Review of Regional Weather for May 2023


1. Overview

1.1 During May 2023, below- to near-average rainfall was recorded over much of 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 Myanmar and southern Thailand. Elsewhere in Mainland Southeast Asia, the rainfall was below- to near-average. Over the Maritime Continent, the largest positive anomalies (wetter conditions) were recorded over eastern Borneo with the largest negative anomalies over parts of western 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, and no widespread regional anomalies over the Maritime Continent, is broadly consistent with the predictions from the subseasonal weather outlooks for May 2023 (1 – 14 May 2023 and 15 – 28 May 2023).

Figure 1: Rainfall anomalies for May 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. Near- to below-average temperatures were recorded over the southern Maritime Continent. The warmest anomalies (more than 2 °C above-average) were recorded over southern parts of the Malay Peninsula, Lao PDR and northeast Thailand.



Figure 2: Temperature anomalies for May 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 May 2023. An MJO signal propagated eastwards from the Maritime Continent (Phase 4), reaching the Western Pacific (Phases 6 and 7) during third week of May, and then the Western Hemisphere (Phase 8) at the end of the month. Typically for May, Phase 4 brings wetter conditions to much of the Maritime Continent, and Phase 5 tends to bring wetter conditions to the eastern Maritime Continent. Phase 6 tends to bring drier conditions to the western half of the Maritime Continent. Phases 7 and 8 bring drier conditions to much of Southeast Asia.



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 likely in an ENSO neutral state during May. The sea surface temperatures in the Nino3.4 region (used to monitor ENSO) continued to warm, although on average were still within the neutral range. Atmospheric indicators over the tropical Pacific Ocean indicated neutral conditions.