Barotropic and baroclinic tides in Panay Strait, Philippines

Abstract

The Panay Strait constitutes a bathymetrically complex system with intense tidal currents. The four major tidal constituents in the total energy spectra inferred from sea level and current profiles are K1, O1, M2, and S2. Spatially, O1 and M2 dominate over K1 and S2, respectively. The diurnal tide accounts for highest amplitude variability over the shallow shelf while semi-diurnal tides over the deeper channel of the strait. Rotary spectra of surface currents shows inertial frequency peaks and exhibit an unusually broad peak in both the clockwise (CW) and counterclockwise (CCW) rotating components, indicating frequency shift by the vorticity of sub-inertial currents prevalent in the region. Vertically, variance of the horizontal velocity explained by the major tidal constituents peaks in two distinct depth bands; the upper layer centered at 110 m (11% variance) dominated by semi-diurnal tide (M2) and the lower layer at 470 m (26% variance) dominated by diurnal tides (O1). Semi-diurnal tidal current ellipses (M2 and S2) exhibit a dominance of CW motions at near-surface depth (110 m), indicative of downward energy propagation and implying a surface energy source. These features from Acoustic Doppler Current Profiler (ADCP) deployed close to the sill is consistent with the dominant semi-diurnal tide (M2) over the channel of the strait from the High Frequency Doppler Radar (HFDR). Comparison of incoherent to coherent tidal energy shows K1 dominated the incoherent tidal band. Spatially, incoherent energy is dominant over the channel particularly near the sill and the constricted part of the strait. The incoherent portion of the tide is presumably attributable to the surface expression of the internal tide which seems to be generated near the sill and then topographically steered west over the edge of the shallow shelf where incoherent energy is dominant.