Structure and Dynamics of the Surface Branch of the Meridional Cell in the Indian Ocean

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Near-surface eastward flow in the South Indian
Ocean
Jay McCreary, Ryo Furue, and Billy Kessler
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Topics

• Observations and models
• Dynamics
• Indonesian Throughflow
• Basin-wide subduction (LPS theory)
• Localized subduction/convection

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Observations and models
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Annual-mean near-surface circulation
There are extensive regions of eastward flow in
the South and North Pacific Ocean and in the
South Indian Ocean.
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Annual-mean near-surface circulation
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Annual-mean near-surface circulation
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Dynamics Indonesian Throughflow
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Impact of ITF (COCO)
u, T, closed
u, T, open
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Dynamics Basin-wide subduction (LPS theory)
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LPS theory (2½-layer model)
Equations Equations of motion for the analytic
model are
(1)
where the pressure gradients are
and
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LPS theory (2½-layer model)
h equation Equations (1) can be solved for a
single equation in h, yielding
(2)
where
is the speed of the n 2 Rossby waves,
are the depth-averaged geostrophic currents
associated with the Sverdrup circulation, and
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LPS theory (2½-layer model)
Subduction along ys introduces a source of n 2
Rossby signals that extend equatorward due to
dvection by the SIO Subtropical Gyre.
Courtesy of Billy Kessler
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LPS theory (2½-layer model)
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Dynamics Localized subduction/convection
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Subduction (COCO)
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Impact of ACC (COCO)
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Summary
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• There is near-surface eastward flow that overlies
  westward flow across the South Pacific and South
  Indian Oceans. Such flows are referred to as
  Subtropical Countercurrents. The eastward flow
  is more extensive in the South IO than in the
  Pacific.
• The cause of the eastward flow is not yet well
  understand. Several forcing mechanisms have been
  considered i) the ITF, ii) basin-wide subduction
  across the SIO, and iii) localized
  subduction/convection in the southeast IO and
  south of Australia.
• Subduction and convection are sources of
  higher-order (n ? 1) baroclinic waves. Such
  waves propagate westward at their familiar linear
  wave speed AND are advected by the geostrophic
  part of the Sverdrup circulation (LPS theory).
  In the SIO, this property means that they tend to
  circulate around the SIO subtropical gyre, so
  that baroclinic sources in the far-south IO and
  south of Australia can impact flows farther north.

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