Northern Bay of Bengal (BoB) receives a significant amount of freshwater, from precipitation and river discharge, which makes it comparatively low saline, specifically during August/September. However, the factors that determine the dispersal of this freshwater within the BoB remain relatively less addressed. Analysis of the near-surface salinity data from a moored buoy located in the northern BoB during 2011–2019 supplemented with satellite and model data showed prominent interannual variability viz. time of occurrence, persistence and magnitude of freshening. The surface salinity exhibit significant lowering during 2011, 2015 and 2017 with unique prolonged freshening in 2017. The offshore advection of low salinity water is initiated with sustained low wind stress further favoured by existing surface circulation and mesoscale eddies contributing to the unique characteristics. In addition, the maximum freshening observed in 2011 coincides with positive Indian Ocean Dipole event, whereas that of the prolonged freshening in 2017 is favored with anomalously low wind stress.

Wave energy is the cleanest perennial source of energy which is site-specific and highly dynamic in nature. Wave power characteristics in the central Arabian Sea with swell dominant conditions are accurately assessed from a decade of deep water moored buoy measurements. The ECMWF ERA5 global wave model deviations are estimated by comparing with observations for seven common bulk wave parameters. The monthly variation of correlation and error statistics based on 10 years data is analyzed. Wave parameter ratios between the observations and model are derived for each month based on the longterm statistics for seven common parameters. The wave period of chosen numerical model is multiplied by a 'constant' to arrive at the equivalent single-peaked wave energy spectrum to that of the measured spectrum of mixed seas. This constant or wave energy period ratio, is site specific and varies based on the theoretical spectrum used in the model and spectral shape. Hence, it is very important to use the correct constants pertaining to Indian waters for the accurate assessment of wave power in Indian seas. The site specific wave energy period ratios, with respect to different model periods and other observed periods based on the observed interannual statistics are derived. This location is under the influence of monsoons, swells coming from the Southern Indian Ocean as well as cyclones. The seasonal variability of wave characteristics controls the sustained wave power. The maximum power depends on monsoon or intense cyclones that are on the verge of increase due to climate change. The interannual variability of maximum and sustained wave power are quantified for each month, which aids in the design criteria of wave energy converters. Nine predominant cyclones that showed response at the buoy location over the decade are analysed for peak wave power. This study helps in the validation of numerical model as well as in the transformation of deep water wave power to the coastal waters of a given depth using modification coefficient as an approximate estimate.

The moored buoy network in North Indian Ocean is established in 1997 with a primary objective to support cyclone early warning services. The long-term moored buoy measurements provided many new insights of the upper ocean dynamics during the passage of cyclones in North Indian Ocean. However, the realtime availability of the data set was appreciated much more owing to its utility in weather forecasts and ocean state forecast by providing the critical information about the remote marine environment. Controlled high frequency transmission limited to cyclone period was incorporated in moored buoys as a trade off between the operational efficacy and the demand for higher transmission rate. The rapid mode transmission successfully triggered during many cyclones and provided high frequency real-time data sets, which is appreciated by regional and global scientific community.

The moored buoy network in the Indian Ocean revolutionized the observational programs with systematic time-series measurement of in situ data sets from remote marine locations. The real-time meteorological and oceanographic data sets significantly improved the weather forecast and warning services particularly during extreme events since its inception in 1997. The sustenance of the network requires persistent efforts to overcome the multitude of challenges such as vandalism, biofouling, rough weather, corrosion, ship time availability, and telemetry issues, among others. Besides these, the COVID-19 pandemic constrained the normal functioning of activities, mainly by delaying the maintenance of the network that resulted in losing a few expensive buoy system components and precious data sets. However, the improvements in the buoy system, in-house developed data acquisition system, and efforts in ensuring the quality of measurements together with “best practice methods” enabled 73% of the buoy network to be functional even when the cruises were reduced to 33% during the COVID-19 lockdown in 2020. The moored buoys equipped with an Indian buoy data acquisition system triggered high-frequency transmission during the Super cyclone Amphan in May 2020, which greatly helped the cyclone early warning services during the COVID-19 pandemic. The COVID-19 lockdown points toward the reliability and enhanced utility of moored buoy observations particularly when other modes of measurements are limited and necessitates more such platforms to better predict the weather systems. The present study analyzed the enhancement of the buoy program and improvisation of the buoy system that extended the life beyond the stipulated duration and enabled the high-frequency data transmission during cyclones amid the COVID-19 lockdown. The recommendations to better manage the remote platforms specifically in the event of a pandemic based on the operational experience of more than two decades were also presented.

Temperature and salinity are essential ocean variables for understanding the oceans' physical processes. The conductivity and temperature measurements are used for deriving ocean salinity. Conductivity-temperature (CT) sensors mounted on moored buoys are widely used to collect sustained time-series observations of temperature and salinity. However, these measurements are prone to drifts that need to be corrected to ensure data quality. The present study evaluates a field validation technique to correct the drift in subsurface temperature and conductivity measurements that can potentially complement the standard calibration procedure performed by the original equipment manufacturer. The advantage of field validation is that it could be carried out soon after retrieving CT sensors, which ensures that the physical conditions and configuration of the retrieved CT sensors remain unaltered from that of the in-situ conditions. The drift in the CT sensors was analyzed by pre-deployment and post-retrieval field validation of CT sensors. A correction is applied to the raw data assuming a linear trend in drift with time. An ice test to identify and correct the errors in the timestamp of CT measurements is also discussed.

Amphan, a category-5 tropical cyclone, originated over Bay of Bengal (BoB) and had a landfall in West Bengal, India on 20 May, causing havoc in the region. In this study, in-situ buoy and various satellite measurements are used to analyse the ocean condition before and after the storm, primarily from the air-sea interaction perspective. Widespread anomalous warming was observed in BoB before the event, due to high net surface insolation received by the ocean. The warm SST anomalies in the central BoB were coincident with anti-cyclonic warm core eddies, implying availability of higher oceanic heat content. Observations from BD13 buoy, close to the cyclone track showed heating of the overlying atmosphere due to this ocean warming. Strong surface cooling was observed after passage of the cyclone due to wind induced upper-ocean mixing that is stimulated by low stratification in BoB.

The pre-monsoon cyclone Viyaru in the Bay of Bengal during May 2013 traversed a long track from 5 degrees N to 22 degrees N over 7 days with basin-wide response, which was well captured by the time series observations of OMNI buoy network along with satellite data. The differential upper ocean characteristics and its variable response reveal that vertical mixing override horizontal advection during cyclone passage. This study provides insight into the variability in wave spectra, differential response on either side of the track and presence of cold core eddy combined with a thick barrier layer in modulating the upper ocean response.

The Bay of Bengal (BoB) is a low saline basin owing to large influx of freshwater from precipitation and river runoff. To maintain the salt balance of the BoB, the incessant lowering of salinity is to be balanced by the inflow of saltier water into the basin. In the present work, various processes that contribute to the saltening of the BoB, viz. coastal upwelling, eddies and their interaction, lateral advection from Arabian Sea and tropical cyclones are discussed. In the near-shore regions, the coastal upwelling due to wind induced Ekman transport plays a dominant role in increasing the surface salinity. On the other hand, in the open ocean, the divergence induced by eddies and their mutual interaction contributes significantly to the salt water pumping. In the southern BoB, the advection from the Arabian Sea increases the salinity. The formation of cyclones in the BoB also leads to an increase in the surface salinity. However, the magnitude of saltening of the Bay due to these processes varies from north to south. The uplift of saltier water from subsurface levels increases the salinity in the surface layers thereby creating a salinity gradient and a salinity front.