1. Review of Regional Weather Conditions in September 2017
1.1 In September 2017, Southwest Monsoon conditions prevailed over the ASEAN region, and the monsoon rain band migrated further south with most of the shower activities occurring over the equatorial region between 5oS and 15oN. The rainfall distribution for September 2017 is shown in Figure 1.
1.2 During the month, rainfall was below-normal over northern and central Thailand, while normal to above-normal rainfall prevailed for other parts of the northern ASEAN region. In contrast, for the southern ASEAN region, rainfall was mostly above-normal, particularly over Malaysia, Sumatra, Sulawesi and Papua. Figure 2 shows the percentage normal of rainfall for September 2017.
1.3 In September 2017, there were several tropical cyclones that brought heavy rainfall to Asia Pacific region. Among these tropical cyclones, only typhoon ‘Doksuri’ brought heavy rainfall and strong winds to the Philippines and Vietnam. “Doksuri” developed on 10 September 2017, and made landfall over Luzon, the Philippines before moving westwards over northern Philippines and into the South China Sea. ’Doksuri’ intensified into a typhoon as it continued to track west-northwestward over the South China Sea towards Vietnam. On 15 September 2017, Typhoon Doksuri made landfall near the north central regions of Vietnam. In addition to the torrential rain and strong winds, the coastal provinces of Vietnam experienced storm surges which accompanied the typhoon, Typhoon Doksuri weakened and dissipated shortly after making landfall.
1.4 The prevailing winds during September 2017 were predominantly from the southeast or southwest, particularly over the southern ASEAN region. In the northern ASEAN region, winds were light and blew from the south or southwest. Broad easterly wind anomalies were observed over the western Pacific Ocean and parts of the South China Sea. Figure 4 shows the average and anomalous winds at 5000 feet.
1.5 In September 2017, the equatorial Pacific Ocean’s sea-surface temperature (SST) over the Nino 3.4 region remained at neutral (neither El Niño nor La Niña) values. However, in recent weeks, there has been a gradual cooling of the sea surface temperatures, and the Nino 3.4 index dipped to negative values. However, atmospheric conditions, such as trade winds and cloudiness, over the equatorial Pacific have largely remained neutral.
1.6 The Madden Julian Oscillation (MJO) was weak and non-discernible in September 2017. Although there was a faint and brief MJO signal in Phase 3 in mid-September 2017, overall, the MJO did not have a significant influence on the weather conditions over the southern ASEAN region.during the month.
2. Review of Land/Forest Fires and Smoke Haze Situation
2.1 Hotspot activities in the northern ASEAN region were generally subdued in September 2017 due to wet weather conditions. In the southern ASEAN region, there were isolated or scattered hotspots detected mainly in southern Sumatra and West Kalimantan but the region was relatively wetter than usual during the month.
2.2 On a few days in the third week of September 2017, Kalimantan experienced a brief period of dry weather conditions. This led to an increase of hotspot activities in West Kalimantan on 22 and 23 September 2017, and localised smoke haze and plumes were seen emanating from some of the hotspots. The hotspots were short-lived as showers on subsequent days helped to bring an improvement to the hazy conditions. Satellite images depicting some of the hotspot activities over parts of the ASEAN region during September 2017 are shown in Figure 6 – Figure 10.
2.3 The hotspot distribution charts and daily hotspot charts for September 2017 are shown in Figure 11, Figure 12 and Figure 13 respectively.
3. Status of El Niño/La Niña and Indian Ocean Dipole
3.1 International climate centres have projected that the sea surface temperature would continue to cool in the second half of 2017. During this period, latest experts’ consensus favour neutral conditions or weak La Niña conditions.3.2 Typically, El Niño brings drier-than-normal rainfall conditions to most parts of Southeast Asia during the Southwest 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
3.2 In September 2017, the Indian Ocean Dipole (IOD) index remained in the neutral state (Figure 14). In the next few months, international climate models forecast the IOD to remain neutral.