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Subseasonal Weather Outlook and Review

Review of Regional Weather for August 2022

 

1. Overview

1.1 During August 2022, much of the ASEAN region experienced above-average rainfall (Figure 1). The largest positive anomalies (wetter conditions) were recorded over the southern parts of Myanmar, western Borneo, and the Maluku Islands based on both satellite-derived rainfall estimates datasets (GSMaP-NRT and CMORPH-Blended). In contrast to the above-average rainfall, parts of northern Myanmar, the Malay Peninsula, northern Borneo, the Philippines, and Papua experienced below- to near-average rainfall, or a mix of between above- and below-average rainfall. The two datasets agree well over most of the ASEAN region. However, there are some discrepancies over the Philippines and west Papua where GSMaP-NRT recorded near- to above-average rainfall whereas CMORPH-Blended recorded below- to near-average rainfall.

1.2 The observed rainfall anomaly pattern of above-average rainfall for much of the Maritime Continent region is broadly consistent with the predictions from the subseasonal weather outlooks for August 2022 (25 July – 7 August, 8 – 21 August and 22 August – 4 September). While overall for August much of Mainland Southeast Asia experienced above-average rainfall as well, the subseasonal outlooks predicted wetter conditions in the second week, and drier conditions towards the end of the month.

era5_rainfall_anomalies
cmorph_bld_rainfall_anomalies
Figure 1: Rainfall anomalies for August 2022 based on GSMaP-NRT data (left) and CMORPH-Blended data (right). The climatological reference period is 2001-2021. Green colour denotes above-average rainfall (wetter), while orange denotes below-average rainfall (drier).

 

1.3 The northernmost and southernmost parts of the ASEAN region experienced above-average temperatures during August 2022, with below- to near-average temperatures elsewhere (Figure 2). The warmest anomalies were recorded over northern Myanmar and Papua, with the coolest anomalies over parts of Sumatra and Cambodia. Most of the cooler anomalies occurred for areas that also experienced above-average rainfall.

era5_temperature_anomalies

 

Figure 2: Temperature anomalies for August 2022 based on ERA-5 reanalysis. The climatological reference period is 2001-2021. Red colour denotes above-average temperature (warmer), while blue denotes below-average temperature (colder).

 

2. Climate Drivers

2.1 There were some signs of a Madden-Julian Oscillation (MJO) signal over the Maritime Continent (Phases 4 and 5) in the first week of August, however there was no discernible signal by the second week, based on the RMM Index (Figure 3). An MJO signal emerged over the Indian Ocean (Phase 2) towards the end of August, its strength briefly amplifying at the start of the last week in August, before becoming weak again at the end of the month. Typically, Phase 2 tends to bring drier conditions over the northeastern ASEAN region and wetter conditions for the western Maritime continent at this time of year, while Phases 4 and 5 bring wetter conditions to northern parts of the Maritime Continent. Therefore, while there was little MJO activity for most of August, the MJO signals near the start and end of the month may have contributed to the wetter-than-average conditions.

mjo_phase_diagram

 

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. Sea surface temperatures in the Nino3.4 region (used to monitor ENSO) and the atmospheric indicators over the tropical Pacific Ocean remain consistent with La Niña conditions. La Niña events tend to bring wetter-than-average conditions to much of the Maritime Continent during the June to August period. Over the Indian Ocean, a negative Indian Ocean Dipole (IOD) was present. Negative IOD tends to bring wetter-than-average conditions for much of Southeast Asia.