1. Review of Regional Weather Conditions in August 2017
1.1 Southwest Monsoon conditions continued to prevail in the region in August 2017. The monsoon rain band was located in the Northern Hemisphere, between latitude 5oN and 20oN, and brought extensive rainfall over the region. Heavy rain fell over the coastal regions of Myanmar and the northern parts of the Philippines. For the southern ASEAN region, dry weather conditions were experienced over parts of southern Sumatra and Java. The rainfall distribution for August 2017 is shown in Figure 1.
1.2 In August 2017, rainfall was near-normal to above-normal over the northern ASEAN region except in Cambodia where rainfall was below-normal. In the southern ASEAN region, near-normal to above-normal rainfall prevailed except in southern Sumatra and parts of Java where rainfall was below-normal. Figure 2 shows the percent normal of rainfall for August 2017.
1.3 Two tropical storms – Hato and Pakhar were brewing in August 2017 over the western Pacific Ocean. Tropical Storm Hato developed to the northeast of the Philippines on the 20 August 2017. It tracked westwards and made landfall over the coast of southern China on the 23 August 2017. Tropical Storm Pakhar developed over the sea areas to the east of Luzon, the Philippines, on the 25 August 2017, and tracked north-westwards, crossing over northern Philippines before making landfall over the coast of southern China on the 27 August 2017. Both tropical storms brought heavy rains to Hong Kong and widespread shower activities over the northern ASEAN region.
1.4 The prevailing winds in August 2017 were predominantly from the southwest in the northern ASEAN region, and from the southeast in the southern ASEAN region. Figure 3 shows the average and anomalous winds at 5000 feet. In the month of August 2017, anomalous northeasterly to easterly winds dominated over the South China Sea and the region east of the Philippines. Furthermore, an anomalous cyclonic circulation was observed over the South China Sea, east of Hainan Island, associated with the tropical storms Hato and Pakhar.
1.5 In August 2017, the equatorial Pacific Ocean’s sea-surface temperature (SST) over the Nino3.4 region was close to the El Niño Nino 3.4 index threshold. Most of the atmospheric indicators such as wind and cloudiness were fluctuating about the neutral values.
1.6 During the month, the Madden Julian Oscillation (MJO)1 was generally weak and non-discernible except for a brief period in Phase 2 towards the end of August 2017. Overall, the contribution by the MJO on the weather over the Maritime continent in August 2017 was not significant.
2. Review of Land/Forest Fires and Smoke Haze Situation
2.1 In the first half of August 2017, the dry weather conditions led to an increase in hotspot activities in both the northern and southern ASEAN regions. On the 4 August 2017 and 6 August 2017, there were significant hotspot activities detected over Borneo and Sumatra respectively but no smoke haze was observed.
2.2 In the second half of August 2017, tropical storms Hato and Pakhar brought widespread showers to the region and subdued the hotspot activities in the northern ASEAN region. In contrast, some brief episodes of dry weather conditions over the southern ASEAN region led to an increase in hotspot activities, mostly in southern Sumatra and Borneo. Satellite images depicting some of the hotspot activities over parts of the ASEAN region in August 2017 are shown in Figures 6 – 9.
2.3 The hotspot distribution and daily hotspot charts for August 2017 are shown in Figure 10 and Figure 11 and 12 respectively.
3. Outlook of El Niño/La Niña and Indian Ocean Dipole
3.1 Climate models from major climate centres indicate that the neutral (neither El Nino nor La Nina, conditions are expected to persist for the rest of 2017.
3.2 Typically, El Niño brings drier-than-normal rainfall conditions to most parts of Southeast Asia during the Southwest 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.
3.3 In the southern ASEAN region, occasional periods of drier weather can still be expected, and could lead to an occurrence of transboundary haze in the region. Hence, vigilance should be maintained for any escalation in hotspot activities in the fire-prone areas in the coming months.
3.4 In August 2017, the Indian Ocean Dipole (IOD) index was neutral (Figure 13). In the next few months, climate models from major climate centres project the IOD to remain neutral, with the possibility that a positive IOD may develop towards the end of 2017.