What drives the Biological productivity of the Bay of Bengal and Arabian Sea?

National Institute of OceanographyDona Paula, Goa-403 004, India

S. Prasanna Kumar

4 October 2006 SIBER Workshop

Summer (July-August 1996, SK-115)Winter (February 1995/7, SK-99 & 105)Fall (Sept-Oct 1992/3, SK-77 & 87)Spring (April-May 1994, SK-91)

Summer (July-August 2001, SK-166)Winter (Nov2005-Jan 2006, SS-240)Fall (Sept-Oct 2002, SK-182)Spring (April-May 2003, SK-191)

40 45 50 55 60 65 70 75 800

5

10

15

20

25

Indian JGOFS

80 85 90 95 10005

10152025

BOBPS

• Northern Indian Ocean comprises of two tropical basins – Arabian Sea & Bay of Bengal

• Both the basins are forced by seasonally reversing monsoon wind system. Accordingly, the surface circulation of these basins also reverses seasonally

• In spite of these similarities in location and forcing, Bay of Bengal is traditionally known to be biologically low productive region while Arabian Sea is one of the most productive region of the world Ocean

SeaWiFS 1998-2006

June-SeptNov-Feb

ICOADS January ICOADS July

Despite the, Low chlorophyll & Productivity Average annual fluxes of organic carbon reach comparable rates in both Arabian Sea & Bay of Bengal ……..this is intriguing

Ramaswamy and Nair, 1994

SeaWiFS 1998-2006

0

0.2

0.4

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1

1.2

Sur

face

Chl

a (m

g/m

3 )

Summer Winter Spring

Central Arabian SeaCentral Bay of Bengal

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Inte

grat

ed_C

hl a

(mg/

m2 )

Summer Winter Spring

Central Arabian SeaCentral Bay of Bengal

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Inte

grat

ed_P

P (m

g C

m-2

d-1

)

Summer Winter Spring

Central Arabian SeaCentral Bay of Bengal

Int_

Chl

a (m

g/m

2)Su

rf_C

hla

( mg/

m3)

Int_

PP(m

g C

m-2

d-1

)1992-97 2001-06

Summer (June-September)Very High Chl 20-60 mg/m2Arabian Sea PP 700-1700 mgC/m2/d

Colder SST, Strong winds, northward shoaling MLD& plenty of Nutrients towards the north

Chl a (mg/m³) Aug 1996

NO3 (μM)

11 12 13 14 15 16 17 18 19 20 21

Latitude (N)

-120

-100

-80

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-40

-20

0

Dep

th (

m0

Wind

SST

MLD

Wind Stress-Curl (x10-8 Pascal/m)

High nutrients and chlorophyll in the north can beexplained with the help of wind curl associated withthe Findlater Jet

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Month

1

3

5

7

9

11

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15

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23

25

Latit

ude

(N)

+ve

Findlater Jet

Lateral Advection Wind-driven mixing

Upward Ekman Pumping

Lateral advection + upward Ekman Pumping + Wind-mixing

High Biological

Production in Summeris Driven

by

So What drives the high biological production in summer ?

ASBB

7 8 9 10 11 12 13 14 15 16 17 18 19 20Latitude (N)

-120-100

-80-60-40-20

0M

LD (m

)7 8 9 10 11 12 13 14 15 16 17 18 19 20

Latitude (N)

0

5

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15

Win

d Sp

eed

(m/s

)

7 8 9 10 11 12 13 14 15 16 17 18 19 20Latitude (N)

2728293031323334353637

SSS

(psu

)

7 8 9 10 11 12 13 14 15 16 17 18 19 20Latitude (N)

25

26

27

28

29

30

SST

(o C)

Wind

SST

MLD

SSS

AS

BB Warm SSTBB SST –

2C > AS

Low salinityBB SSS –

3psu < AS

NO3

Temperature

Salinity

SO4

7 8 9 10 11 12 13 14 15 16 17 18 19 20Latitude (N)

