The U.S. National Science Foundation (NSF) announced that it is funding a study with the goal of building a multi-decadal time series of Agulhas Current volume transport, which will contribute to the Global Ocean Observing System. Led by Principal Investigator, Lisa Beal, Ph.D. of the University of Miami's Rosenstiel School of Marine and Atmospheric Science, the international team will include scientists from the National Oceanography Centre (Southampton, United Kingdom) and the University of Cape Town (Cape Town, South Africa).
The Agulhas Current is the "Gulf Stream" of the southern Indian Ocean, carrying warm and salty tropical waters southward along the east coast of Africa as a narrow, fast jet. At the tip of Africa the Agulhas retroflects, looping around to eventually flow eastward toward Australia. This retroflection is unstable and regularly sheds large Agulhas Rings, which carry Indian Ocean waters into the South Atlantic.
"We anticipate this study will shed light on the seasonal to decadal variability of the Agulhas," said Beal. "Locally, the warm waters of the Agulhas effect African rainfall rates, and globally, there is paleo-oceanographic evidence suggesting that changes in the amount of Agulhas water reaching the Atlantic may have triggered the end of the last ice age." In addition, Beal believes there may be other, so far unstudied climatic impacts such as heat transport into the Southern Ocean via the unstable retroflection.
The Agulhas Current Time-series (ACT) project will be conducted in two phases. The first requires the deployment of an array of instruments across the Agulhas Current and along an altimeter ground track, to obtain a three-year series of transport data. Using the in situ measurements gathered, the team will embark on the second phase, to correlate the along-track satellite altimeter data with measured transports to produce a proxy for Agulhas Current transport, which can be extended forwards and backwards in time.
The ACT mooring array will be positioned offshore and to the southwest of East London, South Africa, in up to 4700 m of water. The array will consist of seven full-depth current meter moorings, spanning the mean width of the Agulhas Current, one tide gauge, plus four pressure gauge-equipped inverted echo-sounders (C-PIES) to cover the Current's offshore meandering events cost-effectively.
On each full-depth mooring upward-looking profiling current meters will measure the top 350 m of the water column, where velocities are strongest. Below these, up to six single-point current meters will measure the rest of the water column. All current meters are acoustic, with no moving parts, measuring velocity using the principle of Doppler shift, whereby the frequency of a sound wave changes as it reflects off a moving particle in the ocean.
Offshore recirculations and meandering events will be captured using the C-PIES, which can give information about the transport over the upper 2000 m, when combined with local hydrographic data collected during each mooring cruise. The shallow-water tide gauge will be placed on the continental shelf, allowing for an hourly record of sea surface height shoreward of the Agulhas Current.
The initial deployment cruise is scheduled to leave Cape Town, South Africa, in March 2010, aboard a U.S. University-National Oceanographic Laboratory System (UNOLS) research vessel. The ACT array will be in the water until approximately March 2013.
"Ultimately, a twenty year proxy of Agulhas Current transport will provide an important climate index for the Indian Ocean, which can be linked to other climate indices, such as the Atlantic Meridional Overturning, and hopefully improve our predictive capabilities for the future," Beal added.