Massive SST-front anomaly in the tip of Sumatra waters triggered by extreme positive IOD 2019 event

Western Sumatra typically accepts intense radiation throughout the year, affecting the alteration of sea surface temperature (SST) in the area. This study examines a widespread SST-front in the northeastern Tropical Indian Ocean (NETIO) which is not an essential area of the anomalies that was not predicted by previous studies. The data processing method is based on a remote-sensing approach with SST-front analysis utilizing gradient magnitude (GM). This study reveals the anomaly finding and its mechanism as the impact of extreme positive IOD (IODp) 2019 event modulating the strong ocean–atmosphere interactions in late 2019 to early 2020. However, similar massive phenomena were not found during IODp 2006 or 2015 events. The tip of Sumatra waters showed a high value of GM with more than 0.2°C km−1 in the wide area and well-shaped lengthened towards the Indian Ocean. It has corresponded to cold fronts with minimum SST around 26.0°C and maximum SST reaching 30.0°C, respectively. Cooler SST is associated with an upwelling process that is influenced by reflective upwelling Rossby waves with the speed of 0.24 m s−1, anticyclonic circulation in the Andaman Sea, and strong northeast monsoonal wind with a speed of around 8.0 m s−1 forming a divergent area at NETIO. The correlation of wind component with SST-front is about −0.56(−0.71) for u(v) component. Meanwhile, warmer SST was generated by the local-monsoon shift in influencing the downwelling of the southeast Tropical Indian Ocean (SETIO) by the end of the IODp event. This upwelling–downwelling process is also illustrated by the contrasting sea surface height, which differs by about 0.3 m. It is highlighted that the strong upwelling (NETIO) as the result of the end of extreme IODp 2019 event and downwelling (SETIO) caused an abrupt alteration of water mass in the tip of Sumatra, leading to a massive SST-front.