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Subseasonal Weather Outlook and Review

Review of Regional Weather for April 2022

 

1. Overview

1.1 During April 2022, near- to above-average rainfall was recorded over much of the northern ASEAN region, while a mix of below- to above-average rainfall was recorded over the southern ASEAN region (Figure 1). For the northern ASEAN region, the largest positive anomalies (wetter conditions) were recorded over the Philippines for both satellite-derived rainfall estimates datasets (GSMaP-NRT and CMORPH-Blended), which partly was associated with Tropical Storm Megi during the middle of April. For the southern ASEAN region, negative anomalies (drier conditions) were recorded over the eastern parts. Elsewhere in the southern ASEAN region, rainfall tended to be a mix of below- to above-average. There are some discrepancies between the two datasets, as GSMaP-NRT recorded a mix of below- to above-average rainfall for much of southern Sumatra and Borneo, while CMORPH-Blended recorded below- to near-average rainfall with isolated areas of above-average rainfall over the mentioned regions.

1.2 The observed rainfall anomaly pattern of above-average rainfall for parts of the northern ASEAN region, including the Philippines and a mix of below- to above-average rainfall for the southern ASEAN region is broadly consistent with the predictions from the subseasonal weather outlooks for April 2022 (4 – 17 April, and 18 April – 1 May).

era5_rainfall_anomalies
cmorph_bld_rainfall_anomalies
Figure 1: Rainfall anomalies for the month of April 2022 based on GSMaP-NRT data (left) and CMORPH-Blended data (right). The climatological reference period is 2001-2021. Green colour denotes above-average rainfall (wetter), while orange denotes below-average rainfall (drier).

 

1.3 Most parts of northern ASEAN region recorded below-average temperature in April 2022 (Figure 2). The largest negative anomalies (less than 1°C cooler) were over northeastern Thailand, Lao PDR, and Viet Nam. Elsewhere, there was near- to above-average temperature, with most of the regions recording above-average temperature south of the equator.

era5_temperature_anomalies

 

Figure 2: Temperature anomalies for the month of April 2022 based on ERA-5 reanalysis. The climatological reference period is 2001-2021. Red colour denotes above-average temperature (warmer), while blue denotes below-average temperature (colder).

 

2. Climate Drivers

2.1 The Madden-Julian Oscillation (MJO) signal was weak or indiscernible for much of April 2022, based on the MJO phase diagram below (Figure 3). At the start of the month, the MJO was indiscernible. In the second week of April, there were signs of eastward progression of a signal, however, this signal was weak and remained inside the unit circle. Around the middle of April, the signal strengthened slightly, moving outside the unit circle in the Western Hemisphere (Phase 8) before becoming indiscernible again by the third week in April as it moved towards the Indian Ocean (Phase2). Phase 8 tends to bring drier conditions to much of the Maritime Continent, while Phase 1 tends to bring drier conditions only to parts of the eastern Maritime Continent. Therefore, the MJO may have contributed to the drier conditions in the April in the southeastern 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 tropical Pacific Ocean, La Niña conditions were present. Sea surface temperatures in the Nino3.4 region (used to monitor ENSO) and the atmospheric indicators over the tropical Pacific Ocean were consistent with La Niña conditions. La Niña events tend to bring wetter-than-average conditions to much of Southeast Asia during the March to May period.