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

Transcription

1 4 October 26 SIBER Workshop What drives the Biological productivity of the Bay of Bengal and Arabian Sea? S. Prasanna Kumar National Institute of Oceanography Dona Paula, Goa-43 4, India

2 Summer (July-August 1996, SK-115) Winter (February 1995/7, SK-99 & 15) Fall (Sept-Oct 1992/3, SK-77 & 87) Spring (April-May 1994, SK-91) Summer (July-August 21, SK-166) Winter (Nov25-Jan 26, SS-24) Fall (Sept-Oct 22, SK-182) Spring (April-May 23, SK-191) Indian JGOFS BOBPS

3 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 ICOADS January ICOADS July 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 Nov-Feb June-Sept SeaWiFS

4 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

5 Int_PP (mg C m-2 d-1) Central Arabian Sea Central Bay of Bengal Summer Winter Spring Central Arabian Sea Central Bay of Bengal Summer Winter Spring SeaWiFS Central Arabian Sea Central Bay of Bengal Summer Winter Spring Integrated_Chl a (mg/m2) Surface Chl a (mg/m3) Integrated_PP (mg C m-2 d-1) Int_ Chl a (mg/m2) Surf_Chl a (mg/m3)

6 Summer (June-September)Very High Chl 2-6 mg/m2 Arabian Sea PP 7-17 mgc/m2/d Chl a (mg/m³) Aug 1996 Wind -2-4 Depth (m -6-8 SST NO3 (μm) MLD Colder SST, Strong winds, northward shoaling MLD & plenty of Nutrients towards the north

7 High nutrients and chlorophyll in the north can be explained with the help of wind curl associated with the Findlater Jet Upward Ekman Pumping +ve Wind Stress-Curl (x1-8 Pascal/m) Findlater Jet Lateral Advection Wind-driven mixing Month

8 So What drives the high biological production in summer? High Biological Production in Summer is Driven by Lateral advection + upward Ekman Pumping + Wind-mixing

9 Why Bay of Bengal is less productive during summer? NO SO4 Salinity Stability parameter (E, m -1 ) AS BB Strong Stratification Temperature Surface Chl a (mg/m 3 ) Integrated Chl a (mg/m 2 ) Inegrated PP (mg C m -2 d -1 ) SST ( o C) M L D ( m ) SSS (psu) W ind Speed (m/s) BB AS SSS Wind times BB AS SST 9-11 mg/m mg/m MLD mg C/m2/d mg C/m2/d mg/m mg/m3 AS BB Warm SST BB SST 2C > AS Low salinity BB SSS 3psu < AS 4-5 times ~8 times Prasanna Kumar et al, GRL, 22

10 Low biological production in BB in summer is driven by Strong Stratification arising from Warm and low Salinity waters Comparatively Weak winds are unable to break stratification and inhibits wind-mixing & injection of nutrients from sub-surface

11 Winter (November-February) High Chl 15-5 mg/m2 Arabian Sea PP 35-7 mgc/m2/d Wind NO3 (μm) SST MLD Coldest SST, weak winds, Deep MLD in the north & plenty of Nutrients in the north

12 So What drives the high biological production in winter? High Biological Production in Winter is Driven by Arabian Sea Reduced solar radiation + Increased evaporation Coldest and saltiest waters Winter cooling and convection injects Nutrients

13 Winter Bay of Bengal High Chl mg/m2 PP mgc/m2/d surface_chl a (mg/m 3 ) Integrated_Chl a (mg/m2) Wind Speed (m/s) AS BB NO3 (μm) SSS(PSU) SST(C) MLD(m) Chl a (mg/m³) > 3 psu AS BB

14 Spring (March-May) Very Low Chl 1-15 mg/m2 Intermonsoon PP < 3mgC/m2/d Wind Arabian Sea Bay of Bengal SST Depth(m) -6-8 MLD Warm SST, weak winds, shallow MLD and nutrient depleted upper ocean

15 Signature of eddies in the Bay of Bengal. April-May 23 Temp Spring latitude (N)

16 Temp Fall 22 Southern eddy Northern eddy Coastal eddy

17 Temp Summer 21 July-August 21 Southern eddy Northern eddy Coastal eddy Open Ocean Coastal

18 Chl a mg/m3 Southern eddy Northern eddy Open Ocean What is the implication of eddies in the Bay enhanced biological production? latitude (N) Coastal eddy Coastal Prasanna Kumar et al., GRL, 24 Integrated PP of 22.7 mg C m -2 d -1 in the vicinity of the southern eddy, was more than double the value out side the eddy region (17.2 mg C m -2 d -1 at 12 o N) lowest value of PP (89.4 mg C m -2 d -1 ) at 2 o N, though nitracline shoaled towards the north, the lack of sufficient light BOBPS Integrated PP (328.4 mg C m -2 d -1 ) in the coastal eddy region (17 o N) is at least 8 times higher than that of the non-eddy region (39.7 mg C m -2 d 1 at 15 o N). NO3 μm Open Ocean Coastal

19 CO2 regulation? Seasonal variation of CO2 flux from BOB SW monsoon BOBPS CO 2fluxm m ol/m 2/d Latitude N CO2fluxmmol/m2/d Latitude N Sugandhini Sardesai et al (manuscript under preparation)

20 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 BOBPS

21 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.

22 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..

23 Credits Late Dr. Madhupratap S. Sardesai, NIO Nuncio Murukesh N. Ramaiah, NIO Jayu Narvekar V. Ramaswamy, NIO Jane Paul N.B. Bhosle, NIO Karen Lobo R. Ramesh, PRL Veronica Fernandes J.S. Sarupriaya, NIO Maya Elizabeth Varkey Sharada, CMMACS Jayashankar De Usha Muraleedharan, GU Asha Nadurmath P.M. Muraleedharan, NIO G.N. Nampoothiri, NIO Collaborators Raghu Murtugudde, Univ. Maryland Ajith Subramanyam, Columbia Univ. Ajoy Kumar, Univ.Miami Thoppil Prasad, NRL, MS

24 Summer AS BB Integrated_chl a ( mg/m 2 ) surface_chl a (mg/m 3 ) Summer Winter Spring Winter AS BB Integrated_PP (mg C m-2 d-1) Integrated_chl a (mg/m2) Surf_chl a( mg/m3) Integrated_PP (mg C m-2 d-1) Latitude(N) Integrated _chl a (mg/m2) Surface_chl a (mg/m3) AS BB Spring AS BB Integrated_PP (mg C m-2 d-1) Surface Chl a Integ Chl a Integrated PP

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26 15 Cross-Equatorial section along 77E May-June Temperature Latitude Variability of Nitrate (um) from 5 N to 5 S along77 E -1 Depth(m) Latitude Nitrate D e p t h ( m ) Chlorophyll Chlorophyll a (mg m-3) - along a 77 E