Large-scale mid-depth ocean dynamics: eastern boundary MOC return flows and interior stratification

Abstract

The large-scale Meridional Overturning Circulation (MOC) in the Atlantic Ocean involves surface northward transport to the northern North Atlantic and a southward return flow at mid-depth of the North Atlantic Deep Water (NADW) which forms convectively. In the Indian and Pacific Oceans, a northward bottom water flow is again compensated by a mid-depth return flow toward the Southern Ocean (SO). This zonally-averaged picture of the MOC hides a rich horizontal structure involving concentrated mid-depth flows toward the SO near the western and eastern boundaries, both carrying significant meridional transports. While deep western boundary currents have been understood for a while, the dynamics of their eastern boundary counterparts are still a mystery and the climate models do not simulate such currents at the right location or with the right magnitude. Furthermore, the MOC is an important part of the processes that determine the mid-depth ocean stratification, also involving Southern Ocean winds and eddies, interior vertical mixing and northern high-latitude convection. This thesis has two parts: the first explores the dynamics of mid-depth eastern boundary routes of the MOC; the second studies how the interplay between SO dynamics and interior vertical mixing determines the vertical structure of the mid-depth ocean stratification. The MOC has a complex horizontal structure, and its meridional transport has long been thought to be mainly due to near-surface and deep \textit{western} boundary currents, with a negligible contribution from eastern boundary flows. However, observations have robustly revealed the existence of poleward Deep Eastern Boundary Currents (DEBCs) between 1--4 km depth, in the South Atlantic, Indian, and Pacific Oceans. These deep eastern boundary currents are found to be important pathways of mass, tracer, and heat transports towards the SO. I find that the upstream part of these currents (away from the outflow into SO) is governed by an interior-like vorticity balance ($\beta v\approx f\partial_z w$) over most of their widths, with a narrow layer very close to the solid boundary where friction becomes important, using regional realistic and idealized model simulations. This means that the DEBCs are different from their western boundary counterparts, which are governed by a friction-dominated vorticity dynamics, with contribution from nonlinearity and bottom torque as well. I further find that the vortex stretching driving the upstream DEBCs is maintained by eddy temperature mixing and bottom topography. The outflow into the SO, on the other hand, is found to be directly forced by eddy vorticity transport. This implies that resolving eddies appropriately is crucial for a successful simulation of these DEBCs. The large-scale ocean stratification away from horizontal boundaries and from the SO is very nearly exponential. This was explained via a vertical advective-diffusive balance by Munk (1966). However, later observations showed that open-ocean vertical mixing away from rough topography is one order of magnitude too weak to play a role in Munk's balance. An alternative mechanism for mid-depth ocean stratification involves a balance between the SO winds which steepen the isopycnals there, and eddies which flatten them. The competition of the wind and eddies results in isopycnal slopes that map the SO surface density to a vertical profile in the basin north of the SO. However, it turns out that explaining the observed exponential structure of the stratification still calls for a role for interior vertical mixing, perhaps via boundary-intensified vertical mixing. In this work, I use eddy-permitting idealized simulations to study how the observed exponential structure of mid-depth stratification is determined by an interplay of SO dynamics and interior vertical mixing. I find that the mid-depth stratification north of the SO is exponential only when the diapycnal mixing north of the SO is significant. The SO isopycnal slopes and eddies respond to changes in mixing and stratification north of the SO, even when the SO wind is fixed. The residual overturning circulation strengthens with interior increasing vertical mixing, implying that the balance between SO wind and eddies is still sensitive to interior processes.