Self-organized states of quasi-two-dimensional turbulence in an oceanic basin with topography

Abstract

The evolution of oceanic-scale, quasi-two-dimensional turbulent flows in a closed basin with topography in a rotating system is studied using a shallow-water model. The basin is nearly flat in the central region and has a sloping coastal topography adjacent to the lateral walls. Two problems are analysed for basins in the northern hemisphere (positive Coriolis parameter). The first problem is the slow decay of an initially disordered flow. The main results are (1) the formation of a steady, anticlockwise flow around the basin that follows the topographic contours and (2) the spontaneous generation of an anticyclonic vortex at the nearly flat central part of the domain. This ‘preferred’ configuration was repeatedly found for arbitrary initial conditions with zero circulation and different Reynolds numbers. The well-defined current around the basin is associated with the direction of propagation of topographic Rossby waves along the contours of constant depth. However, when the initial circulation is sufficiently negative, the resulting flow configuration tends to be anticyclonic over the whole domain, including the coastal regions with topography. The second problem is the evolution of an initial flow at rest that is continuously forced until reaching a quasi-stationary turbulent state. In the presence of random forcing (with no preferential direction in time or space), the flow always tends to the preferred configuration found for decaying flows. The results are discussed in light of recent oceanographic observations in different basins. The limitations of the idealised simulations are outlined, together with recommendations for future studies.