bluuebunny/crossref_metadata_2025 huggingface dataset:
Kuroshio temporal and spatial variations south of Japan determined from inverted echo sounder measurements
Authors: Book Jeffrey W. , Wimbush Mark , Imawaki Shiro , Ichikawa Hiroshi , Uchida Hiroshi , Kinoshita Hideki
Published: October 2002

The Affiliated Surveys of the Kuroshio off Ashizuri‐Misaki (ASUKA) project is a collaboration between scientists in Japan and the United States to improve our understanding of the structure and dynamics of the Kuroshio off the coast of southwest Japan. In this project an array of nine inverted echo sounders and nine current meter moorings were deployed for 2 years (1993–1995) along a line extending 1000 km from Shikoku. Numerous hydrographic profiles along this section were used to determine the regional gravest empirical modes (GEMs), which relate vertical profiles of temperature and specific volume anomaly to vertical acoustic echo time. These GEMs were used in conjunction with the inverted echo sounder data to calculate 2‐year time series of entire sections of relative geostrophic velocity and temperature. Current meter data provided absolute velocity references for the geostrophic velocity sections. In addition, a time series of Kuroshio position was determined from the GEM temperature sections. It shows three big (>80 km displacement) and three little offshore meanders of the Kuroshio occurring in February, May/June, and October of both years. This suggests a 4‐month periodicity, possibly phase‐locked to the annual cycle. The meanders may be caused by mesoscale eddies arriving from the east. The observed meanders were preceded by low Kuroshio volume transport and were followed by high Kuroshio volume transport. The Kuroshio volume and temperature transports were highly variable, with respective means of 65 ± 4 Sv and 3.7 ± 0.2 PW and standard deviations of 20 Sv and 0.9 PW.
Upstream control of Agulhas Ring shedding
Authors: Schouten Mathijs W. , de Ruijter Wilhelmus P. M. , van Leeuwen Peter Jan
Published: October 2002

Rings shed in the Agulhas Retroflection region play an important role in the global thermohaline circulation. The shedding of these rings has been considered very irregular. In this paper, we present evidence for remote control of the timing and frequency of the ring shedding events. This turns out to be a far more regular process, at a frequency of 4–5 cycles per year. The movement of the Agulhas Retroflection, and thereby the shedding of rings, is timed by incoming eddies from the upstream regions. Eddies from the Mozambique Channel, and from the East Madagascar current reach the retroflection region at the frequency of 4–5 times per year. The existence of these eddies can be related to incoming Rossby waves that cross the Indian Ocean and reach the Agulhas Current system. These may in turn be part of a basin‐wide oscillation. The irregularity found in ring shedding statistics can be ascribed to processes occurring between the actual shedding and the first unambiguous observation of a separated ring.
Surface salinity variability in the northern North Atlantic during recent decades
Authors: Häkkinen Sirpa
Published: October 2002

The sea surface salinity (SSS) variability in the North Atlantic is investigated using numerical model simulations for the last 50 years based on atmospheric forcing variability from the Comprehensive Atmosphere Ocean Data Set (COADS) and National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis. The focus here is the SSS variability in the subpolar region. The seasonality of the subpolar surface salinity variability is prominent with the maximum standard deviation (SD) occurring in the summer/fall period so that the amplitude of the summer SSS anomalies far exceed those of the wintertime. The interannual SSS variability in the subpolar gyre can be attributed mainly to excess ice melt, while the longer‐term (decadal) variability is associated with meridional overturning variability. In these two hindcasts, the deep mixing (which drives overturning changes) is forced by heat flux variability so that weak subpolar heat loss and subsequent weak overturning are usually manifested in fresh surface conditions in the subpolar gyre within 2 years. This is because the role of deep convection is to mix down the net fresh water input received by the high latitudes. Hence, at the lack of widespread deep mixing, a signal of slowing down overturning, the fresh water input that would be expected based on climatology would accumulate at the surface.
Current variability on a narrow shelf with large ambient vorticity
Authors: Peters H. , Shay L. K. , Mariano A. J. , Cook T. M.
Published: October 2002

Surface and subsurface currents and stratification were observed on and near the narrow shelf off Fort Lauderdale, Florida, in June–August 1999. The Florida Current flowed past, occasionally on the shelf, with speeds at times exceeding 2 m s−1 only 8 km offshore. The typical vorticity associated with the lateral shear of the Florida Current was 4f, where f is the local Coriolis parameter. Two dominant modes of higher frequency current variability were embedded in this low‐frequency flow, a 10‐hour signal with amplitudes of up to 0.5 m s−1 and an equally as strong, almost rectilinear signal with a 27‐hour period, the same as the local inertial period. Both signals were propagating northward along the shore with wavelengths of ∼27 and ∼170 km for the 10‐ and 27‐hour signals, respectively, and with corresponding phase speeds of 0.85 and 1.7 m s−1. The phase trend in east‐west direction of the 10‐hour signal is consistent with unstable, growing waves. The 10‐ and 27‐hour signals appear dominantly barotropic near the coast out to depths of at least 50 m, but the 10‐hour signal displayed a 180° phase change between surface and bottom farther away from the shelf break at 160 m depth. The two dominant signals cannot be attributed to barotropic instability.
Effects of correcting salinity with altimeter measurements in an equatorial Pacific ocean model
Authors: Vossepoel Femke C. , Burgers Gerrit , van Leeuwen Peter Jan
Published: October 2002

