1. Review of Regional Weather Conditions in March 2017
1.1 The Northeast Monsoon conditions in the region gave way to inter-monsoon conditions in the second half of March 2017. During the month, there was a gradual increase of shower activities over parts of the Mekong sub-region, particularly over Thailand, Cambodia and central Vietnam was observed while over most parts of the southern ASEAN region, wet weather conditions continued to prevail. The rainfall distribution for March 2017 is shown in Figure 1.
1.2 In March 2017, rainfall was near to above-normal over most parts of the northern ASEAN region, except in parts of Myanmar, Cambodia and southern Vietnam where below-normal rainfall was received. Likewise in most parts of the Philippines, near- to above-normal rainfall prevailed with most of the rainfall affecting the Mindanao area. In the southern ASEAN region, rainfall received was mostly near to above-normal, and in the second half of March 2017, parts of West Java and Peninsular Malaysia were affected by floods due to the occurrence of heavy thunderstorm activities on some days. Figure 2 shows the percentage normal of rainfall for March 2017.
1.3 The prevailing winds during the month were predominantly from the north-east or east over the northern ASEAN region, and from the west or northwest over the southern ASEAN region. In the second half of March 2017, there was a gradual weakening of winds over the ASEAN region around and north of the Equator. Westerly wind anomalies prevailed over the equatorial region between the Indian Ocean and the eastern parts of Indonesian Archipelago. The anomalous convergence observed over the northern parts of Borneo and southern Philippines could have contributed to above-normal rainfall over those areas. Figure 3 shows the average and anomalous winds at 5000 feet.
1.4 The cool anomalies of the equatorial Pacific Ocean’s sea-surface temperature (SST) over the Niño3.4 region further weakened towards neutral (neither El Niño nor La Niña) values, and most of the atmospheric indicators over the equatorial Pacific were in the near average conditions.
1.5 In early March 2017, the Madden Julian Oscillation (MJO) progressed through Phase 3 to Phase 4 in early March 2017, and this was a contributing factor to the increased rainfall received over the Indonesian Archipelago. The strength of the MJO gradually weakened towards mid-March 2017, and did not have a significant influence on the rainfall in the second half of the month.
2. Review of Land/Forest Fires and Smoke Haze Situation
2.1 In March 2017, the hotspot activities in the northern ASEAN region remained active under the prevailing dry weather conditions. Large clusters of hotspots were detected in Myanmar and northern Thailand compared to February 2017. Most of the hotspots and areas of smoke haze were detected close to the border of Myanmar and Thailand, in the provinces of Mae Hong Son, Chiang Rai and Kamphaeng Phet. In the second half of March 2017, there was a further deterioration in the air quality in northern Thailand, and the PM10 readings exceeded 150ug/m3 (Unhealthy range) on several days in Mae Hong Son and Chiang Rai provinces. In Lao PDR, a high density of hotspots was detected in Xiangkhoang and Xaignabouri pronvinces. On the other hand, there was an increase in shower activities in Cambodia during the month which helped to ease the hotspot situation there.
2.2 In contrast, hotspot activities in the southern ASEAN region remained low and were mostly subdued due frequent shower activities in the region.
2.3 Satellite images depicting some of the hotspot activities over parts of the ASEAN region during March 2017 are shown in Figure 5 – Figure 9.
2.4 The hotspot distribution charts and daily hotspot charts for March 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 Most models from major climate centres indicate the tropical Pacific Ocean will continue to gradually warm over the next 6 months, and there is an increasing chance that weak El Niño conditions could develop in the second half of 2017.3.2 As the seasonal prediction model outlooks of El Niño at this time of the year is known to have the least skill, a more confidence assessment of El Niño risk would be available around mid-2017 onwards.
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 March 2017, the Indian Ocean Dipole (IOD) index was at neutral levels (Figure 13). In the next few months, based on international climate models, the IOD is forecast to remain neutral and it would unlikely have a significant influence on the weather over the ASEAN region. The formation of IOD typically starts around May or June, and peaks between August and October before decaying rapidly between December and April.