Review of Regional Weather for August 2021

1. Overview

1.1 During August 2021, there was above-average rainfall recorded over much of the ASEAN region, while below-average rainfall was recorded over the northeastern ASEAN region (Figure 1). The largest positive anomalies (wetter conditions) were found across the equatorial region for both satellite-derived rainfall estimates datasets (GSMaP-NRT and CMORPH-Blended). The two datasets were also in agreement that the largest negative anomalies over land (drier conditions) were observed over parts of northern Viet Nam. However, there were some discrepancies over the southern Myanmar, where CMORPH-Blended recorded wetter conditions while GSMaP-NRT recorded drier conditions.

1.2 The observed rainfall anomaly pattern of above-average rainfall for much of the ASEAN region is broadly consistent with the predictions from the subseasonal weather outlooks for August 2021 (26 July – 8 August, 9 – 22 August and 23 August – 5 September).

Figure 1: Rainfall anomalies for the month of August 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 the northern ASEAN region and southern Maritime Continent experienced above-average temperatures during August 2021 (Figure 2). The warmest anomalies (above-average temperature) were recorded over northern Viet Nam, where the drier conditions were observed. For the equatorial region and central and northern Myanmar, near- to below-average temperature was recorded.



Figure 2: Temperature anomalies for the month of August 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 August 2021, a Madden-Julian Oscillation (MJO) signal was present in the Western Hemisphere (Phase 8, Figure 3), which weakened by the middle of the first week based on the RMM plot. The signal emerged over the Indian Ocean (Phase 2) in the second week of August, with a rapid strengthening of the signal in the RMM plot and slow eastward propagation. This MJO signal weakened in the second half of August while it was still over the Indian Ocean. The mix of below- to above-average rainfall for the ASEAN region is indicative of the MJO signal: Phase 2 tends to bring drier conditions over the northeastern ASEAN region and wetter conditions for the western Maritime Continent at this time of the year.



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 The equatorial Pacific Ocean was in the ‘ENSO-neutral’ state while a negative Indian Ocean Dipole (IOD) was present in August. At the seasonal timescale, negative IOD events tend to bring wetter conditions to the southern Maritime Continent.