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Journal of Sea Research
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Oxycline variability in the central Arabian Sea: An Argo-oxygen study
Satya Prakash a,⁎, T.M. Balakrishnan Nair a, T.V.S. Udaya Bhaskar a, Prince Prakash a, Denis Gilbert b
a Indian National Centre for Ocean Information Services, “Ocean Valley”, P.O. Box 21, IDA Jeedimetla P.O., Hyderabad, 500 055, Indiab Institut Maurice-Lamontagne, Pêches et Océans Canada, Fisheries and Oceans Canada, 850 route de la mer, Mont-Joli, Québec, Canada G5H 3Z4
Article history:Received 24 February 2011Received in revised form 7 March 2012Accepted 12 March 2012Available online 30 March 2012
Keywords:Arabian SeaArgoDissolved OxygenOxygen Minimum Zone
Dissolved oxygen concentration in the oceanic waters plays a vital role in the global carbon cycle. Theaddition of oxygen sensors in the ongoing Argo programme could revolutionize our understanding of theocean's role in climate change. Here we present a first analysis of the oxygen profiles obtained from Argo float2900776, deployed in the oxygen minimum zone of the central Arabian Sea, one of the thickest oxygenminimum zones in the world ocean. Our study shows perennial oxygen minima in the sub-surface waters ofthe central Arabian Sea with strong inter-annual and intra-seasonal variability. The depth of the oxyclinevaries from 60 to 120 m but occasionally it may be as shallow as 40 m. It appears from the present data setthat in the southeastern Arabian Sea the low oxygen water shoals up during the early winter monsoon and itis largely controlled by remote forcing. Small scale localized high wind events can cause further shoaling ofthe oxycline.
The concentration of dissolved oxygen (DO) in the oceanic waters,particularly sub-surface waters, is a sensitive indicator of physical andbiogeochemical changes in the ocean (Joos et al., 2003). Recentobservations and model studies have suggested a decline in theconcentration of DO and expansion of oxygen minimum zones(OMZs) in the tropical ocean over the past 50 years (Peña et al.,2010; Stramma et al., 2008). It is not well understood whether thedecreasing trend in DO is a natural variability or a warming inducedeffect. Joos et al. (2003) suggested that it may be an indication ofchange in the large scale open-ocean circulation. Detailed accounts ofthe processes that may lead to changes in oxygen inventory in the seawater under anthropogenic climate change have been given byFrölicher et al. (2009). In brief, a decrease in O2 solubility and increasein surface stratification because of global warming or a combinationof both may be responsible for such a decline in DO (Frölicher et al.,2009).
A decline in DO of the oceanic waters has significant implications tothe nitrogen and carbon cycles. The reducing condition in the oxygen-depleted (suboxic) environment leads to intense denitrification,wherein microbial conversion of NO3 to molecular nitrogen (N2) takesplace. The ocean is estimated to be losing ~450 Tg N yr−1 of ‘reactive’nitrogen (e.g., nitrate, ammonium and urea) through oceanic denitri-fication (Codispoti, 2007); availability of ‘Reactive’ nitrogen in the sunlit
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layer limits autotrophic production over a large expanse of the worldocean and, therefore, affects carbon sequestration.
The OMZ occupies 1% of the entire volume of the Ocean (Lam andKuypers, 2011) and accounts for 8% of the total oceanic area (Paulmierand Ruiz-Pino, 2009). Occurrence of OMZ in the sub-surface water(~100–1000 m; Morrison et al., 1999) is the result of restrictedventilation and intense bacterial respiration/remineralization: theformer recharges the water column with oxygen whereas the latterconsumes it and causes depletion of oxygen. There is a large variationin the spatial extent (both vertical and horizontal) and strength of OMZbetween ocean basins. The tropical north and south Pacific and thenorthern Indian Ocean are the three major basins in the world Oceanwith pronounced OMZs. The tropical north and south Pacific(DOb4.5 μmol/kg) are associated with highly productive easternboundary upwelling systems (Lam and Kuypers, 2011) whereas thenorth Indian Ocean is influenced by seasonally reversing monsoonwinds. Another OMZ of relatively smaller areal extent and lesserstrength (DO>40 μmol/kg) is associated with the Benguela upwellingsystem along the continental margin of Namibia in the eastern tropicalSouth Atlantic Ocean (Karstensen et al., 2008).
