The Interaction of Borneo Vortex (BV) and Cold Surge (CS) in Causing Heavy Rainfall in Several Regions Over Kalimantan, Sumatra, and Java Island (Case Study: January 8–10, 2021)

On January 8–10, 2021, there was heavy rainfall, which was also identified as the Borneo vortex (BV) and cold surge (CS) in Sumatra, Java, and Kalimantan. This paper examines the interaction mechanism of CS and BV in influencing extreme rainfall in Kalimantan, Sumatra, and Java during this period using analytical methods. This research uses brightness temperature or cloud top temperature (TBB) data from Himawari-8, rainfall data from GSMaP, and several atmospheric parameters from ECMWF ERA5. The research results show that CS + BV strengthens convective activity and causes high rainfall in the SCS near the north and west coasts of Kalimantan Island as a CS route. Increased convection activity and rainfall on the second and third days during CS + BV in SCS near the coast of Brunei and the Malaysian peninsula. The influence of CS on convective activity and rainfall is also significant in Sumatra and Java, even though there is blocking by BV, because there is support from westerly winds from the MJO that meet CS winds on the islands of Sumatra and southern Sumatra, which push CS winds toward Java Island and its surroundings. The increase in rainfall on the west coast of West Kalimantan is due to the location where the CS and BV winds meet, and the BV shifts because it is attracted by the CS. This also makes the impact of BV on rainfall insignificant on Kalimantan Island. The strengthening of moisture transport during CS + BV on January 8–10 was supported by moisture transport from the Indian Ocean, which was thought to be caused by the MJO. When CS + BV, there is movement of high relative humidity along the CS path. The very cold SST anomaly in the SCS is one of the triggers for the CS wind to move southward, which has a warmer SST anomaly.