Review of Regional Weather for October 2023


1. Overview

1.1 During October 2023, a mix of below- to above-average rainfall was recorded over Southeast Asia (Figure 1). Over much of Mainland Southeast Asia, near- to above-average rainfall was recorded. Over much of the Maritime Continent, below-average rainfall was recorded. For Southeast Asia, the largest positive (wetter) anomalies were recorded over central Viet Nam 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 Sumatra and the Philippines (in both GSMaP-NRT and CMORPH-Blended).

1.2 The observed rainfall anomaly pattern of above-average rainfall over much of Mainland Southeast Asia and below-average rainfall over the Maritime Continent are broadly consistent with the predictions from the subseasonal weather outlooks for October 2023 (2 – 15 October 2023 and 16 – 29 October 2023).

Figure 1: Rainfall anomalies for October 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 the Maritime continent and much of Mainland Southeast Asia (Figure 2). The warmest anomalies (more than 2°C above average) were recorded over southern Sumatra.



Figure 2: Temperature anomalies for October 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 No active Madden-Julian Oscillation (MJO) was observed for much of October 2023. An MJO was present in the Maritime Continent (Phase 5) on the first two days of November before decaying. There were signs of an MJO signal developing over the Western Pacific and Western Hemisphere (Phases 7 and 8) in the fourth week. Typically for October, Phase 7 tends to bring drier conditions to the western Maritime Continent and Phase 8 tends to bring drier conditions to much of the Maritime Continent.



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 October 2023. Sea surface temperatures in the Nino3.4 region (used to monitor ENSO) continue to show El Niño conditions, with key atmospheric indicators (cloudiness and trade winds) also supporting these conditions. A positive Indian Ocean Dipole is also present. 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.