1. Review of Regional Weather Conditions in November 2017
1.1 In November 2017, Southeast Asia experienced inter-monsoon conditions in the first half of the month where the low level winds were generally light and variable in direction. A gradual strengthening of winds to blow predominantly from the northeast in the second half of November 2017 signalled the start of the Northeast Monsoon season. In the latter half of the month, the monsoon rain band was observed to shift southwards to lie between latitudes 10 o S and 10 o N. Most of the rainfall in November 2017 was recorded over the southern ASEAN region. The rainfall distribution for November 2017 is shown in Figure 1.
1.2 In the last week of November 2017, an intensification of a high pressure system in northern Asia led to a strengthening of northeast monsoon winds or a monsoon surge over the South China Sea. The surge brought prolonged moderate to heavy rainfall over southern Thailand and the north-eastern parts of Peninsular Malaysia. Widespread flooding was reported in Songkhla, Thailand, as well as in parts of Kelantan and Terengganu, Peninsular Malaysia.
1.3 In November 2017, rainfall was above normal in central Vietnam, southern Thailand and in the northern parts of the Philippines, and below normal in northern Thailand, Lao PDR and northern Vietnam. For the southern ASEAN region, normal to above-normal rainfall was recorded over most parts of the region. Figure 2 shows the percent of normal rainfall for November 2017.
1.4 On 1 November 2017, a tropical depression developed to the west of Cebu, the Philippines. The system tracked westward over the South China Sea where it intensified into Typhoon “Damrey” on 3 November 2017. Typhoon “Damrey” brought rough seas and strong winds along its path and made landfall in central Vietnam on 4 November 2017. The typhoon brought heavy rains and strong winds, and caused widespread flooding over many provinces in central Vietnam before dissipating over land on 5 November 2017.
1.5 In November 2017, the prevailing winds over the northern ASEAN region blew mainly from the northeast or east while winds in the southern ASEAN region were generally light and blew from the west or northwest. Broad easterly wind anomalies were observed over the western Pacific Ocean while westerly or north-westerly wind anomalies extended from the Indian Ocean to the Java Sea. The presence of the monsoon trough, coupled with the convergence of winds and strong solar heating enhanced the convective activity over southern Southeast Asia. Figure 4 shows the average and anomalous winds at 5000 feet.
1.6 During the month, the equatorial Pacific Ocean’s sea-surface temperature (SST) over the Nino 3.4 region remained at borderline La Niña values, with the 3-month (Aug-Sep-Oct) Nino 3.4 index at -0.46. Atmospheric conditions, such as trade winds and cloudiness, over the equatorial Pacific were suggestive of weak La Niña conditions.
1.7 The Madden Julian Oscillation (MJO) was weak and non-discernible throughout most parts of November 2017. However, signs of Phase-4 MJO were observed in late-November which coincided with a monsoon surge. The development of Phase-4 MJO could have enhanced the intensity of the monsoon surge that affected the region.
2. Review of Land/Forest Fires and Smoke Haze Situation
2.1 With the gradual transition to the traditional dry season in the northern ASEAN region, an increase in hotspot activities was observed in the Mekong sub-region. The hotspots were detected mainly in the northern parts of the Mekong sub-region but there were no significant smoke haze emanating from these hotspots.
2.2 In the southern ASEAN region, the prevailing wet weather conditions helped subdue hotspot activities. Satellite images depicting some of the hotspot activities over parts of the ASEAN region during November 2017 are shown in Figure 6 – Figure 10.
2.3 The hotspot distribution charts and daily hotspot charts for November 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 centers have indicated that for the rest of 2017, the tropical Pacific Ocean will continue to cool, and that there would be at least a 60% chance of a weak La Niña developing by December 2017. Expert consensus have also projected that if a La Niña develops, the event is not expected to last beyond March 2018.
3.2 The region is currently experiencing Northeast Monsoon conditions. Typically, the impact from La Niña for Southeast Asia is wetter-than-normal rainfall conditions. The impact of La Niña on the weather over the near-equatorial region is usually less pronounced during the Northeast Monsoon as compared to the Southwest Monsoon (Jun – Sep).
3.2 In November 2017, the Indian Ocean Dipole (IOD) index was at neutral levels (Figure 14). In the coming months, models from international climate centres forecast the IOD to remain neutral and that it would have a significant influence on the weather in the region over the next few months. The formation of IOD typically starts around May or June, and peaks between August and October before decaying rapidly between December and April.