Review of Regional Weather for September 2022


1. Overview

1.1 During September 2022, much of the ASEAN region experienced above-average rainfall (Figure 1). The largest positive anomalies (wetter conditions) were recorded over parts Viet Nam and southern Borneo based on both satellite-derived rainfall estimates datasets (GSMaP-NRT and CMORPH-Blended). In contrast to the above-average rainfall, much of Peninsular Malaysia, the Philippines as well as northern Myanmar, northern Laos PDR, northern Sumatra, and northern Papua experienced below- to near-average rainfall. The two datasets agree well over most of the ASEAN region. However, there are some discrepancies over the central Myanmar and northern Laos PDR where GSMaP-NRT recorded mix of below- to above-average rainfall whereas CMORPH-Blended recorded near-average rainfall.

1.2 The observed rainfall anomaly pattern of above-average rainfall for most of the Maritime Continent region are broadly consistent with the predictions from the subseasonal weather outlooks for September 2022 (5 – 18 September and 19 September – 2 October). For Mainland Southeast Asia, the southern parts overall experienced above average rainfall in September, where the subseasonal outlooks also predicted wetter conditions in the first half of the September. While the northern parts of Mainland Southeast Asia experienced a mix of below- to -above average rainfall for September, the subseasonal outlooks predicted drier conditions towards the end of the month.

Figure 1: Rainfall anomalies for September 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 The southeastern parts of the Maritime Continent and northwestern Myanmar experienced above-average temperatures during September 2022, with below- to near-average temperatures elsewhere (Figure 2). The warmest anomalies were recorded over Papua, with the coolest anomalies over parts of Thailand, Cambodia, southern Borneo, and southern Sumatra. Most of the cooler anomalies occurred for areas that also experienced above-average rainfall.



Figure 2: Temperature anomalies for September 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 most of September 2022, based on the RMM Index below (Figure 3). As there was no significant MJO activity in the entire September, it would have little contribution to rainfall conditions over the ASEAN region.



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 remain consistent with La Niña conditions. La Niña events tend to bring wetter-than-average conditions to much of the Maritime Continent during the September to November period. Over the Indian Ocean, a negative Indian Ocean Dipole (IOD) was present. Negative IOD tends to bring wetter-than-average conditions for much of Southeast Asia.