Around the South Pole, Earth wears a swirling salty belt known as the Antarctic Circumpolar Current (ACC). This wind-driven current is the planet’s strongest, a powerful regulator of ocean heat and carbon content. The high volume of seawater that the ACC moves as it rotates around the South Pole also affects ocean stratification, which is the separation of water layers with different temperatures, salinities, and densities. Because of this, the influence of the ACC is global, and the physical processes that drive ACC motion may have implications for global ocean heat content and atmospheric carbon dioxide (CO2) content.
Here Marshall et al. examine one force behind the current: the turbulent ocean eddies contained within the large-scale, generalized motion of the ACC. Previous research has shown that volume transport is more sensitive to changes in surface winds when models resolve these eddies, a process known as “eddy saturation.” The authors craft a simple model to examine the relationship between volume transport, surface wind stress, and eddy energy and identify how ocean eddies contribute to the ACC’s global influence.
The model incorporated three major variables. The first was zonal momentum budget, the balance between momentum added to the eddies by surface wind stress, transferred downward by eddies, and lost via topographic barriers along the ocean floor. The second was the relationship between eddy form stress—the mechanism of momentum transfer between the surface and the seafloor—and eddy energy. Last, the authors included the eddy energy budget, which is primarily driven by the instability of flow in the ACC.
The model offered several new insights. It helped to explain the physics of eddy saturation and demonstrated that eddy energy increases with increased wind stress on the surface water. Somewhat surprisingly, however, the model also showed that an increase in friction along the sea bottom increased the dissipation of eddy energy, which translated into increased volume transport around the pole.
According to the authors, because of the ACC’s high-volume transport, this eddy energy dissipation in the Southern Ocean may have an outsized impact on global oceanic stratification and heat content. Because ocean stratification in turn impacts the ocean carbon cycle, the authors emphasize the potential role of these eddies in influencing atmospheric CO2. Further research into the mechanisms behind the current’s movement will help scientists to more fully understand the complex interactions between Earth’s oceans and atmosphere. (Geophysical Research Letters, https://doi.org/10.1002/2016GL071702, 2017)
—Lily Strelich, Freelance Writer