Baroclinic Deformation Radius Research Articles

Downwelling in Basins Subject to Buoyancy Loss

Abstract Recent observational, theoretical, and modeling studies all suggest that the upper part of the downwelling limb of the thermohaline circulation is concentrated in strong currents subject to buoyancy loss near lateral boundaries. This is fundamentally different from the traditional view that downwelling takes place in regions of deep convection. Even when resolving the buoyant boundary currents, coarse-resolution global circulation and climate models rely on parameterizations of poorly known turbulent mixing processes. In this study, the first direct measurements of downwelling occurring within a basin subject to buoyancy loss are obtained. Downwelling is observed near the basin’s vertical wall within the buoyant boundary current flowing cyclonically around the basin. Although the entire basin is cooled, large-scale mean downwelling is absent in the basin interior. Laboratory rotating experiments are conducted to explicitly resolve the turbulent mixing due to convective plumes and the baroclinic eddies generated by the boundary current, and to identify where downwelling takes place. Small vertical velocities can be measured more reliably in the laboratory than in many numerical calculations, whereas the measurement of these small vertical velocities is still a challenge for field experiments. Downwelling is observed near the vertical wall within a boundary layer with a thickness that scales with the baroclinic Rossby radius of deformation, consistent with the dynamical balance proposed by a previous numerical study. Hence, downwelling in the Labrador Sea and Lofoten Basin cyclonic boundary currents may be concentrated in a baroclinic Rossby radius of deformation thick boundary layer in regions with large eddy generation.

A Geostrophic Adjustment Model of Two Buoyant Fluids

Abstract A combination of analytical calculations and laboratory experiments has been used to investigate the geostrophic adjustment of two buoyant fluids having different densities in a third denser ambient fluid. The frontal position, the depth profile, and the horizontal and vertical alignments of the two buoyant fluids at the final equilibrium state are determined by the ratio of the baroclinic Rossby radii of deformation Γ1 = λ31/λ21 and Γ2 = λ32/λ21 and the Burger numbers B1 = λ31/L1 and B2 = λ32/L2 of the two buoyant fluids, where is the baroclinic Rossby radius of deformation between fluids i and j. The buoyant fluids 1 and 2 have densities ρ1 and ρ2 (ρ1 < ρ2), respectively; the ambient denser fluid has density ρ3; g′ is the reduced gravity; H and L are the buoyant fluids’ initial depth and width, respectively; and f is the Coriolis parameter. Laboratory rotating experiments confirmed the analytical prediction of the location of the two fronts. After reaching geostrophic equilibrium, the two buoyant currents align mainly horizontally when the extent of the fronts between fluids 1 and 3 and between fluids 2 and 3 is large compared to the extent of the front between fluids 1 and 2: that is, large values of λ31 and λ32 compared to λ21 or equivalently Γ1 ≫ 1 and Γ2 ≫ 1. Alternatively, if the extent of the fronts between the three fluids is similar (i.e., Γ1 ≈ Γ2 ≈ 1), the buoyant currents align mainly vertically. Furthermore, the Burger number of the lightest fluid B1 controls the distance of the inner front from the coast, while B2 controls the offshore extent of the outer front.

Hysteresis and Dynamics of a Western Boundary Current Flowing by a Gap Forced by Impingement of Mesoscale Eddies

