Review of Regional Haze Situation for September 2023
1.1 Southwest monsoon conditions persisted over the ASEAN region in September 2023. The prevailing low-level winds blew mainly from the southeast over areas south of the Equator and turned to blow from the southwest or west over areas north of the Equator (Figure 1). There were two notable tropical cyclones during the review period, namely Typhoon Saola and Typhoon Haikui, which developed one after another from late August to early September 2023 (Figure 2). Heavy rains and strong winds brought by the storms had affected many areas in northern parts of the ASEAN region, particularly the Philippines, where significant events of floods and landslides were reported.
1.2 The southern ASEAN region remained under its traditional dry season throughout September 2023. Dry conditions prevailed over many parts of the region while there were isolated shower activities over parts of northern Sumatra, Malaysia, and northeastern Kalimantan (Figure 1). Over the fire-prone areas in Sumatra and Kalimantan, hotspot activity gradually escalated from middle of the month and the overall hotspot count for the month of September 2023 was the highest since 2019 (Figure 3). Most of the hotspots with high fire intensity were detected in southern Sumatra and southern Kalimantan while isolated hotspots were also detected in central Sumatra and western Kalimantan on some days (Figure 4 and 5). During the first three weeks of September 2023, slight to moderate smoke plumes were observed mainly in the Indonesian provinces of South Sumatra, South Kalimantan, Central Kalimantan, and West Kalimantan. On some days, transboundary smoke haze was observed to drift northwards from West Kalimantan into western Sarawak (Figure 8). Towards the last week of the month, the smoke haze emanating from persistent hotspot clusters in southern Sumatra and southern Kalimantan became denser and more widespread and was observed to drift towards the northwest and north (Figure 6, 7, and 8). On 29 Sep 2023, ASMC activated Alert Level 2 for Sumatra, indicating a risk of transboundary haze for the region.
1.3 The weather was generally wet over the northern ASEAN region in September 2023 as its traditional wet season prevailed. Hotspot activity was subdued by widespread rains and the overall hotspot count in September 2023 was comparable to the count in previous years (Figure 9). While there were isolated hotspots detected in parts of central Thailand and Viet Nam, these hotspots were mostly short-lived and with low fire intensity (Figure 4 and 5). No significant smoke haze was observed over the region during the review period (Figure 6).
