Impact of cyclones on the Bay of Bengal chlorophyll variability using remote sensing satellites

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1 Indian Journal of Geo-Marine Sciences Vol. 40(6), December 2011, pp Impact of cyclones on the Bay of Bengal chlorophyll variability using remote sensing satellites R.K.Sarangi Marine and Earth Sciences Group, Remote Sensing Applications Area, Space Applications Center (ISRO), Ahmedabad, India [ Received 12 April 2010 revised 7 January 2011 Impact of three different cyclones on the Bay of Bengal water and its surface chlorophyll concentration has been studied in three different zones during October-December 2000 using 24 scenes of IRS-P4 Ocean Color Monitor (OCM). The chlorophyll concentration found to be increasing up to 5.0 mg/m 3 with effect of cyclones and hurricanes. The data has been correlated with Sea Surface Temperature (SST) and wind speed from NOAA-Pathfinder-5 and Quickscat scatterometer, respectively. There has been observation of 2-3 C decrease in SST with movement of cyclone. The wind speed has peaked up to m/sec in the cyclonic zones. [Keywords: chlorophyll, ocean colour, upwelling, Phytoplankton, cyclone] Introduction Measurements of ocean colour and the fate of light in the ocean are extremely useful for describing biological dynamics in surface waters 1-2 and thus, the oceanographic communities has made substantial commitment to remote sensing of ocean colour from space 3-4. Regional scale ocean colour images are particularly useful for spaceborne studies of ocean margins because of the importance and dominance of meso-scale features (upwelling plumes, eddies, filaments and river plumes) in continental shelf and slope waters 5. The oceanic and coastal processes rapidly alter the optical properties of waters and these effects get manifested in the colour of the water 6. Synoptic measurements of the phytoplankton concentration are required adequately to understand the spatial scale ocean productivity. Phytoplankton have a short lifespan of about 1-10 days, and are subjected to the vagaries of ocean currents at a range of spatial and temporal scales. Consequently, the ocean pigment field de-correlates with itself on time scales of about 1-10 days and on spatial scales of about 1-5 km, depending on the environmental conditions 7. So, the cyclones and cyclonic storms in the ocean surface and the induced current on water surface tend to move away the ocean color features and the pigment concentrations. The physical forcing and its impact has been a study of concern. The Bay of Bengal (BoB) is a semi-enclosed tropical basin in the northern Indian Ocean. It experiences seasonal changes in circulation and weather due to the seasonally reversing monsoons. The Bay of Bengal is activated by the cyclonic winds, storms, gyres and sea surface is rough compared to the Arabian Sea. Biological productivity in the BoB is believed to be considerably less than in the Arabian Sea, particularly during the summer monsoon 8-9. A possible reason for this difference is that the Bay receives large quantities of freshwater from rainfall and rivers 10. The surface primary productivity is controlled by the oceanic features, which can vary from season to season. There are a limited number of studies for Bay of Bengal waters with reference to ocean colour remote sensing. The occurrence of cyclones and its impact are frequent in the Bay of Bengal during the southwest as well as northeast monsoon seasons and hit the coast and devastate the coastal resources, as exemplified in terms of Orissa super cyclone of October-November 1999 and many other cyclones hitting the Orissa, Andhra and Tamil nadu coasts. There has been observation of damage and even impact of cyclone on the ocean surface chlorophyll concentration and its increase in the coastal water 11. The physical parameters like sea surface temperature, wind speed and direction and its relationship to the cyclonic forcing would act as corollary data source to the ocean color studies. The ocean color, its productivity and enhancement of fishery resources would imply on the scientific as well as societal applications as far as the cyclones in our water is concerned. Present study is an attempt to examine the following phenomena.(i) Analysis of the IRS-P4 OCM data during the period October-December 2000 and detection of the

