less

satellite tracked drift buoy observations of the near surface flow in the eastern mid latitude north pacific the surface and near surface 30 m currents of the eastern mid latitude pacific obtained from 32 satellite tracked drifting buoys during the period june 1976 through august 1977 are discussed these observations of oceanic currents are merged with and are compared to fleet numerical weather center s daily estimates of surface winds for the same period statistical comparisons of these data sets reveal that the drifters move systematically approximately 30 to the right of the surface wind at approximately 1 5 of the wind speed this relationship between drifter movement and surface winds occurs during the entire period except for the summer months of 1976 when independent of the surface winds the buoy trajectories indicate mesoscale eddy motions with 100 km and 30 day time and space scales respectively drifter liftimes exceeded that of the 9 m diameter parachute drogues set at 30 m depth thus enabling a comparison of the currents derived from the drifter motions at the surface and 30 m these comparisons reveal no systematic difference in the drifter motions with and without drogues these results suggest a rather uniform flow in the upper 30 m during periods of steady winds the estimates of the nature of near surface currents obtained from the drifters differ from those predicted by ekman s classical theory of wind driven flow in that there is no evidence of either a reduction in speed or change in direction of the currents over the upper 30 m

stationary and traveling mesoscale perturbations in the kuroshio extension current abstract several hundred xbt observations in the western north pacific were collected each month over a 2 year period 1976 78 they have been assembled into a monthly map sequence of thermocline temperature in the kuroshio extension current having 100 km spatial resolution the overall time men of the maps exhibits mesoscale 200 600 km perturbations which correlate with several major bathymetric features especially the shatsky rise time variability about the mean decreases significantly cast of the shatsky rise in addition time variable mesoscale disturbances propagate zonally westward at 3 8 cm s 1 attempts to explain the observed propagation phase velocity through simple analytic baroclinic rossby wave theory lead to the implication that there exists in the region an eastward deep mean flow of 3 or 4 cm s 1 direct current measurements of long duration are required to help resolve and explain these observations

the baroclinic stability of the atlantic north equatorial current abstract polymode array iii cluster c was a collection of four current meter moorings that were placed in the atlantic north equatorial current for one year in may 1978 they were designed to look for indications of baroclinic instability such as downgradient eddy heat fluxes and upward phase propagation however fu keffer and other investigators find that the eddy heat fluxes tended to be more across gradient and sometimes even upgradient they also find only slight indications of upward phase propagation in this study the linear stability model of gill et al is applied to determine the expected heat fluxes and the modal shapes and scales due to linear amplification it appears that the expected fluxes should be detectable modal shapes are similar to the observed shapes but the fastest growing mode is only about half the size of the observed correlation scale although these discrepancies could be explained by simple statistical variability the supercriticality of the mean flow the large obser

the relationship between deep ocean currents and winds east of barbados abstract observations of currents collected at the polymode array iii cluster c 16 n 54 w are compared with radiosonde winds measured at barbados 13 n 62 w using a linear response analysis the winds and the currents are coherent throughout the water column 5400 m over the subinertial frequency range of 0 025 to 0 25 cpd the coherence is highest between the east west wind stress and north south currents over smooth topography the wind related ocean currents have a rms of 2 5 cm s 1 at 500 m and 1 1 cm s 1 at 4000 m and account for approximately one third of the total eddy kinetic energy the wind current phase is depth independent and does not vary significantly over the 200 km horizontal scale of the array the response amplitude is surface intensified and increases with decreasing frequency which is consistent with a deterministic theoretical model the coherence values and response estimates suggest an off resonant barotropic response to large scale wind forcing

the eddy radiation field of the gulf stream as measured by ocean acoustic tomography the gulf stream system is one of the most important components of the north atlantic circulation reciprocal acoustic transmissions have been analyzed to determine the structure and variability of temperature current velocity and relative vorticity in a region just south of the gulf stream observational evidence is presented that the gulf stream is the source of the energetic eddy variability found in the atlantic interior through highly nonlinear processes eddy activity energy radiates away from the stream southward via rossby wave packets

