Earth’s five subtropical gyres—large current systems north and south of the tropics—encircle much of the surface water of the open ocean. Known for their low biological activity, these “desert” waters nonetheless contribute significantly to marine productivity because of their vast size. New research by Doddridge and Marshall reveals how eddies might affect nutrient levels within these subtropical surface waters.
The structure of a subtropical gyre includes both a rotating ring of currents and the calmer surface waters within. Winds over the gyres drive a process called Ekman pumping, in which some of the encircled water is drawn deeper into the ocean, out of the gyre system. This vertical flow carries nutrients required for biological activity, contributing to the low nutrient levels often seen in subtropical gyres.
An earlier study that included the current researchers described a previously unrecognized process in which medium-sized (or mesoscale) eddies counteract Ekman pumping and cancel out the associated downward transport of water, which it called “eddy cancellation.” These mesoscale eddies are smaller circular currents, typically less than 100 kilometers across, that may branch off from the main gyre flow and last anywhere from several days to a few months.
The authors have developed a computer model to investigate the impact of eddy cancellation on nutrient transport within an ocean gyre. In a simplified representation of actual gyre structure, the model has an upper water layer representing the euphotic zone, the light-bathed layer where photosynthesis occurs, and a lower mode water layer, an expansive zone with homogeneous properties such as temperature and salinity, characteristic of subtropical gyres.
Using this model, the scientists explored how nutrient concentration within the simulated gyre changed in response to alterations in different model parameters. Nutrients that fell beneath the mode water layer were considered lost to the abyss, no longer contributing to biological productivity in the gyre system.
Their analysis revealed the importance of two parameters: the velocity of Ekman pumping after accounting for eddy cancellation and the thickness of the mode water layer. Initially, increasing the value of either of these two parameters reduced nutrient concentration in the gyre waters. But past a critical threshold, increasing them resulted in higher nutrient levels.
Increasing the Ekman pumping velocity beyond the threshold value increased nutrient concentration by transporting nutrients horizontally into the gyre from adjacent waters. The effect of the mode water layer was less straightforward. In the model, a very thin mode water layer was associated with the diffusion of nutrients up from the abyss and into the gyre system. Thus, increasing the thickness initially reduced nutrient concentration. But past a certain threshold, a thicker mode water layer was associated with reduced flow of nutrients into the abyss and therefore increased biological productivity in the gyre. However, strong Ekman pumping suppressed these effects.
The researchers found support for their model results in the subtropical North Atlantic gyre. Real-world data collected by satellites, free-floating Argo instruments, and research ships showed a correlation between thicker mode water and higher biological productivity, consistent with a small Ekman pumping velocity after accounting for eddy cancelation.
The researchers acknowledge that their model simplifies the structure of gyres, and their observational data have some noisiness. Still, the findings suggest that by countering Ekman pumping, mesoscale eddies may play an important role in setting nutrient concentrations within subtropical gyres and that nutrient recycling within these systems is more effective than expected. (Journal of Geophysical Research: Oceans, https://doi.org/10.1029/2018JC013842, 2018)
—Sarah Stanley, Freelance Writer