Submesoscale variability of volume transport in Malacca and Singapore Straits

Volume Transport (VT) throughout various sections of South China Sea (SCS) is an important ocean characteristic, defining net flux of water masses, that may also involve heat, salt, as well as admixture transport of dissolved nutrients, suspended sediments and anthropogenic substances. Due to large scale of predominant coupled ocean/atmosphere climate phenomena (e.g., monsoon, ENSO, POD, IOD), variability of VT in the SCS basin is considered commonly at seasonal and/or interannual scales, using monthly or annual resolutions. This approach is justified for large VT rates of Luzon and Karimata Straits, both defining SCS throughflow (SCSTF) having long residence time. In contrast, Malacca and Singapore Straits (MSS) VT contribute just a small fraction of SCSFT, and additionally subjected to similar order local phenomena at daily or even hourly scales. At these scales the VT contribution also is in par with predominant astronomic tide in MSS, thus opening avenue for combination of the two otherwise independent phenomena.  To develop the approach further, ocean model NEMO is run at mesoscale resolution for the past period 1990-2024, driven by global  NEMO (ECMWF) model at the lateral boundaries and ERA atmospheric forcing at the ocean surface. Even though the research focuses on Singapore Strait, Malacca Strait is included due to dominant regional and local phenomena affecting both water bodies. In order to elucidate variability of submesoscale VT at different cross-sections of MSS, computed daily currents and sea levels are analysed in par with atmospheric forces with the goal to obtain trend, variability and extremes of VT in MSS at daily-to-interannual resolutions. New phenomenon (coined Singapore Strait Reflux, or simply Reflux) is discovered computationally and using  data analysis  – which is a temporal reversal of VT in Singapore Strait against dominant east-to-west direction. The Reflux episodes lasting from days to weeks may occur any time of a year due to coincidence and interplay of different scale phenomena, affecting MSS  from north (Indian Ocean via Andaman Sea), from south (Riau Islands) and from the east (Anambas Archipelago in SCS). The research focuses on understanding of Reflux genesis and forecasting capabilities.

This project is funded by the Research, Innovation and Enterprise 2025 Coastal Protection and Flood Management Research Programme of Singapore. The authors also thank Low K.S.,  Sasmal K. for their support in the idea discussions.