Dynamics of largescale winddriven circulation off the Indian coast

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Celebrating the Monsoon Indian Institute of
Science 26 July 2007
Dynamics of large-scale wind-driven circulation
off the Indian coast
D. Shankar National Institute of
Oceanography Dona Paula, Goa 403
004 shankar_at_nio.org
Collaborators over the years

• NIO Satish Shetye, Satheesh Shenoi, A.S.
  Unnikrishnan, D. Sundar, G.S. Michael, M. Aparna,
  I. Suresh
• CAOS, IISc P.N. Vinayachandran, J. Kurian
• U.S.A. J.P. McCreary, Jr., Weiqing Han
• France Fabien Durand, Daniel Nethery
• Yemen M. Al Saafani

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Outline

• Mausam The seasonal cycle off the Indian coast
• The long march A brief history
• Beyond mausam Through the looking glass

• Small basin
• Tropical
• Time-dependent winds

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Mausam The seasonal cycle
Wind stress
Josey et al. (1996)
4
West coast
Vertical section of temperature during summer
monsoon
East coast
Upwelling
Shetye et al. (1990)
Shetye et al. (1991)
5
Ship drifts
Mariano et al. (1995)
6
Sea-level anomalies from altimeter
(TOPEX/Poseidon)
High
Low
7
Schematic of circulation during summer monsoon
WICC
SMC
EICC
1
4
LL
14
SMC
12
SC
5
ECC
21
SEC
13
20
EACC
8
Schematic of circulation during winter monsoon
5
WICC
EICC
7
8
LH
WMC
11
1220
WJ
SC
5
ECC
SEC
13
20
EACC
9
Schematic of circulation during inter-monsoon
WICC
10
3
EICC
5
LH
WMC
WMC
1220
WJ
SC
5
ECC
SEC
13
20
EACC
10
OGCM currents (5 m)
OGCM currents (35 m)
Shankar et al. (2002)
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OGCM currents (050 m average)
Shankar et al. (2002)
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Surface (sea level) Small displacement
Forcing mechanisms Analysis of processes with a
simpler model One-and-half layer reduced-gravity
model
Interface Large displacement
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Nonlinear
Linear
Nonlinear simulation
Linear simulation
Shankar et al. (2002)
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Mechanisms Forcing only by winds blowing along
Indian west coast
Weak LH
Weak LL
Shankar et al. (2002)
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Mechanisms Forcing only by winds blowing along
Indian east coast
Strong LH
Strong LL
Shankar et al. (2002)
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Mechanisms Forcing only by winds blowing along
coast of north Indian Ocean
WMC and SMC south of Sri Lanka in correct
direction
Shankar et al. (2002)
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Mechanisms Forcing by other processes (Ekman
pumping, equatorial winds no coastal winds)
SMC south of Sri Lanka flows opposite to observed
direction
No link between Arabian Sea and Bay of Bengal
without east-coast winds
Shankar et al. (2002)
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SMC
WMC
Observed currents south of Sri Lanka (ADCP
measurements)
SMC
WMC
Schott et al. (1994)
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The Lakshadweep High and Low The simplest
physical analog
A mathematical boundary
Oscillating zonal current (forcing) south of Sri
Lanka
SMC/WMC
Shankar and Shetye (1997)
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60-day period
Annual period
Trapping poleward of critical latitude
High
Low
30-day period
Coastally trapped
Shankar and Shetye (1997)
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Equatorial Rossby wave
Equatorial Kelvin wave
Equatorial Kelvin and Rossby waves, coastal
Kelvin waves
Reflection at equatorial eastern ocean boundary
Reflected equatorial Rossby wave
Coastal Kelvin wave
McCreary (1976) After Gill (1982)
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The leaky wave-guide

• Coastal Kelvin waves
• Equatorial Kelvin waves
• Equatorial Rossby waves

Gap between equatorial and coastal wave-guides
The existence of the leaky wave-guide merges the
Arabian Sea, the Bay of Bengal, and the
equatorial Indian Ocean into a single dynamical
entity, which must be modelled as a whole even to
simulate the seasonal circulation in its parts.
Shankar et al. (2002)
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The long march A sketchy history

• Theory
• Matsuno (1966) Trapped waves on an equatorial
  ß-plane
• Moore (1968) Reflection of equatorial waves at
  an eastern ocean boundary
• Lighthill (1969) Faster propagation near the
  equator, rapid response of the ocean to the
  changing monsoon winds