In this paper, we study the consequences of making salinity corrections in a tropical Pacific ocean model run for the period 1993–1997. Salinity and temperature corrections are obtained by assimilating temperature profile data and TOPEX/Poseidon sea level observations in the ocean general circulation model of the National Centers for Environmental Prediction (NCEP). The results are compared to two model runs in which no salinity corrections have been made, one using only temperature data and the other using both temperature and sea level data. The salinity correction sharpens the salinity front at the eastern edge of the western Pacific fresh pool, leading to improved patterns of salinity variability in the model. In addition, the salinity correction allows the model to follow both the temperature and the sea level more closely and estimates the heat content more accurately compared to the case when only temperature is corrected. In the western Pacific, the zonal pressure gradient resulting from the combined temperature and salinity corrections causes an acceleration of the Equatorial Under Current (EUC) that differs from the case when correcting temperature only. In the eastern Pacific, zonal current changes are similar for both runs in which sea level has been assimilated, and are directly related to local changes in surface pressure. We do not observe a remote effect of the salinity corrections on the zonal current structure in the eastern Pacific.
The spring transition from horizontal to vertical thermal stratification on a midlatitude continental shelf
Authors: Morey Steven L. , O'Brien James J.
Published: October 2002

The spring transition from wintertime horizontal thermal stratification to the summertime vertically stratified state on a wide, sloping midlatitude continental shelf is investigated. The sigma z‐level model (SZM), a new three‐dimensional primitive equation numerical ocean model employing a hybrid sigma coordinate and z‐level coordinate in the vertical, simulates the continental shelf region. A simple analytical model to describe the heating rate of a well‐mixed water column is applied to the problem to illustrate the importance of bottom slope in determining the evolution of the horizontal thermal stratification. The applicability of this simple model to continental shelf dynamics forced by realistic surface fluxes is studied with a “pseudo” two‐dimensional (x‐z) form of the SZM forced with daily heat flux and wind stress records from the West Florida Shelf. A term‐by‐term analysis of the depth‐averaged time‐integrated temperature equation quantifies the contribution of each physical process to the evolution of the horizontal temperature field. The simple model used to describe the heating rate of a homogeneous water column appears as the dominant balance in the integrated temperature equation. Cross‐shelf temperature advection associated with upwelling and downwelling alters the horizontal depth‐integrated temperature over the shelf predicted by the simple model. Two processes describe the spring transition. The erosion of the wintertime horizontal thermal gradient occurs on a timescale of a few months when the water column is being heated and mixed periodically. The second process is the formation of the seasonal thermocline following the last strong mixing event. This occurs on a timescale of less than a week under a stabilizing heat flux and light winds completing the spring transition from horizontal to vertical thermal stratification.
Winter cascading of cold water in Lake Geneva
Authors: Fer Ilker , Lemmin Ulrich , Thorpe S. A.
Published: October 2002

During the winters of 1998 and 1999, observations were made of the cascading of cold water from the nearshore, shallow “shelf” zones and down the sloping sides of Lake Geneva. Cascading starts on the average 10 hours after the onset of surface cooling. The draining cold water descends like a gravity current, and the downslope speed of the head of these slugs of cold water, U, has a mean value of 5.2 cm s−1, with slugs persisting, on the average, for 8 hours. When the Monin‐Obukov length scale at the water surface, L, is negative, implying convection occurs, and /|L|> 1,where is the mean shelf depth, the nondimensionalized speed of the front of “slugs,” U/b1/3 is found to be 1.3 ± 0.4, where b is the surface buoyancy flux integrated over the time period from one slug to the next. Each slug is unsteady, the head being followed by several fronts in which the temperature of the current decreases and its thickness increases. These fronts travel faster than the mean flow by a factor of r = 1.38 ± 0.3. Dynamical similarities are found with roll waves observed in turbulent open channel flows. The circulation induced by the cascade is found to give a positive skewness to the time derivatives of near‐surface temperature in shallow waters, in contrast with negative values close to the slope. The volume of cold water carried by a slug increases with downslope distance as a consequence of turbulent entrainment and the contribution of convectively unstable plumes from the surface. The average volume carried by the slug across the 21 m depth contour is about 1.9 times the volume of water in shallower water (i.e., that on the shelf between shore and a depth of 21 m), implying that cascading is an efficient means of flushing shelf water. Integrated around the lake the mean total volume flux amounts to 11.5 the average winter river inflow.
Mesoscale variability in the Alboran Sea: Synthetic aperture radar imaging of frontal eddies
Authors: Font Jordi , Rousseau Stephan , Shirasago Bernardo , García‐Górriz Elisa , Haney Robert L.
Published: October 2002

