Long Wave Dynamics of the Indian Ocean Dipole Events

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Long Wave Dynamicsof the Indian Ocean Dipole
Events

• Dongliang Yuan
• Institute of Oceanology, Chinese Academy of
  Sciences

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Objectives

• How do the equatorial Rossby waves reflected at
  the western boundaries?
• How does the reflection shape the interannual
  variations of the equatorial IO circulation?
• What are the dynamics that control the
  termination of the IOD events?
• What are the roles of ISO in the termination of
  the IOD events?
• This study will Focus on the SSHA

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Linear theory

• Vertical
• are orthogonal base mode functions.
• un,vn,pn, satisfy linear shallow water equations.
• At the W.B. (Cane and Sarachik, 1977)
• Horizontal
• --orthonormal, expressed in Hermite
  functions

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Theory of vertical mode separation
Gzonal nonlinear term
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(No Transcript)
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Linear reflection theory

• Only long equatorial Kelvin and Rossby and short
  Rossby waves involved in the reflection
• Short Rossby waves merge the difference between
  the incoming and outgoing long waves
• Nonlinearity is omitted
• Longshore wind effects and Kelvin waves not
  considered

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Benchmark EXP00

• LASG ocean model LICOM (eta-coordinate with free
  SL and nearly z-level vertical coordinate)
• 0.5ºX0.5º horizontal resolution
• 30 layers in the vertical
• 65ºN--75ºS quasi-global
• ECMWF ERS40 winds
• Relaxation to SST and SSS
• EXP01 same as EXP00 except that windstress
  variations of less than 90 days of periods are
  excluded.

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Vertical profiles
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SSHA
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Windstres anomalies During 1997-98 IOD
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1st Baroclinic mode waves
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WB reflection
Decomposesd
Reflected K
difference
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SSHA and windstress anomalies in 1997
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SSHA and windstress anomalies in 1998
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1994-1995 IOD event
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1st Baroclinic mode waves in 1994-95 IOD
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SSHA and windstress anomalies in 1994
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SSHA and windstress anomalies in 1995
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EXP00
EXP00-EXP01
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1st baroclinic mode waves induced by ISO
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1st baroclinic mode waves forced by ISO
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EXP01 (no ISO forcing)
EXP00-EXP01
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Summary

• Two Kelvin waves emit from the W. Boundary during
  decay of IOD One in winter, reflected from the
  equatorial Rossby waves one in spring reflected
  from the off-equatorial Rossby waves in the S.
  IO.
• The 1997-98 IOD is enhanced by the propagation of
  the equatorial waves in the early stage, but is
  terminated by the reflection of the off
  equatorial Rossby waves
• The duration of the 1997-98 IOD is determined by
  the difference of the Equatorial and off
  Equatorial Rossby wave speeds
• ISO events are found to play a minor role in the
  interannual SLA of the equatorial IO.
• The ISO events are not the primary process to
  terminate the 1994 and 1997-98 IOD events.

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Thank You!