Category Archives: Regional Weather Review for Year 2018

July 12, 2018

Review of Regional Weather Conditions for Second Fortnight of June 2018

1. Review of Regional Weather Conditions for Second Fortnight of June 2018

1.1 The southwest monsoon conditions prevailed in the second fortnight of June 2018. Shower activities were observed over many parts of the region.

1.2 During this period, there were isolated showers over parts of the northern ASEAN region but wetter weather was observed over coastal areas of Myanmar. In southern Myanmar, heavy rains reportedly triggered floods and landslides during the beginning of the second fortnight. Based on climatology, the wet conditions over coastal Myanmar is typical during this period of the season. Most parts of the Mekong sub-region experienced below-normal to near-normal rainfall, while northern and coastal areas of Myanmar, eastern Cambodia and southern Vietnam received above-normal rainfall.

1.3 In the southern ASEAN region, large scale convergence of winds caused by a circulation west of Borneo brought widespread shower activities over the region. Most areas experienced wetter than usual conditions, except for central Sumatra, central Kalimantan and Java where below-normal rainfall were received.

1.4 The rainfall distribution and the percentage of normal rainfall for the second fortnight of June 2018 are shown in Figures 1 and 2.

Figure 1: Daily average rainfall for the ASEAN region in the second fortnight of June 2018. (Source: JAXA Global Satellite Mapping of Precipitation)

Figure 2: Percentage of Normal Rainfall for 15 – 30 June 2018. The rainfall data may be less representative for areas with a less dense rainfall network. (Source: IRI NOAA/NCEP CPC Unified Precipitation Analyses)

1.5 During the second fortnight of June 2018, weather in the region was influenced by Tropical Storm “Gaemi” and Typhoon “Prapiroon” as well. Rainbands associated with “Gaemi” brought widespread showers over parts of the northern ASEAN region, and persistent wet weather over Sumatra, Peninsular Malaysia and Singapore in the southern ASEAN region. Toward the end of June 2018, rain bands from “Prapiroon” also brought widespread showers over most parts of the ASEAN region.

Figure 3: Historical storm track for Tropical Storm Gaemi and Typhoon Prapiroon. (Source: JAXA)

1.6 Prevailing winds over the northern ASEAN region and the South China Sea were weaker than usual and blew predominantly from the southwest or west. Over the southern ASEAN region, easterly to southeasterly winds prevailed. Figure 4 shows the average and anomalous winds at 5000 feet.

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Figure 4: 5000 ft average winds (left) and anomaly (right) for 15 – 30 June 2018. (Source: JMA)

1.7 The El Niño-Southern Oscillation (ENSO) remained in the neutral state (neither El Niño nor La Niña). The atmospheric and oceanic indicators of ENSO such as sea surface temperature and trade winds were also indicative of neutral conditions, although the sea surface temperatures in the eastern Pacific Ocean have been observed to be warming slowly.

1.8 The Madden Julian Oscillation (MJO) progressed from Phase 1 to Phase 2 before the signal weakened toward the end of the month. Typically, the MJO in Phases 1 and 2 brings less rainfall over the coastal Myanmar region and the South China Sea, and the MJO in Phase 2 would bring slightly enhanced rainfall around Sumatra and Borneo. In the second fortnight of June 2018, an increase in rainfall was observed along the belt between the Equator and 5°S.

Figure 5: The MJO phase diagram for June 2018 (green). The diagram illustrates the movement of the MJO through different phases, which correspond to different locations along the equator. The distance of the index from the centre of the diagram is correlated with the strength of MJO. When the index falls within the circle, the MJO is considered weak or indiscernible. (Source: Bureau of Meteorology).

June 11, 2018

Review of Regional Weather and Smoke Haze for May 2018

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1. Review of Regional Weather Conditions in May 2018

1.1 Inter-monsoon conditions prevailed in May 2018, and most parts of the ASEAN region experienced an increase in shower activities. The increased shower activities brought an end to the dry season in the northern ASEAN region.

