1. Review of Regional Weather Conditions in July 2017
1.1 Southwest Monsoon conditions continued to prevail in the region in July 2017. The monsoon rain band shifted further north and brought extensive rainfall over the region between latitudes 5oN and 20oN. Heavy rain fell over the coastal regions of Myanmar, Vietnam, the northern South China Sea and the northern parts of Philippines. For the southern ASEAN region, particularly over parts of Sumatra and Java, experienced dry weather conditions in the second half of the month. The rainfall distribution for July 2017 is shown in Figure 1.
1.2 In July 2017, rainfall was above normal over the northern ASEAN region. In the southern ASEAN region, near-normal to above-normal rainfall prevailed except in northern Sumatra, Sarawak, northern Kalimantan and parts of Java where rainfall was below-normal. Figure 2 shows the percentage normal of rainfall for July 2017.
1.3 Two tropical storms – Talas and Sonca formed over the South China Sea, east of Vietnam between 15 – 16 July 2017 and 21 – 25 July 2017 respectively. Tropical Storm Talas brought heavy rain and strong winds over the northern parts of Vietnam as it tracked westward on 16 July 2017. The storm dissipated the next day as it made landfall in northern Vietnam. Tropical Storm Sonca tracked westwards, similar to that of Talas except that Sanca’s impact was more widespread and brought heavy thunderstorms to parts of Thailand, Lao PDR and Vietnam.
1.4 The prevailing winds during July 2017 were predominantly from the southwest in the northern ASEAN region, and from the southeast or southwest in the southern ASEAN region. An anomalous cyclonic circulation associated with Tropical Storms Talas and Sonca was present over the South China Sea. Figure 4 shows the average and anomalous winds at 5000 feet.
1.5 In July 2017, the equatorial Pacific Ocean’s sea-surface temperature (SST) over the Nino3.4 region was warmer than average and close to the El Niño Nino 3.4 index threshold. However, these SST patterns over the tropical Pacific Ocean did not indicate developing El Niño conditions, and the atmospheric indicators such as cloudiness and winds over the equatorial Pacific were at neutral El Niño Southern Oscillation (ENSO) levels.
1.6 During the month, the Madden Julian Oscillation (MJO) was largely weak and non-discernible except for a few days around mid-July 2017 where the MJO strengthened as it emerged in Phase 3 bringing an increase in rainfall over the western Maritime continent. Overall, the contribution by the MJO on the weather over the Maritime continent in July 2017 was not significant.
2. Review of Land/Forest Fires and Smoke Haze Situation
2.1 In the first half of July 2017, hotspot activities in both the northern and southern ASEAN regions remained subdued due to wet weather conditions.
2.2 In the second half of July 2017, dry weather conditions set in over parts of Sumatra and Kalimantan. Significant hotspot activities were first observed in West Aceh Regency, Sumatra, and between 23 and 26 July 2017, hazy conditions were reported with visibility reduced to a low of 2-3 km during the period. With the occurrence of showers in the days following, 26 July 2017, the haze situation in West Aceh Regency improved. However, in the central and southern parts of Sumatra, the dry weather conditions in the last week of the month brought an increase in hotspot activities which persisted until the end of July 2017. Satellite images depicting some of the hotspot activities over parts of the ASEAN region during July 2017 are shown in Figure 6 – Figure 10.
2.3 The hotspot distribution charts and daily hotspot charts for July 2017 are shown in Figure 11, Figure 12 and Figure 13 respectively.
3. Status of El Niño/La Niña and Indian Ocean Dipole
3.1 International climate centres have forecast that the observed warming of the tropical Pacific Ocean would subside and that neutral ENSO conditions are expected to prevail in the second half 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 Despite the low likelihood of an El Niño developing, periods of dry weather condition can still be expected and could lead to the 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.
3.3 In July 2017, the Indian Ocean Dipole (IOD) index remained in the neutral state (Figure 14). In the next few months, international climate models forecast the IOD to remain neutral, with the possibility that a positive IOD may develop towards the end of the year.