A scheme is presented for assimilating expendable bathythermographic data into HYCOM, an oceanic circulation model featuring a hybrid vertical coordinate. The scheme is fully multivariate, using observations of temperature to correct density, pressure, salinity, and momentum, in addition to temperature. Central to the scheme is the estimation of companion profiles of salinity and potential density. The potential density profiles are used to estimate the thicknesses of the model's layers, so that layer-averaged values of potential density and potential temperature can be computed. These derived data and the derived layer thicknesses are assimilated via optimal interpolation. Salinity corresponding to the corrected potential density and potential temperature fields is determined by the equation of state of seawater, and corrections to the momentum field are computed geostrophically from the corrections to the pressure field. The scheme is illustrated using data from March 1995 in the Atlantic Ocean.

The impact of the nonlinearity in the equation of state associated with the change in the thermal expansion coefficient with temperature on the structure of fingers growing from an interface between two mixed layers is investigated using a numerical model. It is shown that the nonlinearity acts to enhance the buoyancy force acting on the descending fingers with respect to that acting on the ascending fingers, resulting in narrower and faster-growing descending fingers than ascending fingers. The results are discussed with emphasis on the vertical variability of properties along the fingers.

Abstract. An existing particle-in-cell (PIC) numerical method developed for the study of two-layer mesoscale motions with outcropping pycnocline is applied to lens-like anticyclonic vortices and buoyant coastal currents. From a first series of experiments investigating the evolution of an initially elongated lens-like anticyclone, it is found that motions induced in the lower layer act only to increase the rotation of the vortex structure and do not appear to affect the process of eccentricity reduction (partial axisymmetrization). Eccentricity reduction, if any, produces a final vortex of aspect ratio between 1.8 and 1.9, a value that is very close to the stability threshold of large, reducedgravity lenses. A second series of experiments devoted to vortex mergers determines how the maximal separation distance for which two circular anticyclonic lenses merge spontaneously varies with vortex size (ratio of lens radius to deformation radius) and stratification (ratio of lens central thickness to ocean depth). A third series of experiments considers the interaction of a lens vortex in the upper layer with a potential-vorticity anomaly in the lower layer. "Second-hand" relative vorticity, generated in the lower layer under the action of vertical stretching induced by the movement of the upper-layer vortex, interacts with "first-hand" relative vorticity, created by the existing potential-vorticity, to create effects similar to those predicted by studies of two-layer point vortices (hetons). Finally, the PIC method is generalized to simulate the finite-amplitude instability of a buoyant geostrophic/hydrostatic intrusion flowing along a vertical coastal wall. Those results, however, are reported here more as a demonstration on how the PIC method can be extended to include coastal boundaries than as a thorough investigation of coastalcurrent instabilities. IntroductionA substantial portion of the ocean's energy is contained in the so-called mesoscale activity, the internal weather of the sea. These motions, with length scales of the order of the first baroclinic radius of deformation (20-200 km), are mostly comprosed of jets and vortices [Robinson, 1983]. Their length scale implicates geostrophy, and, consequently, their relative high velocities are accompanied by substantial vertical displacements of density surfaces, causing on numerous occasions their outcrop through the surface. Examples are the north wall of the Gulf Stream, the periphery of its detached rings, the offshore edge of coastal intrusions, and upwelling fronts. We shall use here the word "front" to describe the surface outcropping of a pycnocline or, in the context of a layered model, the surfacing of a density interface. Indeed, such outcropping causes important horizontal density gradients along the surface and corresponds to a frontal zone.Since geostrophy places a high degree of relation between horizontal density gradients and velocities (via the thermalwind balance), fronts are accompanied by flow fields that are highly sheared in the horizontal a...

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The evolution of ngers in a double-diffusivesystem is investigatedusing numerical integration of two-dimensional equations of motion for an incompressible, Boussinesq uid. The computational domain is periodic in the horizontal direction and is closed with no-ux boundary conditions in the vertical direction. The main result of this study is the evolution of the system from initially linear pro les for both fast and slow diffusing components to a layered state characterized by a nger zone sandwiched between two mixed layers. The horizontal boundaries in this system play an important role in the development of the layered state. At the top and bottom boundaries, light and heavy nger uxes accumulate leading to the formation of mixed layers exhibiting larger-scale eddies. In the quasi-equilibrium state, the nger zone is characterized by broken wiggly ngers which do not extend across the entire interface. The salinity ux at the mid-depth of the domain is approximately proportional to the 4 3 power of the salinity difference between the mixed layers, except for the initial phase and for the run-down phase.