Chiranjivi Jayaram

Daily composite wind fields from Oceansat-2 scatterometer

Oceansat-2 scatterometer (OSCAT) is an active microwave sensor, intended to provide ocean surface wind vectors over the global oceans. In the present work, an attempt has been made to generate daily composites of OSCAT Level-3 (L3) wind vectors using Data-Interpolating Variational Analysis (DIVA) method from ascending and descending passes over the Indian Ocean region. This could be useful for operational purposes and in generating value-added products like wind stress and curl of wind stress. The daily composite wind vectors of zonal (U) and meridional (V) components have been validated by comparing with Advanced Scatterometer (ASCAT) and wind from in situ buoys for the year 2012. Wind composites thus generated using DIVA are found to match well with in situ, and ASCAT wind products. Minor deviations are observed with respect to ASCAT wind, which could be attributed to the difference in interpolation techniques used for the two scatterometer products. Given that the repeat period of ASCAT is 5 days and that of OSCAT is only 2 days, OSCAT wind products could be conveniently used for real-time met-ocean studies.

Delineation of spatio-temporal changes of shoreline and geomorphological features of Odisha coast of India using remote sensing and GIS techniques

Odisha coast is a dynamic region wherein both natural and anthropogenic processes affecting the shape and position of the shoreline vis-à-vis the coastal processes are prevalent in this region. Geomorphology such as lagoons, tidal flats, mangrove swamps and shoreline is a rapidly changing phenomenon along this coast mainly due to tides, waves, oceanic currents and storm surges. Keeping in view of the above, the present study mainly focused on decadal changes of shoreline and land cover changes, and delineation of geomorphological units along the Odisha coast using geospatial technologies for the period 1990–2009. Spatio-temporal analysis of remote sensing data indicated notable shoreline changes at cyclone land fall locations along the Odisha coast, wherein erosion (loss) and accretion (gain) rates were observed to be high at Paradeep and Kendrapada. During the study period, the rate of accretion was observed to be higher during 1999–2009 compared to 1990–1999. Coastal geomorphology landforms distributions indicate that the swale complex, older and younger coastal plains, tidal flats and mangrove swamp are the dominant features. Long-term land cover analysis indicated that the vegetation cover loss happened in the high vegetated areas during the period 1990–1999 and regeneration during the period 1999–2009.

Interannual variability of chlorophyll-a concentration along the southwest coast of India

The interannual variability of chlorophyll concentration along the southwest coast of India is studied using remote-sensing data from SeaWiFS. The data are analysed in conjunction with satellite-measured sea surface winds. The satellite-measured chlorophyll data for a period of 10 years from 1998 to 2007 were made use of for indexing the maximum offshore extent of chlorophyll along the coast for each month. From the empirical orthogonal functional analysis of chlorophyll data, it is observed that the dominant mode is annual. Interestingly, intraseasonal variability and the influence of climatic events like El Niño are observed in the secondary principle component of the time series. The variability of chlorophyll coincided well with variability of Ekman transport all along the coast with higher chlorophyll (>1 mg m−3) when the Ekman transport is greater than 1000 kg/m/s. During the years 2005–2007, reduction in the meridional (along shore) component of wind resulted in reduction of Ekman transport, the phenomenon which leads to a decrease in chlorophyll. This is due to the reduction in the amount of nutrients that entrained to surface layers during upwelling of the southwest monsoon. The chlorophyll-a is minimum when Ekman transport is less than 0.5 kg/m/s on the normalized scale. For higher values of chlorophyll, the Ekman transport is higher, indicating the contribution of wind in enhancing the already upwelled chlorophyll production. The smaller value of R 2 infers that there exist other forces as well involved in augmenting the surface chlorophyll. The enhanced knowledge on the offshore extent and the intraseasonal and interannual variability of chlorophyll can provide valuable inputs on fisheries and primary productivity for this region.

Analysis of gap-free chlorophyll-a data from MODIS in Arabian Sea, reconstructed using DINEOF

ABSTRACT The chlorophyll-a concentration (chl-a), which is an index of phytoplankton pigment present in the oceans, is considered as a key indicator of health of marine ecosystems that could have direct effect on the human life. In this study, spatial and temporal variability of chl-a in the Arabian Sea (AS) is examined using reconstructed cloud-free ocean colour data for the period 2002–2015. Data Interpolating Empirical Orthogonal Function method is used to reconstruct the missing data. Subsequently, wavelet analysis is applied on the reconstructed data to assess the temporal variability in terms of seasonal, intra-seasonal, and interannual variability of chl-a in the AS. Wavelet analysis clearly depicted the low-frequency, stationary modes or approximation levels inferring the monthly, seasonal, and annual mean of the signal, while the high-frequency, non-stationary modes indicated the local abnormalities. From the analysis of gap-free data, the presence of biennial mode of variability in the northern AS chl-a is observed. The analysis further showed the existence of intra-seasonal oscillations in the northern AS during summer monsoon and single dominant peak during winter monsoon. Chl-a appeared to decline slightly during the entire study period across all the selected regions of the AS. Also, it is observed that chl-a in the northwestern region is highly dynamic than in the other regions of the AS.