The northern Indian Ocean comprises of two major ocean basins:the Arabian Sea in the west and the Bay of Bengal in the east. Thoughthey are situated at similar latitude, their response to the monsoonwinds is quite different. Both of these basins experience severeoxygen depletion at the sub-surface layer but the vertical extent andstrength of the OMZ is more pronounced in the Arabian Sea than theBay of Bengal (Naqvi et al., 2010). The Bay of Bengal is highlystratified due to input of fresh water through river discharges and,hence, productivity is considerably less compared to the Arabian Sea(Prasanna Kumar et al., 2002). In the Arabian Sea, on the other hand,
Fig. 1. Location of Argo float (WMO 2900776) in the Arabian Sea (Blue cross). Small black dots show geographical limits of oxygen minimum zone, outlined by Naqvi (1991),demarketed by 1 μM NO2
− contour. Star (red color) shows the location of an un-named cyclone (05A) 28th October to 2nd November 2007.
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the surface biological productivity is strongly influenced by seasonalmonsoon (Wiggert et al., 2005): strong upwelling during the summer(southwest) monsoon and convective mixing during the winter(northeast) monsoon bring nutrient into the sunlit zone and triggerhigh biological productivity, often leading to formation of phytoplank-ton bloom in the surface layer. Prakash et al. (2008) reported exportproduction of up to 12 mmol N m−2 d−1 during a Noctiluca bloom inthe northern Arabian Sea during the winter monsoon. Enhancedvertical fluxes of organic matter and associated remineralisationlead to increased oxygen demand in the sub-surface layer, resulting information of suboxic (DOb10 μmol kg−1) subsurface waters. Also,being landlocked from all the sides, the circulation in the northernIndian Ocean is sluggish; restricted ventilation and high oxygendemand in the sub-surface layer produce intense oxygen deficiency inthe Arabian Sea (Naqvi et al., 2005). It is a well developed OMZ (~150–1000 m) with one of the highest volume of suboxic (b5 μmol kg−1)waters (Morrison et al., 1998). Though the OMZ in the Arabian Seaappears to be seasonally stable, the oxycline depth seems to have strongseasonality with somewhat larger variations. A detailed account ofhydrography and characteristics of the Arabian Sea OMZ have beengiven by Morrison et al. (1998, 1999).
Considering its vital role in the global carbon cycle and its usefulnessas a tool to understand the ocean's role in climate regulation, it is ofutmost importance to monitor oxygen concentration throughout theworld ocean, particularly in the northern Indian Ocean, over increasedtemporal and spatial resolution and understand its annual cycle. TheArgo programme provides an immense opportunity for real-timemonitoring of salinity and temperature in the upper 2000 m of theocean and has made the ocean's interior accessible to observations atadequate temporal and spatial scales. The addition of biogeochemicalsensors such as for dissolved oxygen has the potential to revolutionizeour understanding of the ocean (Gruber et al., 2010; Körtzinger et al.,2005). The Argo Project, underway since 2000 (Gruber et al., 2010),aims to maintain a network of 3000 Argo floats that provide real-timemonitoring of temperature and salinity of the global upper ocean. As theprogram matures more and more floats with additional sensors likedissolved oxygen, chlorophyll, turbidity etc are being deployed in theglobal ocean. India has also actively contributed to the global Argoprogramme and has deployed some Argo floats, equipped with oxygen
Fig. 2. Time series plot of temperature, salinity and dissolved oxygen in the upper 250 m obtthe upper panel shows mixed layer depth (as calculated using the 1 °C criterion).
sensors, in the Arabian Sea and Bay of Bengal. Here we analyze anddiscuss results from one such Argo float (WMO ID 2900776) deployedin the central Arabian Sea.
