Effects of the asymmetry between surface and interior flow on the dynamics of a thermohaline loop

Large-scale overturning cells in the ocean typically combine an essentially horizontal surface branch and an interior branch below, where the circulation spans both horizontal and vertical scales. The aim of this study is to analyze the impact of this asymmetry between the two branches by “folding” a one-dimensional thermohaline loop, such that its lower part remains vertical while its upper part is folded down into the horizontal plane. It is found that both the transitory response and the distribution of thermohaline properties are modified significantly when the loop is folded. In some cases, velocity oscillations are induced during the spinup that were not seen in the unfolded case. This is because a circular loop allows for compensations between the density torques produced above and below the heat forcing level, while such compensations are not possible in the folded loop because of the horizontal direction of the surface circulation. Furthermore, the dynamical effects associated with nonlinearities of the equation of state are significantly altered by the folding. Cabbeling tends to decelerate the flow in the folded loop, instead of accelerating it as in the circular case, and can also act to dampen velocity oscillations. Thermobaricity also alters the loop circulation, although comparatively less.