manuscript  submitted to
J. Geophys. Res., WOCE South Atlantic special issue,
February 5, 1998

Frictionally modified flow in a deep ocean channel:
Application to the Vema Channel

by
   Johann H. Jungclaus
and
   Michael Vanicek

Institut für Meereskunde an der Universität Kiel
Düsternbrooker Weg 20
24105 Kiel
Germany

email: jjungclaus@ifm.uni-kiel.de

Abstract
--------
The modification of the exchange flow in a deep southern hemisphere passage,
resembling the Vema Channel, by frictionally induced secondary circulation is 
investigated numerically. The hydrostatic primitive equation model is a 
two-dimensional (a cross channelsection) version of the sigma - coordinate 
Princeton Ocean Model.  The time dependent response of a stratified along 
channel flow, forced by barotropic or baroclinic pressure gradients, is 
examined. Near the bottom, where the along channel flow is retarded, there is
cross channel Ekman flux which is associated with downwelling on the eastern 
side and upwelling on the western side of the channel. In the presence of 
stratification the cross channel flow rearranges the density structure which 
in turn acts on the along channel velocity via the thermal wind relation. The 
modification is shown to be part of an adjustment process that crucially 
influences the along channel flow not only within the bottom boundary layers 
but over the width of the channel and eventually shuts down the cross isobath 
Ekman flux.
Complementing earlier studies on the evolution of bottom boundary layers and 
subsequent shutdown this paper adds the peculiar case where upwelling and 
downwelling favorable flow regions overlap and interact in a narrow channel. 
In the case of baroclinically driven flow of Antarctic Bottom Water through 
the Vema Channel the model reproduces the observed shape of the deep temperature
profiles and their cross channel asymmetry. The model offers an explanation that
is alternative, or supplementary,  to inviscid multilayer hydraulic theory which
was put forward in earlier studies. It explains the extremely thick bottom 
boundary layers in the center and on the western slope of the channel. The deep
thermocline is spread out in the west and sharpened in the east and the coldest
water is found on the eastern side of the deep trough. The modified density 
field reduces the along channel flow near the bottom and focuses it into a 
narrow jet on the eastern side of the channel.