In autumn 1992 the entire Alboran Sea (western Mediterranean) was sampled by the Spanish R/V García del Cid. The incoming jet of Atlantic water, its associated meandering front, and the two big anticyclonic gyres were described from conductivity‐temperature‐depth (CTD) and acoustic Doppler current profiler (ADCP) data. Smaller‐scale eddies were also observed. Additionally, 36 ERS‐1 synthetic aperture radar (SAR) scenes were obtained from mid‐September to mid‐October. The SAR images capture these features when wind conditions are suitable. The current shear is mainly depicted as narrow lines of low backscatter because of the damping of waves by natural surface films. These lines delineate the northern border of both gyres and the beginning of the alongslope Algerian current at the eastern limit of the Alboran Sea. ADCP observations confirm that lines on SAR imagery follow the direction of the surface currents. The two gyres present high backscatter values in their center, while their frontal boundaries appear modulated by the variation of the marine atmospheric boundary layer stratification due to the surface thermal front. SAR has observed small spiral eddies that were not evidenced by the almost contemporaneous but too coarse CTD in situ sampling. Good spatial correspondence between radar‐detected and in situ‐measured structures occurs when comparing SAR images to the surface dynamic topography, rather than strictly surface water characteristics.
Spaceborne synthetic aperture radar observations of ocean waves traveling into sea ice
Authors: Schulz‐Stellenfleth Johannes , Lehner Susanne
Published: October 2002

Damping of ocean waves by sea ice is studied using spaceborne synthetic aperture radar (SAR) images of the marginal ice zone (MIZ) acquired by the European Remote sensing satellite ERS‐2. SAR imaging of waves damped by sea ice is analyzed theoretically. The impact of sea ice on the azimuthal cutoff is studied by simulation of the azimuthal SAR image autocorrelation function as well as the two‐ dimensional SAR image spectrum. Typical imaging artifacts like spiky wave crests and wave refraction seen on SAR scenes of the MIZ are reproduced by simulation. Sensitivity studies are performed using models for wind sea and swell systems. It is shown that the degradation of the azimuthal SAR image resolution is dominated by the orbital velocity variance of the waves, while the coherence time of the complex radar reflectivity has a minor impact. A first‐order analysis of wave damping observed on SAR scenes is carried out using a technique that was originally developed for wind estimation by Kerbaol et al. [1998]. The method does not require a priori information and is insensitive to real aperture modulation. The azimuthal SAR image cutoff wavelength is estimated and related to the orbital velocity variance of the sea surface by regression. The model is fitted on the basis of a global set of model ocean wave spectra. The technique is applied within the sea ice and in the open water in front of the ice boundary. On the basis of simple models for wind sea and swell, ocean wave attenuation rates are obtained from the observed orbital velocity decrease of waves entering the ice. The required wind information is derived from calibrated SAR data using the CMOD method. Two case studies showing examples from the Greenland and the Weddell Seas are given. It is shown that the estimated wave damping is consistent with damping parameters found in earlier field campaigns carried out in the Weddell Sea and the Bering Sea. An inversion technique providing estimates for the two‐dimensional wave spectra behind and in front of the ice boundary as well as a two‐dimensional filter function characterizing the sea ice impact is introduced. The technique is based on simultaneous inversion of the two‐dimensional image spectra in the open water and within the ice using a priori information from an ocean wave model. It is shown that the technique gives results consistent with the first‐order analysis based on the cutoff estimation.
Laboratory tests on ridging and rafting of ice sheets
Authors: Tuhkuri Jukka , Lensu Mikko
Published: October 2002

The mechanical deformation of a sea ice cover takes place through ridging and rafting. These processes have been studied in an ice basin by pushing two identical ice sheets together. Nonuniform ice sheets consisting of floes of thickness t1 and thin ice of thickness t2 connecting the floes were used. The major thickness t1 and the thickness ratio t2/t1 were varied. Ice sheets of uniform thickness (t2/t1 = 1) never formed ridges; they only rafted. However, when ice sheets of nonuniform thickness were used, initial rafting transformed into ridging. In general, high values of t1 and low values for t2/t1 favored ridging, while low values of t1 and high values for t2/t1 favored rafting. The forces during the tests were measured. During the initial rafting stage the force increased linearly with displacement. The experiments also suggest that the ridging force has a maximum value. This limit can be related to horizontal growth of the ridge or onset of ridging in another site. The relation between force and ice sheet thickness has also been analyzed. Further, from the force and the measured ridge profiles it was possible to estimate the ratio of work to change in potential energy. This ratio was about 15 for ridging and about 35 for rafting.
Assimilation of sea surface salinity in a tropical Oceanic General Circulation Model (OGCM): A twin experiment approach
Authors: Durand Fabien , Gourdeau Lionel , Delcroix Thierry , Verron Jacques
Published: October 2002

