The Agulhas Undercurrent Experiment (AUCE)

14th February - 17th March 2003, Cape Town to Cape Town
Chief Scientist: Lisa Beal
 
Schematic of ocean currents around South Africa  
 
A Schematic of the ocean currents around South Africa. The Agulhas Current is the Western Boundary Current of the subtropical gyre in the south Indian Ocean. The Agulhas Undercurrent flows beneath it and in the opposite direction.                                                               photo of LADCP





A Lowered Acoustic Doppler Current Profiler (yellow instrument) mounted at the center of a CTD rosette frame.

The battery pack is at the base of the frame to the right. This LADCP operates at 150 kHz with four transducers pointing at 30° to the vertical.         





What is the Agulhas Undercurrent and why is it important?
ACE LADCP velocity section

In March 1995 the first observations of the full-depth velocity structure of the Agulhas Current were made using a Lowered Acoustic Doppler Current Profiler (LADCP). During the experiment a deep counter-current was measured beneath the Agulhas Current, a feature not previously observed and now known as the Agulhas Undercurrent (Beal and Bryden, 1997).

In the image to the right the undercurrent can be seen in blue, next to the continental slope of South Africa and below the Agulhas Current (red) at the surface. Colours change for every 10 cm/s change in along-stream current.

The primary reason that the Agulhas Undercurrent is of interest to oceanographers and climatologists is its potential role in the global thermohaline circulation. At 32 S it carries North Atlantic Deep Water (NADW) and Antarctic Intermediate Water (AAIW) quickly northward, indicating that the western boundary could be an important route for ventilation of the deep Indian Ocean. It has been estimated that the Agulhas Undercurrent drives 40% of the overturning circulation of the Indian Ocean (Bryden and Beal, 2001).

In addition to fundamental questions of its global importance, there are many specific questions about the Undercurrent's dynamics to be addressed (Beal and Chereskin, 2001). With observations of undercurrents now made in the Gulf Stream, the Brazil, East Australia, Mozambique, Somali and Agulhas Currents it seems likely that they are ubiquitous to western boundary current systems. What is the mechanism for this? How similar is the Agulhas Undercurrent to these other bottom trapped flows? Does it entrain waters, rise or deepen as it flows northward? How much does its transport vary over time? Is its speed related to the steepness of the slope, and is it always attached to the slope? Does the Undercurrent conserve potential vorticity along its path? What happens to the north as the continental shelf widens and the Natal Valley shallows?


The Field Program

station map






Positions of stations occupied during AUCE in February -March 2003. Red dots mark positions of CTDO/LADCP stations. Yellow diamonds show the position of the moored current meter array.


From north to south the sections are called "Richards Bay", "Port Shepstone", "East London", and "Port Elizabeth" and they are nominally at 30, 32, 34, and 36°S.










Melville







In February/March 2003 shipboard hydrographic work was completed off the east coast of South Africa, between 29 S and 37 S, in order to address all the questions above and learn more about the Indian Ocean's western boundary current system.

Fieldwork was conducted aboard R/V Melville, one of the UNOLS fleet, operating out of Scripps Institution of Oceanography in San Diego, California.

Four high resolution, cross-stream sections of CTDO/LADCP stations were completed across the Agulhas Current and Undercurrent at nominally 30, 32, 34, and 36°S. The offshore section, which closes the survey region into three boxes, will allow the calculation of entrainment, heat and freshwater budgets for the entire WBC system, quantifying the thermohaline fluxes due to both the Agulhas Current and its Undercurrent. In addition an along-slope survey was conducted following the path of the Undercurrent, assuming it to be strongly topographically controlled, with the intention of defining its latitudinal extent.

Finally, the variability and long-term mean transport of the Undercurrent, which cannot be assessed from one-time surveys, will be monitored by a moored current meter array which was deployed across the Undercurrent at 32 S (off Durban, South Africa).



Mooring Operations: The instruments are Rotary Current Meters (RCMs) with temperature and salinity sensors. The silver tube is an acoustic release. Flotation is vacuum-filled glass spheres. The anchor was disused train wheels. The mooring team for AUCE is based at Southampton Oceanography Centre.

mooring1 mooring2
                                        mooring4 mooring3 mooring5









CTD Operations: the 'package' consisted of 24 sampling bottles, a seabird CTD with oxygen sensor and a redundant set of sensors, the LADCP, and an altimeter for height-off-bottom.

CTD1 CTD2

CTD3 LADCP


Preliminary Results

ADCP vector map





Preliminary results from AUCE indicate that the Agulhas Undercurrent does flow continuously northeastward along the African continental slope. At 36 S (Port Elizabeth) and 32 S (Port Shepstone) it is clearly visible, at 34 S (East London) it is very weak, perhaps because the Agulhas Current is pushed onshore by a cyclone during the survey, and at 30 S (Richards Bay) the undercurrent has moved offshore due to the widening slope and shallower water depths in the northern reaches of the Natal Valley.

