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Subseasonal Weather Outlook and Review

Review of Regional Weather for April 2024

 

1. Overview

1.1 During April 2024, much of the northern ASEAN region experienced below-average rainfall, while much of the southern ASEAN region experienced above-average rainfall (Figure 1). Based on both GSMaP-NRT (Figure 1, left) and CMORPH-Blended (Figure 1, right) satellite-derived rainfall estimates, below-average rainfall was recorded over much of the northern ASEAN region, as well as the western Maritime Continent and parts of Borneo. The largest negative (drier) anomalies were recorded over southern Thailand, Cambodia, and parts of central Borneo. In contrast, most of the southern ASEAN region experience near- to above-average rainfall, although GSMaP-NRT shows larger positive (wetter) anomalies over Java and Sulawesi (Figure 1, left) compared to CMORPH-Blended (Figure 1, right).

1.2 The observed rainfall anomaly pattern of below-average rainfall over much of the northern ASEAN region and above-average rainfall over the southern ASEAN region is consistent with the predictions from the subseasonal weather outlooks for April 2024 (1 – 14 April 2024 and 15 – 28 March 2024).

era5_rainfall_anomalies
cmorph_bld_rainfall_anomalies
Figure 1: Rainfall anomalies for April 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 April 2024 (Figure 2). The warmest anomalies (more than 3°C above average) were recorded over eastern Thailand, parts of Lao PDR, and northern and central Viet Nam. Based on ERA-5 reanalysis, the April 2024 was one of the warmest on record, with temperatures for most of the region in the top 10% warmest values for April.

era5_temperature_anomalies
Figure 2: Temperature anomalies for April 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 inactive for much of April 2024, based on the RMM index (Figure 3). In the first week of April, an MJO signal propagated eastwards through the Indian Ocean (Phases 2 and 3). The signal reached the Maritime Continent (Phase 4) at the start of week 2, after which point the signal rapidly weakened and became indiscernible. It was not until the last week of April when there were signs of an MJO signal again developing over the Indian Ocean (Phase 3). For April, Phase 2 tends to bring wetter conditions for the western Maritime Continent, while Phase 3 tends to bring wetter conditions to the southern ASEAN region.

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 El Niño conditions over the equatorial Pacific continued weakening during April 2024, although still likely contributed to the warmer temperatures observed in Figure 2. At this time of year, warmer temperatures are typical observed when there has been a El Niño event (even if the event is weakening).