Posted by Olaf Boebel on May 11, 2001 at 01:11:09:
This movie provides a most pleasing visualization of the Agulhas Current just south of Durban. Apart from the passing of several "events" it clearly demonstrates the stability of the AC in location and strength. When in its normal position at the shelf break, surface velocities of the AC peak at 1.8 m/s throughout the duration of the measurements.
The "events" appear to present 2 types:
A) Short events: (less than a week) of small amplitude (the current is pushed offshore by less than 50km). These are probably small instabilities, previously called shear edge eddies. It could be interesting to see if the occurrence of these displacements is (lag-) correlated to an intrinsic property of the current, such as its KE.
As short events I would consider: 43-50, 54-55, 105
B) Long events (fortnight and longer) have a larger amplitude (initially >200 km). It seems there is an asymmetry in the temporal evolution of the displacement: While the current moves offshore (disappears out of the frame) in a day or few, its return back to the cost is slower, about 10 days plus.
This later observation is in agreement with the shape of Natal Pulses observed in this region: the AC flows around a tear drop shaped inner cyclonic circulation. The leading edge of the pulse is nearly perpendicular to the coastline, while the tailing edge slowly approaches the coast. Could the fact that the "fallback" is depicted more in discrete steps (there is a preferred mode at 100km offshore) than continuously be a result of the interpolation scheme? Maybe a space-time interpolation scheme (assuming that the data is resolved higher than daily) could improve this?
As long events I would consider; 27-40, 169-196, 235-254 (the latter being one rather than two events)
There is one intermediate event (7day duration, displacement to 80km), which probably is a pulse in its early stage.
What are the effects of the extrapolation? E.g. there is an apparent persistent northward flow round about 1200m adjacent to the shelf, though comparison with the location of current meters shows that there is no instrument there to support this. Could this be a residue of the extrapolation scheme?
Similarly, the sharpness of the AC at the inshore side is imposed by the boundary condition of no flow at the shelf. Are there alternatives to this choice?
I am looking forward to the upcoming analysis, it promises to resolve a number of open questions with re to the Agulhas Current.
Regards, Olaf