Review of Regional Weather for October 2024 |

Review of Regional Weather for October 2024

1. Overview

1.1 During October 2024, the ASEAN region experienced a mix of below- and above-average rainfall. For the Maritime Continent, much of the western region experienced below-average rainfall while much of the eastern region as well as northern and central Philippines experienced above-average rainfall (Figure 1). Above-average rainfall was recorded over much of southern Mainland Southeast Asia, with below-average rainfall over parts of northwestern and eastern Mainland Southeast Asia, although there was disagreement over central Viet Nam with GSMaP-NRT (Figure 1, left) recording below-average rainfall, while CMORPH-Blended (Figure 1, right) recorded above-average rainfall. The largest negative (drier) anomalies were recorded over southern Sumatra in both GSMap-NRT and CMORPH-Blended. The largest positive (wetter) anomalies were recorded over central Philippines (in both GSMap-NRT and CMORPH-Blended), partly associated with Tropical Storm Trami and Typhoon Kong-rey in October 2024.

1.2 The observed rainfall anomaly pattern of a mix of below- and above-average rainfall in the ASEAN region, with below-average rainfall over the western Maritime Continent is consistent with the predictions from the subseasonal weather outlooks for October 2024 (30 September – 13 October 2024 and 14 – 27 October 2024).

Figure 1: Rainfall anomalies for October 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 the much of ASEAN region in October 2024 (Figure 2). The warmest anomalies (more than 1°C above average) were recorded mainly over southern Sumatra based on ERA-5 reanalysis.

Figure 2: Temperature anomalies for October 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 2.1 The Madden-Julian Oscillation (MJO) was active from the end of the second week of October 2024, based on the RMM index (Figure 3). An MJO signal was present over the Western Hemisphere (Phase 1) at the start of October, which quickly weakened and remained inactive for much of the first two weeks of October. At the end of the second week, the MJO signal emerged over the Maritime Continent (Phases 4 and 5), propagated eastwards through the Maritime Continent (Phases 4 and 5) in Week 3, and through the Western Pacific (Phases 6 and 7) during the last week of the month. For October, Phases 4 to 5 tend to bring wetter conditions to parts of southern Mainland Southeast Asia and the Philippines (in line with Figure 1), Phase 6 tends to bring wetter conditions over the Philippines, and Phase 7 tends to bring drier conditions to 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 The tropical Pacific was in an ENSO neutral state during October.