Dissipation of mesoscale eddies at a western boundary via a direct energy cascade.

The fate of mesoscale eddy kinetic energy represents a large source of uncertainty in the global ocean energy budget. Satellite altimetry suggests that mesoscale eddies vanish at ocean western boundaries. However, the fate of the eddies' kinetic energy remains poorly known. Here we show that the generation of small-scale turbulence as eddy flow impinges on the steep and corrugated slope of an ocean western boundary plays a dominant role in the regional decay of mesoscale eddy kinetic energy. We compare altimetry-based estimates of mesoscale eddy kinetic energy decline with measurements of turbulent dissipation. Mesoscale eddies are found to decay at a rate of 0.016 ± 0.012 GW and 0.023 ± 0.017 GW for anticyclonic and cyclonic eddies, respectively, similar to the observed turbulent dissipation rate of 0.020 ± 0.011 GW. This demonstrates that a major direct transfer of mesoscale eddy kinetic energy to small, dissipative scales can be effectively triggered by the eddies' interaction with the western boundary topography.

High-latitude ocean ventilation and its role in Earth's climate transitions.

The processes regulating ocean ventilation at high latitudes are re-examined based on a range of observations spanning all scales of ocean circulation, from the centimetre scales of turbulence to the basin scales of gyres. It is argued that high-latitude ocean ventilation is controlled by mechanisms that differ in fundamental ways from those that set the overturning circulation. This is contrary to the assumption of broad equivalence between the two that is commonly adopted in interpreting the role of the high-latitude oceans in Earth's climate transitions. Illustrations of how recognizing this distinction may change our view of the ocean's role in the climate system are offered.This article is part of the themed issue 'Ocean ventilation and deoxygenation in a warming world'.