Category Archives: Fortnightly Weather Review

1.1 Southwest Monsoon conditions prevailed. There were anomalously strong southwesterly winds over the northern ASEAN region in the second fortnight of August 2019. In the southern ASEAN region, winds were blowing mainly from the southeast.

Figure 1: 5000 ft average winds (left) and anomalies (right) for 16 – 31 August 2019 (Source: JMA)

1.2 In the second half of August 2019, rainfall was generally above average, except in Cambodia, and the southern parts of Thailand and Viet Nam. Tropical Storm Podul, which formed over the Western Pacific Ocean, made landfall in Luzon, the Philippines on 28 August 2019 before tracking westward across the South China Sea. Tropical Storm Podul weakened rapidly after making landfall at northern Viet Nam on 30 August 2019, and the storm contributed to stronger monsoon winds and above-average rainfall over the Mekong sub-region and the Philippines.

Figure 2: Historical track of Tropical Storm Podul. (Source: JAXA)

1.3 In the southern ASEAN region, drier-than-average conditions were observed over many parts of Sumatra, west coast of Peninsular Malaysia, eastern Borneo and Java. The drier-than-average conditions could be partly attributed to the presence of Indian Ocean Dipole in the positive phase.

Figure 3: Daily average rainfall for the ASEAN region in the second fortnight of August 2019 (Source: JAXA Global Satellite Mapping of Precipitation)

Figure 4: Percent of average rainfall for 16 – 31 August 2019. The rainfall data may be less representative for areas with a less dense rainfall network. (Source: IRI NOAA/NCEP CPC Unified Precipitation Analyses)

1.4 During the review period, the Madden-Julian Oscillation (MJO) propagated through the Indian Ocean and Western Maritime Continent. Although the signal was not strong (Figure 5), it might have influenced rainfall over parts of the northern ASEAN region.

Figure 5: The MJO phase diagram (green for August 2019). 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)

1.1 Southwest Monsoon conditions prevailed in the first half of August 2019. In the northern ASEAN region, the prevailing winds were blowing mostly from the southwest or west. During the fortnight, the westerly winds were anomalously strong and stronger-than-usual winds were also observed over the Philippines. The stronger winds could have been due to the presence of Tropical Storm Wipha over the South China Sea in early August 2019 and Typhoon Lekima over the western Pacific Ocean from 4 August till 12 August 2019. Wipha made landfall in northern Viet Nam on 3 August, bringing heavy rain and flash floods to the area. The rainband associated with the storms contributed to the well above-average rainfall recorded over northern Philippines and many parts of the Mekong sub-region.

1.2 In the southern ASEAN region, winds were blowing mainly from the southeast or southwest. The dry conditions observed since July 2019 persisted into the first half of August 2019. Well below-average rainfall was observed over many areas. The dry conditions could be attributed partly to the presence of the Indian Ocean Dipole (IOD) in the positive phase.

Figure 1: 5000 ft average winds (left) and anomalies (right) for 1 – 15 August 2019 (Source: JMA)

Figure 2: Daily average rainfall for the ASEAN region in the first fortnight of August 2019 (Source: JAXA Global Satellite Mapping of Precipitation)

Figure 3: Percent of average rainfall for 1 – 15 August 2019. The rainfall data may be less representative for areas with a less dense rainfall network. (Source: IRI NOAA/NCEP CPC Unified Precipitation Analyses)

1.3 There were signs of the Madden-Julian Oscillation (MJO) appearing briefly in Phases 3 and 4 (6th – 11th August), although MJO activity was insignificant by the end of the fortnight. The enhanced shower activities in the northern ASEAN region were consistent with the typical rainfall response from MJO in Phases 3 and 4 during this period.

Figure 4: The MJO phase diagram (blue for August 2019). 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)

1.4 In the first fortnight of August 2019, the El Niño – Southern Oscillation (ENSO) remained neutral (i.e neither El Niño nor La Niña conditions).

1.1 Southwest Monsoon conditions continued to prevail in the second half of July 2019. The prevailing winds over the ASEAN region were blowing mainly from the southeast or southwest. During this period, rain bands associated with Tropical Storm Danas and Tropical Storm Wipha, which developed over the Philippine Sea and the South China Sea respectively, brought rainy weather to the northern ASEAN region. Above average rainfall was recorded in some parts of the northern ASEAN region.

1.2 In the southern ASEAN region, drier-than-normal conditions persisted over Peninsular Malaysia, Sumatra, Java and parts of Kalimantan. These conditions could be partly attributed to the presence of the Indian Ocean Dipole (IOD) which is currently in its positive phase.

Figure 1: 5000 ft average winds (left) and anomalies (right) for 16 – 31 July 2019 (Source: JMA)

Figure 2: Tracks of Tropical Storm Danas and Tropical Storm Wipha

Figure 3: Daily average rainfall for the ASEAN region in the second fortnight of July 2019 (Source: JAXA Global Satellite Mapping of Precipitation)

Figure 4: Percent of average rainfall for 16 – 31 July 2019. The rainfall data may be less representative for areas with a less dense rainfall network. (Source: IRI NOAA/NCEP CPC Unified Precipitation Analyses)

1.3 The Madden-Julian Oscillation (MJO) was in Phase 1 and became insignificant towards the last week of the month. The MJO did not have a significant impact on the weather over the region during the past fortnight.

Figure 5: The MJO phase diagram (green for July 2019). 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)

1.4 In the tropical Pacific Ocean, there was further cooling of the surface and there was no significant atmospheric coupling. The El Niño – Southern Oscillation (ENSO) remained neutral in the second half of July 2019.