horizontal eddy fluxes of momentum and kinetic energy in the near surface of the kuroshio extension abstract sequential monthly mean temperature fields in the mid latitude western north pacific 30 45 n 140 180 e constructed from transpac xbt expendable bathythermograph data over a two year period are employed to analyze the interaction between mesoscale variability and the mean flow of the kuroshio extension an empirical temperature dynamic height regression and the geostrophic approximation lead to the computation of mean and eddy horizontal velocity components over the region from which horizontal eddy processes in both the mean momentum and the mean kinetic energy balances of the kuroshio extension are estimated the kuroshio extension generally loses mean kinetic energy as it travels from the coast of japan from 140 to 180 e with much larger eddy kinetic energy west of the shatsky rise 160 e than east of there concentrating upon zonally averaged quantities in both subregions east and west of the shatsky rise horizontal eddy momentum fluxes due to transient mesoscale variability i e

observed and modeled mesoscale variability near the gulf stream and kuroshio extension our earliest intercomparisons between western north atlantic data and eddy resolving two layer quasi geostrophic symmetric double gyre steady wind forced numerical model results focused on the amplitudes and largest horizontal scales in patterns of eddy kinetic energy primarily abyssal here intercomparisons are extended to recent eight layer model runs and new data which allow expansion of the investigation to the kuroshio extension and throughout much of the water column two numerical experiments are shown to have realistic zonal vertical and temporal eddy scales in the vicinity of the kuroshio extension in one case and the gulf stream in the other model zonal mean speeds are larger than observed but vertical shears are in general agreement with the data a longitudinal displacement between the maximum intensity in surface and abyssal eddy fields as observed for the north atlantic is not found in the model results the numerical simulations examined are highly idealized notably with respect to basin shape topography wind forcing and of course dissipation therefore the zero order agreement between modeled and observed basic characteristics of mid latitude jets and their associated eddy fields suggests that such properties are predominantly determined by the physical mechanisms which dominate the models where the fluctuations are the result of instability processes the comparatively high vertical resolution of the model is needed to compare with new higher resolution data as well as for dynamical reasons although the precise number of layers required either kinematically or dynamically or numerically has not been determined we estimate four to six when no attempt is made to account for bottom or near surface intensified phenomena

the velocity structure of the upper ocean in the presence of surface forcing and mesoscale oceanic eddies one goal of the joint air sea interaction experiment jasin was to investigate the structure of the near surface velocity field and to attempt to quantify what fraction of that field was related to the local wind toward that end in the late summer of 1978 two moorings were deployed in the northern rockall trough with oceanographic instrumentation concentrated in the upper 100m of the ocean simultaneous observations were made of the surface winds at each mooring and adjacent to one of the moorings of the velocity field at depths from 79 to 1000m energetic eddy like circulation dominated the velocity field in the jasin area at depths shallower than approximately 800m however both the velocity and the vertical shear of horizontal velocity showed variability that increased with proximity to the surface empirical orthogonal functions computed to separate the velocity data into uncorrelated modes of variability showed that over 97 of the variability in the upper 300m was distributed among only three vertical modes the first function had little depth dependence the second had strong depth independent flow in the depth range of the mixed layer and weak flow in the opposite direction at all depths below and the third had strong flow near the surface strong flow in the opposite direction just below the base of the mixed layer and weaker flow at all other depths function 1 alone provided a near complete description of the velocity variability below 85m where the flows associated with the eddy like circulation and the barotropic semidiurnal tide were the dominant components at 85m and above all three functions were necessary to provide a complete description temporal variability of function 2 was coherent with the local wind stress at the inertial frequency but at lower frequencies resulted in transport in the mixed layer to the southeast that was not coherent with the local wind low frequency temporal variability of function 3 was coherent with the local wind stress at these frequencies the velocity vector of function 3 nearest the surface was directed to the right of the wind stress vector and the velocity vector just below the base of the mixed layer was directed to the left of the wind stress thus forcing by the local wind can account for some but not all of the increased variability found near the surface

