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.

Subseasonal Weather Outlook and Review