How is density compensation created in the ocean mixed layer?

TL;DR

This paper investigates how turbulent mixing influences horizontal density compensation in the ocean's upper layer, introducing a new non-linear diffusion scheme and analyzing its effects at different scales.

Contribution

It introduces a novel non-linear horizontal diffusion scheme and analyzes the role of mixing in creating density compensation at submesoscales.

Findings

Horizontal mixing with constant diffusivities does not significantly change density gradients.

The new non-linear scheme can produce compensated gradients at scales of 10 km or less.

External forcing is necessary to reproduce observed compensation at larger scales.

Abstract

This study examines the impact of turbulent mixing on horizontal density compensation in the upper ocean. A series of simulations model the role of mixing in scenarios initialized with geostrophically-adjusted compensated and uncompensated thermohaline gradients. Numerical experiments isolate the influence of mixing on these gradients using idealized conditions with zero surface heat and momentum flux. Prompted by theoretical considerations and observed consequences of mixing, a new non-linear horizontal diffusion scheme is introduced as an alternative to the standard Laplacian diffusion. Results suggest that when horizontal mixing is parameterized using constant diffusivities, horizontal density compensation is substantially unchanged as the gradients erode. Simulations using the new scheme, which parameterizes mixing with horizontal diffusivities scaled by squared buoyancy gradient, suggest that horizontal mixing can produce compensated gradients during this decay, but only at scales of 10 km and less. Reducing vertical mixing to small background values has a similar effect, increasing the degree of compensation at submesoscales. Reproducing observed compensated thermohaline variability in the mixed layer at scales greater than 10 km requires external forcing. These results show important influences of mixing on density compensation within the ageostrophic submesoscale regime. In the transition from a horizontally compensated mixed layer to a partially compensated thermocline, advection must play an important role; mixing alone is insufficient.