On the nature of tidal asymmetry in the Gulf of Khambhat, Arabian Sea using HF radar surface currents

The hourly ocean surface current observations from available High Frequency Radars (HFR) have been analyzed to i) quantify the tidal constituents, ii) investigate the role of the bathymetry on shallow water tides, and iii) identify the nature of tidal asymmetry in the inverted funnel-shaped Gulf of Khambhat (GoKh) on the western shelf of India during May 2012. The HFR observations captured strong currents (similar to 2.0 m s(-1)), showing significant spring - neap variations. Tidal harmonic analysis of the HFR currents showed that the tidal currents at the mouth are dominated by the M2 constituent with amplitude similar to 4.5 times higher than that of K1. Among the shallow water constituents, M4 dominates the entire Gulf with significant amplitudes (similar to 10 cm s(-1)) and gets amplified towards the head of the Gulf (similar to 30 cm s(-1)), followed by MS4. Comparisons with sea level heights from tide gauges show a higher correlation for the major (>0.97) and shallow water (>0.96) tidal constituents in terms of the order of amplitudes of the tidal constituents. The circulation pattern in the Gulf is predominantly driven by tidal currents, with the total tidal variance being similar to 90% of the total current variance. The major contributions are from the M2 tidal currents (70-90%), except at the head, where the M4 component contributes significantly up to 40%. The interactions with shallow bathymetry are responsible for the amplification of tidal currents in this converging channel with negligible impact from stratification during May. The harmonic analysis-derived relative surface phase (2M2 - M4) and higher M4/M2 amplitude ratio at the head of the Gulf reveal that the Gulf is highly tidally asymmetric and flood dominant, indicating stronger non-linear tidal distortions in the nearshore regions. A generalized skewness-based analysis confirms the flood-dominance with positive values of a morphology-dependent asymmetry factor and skewness. Two tidal combinations (M2/M4 overtide and M2/S2/MS4 compound tide) are identified as the major contributors to the flow velocity skewness in the GoKh with mixed, mainly semi-diurnal tidal regime.