1.2 In May 2018, most parts of the Mekong sub-region received below-normal to near-normal rainfall except for the northern parts of Myanmar, eastern Cambodia and southern Vietnam where above-normal rainfall was recorded.

1.3 In the southern ASEAN region, below-normal rainfall was experienced over the northern parts of Sumatra, eastern Peninsula Malaysia, Java, and parts of Kalimantan. Elsewhere, near-normal to slightly above-normal rainfall were received.

1.4 The rainfall distribution for May 2018 and the percentage of normal rainfall for May 2018 are shown in Figure 1 and 2.

Figure 1: Daily average rainfall for the ASEAN region in May 2018. (Source: JAXA Global Satellite Mapping of Precipitation).

Figure 2: Percent of Normal Rainfall for May 2018. The rainfall data Jun be less representative for areas with a less dense rainfall network.

1.5 In the last week of May 2018, a low-pressure system developed over the Bay of Bengal and tracked north to northeast-wards toward Myanmar before making landfall around end May. The rain band associated with the low pressure system brought strong winds and heavy rain over many parts of Myanmar, including Yangon and Mandalay.

1.6 Over the South China Sea and the southern part of the Mekong sub-region, the northeasterly/easterly wind component was anomalously strong. Prevailing winds typically blow from the southwest or west in May, and the wind anomaly in May 2018 could be due to a persistent ridge over the southern coast of China from which there was an outflow of northeasterly and easterly winds. Figure 3 shows the average and anomalous winds at 5000 feet.

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Figure 3: 5000 ft average winds (left) and anomaly (right) for May 2018.

1.7 The El Niño-Southern Oscillation (ENSO) remained neutral (neither El Niño or La Niña) in May 2018. The atmospheric and oceanic indicators of ENSO such as sea surface temperature and trade winds were also indicative of neutral conditions.

1.8 In May 2018, the Madden Julian Oscillation (MJO) progressed from Phase 1 to Phase 4 from the second week of the month. The MJO in Phase 3 and 4 typically brings slightly enhanced rainfall over the southern ASEAN region during this time of the year. In the last week of May 2018, an increase in rainfall was observed over parts of the southern ASEAN region.

Figure 4: The MJO phase diagram for May 2018 (green). The diagram illustrates the movement of the MJO through different phases, which correspond to different locations along the equator. The distance of the index from the centre of the diagram is correlated with the strength of MJO. When the index falls within the circle, the MJO is considered weak or indiscernible. (Source: Bureau of Meteorology).

2. Review of Land/Forest Fires and Smoke Haze Situation

2.1 In May 2018, the increase in shower activities in the northern ASEAN region brought an end to the dry season. There was a significant reduction in hotspot activities and isolated hotspots were detected in the Mekong sub-region.

2.2 In the southern ASEAN region, hotspot activities were generally subdued due to the prevailing wet weather conditions in the region. Isolated hotspots were detected in Peninsula Malaysia, Kalimantan, as well as central and southern Sumatra during brief periods of drier weather. On few occasions, localised smoke plumes were observed in the vicinity of some of the hotspots in central and southern Sumatra.

2.3 Satellite images showing the hotspots detected in the ASEAN region in May 2018 are given in Figure 5 to Figure 8.

Figure 5: NOAA-19 satellite image on 21 May 2018 shows wet weather conditions over most parts of the northern ASEAN region.

Figure 6: NOAA-19 satellite image on 27 May 2018 shows a low pressure system over Bay of Bengal. The low pressure system affected Myanmar in the last week of May and brought widespread rainfall over most parts of Myanmar.

Figure 7: NOAA-19 satellite image on 10 May 2018 shows isolated hotspots in central Sumatra. A thin smoke plume can be observed emanating from one of the hotspot.

Figure 8: NOAA-19 satellite image on 20 May 2018 shows shower activities affected most parts of the southern ASEAN region.

2.4 The hotspot distribution and daily hotspot charts for May 2018 are shown in Figure 9, Figure 10 and Figure 11.

Figure 9: NOAA-19 hotspots distribution in May 2018.