Observing ocean surface salinity in the global ocean is a challenging issue for future years' oceanographic activities. It is motivated by the active role of salinity that is now well recognized in ocean dynamics and ocean/atmosphere exchanges. This is particularly evident in the case of the El Niño Southern Oscillation (ENSO) phenomenon in the tropical Pacific Ocean. Improvements to ocean state estimation and predictions will require that salinity observations must be taken into proper account in conjunction with temperature and altimetric data. The sensitivity of a primitive equation model of the tropical Pacific Ocean to sea surface salinity (SSS) is studied through the use of a data assimilation technique in the rather academic “twin experiment” context. The data assimilation technique used, the Singular Evolutive Extended Kalman (SEEK) filter, is derived from the conventional Kalman filter theory. The paper explains why such a sophisticated technique is necessary. Indeed, an empirical scheme such as the Newtonian relaxation method, used in the same conditions, fails to constrain either the observed (surface) variable or the other components of the state vector. Within the experimental context chosen, the assimilation of SSS data with the SEEK filter is able to constrain most of the model variables linked with the SSS signal. SSS information, in particular, appears relatively successful in restoring zonal velocity, which is an important variable in warm/fresh pool migration, and in simulating a barrier layer in the atmospheric convergence zones. The final analysis errors are small and stable over time. This is widely true when simulating satellite SSS observations based on the GODAE criteria (0.2 psu error, 200 km, 10 days), which shows the potential of these observations. To extend these results to a real context, the problems of model‐data bias and unknown error covariances must be addressed as they are actually a strong limitation in assimilation performance when assimilating any real data set.
Mechanisms of the 1997–1998 El Niño–La Niña, as inferred from space‐based observations
Authors: Picaut Joel , Hackert Eric , Busalacchi Antonio J. , Murtugudde Ragu , Lagerloef Gary S. E.
Published: October 2002

The intensity of the 1997 El Niño and the 8°C sudden drop in sea surface temperature (SST) around 0°–130°W during the turn into La Niña in 1998 were a surprise to the scientific community. This succession of warm and cold events was observed from start to finish with a comprehensive set of remotely sensed and in situ observations. In this study we employ space‐based observations to demonstrate, for the first time, their maturity in capturing the preconditioning, onset, evolution, and decay of the 1997 El Niño and its transition into the 1998 La Niña. An accumulation of warm water in the west and equatorial wave reflection on the western ocean boundary appeared favorable for the development of El Niño. However, the action of a series of westerly wind bursts from December 1996 to June 1997, notably in March 1997, was instrumental in setting up this huge El Niño. The westerly wind bursts excited equatorial downwelling Kelvin waves and advected the eastern edge of the warm pool eastward, which triggered a distinct warming over the central and eastern parts of the equatorial basin. Once these warmed regions joined, the coupling between the SST and surface winds was fully effective, and El Niño reached its mature phase. By that time much of the warm waters of the western equatorial Pacific was transferred toward the east by surface eastward currents. The demise of El Niño and its turn into La Niña in spring 1998 were due to the arrival in the east of various interrelated phenomena. Upwelling was brought from the west by favorable off‐equatorial wind stress curl and equatorial Kelvin waves generated by easterly winds and wave reflection on the western ocean boundary. Additional upwelling was brought from the east by equatorial Rossby waves generated by westerly winds. These various upwelling signals were added to the general uplifting of the thermocline because of the slow discharge of the upper layer of the equatorial basin by diverging surface currents. A series of equatorial Kelvin and Rossby waves, characterized by upwelling and opposite surface currents, led to the breakup of the warm waters, the surfacing of the thermocline, and the drastic drop in SST in May 1998 around 0°–130°W. With the arrival of cold water in the east the easterly winds expanded from the west, and La Niña turned into a growing mode. This view of the 1997–1998 El Niño–La Niña, afforded from space, enables the testing of various El Niño theories.
Morphological response to a North Sea bed depression induced by gas mining
Authors: Fluit Caroline C. J. M. , Hulscher Suzanne J. M. H.
Published: October 2002

Gas mining leads to saucer‐like surface depressions. In the North Sea, gas is currently mined at several offshore locations. The associated bed depression has a similar spatial extent as offshore tidal sandbanks, which are large‐scale bed patterns covering a significant part of the North Sea bottom. The morphological time scales of bed depressions and tidal sandbanks are similar, so that significant interaction between these features is expected. In this paper we allow the bed depression to become morphologically active. A simple depression model based on a homogeneous soil is tuned with data of a bed depression near the Dutch barrier island of Ameland. Next, this subsidence model is included in a morphodynamic model. We show that this model is able to explain tidal sandbanks, which represent natural bed behavior. Here we approximate the solution by an expansion up to first order. The zeroth‐order solution of the model is a flat bed with a spatially uniform, time‐independent current. The first‐order solution is investigated using a Fourier transformation. In general, we observe significant interaction between the bed depression and the natural sandbank formation process. The process of induced bed depression triggers and intensifies the natural morphological behavior of the offshore seabed. The model also shows essential differences between modeling a morphodynamically active marine bottom depression and a bottom depression below the threshold for sediment motion. The maximum bed level depression in the active case is significantly larger, and the circular shape of depression contours is affected by stretching toward the preferred orientation of the tidal sandbank formation process.
Box inverse models, altimetry and the geoid: Problems with the omission error
Authors: Losch Martin , Sloyan Bernadette M. , Schröter Jens , Sneeuw Nico
Published: October 2002