2. Methodology
The dissolved oxygen data used in this study was obtained fromArgo float (WMO ID 2900776) deployed in the central Arabian Sea,which drifts at 2000 m water depth. It is equipped with an Aanderaaoxygen optode 3830 sensor along with Seabird CTD for traditionaltemperature, salinity and depth measurements. It records profiles ofthe above listed parameters at every 10 day interval; in total 110profiles over a period spanning from 01-Apr-2007 to 26-Mar-2010were extracted and analyzed for this study. Temperature and salinityprofiles from the present float were quality controlled based on realtime quality control tests (Wong et al., 2010) prescribed by the inter-national Argo Data management Team (ADMT) but unlike temperatureand salinity, there is no prescribedmethod suggested for quality controlof dissolved oxygen data. However, for the present study dissolvedoxygen data was quality controlled as per Martz et al. (2008): the datawas visually checked for its correctness and bad values appearingas spikes were removed. The factory calibrated oxygen Optode wasdirectly deployed in the oceanwithout any further user calibration. Theoxygen readings from the optode were corrected for salinity andpressure effects: the salinity compensation was done according to theprocedure described in the operating manual for the model 3830optode sensor and the pressure compensation (by 3.2%) was done assuggested by Uchida et al. (2008).
The sea level anomaly (SLA) data used in this study was derivedfrom the monthly merged data from multi-satellite sensors (TOPEX/Poseidon, ERS and Jason) and has spatial resolution of 1/3 degree.Surface wind data were derived from the QuikSCAT scatterometer thatmeasures the ocean surface winds (~10 m) around the globe and isavailable from July, 1999 to November, 2009 as wind vector. TheQuikSCAT scatterometer-derived surface wind fields in the Arabian Seafor the period of April, 2007 to November, 2009 are used in this study.We have used Level 3 daily gridded (0.5° by 0.5°) ocean wind vectordata available from the web site http://apdrc.soest.hawaii.edu/dods/public_data/satellite_product. We also used 8-day composites Level 3
ained from an oxygen sensor-equipped Argo float (WMO ID 2900776). The white line in
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Standard Mapped Image (SMI) data of SeaWiFS with 9-km resolution.The data were obtained from the National Aeronautics and SpaceAdministration (NASA) Ocean Color website (http://oceancolor.gsfc.nasa.gov).
3. Results and discussion
The present float was deployed in the southern sector of the oxygenminimum zone. The spatial extent of the OMZ as outlined by Naqvi(1991) and position of the Argo float are shown in Fig. 1. The annualcycle of temperature, salinity and dissolved oxygen for the upper 200 mare shown in Fig. 2. Temperature and salinity structures in the upperlayer illustrate the typical variability of the water column for the centralArabian Sea over an annual cycle. Mixed layer depth (MLD) variesbetween 20 m to 110 m, being deeper during the summer (~100 m) andwinter monsoons (~80 m) and shallower during the inter-monsoons(~30 m). MLD was defined as per Prasanna and Narvekar (2005); theyobserved, after analysis of several temperature, salinity and densityprofiles, that irrespective of the season or location the isothermal,isohaline and isopycnal layers coincide in the central Arabian Sea and,therefore, they defined MLD as the depth at which the temperature is1 °C less than the surface value. Since our float was also from the centralArabian Sea, we adopted the same criterion. Prasanna and Narvekar(2005) also reported similar seasonal variations in the mixed layeri.e., deeper during the summer and winter monsoons and shallowerduring the inter-monsoons, in the central Arabian Sea after analyzinghydrocasts and CTD data set from 1976 to 1997. They attributed theseasonality in the observed MLD to time varying atmospheric forcingsuch as wind speed. The observed deeper MLD during the southwestmonsoon is caused by negative wind stress curl and consequently byconvergence of Ekman transport in this region which lies south of theFindlater jet. The winter monsoon is characterized by cool drynortheasterly wind causing convective mixing which results in deepen-ing of the mixed-layer (Madhupratap et al., 1996). During the inter-monsoons thewind areweak andhence, theMLD is shallower. A possiblerole of remote forcing, such as the effect of westward propagating Rossbywaves, can also play a vital role (Rao et al., 2010). The salinity of the upper100 mwater is higher during June–December than during January–May.This is due to equator-ward flow of Arabian Sea high salinity waterduring the summer monsoon and pole-ward flow of low salinity waterfrom the Bay of Bengal into the Arabian Sea during the winter. Sarma(2002) also reported throughmodel studies that along 10°N at the depthof 150–250 m water flows southward from January to May and flowsnorthward from June to December.