2 SARANGI: IMPACT OF CYCLONES ON THE BAY OF BENGAL CHLOROPHYLL VARIABILITY 795 cyclone induced high chlorophyll concentration features along the tracks produced in IMD report 12. (ii) Correlate the OCM derived chlorophyll images with Pathfinder-4 retrieved Sea Surface Temperature (SST) images. (iii) Analyze the NASA Quickscat scatterometer retrieved wind speed and wind vector data and interpret with the chlorophyll images to understand the direction and movement of the high chlorophyll features. The physical parameters such as SST, wind vector maps gets correlated with ocean chlorophyll-a retrieved from satellites called ocean colour images. The seasonal features and patterns generated by the wind, currents has the link with ocean chlorophyll and SST in the basin. So, the information based on the relationship between ocean chlorophyll with wind speed/vector and SST data would be significant in regional scales. Materials and Methods Bay of Bengal is unique due to the seasonal reversing monsoon winds. Due to this, the ocean productivity is enhanced in the two major monsoon systems, the southwest monsoon and the northeast monsoon. Bay of Bengal is poured with large volume of fresh water from rainfall and from the rivers like Hoogly, Ganges, Brahmaputra, Airawati, Mahandi, Godavari, Krishna, Kaveri and so on 10. The riverine inputs are a major potential source of nutrients such as nitrate, phosphate and silica to the Bay. Also, one of the major suppliers of the nutrients to the surface in BoB is mixing due to cyclones, frequent during postmonsoon season 13. The formation of localized intense blooms and also the intensification of bloom generated by anticyclonic gyre is known due to injection of nutrients by cyclonic activity 14. Specifically, the strong stratification due to the barrier layer impedes the transfer of nutrients into the euphotic zone from below 15. Three cyclones have been detected in the basin and the impact on chlorophyll has been studied. The IRS-P4 OCM derived chlorophyll images have been generated during the cyclone period in the eastern part of the Bay of Bengal [Latitude N; Longitude E] for the dates October 12, 14, 18 and 22 (Path 11, Row 13 and 14). The 2 nd recorded cyclone during the period October 24-31, 2000 (Path 10, 11 and Row 13) covering waters off west Bengal and Bangladesh coast [Latitude N; Longitude E] and the third cyclone during the period December 23-30, 2000 (path 10, 11 and row 14) in central Bay of Bengal [Latitude 5-18 N; Longitude E] has been analyzed. The ocean colour monitor (OCM) of the Indian Remote Sensing satellite IRS-P4 is optimally designed for the estimation of chlorophyll in coastal and oceanic waters, detection and monitoring of phytoplankton blooms, studying the suspended sediment dynamics and the characterization of the atmospheric aerosols. The retrieval of ocean colour parameters such as phytoplankton pigment (chlorophyll-a) in oceanic waters, involves two major steps like atmospheric correction of visible channels to obtain normalized water leaving radiances in shorter wavelengths and second application of the bio-optical algorithm for retrieval of phytoplankton pigment concentration. In the ocean remote sensing, the signal received at the satellite altitude is dominated by radiances contribution through atmospheric scattering processes and only 8-10% signal corresponds to oceanic reflectance. Therefore it has been mandatory to correct the atmospheric effect to retrieve any quantitative parameter from space. The OCM scenes were corrected for atmospheric effects of Rayleigh and aerosol scattering using an approach called long wavelength atmospheric correction method. The approach used the two near infra red channels at 765 and 865 nm to correct for the contribution of molecular and aerosol scattering in visible wavelengths at 412, 443, 490, 510, and 555 nm The water leaving radiances derived from atmospheric correction procedure was used to compute chlorophyll-a pigment concentration. A number of bio-optical algorithms for retrieval of chlorophyll have been developed to relate measurements of ocean radiance to the in situ concentrations of phytoplankton pigments. The proposed empirical algorithm (also known as Ocean 2 or OC2) has been operated on SeaWiFS ocean colour data 19. This algorithm captures the inherent sigmoid relationship between R rs 490/ R rs 555 band ratio and chlorophyll concentration C Where R rs is the remote sensing reflectance. The algorithm operates with five coefficients and has following mathematical form. C = [ * X * X * X3] where C is chlorophyll concentration in mg m -3 and X = log 10 [R rs 490/R rs 555], where R rs is remote sensing reflectance. This algorithm has been presently used for generating the chlorophyll maps, using IRS-P4 OCM derived water leaving radiances. The chlorophyll retrieval accuracy obtained is within +35 percent 20.

