Review of Regional Weather for December 2022
1.1 During December 2022, a mix of below- to above-average rainfall was recorded over the Maritime Continent, while near-average rainfall was recorded over much of Mainland Southeast Asia, except for coastal parts of eastern Mainland Southeast Asia where above-average rainfall was recorded (Figure 1). The largest positive anomalies (wetter conditions) were recorded over northwestern Continent, including Malay Peninsula for both the GSMaP-NRT (Figure 1, left) and CMORPH-Blended (Figure 1, right) satellite-derived rainfall estimates. The largest negative anomalies (drier conditions) were recorded over the southern Borneo, with CMORPH-Blended data showing relatively drier anomalies as compared to GSMaP-NRT data.
1.2 The observed rainfall anomaly pattern of a mix of below- to above-average rainfall for the Maritime Continent and the above-average rainfall for eastern Mainland Southeast Asia are broadly consistent with the predictions from the subseasonal weather outlooks for December 2022 (28 November – 11 December 2022, 12 – 25 December 2022 and 26 December 2022 – 8 January 2023).
1.3 Warmer-than-average temperatures were recorded over northwestern Mainland Southeast Asia and parts of southeastern Maritime Continent, whereas below- to near-average temperatures were observed elsewhere over the ASEAN region. The warmest anomalies were recorded over Myanmar and the coolest over the eastern Mainland Southeast Asia.
2. Climate Drivers
2.1 An Madden-Julian Oscillation (MJO) signal emerged over the Indian Ocean (Phase 3) and the Maritime Continent (Phase 4) at the end of the first week of December, based on the RMM index (Figure 3). This signal weakened and went inside the RMM unit circle at the end of the second week of December. At the end of the third week, an MJO signal reappeared over the Maritime Continent (Phases 4 & 5), which strengthened and propagated eastwards towards the Western Pacific (Phase 6) over the last week of December. Typically, during this time of the year, Phases 4 and 5 bring wetter conditions for most of the ASEAN region whereas Phase 6 tends to bring wetter conditions for eastern Maritime Continent. Therefore, the MJO may have contributed to the wetter conditions over northern parts of the Maritime Continent region December 2022.
2.2 La Niña conditions were present over the tropical Pacific. 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, though there are signs of weakening. La Niña events tend to bring wetter-than-average conditions to much of Southeast Asia during the September to November period.