Abstract:
A numerical model of the tropical Atlantic ocean is used to investigate the upper
layer pathways of the Meridional Overturning Circulation (MOC) in the tropical
Atlantic. The main focus of this thesis is on those parts of the tropical circulation
that are thought to be important for the MOC return flow, but whose dynamics
have not been understood yet.
It is shown how the particular structure of the tropical gyre and the MOC act
to inhibit the flow of North Atlantic water into the equatorial thermocline. As a
result, the upper layers of the tropical Atlantic are mainly fed by water from the
South Atlantic. The processes that carry the South Atlantic water across the tropical
Atlantic into the North Atlantic as part of the MOC are described here, and
three processes that were hitherto not understood are explained as follows:
The North Brazil Current rings are created as the result of the reflection of
Rossby waves at the South American coast. These Rossby waves are generated by
the barotropically unstable North Equatorial Countercurrent. The deep structure of
the rings can be explained by merger of the wave's anticyclones with the deeper intermediate
eddies that are generated as the intermediate western boundary current
crosses the equator.
The bands of strong zonal velocity in intermediate depths along the equator have
hitherto been explained as intermediate currents. Here, an alternative interpretation
of the observations is offered: The Eulerian mean flow along the equator is negligible
and the observations are the signature of strong seasonal Rossby waves. The
previous interpretation of the observations can then be explained as aliasing of the
tropical wave field.
The Tsuchyia Jets are driven by the Eliassen-Palm flux of the tropical instability
waves. The equatorial current system with its strong shears is unstable and generates
tropical instability waves. These waves cause a poleward temperature flux
which steepens the isotherms which in turn generates are geostrophically balanced
zonal flow. In the eastern part of the basin this zonal flow feeds the southeastward
flow of the equatorial gyre.
