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Subseasonal Weather Outlook and Review

Review of Regional Weather for July 2023

 

1. Overview

1.1 During July 2023, a mix of below- to above-average rainfall was recorded over Southeast Asia. Over Mainland Southeast Asia, below-average rainfall was recorded over the northeastern 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 above-average rainfall, while CMORPH-Blended indicating well below-average rainfall. Over the Maritime Continent, above-average rainfall was recorded over much of the western half of the region, with below-average rainfall over much of the eastern half. The largest positive (wetter) anomalies were recorded over parts of southern Myanmar, southern Viet Nam, and northern Philippines 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 Papua (CMORPH-Blended) and northern Viet Nam and coastal Myanmar (both GSMaP-NRT and CMORPH-Blended).

1.2 The observed rainfall anomaly pattern of a mix of below- to near-average rainfall over the ASEAN region is broadly consistent with the predictions from the subseasonal weather outlooks for July 2023 (26 June – 9 July 2023, 10 – 23 July 2023 and 24 July – 6 August 2023).

era5_rainfall_anomalies
cmorph_bld_rainfall_anomalies
Figure 1: Rainfall anomalies for July 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 The observed rainfall anomaly pattern of a mix of below- to near-average rainfall over the ASEAN region is broadly consistent with the predictions from the subseasonal weather outlooks for July 2023 (26 June – 9 July 2023, 10 – 23 July 2023, and 24 July – 6 August).

era5_temperature_anomalies

 

Figure 2: Temperature anomalies for July 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 At the start of July, the Madden-Julian Oscillation (MJO) signal was present over the Indian Ocean (Phase 2). This signal decayed during the first week of July, with no significant MJO activity for the rest of the month. Typically for June, Phase 2 tends to bring drier conditions in northern Southeast Asia and bring wetter conditions to the western Maritime Continent (somewhat discernible in Figure 1).

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