Review of Regional Weather for February 2021

1. Overview

1.1 During February 2021, the western Maritime Continent received less rainfall than average for this time of year (Figure 1). The largest negative anomalies (drier conditions) were recorded over western Borneo based on both satellite-derived rainfall estimates datasets (GSMaP-NRT and CMORPH-Blended). Above-average rainfall was recorded for southern Indonesia, northern Borneo, and the Philippines, with the largest positive anomalies over the central Philippines in part due to Tropical Storm Dujuan. A mix of below- to above-average rainfall was recorded over the rest of the Maritime Continent. For Mainland Southeast Asia, rainfall anomalies were negligible for most of the region, as expected during the dry season for the northern ASEAN region, except for the north-eastern region where positive anomalies were recorded.

1.2 The observed large-scale rainfall anomaly pattern (i.e. below-average rainfall in western Maritime Continent, and above-average rainfall in southern Indonesia, northern Borneo and the Philippines) is broadly consistent with the predictions from the subseasonal weather outlooks for February 2021 (25 Jan – 7 Feb, 8 – 21 Feb, 22 Feb – 7 Mar).

Figure 1: Rainfall anomalies for the month of February 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 were recorded over central and eastern parts of Mainland Southeast Asia in February 2021 (Figure 2). In contrast, warmer-than-average temperatures were recorded over parts of northern Myanmar. Over the Maritime Continent, temperatures were closer to average, with above-average temperature in northern Sumatra, western Borneo, and parts of eastern Indonesia.



Figure 2: Temperature anomalies for the month of February 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 the RMM index plot suggests that a Madden-Julian Oscillation (MJO) signal was present in the Western Pacific during February (Figure 3, phases 6 and 7), the presence of La Niña and high frequency tropical waves could have interfered with the signal. There was a weakening of the RMM index in Phase 7 from the middle of the month with no eastward propagation, followed by a westward propagation to Phase 6. Eastward propagation of the signal was seen at the end of February. 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, consistent with below-average rainfall observed in the western Maritime Continent (Figure 1).



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) 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.