Deep-sea moorings in the equatorial Indian Ocean were first deployed by India in the year 2000, and currents were measured at three locations (77$^{\circ}$E, 83$^{\circ}$E, and 93$^{\circ}$E) on the equator. In this paper, we present two decades of current observations from these moorings and discuss how the moorings have evolved with time. The observations show that the 180-day (90-day) period dominates the surface and mid-depth (bottom) circulation. Though the Wyrtki Jets are strong, the near-surface currents do not show any clear semi-annual reversals. The reversals become evident only below 100 m.

$\bf{Highlights}$

$\bullet$ Twenty years of ocean current data collected at the equatorial Indian Ocean

$\bullet$ Bottom current data show strong intraseasonal variability

$\bullet$ Upward phase propagation associated with the semi-annual cycle more prominent for the mid-depth currents

$\bullet$ The semi-annual reversals not evident every year near the surface

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.

Data from a high-resolution sediment core off Goa in the eastern Arabian Sea (EAS) show that the Holocene surface-salinity variation off Goa contains four alternating high- and low-salinity events. These events are in contrast with the Bay-of-Bengal (BoB) surface-salinity variation after 5 ka BP, suggesting an asymmetry in the rainfall associated with the Indian summer monsoon over the eastern and western parts of the Indian subcontinent and its surrounding seas. This zonal asymmetry in rainfall is also seen in modern rainfall data. The historical rainfall over the Indian subcontinent indicates that the Northwest India and West Peninsular India and their rainfall subdivisions, which feed freshwater to the EAS, are mutually strongly correlated, but they are not correlated with Northeast India and North Central India and their subdivisions, which feed freshwater to the BoB. This mid-Holocene zonal asymmetry in rainfall over the eastern and western parts of the subcontinent appears to have sustained through to modern times. The Holocene salinity events off Goa are closely comparable to the evolution of Harappan Civilization in the Indus Valley, suggesting that the Holocene salinity variation in the EAS is connected to, and is a reliable indicator of, rainfall over the Harappan Civilization Region.

$\bf{Highlights}$

$\bullet$ High-resolution core data off Goa show four alternating high- and low-salinity events during the Holocene.

$\bullet$ These events are coherent with the Bay of Bengal (BoB) surface-salinity variation till ~5 ka BP, but diverge thereafter.

$\bullet$ This zonal contrast between the eastern and western parts of the Indian subcontinent is also seen in modern rainfall data.

$\bullet$ This zonal asymmetry in rainfall may be associated with the northward propagation of rain bands and northwestward movement of low-pressure systems.

$\bullet$ The analysis favours a flood-forced decline of the Harappan Civilisation.

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.