Dynamics of large-scale wind-driven circulation off the Indian coast

1. 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@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 http://www.nio.org

2. 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

3. Mausam: The seasonal cycle Wind stress Josey et al. (1996)

4. Mausam … West coast Vertical section of temperature during summer monsoon East coast Upwelling Shetye et al. (1990) Shetye et al. (1991)

5. Mausam … Ship drifts Mariano et al. (1995)

6. Mausam … Sea-level anomalies from altimeter (TOPEX/Poseidon) High Low

7. Mausam … Schematic of circulation during summer monsoon WICC SMC EICC 1 4 LL 14 12 SMC SC 5 ECC 21 SEC 13 20 EACC

8. 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

9. 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

10. Mausam … OGCM currents (5 m) OGCM currents (35 m) Shankar et al. (2002)

11. Mausam … OGCM currents (0–50 m average) Shankar et al. (2002)

12. 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

13. Mausam … Nonlinear Linear Nonlinear simulation Linear simulation Shankar et al. (2002)

14. Mausam … Mechanisms: Forcing only by winds blowing along Indian west coast Weak LH Weak LL Shankar et al. (2002)

15. Mausam … Mechanisms: Forcing only by winds blowing along Indian east coast Strong LH Strong LL Shankar et al. (2002)

16. 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)

17. 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)

18. Mausam … SMC WMC Observed currents south of Sri Lanka (ADCP measurements) SMC WMC Schott et al. (1994)

19. 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)

20. Mausam … 60-day period Annual period Trapping poleward of critical latitude High Low 30-day period Coastally trapped Shankar and Shetye (1997)

21. 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)]

22. 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)

23. 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

24. 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)

25. 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)]

26. 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)

27. 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

28. 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

29. 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

30. The long march … Hindsight: An exact science? Putting old observations back together again! The Lakshadweep Low Sundararamam and Murthy (1968)

31. 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

32. 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

33. 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)

34. 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

35. 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

36. Celebrating the Monsoon Indian Institute of Science 26 July 2007 Thank you Happy birthday, C(H?)A(O)S http://www.nio.org