0.0

0.5

1.0

1.5

Surf

ace

Chl

a (m

g/m

3 )

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100In

tegr

ated

Chl

a (m

g/m

2 )

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Ineg

rate

d PP

(mg

C m

-2 d

-1)

7 8 9 10 11 12 13 14 15 16 17 18 19 20Latitude (N)

7 8 9 10 11 12 13 14 15 16 17 18 19 20Latitude (N)

0.06-0.3 mg/m30.3-1.1 mg/m3

9-11 mg/m226-60 mg/m2

4-5 times

3-5 times

90-220 mg C/m2/d770-1700 mg C/m2/d

~8 times

Why Bay of Bengal is less productive during summer?

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Dep

th (m

)

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Stability parameter (E, m-1)

Strong Stratification

AS BB

BB

AS

Prasanna Kumar et al,GRL, 2002

Low biological production in BB in summer is driven by

Strong Stratification arising from Warm and low Salinity watersComparatively Weak winds are unable to break stratification andinhibits wind-mixing & injection of nutrients from sub-surface

Winter (November-February) High Chl 15-50 mg/m2Arabian Sea PP 350-700 mgC/m2/d

Coldest SST, weak winds, Deep MLD in the north& plenty of Nutrients in the north

SST

Wind

MLD

NO3 (μM)

High Biological

Production in Winteris Driven

by

Reduced solar radiation + Increased evaporationColdest and saltiest watersWinter cooling and convection injects Nutrients

So What drives the high biological production in winter ?

Arabian Sea

ASBB

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Latitude (N)

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0

Dep

th (m

)

7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22Latitude (N)

2425262728293031

SS

T(0 C

)

7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22Latitude (N)

-80-70-60-50-40-30-20

MLD

(m)

7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22Latitude (N)

0

5

10

15

20

Win

d Sp

eed

(m/s

)

Chl a (mg/m³)

NO3 (μM)

Winter High Chl 12-26 mg/m2Bay of Bengal PP 330-640 mgC/m2/d

7 9 11 13 15 17 19Latitude (N)

-120

-100

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-60

-40

-20

0

Dep

th (m

)

9 10 11 12 13 14 15 16 17 18 19 20 21Latitude (N)

01020304050

Inte

grat

ed_C

hl a

(mg/

m2 )

9 10 11 12 13 14 15 16 17 18 19 20 21Latitude (N)

0

0.5

1

1.5

surf

ace_

Chl

a (

mg/

m3 )

ASBB

7 8 9 10 11 12 13 14 15 16 17 18 19 20 21Latitude (N)

32

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37

SS

S(P

SU

) > 3 psu

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Latitude (N)

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Dep

th (m)

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Latitude (N)

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Dep

th (m

)

7 9 11 13 15 17 19Latitude (N)

-120

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0

Dep

th (m

)

Arabian SeaBay of Bengal

Spring (March-May) Very Low Chl 10-15 mg/m2Intermonsoon PP < 300mgC/m2/d

Wind

SST

MLD

Warm SST, weak winds, shallow MLD and nutrient depleted upper ocean

April-May 2003

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latitude (N)

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Dep

th (

m)

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Latitude (N)

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Dep

th (

m)

Temp Spring 2003

Signature of eddies in the Bay of Bengal…….

Coastal eddySouthern eddy Northern eddy

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Latitude (N)

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Dep

th (

m)

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Latitude (N)

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Temp Fall 2002

Coastal eddyNorthern eddySouthern eddy

Temp Summer 2001

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Latitude (N)

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Dep

th (m

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Latitude (N)

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Dep

th (m

)

Open Ocean Coastal

July-August 2001

Integrated PP of 220.7 mg C m-2

d-1 in the vicinity of the southern eddy, was more than double the value out side the eddy region(107.2 mg C m-2 d-1 at 12oN)

lowest value of PP (89.4 mg C m-2

d-1) at 20oN , though nitracline shoaled towards the north, the lack of sufficient light

Integrated PP (328.4 mg C m-2 d-1) in the coastal eddyregion (17oN) is at least 8 times higher than that of the non-eddy region (39.7 mg C m-2 d1 at 15oN).