Abstract Hysteresis of a western boundary current (WBC) flowing by a wide gap of a western boundary and the dynamics of the WBC variations associated with the impingement of mesoscale eddies from the eastern side of the gap are studied using a 1.5-layer reduced-gravity quasigeostrophic ocean model. The study focuses on two issues not covered by existing studies: the effects of finite baroclinic deformation radii and time dependence perturbed by mesoscale eddies. The results of the study show that the hysteresis of the WBC of finite baroclinic deformation radii is not controlled by multiple steady-state balances of the quasigeostrophic vorticity equation. Instead, the hysteresis is controlled by the periodic penetrating and the leaping regimes of the vorticity balance. The regime of the vorticity balance inside the gap is dependent on the history of the WBC evolution, which gives rise to the hysteresis of the WBC path. Numerical experiments have shown that the parameter domain of the hysteresis is not sensitive to the baroclinic deformation radius. However, the domain of the periodic solution, which is determined by the lower Hopf bifurcation of the nonlinear system, is found to be sensitive to the magnitude of the baroclinic deformation radius. The lower Hopf bifurcation from steady penetration to periodic penetration is found to occur at lower Reynolds numbers for larger deformation radii. In general, the lower Hopf bifurcation stays outside the hysteresis domain of the Reynolds number. However, for very small deformation radii, the lower Hopf bifurcation falls inside the hysteresis domain, which results in the transition from the leaping to the penetrating regimes of the WBC to skip the periodic regime and hence the disappearance of the upper Hopf bifurcation. Mesoscale eddies approaching the gap from the eastern basin are found to have significant impact on the WBC path inside the gap when the WBC is at a critical state along the hysteresis loop. Cyclonic (anticyclonic) eddies play the role of reducing (enhancing) the inertial advection of vorticity in the vicinity of the gap so that transitions of the WBC path from the leaping (periodic penetrating) to the periodic penetrating (leaping) regimes are induced. In addition, cyclonic eddies are able to induce transitions of the WBC from the periodic penetrating to the leaping regimes through enhancing the meridional advection by its right fling. The transitions are irreversible because of the nonlinear hysteresis and are found to be sensitive to the strength, size, and approaching path of the eddy.

Exchange across the shelf break at high southern latitudes

Abstract. Exchange of water across the Antarctic shelf break has considerable scientific and societal importance due to its effects on circulation and biology of the region, conversion of water masses as part of the global overturning circulation and basal melt of glacial ice and the consequent effect on sea level rise. The focus in this paper is the onshore transport of warm, oceanic Circumpolar Deep Water (CDW); export of dense water from these shelves is equally important, but has been the focus of other recent papers and will not be considered here. A variety of physical mechanisms are described which could play a role in this onshore flux. The relative importance of some processes are evaluated by simple calculations. A numerical model for the Ross Sea continental shelf is used as an example of a more comprehensive evaluation of the details of cross-shelf break exchange. In order for an ocean circulation model to simulate these processes at high southern latitudes, it needs to have high spatial resolution, realistic geometry and bathymetry. Grid spacing smaller than the first baroclinic radius of deformation (a few km) is required to adequately represent the circulation. Because of flow-topography interactions, bathymetry needs to be represented at these same small scales. Atmospheric conditions used to force these circulation models also need to be known at a similar small spatial resolution (a few km) in order to represent orographically controlled winds (coastal jets) and katabatic winds. Significantly, time variability of surface winds strongly influences the structure of the mixed layer. Daily, if not more frequent, surface fluxes must be imposed for a realistic surface mixed layer. Sea ice and ice shelves are important components of the coastal circulation. Ice isolates the ocean from exchange with the atmosphere, especially in the winter. Melting and freezing of both sea ice and glacial ice influence salinity and thereby the character of shelf water. These water mass conversions are known to have an important effect on export of dense water from many Antarctic coastal areas. An artificial dye, as well as temperature, is used to diagnose the flux of CDW onto the shelf. Model results for the Ross Sea show a vigorous onshore flux of oceanic water across the shelf break both at depth and at the surface as well as creation of dense water (High Salinity Shelf Water) created by coastal polynyas in the western Ross Sea.

