Peculiar Countercurrents in the Subtropical Oceans
Ryo Furue and Jay McCreary
The advent of high-resolution satellite data and
vast Argo and drifter data sets is yielding new
information about the world ocean circulation,
such as the discovery of a broad eastward
geostrophic surface flow with embedded jet(s) in
the equatorward half of the South Indian Ocean
Subtropical Gyre. The current flows against what
the Sverdrup theory of wind-driven circulation
predicts.
Furue and colleagues have studied this puzzling
flow. Various solutions to layer-ocean models and
ocean general circulation models give insight into
the phenomenon.
The figure shows zonal velocity (in cm/s, shading) and
velocity vectors at the depth of 70 m (top panel) and
740 m (bottom panel) for a solution to an idealized
OGCM.
In their idealized solutions, the broad eastward
flow is produced by the subduction of thick water
columns poleward of the flow, and an eastwardsoutheastward jet is generated by sinking at the
eastern boundary. Consistent with dynamics of
large-scale circulation, these eastward surface
currents are accompanied by subsurface westward
currents.
Further study revealed that the basic structure
generating the broad flow also occurs in other
ocean basins, but the flows are not as conspicuous
there as in the South Indian Ocean, where the
subducted layer is so thick.