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Subseasonal Weather Outlook and Review

Review of Regional Weather for June 2023

 

1. Overview

1.1 During June 2023, a mix of below- to above-average rainfall was recorded over the ASEAN region. Over Mainland Southeast Asia, above-average rainfall was recorded in the eastern parts of the region, while a mix of below- to above-average rainfall was recorded over the rest of the region. Over the Maritime Continent, above-average rainfall was recorded over the western Maritime Continent, with a mix of below- to above-average rainfall elsewhere. The largest positive anomalies were over northeastern 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 the ASEAN region is broadly consistent with the predictions from the subseasonal weather outlooks for June 2023 (29 May – 11 June 2023, 12 – 25 June 2023 and 26 June – 9 July 2023).

era5_rainfall_anomalies
cmorph_bld_rainfall_anomalies
Figure 1: Rainfall anomalies for June 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.

era5_temperature_anomalies

 

Figure 2: Temperature anomalies for June 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) was active at the start of June 2023. An MJO signal propagated eastwards from the Western Hemisphere and Africa (Phase 1) and to the Indian Ocean (Phases 2 and 3) in the first half of June. In Week 3, the MJO signal weakened and became insignificant. An MJO signal re-emerged in Week 4 over the Western Hemisphere and Africa (Phase 1) and propagated eastwards to the Indian Ocean (Phase 2). Typically for June, Phases 1 and 2 tends to bring drier conditions in northern Southeast Asia while Phases 2 and 3 tend to 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 Over the past month, there have been signs of El Niño-like conditions. El Niño sea surfaces temperatures indices are near or exceeding El Niño thresholds, whereas key atmospheric indicators (cloudiness and wind anomalies) displayed neutral conditions for most of June.