We consider the dispersion of particles in potential vorticity (PV)-conserving ows. Because par... more We consider the dispersion of particles in potential vorticity (PV)-conserving ows. Because particle drift is preferentially along the mean PV contours, Lagrangian dispersion is strongly anisotropic. If the mean PV eld moreover is spatially variable, as when there is topography, the anisotropy is more clearly visible in the dispersion of displacements along and across the mean PV eld itself. We examine several numerical examples of unforced barotropic ows; in all cases, this ‘‘projected’’ dispersionis more anisotropicthan that in cartesian (x, y) coordinates.What differs is the rate at which spreadingoccurs, both along and across contours.The method is applicable to real data, as is illustrated with oat data from the deep North Atlantic. The results suggest a preferential spreading along contours of (barotropic) f/H.
We attempt to capture the variability of the depth-integrated flow in the Nordic Seas and Arctic ... more We attempt to capture the variability of the depth-integrated flow in the Nordic Seas and Arctic ocean using an integral model based on the shallow water equations. The model is driven by observed winds or ice motion, and uses realistic bottom topography. It has one free parameter, the coefficient of the (linear) bottom drag. The data to which we compare
A previously published method by Wang et al. for predicting subsurface velocities and density fro... more A previously published method by Wang et al. for predicting subsurface velocities and density from sea surface buoyancy and surface height is extended by incorporating analytical solutions to make the vertical projection. One solution employs exponential stratification and the second has a weakly stratified surface layer, approximating a mixed layer. The results are evaluated using fields from a numerical simulation of the North Atlantic. The simple exponential solution yields realistic subsurface density and vorticity fields to nearly 1000 m in depth. Including a mixed layer improves the response in the mixed layer itself and at high latitudes where the mixed layer is deeper. It is in the mixed layer that the surface quasigeostrophic approximation is most applicable. Below that the first baroclinic mode dominates, and that mode is well approximated by the analytical solution with exponential stratification.
Agulhas, Madagascar and Mozambique eddies are vortices which form in the southwest Indian Ocean. ... more Agulhas, Madagascar and Mozambique eddies are vortices which form in the southwest Indian Ocean. All are in excess of 100 km in size, extend deep in the water column and drift westward from their respective formation sites. We suggest all three phenomena may originate from discontinuities in the wind-driven Sverdrup circulation. These discontinuities produce westward-flowing jets which are barotropically unstable, and thus generate vortices. We illustrate the idea using a linear analytical model and a nonlinear numerical model. The linear model produces westward jets off the northern and southern tips of Madagascar, and off the southern tip of South Africa. All these are unstable by the Rayleigh-Kuo criterion. Vortices of realistic size form in all three locations in the numerical model and subsequently drift westward, as observed. The primary shortcoming in the models is their failure to produce a strong retroflection of the Agulhas Current. Further model simulations suggest that b...
The dispersion of pairs of synthetic particles, advected with ECMWF winds, is examined. The parti... more The dispersion of pairs of synthetic particles, advected with ECMWF winds, is examined. The particles were deployed at three latitudes and on three potential temperature surfaces in both hemispheres. Separation statistics are calculated and evaluated in relation to 2D turbulence theory and to Eulerian structure functions calculated directly from the wind data. At the smallest sampled scales (100–1000 km), the pair-separation velocities are correlated, and the dispersion is laterally isotropic, at least at the higher latitudes. At larger scales, the dispersion is zonally anisotropic, and the pair velocities are uncorrelated. In all cases, the dispersion grows exponentially in time, and the second-order Eulerian structure functions consistently increase as separation squared. This implies nonlocal dispersion, which obtains with energy spectra at least as steep as K−3. Regional variations are seen in the parameters however. The e-folding times and the maximum scales for exponential gro...
