Systematic Validation of Conductivity and Temperature from Ocean moored buoy data in the northern Indian Ocean with in situ ship based measurements

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1 Indian Journal of Geo-Marine Sciences Vol. 45(2), February 2016, pp Systematic Validation of Conductivity and Temperature from Ocean moored buoy data in the northern Indian Ocean with in situ ship based measurements Simi Mathew *, V. R. Shamji 1, G. Vengatesan, M. ArulMuthiah & R. Venkatesan Ocean Observation Systems, National Institute of Ocean Technology, Pallikaranai P.O., Chennai , India 1 Faculty of Maritime Studies, King Abdulaziz University, P. O. Box Jeddah 21589, Kingdom of Saudi Arabia. *[ simi@niot.res.in] Received 29 Octoberber 2014; revised 20 May 2015 A continuous data on the surface meteorological, surface and subsurface data on temperature, salinity and current is available online with the deployment of twelve moored buoys; Ocean Moored Network for the Northern Indian Ocean (OMNI) with seven buoys in the Bay of Bengal and five in the Arabian Sea. This network of OMNI buoys has been providing data which are of great relevance to the climate research community to constantly monitor the seasonal, intra-seasonal, annual and inter-annual variations in the northern Indian Ocean. In the long run the data accuracy is another important part of the program, to ensure the quality of the data delivered from these buoys, especially for the sub-surface data where satellite cannot reach. Conductivity-Temperature-Depth profile is done adjacent to the buoy locations during various phases of service of the buoys. This operation carried out in three phases in the Bay of Bengal has been compared with the buoy data, in order to ensure the quality of the conductivity-temperature measurements taken by the OMNI buoys. The high correlation between both the data sets ensures the quality of the data delivered by the Conductivity- Temperature sensors attached along the mooring line of the OMNI buoys. [Keywords: OMNI, CTD, BoB, SST] Introduction The Ministry of Earth Sciences, Government of India has initiated the National Data Buoy Program (NDBP) in 1997 under National Institute of Ocean Technology (NIOT), Chennai to make real time surface meteorological and near-surface oceanographic measurements. This has been later evolved as comprehensive ocean observation network program involving moored buoys which can provide continuous real time data on subsurface temperature, salinity and currents, named as the OMNI buoys. At present twelve OMNI buoys are being operational and they are deployed at seven specific locations in the Bay of Bengal (BoB) and five specific locations in the Arabian Sea (AS) as shown in figure 1. In addition to that, NIOT still maintains seven met-ocean buoys mostly for the coastal waters. In the northern BoB, two OMNI buoys are deployed adjacent to each other for the uninterrupted flow of data from this location, due its significance for various research interests. The lifetime of the buoy depends on the sensor performance, maintenance on timely basis and vandalism issues. The continuous inflow of data from these buoys has Fig. 1 The OMNI buoy locations in the Arabian Sea and Bay of Bengal made a leap in the studies of ocean currents, evolution of warm pool in the AS and BoB 1,2, intra-seasonal oscillations 3, barrier layer formation, propagation of planetary waves, fresh water influx 4 and study on the genesis and responses of the cyclones in the both the seas 5,6.

