Review of Regional Weather for May 2024


1. Overview

1.1 During May 2024, much of the equatorial region experienced above-average rainfall, while much of the southern parts of the Maritime Continent and the Philippines experienced below-average rainfall (Figure 1). A mix of near- and above-average rainfall was recorded over much of Mainland Southeast Asia, although there was disagreement over Cambodia with GSMaP-NRT (Figure 1, left) recording near- to above-average rainfall, while CMORPH-Blended (Figure 1, right) recorded below-to near-average rainfall. The largest negative (drier) anomalies were recorded over southern Philippines, Java, and parts of northeastern Borneo in GSMap-NRT (Figure 1, left) and CMORPH-Blended (Figure 1, right). The larger positive (wetter) anomalies were recorded over Sumatra and central Borneo (in both GSMap-NRT and CMORPH-Blended).
1.2 The observed rainfall anomaly pattern of a mix of below- and above-average rainfall over the northern ASEAN region and above-average rainfall over the equatorial region is consistent with the predictions from the subseasonal weather outlooks for May 2024 (29 April – 12 May 2024 , 13 – 26 May 2024 and 27 May – 9 June 2024).

Figure 1: Rainfall anomalies for May 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 ASEAN region in May 2024 (Figure 2). The warmest anomalies (more than 2°C above average) were recorded over southern Thailand and northeastern Borneo. Based on ERA-5 reanalysis, Temperatures in May 2024 were among the 10% warmest values for May over southern Sumatra, southern Thailand and Myanmar, parts of Borneo and most of the Philippines.

Figure 2: Temperature anomalies for May 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 The Madden-Julian Oscillation (MJO) was active for much the second half of May 2024, based on the RMM index (Figure 3). In the first week of May, an MJO signal propagated eastwards through the Maritime Continent (Phases 4 and 5) before becoming inactive. In the second half of the May, the signal reached re-emerged over the Indian Ocean (Phases 2 and 3) and remained there until the end of the month. For May, Phases 2 to 5 tend to bring wetter conditions to parts of the Maritime Continent (the western Maritime Continent in Phase 2, much of the region for Phases 3 and 4, and the eastern Maritime Continent in Phase 5). Therefore, the MJO may have contributed to the wetter conditions in the equatorial region in May.



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 event has now ended, although the lingering effect of the El Niño still likely contributed to the warmer temperatures observed in Figure 2.