Figure 10: Hotspot Counts in Cambodia, Lao PDR, Thailand, Vietnam and Myanmar in May 2018.

Figure 11: Hotspot Counts in Sumatra, Borneo and Peninsular Malaysia in May 2018.

3. Status of El Niño/La Niña and Indian Ocean Dipole

3.1 Based on projections from major international climate centres, the prevailing neutral (neither El Niño nor La Niña) conditions are forecast to continue at least until October 2018. Towards the end of the year, some models have indicated that there is a slight chance of a weak El Niño developing. El Niño typically brings drier than normal conditions to Southeast Asia during the Southwest Monsoon season from June to October. However, the impact of El Niño is less obvious when it coincides with the Northeast Monsoon season from November to February.

3.2 In May 2018, the Indian Ocean Dipole (IOD) index remained at neutral levels (Figure 12). The IOD is likely to remain neutral for the next few months according to predictions from major climate centres.

Figure 12: Indian Ocean Dipole (IOD) index time series. The IOD index was at neutral levels in May 2018. (Source: Bureau of Meteorology, Australia).

May 14, 2018

Review of Regional Weather and Smoke Haze for Apr 2018

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1. Review of Regional Weather Conditions in April 2018

1.1 The Northeast Monsoon conditions gave way to the inter-monsoon conditions in April 2018. The inter-monsoon period is characterised by afternoon showers with winds that are generally light and variable in direction. In April 2018, a gradual increase in shower activities was observed over parts of the Mekong sub-region, while in the southern ASEAN region, conditions were generally wet.

1.2 In the northern ASEAN region, most parts of the Mekong sub-region received near-normal to above-normal rainfall, except for parts of Myanmar, eastern Thailand and southern Vietnam.

1.3 In the southern ASEAN region, Peninsula Malaysia and Sumatra received below-normal rainfall. Near normal to below-normal rainfall were received over southern Kalimantan and Java.

1.4 The rainfall distribution for April 2018 and the percentage of normal rainfall for April 2018 are shown in Figure 1 and 2.

Figure 1: Daily average rainfall for the ASEAN region in April 2018. (Source: JAXA Global Satellite Mapping of Precipitation).

Figure 2: Percent of Normal Rainfall for April 2018. The rainfall data may be less representative for areas with a less dense rainfall network.

1.5 In April 2018, prevailing winds in the region were predominantly blowing from the northeast or southeast, with comparatively weaker winds within the equatorial belt between 5ºN and 5ºS. Stronger northeasterly winds were observed over the South China Sea, while anomalously strong westerly winds continued to be observed to the east of Sulawesi since March 2018. Figure 3 shows the average and anomalous winds at 5000 feet.

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Figure 3: 5000 ft average winds (left) and anomaly (right) for April 2018.

1.6 During the month, the El Niño-Southern Oscillation (ENSO) remained at neutral (neither El Niño or La Niña) conditions. The sea surface temperature over the central tropical Pacific Ocean, as well as the cloudiness and trade wind conditions were also indicative of neutral conditions.

1.7 The Madden Julian Oscillation (MJO) progressed from Phase 7 to Phase 2 during the first half of the month before propagating to Phase 3 during the third week of April 2018. The MJO in Phase 2 and 3 typically brings suppressed rainfall over the area around the Philippines, and slightly enhanced rainfall over the southern ASEAN region during this time of the year. However, the MJO event in April 2018 did not have a significant influence on the weather patterns in the ASEAN region. For the rest of the month, the MJO signal remained weak.

Figure 4: The MJO phase diagram for April 2018 (green). The diagram illustrates the movement of the MJO through different phases, which correspond to different locations along the equator. The distance of the index from the centre of the diagram is correlated with the strength of MJO. When the index falls within the circle, the MJO is considered weak or indiscernible. (Source: Bureau of Meteorology).

