Review of Regional Haze Situation for May 2023

1.1 Inter-monsoon conditions prevailed over the ASEAN region in May 2023 and the wind conditions were light and variable in direction on most days (Figure 1). Two tropical cyclones affected parts of the region in May 2023, namely Tropical Cyclone Mocha over Bay of Bengal (Figure 2) and Super Typhoon Mawar over the western Pacific Ocean (Figure 3). The storms brought brief periods of widespread rainfall and strong winds over Myanmar and the Philippines, respectively. According to the Joint Typhoon Warning Centre (JTWC), Super Typhoon Mawar was the strongest Northern Hemisphere tropical cyclone ever recorded in the month of May. By the end of May 2023, the inter-monsoon period had gradually transitioned into the Southwest Monsoon for most parts of the ASEAN region.

1.2 Over the northern ASEAN region, dry conditions continued over much of the Mekong sub-region in early May 2023. Elevated hotspot activity persisted in many parts of the sub-region, particularly in Lao PDR and Myanmar (Figure 4). On many days, isolated to scattered hotspots were detected in northern and western parts of the sub-region, where moderate to dense smoke haze, occasionally transboundary, was observed (Figure 6 – 9). Following the return of showers from the second week of May 2023, hotspot activity over the Mekong sub-region had steadily decreased. The ASMC Alert Level for the sub-region was downgraded to Alert Level 1 on 9 May 2023, and subsequently the end of dry season for the sub-region was declared on 24 May 2023.

1.3 For the southern ASEAN region, the weather was rainy and cloudy for most of May 2023. However, periods of drier conditions began to set in over some parts of the region from last week of the month, as monsoon rain band shifted further north from the Equator. Few to isolated hotspots, some with localised smoke plumes, were observed in the fire-prone areas of Sumatra, Kalimantan, and Peninsular Malaysia. A slight increase in hotspot activity was also detected, following the gradual onset of Southwest Monsoon conditions (Figure 5). On 29 May 2023, ASMC activated Alert Level 1 for the southern ASEAN region, signaling the start of the traditional dry season.


Figure 1: Average Daily Rainfall and Mean Winds for May 2023. (Source: JAXA GsMaP and GFS, respectively)


Figure 2: Tropical Cyclone Mocha over the Bay of Bengal and made landfall over Myanmar on 15 May 2023. (Source: Himawari-9 satellite surveillance)


Figure 3: Super Typhoon Mawar over the western Pacific Ocean brought widespread rainfall and strong winds over the Philippines during the last week of May 2023. (Source: Himawari-9 satellite surveillance)


Figure 4: Hotspot counts for the northern ASEAN region for May (2019-2023) based on NOAA-20 satellite surveillance. Hotspot counts were generally comparable to the counts in previous years, with majority of the hotspots detected in Lao PDR and Myanmar.


Figure 5: Hotspot counts for the southern ASEAN region for May (2019-2023) based on NOAA-20 satellite surveillance. Hotspot counts were slightly higher than the counts in previous years due to the gradual onset of the traditional dry season towards the end of May 2023.


Figure 6: Distribution of hotspots detected in May 2023 based on NOAA-20 satellite surveillance. Scattered hotspots were persistent over northern parts of Thailand and Lao PDR, as well as in parts of Myanmar and Viet Nam. Elsewhere in the ASEAN region, there were isolated and short-lived hotspots over the fire-prone areas in southern Sumatra and western Kalimantan.


Figure 7: Maximum fire intensity in May 2023 based on NOAA-20 satellite surveillance. The overall fire intensity was moderate over much of the Mekong sub-region, but particularly high over northern Lao PDR where clusters of hotspots persisted.


Figure 8: Number of days with moderate to dense smoke haze observed in May 2023 based on satellite imagery, ground observations, and air quality reports. Hazy conditions persisted mainly over western and northern parts of the Mekong sub-region.


Figure 9: Moderate smoke haze was observed in eastern Myanmar, northern Thailand and northern Lao PDR on 6 May 2023. Blown by the prevailing winds, transboundary smoke haze was observed to drift eastwards across northern Thailand, northern Lao PDR, and parts of Viet Nam. (Source: NOAA-20 satellite surveillance)