We attempt to capture the variability of the depth-integrated flow in the Nordic Seas and Arctic ... more We attempt to capture the variability of the depth-integrated flow in the Nordic Seas and Arctic ocean using an integral model based on the shallow water equations. The model is driven by observed winds or ice motion, and uses realistic bottom topography. It has one free parameter, the coefficient of the (linear) bottom drag. The data to which we compare
We examine the relative motion of pairs and triplets of surface drifters in the Gulf of Mexico. T... more We examine the relative motion of pairs and triplets of surface drifters in the Gulf of Mexico. The mean square pair separations grow exponentially from the smallest resolved scale (1 km) to 40-50 km, with an e-folding time scale of 2-3 days. Thereafter, the dispersion exhibits a power law dependence on time with an exponent of between 2 and 3
We propose a new method for obtaining average velocities and eddy diffusivities from Lagrangian d... more We propose a new method for obtaining average velocities and eddy diffusivities from Lagrangian data. Rather than grouping the drifter-derived velocities in uniform geographical bins, as is commonly done, we group a specified number of nearest-neighbor velocities. This is done via a clustering algorithm operating on the instantaneous positions of the drifters. Thus it is the data distribution itself which
The problem of forced, geostrophic turbulence in a basin on the beta-plane is considered. Of part... more The problem of forced, geostrophic turbulence in a basin on the beta-plane is considered. Of particular interest is the question of whether an inverse cascade of energy can produce jets in the presence of meridional barriers (as occur in most ocean basins). Theory and numerical results suggest it does not. The reason apparently is that the inverse cascade is halted by basin normal modes rather than by free Rossby waves. Because basin modes have a dispersion relation which is effectively isotropic, so the statistically stationary field is also isotropic. As discussed previously, the turbulent basin flow also exhibits a time mean circulation similar to the Fofonoff solution; however, we show that this mean flow does not significantly alter the basin modes in the interior. A relatively novel feature is the use of Eulerian velocity statistics to quantify the wave and turbulence characteristics. These measures are more suitable to this environment than measures like wavenumber spectra, g...
We consider the dispersion of particles in potential vorticity (PV)-conserving ows. Because par... more We consider the dispersion of particles in potential vorticity (PV)-conserving ows. Because particle drift is preferentially along the mean PV contours, Lagrangian dispersion is strongly anisotropic. If the mean PV eld moreover is spatially variable, as when there is topography, the anisotropy is more clearly visible in the dispersion of displacements along and across the mean PV eld itself. We examine several numerical examples of unforced barotropic ows; in all cases, this ‘‘projected’’ dispersionis more anisotropicthan that in cartesian (x, y) coordinates.What differs is the rate at which spreadingoccurs, both along and across contours.The method is applicable to real data, as is illustrated with oat data from the deep North Atlantic. The results suggest a preferential spreading along contours of (barotropic) f/H.
We attempt to capture the variability of the depth-integrated flow in the Nordic Seas and Arctic ... more We attempt to capture the variability of the depth-integrated flow in the Nordic Seas and Arctic ocean using an integral model based on the shallow water equations. The model is driven by observed winds or ice motion, and uses realistic bottom topography. It has one free parameter, the coefficient of the (linear) bottom drag. The data to which we compare
A previously published method by Wang et al. for predicting subsurface velocities and density fro... more A previously published method by Wang et al. for predicting subsurface velocities and density from sea surface buoyancy and surface height is extended by incorporating analytical solutions to make the vertical projection. One solution employs exponential stratification and the second has a weakly stratified surface layer, approximating a mixed layer. The results are evaluated using fields from a numerical simulation of the North Atlantic. The simple exponential solution yields realistic subsurface density and vorticity fields to nearly 1000 m in depth. Including a mixed layer improves the response in the mixed layer itself and at high latitudes where the mixed layer is deeper. It is in the mixed layer that the surface quasigeostrophic approximation is most applicable. Below that the first baroclinic mode dominates, and that mode is well approximated by the analytical solution with exponential stratification.