2 SIMI et al.: SYSTEMATIC VALIDATION OF CONDUCTIVITY AND TEMPERATURE FROM OMNI BUOY DATA 225 Materials and Methods OMNI buoy measures conductivity and temperature upto 500m depth with the help of Conductivity Temperature (CT) sensors attached to the mooring line at discrete depths at 5, 10, 15, 20, 30, 50, 75,100, 200 and 500m as shown in figure 2. Data is collected every one hour and transmitted through satellite to the land station once in every three hours. However as the dynamic ranges of these ocean parameters are very large and thereby sea truth validation assumes high priority. It is in this context the Conductivity Temperature Depth (CTD) surveys were carried out in a proximate location to the buoy to provide the sea truth data to validate the sub-surface CT sensor data. The CTD surveys were carried out under the deployment and maintenance of data buoy program by the NIOT. This was done in several phases, in this paper we are trying to elucidate the inter-comparison exercise conducted during three phases in the BoB; one during the deployment of the new four OMNI buoys during May-June, 2011in the BoB. The second phase was carried out during the maintenance of RAMA buoys in the BoB, during Fig. 2 Schematic diagram of the OMNI buoy. August-September, Third one was during April-May, 2012 during the service of the OMNI buoys after one year of performance in the sea. Similar kind of exercise was done in the AS too; one during the deployment of new buoys during October, 2012 and another one during the maintenance of the buoys during September- October, Ship based CTD samplings were carried out in close proximity to the buoy location, mostly within 5km of the selected OMNI buoy location. Ship-borne CTD, onboard SagarKanya and SagarNidhi was released mostly upto 3500m depth, depending on the prevailing weather conditions. Data were logged in with high resolution and processed on board ship lab using Sea Bird software. Processed data consists of depth, conductivity, temperature, salinity, dissolved oxygen, chlorophyll etc. The conductivity and temperature data from the OMNI buoy is validated through intercomparison with in situ CTD profiler data. Sensor types used in the both are of the same make (Sea Bird); SBE37-IM is used in the OMNI buoys and SBE-9 is the one used in the ship. Results and Discussions The Inter-comparison During May-June, 2011 a major cruise operation was conducted in the BoB for the deployment of the four new OMNI buoys. The CT sensors attached to the mooring of the OMNI buoys are tested and calibrated before deployment. CTD records data with very high vertical resolution of even less than 1m, depending on the speed with which it is lowered from surface to the desired depth. During the first phase which was during the onset of the Southwest Monsoon (SWM) season; the temperature and conductivity profiles at all the four buoy locations showed well mixed upper layer. The CTD sampling was carried out within one hour after deployment of BD13 at N, E on 5th June Ship CTD as well as the buoy recorded a well mixed upper layer with an isothermal layer depth (ILD) of around 50m. Both the data were in good agreement even up to 500m depth as shown in figure 3. CTD measurements taken at the other OMNI buoy locations too showed very good correlation of more than OMNI buoys BD10 and BD11 located in the north-central and western BoB, respectively; showed double thermocline structure, which were reflected in both the datasets. BD08 buoy which is located 370 km

3 226 INDIAN J. MAR. SCI., VOL. 45, NO. 2 FEBRUARY 2016 south of the Ganges-Brahmaputra river-mouth recorded shallow ILD of around 30 m depth, with a sub-surface maximum in conductivity, which was in agreement with the CTD measurements too. Non-linearity in the temperature and salinity measurements can be expected in regions of strong river discharges especially for normal discharge years 7. Good correlation in the data ensures the quality of the OMNI buoy data which provides valid information on the impact of the strong river discharges especially in the northern BoB. temperature and salinity as the SWM season progress and the same was reflected in the buoy data too. But there was a mismatch in the conductivity values along the halocline. Since in the halocline region where the salinity varies rapidly with depth, even small vertical displacement of the mooring, due to the mooring scope of S type mooring will result in large variation in the conductivity values. Best option in such cases will be to go for daily average values wherein the pressure recorded at 500m depth by the Conductivity Temperature Depth (CTD) sensor is within a desired limit. Strong freshwater influx during the SWM season has reduced the ILD with respect to phase 1 measurements. CTD survey at south location, near BD 13 buoy showed an increase in the ILD compared to the other buoys. A sub-surface maximum in conductivity were also recorded by both the measurements. Inter-comparison showed good correlation even after three months of the buoy s performance at this specific location. A sharp gradient in the thermocline and halocline at the BD11 location during phase 2 of the cruise replaced the double thermocline and halocline present during the phase 1 as shown in figure 4. Both the CTD and CT sensors were in good agreement with each other with an over-all correlation of Fig. 3 Inter-comparison between the ship CTD and the OMNI buoy data during the first phase of the cruise. The second phase of CTD survey was carried out during August-September, 2011; the peak of SWM season and on completion of 3 months of successful functioning of buoys at sea. Measurements obtained near to all the four buoy locations were compared, except for the BD10 location whose CT sensors malfunctioned. A ship based CTD sampling conducted near the BD08 location recorded the decrease in surface Fig. 4 Inter-comparison between the ship CTD and the OMNI buoy data during the second phase of the cruise.

4 SIMI et al.: SYSTEMATIC VALIDATION OF CONDUCTIVITY AND TEMPERATURE FROM OMNI BUOY DATA 227 Table 1 The drag in conductivity and temperature as obtained from calibration sheets of CT sensors retrieved from BD12 OMNI buoy. Depth of CT(m) Serial Number Drift since last calibration ( C/Year) - as received Third phase of inter-comparison exercise was carried out in the BoB during April-May, 2012; almost after one year of successful operation of the buoys in the BoB. During this cruise, CTD operation was carried out near BD08, BD10 and BD12 buoy locations. The good correlation between CTD and buoy data proved the good quality of data delivered from the buoys even after one year of operation in the sea. High correlation at BD08 location, confirms the mismatch in the conductivity data found at 20m and 30m depth during the second phase of intercomparison exercise was due to the vertical displacement of the mooring along with the sharp halocline structure present at that time. The CT sensors of the BD10 OMNI buoy were working fine after maintenance operation during August-September, So an intercomparison was carried out Drift since last calibration ( C/Year) -final Drift since last Drift since last calibration calibration (psu/year) (psu/year) - final - as received 5 SBE37SIP NA 10 SBE37IM SBE37IM SBE37IM SBE37IM SBE37IM SBE37IM SBE37IM SBE37IM SBE37IM Fig. 5 Inter-comparison between the ship CTD and the OMNI buoy data in the Bay of Bengal during the service of the buoys during April-May, 2012.

