A mechanism for establishment and maintenance of the meridional overturning in the upper ocean

Abstract

A two-dimensional analytical residual-mean model of the meridional overturning in the upper ocean is presented which illustrates dynamics of the interaction between the Northern and Southern hemispheres. The theory is based on the semi-adiabatic approximation in which all diabatic processes are confined to the upper mixed layer. The overturning circulation is driven directly by the wind forcing which, in our model, is affected by the sea-surface temperature distribution. The surface boundary conditions are symmetric with respect to the equator, and therefore one of the steady state solutions represents a symmetric flow characterized by the absence of the inter-hemispheric buoyancy fluxes. However, linear stability analysis, which takes into account both mechanical and thermodynamic forcing at the sea surface, indicates that the symmetric configuration such as this is unstable. The instability results in transition to the asymmetric regime with finite cross-equatorial exchange flows and heat transfer. Weakly nonlinear instability theory makes it possible to estimate the equilibrium fluxes in the new asymmetric steady states; for the oceanographically relevant range of parameters our model predicts the meridional overturning of about 10 Sv. While earlier studies considered the role of salt advection in spontaneous symmetry breaking, our study relies on a positive feedback between atmospheric winds and the oceanic meridional circulation.