3 796 INDIAN J. MAR. SCI., VOL. 40, NO. 6, DECEMBER 2011 The chlorophyll data has been correlated with corollary information from sea surface temperature (SST) and wind speed from NOAA-Pathfinder-4 and Quickscat scatterometer, respectively. The Pathfinder- 4 sensor data of resolution 4 kilometer has been used and provides SST with accuracy of 0.3 C ( Wind speed measurements of 3 to 20 m/second range, with an accuracy of 2 m/second with its direction, with an accuracy of 20 degrees with wind vector resolution of 25 kilometers has been the target of retrieval. Scatterometer data products have been archived through the JPL Physical Oceanography Distributed Active Archive Center (PO.DAAC), data distribution site: Results and Discussion The cyclone-1 is marked in the northeast region in the Bay of Bengal water off the Myanmar and Bangladesh coast 12. There has been observation of high chlorophyll concentration in the OCM derived images during October (Fig. 1). The chlorophyll concentration has been hiked up to 5.0 mg/m 3 and spread in the northeast part of the Bay of Bengal (Fig. 1). There has been observation of clouds in the surrounding water. The cyclone-2 is well observed with its impact in northern Bay 12. The square box shown on the 1 st image indicating higher chlorophyll concentration (~5.0 mg/m 3 ) marked with Box-1, same box is also marked on the image of st October 2000 showed the dense nature of Fig. 1 IRS-P4 OCM derived chlorophyll images of the northeast Bay of Bengal indicating cyclon-1 induced chlorophyll features during October 12-22nd 2000.

4 SARANGI: IMPACT OF CYCLONES ON THE BAY OF BENGAL CHLOROPHYLL VARIABILITY 797 chlorophyll and its accumulation in the near coastal water. This water is assumed to be highly productive and can be used for high fish catch in the post-cyclone conditions (Fig. 2). The shifting of the high chlorophyll concentration dense feature and its accumulation shown, covering the rectangular box (1). The shifting of the moderate range chlorophyll concentration ( mg/m 3 ) from the deeper water shifted towards coast and this is well observed, marked with another box (No.2) in the th October and 3 rd image (30-31 st October). The concentration observed to be low in the box-2 in the 3 rd image. So, the shifting of the denser surface chlorophyll has been well observed and documented. So, the coastal water near the coast becomes highly productive after every cyclone, so high fish catch is expected and could be achieved (Fig.2). Coastal water has been triggered to indicate chlorophyll concentration ~5.0 mg/m 3 with effect of cyclone. The patch indicated with circle (Number 3) with higher chlorophyll concentration (>1.0 mg/m3) has seen to be shifted towards coast and northwards around Dhamra coast and around Sunderbans of Bengal coast, seen in the 3 rd image, October 2000 (Fig. 2). The cyclone-3 and its impact has been observed in the central Bay of Bengal starting around 23 rd December 2000 and prolonged upto 30 th December The movement is held up to the Indian east coast Fig.2 IRS-P4 OCM derived chlorophyll images of the northern Bay of Bengal indicating cyclone-2 induced chlorophyll features during October 24-31st 2000.

5 798 INDIAN J. MAR. SCI., VOL. 40, NO. 6, DECEMBER 2011 Fig.3 IRS-P4 OCM derived chlorophyll composite images of the Bay of Bengal indicating cyclone-3 induced chlorophyll features during December 23-30th 2000.