Fig. 3. Oxygen saturation (in %)
Data from the optode sensor attached to Argo float 2900776provides high vertical resolution data from the oxygen minimumzone. The dissolved oxygen concentration in the subsurface waters(100 m and below) confirms the presence of a perennial oxygenminimum zone in the central Arabian Sea. Though the presence of theOMZ in the subsurfacewaters has been reportedwidely in the literature(de Sousa et al., 1996;Morrison et al., 1998, 1999; Sarma, 2002), there isconsiderable debate on how it is maintained throughout the year.Sluggish circulation (Swallow, 1984) and a coupling of biology andphysical processes (Sarma, 2002) are the main processes responsiblefor the intense OMZ in the Arabian Sea. Olson et al. (1993) suggestedthat regeneration of enhanced monsoon-induced surface productivityinto the subsurface layer does not necessarily cause sustenance of theOMZ in this basin. Sarma (2002) also emphasized that both physicaland biological processes play important roles inmaintainingOMZ in theArabian Sea, the former being responsible for maintaining the OMZduring the inter-monsoon seasons.
The Joint global ocean Flux study (JGOFS) did find strong seasonalityin the productivity pattern of the Arabian Sea but failed to find similarseasonality in the dissolved oxygen concentration in the sub-surfacewaters (Morrison et al., 1999) probably because of limitation in analy-tical techniques. de Sousa et al. (1996), however, did find seasonality insub-surface DO in the eastern Arabian Sea. The high resolution Argo-oxygen data suggests large variations and strong seasonality in oxyclinedepth in the central Arabian Sea. The surface layer is saturated withdissolved oxygen (>180 μ mole/kg) but its concentration decreasesrapidly to sub-oxic concentration (b10 μ mole/kg) in the sub-surfacelayer. Fig. 3 shows oxygen saturation (in %) for the upper 250 m. Theoxycline varied between 60 m to 120 m depth, except in 2007 whenduring the months of November–December it became as shallow as40 m. In general, the Argo-oxygen profiles suggest semi-annualoscillation in oxycline depth: it is shallower (~60 m) during themonthsof November–January and deeper (~120 m) during April–May. Smallphases of decline in the oxycline, though for relatively smaller periods,may also be seen during the month of late February–early March andJuly–August when it suddenly shoals to 60 m but again deepens to120 m within a short span of time.
The oxycline depth shoals up every year during the early wintermonsoon, as evident from the oxygen profiles which indicate inten-sification of the OMZ in the upper layer. Intensification in reducingcondition in the Arabian Sea during the NE monsoon has been widelyreported and has been attributed to sluggish renewal during thisseason (Naqvi et al., 2005; Sarma, 2002) whereas during the SW mon-soon northward flowing sub-surface currents seem to supply oxygen
Fig. 4. The time–longitude section of Sea Surface Height Anomaly (SSHA) duringJanuary 2007 to December 2009 along the band 9°N to 11°N.
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(Swallow, 1984). This process is alsomanifested in the salinity structureof the upper 100 m (Fig. 2b). We also analyzed sea level anomaly andsatellite derived chlorophyll data to investigate and further understandthe processes responsible for the anomalous decrease in the oxyclinedepth and strengthening of the oxygen minima during this period. Seasurface height anomaly (SSHA) was negative during these months. Thetime–longitude section of SSHA during January 2007 to December 2009along the band 9°N to 11°N is shown in Fig. 4. The observed negative seasurface anomaly suggests a lowering of the sea level which in-turn isclosely related to a decrease in the thermocline depth during themonths of November/December. It is reflected in the temperatureprofiles as well. It may be caused by westward propagating upwellingRossby waves, as evident from Fig. 4. The role of Rossby waves in thetransient adjustment of ocean circulation is well understood; it caneither raise or lower the thermocline, leading to a decrease/increase inthe sea surface height. A recent study also shows that this part of theArabian Sea is characterized by westward propagating upwellingRossby waves, triggered off the west coast of India and strengthenedby the local wind stress curl (Rao et al., 2010). The observed decrease inthe oxycline depth is possibly caused by upwelling Rossby waveswherein lowering of the thermocline brings low-oxygen water to amuch shallower level.