gulf stream meanders observations on propagation and growth we present a new method for continuously tracking the location of the gulf stream using a moored array of inverted echo sounders time series of lateral displacements of the front shown accurate to 8 km have been collected along three sections spaced 100 150 and 200 km downstream ne of cape hatteras north carolina for a period of 12 months these records are highly coherent at all periodicities longer than 4 days from the observed phase lags a dispersion relationship is presented for the meanders as the period and wavelength t h increase from 4 days 180 km to 30 days 600 km the phase speed decreases smoothly from 40 to 20 km d the meanders exhibited rapid growth at periods longer than 4 days doubling in variance in each 50 km step downstream this downstream growth is most appropriately described by a spatial e folding scale k 400 km for t x 14 days 400 km from an array of seven inverted echo sounders ies and three deep current meters deployed 100 to 200 km northeast of cape hatteras in 2000 4000 m water this array continu ously monitored the path of the gulf stream and the deep currents under it on four deployment and recovery cruises to the area expendable bathythermograph traces xbt and hydrographic data were collected to survey the temperature and density structure along the stream path and on sections normal to it after a brief review of earlier studies on meanders this paper presents a discussion of the measurement techniques using ies in the gulf stream we show by direct compari sons with traditional measurement methods that 1 each ies produces a continuous record of the thermocline depth here taken to be the 15oc isotherm for subsequent comparison with standard methods of determining the position of the north wall of the stream with accuracy of 20 m in a range of variation of 800 m and 2 the array of ies s paired across the stream produces continuous estimates of the stream position with 8 km accuracy the results from the three ies sections located 100 150 and 200 km downstream of cape hatteras show very high coherence throughout the array with rapid 20 to 40 km d l translation of the meanders downstream accompanied by rapid growth of the lateral displacements doubling in variance with each 50 km step downstream in a companion paper w e johns and d r watts manuscript in preparation 1982 we discuss the current meter records and the relationship of the deep currents to gulf stream meanders 1981 they have wavelengths of 150 200 km and propagate downstream at rates of approximately 30 40 km d downstream of cape hatteras the envelope of meandering broadens greatly to 200 300 km figure 1 and the stream can shift through more than its own width in the early shipboard surveys fuglister and worthington 1951 fuglis ter 1963 the path of the stream south of nova scotia was dominated by long wavelength 400 km large amplitude meanders those which were studied in detail were found to be quasi stationary and were characterized by a slowly evolving baroclinic field indicating temporal growth far ther upstream near 72ow however significant eastward propagation was observed one meander crest moved at a rate of approximately 17 km d for 3 days fuglister and worthington 1951 hansen 1970 later conducted monthly path surveys between 75 o and 55ow for 1 year with weekly surveys over a 2 month period using a towed thermistor he found meanders on appreciably shorter scales with wave lengths ranging from 200 to 400 km mean downstream propagation rates of 5 10 cm s and spatial growth rates of 2 x 10 3 km were estimated by constructing lines of constant phase between features on successive path se quences the rapid evolution between monthly paths how ever produced considerable ambiguity in identifying and tracking events weekly paths evolved more continuously and were apparently not aliased by higher frequency wave number phenomena

detecting a salt dome overhang with magnetotellurics 3d inversion methodology and synthetic model studies detecting a salt dome overhang is known to be problematic by seismic methods alone we used magnetotellurics mt as a complementary method to seismics to investigate the detectability of a salt dome overhang a comparison of mt responses for 3d synthetic salt models with and without overhang shows that mt is very sensitive to shallow salt structures and suggests that it should be possible to detect an overhang to further investigate the resolution capability of mt for a salt dome overhang we performed a 3d mt inversion study and investigated the impact of model parametrization and regularization we showed that using the logarithms of the conductivities as model parameters is crucial for inverting data from resistive salt structures because in this case commonly used tikhonov type stabilizers work more equally for smoothing the resistive and conductive structures the use of a logarithmic parametrization also accelerated the convergence and produced better inversion results when the laplace operator was used as a regularization functional we still observed that the inversion algorithm allows spatial resistivity gradients these spatial gradients are reduced if a regularization based on first derivatives in contrast to the laplace operator is introduced to demonstrate the favorable performance when logarithmic parametrization and gradient based regularization are employed we first inverted a data set simulated for a simple model of two adjacent blocks subsequently we applied the code to a more realistic salt dome overhang detectability study the results from the detectability study are encouraging and suggest that 3d mt inversion can be applied to decide whether the overhang is present in the shallow salt structure even in the case when only profile data are available however to resolve the overhang a dense mt site coverage above the flanks of the salt dome is required