Linear Waves in Midlatitudes on the Rotating Spherical Earth

Abstract The linear waves of the shallow water equations in a zonal channel in midlatitudes on the rotating spherical earth are investigated analytically and numerically by solving several relevant eigenvalue problems. For baroclinic deformation radii in the ocean, the phase speed of long Rossby waves in a sufficiently wide channel on the sphere can be 5 times that of their harmonic β-plane counterparts. The difference between the two phase speeds increases with the channel width and decreases with 1) the latitude of the equatorward wall, 2) the radius of deformation, and 3) the mode number. For Poincaré (inertia–gravity) waves, the phase speed on the sphere is slightly lower than that of harmonic waves on the β plane. The meridionally dependent amplitude of the meridional velocity is identical for both waves and is trapped near the equatorward wall—that is, its amplitude is maximal within a few deformation radii from this wall. The phase speeds of the Kelvin and anti-Kelvin waves on a sphere are determined by the latitudes of the equatorward and poleward walls, respectively, where they attain their maximal height amplitude. Accordingly, the phase speed of the anti-Kelvin wave is larger than that of the westward-propagating Poincaré waves in a certain wavenumber range, whereas the phase speed of eastward-propagating Poincaré waves does not approach that of the Kelvin wave even at large wavenumbers. Analytical expressions for the phase speed of trapped Poincaré and Rossby waves are obtained for small deformation radii in wide channels by approximating the meridional velocity’s eigenfunction by an Airy function that decays with distance from the equatorward wall. The exact latitude of the poleward wall does not affect the solution, provided it is several deformation radii away from the equatorward boundary and the exact channel width increases with the radius of deformation. For a sufficiently small radius of deformation, such as that observed in the ocean, the solution is trapped, even for very narrow channels, and the phase speed is only slightly larger than that of harmonic waves.

Mesoscale‐resolving simulations of summer and winter bora events in the Adriatic Sea

This paper presents simulations of the Adriatic Sea response to two distinct bora wind events, one in summer (11–20 August 2001), when the water is stratified, and the other in winter (11–20 February 2003), when it is vertically homogeneous. The simulations employ the DieCAST model on a 1.2‐min grid (about 2‐km resolution) and resolve the mesoscale variability because the grid size falls below the first baroclinic deformation radius (about 5–10 km) and the model has very low horizontal dissipation. The model is initialized with seasonally averaged temperature and salinity data and spun up with climatological winds. The summer of August 2001 event leads to the generation of a coastal current directed paradoxically to the left of the wind and identified with the summertime Istrian Coastal Countercurrent. Analysis of the physics simulated by the model leads to the conclusion that this current is caused by baroclinic geostrophic adjustment of the Istrian coastal waters following a rapid but strong wind impulse. According to both satellite observations and model simulations, the current persists for more than a week after the bora event. The winter event of February 2003 generates a slightly less complicated situation because the shallow northern Adriatic is then vertically homogeneous, but in situ observations at the time of this particular event permit a comparison of model results with observations and thus an evaluation of the model performance. In particular, the model simulates correctly a series of currents, bifurcations, and confluences of the wind‐driven currents across the northern Adriatic basin. A lesson learned is that a bora event, though generally strong, especially in winter, leaves a legacy that does not obliterate completely what existed prior to the event. In other words, the state of the northern Adriatic basin is fashioned by sequential events, and one bora event may not be viewed in isolation but must be considered as one episode in an unfolding succession of events.

Eddy statistics from Lagrangian drifters and hydrography for the northern Gulf of Mexico slope

Cyclones and anticyclones on the northern Gulf of Mexico slope have been statistically characterized using upper layer drifter orbits and repeated hydrographic surveys. A total of 243 slope eddy drifter orbits (151 cyclones and 92 anticyclones) were analyzed in terms of size (mean radius and eccentricity) and rotation period. Orbits in large Loop Current anticyclones were excluded. Eddies of both signs scale approximately as the baroclinic radius of deformation with relatively smaller radii on the upper slope, where water depths are less than 1000 m. The median radius for all eddies was ∼30 km, and median periods were 12 and 13.5 days for cyclones and anticyclones, respectively. Anticyclones also tend to be more circular than cyclones. Relative vorticity maps were constructed for seven hydrographic surveys, which mapped the slope at 4–5 month intervals. A total of 26 cyclones and 15 anticyclones had statistics that were consistent with drifter orbit analyses. The surveys indicate that at any one time, between 2 and 5 cyclones and 1 and 3 anticyclones may be present. Both drifters and surveys indicate that cyclones have a slight numerical dominance.