6 SARANGI: IMPACT OF CYCLONES ON THE BAY OF BENGAL CHLOROPHYLL VARIABILITY 799 around Chennai coast and the east coast of Srilanka 12. The water was covered with clouds initially on 23 rd October. With the progress of cyclone, the high chlorophyll concentration (~4.0 mg/m 3 ) has been well observed off the Indian southeast coast and Srilankan east coast (Fig. 3). These areas become productive with effect of cyclones and can be the potential source for fishery and high catch just after the cyclone event. So, the ocean color images have the potential to indicate the productive zones in terms of fishery. Tables 1-3. Show the IRS-P4 OCM derived chlorophyll concentration (mg/m 3 ) for selected points in the cyclone affected waters in the Bay of Bengal. The increase in chlorophyll concentration has been overall marked in the three cyclonic regions and six the stations (1-6), marked as yellow digits, two stations in each cyclones and the derived data from respective chlorophyll images has been displayed in tabular form above (Tables 1-3). The chlorophyll has been ranged mg/m 3 with effect of cyclones. The cyclones 1 and 2 have affected mostly the coastal and shallower water of northern Bay of Bengal and the cyclone-3 has affected the deeper water and crossed Srilanka and Tamil nadu coast. So, cyclones 1&2 enhanced to higher and cyclone-3 enhanced to moderate and high chlorophyll concentration. So, the post cyclone regions needs to be identified as suitable potential fishing zones (PFZs) and can be forecasted. To correlate the chlorophyll high concentration patches with physical parameters, satellite derived information for SST and wind speed has been displayed in Figures 4 and 5. The SST data was rare to get Fig. 4 Pathfinder-5 retrieved sea surface temperature (SST) maps of the Bay of Bengal water in cyclonic phase during December 23-31st Tables 1 3. Show the IRS-P4 OCM derived chlorophyll concentration (mg/m3) for selected points in the cyclone affected waters in the Bay of Bengal. Table 1 Cyclone-1 (October 12-22, 2000) Point Latitude ( N) Longitude ( E) Table 2 Cyclone-2 (October 24-31, 2000) Point Latitude ( N) Longitude ( E) Table 3 Cyclone-3 (December 23-30, 2000) Point Latitude ( N) Longitude ( E)

7 800 INDIAN J. MAR. SCI., VOL. 40, NO. 6, DECEMBER 2011 cloud-free and good quality datasets. So, the December 2000 datasets have been obtained for this study purpose. There has been cooling of water in the Indian east coast, ~4 C deviation from the central Bay of Bengal, which has been most probably being influenced by the cyclonic wind storms and the churning and vertical stratification of water (Fig. 4). The wind speed and direction has been well explanatory about the cyclone intensity, its eye and the movement direction. As it has been observed from the chlorophyll images, the cyclone-1 was in the northeast coast, 2 nd cyclone was in the northern Bay of Bengal and the third was in the central Bay and approached towards Indian southeast coast. Similar information has been inferred from the scatterometer retrieved wind speed images (Fig. 5). The Highest wind was observed to be around 15 m/sec. The forcing of the chlorophyll patched from the offshore to coast has been well understood with the wind speed data. Fig.5 Quickscat scatterometer derived wind speed and wind vector maps during October and December 2000 over the Bay of Bengal water (m=morning, e=evening pass).

