Mesoscale Mesoscale Eddies inEddies in

Indian Ocean Indian Ocean

Cdr Manoj Kumar SinghCdr Manoj Kumar SinghIndiaIndia

Geography of NW Indian OceanGeography of NW Indian Ocean

Land locked from Land locked from three sidesthree sides

GulfsGulfs Marginal SeasMarginal Seas

Annual Weather PatternAnnual Weather Pattern Four seasons with Reversing Monsoon Wind Four seasons with Reversing Monsoon Wind

PatternPattern Northeast Monsoon, December to FebruaryNortheast Monsoon, December to February Transition season, March and MayTransition season, March and May Southwest Monsoon, June to SeptemberSouthwest Monsoon, June to September Transition season, October and NovemberTransition season, October and November

SW Monsoon NE

Monsoon

Wind Stress - JanWind Stress - Jan

Wind Stress - FebWind Stress - Feb

Wind Stress - MarWind Stress - Mar

Wind Stress - AprWind Stress - Apr

Wind Stress - MayWind Stress - May

Wind Stress - JunWind Stress - Jun

Wind Stress - JulWind Stress - Jul

Wind Stress - AugWind Stress - Aug

Wind Stress - SepWind Stress - Sep

Wind Stress - OctWind Stress - Oct

Wind Stress - NovWind Stress - Nov

Wind Stress - DecWind Stress - Dec

Wind Stress - JanWind Stress - Jan

Wind Stress - FebWind Stress - Feb

Wind Stress - MarWind Stress - Mar

Wind Stress - AprWind Stress - Apr

Wind Stress - MayWind Stress - May

Wind Stress - JunWind Stress - Jun

Wind Stress - JulWind Stress - Jul

Wind Stress - AugWind Stress - Aug

Wind Stress - SepWind Stress - Sep

Wind Stress - OctWind Stress - Oct

Wind Stress - NovWind Stress - Nov

Wind Stress - DecWind Stress - Dec

Somali CurrentSomali Current

The Somali current system forms part of the The Somali current system forms part of the circulation of the Indian Ocean, and is circulation of the Indian Ocean, and is remarkable for the fact that the annual cycle remarkable for the fact that the annual cycle includes a reversal of the entire current system includes a reversal of the entire current system

The Somali Current (SC) is the Western The Somali Current (SC) is the Western boundary current of the Northern Indian ocean – boundary current of the Northern Indian ocean – the equivalent of the Gulf stream in the North the equivalent of the Gulf stream in the North Atlantic, or the Kuroshio in the North Pacific. Atlantic, or the Kuroshio in the North Pacific. However the SC is unique in that it is not present However the SC is unique in that it is not present during the whole year during the whole year

As a result of strong SW monsoon winds, the SC As a result of strong SW monsoon winds, the SC develops during May ~2.0 m/s develops during May ~2.0 m/s

Strongest as a Northwards jet from June through Strongest as a Northwards jet from June through to September (~3.5 m/s and more for June, to September (~3.5 m/s and more for June, observed during the Indian Ocean Experiment -observed during the Indian Ocean Experiment -INDEX, 1976-1979). INDEX, 1976-1979).

During December, the flow is weaker (~0.7-1.0 During December, the flow is weaker (~0.7-1.0 m/s) , and often turns southwards with the m/s) , and often turns southwards with the Northeast Monsoon WindsNortheast Monsoon Winds

Somali CurrentSomali Current

Somali Eddies Somali Eddies In general a two Gyre SystemIn 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)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 eddyAnticyclonic eddy

SG

Strong / Normal Monsoon yearsStrong / Normal Monsoon years All the three GW, SG and SEAll the three GW, SG and SE

Weak Monsoon years in general Weak Monsoon years in general Single eddy (GW)Single eddy (GW)

Eddy evolution - varies from year to year Eddy evolution - varies from year to year StrengthStrength LocationLocation NumberNumber TimeTime

Annual Variability – Somali EddiesAnnual Variability – Somali Eddies

Interannual variability in Somali Current : 1954-1976 : Mark M Luther

WOCE Indian Ocean ExpeditionWOCE Indian Ocean Expedition

Well Marked Eddy – Sep 1995Well Marked Eddy – Sep 1995

Two Eddies (1997) – Mark M LutherTwo Eddies (1997) – Mark M Luther

Model generate

d - eddies

Satellite observed

Decay Mechanism Decay Mechanism

CoalescenceCoalescence

Baroclinic instability triggered Baroclinic instability triggered by seasonal Rossby wave by seasonal Rossby wave from west coast of Indiafrom west coast of India

NE Monsoon EddiesNE Monsoon Eddies

Cyclonic eddyCyclonic eddy Diameter ~1000 kmDiameter ~1000 km 4-10 deg N & 50-60 deg E 4-10 deg N & 50-60 deg E

