Review of Regional Weather for May 2021

1. Overview

1.1 During May 2021, there was a mix of below- to above-average rainfall over the ASEAN region. For the equatorial region, the GSMaP-NRT dataset (Figure 1a) shows near- to above-average rainfall (except for parts of central Borneo), while the CMORPH-Blended dataset (Figure 1b) shows a mixture of below- and above-average rainfall. The largest positive anomalies (wetter conditions) were recorded over southern Philippines for both satellite-derived rainfall estimates datasets, due to Tropical Depression 03W. Elsewhere in the Maritime Continent, the southern Maritime Continent and northern Philippines experienced below-average rainfall, with the largest negative anomalies (drier conditions) recorded over northern Philippines. For Mainland Southeast Asia, parts of central Mainland Southeast Asia experienced below-average rainfall, with CMORPH-Blended showing a larger proportion of Thailand receiving below-average rainfall, while GSMaP-NRT extended the below-average rainfall over coastal Myanmar. Near- to above-average rainfall was experienced elsewhere in Mainland Southeast Asia.

1.2 The observed rainfall anomaly pattern (i.e. below-average rainfall over southern Maritime Continent, above-normal rainfall in the equatorial region) is broadly consistent with the predictions from the subseasonal weather outlooks for May 2021 (3 – 16 May, 17 – 30 May).

Figure 1: Rainfall anomalies for the month of May 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 of Mainland Southeast Asia experienced above-average temperatures during May 2021, except for parts of Myanmar, Cambodia, and southern Viet Nam (Figure 2). Above-average temperatures were also experienced in northern Philippines and the southern Maritime Continent. Elsewhere, near-average temperatures were experienced, apart from below-average temperature for southern Philippines associated with the heavy rainfall.



Figure 2: Temperature anomalies for the month of May 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 May 2021, a strong Madden-Julian Oscillation (MJO, Figure 3) signal was present in Phase 1 (Western Hemisphere). The signal then propagated eastward into the Indian Ocean (Phases 2 and 3), where it became less coherent for some time before strengthening again and propagating eastwards to the Maritime Continent (Phases 4 and 5). It subsequently weakened upon approaching Phase 6 with no coherent MJO signal for the rest of the month based on the RMM index. Phases 3 and 4 tend to bring wetter conditions to much of the Maritime Content, 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 of the Maritime Continent.



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 decayed and returned to ENSO-neutral conditions by the beginning of June. The sea surface temperatures in the central and eastern Pacific Ocean continued to warm, with atmospheric indicators (cloudiness and wind anomalies) remaining consistent with the ENSO-neutral conditions.