Coupling Between Abyssal Boundary Layers and the Interior Ocean in the Absence of Along-Slope Variations

TL;DR

This paper develops a multi-dimensional boundary layer theory to better understand how abyssal boundary layers interact with and influence the large-scale interior ocean circulation, moving beyond previous 1D models.

Contribution

It introduces a generalized boundary layer framework in 2D and 3D, capturing the coupling between boundary layers and interior flow, which was not possible with earlier 1D theories.

Findings

Boundary layer transport influences interior circulation through horizontal variations.

The new framework captures BL--interior coupling in stratified conditions.

Explicit formulas for boundary layer transport are derived.

Abstract

To close the overturning circulation, dense bottom water must upwell via turbulent mixing. Recent studies have identified thin bottom boundary layers (BLs) as locations of intense upwelling, yet it remains unclear how they interact with and shape the large-scale circulation of the abyssal ocean. The current understanding of this BL--interior coupling is shaped by 1D theory, suggesting that variations in locally produced BL transport generate exchange with the interior and thus a global circulation. Until now, however, this picture has been based on a 1D theory that fails to capture the local evolution in even highly idealized 2D geometries. The present work applies BL theory to revised 1D dynamics, which more naturally generalizes to two and three dimensions. The BL is assumed to be in quasi-equilibrium between the upwelling of dense water and the convergence of downward buoyancy fluxes. The BL transport, for which explicit formulae are presented, exerts an influence on the interior by modifying the bottom boundary condition. In 1D, this BL transport is independent of the interior evolution, but in 2D the BL and interior are fully coupled. Once interior variables and the bottom slope are allowed to vary in the horizontal, the resulting convergences and divergences in the BL transport exchange mass with the interior. This framework allows for the analysis of previously inaccessible problems such as the BL--interior coupling in the presence of an exponential interior stratification, laying the foundation for developing a full theory for the abyssal circulation.