Meeting Documents

Boundary Pressure: A Unique Window into Atlantic Transport Variability

Abstract

Away from continental boundaries, the variability of the global ocean is often dominated by eddies. Despite this interior turbulence, ocean boundary pressures on opposing sides of a basin can vary coherently over interannual to decadal timescales, while exhibiting large-scale (~10⁴ km) spatial structure. As part of the OceanBound project, we use an adjoint model to directly quantify the sources of variability for Atlantic boundary pressures and the associated geostrophic transport.

In the ECCO state estimate (1992-2017), non-equatorial boundary pressure differences across the Atlantic account for 60–90% of the meridional transport variability, both on interannual and subannual timescales. We run the ECCO model in adjoint mode to quantify the linear sensitivity of across-basin pressure differences to wind stress and buoyancy forcing on a 10-year timescale.

We focus on two latitude ranges where boundary pressure estimates of meridional transport variability are particularly robust. The first is centred at 26.5°N, aligning with the RAPID array, and the second at 26.5°S, overlapping with the SAMBA array. In both cases, surface winds are the dominant driver of boundary pressure variability. Along-slope winds are above the Atlantic continental boundaries and interior zonal winds at the local latitudes drive the year-to-year variability.

Surface buoyancy fluxes play a limited role in driving boundary pressure variability. Surface heat fluxes only influence boundary pressure variability in the Northern Hemisphere. Ocean warming at the local latitudes alongside cooling at remote northern latitudes induce a northward geostrophic transport in the upper 1000m of the North Atlantic. In all cases, the effect of surface freshwater fluxes is negligible.

Forward perturbation experiments reveal the mechanisms behind these sensitivities. The experiments are also used to assess effects beyond the 10-year timescale of the adjoint sensitivities and to identify any non-linear feedbacks.

Plain-language Summary: Away from the coastlines, the variability of the ocean is often controlled by small-scale, short-lived, and unpredictable circulations called eddies. Despite being geographically separated by this chaotic behaviour, coastal ocean pressures on opposite sides of the Atlantic can vary coherently over large distances. This is significant as the evolution of ocean pressure can control the volume of water circulated in the Atlantic.

We use an ocean model to identify the surface conditions (e.g. winds, heating, and cooling) which drive variability in ocean pressures. In this model, ocean pressure differences across the Atlantic can describe 60-90% of the variability in northward ocean transport. The vast majority of this variability originates from variable winds following the Atlantic coastline and varying eastward winds above the interior Atlantic. Heating and cooling effects have a limited influence in the North Atlantic, but practically describe none of the variability seen in the Southern Hemisphere.