1. Review of Regional Weather Conditions in May 2017
1.1 May 2017, inter-monsoon conditions, characterised by light winds and afternoon shower activities, prevailed over the ASEAN region. During this period, rainfall activities increased over the northern ASEAN region. In the southern ASEAN region, drier weather conditions began to set in over the surrounding region of Java Sea. The rainfall distribution for May 2017 is shown in Figure 1.
1.2 In May 2017, most parts of the ASEAN region received near normal to above normal rainfall. In particular, rainfall was above normal over the eastern parts of the Philippines, southern Vietnam, Thailand, and over most parts of Sumatra and Kalimantan. Figure 2 shows the percentage normal of rainfall for May 2017.
1.3 The prevailing winds during May 2017 were predominantly from the southeast or south in the northern ASEAN region. In the southern ASEAN region, prevailing winds continued to remain light but blew from the southeast or southwest. An anomalous cyclonic circulation observed over the Andaman Sea brought enhanced rainfall activities over southern Thailand. Figure 3 shows the average and anomalous winds at 5000 feet.
1.4 In Bay of Bengal, a low pressure system began to develop on 28 May and quickly strengthened into Cyclone Mora the next day. As the cyclone tracked northward toward Bangladesh, it brought strong winds and heavy rains over Arakan and Chin states in Myanmar. Cyclone Mora eventually made landfall in Bangladesh on 30 May and dissipated the next day.
1.5 The equatorial Pacific Ocean’s sea-surface temperature (SST) over the Nino3.4 region remained at neutral (neither El Niño nor La Niña) values. Atmospheric indicators such as cloudiness and winds over the equatorial Pacific were in the near average conditions.
1.6 In the first half of May 2017, the Madden Julian Oscillation (MJO) , in Phase 8 and Phase 1, did not have significant impact on the weather patterns in the Maritime Continent. The MJO subsequently weakened and was non-discernible during the third week of May. By the last week of May, the MJO redeveloped and emerged in Phase 2 and 3, which brought enhanced rainfall activities particularly over the surrounding region of Indian Ocean and Andaman Sea.
2. Review of Land/Forest Fires and Smoke Haze Situation
2.1 In the northern ASEAN region, isolated to scattered hotspots with localised smoke haze were observed in the northern half of Myanmar, Lao PDR and Vietnam in early May 2017. Increased shower activities over the northern ASEAN region in the second week of May 2017 signalled an end to the traditional dry season. The hotspot activities in the region were then generally subdued by shower activities.
2.2 In the southern ASEAN region, hotspot activities remained subdued due to wet weather conditions. Nonetheless, isolated hotspots were detected in parts of Sumatra on some days. Satellite images depicting some of the hotspot activities over parts of the ASEAN region during May 2017 are shown in Figure 5 – Figure 9.
2.3 The hotspot distribution charts and daily hotspot charts for May 2017 are shown in Figure 10, Figure 11 and Figure 12 respectively.
3. Status of El Niño/La Niña and Indian Ocean Dipole
3.1 International climate centres are projecting a near equal chance of the El Niño Southern Oscillation (ENSO) remaining neutral and a chance of weak El Niño developing in the latter half of the year.
3.2 Should an El Niño develop, the conditions are expected to be weak and the impact over the Maritime Continent is unlikely to be significant.
3.3 Typically, El Niño brings drier-than-normal rainfall conditions to most parts of Southeast Asia, except over the near-equatorial region where the impact is less pronounced during the Northeast Monsoon season. During La Niña events, the opposite, i.e wetter-than-normal condition, normally occurs. Locally specific impact differs from place to place and for different seasons.
3.4 In May 2017, the Indian Ocean Dipole (IOD) index continued to remain at neutral levels (Figure 13). In the next few months, based on international climate models, the IOD is forecast to remain neutral, with some chance that a positive IOD may develop toward the end of the year. The IOD is unlikely have a significant influence on the weather over the ASEAN region for the next few months.