1. Review of Regional Weather Conditions in May 2018
1.2 In May 2018, most parts of the Mekong sub-region received below-normal to near-normal rainfall except for the northern parts of Myanmar, eastern Cambodia and southern Vietnam where above-normal rainfall was recorded.
1.3 In the southern ASEAN region, below-normal rainfall was experienced over the northern parts of Sumatra, eastern Peninsula Malaysia, Java, and parts of Kalimantan. Elsewhere, near-normal to slightly above-normal rainfall were received.
1.4 The rainfall distribution for May 2018 and the percentage of normal rainfall for May 2018 are shown in Figure 1 and 2.
1.5 In the last week of May 2018, a low-pressure system developed over the Bay of Bengal and tracked north to northeast-wards toward Myanmar before making landfall around end May. The rain band associated with the low pressure system brought strong winds and heavy rain over many parts of Myanmar, including Yangon and Mandalay.
1.6 Over the South China Sea and the southern part of the Mekong sub-region, the northeasterly/easterly wind component was anomalously strong. Prevailing winds typically blow from the southwest or west in May, and the wind anomaly in May 2018 could be due to a persistent ridge over the southern coast of China from which there was an outflow of northeasterly and easterly winds. Figure 3 shows the average and anomalous winds at 5000 feet.
1.7 The El Niño-Southern Oscillation (ENSO) remained neutral (neither El Niño or La Niña) in May 2018. The atmospheric and oceanic indicators of ENSO such as sea surface temperature and trade winds were also indicative of neutral conditions.
1.8 In May 2018, the Madden Julian Oscillation (MJO) progressed from Phase 1 to Phase 4 from the second week of the month. The MJO in Phase 3 and 4 typically brings slightly enhanced rainfall over the southern ASEAN region during this time of the year. In the last week of May 2018, an increase in rainfall was observed over parts of the southern ASEAN region.
2. Review of Land/Forest Fires and Smoke Haze Situation
2.1 In May 2018, the increase in shower activities in the northern ASEAN region brought an end to the dry season. There was a significant reduction in hotspot activities and isolated hotspots were detected in the Mekong sub-region.
2.2 In the southern ASEAN region, hotspot activities were generally subdued due to the prevailing wet weather conditions in the region. Isolated hotspots were detected in Peninsula Malaysia, Kalimantan, as well as central and southern Sumatra during brief periods of drier weather. On few occasions, localised smoke plumes were observed in the vicinity of some of the hotspots in central and southern Sumatra.
2.3 Satellite images showing the hotspots detected in the ASEAN region in May 2018 are given in Figure 5 to Figure 8.
2.4 The hotspot distribution and daily hotspot charts for May 2018 are shown in Figure 9, Figure 10 and Figure 11.
3. Status of El Niño/La Niña and Indian Ocean Dipole
3.1 Based on projections from major international climate centres, the prevailing neutral (neither El Niño nor La Niña) conditions are forecast to continue at least until October 2018. Towards the end of the year, some models have indicated that there is a slight chance of a weak El Niño developing. El Niño typically brings drier than normal conditions to Southeast Asia during the Southwest Monsoon season from June to October. However, the impact of El Niño is less obvious when it coincides with the Northeast Monsoon season from November to February.
3.2 In May 2018, the Indian Ocean Dipole (IOD) index remained at neutral levels (Figure 12). The IOD is likely to remain neutral for the next few months according to predictions from major climate centres.