Transport and Dynamics of the Panay Sill Overflow in the Philippine Seas

Abstract

Observations of stratification and currents between June 2007 and March 2009 reveal a strong overflow between 400- and 570-m depth from the Panay Strait into the Sulu Sea. The overflow water is derived from approximately 400 m deep in the South China Sea. Temporal mean velocity is greater than 0.75 m s?1 at 50 m above the 570-m Panay Sill. Empirical orthogonal function analysis of a mooring time series shows that the flow is dominated by the bottom overflow current with little seasonal variance. The overflow does not descend below 1250 m in the Sulu Sea but rather settles above high-salinity deep water derived from the Sulawesi Sea. The mean observed overflow transport at the sill is 0.32 ? 106 m3 s?1. The observed transport was used to calculate a bulk diapycnal diffusivity of 4.4 ? 10?4 m2 s?1 within the Sulu Sea slab (?575?1250 m) ventilated from Panay Strait. Analysis of Froude number variation across the sill shows that the flow is hydraulically controlled. A suitable hydraulic control model shows overflow transport equivalent to the observed overflow. Thorpe-scale estimates show turbulent dissipation rates up to 5 ? 10?7 W kg?1 just downstream of the supercritical to subcritical flow transition, suggesting a hydraulic jump downstream of the sill.