Satellite remote sensing is being effectively used in monitoring the ocean surface. Among

the ocean observing satellite sensors, ocean colour sensors make use of visible band of

electromagnetic spectrum (shorter wavelength). The use of shorter wavelength ensures

fine spatial resolution of these parameters to depict oceanographic characteristics of any

region having significant spatio-temporal variability. The Southeastern Arabian Sea (SEAS;

encompassing between 70.5-77.5oE longitude and 8-15oN latitude) is such an area showing

very significant spatio-temporal oceanographic and atmospheric variability due to the

seasonally reversing surface winds and currents (Shankar et al., 2002; Shetye et al., 1990

Salim, Shaleen

Sanilkumar, K V

CMFRI Digital Repository

Winter School on Structure and Function of the Marine Ecosystem : Fisheries 59Spatial and temporal variability of Chlorophyll-A concentration in the South Eastern Arabian Sea (SEAS)1. IntroductionSatellite remote sensing is being effectively used in monitoring the ocean surface. Amongthe ocean observing satellite sensors, ocean colour sensors make use of visible band ofelectromagnetic spectrum (shorter wavelength). The use of shorter wavelength ensuresfine spatial resolution of these parameters to depict oceanographic characteristics of anyregion having significant spatio-temporal variability. The Southeastern Arabian Sea (SEAS;encompassing between 70.5-77.5oE longitude and 8-15oN latitude) is such an area showingvery significant spatio-temporal oceanographic and atmospheric variability due to theseasonally reversing surface winds and currents (Shankar et al., 2002; Shetye et al., 1990)(Figure 1). Consequently, the region is enrichedwith features like upwelling, sinking, eddies,fronts, etc. Among them, upwelling bringsnutrient-rich waters from subsurface layers tosurface layers. During this process primaryproduction enhances, which is measured inocean colour sensors as high values ofchlorophyll a (Chl a) (Banse, 1959, Johannessenet al., 1987, Shetye et al., 1990), satelliteobservations (Jayaram, 2011) and modelstudies (Shankar et al., 2002, Haugen et al.,2002; Shaji and Gangopadhyay, 2007). Thepresent work analyses the spatial and temporalvariability in Chl a provided by satellite oceancolour sensors to understand oceanographicvariability in the SEAS.2. Dataa. Chl aGlobal Area Coverage on monthly Chl a at 9x9 km2 of SeaWiFS sensor during the periodSeptember 1997- December 2010 were extracted from the NASA website (http://oceandata.sci.gsfc.nasa.gov/).Fig. 1. Study area (SEAS) is demarcated using theviolet shaded box in the Arabian Sea02C H A P T E R09C H A P T E RSPATIAL AND TEMPORAL VARIABILITY OFCHLOROPHYLL-A CONCENTRATION IN THESOUTH EASTERN ARABIAN SEA (SEAS)Shalin S.1 and K. V. Sanilkumar21ICAR-Central Marine Fisheries Research Institute 2Naval Physical and Oceanographic Laboratory, KochiWinter School on Structure and Function of the Marine Ecosystem : Fisheries60Spatial and temporal variability of Chlorophyll-A concentration in the South Eastern Arabian Sea (SEAS)b. WindDaily averaged wind data on zonal and meridional scale were extracted from QuikSCATfor the period 19 July 1999 – 19 November 2009 (ftp://www.ssmi.com). Monthly wind wasgenerated from the above dataset using ferret software. The alongshore component wascalculated for each area using the monthly averaged dataset.c. SSTSST from MODIS / AQUA sensor at 9 km spatial resolutions were downloaded fromocean colour website (http://oceandata.sci.gsfc.nasa.gov) on monthly scale.d. Sea Surface Height Anomaly (SSHA)Monthly SSHA data during January 1992 - February 2010 were downloaded from AVISO(www.aviso.soest.hawaii.edu). The SSHA data obtained from AVISO is the merged productof TOPEX, ERS and Jason-1 altimetry. The data has a spatial resolution of 0.33o.e. Rain dataWeekly rainfall data from TMI sensor (December 1997 – February 2011) at 0.25o x 0.25ospatial resolution was downloaded to generate monthly rainfall rate.3. Results3.1. Spatial variability - Climatology of Chl a in the study areaDistribution of Chl a is examined to understand the climatology of these parameters inthe study area. In this regard, the monthly averages generated from the data provided bythe sensors SeaWiFS is used. The spatial distribution of Chl a brought out seasonal as wellas regional scale variability along the SEAS with maximum ranges of the variability duringthe southwest monsoon period (Figure 2).Fig. 