Study of Internal Tides characteristics in the Eastern Arabian Sea

Internal tides, internal waves of tidal frequency, are generated by the flow of barotropic tidal currents over topography. Arabian Sea is a region of the northwestern Indian Ocean bounded to the east by the Indian peninsula. Though Arabian Sea and Bay of Bengal reside almost at the same latitudinal belt yet there is difference in the tide’s properties at the two basins. Internal tides in the Arabian Sea are complex and recent modeling studies have suggested that the semidiurnal internal tides show the largest seasonal variability among other regions in the world. However, the generation and propagation mechanism of internal tides, as well as their temporal variability, are unknown in this region. Therefore, we used in-situ observations collected at AD09 (8°N, 73°E) from November 2018 to December 2019 in this study. Salinity has a major role in governing the near-surface stratification, whereas temperature fluctuations govern the subsurface stratification at this location. The rectilinear zonal flow dominates the ellipticity of both semidiurnal and diurnal motions, indicating the generation of internal tides at the slopes. The maximum isopycnal displacement is observed during April at 100 m depth. Furthermore, the semidiurnal barotropic tides rotates in the clockwise direction, while the diurnal rotates in a counter-clockwise direction. Moreover, baroclinic semidiurnal tidal currents rotate anticlockwise at all depths, whereas diurnal tidal currents rotate both clockwise and anticlockwise at various depths. The strongest baroclinic currents, based on the magnitude of the semi-major axis for K₁ are found near 100 m, dominated by rectilinear flow, whereas for M₂, they are found at depths below 125 m. The maximum kinetic energy of the internal wave is observed at 90 m depth, and the analysis shows both diurnal and semidiurnal frequency dominates in the Arabian Sea, as the constituents M₂, S₂, K₁, and O₁ forms the most energetic part of the spectrum. In contrast, on the eastern part of the Indian peninsula in the Andaman Sea and Bay of Bengal, semidiurnal frequency dominates. Arabian Sea exhibits remarkable seasonal variability driven by the Indian monsoon system and seasonal variations in stratification influence the properties of internal tides in this basin. Hence, the study provides a major insight into the characteristics of internal tides over eastern Arabian Sea region.