Atmospheric Conditions Associated with Northerly Surge, Borneo Vortex and Madden Julian Oscillation During the Extreme Rainfall Cases in Early 2021 Over the Western Part of the Maritime Continent

During early 2021 (January to February), high rainfall intensity resulted in flooding over several locations over the western part of the Maritime Continent (East Aceh, South Borneo, Aceh Tamiang, and Jakarta). The atmospheric conditions were specifically different, but the pattern of strong northeasterly wind over the South China Sea (SCS) appears during these all flooding events. During the flooding event over East Aceh and South Borneo, the high rainfall intensity is mainly associated with the vortex over the Borneo Island, denoted by high wind speed (more than 10 m/s), closed-counterclockwise circulation, and high vorticity over Borneo (over 25 × 10^–6 s⁻¹). Moreover, during the Aceh Tamiang’s flooding, there was a local convective activity and strong wind from the Indian Ocean. The wind from the Indian Ocean deflected southward along the west coast of Sumatera Island. This northwesterly wind over the Indian Ocean (west part of Sumatra Island) was stronger than the other flooding cases. Furthermore, the vorticity magnitude over this area (18 × 10^–5 s⁻¹) was also considerably higher than the other cases. Moreover, during the Jakarta flooding event, the Borneo vortex did not appear, and the northeasterly wind over the South China Sea turned to southward wind and elongated to the southern part of Java Island. The meridional SST gradient along the 105° E resulted in high wind speed over the western part of the Java Island and played a role in low-level wind convergence at the beginning of the Jakarta flooding episode.