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Latitude (N)

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Dep

th (m

)

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latitude (N)

-120

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0

Dep

th (m

)

Southern eddyNorthern eddy Coastal eddy

Open Ocean Coastal

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0

Dep

th (m

)

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Latitude (N)

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0

Dept

h (m

)

CoastalOpen Ocean

NO3 μM

What is the implication of eddies in the Bay…………enhanced biological production ?

Chl a mg/m380 85 90 95 100

05

10152025

BOBPS

Prasanna Kumar et al., GRL, 2004

…………CO2 regulation ?

Seasonal variation of CO2 flux from BOBSW monsoon

-16-10

-428

1420

6 8 10 12 14 16 18 20

Latitude°N

CO

2flu

xmm

ol/m

2/d

-202468

10

10 12 14 16 18 20

Latitude°N

CO

2flu

xmm

ol/m

2/d

Sugandhini Sardesai et al (manuscript under preparation)

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10152025

BOBPS

What is the implication of eddies in the Bay?

• Eddy pumping leads to enhanced Biological Production

• Pumping of Colder CO2 richer water to the upper layers may lead to CO2 out-gassing

• This may alter Bay from a CO2 sink to CO2 source at least locally

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10152025

BOBPS

Summary

• Our understanding of Seasonal variability of biological productivity in the Arabian Sea is better now………but…….

• In the Bay of Bengal, where fresher waters remain in the upper ocean perennially, any mechanism which is capable of breaking the strong stratification would alter the biogeochemistry.

• One such mechanism is the cold-core eddies and we are beginning to understand their potential role.

Task ahead………..

• Is the biological production in the Arabian Sea limited by iron during any season?

• We do not yet understand the role of cyclones in altering the biogeochemistry of the Bay of Bengal.

• Microbial loop is an important link in the sustenance of zooplankton biomass in the Arabian Sea, but what is its role in the Bay, we need to address.

• Time to look at the biogeochemistry of the equatorial Indian Ocean and its link to climate………..

CreditsLate Dr. MadhupratapS. Sardesai, NIO Nuncio MurukeshN. Ramaiah, NIO Jayu NarvekarV. Ramaswamy, NIO Jane PaulN.B. Bhosle, NIO Karen LoboR. Ramesh, PRL Veronica FernandesJ.S. Sarupriaya, NIO Maya Elizabeth VarkeySharada, CMMACS Jayashankar DeUsha Muraleedharan, GU Asha NadurmathP.M. Muraleedharan, NIOG.N. Nampoothiri, NIO

CollaboratorsRaghu Murtugudde, Univ. MarylandAjith Subramanyam, Columbia Univ.Ajoy Kumar, Univ.Miami

Thoppil Prasad, NRL, MS

Summer AS BB

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) 9 10 11 12 13 14 15 16 17 18 19 20 21

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Inte

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(mg/

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Winter AS BB

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chl a

( mg/

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grat

ed _

chl a

(mg/

m2 ) Spring AS

BB

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a (m

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grat

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ASBB

Surf

ace

Chl

aI n

t eg

Ch l

aIn

tegr

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PP

-5 -4 -3 -2 -1 0 1 2 3 4 5

Latitude

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th (m

)

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Latitude

Dep

th(m

)

Variability of Nitrate (uM) from 5°N to 5°S along77°E

-1000-800-600-400-200

-4.0 -3.0 -2.0 -1.0 0.0 1.0 2.0 3.0 4.0-150

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Dep

th (m

)

Chlorophyll a (mg m-3) - along 77 ENitrateTemperature Chlorophyll a70 75 80 85 90

-10

-5

0

5

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Cross-Equatorial section along 77EMay-June 2005