Review of Regional Weather for April 2021

1. Overview

1.1 During April 2021, much of the Mainland Southeast Asia and the Malay Peninsula experienced above-average rainfall (Figure 1). The largest positive anomalies (wetter conditions) were recorded over northern parts of Thailand and Lao DPR for both satellite-derived rainfall estimates datasets (GSMaP-NRT and CMORPH-Blended), with GSMaP-NRT also recording larger anomalies over Peninsular Malaysia and eastern side of northern Sumatra. Large positive anomalies were also recorded over the eastern part of central Philippines and the region around Nusa Tenggara due to Typhoon Surigae and Tropical Cyclone Seroja, respectively. Much of the rest of the Maritime Continent experienced below-average rainfall.

1.2 The observed large-scale rainfall anomaly pattern (i.e. above-average rainfall over the northern ASEAN region and drier conditions in the southern ASEAN region) is broadly consistent with the predictions from the subseasonal weather outlooks for April 2021 (5 – 18 Apr, 19 Apr – 2 May).

Figure 1: Rainfall anomalies for the month of April 2021 based on GSMaP-NRT data (left) and CMORPH-Blended data (right). The climatological reference period is 2001-2020. Green colour denotes above-average rainfall (wetter), while orange denotes below-average rainfall (drier).


1.3 Most parts of the northern ASEAN region experienced below-average temperatures during April 2021, apart from northern parts of Myanmar and Viet Nam where above-average temperatures were recorded (Figure 2). In contrast, most parts of the southern ASEAN region experienced near- to above-average temperatures.



Figure 2: Temperature anomalies for the month of April 2021 based on ERA-5 reanalysis. The climatological reference period is 2001-2020. Red colour denotes above-average temperature (warmer), while blue denotes below-average temperature (colder).


2. Climate Drivers

2.1 At the start of April 2021, a Madden-Julian Oscillation (MJO, Figure 3) signal was present in Phase 5 (Maritime Continent). The signal then propagated eastward into the Western Pacific (Phases 6 and 7). The signal became less coherent for a time in Phase 7, before strengthening again and propagating eastwards into the Western Hemisphere (Phase 8) at the end of the month. Phase 5 tends to bring wetter conditions to the eastern half of the Maritime Content, while Phase 6 tends to bring drier conditions to the western half. Phases 7 and 8 tend to bring drier conditions to much of the Maritime Continent, similar to what is observed from the rainfall anomalies in Figure 1.



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).


2.2 Over the tropical Pacific Ocean, La Niña conditions were present but weakening. Overall, the sea surface temperatures in the central and eastern Pacific Ocean remained cool, with atmospheric indicators (cloudiness and wind anomalies) remaining consistent with the weakening La Niña conditions. La Niña events tend to bring wetter-than-average conditions to parts of Southeast Asia during the March to May period.