Review of Regional Weather for October 2021
1.1 During October 2021, above-average rainfall was recorded over much of the northern ASEAN region, while a mix of below- to above-average rainfall was recorded over the southern ASEAN region (Figure 1). For the northern ASEAN region, above-average rainfall was recorded apart from over central and northern Myanmar and parts of the Philippines. The largest positive anomalies (wetter conditions) were recorded over eastern Mainland Southeast Asia for both satellite-derived rainfall estimates datasets (GSMaP-NRT and CMORPH-Blended), which was associated with Severe Tropical Storm Kompasu. A second tropical storm, Tropical Storm Lionrock, crossed the central Philippines at the start of October, contributing to the large positive anomalies in the ocean region around the Philippines (GSMaP-NRT). For the southern ASEAN region, negative anomalies (drier conditions) were recorded over western parts as well as northern Borneo, although the negative anomalies were larger based on CMORPH-Blended data compared to GSMaP-NRT. Elsewhere in the southern ASEAN region, rainfall tended to be near-to above-average during October 2021.
1.2 The observed rainfall anomaly pattern of above-average rainfall for much of the northern ASEAN region and a mix of below- to above-average rainfall for the southern ASEAN region is broadly consistent with the predictions from the subseasonal weather outlooks for October 2021 (4 – 17 October and 18 – 31 October).
1.3 Most of the ASEAN region experienced near to above-average temperatures during October 2021 (Figure 2). The exception was over eastern parts of Mainland Southeast Asia, which experienced below- to near-average temperatures. The largest positive anomalies (warmer temperatures) were over northern Myanmar and the western Maritime Continent where negative rainfall anomalies (drier conditions) were also recorded.
2. Climate Drivers
2.1 The Madden-Julian Oscillation (MJO, Figure 3) signal was present over the Maritime Continent at the start of October 2021 (Phases 4 and 5). However, this signal weakened rapidly as it approached the Western Pacific (Phase 6) likely influenced by the background La Niña conditions. For the rest of October, there was no discernible MJO signal, based on the MJO phase diagram below. Phases 4 and 5 tend to bring wetter conditions to parts of the ASEAN region around 10°N, while Phase 5 tends to bring drier conditions to the western Maritime Continent.
2.2 By the end of October 2021, La Niña conditions were present in the tropical Pacific. Sea surface temperatures in the Nino3.4 region (used to monitor ENSO) continued to cool, with some signs of La Niña conditions in the atmosphere. A weak negative Indian Ocean Dipole (IOD) was present and at the seasonal timescale, negative IOD events tend to bring wetter conditions to the southern Maritime Continent.