When one combines satellite altimetry and a geoid model to improve estimates of the ocean general circulation from hydrographic data with a box inverse model, there arises a problem of different resolution and representation of the data types involved. Here we show how this problem can lead to an artificial leakage of the error estimates of short‐scale (high degree) spherical harmonic functions into long wavelength (low wave number) Fourier functions. A similar paradox effect can be seen in an idealized box inverse model constrained by additional sea‐surface topography data of low, medium, and high resolution: When more information is added in the form of additional smaller scales, the error of a transport estimate eventually increases. Consequently, including the large geoid omission errors associated with smaller scales in a box inverse model of the Southern Ocean increases the posterior errors of transport estimates over those of a model that does not include the geoid omission error. We do not claim that including or excluding the geoid omission error is correct. Instead, we juxtapose two different ways of estimating the geoid errors to demonstrate the effect that the omission error might have on the long, supposedly well‐known, scales. How (or if) to properly account for the geoid omission error must be the topic of further research. A proper treatment of the geoid model errors is demanded when one evaluates the errors of absolute sea‐surface topography data.
Interannual sea surface salinity and temperature changes in the western Pacific warm pool during 1992–2000
Authors: Delcroix Thierry , McPhaden Michael
Published: October 2002

Sea surface salinity (SSS) and sea surface temperature (SST) in the western Pacific warm pool (130–180°E; 10°N–10°S) are analyzed for the period 1992–2000 taking advantage of complementary data from the ship of opportunity program and the Tropical Atmosphere‐Ocean (TAO)‐Triangle Trans‐Ocean Buoy Network (TRITON) array of moored buoys. Covariability of these variables with surface wind stress, surface zonal currents, evaporation, precipitation, and barrier layer thickness is also examined. These fields all go through large oscillations related to the El Niño Southern Oscillation (ENSO) cycle, most notably during the record breaking 1997–1998 El Niño and subsequent strong 1998–2000 La Niña. East of about 160°E, during El Niño, precipitation minus evaporation increases in the equatorial band, in conjunction with anomalous increases in westerly winds, eastward surface currents, SST, and decreases in SSS. Opposite tendencies are evident during La Niña. Peak to peak 2°N–2°S averaged variations reached as much as 1.2 m s−1 for zonal currents and 1.5 practical salinity units (psu) for SSS. West of about 160°E, SST cools during El Niño and warms during La Niña, opposite to what occurs further east. To understand these SST tendencies west of 160°E, a proxy indicator for barrier layer formation is developed in terms of changes in the zonal gradient of SSS (∂S/∂x). Zonal SSS gradients have been shown in modeling studies to be related to barrier layer formation via subduction driven by converging zonal currents in the vicinity of the salinity front at the eastern edge of the warm pool. Correlation between changes in ∂S/∂x and changes in SST a few degrees longitude to the west is significantly nonzero, consistent with the idea that increased barrier layer thickness is related to warmer SSTs during periods of westward surface flow associated with La Niña, and vice versa during El Niño. Direct evidence of barrier layer thickness variations in support of this hypothesis is also presented.
Modeling bottom roughness in the presence of wave‐generated ripples
Authors: Styles Richard , Glenn Scott M.
Published: October 2002

Near‐bottom wave, current and microtopography measurements have been obtained at the LEO‐15 site off the southern coast of New Jersey. The measurements are used in conjunction with a recently calibrated continental shelf bottom boundary layer model to refine estimates of bottom roughness for a rippled bed under combined wave and current flows. In the presence of nearly two‐dimensional wave‐generated ripples, application of the Grant and Madsen [1979] type bottom boundary layer model produces estimates of the bottom roughness felt by the current that are several times greater than established formulations for rippled beds under waves. Modifying one of the primary length scales in the eddy viscosity profiles returns estimates that are more consistent with the wave roughness results. Application of the roughness model in conjunction with a refined empirical ripple geometry model produces highly accurate estimates of measured shear velocities and apparent roughnesses during a moderate storm. The demonstrated accuracy of this simple combined roughness and ripple geometry formulation reveals its utility when considering applications of coupled bottom boundary layer–shelf circulation models.
Backward Fokker‐Planck equation for determining model valid prediction period
Authors: Chu Peter C. , Ivanov Leonid M. , Fan Chenwu
Published: October 2002

