1. Review of Regional Weather Conditions in January 2018
1.1 The prevailing Northeast monsoon conditions over Southeast Asia strengthened in January 2018. Wet weather conditions were mostly over the Philippines and the southern ASEAN region, while drier weather conditions were experienced over the northern ASEAN region, particularly over Lao PDR and Cambodia. The rainfall distribution for January 2018 is shown in Figure 1.
1.2 In January 2018, there were two occurrences of monsoon surges which brought strong winds and moderate to heavy rainfall over the equatorial region, in particular over Borneo, Peninsular Malaysia and Singapore.
1.3 Southern ASEAN region received near to above-normal rainfall, except for the southern half of Sumatra, southwest parts of Borneo, parts of Sulawesi and Java, where below-normal rainfall were received. In the northern ASEAN region, below-normal rainfall was received over parts of Cambodia, Thailand, Laos PDR and Vietnam. Figure 2 shows the percent of normal rainfall for January 2018.
1.4 On 2 January 2018, Typhoon “Bolaven” (Figure 3) developed over the southern part of the Philippines and tracked westwards bringing heavy rainfall over the southern Philippines and the South China Sea. Typhoon “Bolaven” moved into the South China Sea on 3 January 2018 and dissipated over the sea before it reached the eastern coast of Vietnam.
1.5 The prevailing winds in January 2018 were mainly from the northeast or east over the northern ASEAN region and the South China Sea while, westerly and northwesterly winds were observed over the southern ASEAN region. In addition, strong anomalous westerly winds were recorded between latitudes 5o N and 10o S, extending from the Indian Ocean to 125o E. Figure 4 Figure 5 shows the average and anomalous winds at 5000 feet. The convergence between the northeast winds over the northern ASEAN region and the westerly winds over the southern ASEAN region contributed to the observed above-normal rainfall around the equatorial region.
1.6 The sea surface temperatures (SSTs) over the Niño 3.4 region in the equatorial Pacific Ocean remained at La Niña values, with stronger than average trade winds and below-average cloudiness over the equatorial Pacific Ocean, indicative of La Niña conditions. Typically, La Niña brings wetter-than-normal rainfall conditions to most parts of Southeast Asia, and has a less pronounced impact on the weather over the near-equatorial region during the Northeast Monsoon season.
1.7 The Madden Julian Oscillation (MJO) was active throughout most of January 2018, propagating from a start phase of Phase 2 and ending in Phase 6 towards the end of the month. The monsoon surges that affected the region in the first half of the month coincided with MJO phases 2 and 3. This brought heavy rainfall over the southern parts of the South China Sea. As the MJO shifted to phases 5 and 6, the rainfall conditions over most parts of ASEAN were suppressed.
2. Review of Land/Forest Fires and Smoke Haze Situation
2.1 The dry weather conditions that prevailed in the northern ASEAN region in December 2017 continued into January 2018. This led to an increase in hotspot activities in the northern ASEAN region. In parts of Cambodia, Thailand and Lao PDR, isolated to scattered hotspots with occasional smoke plumes and haze were observed.
2.2 Wet weather conditions persisted in the southern ASEAN region during the first half of the month, which helped to subdue hotspot activities there. However, in the second half of the month, dry weather conditions in the region which coincided with the suppressed MJO phases 5 and 6, contributed to the emergence of some hotspots observed in parts of Sumatra, Borneo and Sulawesi. Satellite images of hotspots detected in the ASEAN region in January 2018 are shown in Figure 6 to Figure 9.
2.3 The hotspot distribution and daily hotspot charts for January 2018 are shown in Figure 10, Figure 11 and Figure 12.
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 April 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 January 2018, the Indian Ocean Dipole (IOD) index remained at neutral levels (Figure 13). 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. The formation of IOD typically starts around May or June, and peaks between August and October before decaying rapidly between January and April.