5 228 INDIAN J. MAR. SCI., VOL. 45, NO. 2 FEBRUARY 2016 during this cruise and the correlation was pretty high as shown in figure 5, even the fine features in the upper 75m column was very well captured. At BD12 location the sensors were replaced with new sensors during this cruise, since the sensors have completed two years of life after the first calibration. Retrieved sensors from BD12 were sent back to the manufacturers for re-calibration. Drift values noted by the manufacturer for the set of sensors used in BD12 are given in Table 1. The drift values are only in the third decimal place for conductivity measurements while in the fourth and fifth decimal place for temperature measurements. The choice whether to in-corporate those drift values in the measurements solely depends on the accuracy level with which the user aimed at, depending on the project. Inter-comparison near BD12 location were carried out with two sets of data, one which has completed its two years life and also with the new set of CT sensors attached to the BD12 buoy during this cruise as represented by the last two panels in the figure 5. CT sensor at 5m was not working during preservice and it was replaced with new one during the service. The only CT sensor which was showing deviation of more than 2 ms/cm in conductivity value was the one at 75m depth. It has nothing to do with the drift values since the drift value noted for the particular sensor at 75m depth with serial numbersbe-37 IM 7985 is very low as listed in Table 1. Difference in conductivity value must have arised due to the time difference between the CTD observations and buoy data prior to the service. The buoy data before service corresponds to 22 GMT on 29thApril and the CTD data is for 13GMT on 30th April. The time series data from 75m from the BD12 buoy shows an increase in the conductivity value for a short period and it is reflected in the intercomparison plot too. On the other side the time difference between CTD data and post-service sub-surface CT sensor data from the BD12 location are only 1 hour and both the data correlated very well with correlation coefficient of Even the sub-surface increase in the conductivity values were very well captured inboth the measurements prior to and after service of the BD12 buoy as shown in the last two panels of figure 5. Discussion Seven OMNI buoys are placed in the BoB in order to closely monitor the temporal and spatial variations in the temperature and salinity structure of the BoB. These parameters are very crucial for studying the driving forces behind the seasonal, intra-seasonal and inter-annual variability in the BoB. The only source for subsurface data is either through Argo, moored buoys or the data collected during major experiments. Validation operation of the subsurface CT sensors of the OMNI buoys has been carried out regularly with an interval of 6 to 12 months time in order to ensure the good quality of the data. Drag values in the CT sensors are also obtained from the manufacturers while the sensors are re-calibrated after one year of operation in the sea. The CT sensor seems to be perfectly match with the ship based CTD observations even after one full year of operation in the sea which gives good confidence over the sub-surface CT sensor data delivered by the OMNI buoys. Acknowledgement We thank Ministry of Earth Sciences (MoES) for extending funding support to this program.we express our sincere thanks to Dr. M. A. Atmanand, Director, N.I.O.T. for his valuable guidance. We sincerely acknowledge the team efforts of NIOT technical and administrative staff and FugroOceanor, Norway who provided the OMNI buoys. Special thanks are due to Dr. DamodarShenoy, N.I.O., Goa for his valuable suggestions. Thanks are due to MoES, INCOIS and members of expert committee for the support of this project and for extending the ship time. References 1 Sengupta, D., P. K. Ray and G. S. Bhat, Spring warming of the eastern Arabian Sea and Bay of Bengal from buoy data. Geophys. Res. Lett., 29 (15) (2002), 1734, doi: /2002GL J. Vimala, R. Venkatesan, G. Latha and R. R. Rao, Observed buildup and collapse of warm pool in eastern Arabian Sea and Bay of Bengal from moored buoy SST records from , The International Journal of Ocean and Climate Systems, 13-22, Vol. 5(2014), doi: / Sengupta, D. and Ravichandran, M., Oscillations of Bay of Bengal Sea surface temperature during the 1998 summer monsoon. Geophys. Res. Lett., 28 (10) (2001),

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