A new concept, valid prediction period (VPP), is presented here to evaluate ocean (or atmospheric) model predictability. VPP is defined as the time period when the prediction error first exceeds a predetermined criterion (i.e., the tolerance level). It depends not only on the instantaneous error growth but also on the noise level, the initial error, and the tolerance level. The model predictability skill is then represented by a single scalar, VPP. The longer the VPP, the higher the model predictability skill is. A theoretical framework on the basis of the backward Fokker‐Planck equation is developed to determine the mean and variance of VPP. A one‐dimensional stochastic dynamical system [Nicolis, 1992] is taken as an example to illustrate the benefits of using VPP for model evaluation.
Temporal and spatial variability of surface fluxes over the ice edge zone in the northern Baltic Sea
Authors: Brümmer B. , Schröder D. , Launiainen J. , Vihma T. , Smedman A.‐S. , Magnusson M.
Published: October 2002

Three land‐fast ice stations (one of them was the Finnish research ice breaker Aranda) and the German research aircraft Falcon were applied to measure the turbulent and radiation fluxes over the ice edge zone in the northern Baltic Sea during the Baltic Air‐Sea‐Ice Study (BASIS) field experiment from 16 February to 6 March 1998. The temporal and spatial variability of the surface fluxes is discussed. Synoptic weather systems passed the experimental area in a rapid sequence and dominated the conditions (wind speed, air‐surface temperature difference, cloud field) for the variability of the turbulent and radiation fluxes. At the ice stations, the largest upward sensible heat fluxes of about 100 Wm−2 were measured during the passage of a cold front when the air cooled faster (−5 K per hour) than the surface. The largest downward flux of about −200 Wm−2 occurred during warm air advection when the air temperature reached +10°C but the surface temperature remained at 0°C. Spatial variability of fluxes was observed from the small scale (scale of ice floes and open water spots) to the mesoscale (width of the ice edge zone). The degree of spatial variability depends on the synoptic situation: during melting conditions downward heat fluxes were the same over ice and open water, whereas during strong cold‐air advection upward heat fluxes differed by more than 100 Wm−2. A remarkable amount of grey ice with intermediate surface temperature was observed. The ice in the Baltic Sea cannot be described by one ice type only.
Sea level variability at Ulleung Island in the East (Japan) Sea
Authors: Kim Kuh , Cho Yang‐Ki , Choi Byoung‐Ju , Kim Young‐Gyu , Beardsley Robert C.
Published: October 2002

Monthly adjusted sea level (ASL) was analyzed to examine the characteristics of sea level variation at Ulleung Island in the East (Japan) Sea from 1979 to 1992. It is remarkable that the nonseasonal variation is comparable with the mean seasonal variation in amplitude at Ulleung Island, whereas the mean seasonal variation is dominant along the eastern coast of the Korean peninsula. At Ulleung Island the variation in the mean seasonal, interannual, and short‐term component variability is 54, 13, and 30% of the total ASL variance, respectively. ASL variability is highly correlated with the upper layer heat content around Ulleung Island. There are two factors that contribute to large nonseasonal variation of the ASL around the island. One is the location of the polar front that divides the northern cold water region and the southern warm water region. The other is the presence of a warm eddy that frequently forms near Ulleung Island. When the northern cold water region extends southward to Ulleung Island, the heat content of the upper layer decreases, and ASL around the island falls. In contrast, when the southern warm water region extends northward to the island, the heat content increases, and ASL rises greatly. The movement of the front with a period of 1–2 years causes interannual variation of ASL. When a warm eddy approaches Ulleung Island, the heat content increases, and ASL rises. The short‐term variation in ASL is caused primarily by the migration of the warm eddy around Ulleung Island.
Computation methods of major tidal currents from satellite‐tracked drifter positions, with application to the Yellow and East China Seas
Authors: Lie Heung‐Jae , Lee Seok , Cho Cheol‐Ho
Published: October 2002

Two practical techniques of “correction method” and “fitting method” for computing major diurnal and semidiurnal tidal currents from irregular satellite‐tracked drifter position data have been developed and then applied to the Yellow and East China Seas to compute M2 and K1 currents. The correction method calculates the most dominant diurnal and semidiurnal tidal currents using the correction coefficients, defined amplitude ratios, and phase differences between the major tidal currents. On the other hand, the fitting method calculates major tidal currents by fitting the amplitudes and phases to a given polynomial function in space, without utilization of the correction coefficients. Validation tests for sampling intervals, data truncation, effects of low‐frequency currents, and position errors are performed to improve computation results. Both methods enable us to evaluate effectively and correctly strong tidal currents provided that the signal‐to‐noise level in drifter positions is not significantly high. The spatial pattern and current ellipses of M2 and K1 tidal currents from drifter positions in the Yellow Sea are fairly well consistent with those from moored current data. Application of the two methods to the Yellow and East China Seas exhibits commonly that M2 and K1 currents rotate clockwise in the East China Sea but counterclockwise in the Yellow Sea interior, with rotation direction change around 35° and 34° N, respectively.
The Canada Basin, 1989–1995: Upstream events and far‐field effects of the Barents Sea
Authors: McLaughlin Fiona , Carmack Eddy , Macdonald Robie , Weaver Andrew J. , Smith John
Published: October 2002