The shoaling of the thermocline also causes entrainment of nutrientsand triggers high biological productivity. Murtugudde et al. (1999)observed high chlorophyll (>1 mgm−3) in the latitudinal band of 5°Nto 10°N and proposed that the bloom is caused by vertical displacementof the thermocline due to local wind stress curl in the region.Girishkumar et al. (in press) reported anomalous high chlorophyllvalues (>0.4 mgm−3) in satellite deriveddata over the southern Bay ofBengal during thewinter 2006–2007 and attributed it to shaoling of thethermocline due to westward propagating upwelling Rossby waves.The shaoling of the thermocline brings low-oxygenwater to a shallowerlevel. Since the deeper water is enriched in nutrients, mainly nitrate, ittriggers high surface productivity, and associated remineralisation cancause further reduction in DO (Naqvi et al., 2005) at intermediatedepths in already oxygen depleted waters.
The year 2007 was unique. During this year, the doming of thethermocline was intensified by an un-named tropical storm 05A(hereafter TS05A), which formed and developed in southeasternArabian Sea during Oct 28–Nov 2. TS05A originated with a maximumsustained wind (MSW) of 16 m s−1 near 66.30°E 10.60°N on October27, 2007 and rapidly intensified to a storm with MSW up to 23 m s−1
on October 29, 2007. It weakened gradually after November 01, 2007and moved towards the west. A stronger positive wind stress curl ofmagnitude 4×10−6 N m−3 appeared over the southeastern ArabianSea duringOctober 28–November 02 (Fig. 5 in color shading). Profiles oftemperature and dissolved oxygen before and after the storm passedare shown in Fig. 6. It is evident from the profiles that the storm had asignificant effect on the thermocline, mixed layer depth and oxycline;the thermocline also shoaled up after the cyclone passed. The sea watertemperature at 65 m depth decreased drastically from 26.8 °C on 28thOctober to 21.5°°C on 11th November, 2007. The MLD also decreasedfrom 50 m on 28th October to 20 m on 7th November. The anoxic(DOb1 μ mole/kg) sub-surface water became shallower by more than50m after the cyclone passed: it was at the depth of ~110 m on 28thOctober which shoaled up to 65 m on 11th November after the stormpassed. The thermocline and oxycline then deepened back, as evidentfrom the profile of 27th November 2007. The temperature-dissolvedoxygen plot (Fig. 7) for the sub-surface samples also shows gradualdecrease in DO, with decreasing temperature in the sub-surface layerbefore the storm (i.e., on 28th October) but a sharp decline in both, thetemperature and DO, immediately after the storm (on 7th November).This is due to isopycnal displacement effect which has broughtrelatively cooler, oxygen poor water to a shallower level. As discussedearlier, this period of the year witnesses a lowering of the thermoclinein the study area. In the year 2007 due to the occurrence of an un-
Fig. 5. Wind stress curl (in units of×10-6 N/m3 shown in color shading) during un-named tropical storm 05A in the Arabian Sea. Wind direction is indicated by the arrows. Symbolwhite star on upper right figure depicts float location on 28th October.
Fig. 6. Temperature and dissolved oxygen profiles before and after the tropical storm 05Aevent. The anoxic water became shallower by more than 50 m after the cyclone passed.Mixed layer depth decreased from 50 m on 28th October to 20 m on 7th November. Thewater temperature decreased bymore than 6° at the depth of 65 mafter the cyclonepassed.
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named storm the situation got aggravated with upward transport ofsub-oxic water to a much shallower level. It also helped produce bloomlike condition in the surface layer by bringing ample nutrient into it.Satellite derived ocean color data shows higher concentrations (>1 mg/m3) of chlorophyll in the cyclone influenced region, which appeared asa bloom over an area of 200 km in diameter. Prior to the cyclone, thesurface chlorophyll concentration ranged between 0.1 and 0.4 mgm−3.Fig. 8a and b depicts the distribution of chlorophyll before (16–23October 2007) and after (01–08 November 2007) the passage of thestorm, respectively. Owing to overcast skies and interference fromclouds, a cloud-free ocean color data could not be obtained during thecyclone (24–31 October 2007).