Dispersion curves

• Small basin
• Tropical
• Time-dependent winds

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Reflection at equatorial eastern ocean boundary
Observations
Poleward propagation of sea-level anomalies
associated with ENSO (Enfield and Allen, 1980
Chelton and Davis, 1982)
Chelton and Davis (1982)
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Equatorial Rossby wave
Equatorial Kelvin wave
Reflection at equatorial eastern ocean boundary
Modelling (Hurlburt et al., 1976 McCreary,
1976)
Reflected equatorial Rossby wave
Coastal Kelvin wave
McCreary (1976) After Gill (1982)
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• The Indian Ocean
• The International Indian Ocean Expedition
  (1960s)
• Lighthill (1969) Rapid response of the Somali
  Current to the onset of the summer monsoon
• Cox (1970) OGCM simulation of the Somali
  Current
• Considerable work on the equatorial Indian Ocean
• Observations Wyrtki (1973)
• Theory OBrien and Hurlburt (1974)

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• The West India Coastal Current
• Banse (1959) Undercurrent along west coast of
  India
• Sharma (1968) and Banse (1968) Upwelling
  stronger in the south, propagates from south to
  north, flow against wind observed
• Several cruises, hydrographic observations
• Interpretation of WICC in terms of classical
  eastern-boundary upwelling systems
• Banse (1959) Difference between WICC and other
  eastern boundary systems prevailing winds
  comparable during winter, circulation comparable
  during summer
• WICC like other eastern-boundary currents only
  during the summer monsoon (Shetye and Shenoi,
  1988)
• Poleward WICC during winter driven by alongshore
  salinity (and therefore density) gradient (Shetye
  et al., 1991) like Leeuwin Current off west
  coast of Australia (McCreary et al., 1986)
• The Lakshadweep High (Bruce et al., 1994) and
  Low (Shankar and Shetye, 1997) Annual cycle of
  sea level off southwest India and relation to WICC

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• Remote forcing
• Potemra et al. (1991) and Yu et al. (1991)
  Remote forcing from the equator affects East
  India Coastal Current (EICC)
• McCreary et al. (1993) Comprehensive numerical
  simulation of the dynamics of the Indian Ocean
  confirms importance of remote forcing for the
  EICC, suggests remote forcing important for the
  WICC
• Shankar et al. (1996) and McCreary et al.
  (1996) Analysis of relative importance of EICC
  forcing mechanisms using a linear, continuously
  stratified model
• Vinayachandran et al. (1996) Analysis of EICC
  forcing mechanisms using an OGCM

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• Remote forcing of WICC
• McCreary et al. (1993), Shankar and Shetye
  (1997), and Shankar et al. (2002) Analysis of
  WICC forcing mechanisms
• McCreary et al. (1993) and Shankar and Shetye
  (1997) Alongshore pressure gradient for WICC set
  up by coastal Kelvin waves propagating along the
  east and west coasts of India, not by salinity
  gradient as envisaged by Shetye et al., 1991
• Not a classical eastern-ocean boundary system
  Communication between EICC and WICC via SMC/WMC
  south of Sri Lanka makes west coast of India
  different

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Hindsight An exact science? Putting old
observations back together again!
The Lakshadweep Low
Sundararamam and Murthy (1968)
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Beyond mausam Through the looking glass

• Inter-annual and intra-seasonal variability
• The leaky wave-guide is successful in explaining
  the observations at the seasonal time scale
• What happens at inter-annual and intra-seasonal
  time scales?
• Observations are scanty
• Sources altimeter, a few current-meter
  measurements

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Visakhapatnam
Chennai
Inter-annual variability in sea level along
Indian west coast
Kochi
Range of interannual Variability comparable to
that of the seasonal cycle
Mangalore
Mumbai
Kandla
Clarke and Liu (1994)
Karachi
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The Lakshadweep High and Low The simplest
physical analog
A mathematical boundary
Oscillating zonal current (forcing) south of Sri
Lanka
SMC/WMC
Shankar and Shetye (1997)
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Summary

• Basic linear theory for understanding the
  dynamics of the WICC was in place by the end of
  the 1960s
• Matsuno (1966), Moore (1968), Lighthill (1969)
• Initial application to the Somali Current and
  the equatorial Indian Ocean
• Theory applied first in the Pacific, then in the
  Atlantic, then in the Indian Ocean
• Observations more in the Pacific and Atlantic
  Oceans
• Same groups involved in the initial theoretical
  development in all three basins OBrien et al.,
  McCreary et al.
• Detailed hydrographic observations off the
  Indian coast in the late 1980s and 1990s map the
  seasonal cycle, and theoretical development
  follows

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The years ahead

• Ocean colour A guide to ocean physics
• Bifurcation of the EICC east of Sri Lanka
  (Vinayachandran et al., 2005)
• Margins Altimetry Project TOPEX/Poseidon
  resurrected
• Mapping the variability of the EICC and WICC
  (Durand et al., 2007)
• Current-meter and ADCP measurements
• OGCMs with high enough resolution
• Simple models still have a role

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Celebrating the Monsoon Indian Institute of
Science 26 July 2007
Thank you
Happy birthday, C(H?)A(O)S
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