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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 . Sh ankar National Institute of Oceanography Dona Paula, Goa 403 004 sh ankar @nio.org. Collaborators over the years.

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slide1

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

[email protected]

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

http://www.nio.org

slide2

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
slide3

Mausam: The seasonal cycle

Wind

stress

Josey et al.

(1996)

slide4

Mausam …

West

coast

Vertical section of temperature

during summer monsoon

East

coast

Upwelling

Shetye et al. (1990)

Shetye et al. (1991)

slide5

Mausam …

Ship

drifts

Mariano et al.

(1995)

slide6

Mausam …

Sea-level anomalies from altimeter (TOPEX/Poseidon)

High

Low

slide7

Mausam …

Schematic of circulation

during summer monsoon

WICC

SMC

EICC

1

4

LL

14

12

SMC

SC

5

ECC

21

SEC

13

20

EACC

slide8

Mausam …

Schematic of circulation

during winter monsoon

5

WICC

EICC

7

8

LH

WMC

11

12–20

WJ

SC

5

ECC

SEC

13

20

EACC

slide9

Mausam …

Schematic of circulation

during inter-monsoon

WICC

10

3

EICC

5

LH

WMC

WMC

WJ

12–20

SC

5

ECC

SEC

13

20

EACC

slide10

Mausam …

OGCM currents

(5 m)

OGCM currents

(35 m)

Shankar et al. (2002)

slide11

Mausam …

OGCM currents

(0–50 m average)

Shankar et al. (2002)

slide12

Mausam …

Surface (sea level):

Small displacement

Forcing mechanisms:

Analysis of processes with a simpler model

One-and-half layer reduced-gravity model

Interface:

Large displacement

slide13

Mausam …

Nonlinear

Linear

Nonlinear simulation

Linear simulation

Shankar et al. (2002)

slide14

Mausam …

Mechanisms:

Forcing only by winds blowing along Indian west coast

Weak LH

Weak LL

Shankar et al. (2002)

slide15

Mausam …

Mechanisms:

Forcing only by winds blowing along Indian east coast

Strong LH

Strong LL

Shankar et al. (2002)

slide16

Mausam …

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)

slide17

Mausam …

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)

slide18

Mausam …

SMC

WMC

Observed currents south of Sri Lanka (ADCP measurements)

SMC

WMC

Schott et al. (1994)

slide19

Mausam …

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)

slide20

Mausam …

60-day period

Annual

period

Trapping poleward of critical latitude

High

Low

30-day period

Coastally trapped

Shankar and Shetye (1997)

slide21

Mausam …

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)]

slide22

Mausam …

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 seasonalcirculation in its parts.

Shankar et al. (2002)

slide23

The long march: A sketchy history

  • Theory
  • Matsuno (1966): Trapped waves on an equatorial β-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
slide24

The long march …

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)

slide25

The long march …

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)]

slide26

The long march …

  • 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: O’Brien and Hurlburt (1974)
slide27

The long march …

  • 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
slide28

The long march …

  • 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
slide29

The long march …

  • 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
slide30

The long march …

Hindsight: An exact science?

Putting old observations back together again!

The Lakshadweep Low

Sundararamam and Murthy (1968)

slide31

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
slide32

Beyond mausam …

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

slide33

Beyond mausam …

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)

slide34

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: O’Brien 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
slide35

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
slide36

Celebrating the Monsoon

Indian Institute of Science

26 July 2007

Thank you

Happy birthday,

C(H?)A(O)S

http://www.nio.org

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