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Subseasonal Weather Outlook and Review

Review of Regional Weather for November 2020

1. Overview

1.1 During November 2020, much of the eastern parts of Mainland Southeast Asia and the Philippines experienced above-average rainfall (Figure 1). The largest positive anomalies (wetter conditions) were recorded over northern Philippines and central and southern Viet Nam based on both satellite-derived rainfall estimates datasets (GSMaP-NRT and CMORPH-Blended), linked to the tropical storms and cyclones in the first half of November 2020 (including Tropical Cyclone Goni, Tropical Storm Atsani, and Tropical Cyclone Vamco). Elsewhere in Mainland Southeast Asia, near-average rainfall was experienced in much of the region. A mix of above- and below-average rainfall was experienced for most regions in the equatorial region and south of the equator, with the largest region of above-average rainfall in the western Maritime Continent.

1.2 The observed large-scale rainfall anomaly pattern (i.e. above-average rainfall in regions linked to tropical storm and cyclone activity) is broadly consistent with the predictions from the subseasonal weather outlooks for November 2020 (2 – 15 Nov, 16 – 29 Nov).

era5_rainfall_anomalies
cmorph_bld_rainfall_anomalies
Figure 1: Rainfall anomalies for the month of November 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 some regions of the eastern half of Mainland Southeast Asia in November 2020 (Figure 2), in particular Cambodia. In contrast, warmer-than-average temperatures were recorded over northern Viet Nam and Lao PDR, northwestern Thailand and parts of Myanmar. Elsewhere, the temperature for November was closer to average.

era5_temperature_anomalies

 

Figure 2: Temperature anomalies for the month of November 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 Starting off as a weak signal in the Western Pacific (Phase 7), the Madden-Julian Oscillation (MJO) signal strengthened in the Western Hemisphere (phases 8 and 1) during the first half of November (Figure 3), and continued propagating eastward through the Indian Ocean (phases 2 and 3) in the second half and eventually weakened as it approached the Matitime Continent (Phase 4) at the end of the month. Phases 8 and 1 typically bring below-average rainfall for much of Southeast Asia during this time of the year, while Phases 2 and 3 tend to bring wetter conditions to parts of the western Maritime continent (observed in the rainfall anomalies for November 2020, Figure 1) and drier conditions to eastern parts.

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.