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Subseasonal Weather Outlook and Review

Review of Regional Weather for October 2025

 

1. Overview

1.1 During October 2025, a mix of below- to above-average rainfall was recorded for Southeast Asia (Figure 1). For the Maritime Continent, most of the southern and eastern regions recorded positive (wetter) anomalies in both datasets (GSMaP-NRT and CMORPH-Blended), with below-average rainfall recorded over the western region and parts of the Philippines. Over Mainland Southeast Asia, below-average rainfall was recorded over the southern region, with near- to above-average rainfall elsewhere. Generally, CMORH-Blended (Figure 1, right) recorded more extensive negative (drier) anomalies over the western Maritime Continent and southern Mainland Southeast Asia. The largest positive (wetter) anomalies were recorded over eastern Mainland Southeast Asia, with the largest negative (drier) anomalies over the western Maritime Continent.

1.2 The observed rainfall anomaly pattern of above-average rainfall over the southern and southeastern Maritime Continent is consistent with the predictions from the subseasonal weather outlooks for October 2025 (1 – 12 Oct 2025 and 13 – 26 Oct 2025). However, the below-average rainfall observed was not predicted in the subseasonal weather outlook. The observations are also somewhat consistent with the seasonal outlook for October 2025, which predicted an increase in chance of above-normal rainfall in the southern and eastern Maritime Continent, and an increase in chance of below-normal rainfall in the western Maritime Continent.

era5_rainfall_anomalies
cmorph_bld_rainfall_anomalies
Figure 1: Rainfall anomalies for October 2025 based on GSMaP-NRT data (left) and CPC Global Unified Gauge-based Analysis data (right). The climatological reference period is 2001-2024. Green colour denotes above-average rainfall (wetter), while orange denotes below-average rainfall (drier).

 

1.3 Near- to above-average temperatures were recorded over most of Southeast Asia in October 2025, with the exception of below-average temperatures recorded over Papua (Figure 2). The warmest anomalies (1°C – 2°C above average) were recorded over northern Myanmar, while the coolest anomalies (0.5°C – 1°C below average) were recorded over northern Papua.

era5_temperature_anomalies
Figure 2: Temperature anomalies for October 2025 based on ERA-5 reanalysis. The climatological reference period is 2001-2024. Red colour denotes above-average temperature (warmer), while blue denotes below-average temperature (colder).

 

2. Climate Drivers

2.1 At the start of October, the Madden-Julian Oscillation (MJO) decayed over the Indian Ocean (Phase 2) and remained inactive in Week 1 and start of Week 2 based on the RMM diagram (Figure 3). An MJO signal then emerged over the Western Hemisphere (Phase 1) towards the end of Week 2, propagating eastwards to the Indian Ocean (Phases 2 and 3) in Week 3 and to the Maritime Continent (Phases 4 and 5) in Week 4. At this time of the year, Phases 1 and 2 tend to bring drier conditions to the eastern Maritime Continent, while Phases 4 and 5 tend to bring wetter conditions to much of the Maritime Continent. However, these patterns are not clear in Figure 1, indicating other drivers and weather systems likely had a stronger influence on the region’s rainfall.

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 A negative Indian Ocean Dipole (IOD) was present in October 2025. Negative IOD events tend to bring wetter conditions to parts of the region, particularly the southern Maritime Continent, in line with the positive rainfall anomalies in Figure 1. La Niña conditions were also present. La Niña events tend to bring wetter-than-average conditions to much of the Maritime Continent during this time of the year.