For the past two decades, BIOS scientists have stood behind the idea that mesoscale eddies are a driving force in coastal and open ocean processes, including biogeochemical cycling and the global carbon cycle. As research technologies improved over this time period it became apparent that, not only were they correct, but that eddies are far more important to ocean and climate systems than previously imagined.
Eddies are one of the many physical features that influence the transport of water in the ocean (currents and gyres being other examples). While most eddies are relatively small—on the scale of centimeters to meters in diameter—and fleeting, some eddies are on the scale of 10-100s of kilometers and last for days or even months. These large, persistent eddies are called “mesoscale” eddies and they are ubiquitous across all major areas of the ocean, including the polar regions.
While the mechanics of eddy formation are still an area of active research, scientists have identified three distinct types of open ocean mesoscale eddies:
1. Cyclonic – these eddies exhibit a counter-clockwise rotation and, due to the Coriolis Effect, push water away from their center and allow upwelling from the deeper ocean, creating a region of anomalously cold water. Cyclonic eddies are considered “divergent systems” and are slightly lower than the surrounding ocean (“sea level depressed”);
2. Anti-cyclonic – these eddies exhibit a clockwise rotation that creates a “convergent system” in which water is pushed toward the center, resulting in a region of warmer water. As the water is pushed inward a slight bulge is created, causing the eddy to be slightly higher than sea level (“sea level elevated”);
3. Mode water/lens – these eddies are created when a lens of water is pushed between density layers in the ocean, such that the ocean above the lens to upwells and resembles a cyclonic eddy, while the ocean below the lens downwells and resembles an anti-cyclonic eddy. In these situations the net effect is that of a weak anti-cyclonic eddy with a moderately elevated sea level. Mode water/lens eddies are of particular interest to oceanographers because of elevated primary production (phytoplankton) levels associated with the upwelled water.
Scientists at BIOS are combining spatial data (e.g., measurements collected as part of the Bermuda Atlantic Time-series Study, or BATS) with specialized computer models to better understand how nutrient material moves, or fluxes, horizontally through mesoscale eddy fields. They’re attempting to balance the ocean’s nutrient budget and, in doing so, are bringing eddies out from the footnote of oceanographic texts and into the forefront of ocean systems research.