2. Monthly climatology of Chl a in the SEAS. The upper panels represent the distribution patternduring January to June and the lower panel during July to December.Winter School on Structure and Function of the Marine Ecosystem : Fisheries 61Spatial and temporal variability of Chlorophyll-A concentration in the South Eastern Arabian Sea (SEAS)In general, the values of Chl a are high near the coast during all the months, whichdecreased towards offshore. During May, a marginal increase in Chl a sets in and this trendcontinues up to August. In addition, a significant cross-shore gradient is established in thesouthern area by June, which moves northward to encompass the entire study area byAugust and undergoes gradual decay during subsequent months. Similar to its intensification,the decay also starts from the south and moves northward. The gradient starts weakeningfrom September and disappears from the southern area by October. By November, it fadesoff the northern area too. Thereafter, low Chl a values continue up to April all through thearea.3.2. Temporal variabilityMost of the oceanographic and atmospheric parameters exhibit temporal variability.The time series observations on Chl a as obtained from SeaWiFS images during September1997 – December 2010 is analysed to understand the temporal variability on Chl a. Spatialdistribution has established the existence of large spatio-temporal variability in the studyarea especially during the southwest monsoon. Monthly Chl a were averaged for the selectedarea to explain their temporal variability. The influence of local environmental conditions(wind, ocean current, SSH and SST) on these parameters were also examined.In general, Chl a in SEAS showed high values (> 1.20 mg m-3) during June – Septemberand very low values (~0.25 mg m-3) during December – April. May and October / Novembercan be considered as transition periods (Figure 3). The above three phases complete anannual cycle that recurs, while peak values occur during one of the months and found tovary each years between July and September (Table 1). In the following section, thecorresponding supporting parameters representing oceanographic and atmosphericconditions were examined, to understand their influence on Chl a distribution.The supporting parameters showed that in general, low SST/ SSHA and high equatorwardcomponents of currents and winds existed during the southwest monsoon. SST was foundto follow a bi-annual oscillation, with lowest value in one of the months during the southwestmonsoon, whereas, SSHA showed an annual cycle again with minima in one of the monthsduring the southwest monsoon. The surface alongshore currents change its directionpoleward during the northeast monsoon, but the alongshore winds has equatorwardcomponents irrespective of the season (0 to -5 ms-1). The equatorward winds intensifiedduring the southwest monsoon, except during 2006 and 2007. Thus, in general, thesupporting parameters showed maximum equatorward current / wind and low SST / SSHAduring the southwest monsoon, with their peak in one of the months similar to Chl a. In thiscontext, it is to be noted that this low SST / SSHA and strong equatorward current / windWinter School on Structure and Function of the Marine Ecosystem : Fisheries62Spatial and temporal variability of Chlorophyll-A concentration in the South Eastern Arabian Sea (SEAS)are associated features of upwelling (Bruce et al., 1998; Shankar and Shetye, 1997; Shankaret al., 2002). Therefore, the co-occurrence of high Chl a along with the above maxima/minima of the supporting parameters during the southwest monsoon period suggests closelinkage to them with upwelling. Further, the low SST and highest Chl a values co-occurredwith the highest southward current/ wind in the same month or within the differences ofone month during most of the years. This observation showed the increase in Chl a wouldhave induced by upwelling as these factors are favorable for upwelling.Fig. 3. Temporal variability in (i) Chl a, (ii) SST, (iii) SSH, (iv) alongshore surface current and (v) alongshoresurface wind averaged for SEAS. The dotted vertical lines denote peak values of Chl a. The gap indicatesnon-availability of data. (Figure is redrawn from Shalin and Sanilkumar, 2014).Winter School on Structure and Function of the Marine Ecosystem : Fisheries 63Spatial and temporal variability of Chlorophyll-A concentration in the South Eastern Arabian Sea (SEAS)Table 1. Months during which