2. Review of Land/Forest Fires and Smoke Haze Situation

2.1 Although the hotspot count in the northern ASEAN region in April 2018 was high with a peak of 622 on 23 April, it was lower compared to March 2018 likely due to an increase in shower activities with the onset of the inter-monsoon period in early April 2018. Smoke haze was observed on occasions in the vicinity of the hotspots. In particular, scattered hotspots with moderate smoke haze were observed over a few days in northern Lao PDR in the second week of April 2018.

2.2 In the southern ASEAN region, hotspot activities were generally subdued due to the occurrence of shower activities in the region.

2.3 Satellite images showing the hotspots detected in the ASEAN region in April 2018 are given in Figure 5 to Figure 8.

Figure 5: NOAA-19 satellite image on 2 April 2018 shows isolated hotspots with localised smoke plumes over eastern Lao PDR.

Figure 6: NOAA-19 satellite image on 10 April 2018 shows scattered hotspots over northern Lao PDR. Moderate smoke haze were observed in the vicinity of the hotspots.

Figure 7: NOAA-19 satellite image on 26 April 2018 shows increased shower activities over the Mekong sub-region.

Figure 8: NOAA-19 satellite image on 16 April 2018 shows shower activities affected most parts of the southern ASEAN region.

2.4 The hotspot distribution and daily hotspot charts for April 2018 are shown in Figure 9, Figure 10 and Figure 11.

Figure 9: NOAA-19 hotspots distribution in April 2018.

Figure 10: Hotspot Counts in Cambodia, Lao PDR, Thailand, Vietnam and Myanmar in April 2018.

Figure 11: Hotspot Counts in Sumatra, Borneo and Peninsular Malaysia in April 2018

3. Status of El Niño/La Niña and Indian Ocean Dipole

3.1 Predictions from international major climate centres have indicated that the current neutral conditions are expected to prevail for the next few months.

3.2 In April 2018, the Indian Ocean Dipole (IOD) index remained at neutral levels (Figure 12). Climate models from major climate centres forecast that the IOD is likely to remain neutral for the coming months, with a slight chance that a negative IOD may develop later in the year. The formation of IOD typically starts around May or June, and peaks between August and October before decaying rapidly between February and April. The IOD is not likely to have a significant influence on the weather over the region.

Figure 12: Indian Ocean Dipole (IOD) index time series. The IOD index was at neutral levels in April 2018. (Source:

Bureau of Meteorology, Australia).

April 11, 2018

Review of Regional Weather and Smoke Haze for Mar 2018

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1. Review of Regional Weather Conditions in March 2018

1.1 The Northeast Monsoon conditions continued to prevail in Southeast Asia in March 2018. Dry air from the northern Asian landmass brought dry conditions to many parts of Southeast Asia, extending as far south as over Peninsular Malaysia. The northern ASEAN region remained in the midst of the traditional dry season, while wetter conditions were experienced in the south with the monsoon trough and its associated rain band lying close to the Equator.

1.2 In the northern ASEAN region, drier conditions were observed over Myanmar, northern Thailand and parts of Cambodia and Thailand while near to above-normal rainfall was felt over other parts of the region.

1.3 In the southern ASEAN region, parts of Peninsula Malaysia and northern Sumatra experienced below-normal rainfall.

1.4 The rainfall distribution for March 2018 and the percentage of normal rainfall for March 2018 are shown in Figure 1 and 2.

Figure 1: Daily average rainfall for the ASEAN region in March 2018. (Source: JAXA Global Satellite Mapping of Precipitation).

Figure 2: Percent of Normal Rainfall for March 2018. The rainfall data may be less representative for areas with a less dense rainfall network.

1.5 In March 2018, winds north of the Equator were predominantly from the east or southeast, while at and south of the Equator, the winds were generally from the west or north. During the month, strong anomalous northerly winds were observed over the Philippines. Stronger westerly winds were also observed between latitudes 8ºN and 8ºS, and longitudes 125ºE and 140ºE. Figure 3 shows the average and anomalous winds at 5000 feet.

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Figure 3: 5000 ft average winds (left) and anomaly (right) for March 2018.

