Characteristics of astronomical tides and their modulation on sea level extremes along the Indian coast

In this study, long-term hourly sea-level records from 18 tide gauge stations during 1972-2007 were analyzed to study the characteristics of astronomical tides, sea-level trends, and extremes around India's mainland. The de -tided signals were used to estimate these parameters and study tide and surge interaction. The observed sea level depicts significant variability in daily, seasonal, and inter-annual time scales. Semidiurnal tides are the most dominant among the high-frequency tides, with an amplitude of up to 2.5 m for M2 and 0.75 m for S2 tides in the northwestern and northeastern continental shelf and reduced to 0.25 m in the south. The amplitude of diurnal tides (O1 and K1)is relatively weak (<0.25m) at all the stations. The annual harmonics dominate the seasonal cycle, with an amplitude of 0.7 m at Garden Reach in the northeastern continental shelf, decreases to 0.15 m at Tuticorin in the south, and remains small (<0.1 m) along the west coast of India. The amplitude of lunar nodal and perigee tides are significantly high (up to 25 mm) at several stations compared to the long-term global mean sea level trend (similar to 3.3 mm/y). The sea level trend is significantly positive (up to 4 mm/y) for Sagar Island, Diamond Harbor, Haldia, and Mangalore; negative (up to-3 mm/y) for Garden Reach and Okha, and in-significant (0.5 mm/y) for Mumbai, Vishakhapatnam, and Paradip. Interaction between the semidiurnal tides and surges was intense at most stations, with a high probability of surge peaks during the fall-tide conditions in the northern continental shelf, at rising-tidal conditions for the south-eastern and western peninsula, and co-occurring at high tide for southern stations. The degree of tide-surge interaction increases from south to north with an increase in tidal range and significant nodal and perigee tidal modulation.