Boundary Pressure: A Unique Window into Atlantic Transport Variability

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 investigate how boundary pressure differences reflect meridional transport anomalies in the Atlantic and use an adjoint model to track potential sources of variability.

In the ECCO state estimate, we find that boundary pressure differences across the Atlantic account for 60–90% of the meridional transport variability, both on interannual and subannual timescales. This result is consistent at most latitudes, excluding the equatorial region.

Adjoint model simulations allow us to quantify the linear sensitivity of across-basin pressure differences to surface forcing. 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, we identify surface winds above the continental shelf as the dominant driver of boundary pressure variability, whereas surface buoyancy forcing plays a negligible role.

Sensitivity fields derived from the adjoint model are used to reconstruct the Atlantic boundary pressure differences and, consequently, the significant geostrophic component of meridional transport variability. Forward perturbation experiments further reveal potential mechanisms underlying these sensitivities, as well as any non-linear behaviours.