Review of Regional Weather for March 2021

1. Overview

1.1 During March 2021, much of the land masses of the Maritime Continent experienced above-average rainfall (Figure 1). The largest positive anomalies (wetter conditions) were recorded over western coastal parts of Sumatra, eastern coastal parts of Peninsular Malaysia and western Borneo based on both satellite-derived rainfall estimates datasets (GSMaP-NRT and CMORPH-Blended). For Mainland Southeast Asia, rainfall anomalies were largely negligible for most of the region, as expected during the dry season for the northern ASEAN region, except for the southern tips of Thailand and Myanmar where the dry conditions are more pronounced.

1.2 The observed large-scale rainfall anomaly pattern (i.e. above-average rainfall over land masses of the Maritime Continent is broadly consistent with the predictions from the subseasonal weather outlooks for March 2021 (22 Feb – 7 Mar, 8 – 21 Mar, 22 Mar – 4 Apr), apart from the anomalies for some parts of southern Sumatra and central Java not highlighted in the outlooks.

Figure 1: Rainfall anomalies for the month of March 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 Southeast Asia south of 10°N experienced near-average temperatures during March 2021 (Figure 2). In contrast, warmer-than-average temperatures were recorded over most of Southeast Asia north of 10°N, except for some south-eastern parts of Mainland Southeast Asia where below- to near-average temperatures are observed.



Figure 2: Temperature anomalies for the month of March 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 During the first half of March 2021, the Madden-Julian Oscillation (MJO) signal was weak (Figure 3). In the second half of March, it strengthened in the Western Hemisphere (Phases 8 and 1) and propagated eastward through the Indian Ocean (Phases 2 and 3) and western Maritime Continent (Phase 4). Phase 2 and Phase 3 tend to bring wetter conditions to western Maritime Continent and whole of Maritime Continent respectively.



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 much of Southeast Asia during the September to March period.