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

We present current data from an acoustic Doppler current profiler (ADCP) moored on the continental shelf-break in the Gulf of Mannar (GoM) near Ram Setu during 2010–2011. The observations show that the near-surface, sub-inertial current flows primarily north-westward all round the year. This northwestward flow is punctuated by intraseasonal bursts that last a few weeks. Compared to the current off Kanyakumari, the intraseasonal fluctuations in the GoM are weaker and in the opposite direction. The flip in direction is linked to eddy-like circulations, which cause the current to bifurcate or merge in the region between Kanyakumari and Ram Setu.

$\bf{Highlights}$

$\bullet$ Ocean current data collected in Gulf of Mannar.

$\bullet$ Near-surface current is north-westward over most part of the year.

$\bullet$ Current shows intraseasonal variability.

$\bullet$ Intraseasonal variability in the Gulf of Mannar current is weaker and in opposite direction compared to off Kanyakumari current.

$\bullet$ The opposite direction of Gulf of Mannar and off Kanyakumari currents is linked to local changes in the sea level.

The region between the southern tip of the Indian subcontinent and the equator links the Arabian Sea and the Bay of Bengal. A significant exchange of mass, heat and salt occurs in the region via two major current systems: the monsoon current (dominant annual cycle) and the equatorial current (dominant semi-annual cycle). This paper presents observations from deep-sea mooring programmes and describes the rationale for maintaining a mooring array along 77.4°E to monitor both current systems. Analysis shows that the semi-annual to annual cycle transition occurs at 2–2.5°N, with the winter monsoon current peaking at 5°N, the summer monsoon current at 3–6°N, and the equatorial current at 0–1°N.