1.1 In the second fortnight of June 2019, moderate to strong southwesterly or westerly winds prevailed over the northern ASEAN region. In the southern ASEAN region, winds continued to blow from the southeast or southwest. No significant wind anomalies were observed during this period.

1.2 In the second half of June 2019, the monsoon rain band brought widespread showers over parts of the northern ASEAN region, in particular over the coastal regions. In the southern ASEAN region, most of the rain/showers fell over the equatorial region including Malaysia, Sumatra and Kalimantan. Over Java and the southern parts of Sumatra and Kalimantan, dry weather prevailed and rainfall was below-average.

1.3 In the early half of the fortnight, the Madden-Julian Oscillation (MJO) was in the Maritime Continent but weakened in the later part of the fortnight. There were no clear MJO signals during the period as shown in Figure 4.

1.4 The El-Nino Southern Oscillation (ENSO) is currently in the neutral state. Sea-surface temperature (SST) anomalies remained slightly warmer than average in the central eastern Pacific.

Figure 4: The MJO phase diagram for June 2019 (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 generally considered weak or indiscernible. (Source: Bureau of Meteorology)

1.1 In the second fortnight of May 2019, prevailing winds were mostly from the southwest or west over the northern ASEAN region. In the southern ASEAN region, winds were generally weak and variable in direction over the equatorial region, but moderate to strong south-easterly winds prevailed over the Java Sea and Banda Sea areas.

1.2 In the second half of May 2019, there were shower activities on most days over many parts of the ASEAN region except over the Java Sea area where dry weather prevailed. This was brought about by the incursion of dry continental air mass from Australia, transported by the anomalously strong south-easterly winds over the Java Sea during the review period. Below-average rainfall was also observed in southern Sumatra, parts of Kalimantan, northern Myanmar and eastern Thailand.

1.3 The Madden-Julian Oscillation (MJO) propagated through Phases 8 – 1, and this could have contributed to the drier-than-average conditions over the Java Sea area. The MJO contributed to large-scale wind anomalies over the Indian Ocean and the Java Sea. Toward the end of May, the MJO reached the Indian Ocean (Phase 2).

1.4 Slightly warm sea-surface temperature (SST) anomalies over the Nino3.4 region were still observed. There was a slight weakening of the trade winds in the eastern Pacific Ocean towards the end of May 2019. Overall, there were no clear indications of El Nino conditions as the coupling of the SST and atmosphere over the tropical Pacific remained inconsistent.

Figure 4: The MJO phase diagram for April 2019-May 2019 (green for May). 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)

1.1 Under inter-monsoon conditions, the prevailing winds over the equatorial region were mostly light and variable in direction over the past fortnight. Over the northern ASEAN region, there were anomalous westerly winds (Figure 1).

Figure 1 5000 ft average winds (left) and anomalies (right) for 1 – 15 May 2019 (Source: JMA)

Figure 2: Daily average rainfall for the ASEAN region in the first fortnight of May 2019. (Source: JAXA Global Satellite Mapping of Precipitation)

Figure 3: Percentage of average rainfall for 1 – 15 May 2019. The rainfall data may be less representative for areas with a less dense rainfall network. (Source: IRI NOAA/NCEP CPC Unified Precipitation Analyses)

1.2 There were scattered showers over the region in the first half of May (Figure 2), with the exception of Myanmar where generally dry weather prevailed. Myanmar and the northern parts of Lao PDR and Thailand received below-average rainfall, which could have been due to dry air from the Bay of Bengal that persisted over these areas early in the month. Many areas along the equatorial region (including southern parts of Peninsular Malaysia, Sumatra, and Borneo) received below-normal rainfall (Figure 3), and this could in part be attributed to the Madden-Julian Oscillation (MJO) activities from Phase 5 to Phase 8 during the fortnight.

Figure 4: The MJO phase diagram for March 2019-May 2019 (green for April). 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)

1.3 In the first half of May, although there was a slight cooling of the sea surface, the sea surface temperatures of the equatorial Pacific Ocean remained warmer than average. There was no consistent coupling between the sea surface temperatures and the atmosphere, indicating that full-fledged El Niño conditions have not developed.

1.1 The ASEAN region is under the influence of inter-monsoon conditions. Southwesterly or westerly winds prevailed over the Mekong sub-region, with Myanmar experiencing stronger-than-usual westerly winds along the coast. Winds were generally weak in the southern ASEAN region, except for the area around Papua and south of Kalimantan where there were stronger-than-usual westerly or northwesterly winds due to Typhoon Wallace and a developing low pressure system in the Timor Sea.

1.2 Inter-monsoon conditions contributed to an increase in showers compared to March 2019 over the northern ASEAN region. However, dry weather continued to persist over many areas in Myanmar, northern Thailand and northern Lao PDR.

1.3 In the southern ASEAN region, showers fell over many parts of the region. Drier-than-average conditions were observed over northern Sumatra, east coast of Peninsular Malaysia, Sabah, Sarawak, and West Kalimantan.

Figure 2: Daily average rainfall for the ASEAN region in the First Fortnight of April 2019. (Source: JAXA Global Satellite Mapping of Precipitation)

Figure 3: Percent of average rainfall for 1 – 15 April 2019. The rainfall data may be less representative for areas with a less dense rainfall network. Hatched areas indicate climatology dry mask (average daily rainfall below 1 mm). (Source: IRI NOAA/NCEP CPC Unified Precipitation Analyses)

1.4 The Madden-Julian Oscillation (MJO) signal remained weak during the first fortnight of April 2019 and had no significant influence on the weather over the ASEAN region (Figure 4).

Figure 4: The MJO phase diagram (blue for April 2019). 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)