Ruijter WPM de ,
Dijkstra HA ,
Leeuwen PJ van ,
Schouten MW ,
Vaart PCF van der ,
Weijer W ,
Drijfhout SS
110 p
in Dutch
2001
Toon Nederlands
English Abstract Climate variability at decadal to centennial time
scales is coupled to variations in the Ocean's global scale circulation and
associated thermohaline transports. Interocean exchange of heat and salt
around South Africa is thought to be a key link in the maintenance of the
global overturning circulation of the ocean. It takes place at the Agulhas
Retroflection, largely by the intermittent shedding of enormous rings that
penetrate into the South Atlantic Ocean. This makes it extremely hard to
estimate the inter ocean fluxes. Estimates of direct Agulhas leakage from
hydrographic and tracer data range between 2 and 10 Sv (1 Sv = 106 m3s-1).
The average ring shedding frequency, determined from satellite information,
is approximately six rings per year. Their associated interocean volume
transport is between 0.5 and 1.5 Sv per ring. A number of Agulhas rings
have been observed to cross the South Atlantic. They decay exponentially to
less than half their initial size (measured by their available potential
energy) within 1000 km from the shedding region. Consequently, most of
their properties mix into the surroundings of the Benguela region, probably
feeding directly into the upper (warm) limb of the global thermohaline
circulation. The most recent observations suggest that in the present
situation Agulhas water and Antarctic Intermediate Water are about equally
important sources for the Benguela Current. Variations in the strength of
these may lead to anomalous stratification and stability of the Atlantic at
decadal and longer timescales. Modelling studies suggest that the
Indian-Atlantic interocean exchange is strongly related to the structure of
the wind field over the South Indian Ocean. This leads in the mean to a
subtropical supergyre wrapping around the subtropical gyres of the South
Indian and Atlantic Oceans. However, local dynamical processes in the
highly nonlinear regime around South Africa appear to play a crucial role in
inhibiting the connection between the two oceans. The regional bottom
topography also seems to play an important role in locking the Agulhas
Currents' retroflection. State-of-the-art global and regional
"eddy-permitting" models show a reasonably realistic representation of the
mean Agulhas system; but the mesoscale variability and the local
geometrical and topographic features that determine largely the interocean
fluxes still need considerable improvement. In this report we review most
of the above mentioned aspects of the interocean exchange around South
Africa and report on the main contribution from this NRP II project to: the
estimation of the fluxes into the South Atlantic from different types of
observations, our present level of understanding of the exchange's dynamics
and forcing, its representation in state-of-the-art models, and, finally,
the impact of the Indian-Atlantic fluxes on regional and global scale both
within the Atlantic Ocean and in interaction with the overlying atmosphere
as part of the global climate system.