1.6 During the month, the cool anomalies of the equatorial Pacific Ocean’s sea surface temperature (SST) over the Niño 3.4 region indicated neutral conditions (neither El Niño or La Niña) values. Trade winds and cloudiness over the equatorial Pacific Ocean has returned to near-average levels, which is also consistent with a neutral state.

1.7 During the first half of March 2018, the Madden Julian Oscillation (MJO) progressed through Phase 3, before gradually weakening towards the end of week 2. The MJO in Phase 3 typically brings slightly enhanced rainfall over the equatorial Southeast Asia region during this period of the year. However, the MJO event did not bring significant rainfall anomaly to the region in March 2018. For the rest of the month, the MJO signal remained weak.

Figure 4: The MJO phase diagram for March 2018 (green). The diagram illustrates the movement of the MJO through different phases, which correspond to different locations along the equator. The distance of the index from the centre of the diagram is correlated with the strength of MJO. When the index falls within the circle, the MJO is considered weak or indiscernible. (Source: Bureau of Meteorology).

2. Review of Land/Forest Fires and Smoke Haze Situation

2.1 In March 2018, the hotspot counts in the northern ASEAN region remained elevated as dry weather conditions prevailed. In particular, the number of hotspots observed over Myanmar had increased compared to early 2018. Persistent haze was observed over parts of Myanmar on many days. Elsewhere, scattered hotspots with localised smoke plumes were observed on occasions.

2.2 In the southern ASEAN region, hotspot conditions were generally subdued due to prevailing wet conditions.

2.3 Satellite images showing the hotspots detected in the ASEAN region in March 2018 are given in Figure 5 to Figure 8.

Figure 5: NOAA-19 satellite image on 11 March 2018 shows scattered hotspots over Cambodia. A thin smoke plume was observed in the vicinity of a hotspot.

Figure 6: NOAA-19 satellite image on 20 March 2018 shows scattered hotspots with smoke plumes near the border between Myanmar and Thailand.

Figure 7: NOAA-19 satellite image on 23 March 2018 shows cluster of hotspots with smoke haze in southern part of Lao PDR.

Figure 8: NOAA-19 satellite image on 20 March 2018 shows shower activities affected most parts of the southern ASEAN region.

2.4 The hotspot distribution and daily hotspot charts for March 2018 are shown in Figure 9, Figure 10 and Figure 11.

Figure 9: NOAA-19 hotspots distribution in March 2018.

Figure 10: Hotspot Counts in Cambodia, Lao PDR, Thailand, Vietnam and Myanmar in March 2018.

Figure 11: Hotspot Counts in Sumatra, Borneo and Peninsular Malaysia in March 2018

3. Status of El Niño/La Niña and Indian Ocean Dipole

3.1 Based on experts’ assessments from international climate centres, the prevailing La Niña conditions weakened and transitioned to neutral (neither El Niño nor La Niña) conditions in March 2018. The neutral conditions are expected to last until the third quarter of 2018.

3.2 In March 2018, the Indian Ocean Dipole (IOD) index remained at neutral levels (Figure 12). In the coming months, international climate models forecast the IOD to remain neutral, with some chance that a negative IOD may develop later in the year. The IOD 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 February and April.

Figure 12: Indian Ocean Dipole (IOD) index time series. The IOD index was at neutral levels in March 2018. (Source:

Bureau of Meteorology, Australia).

March 12, 2018

Review of Regional Weather and Smoke Haze for Feb 2018

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1. Review of Regional Weather Conditions in February 2018

1.1 In February 2018, Northeast Monsoon conditions continued to prevail over Southeast Asia with the northern ASEAN region still in the midst of the traditional dry season, and the southern ASEAN region experiencing wet weather conditions or it traditional rainy season.

1.2 In the northern ASEAN region, dry weather conditions were felt mostly over Myanmar, Cambodia and parts of Thailand and Vietnam. With the presence of the monsoon trough and its associated rain band lying close to the Equator, most parts of the southern ASEAN region experienced wet weather conditions in February 2018.

