Multiscale sea level variability on the western Bay of Bengal: A study using tide gauge and satellite observations

The study examines the tidal variability and intraseasonal oscillations (ISOs) of sea level using tide gauge observations collected during the year 2014 at four stations: Gardenreach, Paradeep, Kakinada, and Ennore along the western Bay of Bengal (BoB). In the first part, the tidal analysis of sea level data shows the dominance of M2 tides followed by S2, N2, K1, and O1 at all the stations. The tides are usually semidiurnal in nature over BoB, represented through the form factor, which is maximum over Ennore (0.21) and minimum over Gardenreach (0.10). The tidal amplitudes increase gradually northward along the western BoB due to interaction with the shallow continental shelf. The topography and coastal geometry also impact the shallow-water constituents (M4 and MS4) significantly at all the locations. Another noteworthy finding is the higher amplitude of M2 tide during the post-monsoon seasons because of the higher stratification of the water column. In the second part, the presence of different non-tidal signals from the residual sea level and altimetry gridded data are studied as well. The correlation coefficient between Sea Level Anomaly (SLA) data from altimetry and the Tide Gauge data are as high as 0.94, 0.91, and 0.90 at Paradeep, Kakinada and Ennore, respectively. The spectral analysis ensures the dominance of the signals with periodicity of 20–60 days in the BoB, which is probably caused due to the monsoon ISOs and Madden Julian Oscillations. The 10–20 days oscillations are also observed with significant amplitudes primarily at Gardenreach which attributes to the Quasi-Biweekly Oscillations. The signature of seasonally varying coastally trapped Kelvin Waves is also identified on the western BoB from both the satellite and Tide Gauge data. The strength of the intraseasonal variability significantly increases during negative Indian Ocean Dipole years with respect to the positive years. The study identified the usefulness of the sea level observation to monitor the multiscale variation within the tides to the interannual scales along the Indian coast.