a comparison of the mid latitude eddy fields in the western north atlantic and north pacific oceans abstract latitudinal distributions of estimates of eddy kinetic energy and off diagonal component of the horizontal reynolds stresses are demonstrated to have similar shapes at corresponding generally western locations but at much different depths in the mid latitude north atlantic and north pacific oceans this comparison is based on direct current measurements at abyssal depths in the north atlantic along 55 w and on indirect estimates of upper level velocity differences in the vertical for the north pacific between the izu or izu ogasawara or south honshu ridge and the shatsky rise these sections span the more energetic segments of the kuroshio extension and gulf stream systems

recent long term moored instrument observations in the western north pacific selected results based on the first deployment duration was 298 324 days depending on specific location of an array of 38 current temperature meters on 10 moorings along 152 e from 28 n to 41 n are discussed abyssal eddy kinetic energies along 152 e near the kuroshio extension are a factor of 2 to possibly 5 lower than those previously observed at similar depths close to the energetic regions of the gulf stream the thermocline kinetic energy frequency distribution at 28 n 152 e is similar in shape to that observed at a site near 31 n 70 w but not 28 n 70 w although the total energy levels are quite different however the distance from the corresponding strong current regimes is nearly the same the largest scale latitudinal distribution of the off diagonal component of horizontal reynolds stress has a characteristic general shape that is negative to the south and positive to the north of the gulf stream or kuroshio extension or mid latitude jet at all depths for all longitudes where such data are available in both oceans and in the numerical experiments examined

seismovolcanic signals at deception island volcano antarctica wave field analysis and source modeling the seismovolcanic signals associated with the volcanic activity of deception island antarctica recorded during three antarctic summers 1994 1995 1995 1996 and 1996 1997 are analyzed using a dense small aperture 500 m seismic array the visual and spectral classification of the seismic events shows the existence of long period and hybrid isolated seismic events and of low frequency quasi monochromatic and spasmodic continuous tremors all spectra have the highest amplitudes in the frequency band between 1 and 4 hz while hybrids and spasmodic tremors have also significant amplitudes in the high frequency band 4 10 hz the array analysis indicates that almost all the well correlated low frequency signals share similar array parameters slowness and back azimuth and have the same source area close to the array site the polarization analysis shows that phases at high frequency are mostly composed of p waves and those phases dominated by low frequencies can be interpreted as surface waves no clear shear waves are evidenced from the energy evaluation we have found that the reduced displacement values for surface and body waves are confined in a narrow interval volcano tectonic seismicity is located close to the array at a depth shallower than 1 km the wave field properties of the seismovolcanic signals allow us to assume a unique source model a shallow resonating fluid filled crack system at a depth of some hundreds of meters all of the seismic activity is interpreted as the response of a reasonably stable stationary geothermal process the differences observed in the back azimuth between low and high frequencies are a near field effect a few episodes of the degassification process in an open conduit were observed and modeled with a simple organ pipe

energetics of the surface circulation of the japan east sea during the period of august 1987 to june 1998 96 argos tracked drifters traversed through the japan east sea the displacement data from these drifters were used to compute 1 3 day average current vectors which described the circulation at 15 m depth the drifters that passed through the korea strait via the western channel were caught by the east korean warm current and the drifters that passed through the eastern channel were caught by the tsushima current individual observations of the 3 day average current over 40 cm s 1 were made in the warm waters south of the subpolar front and along the coasts there was a distinct decrease of eddy energy northward across the subpolar front conversion rates of eddy kinetic energy due to eddy mean flow interaction showed significant regions of both dissipation and production

north atlantic subtropical gyre sofar floats tracked by moored listening stations in 1980 sofar sound fixing and ranging floats were tracked acoustically in the western north atlantic entirely by means of moored autonomous listening stations during a 5 month period 17 float trajectories were obtained in the eastern 45 degrees to 65 degrees w gulf stream and subtropical gyre interior at depths of 700 and 2000 meters these mid depth trajectories suggest a time varying gulf stream with instances of both a narrow swift westward recirculation south of the stream and a northeastward penetration into the newfoundland basin a hundredfold increase of eddy kinetic energy was observed at 2000 meters from the gyre interior south of 30 degrees n to the gulf stream

