The Sundarbans Estuarine System (SES), comprising the southernmost part of the Indian portion of the Ganga-Brahmaputra delta bordering the Bay of Bengal, is India’s largest monsoonal, macro-tidal, delta-front estuarine system. The Sundarbans Estuarine Programme (SEP), covering six semi-diurnal tidal cycles during 18–21 March 2011 (the Equinoctial Spring Phase), was the first comprehensive observational programme in the SES. The 30 observation stations, spread over more than 3600 km2km2, covered the seven inner estuaries of the SES: the Saptamukhi, Thakuran, Matla, Bidya, Gomdi, Harinbhanga, and Raimangal. At all stations or time-series locations (TSLs), the water level was measured every 15 min and water samples were collected every hour for estimating salinity. We report the observed spatio-temporal variations of salinity in this paper. The mean salinity over the six tidal cycles decreased upstream and the mean range of salinity over a tidal cycle increased upstream. In addition to this along-channel variation, the mean salinity also varied zonally across the SES. Salinity was lowest in the eastern SES, with the lowest value occurring at the TSLs on the Raimangal. Though higher than at the Raimangal TSLs, the mean salinity was also low at Mahendranagar, the westernmost TSL located on the West Gulley of the Saptamukhi. Salinity tended to be higher in the central part of the SES. CTD (conductivity–temperature–depth) measurements at three stations on the Matla show a well-mixed profile. Only the Raimangal has a freshwater source at its head. Therefore, the upstream decrease of salinity in the SES is likely to be the effect of the preceding summer monsoon, which would have freshened the estuary, and the ingress of salt from the seaward end due to the tide following the cessation of of the monsoon rains. The freshwater inflow from the Raimangal leads to the lowest salinities occurring in the eastern SES. The lower salinity in the western SES also suggests inflow from the Hoogly estuary, whose freshwater source is regulated via the Farakka Barrage. At 20 of the 30 TSLs, the salinity varied semi-diurnally, like the water level, and the maximum (minimum) salinity tended to occur at or around high (low) water. The temporal variation was more complex at the other 10 TSLs. Even at the TSLs at which a tidal stand exceeding 75 min was seen in the water level, the salinity oscillated with a semi-diurnal period. Thus, the salinity variation was unaffected by the stand of the tide that has been reported from the SES.

$\bf{Highlights}$

$\bullet$ Comprehensive description of salinity variability in the Sundarbans Estuarine System (SES)

$\bullet$ Semi-diurnal variation seen at a majority of the stations and the estuaries are well-mixed

$\bullet$ Mean salinity decreases upstream and is lower in the eastern and western SES

$\bullet$ The upstream decrease is due to the preceding monsoon and the tidal ingress of salt

$\bullet$ Direct (indirect) inflow from the Ganga (Hoogly) lowers salinity in the eastern (western) SES

The West India Coastal Current (WICC) flows southward (northward) during summer (winter). We examine the nature of circulation in the region of WICC during an inter-monsoon period using hydrographic data collected during March 6–21, 1994, and archived 1994 daily altimeter data. The hydrographic data did not show any organized northward or southward Cow, implying that the amplitudes of the Rossby and Kelvin waves that make the WICC were negligible. Instead, cyclonic and anticyclonic eddies, well recorded in altimeter data, dominated the circulation. Because eddies occur throughout the year, our analysis highlights the need to study their role in WICC all through the year.