Weather disturbances such as tropical storms and cyclones areknown to cause sudden upsurge in the biological productivity (Naik etal., 2008; Smitha et al., 2006; Vinayachandran and Mathew, 2003) butmost of these observations were largely based on remote sensingandmodeling studies. Naik et al. (2008) provided a rare direct evidence
Fig. 7. Temperature-DO plot for the sub-surface samples before (28th October, 2007)and after (7th and 27th November, 2007) the storm pass.
Fig. 8. Satellite derived surface chlorophyll in the central Arabian sea (a.) before and (b.) after the storm pass.
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of development of bloom due to a cyclonic event; they reportedan increase in the surface chlorophyll concentration, from 0.5 to1.7 mgm−3, after a cyclone passed in the central Arabian Sea. Anincrease in the surface chlorophyll, satellite-derived, was also reportedby Srinivasa et al. (2009) after the passage of tropical cyclone Sidr in theBay of Bengal (lowest pressure of 944 mb). The preset study, however,provided a unique opportunity to understand the sub-surface processesduring such storms because the Argo float under discussion did not driftsignificantly during the storm period, clearly evident from the lat-longposition of the float before and after the cyclone listed in Table 1. It isalso evident from the above table that we are essentially following thesame water mass before and after the storm. However, we should notehere that the drift cycle of the present float is 10 days which limits ourunderstanding on the processes immediately after the storm. Therefore,for a better understanding of the sub-surface processes and effect ofsuch storm on the sub-surface biogeochemistry, we insist that floatswith high profile frequency, drifting atmuch shallower depth (~500 m)should be considered for the future float deployments.
On 27th November 2007, almost a month after the storm passed,the temperature and DO appear to have increased again; both thethermocline and oxycline started to regain their original structure whichis evident from the profile of 7th December. The temperature-DOplot for27th November and 7th December confirms it. The temperature and DOdata line upwell after the stormwhich indicates that remineralisation inthe sub-surface layer, as a result of the bloom-like situation in the surfacelayer immediately after the storm, did not contribute significantlytowards oxygen consumption. It is in apparent contradiction to the viewthat biology/remineralisation contributes significantly towards main-taining the OMZ in the Arabian Sea. It is well known that a combinationof sluggish circulation and sub-surface remineralisation helps maintainthe OMZ in the Arabian Sea, but from the present data set it appears thatcirculation plays a dominant role in generating/maintaining the OMZ.Results from the present Argo-oxygen data underscore the role of sub-surface remineralisation in generating/maintaining oxygen deficiency inthe Arabian Sea. A decrease, though more gentle, in the sea watertemperature and depth of OMZ can also been seen in the years 2008 and2009 but it was more intense in the year 2007.
Table 1List lat.–long. positions of the Argo float before and after the storm.
Date Latitude (°N) Longitude (°E)
Before the storm 28/10/2007 11.58 65.78After the storm 07/11/2007 11.58 65.52After the storm 17/11/2007 11.65 65.54After the storm 27/11/2007 11.65 65.50
The presence of oxygen deficient nutrient rich water close the seasurface has significant implications. The entrainment of the nutrientrich water can fuel biological productivity and can thus play a vitalrole in the carbon budget. On the other hand infusion of oxygendeficient water to a much shallower level can affect nitrogen cyclingwhich cannot only affect the biological production on a longer timescale but also lead to increased flux from the ocean to the atmosphereof the potent green house gas nitrous oxide (Naqvi et al., 2010). It isalso likely to affect marine ecology and fisheries.
4. Conclusion
Argo-derived dissolved oxygen data suggests the perennial presenceof the oxygen minimum zone in the sub-surface layer with strongvariability and seasonality in the oxycline depth. The latter displaysa large semi-annual oscillation, being shallower (~60 m) during themonths of November–January and deeper (~120 m) during April–May.The lowering of the oxycline during the early wintermonsoon appears tobe caused by westward propagating upwelling Rossby waves. Theoxycline was anomalously shallower in the year 2007 when the domingof the thermocline due to the above mentioned phenomenon wasaggravated by the presence of an un-named tropical storm. The oxygendeficient water came as close as 30 m below the sea surface. Such anevent has greater implications on the biological productivity andnitrogencycling.
Acknowledgment
We thank Dr. S.S.C. Shenoi, Director INCOIS for his support andencouragement. We also thank Dr. M. Ravichandran for coordinatingthe Argo programme at INCOIS and deploying Argo floats. This workwas supported by funding from INCOIS. This is INCOIS contributionnumber 101.