a study of the bottom boundary layer over the eastward scarp of the bermuda rise velocity and temperature measurements from the bottom boundary layer bbl over the eastward scarp of the bermuda rise in water depth of 4620 m show little variability over an 8 month period the free stream flow 62 m above the bottom was south southeasterly following the isobaths in the region with an average speed of 22 cm s the current vector in the bbl rotated an average of 5 in a counterclockwise sense between 62 and 0 8 m above the bottom the thickness of the bbl was 40 m and the average magnitude of the bottom stress was 0 7 dynes cm2 mean speed profiles height of the bbl and the magnitude of the bottom stress predicted by a model compare favorably with the observations but the model predicts a rotation of the current vector between 62 and 0 8 m more than twice that measured the time dependent nature of the flow field is also reproduced by the model the bermuda rise data and speed profile measurements at the base of the scotian rise show that the m2 clockwise polarized tide is damped more than the mean current as the bottom is approached in the bbl this phenomenon is reproduced by the model and can be explained by differing effective ekman layer thicknesses associated with tidal and steady components of the flow

a preliminary exploration of the gulf stream system with sofar floats abstract sofar sound fixing and ranging floats deployed for engineering tests during 1977 79 yield the first long term quasi lagrangian observations in the subsurface gulf stream system the character of these float tracks supports the premise that the gulf stream is a persistent large scale vertically coherent jet at depths approximately within and above the main thermocline where mean and eddy kinetic energies are roughly the same and lateral motions of the stream are clearly delineated a float track at thermocline depth is visually coherent with the track of a concurrently launched surface drifter over the larger horizontal scales traversed during the first few months of their trajectories below thermocline depths fluctuation or eddy kinetic energies are normally larger than the mean and a persistent gulf stream is difficult to detect however deep motions that are visually coherent with upper level flows may be observed for an intermediate range of space and time scales eddy kinetic energi

excepftonally strong near bottom flows on the continental rise of nova scotia recent current velocity measurements across the lower continental rise of nova scotia show a deep equatorwardflow with speeds maximum 73 centimeters per second among the highest recorded for the deep sea silicate measurements indicate that this flow usually consists of southern source antarctic bottom water these measurements confirm the existence of a second and deeper western boundary flow that was earlier inferredfrom geological observations

stochastically forced current fluctuations in vertical shear and over topography the effects of constant vertical shear of zonal velocity uniform meridional bottom slope and a deep meridional wall representing the mid atlantic ridge on the response of a linear two layer quasi geostrophic ocean to stochastic wind stress curl forcing are investigated the model is motivated by discrepancies in energy levels and distributions between recent observations and a similar model without shear or topography that predicted remote coherences relatively well the results indicate that the presence of mean shear can significantly enhance the wind generation of surface intensified oceanic kinetic energy at frequencies above the zero shear cutoff for free baroclinic waves the primary effect of the sloping topography is enhanced barotropic response near the barotropic topographic sverdrup resonance where topographic and planetary potential vorticity gradients cancel the wall inhibits the barotropic response and increases coherences on the mooring side of the wall remote coherences for 49 day period fluctuations are due to long barotropic waves as in the previous model so the present model only partially reconciles the previous discrepancies

