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Subseasonal Weather Outlook and Review

Review of Regional Weather for April 2023

 

1. Overview

1.1 During April 2023, below- to near-average rainfall was recorded over Mainland Southeast Asia, with a mix of below- to above-average rainfall over the Maritime Continent. Over Mainland Southeast Asia, below-average rainfall was recorded over most of the central and southern part of the region with the largest negative anomalies (drier conditions) over southern Thailand. Elsewhere in Mainland Southeast Asia, the rainfall was near-average. Over the Maritime Continent, the largest positive anomalies (wetter conditions) were recorded over southern Sumatra with the largest negative anomalies over parts of Borneo for both the GSMaP-NRT (Figure 1, left) and CMORPH-Blended (Figure 1, right) satellite-derived rainfall estimates.

1.2 The observed rainfall anomaly pattern of below- to near-average rainfall over Mainland Southeast Asia, below-average rainfall over central Maritime Continent , and no widespread regional anomalies elsewhere over the Maritime Continent, is broadly consistent with the predictions from the subseasonal weather outlooks for April 2023 (3 – 16 April 2023 and 17 – 30 April 2023).

era5_rainfall_anomalies
cmorph_bld_rainfall_anomalies
Figure 1: Rainfall anomalies for April 2023 based on GSMaP-NRT data (left) and CMORPH-Blended data (right). The climatological reference period is 2001-2022. Green colour denotes above-average rainfall (wetter), while orange denotes below-average rainfall (drier).

 

1.3 Above-average temperatures were recorded over most of Mainland Southeast Asia, with near- to above-average temperature over most of the Maritime Continent. The warmest anomalies (more than 2 °C above-average) were recorded over parts of Lao PDR and northern Thailand.

era5_temperature_anomalies

 

Figure 2: Temperature anomalies for April 2023 based on ERA-5 reanalysis. The climatological reference period is 2001-2022. 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 during April 2023. An MJO signal propagated eastwards from the Maritime Continent (Phase 5), reaching the Western Hemisphere (Phases 8 and 1) during third week of April, and then just entering again the Maritime Continent (Phase 4) at the end of the month. Typically for April, Phases 7, 8, and 1 bring drier conditions to the Southeast Asia, while Phases 3 to 5 bring wetter conditions, particularly in southern Southeast Asia.

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 The tropical Pacific was in an ENSO neutral state during April. Overall, sea surface temperatures in the Nino3.4 region (used to monitor ENSO) and the atmospheric indicators over the tropical Pacific Ocean indicated neutral conditions.