Review of Regional Weather for December 2023


1. Overview

1.1 During December 2023, a mix of below- to above-average rainfall was recorded over the Maritime Continent (Figure 1). Near- to above-average rainfall was recorded over much of the western Maritime Continent and Borneo, with below- to near-average over much of the rest of the region. Over Mainland Southeast Asia, near-average rainfall was recorded for most of the region. The largest positive (wetter) anomalies for December were recorded over the Malay Peninsula and northern Sumatra based on GSMaP-NRT (Figure 1, left) and CMORPH-Blended (Figure 1, right) satellite-derived rainfall estimates. In contrast, the largest negative (drier) anomalies were recorded over central Philippines (in both GSMaP-NRT and CMORPH-Blended), as well as Java and parts of Papua (CMORPH-blended only). The other notable difference between the two datasets was over central Viet Nam where CMORPH-Blended recorded above-average rainfall, while GSMAP-NRT recorded near average.

1.2 The observed rainfall anomaly pattern of above-average rainfall over the western Maritime Continent, below- to near- average rainfall for the rest of the Maritime Continent, and near-average over Mainland Southeast Asia, is broadly consistent with the predictions from the subseasonal weather outlooks for December 2023 (27 November – 10 December 2023, 11 – 24 December 2023 and 25 December 2023 – 7 January 2024).

Figure 1: Rainfall anomalies for December 2023 based on GSMaP-NRT data (left) and CMORPH-Blended data (right). The climatological reference period is 2001-2022. Green colour denotes above-average rainfall (wetter), while orange denotes below-average rainfall (drier).


1.3 Above-average temperatures were recorded over Southeast Asia in December 2023 (Figure 2). The warmest anomalies (more than 2°C above average) were recorded over parts of Mainland Southeast Asia, including parts of Myanmar and northwestern Thailand.



Figure 2: Temperature anomalies for December 2023 based on ERA-5 reanalysis. The climatological reference period is 2001-2022. Red colour denotes above-average temperature (warmer), while blue denotes below-average temperature (colder).


2. Climate Drivers

2.1 A Madden-Julian Oscillation (MJO) signal was present over the Indian Ocean (Phase 3) at the start of December 2023 (Figure 3). This signal propagated eastwards during December, weakening temporarily as it moved through the Western Pacific (Phases 6 and 7) and Western Hemisphere (Phase 8) towards the middle of the month. By the end of December, the MJO signal had strengthened again and returned to the Indian Ocean (Phase 2). Considering the phases with the stronger MJO signal (based on the RMM index), typically for this time of year, Phases 4 and 5 bring wetter conditions for most of the ASEAN region whereas Phases 1 and 2 bring drier conditions to the eastern Maritime Continent, with Phase 2 also bringing wetter conditions to the western Maritime Continent.



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 El Niño conditions persisted over the equatorial Pacific during December 2023. Sea surface temperatures in the Nino3.4 region (used to monitor ENSO) continue to show El Niño conditions, with key atmospheric indicators (cloudiness and trade winds) also supporting these conditions. The positive Indian Ocean Dipole started to weaken in December. El Niño events tend to bring drier and warmer-than-average conditions to much of the Maritime Continent during December – February. Positive Indian Ocean Dipole events also tend to bring drier conditions to most of the southern half of the Maritime Continent during this period.