measurements of deep currents in the central north pacific two deep arrays of current meters at heights of 1200 and 100 m above the bottom were set under the north pacific subtropical gyre near 30 30 n 157 45 w the maximum record length obtained was 19 months autospectra show the following characteristics strong peaks at the m2 s2 tidal frequencies a broad peak at the diurnal inertial peak a spectral gap centered at 0 02 c h 1 and a regular increase in energy with decreasing frequency below the gap rotary components tend to be counterclockwise below 10 2 c h 1 and clockwise above the diurnal inertial frequency the m2 tidal amplitude ranged between 1 0 and 1 6 cm s 1 phase relationships suggest that a significant part of the motion is due to internal wave motion at this frequency low pass filtered velocity statistics show the following evidence of nonstationarity in the variances between the first 9 month and second 10 month arrays significant horizontal variation of mean kinetic energy on scales of 100 km and time space averaged eddy kinetic energy is comparable in magnitude to the lowest values measured under the subtropical gyre in the western north atlantic currents at 1200 m tended to be greater than at 100 m above the bottom spectra of the 100 m records indicate the presence of two peaks at low frequency a dominant and ubiquitous peak in the 1 105 1 175 c d l frequency band and a secondary peak which is shown in three of four moorings in the 1 58 1 75 c d 1 frequency band point estimates of the periods of the peaks give average values of 154 and 67 days comparison with subtropical western north atlantic spectra shows that the temporal mesoscale period 150 d is somewhat higher in the pacific data and it does not show the dominance of meridional motions observed in the atlantic spectra plane wave fits to the 154 day oscillation show the longest wave which closely fits the data has a wave length of 170 km and propagates toward 197 t the westward phase propagation and particle orbits which are transverse to the direction of propagation are consistent with planetary rossby wave dynamics this wave does not fit the dispersion relation for flat bottom planetary waves the observed phase speed is greater than that predicted by the dispersion relation it is shown that either doppler shifting of the frequency by the mean flow or topographic effects might account for the discrepancy

observations of wind forced deep ocean currents in the north pacific this study examines the atmospheric response of the deep ocean using 200 instrument years of moored current measurements from across the breadth of the mid latitude north pacific we have found evidence of a seasonal modulation in the ocean eddy kinetic energy beneath the thermocline at several locations north of 35 n this modulation is often in phase with the local atmospheric forcing function at frequencies between 0 1 and 0 01 cpd correlations between the local wind stress curl and the deep currents have been found for a few locations the local forced response is parallel to the potential vorticity gradient f h and consistent in amplitude with a local topographic sverdrup balance nonlocal forcing in the form of flow along isolines of potential vorticity has been estimated and it is comparable to the observed flow at some locations but the modeled flow is not correlated with the observations throughout the mid latitude north pacific the bottom slope tends to enhance the effect this suggests that topography narrows the available bandwidth for forced barotropic rossby waves facilitating a quasi steady topographic sverdrup response over most of the basin however the enhanced effect reduces the magnitude of the ocean s response to wind forcing nonlocally forced sverdrup currents flow along isolines of f h are the dominant component but are only of the order of 1 cm s 1 locally forced sverdrup currents flow across isolines of f h are typically of the order of 0 1 cm s 1 rms and are not commonly measured hence these wind forced variations are a background level signal in the mid latitude eddy kinetic energy in a few isolated areas the bottom slope reduces the effect in these regions the local sverdrup response is amplified and has been observed

mean circulation and eddy kinetic energy in the eastern north atlantic sixty two satellite tracked buoys have been released in the northern part of the western and central north atlantic during 1981 and 1982 by the institut fur meereskunde universitat kiel federal republic of germany additionally six buoys deployed in spring 1980 by the bedford institute have been taken over mean values and variances about the mean have been calculated for 3 by 3 areas the mean velocities confirm the large scale gyre circulation as deduced from hydrographie data and numerical models the center of the gyre is located at about 33 n eddy kinetic energy decreases east and southward and is concentrated along the north atlantic current it appears that the north atlantic current polar front is the main source of eddy kinetic energy for the northern north atlantic whereas the mid atlantic ridge is no source of eddy energy eddy kinetic energy along the north atlantic current decreases from about 1000 cm2 s 2 near newfoundland to 300 cm2 s 2 west of scotland a homogeneous pool of low eddy kinetic energy less than 100 cm2 s 2 appears in the central north atlantic east of the mid atlantic ridge the level increases towards the european and african shelf where the mean southward flow also becomes stronger the results concern the upper ocean

