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Subseasonal Weather Outlook and Review

Review of Regional Weather for October 2020

1. Overview

1.1 During October 2020, much of the northern part of Southeast Asia experienced above-average rainfall (Figure 1). The largest positive anomalies (wetter conditions) were recorded over the Philippines, Viet Nam, parts of Lao PDR and southern Cambodia based on both satellite-derived rainfall estimates datasets (GSMaP-NRT and CMORPH-Blended). The large positive anomalies in these regions is linked to the high number of tropical storms and cyclones (including Tropical Storm Linfa, Tropical Storm Nangka, Tropical Cyclone Saudel, and Tropical Cyclone Molave). Elsewhere, the equatorial region experienced a mix between above- and below-average rainfall, while wetter-than-average conditions were recorded over much of the region south of the equator.

1.2 The observed large-scale rainfall anomaly pattern (i.e. above-average rainfall in regions linked to tropical storm and cyclone activity as well as south of the equator) is broadly consistent with the predictions from the subseasonal weather outlooks for October 2020 (5 – 18 Oct, 19 Oct – 1 Nov).

era5_rainfall_anomalies
cmorph_bld_rainfall_anomalies
Figure 1: Rainfall anomalies for the month of October 2020 based on GSMaP-NRT data (left) and CMORPH-Blended data (right). The climatological reference period is 2001-2019. Green colour denotes above-average rainfall (wetter), while orange denotes below-average rainfall (drier).

 

1.3 Colder-than-average temperatures were recorded over the eastern half of Mainland Southeast Asia in October 2020 (Figure 2). Parts of Viet Nam, Lao PDR, and Thailand were on average more than 1°C cooler than usual for this time of year. In contrast, warmer-than-average temperatures were recorded over northern Myanmar (more than 1°C warmer than average). Elsewhere, the temperature for October was closer to average.

era5_temperature_anomalies

 

Figure 2: Temperature anomalies for the month of October 2020 based on ERA-5 reanalysis. The climatological reference period is 2001-2019. Red colour denotes above-average temperature (warmer), while blue denotes below-average temperature (colder).

 

2. Climate Drivers

2.1 A Madden-Julian Oscillation (MJO) signal developed in the Maritime Continent (Phase 5) during the first half of October (Figure 3), before propagating eastward through the western Pacific (phases 6 and 7) in the second half. Phases 5 and 6 tend to bring wetter conditions to the eastern half of Southeast Asia, while phases 6 and 7 bring drier conditions to parts of the western 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. The sea surface temperatures in the central and eastern Pacific Ocean continued to 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 November period.