Posted by Harry Bryden on May 17, 2001 at 10:10:13:
In Reply to: Agulhas movie posted by Olaf Boebel on May 11, 2001 at 01:11:09:
Extrapolation is a difficult issue. Our aim has been
to produce reasonable currents from the moored instruments
for as long a time as possible for the entire section from the
surface down to 2400m depth and from the coast offshore to
206 km. We have based the extrapolation procedures both on
the available measurements and on synoptic CTD and LADCP
sections in the region.
The questions seem to be primarily about the extrapolation to
depth used on Mooring B where the moored ADCP provides
measurements in 8m bins only from 56 m to 424m depths. Based
on 1995 LADCP profiles around Mooring B and 1987 and 1995
geostrophic current profiles from stations bracketing the position
of Mooring B, we decided that there was a linear vertical shear in
this region through the thermocline down to 940 m depth and then
there was no shear from 940 m to the bottom. From an eof analysis
of the ADCP profiles, we found the vertical structure at Mooring B to
be a combination of a linearly sheared (12.63 cm/sec per 100m) mean
velocity through the thermocline with perturbations that exhibited a
weak baroclinic structure. We extrapolated the mean ADCP profile to
obtain a mean alongshore current at 940 m depth of 7.21 cm/sec and
extrapolated the first eof profile to obtain a perturbation amplitude at
940 m depth of 0.116 times the perturbation amplitude at 424 m depth.
Thus, at the location of Mooring B, the mean currents are northward
below 880 m depth (the level of 0 mean velocity), in agreement with the
LADCP profiles of the Undercurrent at this location, and the variability
is a scaled version of the variability measured at 424 m depth.