Subseasonal Weather Outlook and Review

1.1 In the second fortnight of October 2019, the prevailing winds over the northern ASEAN region blew from the northeast or east, while the winds over the southern ASEAN region were from the southeast or east. Over the equatorial areas, weaker winds prevailed. There were anomalously strong easterly winds over southern Thailand and Peninsular Malaysia, and stronger-than-usual southeast winds over southern Sumatra.

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

1.2 The presence of the monsoon rain band over the equatorial ASEAN region brought near- to above-average rainfall over Peninsula Malaysia, northern Sumatra, and the western part of Borneo Island. Over the areas surrounding the Java Sea (including southern Sumatra, southern Kalimantan and Java), rainfall was below average. The northern ASEAN region received well below-average rainfall in general, except for Viet Nam and parts of Myanmar. Figures 2 and 3 shows the daily average rainfall and the percentage of average rainfall for the second fortnight of October 2019 respectively.

1.3 Over the tropical Pacific Ocean, El Niño – Southern Oscillation (ENSO) remained neutral. In the Indian Ocean, the Indian Ocean Dipole (IOD) remained strongly positive during the fortnight.

1.4 The Madden-Julian Oscillation (MJO) propagated through Phase 2 during the third week of October 2019, before weakening towards the last week of the month. During this period, the MJO had some influence on the ASEAN region. It brought easterly anomalies and above-average rainfall over the western Maritime Continent.

Figure 4 The MJO phase diagram (green for October 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 In the second fortnight of October 2019, the prevailing winds over the northern ASEAN region blew from the northeast or east, while the winds over the southern ASEAN region were from the southeast or east. Over the equatorial areas, weaker winds prevailed. There were anomalously strong easterly winds over southern Thailand and Peninsular Malaysia, and stronger-than-usual southeast winds over southern Sumatra.

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

1.2 The presence of the monsoon rain band over the equatorial ASEAN region brought near- to above-average rainfall over Peninsula Malaysia, northern Sumatra, and the western part of Borneo Island. Over the areas surrounding the Java Sea (including southern Sumatra, southern Kalimantan and Java), rainfall was below average. The northern ASEAN region received well below-average rainfall in general, except for Viet Nam and parts of Myanmar. Figures 2 and 3 shows the daily average rainfall and the percentage of average rainfall for the second fortnight of October 2019 respectively.

1.3 Over the tropical Pacific Ocean, El Niño – Southern Oscillation (ENSO) remained neutral. In the Indian Ocean, the Indian Ocean Dipole (IOD) remained strongly positive during the fortnight.

1.4 The Madden-Julian Oscillation (MJO) propagated through Phase 2 during the third week of October 2019, before weakening towards the last week of the month. During this period, the MJO had some influence on the ASEAN region. It brought easterly anomalies and above-average rainfall over the western Maritime Continent.

Figure 4 The MJO phase diagram (green for October 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)