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Observations of Surface Currents in Panay Strait, Philippines

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Title:Observations of Surface Currents in Panay Strait, Philippines
Authors:Repollo, Charina Lyn
Date Issued:Dec 2016
Publisher:[Honolulu] : [University of Hawaii at Manoa], [December 2016]
Abstract:High Frequency Doppler Radar (HFDR), shallow pressure gauges (SPG) and Acoustic Doppler Current Profiler (ADCP) time-series observations during the Philippine Straits Dynamics Experiment (PhilEx) were analyzed to describe the tidal and mesoscale currents in Panay Strait, Philippines.
Low frequency surface currents inferred from three HFDR (July 2008 – July 2009), reveal a clear seasonal signal concurrent with the reversal of the Asian monsoon. A mesoscale cyclonic eddy west of Panay Island is generated during the winter Northeast (NE) mon- soon. This causes changes in the strength, depth and width of the intraseasonal Panay coastal (PC) jet as its eastern limb. Winds from QuikSCAT and from a nearby air- port indicate that these flow structures correlate with the strength and direction of the prevailing local wind. An intensive survey in February 8-9, 2009 using 24-hour of successive cross-shore Conductivity - Temperature - Depth (CTD) sections, which in conjunction with shipboard ADCP measurement show a well-developed cyclonic eddy characterized by near-surface velocities of 50 cm/s. This eddy coincides with the intensification of the wind in between Mindoro and Panay Islands generating a positive wind stress curl in the lee of Panay, which in turn induces divergent surface currents. Water column response from the mean transects show a pronounced signal of upwelling, indicated by the doming of isotherms and isopycnals. A pressure gradient then is set up, resulting in the spin-up of a cyclonic eddy in geostrophic balance. Evolution of the vorticity within the vortex core confirms wind stress curl as the dominant forcing.
The Panay Strait constitutes a topographically complex system that is the locale of intense tidal currents. The four major tidal constituents in the total energy spectra inferred from sea level and current profile are K1, O1, M2, and S2. In terms of spatial variability, O1 and M2 are the dominant diurnal and semi-diurnal constituents, respectively. The diurnal tide accounts for the highest variability over the shallow shelf while semi-diurnal tides dominate over the deeper channel of the strait. In addition, inertial frequency peaks and exhibits an unusually broad spectra between the clockwise and counterclockwise components, possibly shifted by the vorticity of sub-inertial currents prevalent in the region. Vertically, major tidal components in the velocity profile appear in two distinct layers: at 110 m, 10.7% of the variance is associated with semi-diurnal tides, and at 470 m, 16.6% of the variance is due to diurnal tides (K1 and O1). Tidal current ellipses of semi-diurnal constituents (M2 and S2) exhibit a dominant clockwise motion in time at near-surface depth (110 m), indicative of downward energy propagation and implying a surface energy source. These features observed in the ADCP deployed close to the sill may explain the dominant semi-diurnal tide from the HFDR over the channel of the strait. Comparison of incoherent to coherent tidal energy shows coherent energy is dominant over the shallow Cuyo shelf for both diurnal and semi-diurnal tides while incoherent energy is stronger over the channel, distinctly over 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 is topographically steered west over the edge of the shallow shelf where incoherent energy is dominant.
Description:Ph.D. University of Hawaii at Manoa 2016.
Includes bibliographical references.
Appears in Collections: Ph.D. - Oceanography

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