References
Codispoti, L.A., 2007. An oceanic fixed nitrogen sink exceeding 400 Tg Na−1 vs theconcept of homeostasis in the fixed-nitrogen inventory. Biogeosciences 4, 233–253.
de Sousa, S.N., Kumar, M.D., Sardessai, S., Sarma, V.V.S.S., Shirodkar, P.V., 1996. Seasonalvariability in oxygen and nutrients in the central and eastern Arabian Sea. CurrentScience 71, 847–851.
Frölicher, T.L., Joos, F., Plattner, G.K., Steinacher, M., Doney, S.C., 2009. Natural variabilityand anthropogenic trends in oceanic oxygen in a coupled carbon cycle–climatemodelensemble. Global Biogeochemical Cycle. doi:10.1029/2008GB003316.
Girishkumar, M.S., Ravichandran, M., Pant, V., in press. Observed chlorophyll-a bloom inthe southern bay of Bengal during winter 2006–2007, Intern. Journal of RemoteSensing. doi:10.1080/01431161.2011.563251.
Gruber, N., Doney, S.C., Emerson, S.R., Gilbert, D., Kobayashi, T., Körtzinger, A., Johnson,G.C., Johnson, K.S., Riser, S.C., Ulloa, O., 2010. Adding oxygen to Argo: developing a
8 S. Prakash et al. / Journal of Sea Research 71 (2012) 1–8
global in-situ observatory for ocean deoxygenation and biogeochemistry. 21–25September 2009 In: Hall, J., Harrison, D.E., Stammer, D. (Eds.), Proceedings ofOceanObs'09. Sustained Ocean Observations and Information for Society, vol. 2.ESA Publication, Venice, Italy, p. WPP-306.
Joos, F., Plattner, G.K., Stocker, T.F., Körtzinger, A., Wallace, D.W.R., 2003. Trends inmarine dissolved oxygen: implications for ocean circulation changes and thecarbon budget. Eos 84 (21), 197–201.
Karstensen, J., Stramma, L., Visbeck, M., 2008. Oxygen minimum zones in the easterntropical Atlantic and Pacific oceans. Progress in Oceanography 77, 331–350.
Körtzinger, A., Schimanski, J., Send, U., 2005. High quality oxygen measurements fromprofiling floats: a promising new technique. Journal of Atmospheric and OceanicTechnology 22, 302–308.
Lam, P., Kuypers, M.M.M., 2011. Microbial nitrogen cycling processes in the oxygenminimum zones. Annual Review of Marine Science 3, 317–345.
Madhupratap, M., PrasannaKumar, S., Bhattathiri, P.M.A., DileepKumar, M.,Raghukumar, S., Nair, K.K.C., Ramaiah, N., 1996. Mechanism of the biologicalresponse to winter cooling in the northeastern Arabian Sea. Nature 384, 549–552.
Martz, T.R., Johnson, K.S., Riser, S.C., 2008. Ocean metabolism observed with oxygensensor on profiling floats in the South Pacific. Limnology and Oceanography 53 (2),2094–2111.
Morrison, J.M., Codispoti, L.A., Gaurin, S., Jones, B., Manghnani, V., Zheng, Z., 1998.Seasonal variation of hydrographic and nutrient fields during the US JGOFS ArabianSea Process Study. Deep Sea Res., Part II 45, 2053–2101.
Morrison, J.M., et al., 1999. The oxygen minimum zone in the Arabian Sea during 1995.Deep Sea Res., Part II 46, 1903–1932.
Murtugudde, R.G., Signorini, S.R., Christian, J.R., Busalacchi, A.J., McClain, C.R., Picaut,1999. Ocean color variability of the tropical Indo-Pacific basin observed bySeaWiFS during 1997–1998 104, 18351–18366.
Naik, H., Naqvi, S.W.A., Suresh, T., Narvekar, P.V., 2008. Impact of a tropical cyclone onbiogeochemistry of the central Arabian Sea. Global Biogeochemical Cycle 22.doi:10.1029/2007GB003028.