Another interesting result is the distinct distribution of water masses across the Agulhas Current, which remain seperate despite high eddy kinetic energy. The dynamical front of the WBC, defined as the region where the relative vorticity (lateral velocity shear) changes sign, separates northern and equatorial waters from those waters from the south and east which are circulated within the subtropical gyre.


This map shows shipboard ADCP velocities, with underway data in black and on-station data in red. The Agulhas Current is oriented southwestward, except at Richards Bay where the continental slope topography guides it to the southeast around the Natal Bight. At the southernmost line, off Port Elizabeth, the current has detached from the slope.



Below are the four cross-sections of the Agulhas Current and Undercurrent all to the same scale. For these illustrations shipboard ADCP and LADCP velocities have been combined to reduce data gaps. Northeastward velocities are in blue, southwestward velocities are in red/yellow. All velocities are cross-track, and close to perpendicular with topography. Velocity contours are in cm/s. Green contours represent neutral surfaces separating layers of, from top to bottom, Tropical Surface Water (TSW), SubTropical Surface Water (STSW), thermocline waters (including SubAntarctic Mode Water), Red Sea (RSW) and AntArctic Intermediate Water (AAIW), and Upper/Lower North Atlantic Deep Water (NADW).

RB LADCP
PS LADCP
30°S:  Richards Bay

Here the Agulhas Current is weakest and confined to 1500 m depth due to the topography of the wide continental slope. The shallow recirculation offshore is common to all sections.

There is an indication a of bottom-intensified current 240 km offshore, possibly representing a weak Agulhas Undercurrent.

Isopycnals rise up 5 m in every kilometre over the slope. Over the shelf, which is wide here extending about 10 km to the west of the origin of this section, is a northward counter current.

 32 S:  Port Shepstone

Now that the continental slope is steep the Agulhas Current is strong and deep and the offshore recirculation is weak.

The Undercurrent is clear between the slope and the WBC above it. The velocity structure is similar to that seen in the original section taken in 1995 and shown at the top of this page. However, the Undercurrent is weaker in 2003.


EL LADCP
PE LADCP
34°S:  East London

The Agulhas Current appears to be modified by a cyclone (sea-surface low) offshore during the occupation of this section. Looking at the ADCP vector map (above) the cyclone is clear. Moreover, water mass properties show that the northward velocities offshore are not recirculating Agulhas waters in this case.

The cyclone has pushed the WBC right over the shelf break making it weak and shallow. The Undercurrent is also weak.
 36°S:  Port Elizabeth

Here the Agulhas Current is the strongest and deepest with a 125 Sv southwestward transport. It has separated from the coast so that the core is on over 3000 m water depth. Isopycnals rise by 12 m per kilometre on the cyclonic or onshore side of the current.

The Undercurrent is attached to a fast subsurface flow which sits at the shelf break. Notice that because of the steeply sloping isopycnals, most of this northward flow is within the intermediate water layer.


 TS diagram
Finally, here is a figure of the water mass properties in the Agulhas Current. The temperature-salinity plot has been colour-coded by the sign of the relative vorticity of each water parcel.

Positive relative vorticity corresponds to the offshore side of the Current, that is, those water parcels offshore (east) of the dynamical front. Notice that these waters are saltier than onshore waters at the surface, and fresher at intermediate levels. They originate in the evaporative region of the eastern subtropical Indian Ocean (STSW) and in the Southern Ocean (AAIW) respectively, and are advected into the Agulhas Current via the subtropical gyre.

Negative relative vorticity corresponds to the onshore side of the WBC jet, which is associated with much stronger lateral velocity shears. Here the surface waters have advected from the equator (TSW) and intermediate waters exhibit the salty influence of RSW, both water masses are advected through the Mozambique Channel into the Agulhas Current.
ACKNOWLEDGEMENTS

Captain Curl and the officers and crew aboard R/V Melville during AUCE were especially friendly, helpful and interested in the science, making the experiment a pleasureable and successful experience.

We are grateful to the personnel of Scripp's Ocean Data Facility for their expertise and hard-working ethic. Especially to Frank Delahoyde who is nothing short of a genius. Special thanks also to the Southampton Oceanography Centre's Mooring team.

Last but not least, huge appreciation goes to the students who took part in AUCE, who processed data and produced plots in near-real time and, equally important, lightened the mood through their upbeat, enthusiastic, and humorous exploits.

This work was supported by NSF grant number OCE-0244769.


REFERENCES

Beal, L. M., and H. L. Bryden, Observations of an Agulhas Undercurrent, Deep-Sea Research, 44, 1715-1724, 1997.

Bryden H. L., and L. M. Beal, Role of the Agulhas Current in Indian Ocean circulation and associated heat and freshwater fluxes, Deep-Sea Research, 48, 1821-1845, 2001.

Beal L. M., and T. K. Chereskin, The Agulhas Undercurrent Experiment, NSF proposal, 2001.