1. Home
  2. Weather
  3. Seasonal Weather Outlook and Review

Subseasonal Weather Outlook and Review

Review of Regional Weather for August 2023

 

1. Overview

1.1 During August 2023, a mix of below- to above-average rainfall was recorded over Southeast Asia. Over Mainland Southeast Asia, below-average rainfall was recorded over central and southern parts of the region, while a mix of below- to above-average rainfall was recorded over the rest of the region. However, there was some discrepancy over Cambodia, with GSMaP-NRT indicating predominantly near- to above-average rainfall, while CMORPH-Blended indicating well below-average rainfall. Over the Maritime Continent, below-average rainfall was recorded over much of the southern half of the region, with below- to above average rainfall over much of the northern half. For Southeast Asia, the largest positive (wetter) anomalies were recorded over parts of western Myanmar for GSMaP-NRT (Figure 1, left) and CMORPH-Blended (Figure 1, right) satellite-derived rainfall estimates. The largest negative (drier) anomalies were recorded over parts of Cambodia (CMORPH-Blended) and southern Sumatra, parts of the Maluku Islands and southern Papua (both GSMaP-NRT and CMORPH-Blended).

1.2 The observed rainfall anomaly pattern of below-average rainfall over the southern half of the Maritime Continent and a mix of below- to near-average rainfall elsewhere is broadly consistent with the predictions from the subseasonal weather outlooks for August 2023 (7 – 20 August 2023 and 21 August – 3 September 2023).

era5_rainfall_anomalies
cmorph_bld_rainfall_anomalies
Figure 1: Rainfall anomalies for August 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 Southeast Asia, apart from over northern Mainland Southeast Asia and the southern Maritime Continent where a mix of below- to above-average temperature was recorded. The warmest anomalies (more than 1°C above average) were recorded over parts of Thailand, Cambodia, central and southern Viet Nam, as well as the central equatorial region.

era5_temperature_anomalies

 

Figure 2: Temperature anomalies for August 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 For much of August, there was no coherent Madden-Julian Oscillation (MJO) signal. While there were some indications of an MJO signal based on the RMM index, there was no eastward propagation until the last week of August. During the last week of August, based on the RMM Index, an MJO signal propagated from the Western Hemisphere (Phase 1) to the Indian Ocean (Phases 2 and 3). Typically for August, Phases 1 and 2 tend to bring drier conditions to northeastern Southeast Asia, while Phases 2 and 3 bring wetter conditions to the western Maritime Continent.

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 El Niño conditions were present during August 2023. Sea surface temperatures in the Nino3.4 region (used to monitor ENSO) were consistent with El Niño conditions, with signs of El Niño-like response in key atmospheric indicators. El Niño events tend to bring drier-than-average conditions to much of the Maritime Continent during the June to August period.