Modeling the mesoscale variability in the Adriatic Sea

A new high-resolution (<2 km) version of the DieCAST fourth-accuracy-order model for the ocean circulation is proposed for the study of the general circulation, mesoscale structures, and their variability in the Adriatic Sea. The model uses mean seasonal data on the temperature, salinity, buoyancy fluxes, and wind. The data of the COAMPS system with a 4-km resolution were used for the simulation of the sea response to the effects of various winds: Sirocco, Maestro, and two types of boras. The mean monthly runoffs from 38 rivers and mean daily runoffs from 12 main rivers throughout the year were given in the model. The conditions at the open boundary of the Strait of Otranto were given on the basis of the hierarchy of two coarser models for the Adriatic and Mediterranean seas. Due to the extremely weak dissipation and the high resolution (the mesh size is less than the baroclinic radius of deformation, 5–10 km), the model allows one to trace the development of a baroclinic instability along the Italian coast, to simulate mesoscale structures associated with the instability, and to estimate the scales of the structures. Mesoscale filaments, meanders, mushroom-like currents, fronts, and intrusions known from satellite observations were simulated and explained. The scenario of the anomalous upwelling near the Italian coast observed in the summer of 2003 was also simulated and analyzed.

Island Wakes in Deep Water

AbstractDensity stratification and planetary rotation distinguish three-dimensional island wakes significantly from a classical fluid dynamical flow around an obstacle. A numerical model is used to study the formation and evolution of flow around an idealized island in deep water (i.e., with vertical island sides and surface-intensified stratification and upstream flow), focusing on wake instability, coherent vortex formation, and mesoscale and submesoscale eddy activity. In a baseline experiment with strong vorticity generation at the island, three types of instability are evident: centrifugal, barotropic, and baroclinic. Sensitivities are shown to three nondimensional parameters: the Reynolds number (Re), Rossby number (Ro), and Burger number (Bu). The dependence on Re is similar to the classical wake in its transition to turbulence, but in contrast the island wake contains coherent eddies no matter how large the Re value. When Re is large enough, the shear layer at the island is so narrow that the vertical component of vorticity is larger than the Coriolis frequency in the near wake, leading to centrifugal instability on the anticyclonic side. As Bu decreases the eddy size shrinks from the island breadth to the baroclinic deformation radius, and the eddy generation process shifts from barotropic to baroclinic instability. For small Ro values, the wake dynamics is symmetric with respect to cyclonic and anticyclonic eddies. At intermediate Ro and Bu values, the anticyclonic eddies are increasingly more robust than cyclonic ones as Ro/Bu increases, but for large Re and Ro values, centrifugal instability weakens the anticyclonic eddies while cyclonic eddies remain coherent.

Simulation and characterization of the Adriatic Sea mesoscale variability

This paper presents simulations of the Adriatic Sea using the DieCAST model applied on a 1.2–min grid (about 2–km resolution). The simulations resolve the mesoscale variability because the grid size falls below the first baroclinic deformation radius (about 5–10 km) and DieCAST has very low horizontal dissipation. The model is initialized with seasonally averaged temperature and salinity data and forced with climatological winds and surface buoyancy fluxes (both heat flux and evaporation minus precipitation). River discharges are varied daily according to a perpetual year for every river, and the open‐boundary conditions at Otranto Strait are obtained by nesting in two larger‐scale models. The present simulations demonstrate that the DieCAST model allows mesoscale instabilities to develop at length scales of 5–20 km and over time scales of a few days. The simulated variability exhibits pronounced similarities with the actual mesoscale variability, in terms of location, nature and temporal evolution of the features. Meanders, swirls and eddies are noted along the relatively smooth Italian coast while offshore jets and filaments better describe the mesoscale activity along the more rugged coast of Croatia. In sum, DieCAST is highly suitable for the study of mesoscale variability in the Adriatic Sea. The present simulations also show that the seasonal hydrography of the Adriatic Sea is intrinsically unstable to mesoscale perturbations, and that the mesoscale variability along the Italian coast is the result of baroclinic instability of the Western Adriatic Current. It is shown how the properties of this instability are related to the local bottom topography.

