Subtidal variability of near-surface salinity in the Cochin estuary, southwest coast of India: Insights from year-long continuous observation

Abstract

A significant knowledge gap exists about the subtidal variations in salinity in Indian monsoonal estuaries due to a lack of continuous data. To address this, we collected high-resolution, long-term near-surface measurements from the Cochin estuary in southwestern India. A wavelet spectral analysis of this data reveals notable subtidal variability in the estuary, with periods ranging from 10 to 90 days. This indicates the presence of intraseasonal oscillations throughout the year, from December 2019 to May 2021. During the wet monsoon season (June–September), subtidal variations in near-surface salinity show amplitudes typically oscillating between 5 and 10 with an average salinity of 6.5, occasionally matching the dominant tidal amplitudes. In the dry season (December to April), the average surface salinity is around 30, and the subtidal oscillation amplitude typically ranges from 3 to 5. We compared the observed subtidal salinity variability with other environmental data, such as rainfall, residual sea-level variability near the estuary's mouth, and water-level variability upstream in the riverine channels. This comparison is used to speculate on the processes controlling the observed subtidal variability. We explored two possible processes that could cause subtidal salinity variability. The first process is the influence of 30–60 day oscillations associated with Monsoon Intraseasonal Oscillations (MISO). MISO involves the northward movement of cloud bands from the equatorial Indian Ocean towards the Himalayan foothills. We present evidence suggesting that coherent oscillations exist in the estuary within the MISO band. The second process relates to the influence of shelf wave propagation along the coast. During the dry season, we observed coherent oscillations in both surface salinity and sea level within a quasi-biweekly (10–20 days) period. The presence of shelf wave propagation along the coast during this season may explain the quasi-biweekly subtidal salinity variations in the estuary. Our findings have broader implications for understanding subtidal estuarine dynamics in monsoonal regimes of the Indian Ocean and other similar regions.