Mesoscale Eddies in Indian Ocean

Cdr Manoj Kumar Singh India

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

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

Wind Stress - Jan

Wind Stress - Feb

Wind Stress - Mar

Wind Stress - Apr

Wind Stress - May

Wind Stress - Jun

Wind Stress - Jul

Wind Stress - Aug

Wind Stress - Sep

Wind Stress - Oct

Wind Stress - Nov

Wind Stress - Dec

Wind Stress - Jan

Wind Stress - Feb

Wind Stress - Mar

Wind Stress - Apr

Wind Stress - May

Wind Stress - Jun

Wind Stress - Jul

Wind Stress - Aug

Wind Stress - Sep

Wind Stress - Oct

Wind Stress - Nov

Wind Stress - Dec

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

Somali Current  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 Eddies  In general a two Gyre System  Southern Gyre (SG) • ~5 Deg N in June • • 500-600 km in diameter Anticyclonic eddy ( Clockwise rotation)  Great Whirl (GW) • Between 9~10 Deg N in July • • • • ~600 km in diameter Anticyclonic eddy (Clockwise rotation) ~200 m deep Max velocity ~80 cm/s

SG

 Socotra Eddy (SE)   10-12 Deg N during late SW monsoon month (Aug-Sep) ~200 Km in diameter  Anticyclonic eddy

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

WOCE Indian Ocean Expedition Well Marked Eddy – Sep 1995

Two Eddies (1997) – Mark M Luther

Model generate d eddies

Satellite observed

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

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

L1 H1

Eddies in Red Sea

H2 L2

 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

Feb :1991 Temp - 100 m

Eddies in Gulf of Aden   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 ** Amy S Bower at el, WHOI

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  Anticyclonic eddy in south

Temp at 250 m

Eddies at a Glance (1993-2002)

Eddies through SST

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

)

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

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.