On the cyclonic eddy generation in Panay Strait, Philippines

High Frequency Doppler Radar (HFDR), shallow pressure gauges and Acoustic Doppler Current Profiler (ADCP) time-series observations during the Philippine Straits Dynamics Experiment (PhilEx) were analyzed to describe the mesoscale currents in Panay Strait, Philippines. Low frequency surface currents inferred from three HFDR (July 2008 { July 2009), revealed a clear seasonal signal in concurrent with the reversal of the Asian monsoon. The mesoscale cyclonic eddy west of Panay Island is generated during the winter northeast (NE) monsoon. This causes changes in the strength, depth and width of the intra-seasonal Panay coastal jet as its eastern limb. Winds from QuikSCAT satellite and from a nearby airport indicate that these flow structures correlate with the strength and direction of the prevailing local wind. An intensive survey of the cyclonic eddy in February 8-9, 2009, obtaining a 24-hour successive cross-shore Conductivity-Temperature- Depth (CTD) sections in conjunction with shipboard ADCP measurements showed a well- developed cyclonic eddy characterized by near-surface velocities reaching 50 cm/s. This observation 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 showed a pronounced signal of upwelling, indicated by the doming of isotherms and isopycnals. A pressure gradient then was sets up, resulting in the spin-up of a cyclonic eddy in geostrophic balance. Evaluation of the surface vorticity balance equation suggests that the wind stress curl via Ekman pumping mechanism provides the necessary input in the formation and evolution of the cyclonic eddy. In particular, the cumulative effect of the wind stress curl plays a key role on the generation of the eddy. The Beta-effect on the other hand may led to propagation of the eddy westward.