1. Review of Regional Weather Conditions in December 2017
1.1 In December 2017, Southeast Asia experienced northeast monsoon conditions with drier weather conditions over the northern ASEAN region, and rainy or wet weather conditions in the southern ASEAN region. Most of the rain during the month fell within the region bounded between 10oS and 10oN of the Equator. The most intense rain that fell during the month was over the central South China Sea and to the east of the Philippines. These areas coincided with the path of the tropical cyclones crossing the central South China Sea, north of Borneo after making landfall in the Philippines. The rainfall distribution for December 2017 is shown in Figure 1.
1.2 In the second half of December 2017, intensification of the high pressure system in northern Asia brought a monsoon surge or a sudden surge of cold winds over the South China Sea. As these strong northeasterly winds blew equatorward over the South China Sea, it gathered moisture and led to the development of persistent rain clouds over large parts of the equatorial region. The surges brought strong winds and spells of moderate to heavy rain to Borneo, Peninsular Malaysia and Singapore. In particular, the eastern coastal states of Peninsular Malaysia and northern coast of Borneo were affected by heavy rainfall and intense floods arising from the monsoon surges.
1.3 Above-normal rainfall was recorded in most parts of the northern ASEAN region except for the eastern parts of Cambodia and the southern parts of Laos and Vietnam where below-normal rainfall was received. Most parts of the southern ASEAN region received normal to above-normal rainfall. Figure 2 shows the percent of normal rainfall for December 2017.
1.4 In December 2017, 3 tropical cyclones brought heavy rainfall and strong winds to the region. On 1 December, Tropical cyclone “Dahlia” (Figure 3) moved over the Indian Ocean, south of Java, bringing heavy rains to Java’s southern coast. On 14 December, a tropical depression (Figure 4) over the western Pacific Ocean, east of Cebu, the Philippines, intensified into Typhoon “Kai-Tak” and tracked over the South China Sea towards Peninsular Malaysia after making landfall in central Philippines. Heavy rain from the bands of “Kai-Tak” fell over the eastern coastal states of Peninsular Malaysia. As “Kai-Tak” dissipated over the South China Sea, another tropical depression over the western Pacific Ocean developed and tracked westwards and intensified into Typhoon “Tembin” over the South China Sea (Figure 4) before dissipating over water as it approached the southern Vietnam.
1.5 In December 2017, prevailing winds blew predominantly from the northeast or east in the northern ASEAN region and from the west or northwest in the southern ASEAN region. Strong anomalous westerly winds were recorded between 5o S and 10o N, extending from the Indian Ocean to the western Pacific Ocean due to the passage of typhoons “Kai-Tak” and “Tembin” in the South China Sea. Figure 5 shows the average and anomalous winds at 5000 feet.
1.6 During the month, sea surface temperatures (SSTs) over the Niño 3.4 region in the equatorial Pacific Ocean remained at La Niña values. The stronger than average trade winds and below-average cloudiness indicate La Niña conditions.
1.7 The Madden Julian Oscillation (MJO) was active throughout most of December 2017, propagating from a start phase of Phase 5, ending in Phase 2 towards the end of the month. The MJO however did not play a significant role in modulating the rainfall over the Southeast Asia regions. The rainfall was more likely modulated by the typhoons and monsoon surges in the region.
2. Review of Land/Forest Fires and Smoke Haze Situation
2.1 Dry weather conditions prevailed over the northern ASEAN region in the second half of December 2017. This led to an increase in the number of hotspots detected, in particular in parts of Thailand, Cambodia and Vietnam. There were no significant smoke plume or haze observed from these hotspots.
2.2 Wet weather conditions prevailed in the southern ASEAN region, which helped to subdue hotspot activities there. Satellite images of hotspots detected in the ASEAN region in December 2017 are shown in Figure 7 to 10.
2.3 The hotspot distribution and daily hotspot charts for December 2017 are shown in Figure 11, Figure 12 and Figure 13.
3. Status of El Niño/La Niña and Indian Ocean Dipole
3.1 Experts from International climate centers assessed that the tropical Pacific Ocean will continue to be cool and La Niña conditions would prevail until about March 2018.
3.2 The region is currently experiencing Northeast Monsoon conditions. Typically, for Southeast Asia, the impact from La Niña is wetter-than-normal rainfall conditions. For the weather over the near-equatorial region, the impact of La Niña is usually less pronounced during the Northeast Monsoon season (Dec – Mar) as compared to the Southwest Monsoon season (Jun – Sep).
3.3 In December 2017, the Indian Ocean Dipole (IOD) index remained at neutral levels (Figure 14). In the coming months, international climate models forecast the IOD to remain neutral and it is not likely to have a significant influence on the weather over the region for 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.