Observed tidal currents on the continental shelf off the west coast of India

Abstract

Observed currents from ADCPs, deployed at 100 m and 150 m depths, on the continental shelf at five different locations along the west coast of India, were used to study the characteristics of both barotropic and baroclinic tidal currents. The observations extended over a 6-month period (March–August), which includes two seasons, pre-monsoon (March–April) and southwest (SW) monsoon (May–August) during 2008, 2009 and 2011. Tidal ellipses, constructed for M₂ and K₁ constituents, show that barotropic tidal currents propagate in along-isobath direction at the southern shelf (off Kollam at about 9°N) and they are oriented more or less in a cross-isobath direction at the northern shelf locations (off Jaigarh and Mumbai at about 17°N and 20°N, respectively). Maximum cross-isobath tidal current is found at northern shelf locations (for instance, about 32 cm s⁻¹ off Mumbai) than those in south (about 10 cm s⁻¹ off Bhatkal, at 13°N). This could be due to the amplification of semidiurnal tidal currents from south to north of the shelf due to an increase in shelf widths towards north. The rotary spectra of baroclinic currents showed large peaks at semidiurnal and diurnal bands with a dominant clockwise rotation showing the presence of strong internal tidal currents. Semidiurnal variability occurs mainly in M₂ and S₂ and diurnal variability occurs mainly in K₁ and O₁. An increase in the amplitude of semidiurnal and diurnal internal tide is apparent when the stratification on the shelf increases from pre-monsoon to SW monsoon period. The presence of strong internal tide during May to August is attributed to increased seasonal stratification on the shelf. EOF analysis showed that the first three modes are sufficient to describe most of the variability in both semidiurnal and diurnal internal tides on the shelf, as they represent about 70–90% of total variance. The small scale vertical shear in the velocity field, induced by diurnal internal tide, is found to be larger than that induced by semidiurnal internal tide.