Naqvi, S.W.A., 1991. Geographical extent of denitrification in the Arabian Sea inrelation to some physical processes. Oceanologica Acta 14, 281–290.
Naqvi, S.W.A., Narvekar, P.V., Desa, E., 2005. In: Robinson, Allen R., Brink, Kenneth H.(Eds.), Coastal biogeochemical processes in the Northern Indian Ocean: The Sea,vol. 14.
Naqvi, S.W.A., Bange, H.W., Farías, L., Monteiro, P.M.S., Scranton, M.I., Zhang, J., 2010.Marine hypoxia/anoxia as a source of CH4 and N2O. Biogeosciences. doi:10.5194/bg-7-2159-2010.
Olson, D.B., Hitchcock, G.L., Fine, R.A., Warren, B.A., 1993. Maintenance of the low-oxygen layer in the central Arabian Sea. Deep Sea Res II 40, 673–685.
Paulmier, A., Ruiz-Pino, D., 2009. Oxygen Minimum Zones (OMZs) in the modernOcean. Progress in Oceanography 80, 113–128.
Prakash, S., Ramesh, R., Sheshshayee, M.S., Dwivedi, R.M., Raman, M., 2008. High newproduction during a Noctiluca scintillans bloom in winter in the northeasternArabian Sea. Geophysical Research Letters 35. doi:10.1029/2008GL033819.
Prasanna, Kumar S., Narvekar, J., 2005. Seasonal variability of the mixed layer in thecentral Arabian Sea and its implication on nutrients and primary productivity.Deep Sea Research II 52, 1848–1861.
PrasannaKumar, S., Muraleedharan, P.M., Prasad, T.G., Gauns, M., Ramaiah, N., DeSousa,S.N., Sardessai, S., Madhupratap, M., 2002. Why is the Bay of Bengal less productiveduring summer monsoon compared to the Arabian Sea? Geophysical ResearchLetters 29 (24), 881–884.
Rao, R.R., Girish Kumar,M.S., Ravichandran,M., Rao, A.R., Gopalakrishna, V.V., Thadathil, P.,2010. Interannual Variability of Kelvin wave propagation in the wave guides of theequatorial Indian Ocean, the coastal Bay of Bengal and the Southeastern Arabian Seaduring 1993–2006. Deep Sea Research I 57, 1–13.
Sarma, V.V.S.S., 2002. An evaluation of physical and biogeochemical processesregulating perennial suboxic conditions in the water column of the Arabian Sea.Global Biogeochemical Cycle. doi:10.1029/2001GB001461.
Smitha, A., Rao, K.H., Sengupta, D., 2006. Effect of 2003 tropical cyclone on physical andbiological processes in the Bay of Bengal. International Journal of Remote Sensing27. doi:10.1080/01431160600835838.
Srinivasa, K.T., Mupparthy, R.S., Nagaraja Kumar, M., Nayak, S.R., 2009. Phytoplanktonbloom due to Cyclone Sidr in the central Bay of Bengal. Journal of Applied RemoteSensing 3 (1), 1–14.
Stramma, L., Johnson, G.C., Sprintall, J., Mohrholz, v., 2008. Expanding oxygen-minimumzones in the tropical oceans. Science. doi:10.1126/science.1153847.
Swallow, J.C., 1984. Some aspects of the physical oceanography of the Indian Ocean.Deep Sea Research 31, 639–650.
Uchida, H., Kawano, T., Kaneko, I., Fukasawa, M., 2008. In situ calibration of optode-basedoxygen sensors. Journal of Atmospheric and Oceanic Technology 25, 2271–2281.doi:10.1175/2008JTECHO549.1.
Vinayachandran, P.N., Mathew, S., 2003. Phytoplankton bloom in the Bay of Bengalduring the northeast monsoon and its intensification by cyclones. GeophysicalResearch Letters 30 (11). doi:10.1029/2002GL016717.
Wiggert, J.D., Hood, R.R., Banse, K., Kindle, J.C., 2005. Monsoon driven biogeochemicalprocesses in the Arabian Sea. Progress in Oceanography 65, 176–213.
Wong, A.R., Keely, T., Carval, and the Argo Data Management Team, 2010. Argo qualitycontrol manual, Report Ver. 2.6, Argo data management, November 2010. 46p.
