1. Mesoscale Eddies in Indian Ocean Cdr Manoj Kumar Singh India

2. Geography of NW Indian Ocean  Land locked from three sides  Gulfs  Marginal Seas

3. Annual Weather Pattern  Four seasons with Reversing Monsoon Wind Pattern Northeast Monsoon, December to February Northeast Monsoon, December to February Transition season, March and May Transition season, March and May Southwest Monsoon, June to September Southwest Monsoon, June to September Transition season, October and November Transition season, October and November SW Monsoon NE Monsoon

4. Wind Stress - Jan

5. Wind Stress - Feb

6. Wind Stress - Mar

7. Wind Stress - Apr

8. Wind Stress - May

9. Wind Stress - Jun

10. Wind Stress - Jul

11. Wind Stress - Aug

12. Wind Stress - Sep

13. Wind Stress - Oct

14. Wind Stress - Nov

15. Wind Stress - Dec

16. Wind Stress - Jan

17. Wind Stress - Feb

18. Wind Stress - Mar

19. Wind Stress - Apr

20. Wind Stress - May

21. Wind Stress - Jun

22. Wind Stress - Jul

23. Wind Stress - Aug

24. Wind Stress - Sep

25. Wind Stress - Oct

26. Wind Stress - Nov

27. Wind Stress - Dec

28. Somali Current  The Somali current system forms part of the circulation of the Indian Ocean, and is remarkable for the fact that the annual cycle includes a reversal of the entire current system  The Somali Current (SC) is the Western boundary current of the Northern Indian ocean – the equivalent of the Gulf stream in the North Atlantic, or the Kuroshio in the North Pacific. However the SC is unique in that it is not present during the whole year

29.  As a result of strong SW monsoon winds, the SC develops during May ~2.0 m/s  Strongest as a Northwards jet from June through to September (~3.5 m/s and more for June, observed during the Indian Ocean Experiment - INDEX, 1976-1979).  During December, the flow is weaker (~0.7-1.0 m/s), and often turns southwards with the Northeast Monsoon Winds Somali Current

31. Somali Eddies  In general a two Gyre System Southern Gyre (SG) Southern Gyre (SG) ~5 Deg N in June~5 Deg N in June 500-600 km in diameter500-600 km in diameter Anticyclonic eddy ( Clockwise rotation)Anticyclonic eddy ( Clockwise rotation) Great Whirl (GW) Great Whirl (GW) Between 9~10 Deg N in JulyBetween 9~10 Deg N in July ~600 km in diameter~600 km in diameter Anticyclonic eddy (Clockwise rotation)Anticyclonic eddy (Clockwise rotation) ~200 m deep~200 m deep Max velocity ~80 cm/sMax velocity ~80 cm/s  Socotra Eddy (SE) 10-12 Deg N during late SW monsoon month (Aug-Sep) 10-12 Deg N during late SW monsoon month (Aug-Sep) ~200 Km in diameter ~200 Km in diameter Anticyclonic eddy Anticyclonic eddy SG

32.  Strong / Normal Monsoon years All the three GW, SG and SE All the three GW, SG and SE  Weak Monsoon years in general Single eddy (GW) Single eddy (GW)  Eddy evolution - varies from year to year Strength Strength Location Location Number Number Time Time Annual Variability – Somali Eddies Interannual variability in Somali Current : 1954-1976 : Mark M Luther

33. WOCE Indian Ocean Expedition Well Marked Eddy – Sep 1995

34. Two Eddies (1997) – Mark M Luther

35. Model generate d - eddies Satellite observed

36. Decay Mechanism  Coalescence  Baroclinic instability triggered by seasonal Rossby wave from west coast of India

37. NE Monsoon Eddies  Cyclonic eddy Diameter ~1000 km Diameter ~1000 km 4-10 deg N & 50-60 deg E (Location of SG in SW monsoon) 4-10 deg N & 50-60 deg E (Location of SG in SW monsoon)  Anticyclonic eddy (Laccadive High-LH) Diameter ~500 km Diameter ~500 km 4-10 deg N & 60-70 deg E (Laccadive Islands) 4-10 deg N & 60-70 deg E (Laccadive Islands) Due to intense negative wind curl off the SW coast of India Due to intense negative wind curl off the SW coast of India Consists multiple eddies Consists multiple eddies Marine Geodesy, 23:167-195,2000. Bulusu Subrahmanyan & Ian S Robinson

38. Eddies in Red Sea  L1 – Mostly in winter due to formation of intermediate water  H1 – Most regular due to coastline and topography variations  H2 & L2 – Non persistent but high horizontal velocities >= 0.5 m/s L1 H1 H2 L2

39.  Mostly Anticyclonic  In 1991 – 2:Cyclonic & 1: Anticyclonic **  Origin is linked with decay of Western Arabian Sea eddies  Dia : 150-250 km  Deep ~1000 m  Speed :0.2-0.3 m/s Eddies in Gulf of Aden Temp - 100 m Feb :1991 ** Amy S Bower at el, WHOI

40. Gulf of Oman  Ras al Hadd jet is highly variable  Strong in SW Monsoon  Reversal of flow along NE Oman coast in Aug is related to intensification and propagation of cyclonic eddy in North cyclonic eddy in North Anticyclonic eddy in south Anticyclonic eddy in south Temp at 250 m

41. Eddies at a Glance (1993-2002)

42. Eddies through SST

43. Eddies in Northern Arabian Sea Winter  Widespread occurrence of mesoscale eddies (warm and cold) in the northern Arabian Sea - Satellite imageries  The size of the cold water eddies in the Arabian Sea ranges from 10 to 150 kms, while the duration of eddies appears to vary from 2 to 18 days.  The warm water eddies are lower than that of cold water eddies.  ARGO floats data indicate that there is a great deal of fluctuation in the mixed layer depths (MLD) particularly during February and March attributable to the occurrence of numerous eddies in this region (IUGG 2003 Abstract, JSP05/03P/C30-003-Somasundar KROTHAPALLI ) (IUGG 2003 Abstract, JSP05/03P/C30-003 -Somasundar KROTHAPALLI )

44. Summary  NW Indian Ocean - a complete Oceanographic Lab with highly variable energetic environment  Large number of eddies present throughout the year with varying Location Location Strength Strength Dimension Dimension Characteristics Characteristics

46. References  Lisa M Beal, Teresa K Chereskin. The volume transport of the Somali Current during 1995 SW Monsoon. Deep Sea research II 50 (2003) 2077-2089.  Amy E Bower & David M Fratantoni. Gulf of Aden eddies and their impact on Red Sea Water. Geophysical Research Letters, Vol 29 No 21,2025.  Bulusu Subrahmanyam & Ian S Robinson. Sea surface height variability in the Indian Ocean from TOPEX/ Poseidon Altimetry and Model simulations. Marine Geodesy, 23:167-195,2000.  Aftab Can, Raj Kumar and Sujit Babu. Kenetic energy variability in the North Indian Ocean using a numerical model. Marine Geodesy, 25:175-186,2002.  University of Miami RSMAS Technical Report 2000-01. Arabian Marginal Seas and Gulfs.  Mark M Luther. Interannual variability in the Somali Current 1954-1976. Nonlinear Analysis 35 (1999) 59-83.  Schott FA, McCreasy Jr JP. The monsoon circulation of the Indian Ocean. Progress in Oceanography 51 (2001) 1-123.  Wirth A, Willebrand J and Schott F. Variability of the great whirl from observations and models. Deep sea Research Ii 49 (2002) 1279-1295.