Oceanic currents spanning the southern Altantic may be driven by giant eddies.

Section of thermohaline circulation driven by transient eddies

28 June 2012

New research has found currents traversing the world’s oceans may not be as stable as previously thought after finding deep currents in the South Atlantic may actually be powered and steered by transient eddies.

In a recently published study lead authored by Erik Van Sebille (CCRC) found the path of deep water across the Atlantic from the Americas to South Africa is controlled by warm-core Agulhas rings - eddies that form south of the African Cape and drift in the opposite direction to the deep current.

Deep currents in the South Atlantic are part of the oceanic overturning circulation, which redistributes heat around the globe and has an important effect on global climate.

While deep water currents usually flow in a pattern that follows the western margins of ocean basins, in the Atlantic some waters veer to the east towards South Africa around the latitude 25°S.

Researchers have known for decades about this deep, southeastward current, but have not understood what drives it. Using a fine-resolution global ocean circulation model, the researchers found the current is driven and steered by an opposing drift and decay of Agulhas rings above it.

Using simulated floats to track the three-dimensional pathway of deep currents across the South Atlantic, the team found a major pathway lined up with the so called Agulhas ring corridor. This finding provided a clue as to how the two are connected, and a mathematical study shows that the upper layer rings force a circulation in the lower layer by squeezing it and changing its vorticity.

This finding suggests that when the number of Agulhas rings changes, there may be a direct affect on the deep branch of the global overturning circulation.

Citation: Van Sebille, E, WE Johns, and LM Beal (2012), Does the vorticity flux from Agulhas rings control the zonal pathway of NADW across the South Atlantic? J. Geophys. Res., 117, C05037.

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