Review of Regional Haze Situation for August 2024
1.1 1.1 Southwest Monsoon conditions prevailed over the ASEAN region in August 2024. The prevailing winds over regions south of the Equator blew mainly from the southeast while the prevailing winds over areas north of the equator blew mainly from southwest to west, with winds turning southerly near the Equator (Figure 1). With the occurrence of Tropical Storms and Typhoons mainly over the northwestern Pacific Ocean, winds over the northern parts of the Philippines would occasionally blow from the east.
1.2 It was a generally wet August with rainfall observed over most areas of the ASEAN region, with higher rainfall over the northern parts of the Mekong sub-region, Sumatra and Borneo and parts of the Philippines and Papua. Flooding and landslides were reported in multiple areas of Thailand during the month (Figure 1). In contrast, drier conditions were observed over Java, the Lesser Sunda Islands, Sulawesi as well as the southern parts of Sumatra (Figure 1).
Figure 1: Average Daily Rainfall and Mean Winds for August 2024. (Source: JAXA GsMaP and GFS, respectively)
1.3 During August 2024, the hotspot count in the ASEAN region was comparable to that of previous years (Figure 2 and 3), except for Viet Nam where the hotspot count in August 2024 is the highest compared to the same month of previous years (Figure 2).
Figure 2: Hotspot counts for the southern ASEAN region for August (2020-2024) based on NOAA-20 satellite surveillance.
Figure 3: Hotspot counts for the northern ASEAN region for August (2020-2024) based on NOAA-20 satellite surveillance.
1.4 In August 2024, isolated to scattered hotspots were mainly detected in parts of the southern ASEAN region, particularly in Java, the Lesser Sunda Islands, the southern parts of Sumatra and the western parts of Kalimantan where hotspot persistency and fire intensity were more pronounced (Figure 4 and 5). Slight to moderate smoke haze was observed over the western parts of Kalimantan, while smoke plumes were observed over the southern parts of Sumatra (Figure 6). While the hotspot activity in the Mekong sub-region was suppressed, isolated hotspots with high persistency and intensity were detected mostly in the central parts of Viet Nam (Figure 4 and 5).
Figure 4: Distribution of hotspots detected in August 2024 based on NOAA-20 satellite surveillance.
Figure 5: Maximum fire intensity in August 2024 based on NOAA-20 satellite surveillance.
Figure 6: Number of days with moderate to dense smoke haze observed in August 2024 based on satellite imagery, ground observations, and air quality reports.
Review of Regional Weather for August 2024
1. Overview
1.1 During August 2024, a mix of below- and above-average rainfall was recorded over the Southeast Asian region (Figure 1). Much of the equatorial region and northern and eastern Mainland Southeast Asia recorded above-average rainfall, while parts of eastern Mainland Southeast Asia, the Philippines, and the southern Maritime Continent recorded below-average rainfall. The largest positive (wetter) anomalies were recorded over the western coast of Myanmar and parts of Borneo based on GSMaP-NRT (Figure 1, left) and CMORPH-Blended (Figure 1, right) satellite-derived rainfall estimates. In contrast, the largest negative (drier) anomalies were recorded over the northern Philippines, central Viet Nam and southern Lao PDR (in both GSMaP-NRT and CMORPH-Blended) and Cambodia (CMORPH-Blended only).
1.2 The observed rainfall anomaly pattern of above-average rainfall over the equatorial region and a mix of below- and above-average rainfall elsewhere are broadly consistent with the predictions from the subseasonal weather outlooks for August 2024 (5 – 18 August 2024 and 19 August – 1 September 2024).
Figure 1: Rainfall anomalies for August 2024 based on GSMaP-NRT data (left) and CMORPH-Blended data (right). The climatological reference period is 2001-2023. Green colour denotes above-average rainfall (wetter), while orange denotes below-average rainfall (drier).
1.3 Above-average temperatures were recorded over most of Southeast Asia in August 2024, apart from over the equatorial region where near- to above-average temperatures were recorded and northern and western Mainland Southeast Asia where below-to above average temperatures were recorded (Figure 2). The regions where below- and near-average temperatures were recorded are in line with those with above-average rainfall. The coolest anomalies were over northern Lao PDR (around 1°C below average), while the warmest anomalies (more than 2°C above average) were recorded over southern Sumatra.
Figure 2: Temperature anomalies for August 2024 based on ERA-5 reanalysis. The climatological reference period is 2001-2023. Red colour denotes above-average temperature (warmer), while blue denotes below-average temperature (colder).
2. Climate Drivers
2.1 A Madden-Julian Oscillation (MJO) signal strengthened over the Western Hemisphere (Phase 1) in the second week of August, after no discernible signal for much of the first week based on the RMM index (Figure 3). This signal propagated eastward for the rest of August, reaching the Indian Ocean (Phase 2) in the third week, and the Maritime Continent (Phase 4) in the last week. Typically for August, Phases 1 and 2 tend to bring drier conditions to northeastern Southeast Asia, while Phases 2 and 3 bring wetter conditions to the western Maritime Continent, and Phase 4 brings wetter conditions to northern parts of the Maritime Continent. MJO activity therefore likely contributed to the above-average rainfall in the equatorial region, and drier conditions over parts of eastern Mainland Southeast Asia and the Philippines.
Figure 3: The MJO phase diagram. The diagram illustrates the movement of the MJO through different phases, which correspond to different locations along the equator (denoted in the text with the first day of the month in blue and the last day of the month in red). The distance of the index from the centre of the diagram is related to the strength of the MJO. Values within the grey circle are considered weak or indiscernible (data from the Bureau of Meteorology, Australia).
2.2 The tropical Pacific was in an ENSO neutral state during August.
Subseasonal Weather Outlook (16 – 29 September 2024)
Issued: 11 September 2024
First forecast week: 16 – 22 September 2024
Second forecast week: 23 – 29 September 2024
Figure 1: Rainfall Outlook
Figure 2: Temperature Outlook
Wetter conditions are predicted for central and eastern Mainland Southeast Asia and the northern half of the Philippines in Week 1 (16 – 22 September). In Week 2 (23 – 29 September), wetter conditions are predicted over much of the Maritime Continent, with the highest likelihood over the western half of the region.
Warmer than usual temperature is predicted over much of the Maritime Continent in Week 1 (16 – 22 September). These warmer conditions are predicted to ease in Week 2 (23 – 29 September), in line with the predicted wetter conditions. Elsewhere, warmer than usual temperature is predicted over southern Mainland Southeast Asia in Week 2.
No clear Madden-Julian Oscillation (MJO) signal was present during the second week of September, based on the RMM index. Most models predict an MJO signal to strengthen over the Maritime Continent (Phase 5), before propagating to the Western Pacific (Phase 6) by the start of Week 1. Most models predict this signal to continue propagating through the Western Pacific (Phases 6 and 7) during Week 1 and become inactive during Week 2.
The outlook is assessed for the region in general, where conditions are relative to the average conditions for the corresponding time of year. For specific updates on the national scale, the relevant ASEAN National Meteorological and Hydrological Services should be consulted.
