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Subseasonal Weather Outlook and Review

Review of Regional Weather for January 2025

 

1. Overview

1.1 During January 2025, much of the Maritime Continent experienced above-average rainfall, with only parts of southern Philippines and the Malay Peninsula experiencing below-average rainfall (Figure 1). Over Mainland Southeast Asia, there were no notable anomalies, typical of the dry season for this region. The largest positive (wetter) anomalies were recorded over the southern tip of Malay Peninsula, eastern coast of Sumatra, and northwest Borneo, while the largest negative (drier) anomalies were recorded over southern Philippines. There were disagreements between GSMaP-NRT and CMORPH-Blended datasets on the extent and magnitude of the dry anomalies over southern Philippines, with GSMaP-NRT showing stronger and more extensive dry anomalies as compared to CMORPH-Blended. More disagreements between the datasets are also evident over Sumatra and western Papua, with CMORPH-Blended tending to show drier conditions over these regions as compared to GSMaP-NRT.

1.2 The observed rainfall anomaly pattern of above-average rainfall over much of the Maritime Continent is consistent with the predictions from the subseasonal weather outlooks for January 2025 (6 – 19 January 2025, and 20 January – 2 February 2025). The observed pattern is also consistent with the January 2025 seasonal outlook of above-normal rainfall over parts of the Maritime Continent.

era5_rainfall_anomalies
cmorph_bld_rainfall_anomalies
Figure 1: Rainfall anomalies for January 2025 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 Below-average temperatures were recorded over much of Mainland Southeast Asia in January 2025 (Figure 2). Elsewhere, positive temperature anomalies were recorded over parts of Myanmar and the eastern and northeastern Maritime Continent. The coldest anomalies (-2°C – -3°C below average) were recorded over parts of northeastern Thailand and northern Cambodia, while the warmest anomalies (1°C – 2°C above average) were recorded over northwestern Myanmar.

era5_temperature_anomalies
Figure 2: Temperature anomalies for January 2025 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 In the first week of the January, no active Madden-Julian Oscillation (MJO) signal was observed. During the second week of the month, an active MJO signal emerged, and was present over the Western Hemisphere (Phases 8 and 1). Then, the signal propagated eastwards through the Indian Ocean (Phases 2 and 3) in the third week of January and continued its eastwards propagation over the Maritime Continent (Phases 4 and 5) in the last week of the month. During this time of the year, Phases 8 and 1 typically bring drier conditions to the Maritime Continent. Phase 2 and 3 and tend to induce wetter conditions over the western Maritime Continent, while Phases 4 and 5 typically bring wetter conditions over much of the Maritime Continent. This observed MJO signal during January is in line with the above-normal rainfall 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 January 2025, La Niña 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. La Niña events tend to bring wetter-than-average conditions to much of the Maritime Continent during this time of the year.