Island Wakes in Deep Water

Abstract

Density 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.

Inertial instability of intense stratified anticyclones. Part 2. Laboratory experiments

“…The dynamical evolution of both eddies was compared to detect the signature of the selective destabilization of anticyclonic eddies by inertial-centrifugal perturbations. Unlike previous numerical studies (Dong et al 2007;Kloosterziel et al 2007), we did not observe a rapid or complete breakdown of anticyclones with a core of negative absolute vorticity (negative potential vorticity). However, in a number of intense anticyclones, we detected an anomalous decay of the mean azimuthal velocity profile in the unstable annulus defined by the generalized Rayleigh criterion χ(r) = (ζ (r) + f ) (2V(r)/r + f ) < 0.…”

“…According to the generalized Rayleigh criterion (Kloosterziel & van Heijst 1991;Mutabazi et al 1992;Sipp et al 2000), these strong anticyclones are well above this inviscid stability limit. But surprisingly, these anticyclones do not break down and they remain circular and coherent for several rotation periods, and there is no signature of small-scale perturbations in the vorticity field, as shown in the numerical simulations of Dong, McWilliams & Shchepetkin (2007). We do, however, detect a cyclone-anticyclone asymmetry.…”

Mesoscale variability in the Arabian Sea from HYCOM model results and observations: impact on the Persian Gulf Water path

“…t (see Dong et al, 2007). The strongest values are found near these coasts and in the Sea of Oman with an order of magnitude of 2.5 × 10 −6 s −1 , positive in summer, negative the rest of the year.…”

Ebb-Tide Dynamics and Spreading of a Large River Plume*

“…We set the background vertical diffusivity of both tracers and momentum at 5 3 10 26 m 2 s 21 , with zero explicit horizontal diffusivity and viscosity (e.g., Dong et al 2007). Bottom stress is parameterized with a quadratic drag law and a drag coefficient of 3 3 10 23 (e.g., Geyer et al 2000).…”