Indian - Atlantic interocean exchange: variability and controls

Abstract

South of Africa, warm Indian Ocean water enters the Atlantic Ocean by means of large Agulhas Rings. These rings, with diameters up to 350 km and reaching all the way to the ocean floor at 5 km depth, form an important link in the global thermohaline circulation, which is the driving force behind the moderate temperatures over Northern Europe.

For six years, the shedding of Agulhas rings has been monitored from space, using satellite altimeter measurements of the sea surface height. Rings can be identified by sea surface elevations on the order of 20-50 cm. The rings are followed on their way westward through the Atlantic, where they gradually decay. The decay is found to take place mainly in the Cape Basin during the first year after formation, probably delivering most of the Ring's thermohaline contents to this region. Also, rings are observed to often split into smaller rings.

The shedding of Agulhas Rings is shown to be related to upstream processes. Especially, eddies from the Mozambique Channel are found to control the timing of Agulhas Ring shedding. These Mozambique Eddies have recently been measured to be dynamically comparable to the energetic Agulhas Rings, to carry tropical water masses southward through the Mozambique Channel, and to induce strong mixing in the channel. They in turn seem to be part of a mode of variability that is found over much of the Indian Ocean.

A four-per-year signal is found to originate in the equatorial part of the Indian Ocean, and to propagate by a sequence of Kelvin and Rossby waves into the Mozambique Channel. Kelvin waves at the equator are forced by the semi-annual westerly windbursts that occur during the change of the monsoons. The frequency of these waves is doubled to four, probably by resonance of an equatorial basin mode. Four Kelvin waves per year are observed to reach the Indonesian coast, where they travel southward as coastal Kelvin waves. These force baroclinic Rossby waves at subtropical latitudes, which travel in westward direction towards the African continent. When they reach Madagascar these waves force the frequency of the Mozambique eddies in the channel between Madagascar and the African continent. A statistically significant correlation between the equatorial forcing and the Mozambique Channel eddies is shown to exist at a lag of 1.25 years.