Meanders and Rings
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The combined float/SSH data set enabled us to
continuously observe the evolution of mesoscale features in
the greater Agulhas region, results of which are
discussed for the major mesoscale events known
for the region-- Agulhas Rings, Agulhas Current Meanders, and
Natal Pulses.
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Agulhas Rings
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During the 3 year experimental period we observed 16 rings to drift through
the Cape Basin. This number is at the lower end of earlier estimates
(4-9 per year, [Lutjeharms, 1996]), however the identification
of a ring shedding event is inherently difficult,
due to numerous rings reattaching to the Agulhas Retroflection
or merging together during their early stage.
The phase propagation of the positive SSH anomalies associated
with rings was 5.2 ± 1.7 cm s-1,
which is lower than estimates in the
literature (6-9 cm s-1, [Olson and Evans, 1986]).
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Agulhas Current Meanders
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5 major Agulhas Current meanders translated south-westward along
the continental shelf break at similar speeds,
averaging to 11.6 ± 0.8 cm s-1. This is in contrast to
earlier observations by other authors [Lutjeharms and Roberts, 1988],
who found a propagation speed of 15-20 cm s-1 for the
region northeast of Port Elizabeth and 5-6 cm s-1 to the
southwest of Port Elizabeth.
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Eddies embedded in major meanders of the Agulhas were shed
from their captured position between the coast and
the Agulhas Current at the southernmost tip of the
Agulhas Bank. In all cases this involved a pinching
off of a juvenile Agulhas Ring, which is why the region
adjacent to the tip of the Agulhas Bank can be viewed as
a crossroad in which Agulhas rings translate north-westward
and cyclonic eddies drift west- or south-westward.
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Natal Pulses
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The total number of shelf-trapped cyclonic events
(11 in 3 years) is similar to earlier estimates of
Natal Pulse frequency (14 in 4 years [de Ruijter et al., 1999]).
Most of the meanders we resolved were only discernable
from Port Elizabeth downstream. This is in contrast to the
concept that all major meanders, Natal Pulses, are formed in the
Natal Bight.
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