Review of Regional Weather for September 2020
1.1 During September 2020, most of the equatorial region and southern parts of the Mainland Southeast Asia experienced above-average rainfall (Figure 1). The largest positive anomalies (wetter conditions) were recorded in the coastal equatorial regions, southern Thailand, southern Cambodia and southern Viet Nam based on both satellite-derived rainfall estimates datasets (GSMaP-NRT and CMORPH-Blended). In contrast, most of the Philippines recorded below-average rainfall. The rest of regions either experienced near-average rainfall for this time of year, or a mix between above- and below-average.
1.2 The observed large-scale rainfall anomaly pattern (i.e. above-average rainfall in the equatorial regions and below-average rainfall in the Philippines) is broadly consistent with the predictions from the subseasonal weather outlooks for September 2020 (7 – 20 Sep, 21 Sep – 4 Oct). The only notable anomalies forecasted for Mainland Southeast Asia were the wetter conditions over its southern and eastern coastal regions (linked to the MJO signal in phases 4 and 5).
1.3 Most parts of equatorial Southeast Asia experienced near-average temperature during September 2020 (Figure 2), coinciding with areas that experienced wetter conditions during the same period. Warmer anomalies (≥ 0.5°C) are concentrated over Mainland Southeast Asia, northern parts of the Philippines and eastern Maritime Continent with the warmest anomalies in September (≥ 2.0°C) in northern Myanmar.
2. Climate Drivers
2.1 A weak and very slow-moving Madden-Julian Oscillation (MJO) signal was present in the Maritime Continent (Figure 3) during the month of September, momentarily strengthened in the middle of September for a few days and also towards the end of the month. At this time of year, phases 4 and 5 typically bring above-average rainfall for coastal regions in Southeast Asia between 5°N and 20°N.