Review of Regional Weather for January 2021

1. Overview

1.1 During January 2021, much of the Maritime Continent received more rainfall than average for this time of year (Figure 1). The exceptions were Sumatra and central parts of Borneo and Papua, which received a mix of below- to above-average rainfall. The largest positive anomalies (wetter conditions) were recorded over Peninsular Malaysia, northern Borneo, and Nusa Tenggara based on both satellite-derived rainfall estimates datasets (GSMaP-NRT and CMORPH-Blended). For the rest of Mainland Southeast Asia, rainfall anomalies were negligible, as expected at during dry season for the northern ASEAN region.

1.2 The observed large-scale rainfall anomaly pattern (i.e. above-average rainfall in parts of the Maritime Continent, in particular southern Indonesia, northern Borneo and the Philippines) is broadly consistent with the predictions from the subseasonal weather outlooks for January 2021 (28 Dec – 10 Jan, 11 – 24 Jan, 25 Jan – 7 Feb).

Figure 1: Rainfall anomalies for the month of January 2021 based on GSMaP-NRT data (left) and CMORPH-Blended data (right). The climatological reference period is 2001-2020. Green colour denotes above-average rainfall (wetter), while orange denotes below-average rainfall (drier).


1.3 Colder-than-average temperatures (more than 1°C cooler) were recorded over most of Mainland Southeast Asia in January 2021 (Figure 2). In contrast, warmer-than-average temperatures (more than 1°C warmer) were recorded over parts of Myanmar. Over the Maritime Continent, temperatures were closer to average, with near- to above-average temperature in eastern part, and below- to near-average temperatures in the western and central parts.



Figure 2: Temperature anomalies for the month of January 2021 based on ERA-5 reanalysis. The climatological reference period is 2001-2020. Red colour denotes above-average temperature (warmer), while blue denotes below-average temperature (colder).


2. Climate Drivers

2.1 While there were some signs that a weak Madden-Julian Oscillation (MJO) signal was present in the Indian Ocean (Figure 3, Phase 3) in the first week of January 2021, this signal weakened as it moved towards the Maritime Continent. In the second half of the month, a moderate strength, very slow-moving MJO signal appeared in the Western Pacific (Phases 6 and 7), although there was no notable eastward progression. Phase 6 typically brings wetter conditions to the eastern-most part of the Maritime Continent, while Phase 7 typically brings drier conditions to the western half of the Maritime Continent at this time of year.



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. The sea surface temperatures in the central and eastern Pacific Ocean remained cool, with atmospheric indicators (cloudiness and wind anomalies) also remaining consistent with La Niña conditions. La Niña events tend to bring wetter-than-average conditions to much of Southeast Asia during the September to March period.