1.3 The rainfall distribution for February 2018 and the percent of normal rainfall for February 2018 are shown in Figure 1 and 2.

Figure 1: Daily average rainfall for the ASEAN region in February 2018. (Source: JAXA Global Satellite Mapping of Precipitation).

Figure 2: Percent of Normal Rainfall for February 2018. The rainfall data may be less representative for areas with a less dense rainfall network.

1.4 In the first half of February 2018, a mass of dry continental air from the northern Asian landmass brought dry and warm conditions to many parts of Southeast Asia, extending from northern Myanmar to as far south as over southern Peninsular Malaysia, Singapore and central Sumatra. During the first two weeks of February 2018, most of the shower activities fell over parts of Southeast Asia, south of the Equator, and near-normal rainfall was recorded there.

1.5 The tropical cyclone activity in February was low. There was only one tropical cyclone, Tropical Storm “Sanba” which developed on 11 February 2018 over the Pacific Ocean, east of Mindanao, the Philippines. Tropical Storm “Sanba” tracked to the west and made landfall over Mindanao, the Philippines two days later. “Sanba” brought heavy rainfall, strong wind gusts of around 75km/h and landslides in parts of the central and southern Philippines as the storm moved west northwest across the Philippines towards the South China Sea before it dissipated off the coast of Vietnam. Figure 3 shows the storm track of Tropical Storm Sanba.

Figure 3: Historical storm track for Tropical Storm Sanba. (Source: JAXA)

1.6 In February 2018, the prevailing winds over Southeast Asia, north of the Equator blew predominantly from the northeast or east, while around and to the south of the Equator, the winds were generally from the west or northwest. During the month, there were strong anomalous westerly winds observed between latitudes 10^o N and 10^o S, and from 120^o E to 170^oE. Figure 4 shows the average and anomalous winds at 5000 feet.

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Figure 4: 5000 ft average winds (left) and anomaly (right) for February 2018.

1.7 During the month, the sea surface temperature (SST) over the Niño 3.4 region in the equatorial Pacific Ocean remained at La Niña values, but the cool anomalies showed a gradually declining trend. Cloudiness over the equatorial Pacific Ocean was slightly below average during the month, indicative of a weakening of the La Niña conditions. Although the SST and cloudiness were suggestive of La Niña conditions, the trade winds over the equatorial Pacific Ocean, were observed to be near-normal, partly due to the occurrence of the westerly wind burst associated with the active MJO phase 7 and 8.

1.8 The Madden Julian Oscillation (MJO) was active throughout the month, propagating from a start phase of Phase 7 and ending in Phase 2 towards the end of the month. In addition to the occurrence of the mass of dry and warm air over most parts of Southeast Asia in the first half of February 2018, rainfall in these areas were also suppressed due to the MJO which was in phase 7 and 8. The shift in the MJO to phase 1 and 2 towards the end of the month contributed to an increase in intense organised convection over the eastern Indian Ocean, and brought an increase in shower activities over the southern ASEAN region.

Figure 5: The MJO phase diagram for February 2018 (green). The diagram illustrates the movement of the MJO through different phases, which correspond to different locations along the equator. The distance of the index from the centre of the diagram is correlated with the strength of MJO. When the index falls within the circle, the MJO is considered weak or indiscernible. (Source: Bureau of Meteorology).

2. Review of Land/Forest Fires and Smoke Haze Situation

2.1 In February 2018, the drier weather in the northern ASEAN region led to escalations in hotspot activities, in particular in Cambodia, Lao PDR, Myanmar and parts of northern Thailand. On some days, smoke plumes were seen emanating from some of these hotspots, and the haze from these fire spots affected the air quality in parts of the northern ASEAN region. Several places such as Lampang in northern Thailand reported moderate to unhealthy air quality levels.

