Water flux from the Andaman Sea to the South China Sea

Andam Sea to South China Sea (SCS) transient mesoscale water flux through the Singapore Strait, defined as reflux, reverses the annual SCS-Malacca Strait throughflow. Despite its dynamical significance, this process has received little attention. This study comprehensively examines its climatic features, driving factors and underlying mechanisms. Results indicate that reflux mainly occurs in summer and is rare in winter. Three types - WCE-, CE-, and E-type - are classified based on the extent of eastward intrusion across the Strait. The proposed physical mechanism is as follows: strong westerly winds drive surface water eastward, causing water accumulation along the western coasts of the Malay Peninsula and Sumatra and thereby elevating sea surface height (SSH) in the Malacca Strait. As SSH increases, the SSH gradient across the Strait reverses, initiating eastward flux. Meanwhile, strong westerly winds blocked by Sumatra deflect the southeastward flow northwestward around the Sunda Strait, intensifying the northward current at the eastern exit of the Singapore Strait, which enhances local Ekman transport and facilitates reflux. Although the same physical process operates in both seasons, the causes of strong westerly winds over the tropical eastern Indian Ocean differ. Summer reflux is favoured by the intensified southwest monsoon, whereas wintertime events are modulated by La Niña conditions, when warm waters and atmospheric heating near Sumatra induce a Gill-type low-level response to the equatorially symmetric heat source. Furthermore, while the considered three reflux types share the same fundamental mechanism, stronger atmospheric and oceanic forcing generates more intense and spatially extensive reflux events.