Vortices and Rossby waves in cylinder wakes on a parabolic β-plane observed by altimetric imaging velocimetry

Abstract

Intense vortices in the wake of a circular cylinder are investigated in a rotating parabolic (polar) β-plane fluid. This system has a background potential vorticity (PV) field that supports Rossby waves and causes vortices to migrate and radiate. A method for imaging rotating flows, which we call “altimetric imaging velocimetry” is employed. Optical color coding of slopes of the free-surface elevation field yields the pressure, geostrophic and gradient wind velocity, and potential vorticity fields with very high spatial resolution, limited largely by the pixel resolution of the available imaging sensors. Cylinder wakes on the polar β-plane exhibit strikingly different regimes as it is translated azimuthally, eastward or westward. Self-arrangement of vortices after the cylinder was stopped drives an intense eastward jet formed by the rows of anticyclones and cyclones on its flanks. In agreement with the idea of a PV staircase, this jet has a strong PV gradient at its center, while PV is homogenized by the vortices on either side. A slowly translating cylinder generates Rossby waves with phase propagation locked to the cylinder, and intermediate cases show a widespread vortex/wave interaction.