small scale cyclones on the periphery of a gulf stream warm core ring small scale cyclones found around gulf stream warm core ring 82b are investigated by using infrared satellite images and current information obtained with an acoustic doppler velocimeter currents in these cyclones reveal speeds ranging from 20 to 80 cm s one small cyclone or ringlet found in june 1982 was studied extensively by removing the basic rotational velocities of 82b the azimuthal velocity field for this ringlet was used with the gradient current equation to calculate the absolute dynamic topography at 100 dbar it was found that the ringlet was 13 dyn cm lower than its surroundings in addition neglect of the centrifugal term would have changed the dynamic topography of the ringlet by 30 from a comparison with ctd data the absolute reference level was determined and a vertical profile of horizontal currents was calculated for the ringlet other cyclones were found throughout the slope water region around warm core ring 82b with observable lifetimes of 1 to 2 weeks the northeast quadrant of 82b was a favored generation site for ringlets two cyclones were observed to form in this region and were advected anticyclonically around 82b typically at any one time six cyclones with diameters of approximately 40 to 50 km can be detected north of the gulf stream by using satellite images

gulf stream fluctuations and meanders over the onslow bay upper continental slope abstract gulf stream fluctuations observed over the 200 and 400 m isobaths off onslow bay north carolina have a prominent weekly time scale the principal fluctuations observed during the 4 month winter experiment are consistent with webster s 1961a description of downstream propagating skewed lateral meanders of the gulf stream over the upper continental slope the subtidal velocity fluctuations were highly coherent over the vertical extent 120 m and over the horizontal extent 64 km of our array the implied downstream propagation speed was 30 km day 1 for the weekly period meanders concurrent satellite images of a sea surface temperature sst meander pattern indicate that subsurface temperature salinity velocity and relative vorticity maxima occurred as meander crests shoreward sst front excursions passed over the experiment site the meandering currents were not coherent with nearby wind stress or coastal sea level fluctuations eddy flux estimates indicate energy conversion from the f

compact intrathermocline eddies in the sargasso sea we report observations of isolated lenses of constant temperature water embedded in the permanent thermocline in the sargasso sea these features are observed to have vertical extents of 220 m horizontal dimensions of 65 km and residence depths near 700 m a dynamic model is constructed which permits a balance among the pressure gradient coriolis and cyclostrophic forces and maintains the lens against gravitational collapse a result is that anticyclonic rim velocities approaching 20 cm s 1 are permitted and that these eddies have finite radii of the order of 50 km at which their thicknesses fall to zero

a seasonal signal in ocean currents to abyssal depths long term 9 32 month current meter records from the eastern north atlantic 41 n 59 n suggest that remote from the influence of the western boundary current eddy kinetic energy ke at all depths is principally a function of wind stress and stratification generated during the winter and early spring propagating to abyssal depths and decaying to a minimum in late summer although this seasonal variation in ke has not previously been described from the midlatitude ocean some theoretical basis for it is presented

vorticity balance for mid ocean mesoscale eddies at an abyssal depth current fluctuations in mid ocean are dominated by mesoscale eddies which have typical time scales of weeks to months and horizontal scales of tens to hundreds of kilometres such mid ocean eddies may have an important part in the general ocean circulation and intensive efforts have been made to observe current and density fluctuations in the world s oceans1 4 data sets describing eddy field in a selected region such as the mode1 and polymode3 regions have been obtained in the western north atlantic the local dynamics of eddy motions however have seldom been discussed on the basis of field measurement data this report examines the vorticity balance for mesoscale eddies as observed from current meter data currents at an abyssal depth were measured at five mooring stations in a cross pattern in the mid ocean of the western north pacific relative vorticity is estimated from horizontal derivatives of 10 day mean velocities its local time change is accounted for by the advection of planetary vorticity within the estimated error the vorticity balance is primarily linear and barotropic for mesoscale eddy motions at this depth

a linear stochastic field model of midlatitude mesoscale variability abstracta semiempirical model of midlatitude sea surface height ssh variability is formulated and tested against two decades of weekly global fields of merged altimeter data the model is constrained to match approximately the observed ssh wavenumber power spectrum but it predicts the spatiotemporal ssh field structure as a propagating damped linear response to a stochastic forcing field an objective coherent eddy identification and tracking procedure is applied to the model and altimeter ssh fields with a focus on eddies with lifetimes l 16 weeks the model eddy dataset reproduces the basic global mean characteristics of the altimeter eddy dataset including the structure of mean amplitude and scale life cycles the number distributions versus lifetime and the distributions of all eddy length scale realizations the model underpredicts the numbers of eddy realizations with large amplitudes and large scales overpredicts the growth of mean amplitude and scale with lifetime and modestly overpre

