Review of Regional Weather for September 2023


1. Overview

1.1 During September 2023, a mix of below- to above-average rainfall was recorded over Southeast Asia (Figure 1). Over much of Mainland Southeast Asia, above-average rainfall was recorded. Over the Maritime Continent, below-average rainfall was recorded over much of the southern of the region, with below- to above average rainfall over the northern half. For Southeast Asia, the largest positive (wetter) anomalies were recorded over coastal Myanmar, southern Viet Nam, and northern Borneo based on GSMaP-NRT (Figure 1, left) and CMORPH-Blended (Figure 1, right) satellite-derived rainfall estimates. In contrast, the largest negative (drier) anomalies were recorded over parts of Cambodia (CMORPH-Blended), southern Borneo, Java, Sumatra, and southern Papua, and parts of Philippines (both GSMaP-NRT and CMORPH-Blended).

1.2 The observed rainfall anomaly pattern of above-average rainfall over most of Mainland Southeast Asia, while below-average rainfall elsewhere over the southern half of the Maritime continent are broadly consistent with the predictions from the subseasonal weather outlooks for September 2023 (4 – 17 September 2023 and 18 September – 1 October 2023).

Figure 1: Rainfall anomalies for September 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 the Maritime Continent, where a mix of below- to above-average temperature was recorded (Figure 2). Near- to above-average temperatures were recorded over Mainland Southeast Asia. The warmest anomalies (more than 1°C above average) were recorded over parts of southern Sumatra and northern Myanmar.



Figure 2: Temperature anomalies for September 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 A Madden-Julian Oscillation (MJO) signal developed over the Indian Ocean (Phase 3) during the first week of September 2023, based on the RMM Index below (Figure 3). During the second week of September, the signal weakened and appeared to stall over Phase 3 and Phase 4. However, towards the end of September the signal began propagating eastwards through the Maritime Continent (Phases 4 and 5), more typical of an MJO signal. Usually for September, Phases 4 and 5 bring wetter conditions to parts of southern Mainland Southeast Asia and the Philippines, somewhat in line with Figure 1, while Phase 3 has little impact on the region’s rainfall.



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 persisted over the equatorial Pacific during September 2023. Sea surface temperatures in the Nino3.4 region (used to monitor ENSO) were consistent with El Niño conditions, with El Niño-like response in key atmospheric indicators. A positive Indian Ocean Dipole also developed. El Niño events tend to bring drier and warmer-than-average conditions to much of the Maritime Continent during September to November. Positive Indian Ocean Dipole events also tend to bring drier conditions to most of the southern half of the Maritime Continent during this period.