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.
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