Spatio-temporal evolution of chlorophyll-a in the Bay of Bengal: a remote sensing and bio-argo perspective

Argo floats equipped with sensors to measure Dissolved Oxygen, Chlorophyll-a and backscattering are deployed in the Arabian Sea, Bay of Bengal and Southern Indian Ocean as part of Indian Argo program. In this study, abnormal chlorophyll-a bloom observed by a float with WMO ID 2902086 deployed in the south central Bay of Bengal is analyzed. High concentration of chlorophyll > 0.8 mg/l is observed during December 2013. This period is also associated with drop in temperature and increase in salinity. Analysis of data from the bio-Argo float has shown the impact of many cyclones and depressions that occurred during the period. Of particular importance is cyclone ‘Madi’, which passed very near to the position of mentioned float, during December 2013. This is also evident from the satellite based wind observations from OSCAT through curl of wind stress and Ekman pumping. The sub-surface chlorophyll bloom is substantiated by the surface chlorophyll-a values of MODIS during the period. Intense mixing caused due to the passage of cyclone might have resulted in mixing of subsurface waters thereby breaking the stratification of otherwise stable surface waters of Bay of Bengal, enhancing the nutrient supply, which resulted in strong chlorophyll bloom. The subsurface chlorophyll structure of Bay of Bengal and its variability during the passage of cyclone is for the first time revealed by the floats equipped with biological sensors. This work reveals the synergistic application of in-situ (Bio- Argo) and satellite data to monitor the changes in subsurface structure during the passage of cyclones.

Evaluation of Aquarius Sea Surface Salinity With Argo Sea Surface Salinity in the Tropical Indian Ocean

The performance of sea surface salinity (SSS) obtained from the Aquarius satellite is evaluated. Daily, weekly, and monthly Aquarius SSS data between September 2011 and August 2013 from the tropical Indian Ocean (TIO) are compared with the surface most salinity observations (<; 5 m) from Argo floats. Statistics between a daily reporting Argo float (WMO 2901329) and daily Aquarius SSS resulted in a root-mean-square error (RMSE), a bias, and a correlation of 0.32 psu, 0.02 psu, and 0.81, respectively. Weekly 1° × 1° spatial resolution SSS was produced from the Argo data using variational analysis and was compared with Aquarius SSS data. RMSE was observed to be between 0 and 0.25 psu over most of the region. Bias was observed to be within 0.3 psu everywhere, except in the southeastern Arabian Sea and the southeastern TIO. Good correlation (> 0.6) was observed everywhere, except the coast of Oman, western equatorial Indian Ocean, and south of 20° S. A seasonal comparison has also revealed that both Aquarius and Argo are in agreement; in addition, the Aquarius SSS clearly shows the seasonal salinity cycle of the TIO.

Remote Sensing of Upwelling in the Arabian Sea and Adjacent Near-Coastal Regions

Upwelling is a dominant mechanism in the Arabian Sea that occurs annually during southwest monsoon summer season. This results in abundance of phytoplankton and zooplankton in the region and has profound influence on the coastal fisheries. During the southwest monsoon, an intense low-level wind jet blows diagonally across the Arabian Sea generating coastal upwelling along the coasts of Somalia, Oman and the southeastern Arabian Sea. In this study, a synergy of different parameters like sea surface winds, chlorophyll (chl-a), sea surface temperature (SST) and sea level anomaly (SLA) retrieved from remote sensing were used to make a more detailed analysis on upwelling features for the summer seasons of the years 1982–2015. From the analysis, it is observed that upwelling in the Arabian Sea is not homogeneous across the basin despite being driven by monsoon winds. During the study period, Ekman transport, SLA and SST anomaly showed positive trend, whereas chlorophyll showed negative trend of varying strengths. Increased Ekman transport has not generated increased productivity indicating the role of other governing mechanisms on the availability of nutrients in the region.

Variability of Total Suspended Matter in the Northern Coastal Bay of Bengal as Observed from Satellite Data

The coastal regions of northern Bay of Bengal (BoB) are one of the most turbid areas owing to the large freshwater discharge from the three major river systems. This study is conducted to investigate the seasonal and interannual variability of total suspended matter (TSM) in the northern BoB. TSM concentration derived from medium resolution imaging spectrometer for the period 2002–2011 is used for this study. TSM concentration is observed to be the highest during summer monsoon season along the north-eastern region, off the Ganges–Brahmaputra river mouth. The variability of the TSM concentration depicts the role of river runoff associated with the summer monsoon in influencing the same. It is observed that the sediment concentration tapers away towards the offshore regions with the maximum extent observed up to 21.5°N latitude. Interannual variability is also observed with highest TSM concentrations occurring in the years 2003, 2008 and 2011 and least during 2004. Time series analysis performed at three major river discharge regions illustrated the distinct and highly variable nature of TSM dynamics prevailing in the northern BoB.