The effects of upstream events on southern Canada Basin waters were examined using physical and geochemical data collected at one location between 1989 and 1995. These events included Atlantic layer warming, relocation of the Atlantic/Pacific water mass boundary, and increased ventilation of boundary current waters. Early signals of change in the Canada Basin first appeared in 1993 along the continental margin and by 1995 were evident in the basin interior and farther downstream. Differences in physical and geochemical properties (nutrients, oxygen, 129I, and CFCs) were observed between 150 and 1600 m of the water column. By 1995 the boundary between Pacific‐ and Atlantic‐origin water was shallower and Atlantic‐origin water occupied more of the Canada Basin water column. Atlantic‐origin lower halocline water was colder and more ventilated, Fram Strait Branch water was colder, fresher, and more ventilated, and Barents Sea Branch water was warmer, fresher, more ventilated, and occupied a larger volume of the water column than in previous years. Water mass analysis showed that the composition of Atlantic‐origin water in the Canada Basin in 1995 included 20% more water from the Barents Sea. Two related events upstream appear to be the source of this change: an atmospheric regime shift in 1989 that increased cyclonic circulation, a transition unprecedented within the latter half of the 20th century in magnitude, geographic reach, and apparent oceanographic impacts, and a large volume of dense water flowed from the Barents Sea to the Arctic Ocean between 1988 and 1989. These two phenomena illustrate the close relationship between atmospheric and oceanic events whose combined influence was evident 6 years later and 5100 km downstream in the Canada Basin at depths >1000 m.
Influence of benthic boundary layer dynamics on wind‐induced currents in the Ebro delta inner shelf
Authors: Jiménez Jose A. , Guillén Jorge , Sánchez‐Arcilla Agustín , Gracia Vicente , Palanques Albert
Published: October 2002

Measurements taken by two instrumented tripods deployed on the Ebro delta inner shelf at 8.5‐ and 12.5‐m depth were used to characterize low‐frequency currents under the action of a moderate eastern storm. Results show that inner shelf currents respond very rapidly to wind action with along‐shelf currents dominating over the across‐shelf ones. Mean along‐shelf velocities of 0.35 and 0.24 m s−1 at the inner and outer position, respectively, were recorded during the event, and they were highly correlated with the along‐shelf wind stress. The measured current profiles were used to derive wind drag coefficients, assuming a balance between the along‐shelf components of the wind and current bottom stresses. Since under eastern wind events, wave action in the Ebro inner shelf is significantly enhanced, wave‐current interaction processes controlling the current bottom stress and sediment mobilization can affect the current structure. During the event, depth‐averaged concentrations in the lower meter of the water column larger than 1 g L−1 were recorded that were also accompanied by the generation of vertical concentration gradients larger than 1 g L−1m−1. When the bottom drag coefficient was obtained from velocity profiles without considering the presence of these gradients, the required wind drag coefficients to fulfil the along‐shelf balance were much higher than the ones derived from wind data, and they vary across the inner shelf. However, when bottom drag coefficients are obtained including stratification effects, the wind drag coefficients fulfilling the along‐shelf balance were of the same order of magnitude as the ones derived from wind data and without any difference at both locations.
A stochastic, multiscale model of microwave backscatter from the ocean
Authors: Plant William J.
Published: October 2002

The conventional view of microwave backscatter from the ocean is based on composite surface and quasi‐specular theories. In this view, backscatter at intermediate incidence angles is due to Bragg scattering from freely propagating short surface waves that are advected and modulated by longer waves. At small incidence angles the scattering process becomes quasi‐specular, coming from small facets aligned normal to the incident waves. The transition between these two processes is said to occur at incidence angles of about 10° to 20°. In this paper we demonstrate that advances in scattering theory and in computing speed make it possible to improve this view. We show that recent scattering theories agree on the form of the backscatter for incidence angles below that where multiple scattering must be considered, i.e., below about 80°. This form involves the Kirchhoff integral multiplied by a coefficient dependent on dielectric constant and incidence angle. We avoid the higher‐order calculations necessary in these theories to include the variable local incidence angle caused by surface wave slopes by applying them over restricted regions of the surface. We successively break the surface into regions from which the scatter comes from small‐, intermediate‐, and large‐scale waves. We show that in this picture, scattering from small‐scale waves is classic Bragg scattering and is very common while from large‐scale waves it is classic quasi‐specular scattering and is rarely important. For intermediate‐scale waves we evaluate the Kirchhoff integral numerically; this type of scattering becomes increasingly important with increasing wind speed. For all scales but the large one we correct the incidence angle for the slopes of all longer waves as required by composite surface theory. On this picture the transition from Bragg scattering to Kirchhoff scattering occurs gradually in a manner that is dependent on incidence angle, azimuth angle, wind speed, and the surface wave spectrum. The model indicates that Bragg scattering is often viable to surprisingly low incidence angles at low wind speeds. The model is sensitive to the wave height variance spectrum over a wide range of wave numbers. We use two recently published forms of this spectrum to compare the predictions of the model to various data that have been collected over the incidence angles range from 0° to 50°. At 0° this model produces a better fit to Ku band data from the TOPEX altimeter than does quasi‐specular theory and does so with no artificial “effective reflection coefficient.” As the incidence angle increases, the model continues to show good agreement with data without an artificial division into “quasi‐specular” and “Bragg” scattering. The advantage of this formulation over a quasi‐specular one is demonstrated by comparing the two models with data on received power taken at 36 GHz for incidence angles between nadir and 30°.
Laboratory measurements of the vertical structure of rip currents
Authors: Haas Kevin A. , Svendsen Ib A.
Published: October 2002

