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Subseasonal Weather Outlook and Review

Review of Regional Weather for December 2024

 

1. Overview

1.1 During December 2024, the western Maritime Continent experienced below-average rainfall, while rest of the Maritime Continent as well as parts of southeastern Mainland Southeast Asia experienced predominately above-average rainfall (Figure 1). Elsewhere in Mainland Southeast Asia, there were no notable anomalies, typical of the dry season for this region. The largest positive (wetter) anomalies were recorded over southern Thailand, southern Viet Nam, and central Borneo, while the largest negative (drier) anomalies were recorded over Sumatra and the southern Malay Peninsula in both the GSMaP-NRT and CMORPH-Blended datasets.

1.2 Tobserved rainfall anomaly pattern of a below-average rainfall over parts of the western Maritime Continent, and above-average rainfall over much of the rest of the Maritime Continent and southeastern Mainland Southeast Asia is consistent with the predictions from the subseasonal weather outlooks for December 2024 (25 November – 8 December 2024, 9 – 22 December 2024), and 23 December 2024 – 5 January 2025). The observed pattern is also consistent with the December 2024 seasonal outlook of above-normal rainfall over most of the Maritime Continent except over parts of Borneo and Sumatra.

era5_rainfall_anomalies
cmorph_bld_rainfall_anomalies
Figure 1: Rainfall anomalies for December 2024 based on GSMaP-NRT data (left) and CMORPH-Blended data (right). The climatological reference period is 2001-2023. Green colour denotes above-average rainfall (wetter), while orange denotes below-average rainfall (drier).

 

1.3 Above-average temperatures were recorded over much of the Maritime Continent and northwest Mainland Southeast Asia in December 2024 (Figure 2), with below-average temperatures recorded over southeast Mainland Southeast Asia. The warmest anomalies (1°C – 2°C above average) were recorded over parts of Myanmar and northwest Thailand and Lao PDR as well as parts of Sumatra. The coolest anomalies (-1°C below average) were recorded over parts of Cambodia, Lao PDR, and Viet Nam.

era5_temperature_anomalies
Figure 2: Temperature anomalies for December 2024 based on ERA-5 reanalysis. The climatological reference period is 2001-2023. 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 slowly propagated from Phase 4 to Phase 7 during December (Figure 3). In the first half of the month, the signal propagated through the Maritime Continent (Phases 4 and 5), reaching the Western Pacific (Phases 6 and 7) during the second half of the month. Phases 4 and 5 typically bring wetter conditions for much of Southeast Asia, while Phase 6 typically bring wetter conditions to the eastern Maritime Continent and Phase 7 typically brings drier conditions to the western Maritime Continent. This pattern of below-average rainfall in the western Maritime Continent and above-average rainfall for much of the rest of the region is in line with the anomalies in Figure 1.

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 In December 2024, La Niña or La Niña-like conditions were present. Sea surface temperatures in the Nino3.4 region (used to monitor ENSO) indicated La Niña conditions and atmospheric indicators (cloudiness and trade winds) also showed signs of La Niña. However, there is some uncertainty as to whether the conditions will persist long enough to declare a La Niña event. La Niña events tend to bring wetter-than-average conditions to much of the Maritime Continent during the June to August period.