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.