maxima/ minima occurred in Chl a, SST, SSHA, current and winds in SEASYear Maximum Minimum Lowest MaximumChl a SST SSHA southerly current southward wind1998 Jul - Oct Jul -1999 Jul - Oct Jul Aug2000 Jul - Aug Jun Aug2001 Sep - Oct Jul Aug2002 Aug Aug Aug Aug Jul2003 Aug Jul Sep Aug Aug2004 Jul Aug Sep Jun Jul2005 Jul Jul Sep Jul Sep2006 Sep Jul Sep Jul Mar2007 Jul Jul Oct Jul Mar2008 Aug Aug Aug Jul Feb2009 Jul Jul Sep Jul Aug2010 Aug Aug - - -ConclusionIn general, Chl a in the SEAS (70.5-77.5oE, 8.0-15.0oN) follows an annual cycle with theirmaxima during the southwest monsoon period. Moreover, significant cross-shore gradientis observed on Chl a during the period, with high values (>1.0 mg.m-3) near the coast. Thisgradient develops in the south by June and propagated north such that it encompasses theentire study area by August. Similarly, the decay of the gradient starts during Septemberfrom south and moves north and completes the process by November. The spatial distributionon the three parameters showed large regional variability.The supporting parameters showed low values of SST and faster southward current/wind during the southwest monsoon period. These are the indicative features of intenseupwelling. In most of the cases, the month of lowest SSTs and faster southward currentscoincided with the month of peak Chl a. Therefore, the increase in Chl a were attributed toenhanced upwelling as evident from the supporting parameters. The present work bringsout the utility of ocean colour data to study the oceanographic features in the SEAS, especiallythe upwelling phenomenon and its spatio-temporal variability.Influence of upwelling in controlling surface chlorophyll was corroborated by time seriesanalysis of monthly data on chla, SST, SSHA, winds, and currents. Peak values of chl-a werealways concomitant with the highest equator-wards surface currents and lowest SST/SSHA.Winter School on Structure and Function of the Marine Ecosystem : Fisheries64Spatial and temporal variability of Chlorophyll-A concentration in the South Eastern Arabian Sea (SEAS)ReferencesBanse K. (1959). On upwelling and bottom- trawling off the southwest coast of India, Journal of MarineBiological Association of India, 1, 33 – 49.Bruce J. G., J. C. Kindle, L. H. Kantha, J. L. Kerling and J. F. Bailey (1998). Recent observations and modelingin the Arabian Sea Laccadive high region, Journal of Geophysical Research, 103 (C4), 7593 – 7600.Haugen V. E., O. M. Johannessen, and G. Evensen (2002). Mesoscale modeling study of the oceanographicconditions off the southwest coast of India, Proceeding of the Indian Academy of Sciences (Earthand Planetary Sciences), 111(3), 321 – 337.Jayaram V. C. (2011). Remote sensing the signatures of upwelling in the southeastern Arabian sea, Ph. D.thesis, 159 pp.Johannessen O. M., G. Subbaraju and J. Blindheim (1987). Seasonal variation of oceanographic conditionsoff the southwest coast of India during 1971-1975, Fiskerdirektoratet Scripts Serial Havundersoekelser,18, 247 – 261.S. Shalin and K.V. Sanilkumar (2014). Variability of chlorophyll-a off the southwest coast of India,International Journal of Remote Sensing, 35:14, 5420-5433, DOI: 10.1080/01431161.2014.926411Shaji C. and A. Gangopadhyay (2007). Synoptic modeling in the eastern Arabian Sea during the southwestmonsoon using upwelling feature models, Journal of Atmospheric and Oceanic Technology, 24,877-893.Shalin S. (2013). Application of ocean colour data to study the oceanographic and atmospheric featuresoff the southwest coast of India with special reference to upwelling. Ph. D. thesis, 141 pp.Shankar D. and S. R. Shetye (1997). On the dynamics of the Lakshadweep high and low in the southeasternArabian Sea, Journal of Geophysical Research, 102(C6), 12551 – 12562.Shankar D., P. N. Vinayachandran and A. S. Unnikrishnan (2002). The monsoon currents in the northIndian Ocean, Progress in Oceanography, 52, 63-120.Shetye S. R., A. D. Gouveia, S. S. C. Shenoi, D. Sundar, G. S. Michael, A. M. Almeida and K. Santanam (1990).Hydrography and circulation off the west coast of India during the southwest monsoon 1987, Journalof Marine Research, 48, 359 – 378.

Spatial and temporal variability of chlorophyll-a concentration in the South Eastern Arabian Sea (SEAS)

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Spatial and temporal variability of chlorophyll-a concentration in the South Eastern Arabian Sea (SEAS)

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