Optical Inhomogeneities Of Water In TheNew York Coastal Zone Determined UsingMultispectral Satellite Data

Processing of the calculated invariant hydro-optic parameters ) , ( I 2 1 λ λ and ) , ( V 2 1 λ λ based on water leaving radiance (WLR) data measured by color scanners in the New York coastal zone has shown that the most significant inhomogeneities are observed at the coastline, while at a distance of 5-7 km from the coast the variability of the above parameters decreases approximately twice. Spectral analysis of optical inhomogeneities along several profiles with respect to the coastline has shown that their characteristic scales also substantially depend on the distance from the coastline and vary from 12 km near the coast at depths of 14-16 m to 40-43 km where the depth is 22-25 m. These values coincide with the estimates of characteristic sizes of mesoscale hydrodynamic processes based on the baroclinic deformation radii. Satellite measurements performed with an interval of about 24 hours have provided some estimates of time-related variability of optical inhomogeneities. The repeatability of the main spectra maximums for the measurements done with an interval of one day testifies to the relative stability of the basic structures of optical inhomogeneities.

Accuracy Assessment of Jason-1 and TOPEX/POSEIDON Along-Track Sea Surface SlopeSpecial Issue: Jason-1 Calibration/Validation

Sea surface slope computed from along-track Jason-1 and TOPEX/POSEIDON (T/P) altimeter data at ocean mesoscale wavelengths are compared to determine the equivalent 1 Hz instrument height noise of the Poseidon-2 and TOPEX altimeters. This geophysical evaluation shows that the Ku-band 1-Hz range noise for both instruments is better than 1.7 cm at 2 m significant wave heights (H1/3), exceeding error budget requirements for both missions. Furthermore, we show that the quality of these instruments allows optimal filtering of the 1-Hz along-track sea surface height data for sea surface slopes that can be used to calculate cross track geostrophic velocity anomalies at the baroclinic Rossby radius of deformation to better than 5 cm/sec precision along 87.5% of the satellite ground track between 2 and 60 degrees absolute latitude over the deep abyssal ocean (depths greater than 1000 m). This level of precision will facilitate scientific studies of surface geostrophic velocity variability using data from the Jason-1 and T/P Tandem Mission.

Lateral diffusivity and Lagrangian scales in the Pacific Ocean as derived from drifter data

The Global Drifter Program/Surface Velocity Program data set is used to derive maps of the lateral diffusivity and Lagrangian scales of velocity, length and time across the whole Pacific. For the lateral diffusivity, the minor principal component of both the Davis' diffusivity tensor and the half growth rate of the single‐particle dispersion tensor were calculated in 5° × 5° bins from drifters for the period 1979–1999. Estimates of the lateral diffusivity, K, obtained by means of the minor principal component approach, are shown to correspond well with other authors' estimates for the Pacific. The highest values of K (2–3 104 m2/s) are found in the eastern equatorial Pacific, while the lowest ones (2–3 103 m2/s) are typical for forty‐odd degrees in latitude northward of the subpolar fronts in both hemispheres and for the subtropics in the eastern part of the South Pacific. High values of K (about 1 × 104 m2/s) are also observed in the Kusoshio/Kuroshio Extension area and in the equatorial Pacific along the whole length of the equator. There are two approximately symmetrical tongues of high K value in the western Pacific centered around 21°N and 24°S, respectively. In general, K has a tendency to increase westward (eastward) in the midlatitudes (equatorial region) and toward the equator in both hemispheres. In the midlatitudes, the Lagrangian length scale L is found to be approximately equal to the first‐mode baroclinic radius of deformation, Ri, while the Rossby number defined as Ro = Tearth/T sinφ, where T is the Lagrangian timescale, Tearth is the period of the Earth's rotation, and φ is the latitude, is less than unity, implying that the quasigeostrophic mesoscale eddies are the main mechanism for lateral mixing. The lateral diffusivity in the midlatitudes is suggested to be parameterized as K = V2Ri, where V2 is the Lagrangian velocity scale inherent to a mesoscale eddy field. In the low latitudes, L becomes less than Ri, and the condition Ro ≤ 1 is no longer valid, which implies the change of the eddy‐controlled mechanism of lateral mixing by another mechanism.