This paper describes the three‐dimensional variation of rip currents through the use of experimental measurements of rip currents in a directional wave basin. The rip currents are unstable, appearing sporadically at any given location. The vertical profile of the rip current is found to vary from depth‐uniform inside the channel to depth‐varying further offshore. Offshore from the channel the rip current has much stronger velocities at the surface than near the bottom. The instantaneous profiles twist rapidly over depth farther offshore and are fairly uniform in the channel. The depth variations are shown to be sensitive to the total volume flux in the rip.
Examination of diffusion versus advection dominated sediment suspension on the inner shelf under storm and swell conditions, Duck, North Carolina
Authors: Lee Guan‐hong , Friedrichs Carl T. , Vincent Chris E.
Published: October 2002

A benthic boundary layer tripod supporting six current meters and three profiling acoustic backscatter sensors (ABS) documented storm and swell conditions during the fall of 1996 at a depth of 13 m on the inner shelf off Duck, North Carolina. Sediment concentration was higher in the wave boundary layer (WBL) during storm conditions but higher ∼40 cm above the bed (cm ab) during swell conditions. To test the applicability of a diffusive balance during storm versus swell, ABS data were used to invert the vertical diffusion equation and solve for eddy diffusivity from 1 to 50 cm ab. During the storm period, diffusivity derived from the ABS up to ∼40 cm ab agreed well with viscosity derived above the WBL from observed current profiles and from the Grant‐Madsen‐Glenn (GMG) model. During the swell period, diffusivity derived from the ABS up to ∼40 cm ab did not agree with observed mean current shear above this level nor with the GMG model. Diffusivity did agree with viscosity derived from shear stress due to waves within the WBL extrapolated to a height greater than the modeled WBL. We speculate that during swell conditions, shedding vortices enhanced mass and momentum exchange, extending the eddy viscosity associated with waves above the predicted WBL; during storm conditions, strong currents prevented vortices from penetrating beyond the predicted WBL. Rouse diffusion models with two‐ and three‐layered eddy diffusivity and combined diffusion‐advection models with one and three‐layer were applied to the observational data set. During the storm the two‐ and three‐layered Rouse models including multiple grain sizes and bed armoring reproduced the observed concentration well. During swell (weak current conditions) all the models considered underpredicted the observed concentration if applied with a standard WBL thickness. To correct this, enhanced vertical exchange was represented by a thickened WBL whenever mean currents were weak relative to the estimated jet velocity associated with wave‐induced vortex shedding. The two‐layer Rouse model then reproduced the concentrations observed during swell remarkably well. This implies that mean sediment suspension dominated by wave‐induced advection may still be approximated by a diffusion‐like process under some circumstances.
A Lagrangian investigation of vertical turbulent heat fluxes in the upper ocean during Tropical Ocean–Global Atmosphere/Coupled Ocean‐Atmosphere Response Experiment (TOGA‐COARE)
Authors: du Penhoat Yves , Reverdin Gilles , Caniaux Guy
Published: October 2002

We investigated the diurnal cycle of the near‐surface ocean temperatures in the western Pacific Warm Pool, using temperature data from drifters with 20 m thermistor chains, during the Coupled Ocean‐Atmosphere Response Experiment (COARE) Intensive Observation Period (December 1992 to February 1993). We estimated turbulent heat fluxes at 20 m as a residual in the hourly heat budget along each drifter trajectory using hourly surface fluxes. Fortnightly averaged turbulent heat fluxes at 20 m were upward during periods of sustained winds, indicating a warming of the daily thermocline by the deeper layers. During calm periods the inferred turbulent heat fluxes were downward. We also calculated the day‐to‐night differences of the turbulent heat fluxes at 20 m as a residual in the budget of the daily cycle of heat content in the upper 20 m. This method largely avoids the uncertainties of surface turbulent heat fluxes. The residual averaged over the 2 month period was negative for the drifters that experienced strong westerly wind conditions (with heat fluxes positive downward). These surprising results for the warm pool region are at odds with what was found earlier from moored measurements in the central Pacific. Interestingly, however, periods characterized by a westerly wind burst during December 1992 to March 1993 contrasted with the periods with weaker winds: during westerly wind bursts the difference was negative, whereas it was positive during calm or warming periods. This is consistent with our analysis of the daily budget and also with other studies in the COARE domain. It confirms that the nighttime mixed layer was usually much deeper than 20 m during this period. However, our estimates are different for drifters deployed north of 1°N than for drifters near or south of the equator, indicating marked spatial variability in the heat budget of the warm pool.