2.2 In the southern ASEAN region, the dry and warm weather experienced in the first two weeks of February 2018 led to an increase in hotspot activities in parts of Sumatra and Kalimantan. The hotspot activities were generally localised and short-lived, and the hotspot activities were subsequently subdued with the return of showers in the region around mid-February 2018.

2.3 Satellite images showing the hotspots detected in the ASEAN region in February 2018 are given in Figure 6 to Figure 9.

Figure 6: NOAA-19 satellite image on 1 February 2018 shows hotspots over parts of eastern Thailand, Lao PDR and Cambodia. Smoke plumes were observed in the vicinity of some of the hotspots.

Figure 7: NOAA-19 satellite image on 15 February 2018 shows scattered hotspots over Cambodia.

Figure 8: NOAA-19 satellite image on 18 February 2018 shows dry weather conditions over the southern ASEAN region.

Figure 9: NOAA-19 satellite image on 21 February 2018 shows isolated hotspots over Myanmar.

2.4 The hotspot distribution and daily hotspot charts for February 2018 are shown in Figure 10, Figure 11 and Figure 12.

Figure 10: NOAA-19 hotspots distribution in February 2018.

Figure 11: Hotspot Counts in Cambodia, Lao PDR, Thailand, Vietnam and Myanmar in February 2018.

Figure 12: Hotspot Counts in Sumatra, Borneo and Peninsular Malaysia in February 2018

3. Status of El Niño/La Niña and Indian Ocean Dipole

3.1 Based on expert assessments from international climate centres, the prevailing La Niña conditions have been weakening in recent weeks, and a return to neutral (neither El Niño nor La Niña) conditions are expected in April 2018.

3.2 Typically, La Niña brings wetter-than-normal rainfall conditions. During the northeast Monsoon season (Dec – Mar), the impact of La Nina on the weather over the Southeast Asia region is usually less pronounced compared to the Southwest Monsoon season (Jun – Sep), especially for areas in the near-equatorial region.

3.3 In February 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 February and April.

Figure 13: Indian Ocean Dipole (IOD) index time series. The IOD index was at neutral levels in February 2018. (Source: Bureau of Meteorology, Australia).

February 21, 2018

Review of Regional Weather and Smoke Haze for Jan 2018

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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.

Figure 1: Daily average rainfall for the ASEAN region in January 2018. (Source: JAXA Global Satellite Mapping of Precipitation).

Figure 2: Percent of Normal Rainfall for January 2018. The rainfall data may be less representative for areas with a less dense rainfall network.

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.

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Figure 3: (Left) Historical storm track for Tropical Cyclone Bolaven. (Source: JAXA); (Right) ) NOAA-19 satellite image on 3 January 2018 shows Typhoon Bolaven, located over the South China Sea.

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 5^o N and 10^o S, extending from the Indian Ocean to 125^o 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.

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Figure 4: 5000 ft average winds (left) and anomaly (right) for January 2018.

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.

Figure 5: Historical storm track for Typhoon Tembin. (Source: JAXA)

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.

Figure 6: NOAA-19 satellite image on 1 January shows wet weather conditions prevailed over most parts of the southern ASEAN region.

Figure 7: NOAA-19 satellite image on 12 January 2018 shows dry condition and scattered hotspots detected over Sub-Mekong region

Figure 8: NOAA-19 satellite image on 26 January 2018 shows isolated hotspots over parts of Myanmar.

Figure 9: NOAA-19 satellite image on 29 January 2018 shows scattered hotspots over the Sub-Mekong region.

2.3 The hotspot distribution and daily hotspot charts for January 2018 are shown in Figure 10, Figure 11 and Figure 12.

Figure 10: NOAA-19 hotspots distribution in January 2018.

Figure 11: Hotspot Counts in Cambodia, Lao PDR, Thailand, Vietnam and Myanmar in January 2018.

Figure 12: Hotspot Counts in Sumatra, Borneo and Peninsular Malaysia in January 2018

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.

Figure 13: Indian Ocean Dipole (IOD) index time series. The IOD index was at neutral levels in January 2018. (Source: Bureau of Meteorology, Australia).