Numerical study on the planetary Rossby modes in the Black Sea

We address in this paper the planetary basin modes and their possible impact on the circulation of Black Sea. These oscillations could become important in semi-enclosed seas where the horizontal scales are smaller than the barotropic radius of deformation and the baroclinic radius of deformation is comparable with the basin scales. The physical balances studied here are based on theoretical results for the Rossby modes in rectangular basins and on model simulations carried out with Bryan–Cox ocean model with eddy resolution. Eddies with horizontal scales of 100–200 km spontaneously form and propagate to the west with phase velocity of few kilometers per day. External mode oscillations dominate the model solution and show close similarity with estimates based on simplistic theoretical models. In the baroclinic oceans these oscillations trigger active baroclinic variability and western propagation, but with strongly modified wave characteristics in comparison to the barotropic case. The comparisons between simulations and data from surveys and altimeters demonstrates that the planetary modes are a dominating form of wave motion in this sea and can explain the westward propagation of signals in its deep part. It is suggested that the excitation of rotational modes can be also important for the dispersion of tracers.

Upper-ocean water mass characteristics of the California current, Summer 1993

Two large, high-resolution upper-ocean surveys of the California Current region were conducted in the summer of 1993. Temperature and salinity were measured from a Seasoar vehicle, and velocity was measured by a ship-borne acoustic Doppler current profiler. Both surveys extended from the continental margin to 128°W, and consisted of zonal sections 28 km apart. The first survey (7–28 June) extended from 39.5°N to 36.25°N, and the second (16 August to 1 September) extended from 39.0°N to 37.0°N. The current field was fairly simple in June, showing an equatorward surface jet with only one gentle meander, a nearly continuous poleward California Undercurrent near the continental margin, and two anticyclonic subsurface eddies offshore of the jet. By August meanders in the surface jet had strengthened, and eddies dominated the flow field. Large-scale trends in the water-mass characteristics (“spiciness”) along three isopycnal surfaces (26.4, 26.0 and 25.6 kg m -3) show that temperature and salinity tend to increase towards the equator and towards shore, consistent with poleward advection along the continental margin. Local minima in spiciness tend to coincide with the equatorward surface jet. Local maxima in spiciness are associated with subsurface anticyclonic eddies which have a core depth of about 150 m, and seem to originate in the California Undercurrent over the continental slope at local latitudes. We identified several different anticyclonic eddies, each with different water-mass characteristics; all of the eddies we observed had diameters larger than the baroclinic radius of deformation. Water-mass characteristics and migration rates are consistent with formation over the continental margin within the preceding 6–8 months.

Eddy Processes in Semienclosed Seas: A Case Study for the Black Sea

Abstract The enclosed boundaries and small scales of some seas lead to the formation of specific physical balances, which motivates the oceanographic interest in studying the dynamics of semienclosed ocean basins. The focus in the paper is on the specific appearances of eddy processes when the basin scales and the ones of the topographic features are comparable with the baroclinic radius of deformation. The Black Sea is used as a test basin. Eddy variability is analyzed using simulation results and compared with existing observations. The Bryan–Cox model with horizontal resolution Δφ = 1/10° and Δλ = 1/6° is forced with annual-mean wind stress data. Buoyancy flux at the sea surface is proportional to the deviation of the model density from the annual-mean climatological data. Sensitivity studies on different forcing and on the topographic control are carried out. Synoptic periods are estimated to be about 0.5 yr. Eddies form in the eastern Black Sea and propagate westward with a speed of about 3 cm s−1. T...