(Location of SG in SW monsoon)(Location of SG in SW monsoon)

Anticyclonic eddy (Laccadive Anticyclonic eddy (Laccadive High-LH)High-LH)

Diameter ~500 kmDiameter ~500 km 4-10 deg N & 60-70 deg E 4-10 deg N & 60-70 deg E

(Laccadive Islands) (Laccadive Islands) Due to intense negative wind Due to intense negative wind

curl off the SW coast of Indiacurl off the SW coast of India Consists multiple eddiesConsists multiple eddies

Marine Geodesy, 23:167-195,2000. Bulusu Subrahmanyan & Ian S Robinson

Eddies in Red SeaEddies in Red Sea L1 – Mostly in winter L1 – Mostly in winter

due to formation of due to formation of intermediate waterintermediate water

H1 – Most regular due H1 – Most regular due to coastline and to coastline and topography variationstopography variations

H2 & L2 – Non H2 & L2 – Non persistent but high persistent but high horizontal velocities horizontal velocities >= 0.5 m/s>= 0.5 m/s

L1

H1

H2

L2

Mostly Anticyclonic Mostly Anticyclonic In 1991 – 2:CyclonicIn 1991 – 2:Cyclonic& 1: Anticyclonic & 1: Anticyclonic **** Origin is linked with Origin is linked with

decay of Western decay of Western Arabian Sea eddiesArabian Sea eddies

Dia : 150-250 kmDia : 150-250 km Deep ~1000 mDeep ~1000 m Speed :0 .2-0.3 m/sSpeed :0 .2-0.3 m/s

Eddies in Gulf of Eddies in Gulf of AdenAden

Temp - 100 m

Feb :1991

**Amy S Bower at el, WHOI

Gulf of OmanGulf of Oman

Ras al Hadd jet is highly Ras al Hadd jet is highly variablevariable

Strong in SW MonsoonStrong in SW Monsoon Reversal of flow along NE Reversal of flow along NE

Oman coast in Aug is Oman coast in Aug is related to intensification related to intensification and propagation of and propagation of cyclonic eddy in Northcyclonic eddy in North Anticyclonic eddy in southAnticyclonic eddy in south

Temp at 250 m

Eddies at a Glance (1993-2002)Eddies at a Glance (1993-2002)

Eddies through SSTEddies through SST

Eddies in Northern Arabian Sea Eddies in Northern Arabian Sea Winter Winter

Widespread occurrence of mesoscale eddies (warm and cold) in the Widespread occurrence of mesoscale eddies (warm and cold) in the northern Arabian Sea - Satellite imageriesnorthern Arabian Sea - Satellite imageries

The size of the cold water eddies in the Arabian Sea ranges from 10 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 to 150 kms, while the duration of eddies appears to vary from 2 to 18 days. 18 days.

The warm water eddies are lower than that of cold water eddies. 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 ARGO floats data indicate that there is a great deal of fluctuation in the mixed layer depths (MLD) particularly during February and the mixed layer depths (MLD) particularly during February and March attributable to the occurrence of numerous eddies in this March attributable to the occurrence of numerous eddies in this region region

(IUGG 2003 Abstract, (IUGG 2003 Abstract, JSP05/03P/C30-003JSP05/03P/C30-003 --Somasundar KROTHAPALLISomasundar KROTHAPALLI ) )

SummarySummary

NW Indian Ocean - a complete NW Indian Ocean - a complete Oceanographic Lab with highly variable Oceanographic Lab with highly variable energetic environmentenergetic environment

Large number of eddies present Large number of eddies present throughout the year with varyingthroughout the year with varying LocationLocation StrengthStrength DimensionDimension CharacteristicsCharacteristics

ReferencesReferences Lisa M Beal, Teresa K Chereskin. The volume transport of the Somali Current during 1995 SW 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.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. 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.Geophysical Research Letters, Vol 29 No 21,2025.

Bulusu Subrahmanyam & Ian S Robinson. Sea surface height variability in the Indian Ocean from 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.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 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.a numerical model. Marine Geodesy, 25:175-186,2002.

University of Miami RSMAS Technical Report 2000-01. Arabian Marginal Seas and Gulfs.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 Mark M Luther. Interannual variability in the Somali Current 1954-1976. Nonlinear Analysis 35 (1999) 59-83.(1999) 59-83.

Schott FA, McCreasy Jr JP. The monsoon circulation of the Indian Ocean. Progress in Schott FA, McCreasy Jr JP. The monsoon circulation of the Indian Ocean. Progress in Oceanography 51 (2001) 1-123.Oceanography 51 (2001) 1-123.

Wirth A, Willebrand J and Schott F. Variability of the great whirl from observations and models. Wirth A, Willebrand J and Schott F. Variability of the great whirl from observations and models. Deep sea Research Ii 49 (2002) 1279-1295.Deep sea Research Ii 49 (2002) 1279-1295.