Review of Regional Weather for December 2021
1.1 During December 2021, coastal central parts of Viet Nam, Peninsular Malaysia and southern half of the Philippines experienced above-average rainfall (Figure 1). The largest positive anomalies (wetter conditions) were recorded over the central Philippines and the Peninsular Malaysia (due to Super Typhoon Rai and Tropical Depression 29W respectively, which made landfall in mid-December), for both satellite-derived rainfall estimates datasets (GSMaP-NRT and CMORPH-Blended). For the rest of the Mainland Southeast Asia, rainfall anomalies were negligible, as expected at the start of dry season for the northern ASEAN region. A mix of above- and below-average rainfall was experienced for most regions in the equatorial region. There is some discrepancy between the two satellite datasets for the below-average rainfall for central parts of Borneo, with more extensive drier conditions based on CMORPH-Blended (Figure 1, right) compared to near- to above-average rainfall for GSMaP-NRT (Figure 1, left). Elsewhere in the southern Maritime Continent, wetter-than-average rainfall conditions were recorded over much of the land masses.
1.2 The observed large-scale rainfall anomaly pattern (i.e. above-average rainfall in regions linked to tropical storm and cyclone activity and below-average rainfall over parts of equatorial region) is broadly consistent with the predictions from the subseasonal weather outlooks for December 2021 (29 November – 12 December, 13 – 26 December, and 27 December – 9 January), apart from the positive anomalies (wetter conditions) for Peninsular Malaysia not highlighted in the outlooks.
1.3 Below-average temperatures were recorded over central and eastern regions of Mainland Southeast Asia in December 2021 (Figure 2). In contrast, above-average temperatures (more than 1°C warmer) were recorded over parts of Myanmar. Elsewhere, near- to slightly above-average temperature were recorded.
2. Climate Drivers
2.1 A Madden-Julian Oscillation (MJO) was present over the Western Pacific (Phase 6) at the start of December 2021, based on the MJO phase diagram below (Figure 3). From second week of December onwards, this signal was not very coherent (very slow eastwards propagation towards Phase 7) and likely influenced by the background La Niña conditions. Phase 6 tends to bring wetter conditions to eastern Maritime Continent, while Phase 7 tends to bring drier conditions to the western Maritime Continent at this time of the year.
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 were consistent with La Niña conditions. La Niña events tend to bring wetter-than-average conditions to much of the Maritime Continent during the December to February period.