Agulhas, Madagascar and Mozambique eddies are vortices which form in the southwest Indian Ocean. ... more Agulhas, Madagascar and Mozambique eddies are vortices which form in the southwest Indian Ocean. All are in excess of 100 km in size, extend deep in the water column and drift westward from their respective formation sites. We suggest all three phenomena may originate from discontinuities in the wind-driven Sverdrup circulation. These discontinuities produce westward-flowing jets which are barotropically unstable, and thus generate vortices. We illustrate the idea using a linear analytical model and a nonlinear numerical model. The linear model produces westward jets off the northern and southern tips of Madagascar, and off the southern tip of South Africa. All these are unstable by the Rayleigh-Kuo criterion. Vortices of realistic size form in all three locations in the numerical model and subsequently drift westward, as observed. The primary shortcoming in the models is their failure to produce a strong retroflection of the Agulhas Current. Further model simulations suggest that b...
The dispersion of pairs of synthetic particles, advected with ECMWF winds, is examined. The parti... more The dispersion of pairs of synthetic particles, advected with ECMWF winds, is examined. The particles were deployed at three latitudes and on three potential temperature surfaces in both hemispheres. Separation statistics are calculated and evaluated in relation to 2D turbulence theory and to Eulerian structure functions calculated directly from the wind data. At the smallest sampled scales (100–1000 km), the pair-separation velocities are correlated, and the dispersion is laterally isotropic, at least at the higher latitudes. At larger scales, the dispersion is zonally anisotropic, and the pair velocities are uncorrelated. In all cases, the dispersion grows exponentially in time, and the second-order Eulerian structure functions consistently increase as separation squared. This implies nonlocal dispersion, which obtains with energy spectra at least as steep as K−3. Regional variations are seen in the parameters however. The e-folding times and the maximum scales for exponential gro...
We attempt to capture the variability of the depth-integrated flow in the Nordic Seas and Arctic ... more We attempt to capture the variability of the depth-integrated flow in the Nordic Seas and Arctic ocean using an integral model based on the shallow water equations. The model is driven by observed winds or ice motion, and uses realistic bottom topography. It has one free parameter, the coefficient of the (linear) bottom drag. The data to which we compare
We examine the relative motion of pairs and triplets of surface drifters in the Gulf of Mexico. T... more We examine the relative motion of pairs and triplets of surface drifters in the Gulf of Mexico. The mean square pair separations grow exponentially from the smallest resolved scale (1 km) to 40-50 km, with an e-folding time scale of 2-3 days. Thereafter, the dispersion exhibits a power law dependence on time with an exponent of between 2 and 3
We propose a new method for obtaining average velocities and eddy diffusivities from Lagrangian d... more We propose a new method for obtaining average velocities and eddy diffusivities from Lagrangian data. Rather than grouping the drifter-derived velocities in uniform geographical bins, as is commonly done, we group a specified number of nearest-neighbor velocities. This is done via a clustering algorithm operating on the instantaneous positions of the drifters. Thus it is the data distribution itself which
The problem of forced, geostrophic turbulence in a basin on the beta-plane is considered. Of part... more The problem of forced, geostrophic turbulence in a basin on the beta-plane is considered. Of particular interest is the question of whether an inverse cascade of energy can produce jets in the presence of meridional barriers (as occur in most ocean basins). Theory and numerical results suggest it does not. The reason apparently is that the inverse cascade is halted by basin normal modes rather than by free Rossby waves. Because basin modes have a dispersion relation which is effectively isotropic, so the statistically stationary field is also isotropic. As discussed previously, the turbulent basin flow also exhibits a time mean circulation similar to the Fofonoff solution; however, we show that this mean flow does not significantly alter the basin modes in the interior. A relatively novel feature is the use of Eulerian velocity statistics to quantify the wave and turbulence characteristics. These measures are more suitable to this environment than measures like wavenumber spectra, g...
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Papers by J. Lacasce