Review of Regional Weather for May 2022
1.1 During May 2022, near- to above-average rainfall was recorded over much of the northern ASEAN region, while a mixture of below- to above-average rainfall was recorded over the southern ASEAN region (Figure 1). For the northern ASEAN region, the largest positive anomalies in rainfall (wetter conditions) were recorded over the southern Myanmar for both satellite-derived rainfall estimates datasets (GSMaP-NRT and CMORPH-Blended). For the southern ASEAN region, above-average rainfall was recorded over central Indonesia and a mix of below- to above-average elsewhere. The two datasets agree well over most of the ASEAN region. However, there are some discrepancies over western Borneo and southern Sumatra where CMORPH recorded below- to near-average rainfall whereas GSMaP-NRT recorded near- to above-average rainfall.
1.2 The observed rainfall anomaly pattern of above-average rainfall for parts of the northern ASEAN region, particularly Myanmar, is broadly consistent with the predictions from the subseasonal weather outlooks for May 2022 (2 – 15 May and 16 – 29 May). The drier conditions predicted over the equatorial region for 16 – 29 May, can be observed over parts of the western and eastern equatorial region, although overall in May it was relatively wet over the central equatorial region.
1.3 Most parts of northern ASEAN region recorded below-average temperature in May 2022 (Figure 2). The largest negative anomalies (2°C cooler than normal conditions for May) were over northeastern Thailand and northern Viet Nam. For most of the southern ASEAN region, normal to above-average temperature were recorded with largest warm anomalies over Peninsular Malaysia and southeastern Indonesia.
2. Climate Drivers
2.1 There was no discernible Madden-Julian Oscillation (MJO) signal at the start of May 2022 based on the MJO phase diagram below (Figure 3). In the first week of May, an MJO signal emerged over the Indian Ocean (Phase 2). For the second week of May, the MJO signal first became relatively weaker as it moved eastwards over the Indian Ocean (Phase 3). This signal then strengthened again and continued moving rapidly eastwards over the Maritime Continent (Phase 4 and 5) and Western Pacific (Phase 6). In the third week of May, the signal moved further east over the Western Pacific (Phase 7) and Western Hemisphere (Phase 8), before weakening and becoming indiscernible. Towards the end of May, an MJO signal emerged again over the Western Pacific (Phases 6 and 7). For the central Maritime Continent, Phases 3, 4, and 5 can all bring wetter conditions, while for the western Maritime Continent, Phases 6 and 7 tend to bring drier conditions. Therefore, the MJO may have contributed to the wetter conditions over parts of the central Maritime Continent and drier conditions over parts of the western Maritime Continent during May.
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. Over the Indian Ocean, there were signs of a negative Indian Ocean Dipole developing in May but the Dipole Mode Index (DMI), which is used to measure the Indian Ocean Dipole (IOD) signal, remained neutral. Negative IOD tends to bring wetter-than-average conditions for much of Southeast Asia.