horizontal reynolds stresses in the central equatorial pacific seven 13 month long current records from the north pacific experiment hawaii to tahiti shuttle experiment during 1979 1980 were analyzed to investigate eddy energetics of the upper ocean within 75 km of the equator profiling current meter data from the experiment were also used in an attempt to relate the eddy motions to the mean zonal flow the current fluctuations are highly anisotropic with zonal current dominating after removal of a linear trend plus annual and semiannual harmonics the residual high frequency current fluctuations are much more nearly isotropic the high frequency eddy kinetic energy decreases by a factor of 2 over the upper 250 m and at 100 m has a relative minimum on the equator the seasonal time scale eddy kinetic energy shows a much more dramatic decrease below the thermocline with the exception of tidal peaks the spectra of zonal current are red while meridional current spectra show a flattening at the lowest resolvable frequencies meridional current was non stationary with an energetic packet of 10 to 15 day period oscillations occurring in early 1980 the off diagonal component of the reynolds stress tensor u calculated from the high frequency residuals is everywhere positive decreasing from north to south and with depth the implied cross equatorial flux of zonal momentum is at odds with eddy viscosity parameterizations which relate u to the mean meridional shear of the zonal current the observed distribution of reynolds stresses might be explained by radiation of energy from an instability of the portion of the south equatorial current north of the equator in all cases the high frequency u time series are significantly skewed toward positive values and show no evidence of periodicity indicating that the process es responsible is are intermittent and broadband this is consistent with the instability mechanism

drifting derelicts in the north atlantic 1883 1902 abstract in december 1883 the u s navy hydrographic office began to publish monthly pilot charts these showed the positions and drifts of abandoned derelict sailing vessels and other danger to navigation in the north atlantic during the years 1887 1893 a total of 1 628 derelict sightings were made the repeated sightings of 200 derelicts identified by name gave the first large scale and long term measurement of ocean trajectories a new chart of these trajectories between 1883 and 1902 is presented showing features of the general ocean circulation and early evidence of the variability of ocean currents

eddy kinetic energy in the north atlantic from surface drifters one hundred ten satellite tracked freely drifting buoys measured velocities and trajectories of the near surface currents in the north atlantic mean velocity values and the velocity variance about the mean were calculated for different regions a horizontal map of eddy kinetic energy was prepared on a 2 2 grid between latitudes 20 and 55 n maximum eddy energy 3000 cm2 s 2 coincides with the high speed gulf stream jet where it begins large amplitude meanders near 37 n 67 w a tongue of high eddy energy coincides with the stream s path eastward and around the grand banks into the newfoundland basin where values of 1000 cm2 s 2 are found a weaker tongue extends eastward across the mid atlantic ridge near 45 n a second weak extension reaches southeastward from the stream and crosses the mid atlantic ridge between 30 and 35 n north and south of the stream eddy energy diminishes rapidly reaching an e folding at 300 km from the axis values of 200 cm2 s 2 were observed in the mid gyre region and 100 cm2 s 2 in the eastern north atlantic and north equatorial current although the gross distribution of eddy energy is similar to that determined from ship drift measurements there are significant differences eddy energy from drifters amounts to about twice the value measured by ship drift in the gulf stream and one half the ship drift values in the mid gyre it is suggested that these differences are due to the horizontal averaging of mesoscale motion and the errors in navigation both of which are problems with the ship drift technique

on the geographical variability of the upper level mean and eddy fields in the north atlantic and north pacific abstract all available historical expendable bathythermograph data are used to compute means and standard deviations in temperature for the upper 500 m in the north atlantic and north pacific to take advantage of marked spatial differences in data coverage a variable grid scheme is employed both surface and subsurface annual mean temperature distributions agree well with published maps of these quantities climatological mean temperature salinity and salinity depth curves are used to infer salinities for the mean temperature profiles in order to compute 0 500 db inferred dynamic height and its standard deviation these mean curves are also used to compute eddy potential energy at 300 m the maps of mean annual inferred dynamic height agree well with traditional dynamic height maps and appear similar to the maps of mean temperature at 400 m the north atlantic eddy potential energy distribution is similar to an earlier calculation maps of the standard deviations in inferred dynamic height also represent