The impact of the planetary β-effect on the vertical structure of a coherent vortex in the South China Sea

Abstract

Vertical axes of mesoscale eddies with the coherent structure were observed to tilt southwestward significantly from the sea surface to bottom in the South China Sea. The dynamic mechanism for vertical tilting of eddies, however, is not well understood. In this study, the influence of the planetary β-effect on the vertical structure of a coherent eddy is investigated based on analytical considerations and numerical results produced by the MIT general circulation model (MITgcm). Both the analytical and numerical results demonstrate that, during the westward propagation of a mesoscale eddy, the momentum advection and divergence in the inner region of the eddy impact the propagation direction of the eddy on a β-plane. In the upper layer of the mesoscale eddy, the eddy propagation is mainly controlled by the β-effect and nonlinear effect (momentum advection). In the lower layer of an anticyclonic eddy, the eddy propagation direction is dominated by the advection and divergence, which leads to the westward shift of the eddy center in the upper layer. For a cyclonic eddy, by comparison, the vertical axis tilts to eastward from surface to bottom. The tilting distance is up to 30 km in a flat bottom ocean with the β value of 2.15 × 10−11 m−1 s−1. In addition, model results indicate that vertical stratification, eddy strength, and rough topography have significant influences on the vertical structure of a mesoscale eddy.