1. Review of Regional Weather Conditions in Jun 2017
1.1 Southwest Monsoon conditions became established over the ASEAN region in June 2017. The intensification of the sub-tropical high pressure system in the southern hemisphere gave rise to persistent southeasterly winds south of the equator and southwesterly winds north of the equator. During this period, the monsoon rain band was located between 5 S and 20 N. Heavy rainfall fell along the coastal areas of Myanmar as well as in the eastern parts of the Maritime Continent. The rainfall distribution for Jun 2017 is shown in Figure 1.
1.2 In June 2017, rainfall was above normal over Myanmar, parts of Vietnam and southern Philippines. In the southern ASEAN region, near-normal to above-normal rainfall prevailed over most parts of the region. However, rainfall was below-normal in northern and southern Sumatra. Figure 2 shows the percentage normal of rainfall for June 2017.
1.3 The prevailing winds during June 2017 were predominantly from the south or southwest in the northern ASEAN region. In the southern ASEAN region, prevailing winds blew from the southeast or south-southwest. There was no significant wind anomaly in the region except for an easterly anomaly over southern Philippines and a southerly anomaly over Myanmar, which could have contributed to the wetter-than-usual conditions in these areas. Figure 3 shows the average and anomalous winds at 5000 feet.
1.4 The equatorial Pacific Ocean’s sea-surface temperature (SST) over the Nino3.4 region continued to remain at neutral (neither El Niño nor La Niña) values. Atmospheric indicators such as cloudiness and winds over the equatorial Pacific were consistent with neutral El Niño Southern Oscillation (ENSO) conditions.
1.5 In June 2017, the Madden Julian Oscillation (MJO) was in Phase 3 for a few days and weakened rapidly in the subsequent days. The MJO remained in a non-discernible state until 9 June 2017. It emerged in Phase 8 and Phase 1 during the second and third week of June 2017 respectively before weakening again. There was no MJO signal until the end of June 2017. Overall, the MJO did not contribute significantly to the weather patterns over the Maritime Continent in June 2017.
2. Review of Land/Forest Fires and Smoke Haze Situation
2.1 In June 2017, hotspot activities in the northern ASEAN region were generally subdued due to the occurrence of increased rainfall brought by the onset of the Southwest Monsoon season.
2.2 In the southern ASEAN region, the number of hotspots detected remained low as wet weather conditions continued to persist, particularly over Sumatra and Kalimantan. Nonetheless, isolated hotspots with localised and short-lived smoke plumes were still observed in parts of Sumatra on a few days. Satellite images depicting some of the hotspot activities over parts of the ASEAN region during June 2017 are shown in Figure 5 – Figure 9.
2.3 The hotspot distribution charts and daily hotspot charts for Jun 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 indicate that the tropical Pacific Ocean will continue to warm gradually in the second half of 2017, and favors neutral ENSO conditions over El Niño.
3.2 Despite the low likelihood of an El Niño developing, periods of dry weather condition are expected and could lead to the occurrence of transboundary haze in the region.
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 June 2017, the Indian Ocean Dipole (IOD) has showed signs of transitioning into a positive IOD event although the IOD index continued to remain in the neutral state (Figure 13). In the next few months, based on international climate models, the IOD is forecast to remain neutral, with possibility that a positive IOD may develop toward the end of the year.