Overturning cells in the Southern Ocean and subtropical gyres

[摘要] The circulation of the subtropical gyres can be decomposedinto a horizontal recirculation along contours of constant Bernoullipotential and an overturning circulation across these contours. Whilethe geometry and topology of Bernoulli contours is more complicated in thesubtropical gyres than in the Southern Ocean, these subtropical overturningcirculations are very much analogous to the overturning cell found in theSouthern Ocean. This analogy is formalised through an exact integralconstraint, including the rectified effects of transient eddies. Theconstraint can be interpreted either in terms of vertical fluxes ofpotential vorticity, or equivalently as an integral buoyancy budgetfor an imaginary fluid parcel recirculating around a closed Bernoullicontour. Under conditions of vanishing buoyancy and mechanicalforcing, the constraint reduces to a generalised non-accelerationcondition, under which the Eulerian-mean and eddy-induced overturningcirculations exactly compensate. The terms in the integral constraintare diagnosed in an eddy-permitting oceanmodel in both the North Pacific subtropical gyre and the Southern Ocean.The extent to which the Eulerian-meanand eddy-induced overturning circulations compensate is discussed in each case.

[学科分类] 海洋学与技术