8 SARANGI: IMPACT OF CYCLONES ON THE BAY OF BENGAL CHLOROPHYLL VARIABILITY 801 Conclusion High chlorophyll concentration increasing up to 5.0 mg/m 3 has been observed during the three cyclonic incidences. Movement and accumulation of dense chlorophyll concentration has been observed in the coastal water. Sea surface temperature has been observed to be reduced up to 24 C around the coastal water as compared to 28 C in the central Bay of Bengal. Wind speed images are seen and well correlated with the movement and direction of high chlorophyll patches, which acted as a triggering force for chlorophyll enhancement and induces the productivity of the coastal water and these postcyclone regions would be the significant potential fishing zones (PFZs) and can be well forecasted for the fisher-folk community of our nation and others. Acknowledgement Author is grateful to Dr R.M.Dwivedi, Head, MCED, Dr. Ajai, Group Director, Marine and Earth Sciences Group, Director, Space Applications Center, Ahmedabad for necessary guidance and facilities for carrying out the work. References 1 Lorenzen C.J, Extinction of light in the ocean by phytoplankton. J. Cons. Int. Explor. Mer., 34 (1972) Smith R.C, Prézelin B.B, Bidigare R R & Baker K S, Bio-optical modeling of photosynthetic production in coastal waters. Limnol. Oceanogr., 34 (1989) Aiken J, Moore G F & Holligan P M, Remote sensing of oceanic biology in relation to global climate change. J. Phycol., 28 (1992) Mitchell B G, Coastal Zone Colour Scanner retrospective, J. Geophys. Res., 99 (1994) Yoder J A, An overview of temporal and spatial patterns in satellite-derived chlorophyll imagery and their relation to ocean processes. In D.Halpern (Ed.), Satellites, Oceanography and Society, Elsevier Science BV, Netherlands, 2000, pp Sarangi R K, Chauhan P & Nayak S R., Phytoplankton bloom monitoring in the offshore water of Northern Arabian Sea using IRS-P4 OCM Satellite data. Ind. J. Mar. Sci., 30:4 (2001) Abbott M R & Zion P M., Satellite observations of phytoplankton variability during an upwelling event. Cont. Shelf Res., 4 (1987) Prasanna Kumar S, Muraleedharan P M, Prasad T G, Gauns M, Ramaiah N, De Souza, S N, Sardesai S & Madhupratap, M., Why is the Bay of Bengal less productive during summer monsoon compared to the Arabian Sea? Geophys.l Res. Lett., 29:24 (2002) 2235, doi: /2002gl Madhupratap M, Prasanna Kumar S, Bhattathiri P M A, Dileep Kumar M, Raghukumar S, Nair K K C & Ramiah N, Mechanism of the biological response to winter cooling in the northeastern Arabian Sea. Nature, 384 (1996) Subramanian V, Sediment load of Indian Rivers. Curr. Sci., 64 (1983) Nayak S R, Sarangi R K & Rajawat A S, Application of IRS-P4 OCM data to study the impact of cyclone on coastal environment of Orissa. Curr. Sci., 80 (2001) Thapliyal V, Desai D S. & Krishnan V, Cyclones and depressions over north Indian Ocean during Mausam, 53:3 (2001) Madhu N V, Maheswaran P A, Jyotibabu R, Sunil V, Ravichandran C, Balasubramaniam T, Gopalakrsishnaan T C. & Nair K K C, Enhanced biological production off Chennai triggered by October 1999 super cyclone (Orissa). Curr. Sci., 82:12 (2002) Vinayachandran P N & Mathew S, Phytoplankton bloom in the Bay of Bengal during the northeast monsoon and its intensification by cyclones. Geophys. Res. Lett., 30:11 (2003) 1572, doi: /2002gl Gomes H R, Goes J I & Saino T, Influence of physical processes and freshwater discharge on the seasonality of phytoplankton regime in the Bay of Bengal. Cont. Shelf Res., 20 (2000) Gordon H R & Wang M, Retrieval of water-leaving radiance and aerosol optical thickness over the oceans with SeaWiFS: A preliminary algorithm. Appl. Opt., 33 (1994) Mohan M, Chauhan P, Raman M, Solanki H U, Mathur A. K, Nayak S R, Jayaraman A, Satheesh S K & Krishnan S K, Initial results of MOS validation experiment over the Arabian sea, atmospheric correction aspects. In proceedings of the First International Workshop on MOS-IRS and ocean colour, DLR, Berlin, 1997, pp Chauhan P, Nagur C R C, Mohan M, Nayak S R & Navalgund R R, Surface chlorophyll distribution in Arabian Sea and Bay of Bengal using IRS-P4 ocean color monitor satellite data. Curr. Sci., 80 (2001) O Reilly J E, Maritorena S, Mitchell B G, Siegel D A, Carder K L, Garver S A, Kahru M & McClain C R, Ocean color chlorophyll algorithms for SeaWiFS. J. Geophys. Res., 103 (1998) Chauhan P, Mohan M, Sarangi R K, Beena Kumari, Nayak S R. & Matondkar S G P, Surface chlorophyll-a estimation in the Arabian Sea using IRS-P4 Ocean Colour Monitor (OCM) satellite data. Int. J. Rem. Sens., 23:8 (2002)