Review of Regional Weather for September 2020

1. Overview

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).

Figure 1: Rainfall anomalies for the month of September 2020 based on GSMaP-NRT data (left) and CMORPH-Blended data (right). The climatological reference period is 2001-2019. Green colour denotes above-average rainfall (wetter), while orange denotes below-average rainfall (drier).


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.



Figure 2: Temperature anomalies for the month of September 2020 based on ERA-5 reanalysis. The climatological reference period is 2001-2019. Red colour denotes above-average temperature (warmer), while blue denotes below-average temperature (colder).


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.



Figure 3: The MJO phase diagram. The diagram illustrates the movement of the MJO through different phases, which correspond to different locations along the equator (denoted in the text with the first day of the month in blue and the last day of the month in red). The distance of the index from the centre of the diagram is related to the strength of the MJO. Values within the grey circle are considered weak or indiscernible (data from the Bureau of Meteorology, Australia).