A subsurface countercurrent along the east coast of Luzon

A hydrographic section in the region east of Luzon was repeated 14 times during the period from 1986 to 1991. The data revealed the existence of a subsurface countercurrent located on the shoreward side of the Kuroshio with its upper boundary at about 500 m. The countercurrent, which should be called the Luzon Undercurrent (LUC), was only about 50 km wide, which is comparable to the baroclinic radius of deformation. Despite considerable variabilities both in velocity profile and intensity, the LUC appears to be a permanent feature. Over the period of observations, the maximum speed in the LUC calculated from the mean temperature and salinity by assuming geostrophy (relative to 2500 db) was 7 cm s −1 at about 700 m and its mean geostrophic volume transport was 3.6 Sv (1 Sv=10 6 m 3 s−1). About 28% of this transport was composed of the low-salinity North Pacific Intermediate Water (NPIW) advected to the south along the coast of Luzon.

Elliptical Warm-Core Rings in a Two-Layer Ocean with Ambient Shear

Abstract The rodon solution for an elliptical vortex with outcropping interface is extended from the reduced-gravity limit to the two-layer model. Motions in the lower layer consist of a reaction to the presence of the upper-layer vortex, or of an independent shear flow, or both. For simplicity and clarity, the analysis is performed in a geostrophic limit, which restricts the attention to vortices somewhat larger than the baroclinic radius of deformation. In a first part, geostrophic equations are established and discussed to identify clearly when the vortex motions induced in the lower layer and the ambient lower-layer shear must be retained or may be neglected. In the second part, the equations are solved for various cases. It is found that lower-layer motions induced by the upper-layer vortex provide a feedback on the latter and that this reaction is a mere increase of the anticyclonic rotation rate of the rodon. Furthermore, this increased rate has a straightforward interpretation in terms of potentia...

Antarctic Intermediate Water circulation in the tropical and subtropical South Atlantic

Recent hydrographic data from the South Atlantic Ventilation Experiment cruises and others are combined with historical data and used to map the isopycnal properties corresponding to the Antarctic Intermediate Water (AAIW) in the Atlantic Ocean. The low salinity of the AAIW extends eastward across the South Atlantic just south of the equator (3–4°S). Evidence of a weak eastward flow just north of the equator (1–2°N) is also shown. Lateral and vertical homogenization of properties in the AAIW is found at the equator between 2°S and 2°N; there is no clear zonal gradient in salinity just along the equator. These observations suggest enhanced mixing within the equatorial baroclinic deformation radius. The South Atlantic tropical gyre is shown to consist of the following three cells: one cyclonic cell centered at about 7°S, another centered at about 19°S in the west and 23°S in the east, and one anticyclonic cell centered at about 13°S. These cells are associated with a westward extension at 10°S of high salinity and low oxygen which originates in the eastern tropical South Atlantic and a front in these properties at about 15°S in the west and about 20°S in the east.

Solitary waves in the gulf of quarnaro revealed by LANDSAT thematic mapper

The propagation of solitary wave trains in the Adriatic Sea is studied by means of LANDSAT Thematic-Mapper images. The baroclinic Rossby radius of deformation has been estimated from the width of a baroclinic current observed along the coast of Croatia using AVHRR data. The phase speed of the linear internal waves has been consequently estimated and the compatibility of the Kortweg-de Vries equations with experimental data has been tested. Satellite-derived wave parameters are in good agreement with a plane-bottom, shallow-water solitary-wave model.