Horizontal and vertical motions of barotropic vortices over a submarine mountain

Abstract

The evolution of barotropic vortices over a topographic, axisymmetric mountain in a homogeneous rotating fluid is studied experimentally. The aim is to identify the main physical processes observed in (i) a horizontal plane of motion, perpendicular to the rotation axis of the system, and (ii) a vertical plane across the diameter of the mountain. The vortices are monopolar cyclones initially generated near or over the topography. Initially, the vortices drift towards the mountain due to the $\ensuremath{\beta} $-effect associated with the topographic slope. On arriving, they turn around the obstacle in an anticyclonic direction, whilst anticyclonic vorticity is generated over the summit. The long-term vorticity distribution is dominated by the original cyclone elongated around the topographic contours and the generated anticyclone over the tip of the topography. In the vertical plane an oscillatory uphill–downhill flow is generated, which is directly related to the drift of the cyclone around the mountain. Depending on the vortex characteristics, the period of the oscillation ranges from 4 to 10 times the rotation period of the system. The horizontal and vertical flow fields are reproduced numerically by using a shallow-water formulation, which allows a detailed view of the vertical motions, hence facilitating the interpretation of the experimental results. In addition, the cyclone–anticyclone pair over the mountain is compared with analytical solutions of topographically trapped waves. A general conclusion is that vertical motions persist for several days (or rotation periods), which implies that this mechanism might be potentially important for the vertical transport over seamounts.

Quasi-geostrophic vortex solutions over isolated topography

“…Similar experiments using a much larger tank and velocimetry measurements, Zavala Sansón et al. (2012) reported the formation of nonlinear, asymmetric dipoles trapped over a submerged Gaussian mountain while rotating clockwise as a whole. The dipole asymmetry and the subinertial angular speed found in the present solutions closely resemble the experimental cases.…”

“…For instance, the rotation periods of dipolar structures around the mountain in two different experiments of Zavala Sansón et al. (2012) were approximately and 4 ‘days’, where one day in the rotating platform was 30 s. The angular speeds were approximately and , respectively (with ; see their figures 6 and 8). The angular speed of our analytical solutions is of the same order: in absolute value, we obtain between about and .…”

“…Such a striking phenomenon led Hide (1961) to suggest that Jupiter's Great Red Spot might be a manifestation of a columnar vortex over a ‘topographical feature’ of the surface underlying the atmosphere (a hypothesis that was soon discarded). More recently, Zavala Sansón, Aguilar & van Heijst (2012) performed laboratory experiments in a large rotating tank, in which cyclonic vortices approached a submerged mountain. The most relevant result was the formation of an asymmetric cyclone–anticyclone pair rotating around the mountain.…”

Travelling vortices over mountains and the long-term structure of the residual flow

“…Furthermore, dissipative effects owing to solid walls and lateral friction limit the duration of the experiments. To our knowledge, the only laboratory evidence of asymmetric dipoles trapped over a mountain was reported in the experiments by Zavala Sansón et al (2012). The experiments were performed in a large rotating tank (the Coriolis platform in Grenoble, France), where the mentioned difficulties were significantly overcome.…”

“…Experiments with monopolar vortices have been carried out using conical (Carnevale et al. 1991), cylindrical (Cenedese 2002) and Gaussian (Zavala Sansón, Barbosa Aguiar & van Heijst 2012) ‘seamounts’, as well as elongated ridges (Zavala Sansón 2002). To our knowledge, however, the case of a dipolar vortex encountering a submerged mountain has not been addressed.…”