Review of Regional Haze Situation for September 2021
1.1 In September 2021, Southwest Monsoon conditions persisted over the ASEAN region as the prevailing winds blew mainly from the southeast or southwest (Figure 1). In particular, there were three tropical cyclones that affected the northern ASEAN region during the review period. In the first half of September 2021, Typhoon Chanthu and Tropical Storm Conson contributed to the increased rainfall over northern parts of the Philippines. In the last week of the review period, Tropical Storm Dianmu made landfall over Viet Nam, and widespread showers with gusty winds caused floods and landslides in Viet Nam, Lao PDR and Thailand. For the southern ASEAN region, rainy weather was observed over many areas despite this period of the year being its traditional dry season. Average daily rainfall was highest in Sabah, western Kalimantan, and central Sumatra, while relatively drier conditions were observed over the Java Sea region.
1.2 For the September 2021 review period, hotspot activity in the ASEAN region was generally subdued due to wet weather. Most hotspots in the southern ASEAN region were detected in Kalimantan. Overall, the number of hotspots for the southern ASEAN region were lower or comparable to previous years’ counts (Figure 2). For the northern ASEAN region, a majority of the hotspots were detected in Viet Nam. The hotspot counts in Viet Nam, Lao PDR and Myanmar were slightly elevated compared to the same review period in previous years, while the hotspot counts elsewhere in the northern ASEAN region were lower compared to the same review period in previous years (Figure 3).
1.3 In September 2021, isolated to scattered hotspots were detected mainly in Viet Nam, Borneo, and southern Sumatra (Figure 4 and 5). On a few days in the month, localized smoke haze was observed to emanate from persistent hotspot clusters in western Kalimantan. Slight smoke plumes were occasionally detected from some of the isolated hotspots in eastern Kalimantan and southern Sumatra. Otherwise, no significant transboundary smoke haze was observed over the ASEAN region during the review period.