Observations of exchange between the South China Sea and the Sulu Sea

[1] The velocity and transport, with thermohaline properties, are determined from simultaneous moored time series within Mindoro, Tablas and Panay Straits that connect the South China Sea (SCS) and Sibuyan Sea to the Sulu Sea. These passages provide the only pathway for the SCS throughflow below thermocline depths and play a critical role in regulating the heat and freshwater balance within the Philippine archipelago. The upper layer in Mindoro Strait has a distinctly seasonal cycle with northward flow during the boreal summer southwest monsoon and southward flow during the winter northeast monsoon. In contrast, upper layer flow in Panay and Tablas Straits is intraseasonal. Extraordinarily strong pulses that begin at intermediate depth in the fall transition and shoal toward the sub-thermocline during the northeast monsoon are found in Mindoro and Panay Straits. These southward flows are strongly correlated to the SCS large-scale circulation and remote wind-forcing off Vietnam. Temperature and salinity in Mindoro support the SCS source of this Subtropical Water. Southward benthic flow in Mindoro and Panay act to ventilate the deeper Sulu Sea. Bottom temperatures suggest the cooler Mindoro overflow mixes with the warmer benthic waters from Tablas before exiting Panay Strait into the Sulu Sea. The 2008 mean annual total-depth transports were very small, O(0.1 Sv), although individual transport estimates of 1–2 Sv were observed in all depth layers over the year. The mooring deployment coincided with a strong La Niña when the Pacific inflow into the SCS is expected to be weaker, and subsequently transports are lower.