Seasonal upper circulation in the Sulu Sea from satellite altimetry data and a numerical model

Abstract

Eight years of Absolute Dynamic Topography from satellite altimetry data are used to study the seasonal variability of the circulation in the Sulu Sea (SS) through Empirical Orthogonal Function (EOF) analysis. It is revealed that first seasonal EOF mode shows a basin‐scale anticyclonic/cyclonic circulation in summer/winter and second seasonal EOF mode shows a weak basin‐scale anticyclonic/cyclonic meander flow from the Mindoro Strait to the Sibutu Passage, and the typical surface circulation structure in the SS is shown as a basin‐scale anticyclonic/cyclonic circulation (or meander) centered at about 120.8°E, 8.6°N in August and December. According to the numerical experiments by a connected single‐layer and two‐layer model, it is shown that the upper circulation in the SS is closely related to the outflow via the Sibutu Passage and seasonal local wind stress. Either an outflow via the Sibutu Passage or the summer monsoon may cause an anticyclonic circulation in the SS, while the winter monsoon may cause a cyclonic circulation. Either an outflow via the Sibutu Passage or the winter monsoon would push the water out of the SS via the Balabac Strait but bring the water into the SS via the Mindoro Strait, while the summer monsoon would bring the water into the SS via the Balabac Strait but push the water out of the SS via the Mindoro Strait. Thus, in summer, the induced anticyclonic circulation with the negative relative vorticity is stronger in the SS but the water transports via the Mindoro and the Balabac straits are less, while in winter, the induced cyclonic circulation with the mainly positive relative vorticity is weaker but the water transports via the Mindoro and the Balabac straits are larger. The inflow via the Mindoro Strait is also significant since the outflow via the Sibutu Passage is mainly compensated by the inflow via the Mindoro Strait. The western strengthening near the Palawan Island and the asymmetry of the circulation in the SS is caused by the β effect. The transport via the Mindoro Strait is generally much larger than that via the Balabac Strait. An inflow into the SS via the Dipolog Strait contributes little to the circulation in the SS except for the current field near this strait. A stronger lower layer current than the upper layer one near the Mindoro Strait is also discussed.