Rossby Award Winner Thesis Abstract
On
the pathways of the return flow of the meridional overturning
circulation in
the tropical Atlantic
by Markus Jochum
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.
