1. Review of Regional Weather Conditions in April 2017
1.1 Inter monsoon conditions, characterised by afternoon showers and winds that are generally light and variable in direction, prevailed over the region in April 2017. During the month, there was a gradual increase in rainfall over parts of the Mekong sub-region, mostly over Thailand, Cambodia and central Vietnam. Wet weather conditions continued to persist in the southern ASEAN region. The rainfall distribution for April 2017 is shown in Figure 1.
1.2 In April 2017, Myanmar, Vietnam and Lao PDR received near-normal to above-normal rainfall. However, over the eastern parts of Thailand and Lao PDR below-normal rainfall was recorded. Near-normal rainfall was received in northern parts of the Philippines while rainfall was above-normal in central and southern Philippines. In the southern ASEAN region, rainfall was near-normal except in northern Sumatra, east coast of Peninsular Malaysia and parts of Sarawak where below-normal rainfall was received . Figure 2 shows the percentage normal of rainfall for April 2017.
1.3 The prevailing winds during April 2017 were predominantly from the east or south-southwest over the northern ASEAN region. In the southern ASEAN region, prevailing winds were light but continued to blow mainly from the west or northwest. Similar to the conditions in March 2017, weak westerly wind anomalies were observed over the equatorial region between the Indian Ocean and the eastern parts of the Indonesian Archipelago. An anomalous cyclonic circulation over southern Thailand also contributed to enhanced rainfall activity in April 2017. Figure 3 shows the average and anomalous winds at 5000 feet.
1.4 On 13 April, a low pressure system developed in the Bay of Bengal and strengthened into Tropical cyclone Maarutha on 15 April 2017. Cyclone Maarutha made landfall in Myanmar the next day bringing widespread showers over most parts of Myanmar. This contributed to about 50 – 400 % of the long-term average rainfall in April over parts of western Myanmar. The cyclone dissipated quickly into a low pressure system on 17 April 2017.
1.5 The equatorial Pacific Ocean’s sea-surface temperature (SST) over the Nino3.4 region continued to warm but remained at neutral (neither El Niño nor La Niña) values. Most of the atmospheric indicators over the equatorial Pacific were in the near average conditions.
1.6 In the first half of April 2017, the Madden Julian Oscillation (MJO) was non-discernible. By late April 2017, while the MJO gradually strengthened, it remained at weak levels and did not have any significant influence on the weather patterns over the southern ASEAN region.
2. Review of Land/Forest Fires and Smoke Haze Situation
2.1 In the northern ASEAN region, an escalation of hotspot activities was observed mainly in Myanmar, Thailand and Lao PDR. During the first half of the month, widespread hotspots with smoke haze were detected along the western Arakan Ranges, eastern border of Myanmar, northern Thai provinces and northern parts of Lao PDR. Nonetheless, the passage of Tropical Cyclone Maarutha brought increased shower activities which helped alleviate the hotspot situation in Myanmar and northern Thailand.
2.2 In the early part of the second half of April 2017, brief periods of dry weather conditions led to an increase in the number of hotspots detected in northern Lao PDR. Moderate smoke haze was seen emanating from these hotspots. However, the fire hotspots were short-lived as they were subdued by onset of shower activities around end April 2017.2.3 In the southern ASEAN region, hotspot activities remained generally subdued due to the rainy weather in the region. Satellite images depicting some of the hotspot activities over parts of the ASEAN region during April 2017 are shown in Figure 5 – Figure 9.
2.4 The hotspot distribution charts and daily hotspot charts for April 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 in the next 6 months. Forecasts suggest El Niño conditions being slightly favoured over neutral conditions from July-August-September.
3.2 However, the seasonal prediction model outlooks of El Niño at this time of the year are the least skillful all year round, a more confidence assessment of El Niño risk would be available from June 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 April 2017, the Indian Ocean Dipole (IOD) index has increased slightly from previous month but continued to remain at neutral levels (Figure 13). In the next few months, based on international climate models, the IOD is forecast to remain neutral, with some chance that a positive IOD may develop later in the year. The IOD is unlikely have a significant influence on the weather over the ASEAN region for the next few months.