Review of Regional Weather for April 2026

1. Overview

1.1 During April 2026, below- to near-average rainfall was recorded over most of Mainland Southeast Asia with a mix of below- to above-average rainfall for the Maritime Continent (Figure 1). Most of southern and central Mailand Southeast Asia recorded below-average rainfall based on both datasets (GSMaP-NRT and CMORPH-CRT). The western and southern Maritime Continent recorded predominately above-average rainfall, with the northeastern part recording below-average rainfall and the central Maritime Continent a mix of below- to above-average rainfall. The largest positive (wetter) anomalies were recorded over Sumatra for both datasets, along with parts of the southern Maritime Continent for CMORPH-CRT, with the largest negative (drier) anomalies over northwestern Borneo for both datasets and Sulawesi for GSMaP-NRT).

1.2 The observed rainfall anomaly pattern of below-average rainfall over southern and central Mainland Southeast Asia as well as over the northern Maritime Continent, is consistent with the predictions from the subseasonal weather outlooks for March 2026 (30 March – 12 April 2026, 13 – 26 April 2026) and 27 April – 10 May 2026). However, the seasonal outlook predicted below-normal rainfall as the dominant tercile for much of the southern ASEAN region.

Figure 1: Rainfall anomalies for April 2026 based on GSMaP-NRT data (left) and CPC CMORPH-CRT 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 Above-average temperature was recorded over parts of northeastern Mainland Southeast Asia, with below- to near-average temperature for southeastern and northwestern Mainland Southeast Asia (Figure 2). Near-to above-average temperature was recorded for much of the ASEAN region. The warmest anomalies (2°C – 3°C above average) were recorded over parts of Viet Nam and Lao PDR, with warm anomalies (1°C – 2°C above average) recorded over parts of Thailand and Cambodia. The coolest anomalies (0.5°C – 1°C below average) were recorded over parts of Myanmar, Papua and around Timor Leste.

Figure 2: Temperature anomalies for April 2026 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 The Madden-Julian Oscillation (MJO) was active over the Western Pacific and Western Hemisphere for much of April. At the start of April, an MJO signal quickly decayed and was inactive by the end of the first week. An MJO signal then developed over the Western Pacific (Phases 6 and 7) in the second week. This signal continued propagating eastward through the Western Hemisphere, reaching the Indian Ocean (Phase 2) by the end of the month. At this time of the year, Phases 7 and 8 tend to bring drier conditions to the Maritime Continent, with Phase 1 also tending to bring drier conditions to the central and eastern Maritime Continent, and Phase 2 bringing wetter conditions to the western Maritime Continent. This is somewhat in line with drier conditions over parts of the eastern Maritime Continent in Figure 1.

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 El Niño – Southern Oscillation (ENSO) was neutral in April, with the Indian Ocean Dipole (IOD) also in the neutral state.

Subseasonal Weather Outlook and Review