Category Archives: Fortnightly Weather Review

1.1 During May 2021, there was a mix of below- to above-average rainfall over the ASEAN region. For the equatorial region, the GSMaP-NRT dataset (Figure 1a) shows near- to above-average rainfall (except for parts of central Borneo), while the CMORPH-Blended dataset (Figure 1b) shows a mixture of below- and above-average rainfall. The largest positive anomalies (wetter conditions) were recorded over southern Philippines for both satellite-derived rainfall estimates datasets, due to Tropical Depression 03W. Elsewhere in the Maritime Continent, the southern Maritime Continent and northern Philippines experienced below-average rainfall, with the largest negative anomalies (drier conditions) recorded over northern Philippines. For Mainland Southeast Asia, parts of central Mainland Southeast Asia experienced below-average rainfall, with CMORPH-Blended showing a larger proportion of Thailand receiving below-average rainfall, while GSMaP-NRT extended the below-average rainfall over coastal Myanmar. Near- to above-average rainfall was experienced elsewhere in Mainland Southeast Asia.
 

1.2 The observed rainfall anomaly pattern (i.e. below-average rainfall over southern Maritime Continent, above-normal rainfall in the equatorial region) is broadly consistent with the predictions from the subseasonal weather outlooks for May 2021 (3 – 16 May, 17 – 30 May).

Figure 1: Rainfall anomalies for the month of May 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 Most of Mainland Southeast Asia experienced above-average temperatures during May 2021, except for parts of Myanmar, Cambodia, and southern Viet Nam (Figure 2). Above-average temperatures were also experienced in northern Philippines and the southern Maritime Continent. Elsewhere, near-average temperatures were experienced, apart from below-average temperature for southern Philippines associated with the heavy rainfall.

 

Figure 2: Temperature anomalies for the month of May 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 At the start of May 2021, a strong Madden-Julian Oscillation (MJO, Figure 3) signal was present in Phase 1 (Western Hemisphere). The signal then propagated eastward into the Indian Ocean (Phases 2 and 3), where it became less coherent for some time before strengthening again and propagating eastwards to the Maritime Continent (Phases 4 and 5). It subsequently weakened upon approaching Phase 6 with no coherent MJO signal for the rest of the month based on the RMM index. Phases 3 and 4 tend to bring wetter conditions to much of the Maritime Content, Phase 5 tends to bring wetter conditions to the eastern half of the Maritime Content, while Phase 6 tends to bring drier conditions to the western half of the Maritime Continent.

 

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 decayed and returned to ENSO-neutral conditions by the beginning of June. The sea surface temperatures in the central and eastern Pacific Ocean continued to warm, with atmospheric indicators (cloudiness and wind anomalies) remaining consistent with the ENSO-neutral conditions.

1.1 During April 2021, much of the Mainland Southeast Asia and the Malay Peninsula experienced above-average rainfall (Figure 1). The largest positive anomalies (wetter conditions) were recorded over northern parts of Thailand and Lao DPR for both satellite-derived rainfall estimates datasets (GSMaP-NRT and CMORPH-Blended), with GSMaP-NRT also recording larger anomalies over Peninsular Malaysia and eastern side of northern Sumatra. Large positive anomalies were also recorded over the eastern part of central Philippines and the region around Nusa Tenggara due to Typhoon Surigae and Tropical Cyclone Seroja, respectively. Much of the rest of the Maritime Continent experienced below-average rainfall.

1.2 The observed large-scale rainfall anomaly pattern (i.e. above-average rainfall over the northern ASEAN region and drier conditions in the southern ASEAN region) is broadly consistent with the predictions from the subseasonal weather outlooks for April 2021 (5 – 18 Apr, 19 Apr – 2 May).

Figure 1: Rainfall anomalies for the month of April 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 Most parts of the northern ASEAN region experienced below-average temperatures during April 2021, apart from northern parts of Myanmar and Viet Nam where above-average temperatures were recorded (Figure 2). In contrast, most parts of the southern ASEAN region experienced near- to above-average temperatures.

 

Figure 2: Temperature anomalies for the month of April 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 At the start of April 2021, a Madden-Julian Oscillation (MJO, Figure 3) signal was present in Phase 5 (Maritime Continent). The signal then propagated eastward into the Western Pacific (Phases 6 and 7). The signal became less coherent for a time in Phase 7, before strengthening again and propagating eastwards into the Western Hemisphere (Phase 8) at the end of the month. Phase 5 tends to bring wetter conditions to the eastern half of the Maritime Content, while Phase 6 tends to bring drier conditions to the western half. Phases 7 and 8 tend to bring drier conditions to much of the Maritime Continent, similar to what is observed from the rainfall anomalies in 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 but weakening. Overall, the sea surface temperatures in the central and eastern Pacific Ocean remained cool, with atmospheric indicators (cloudiness and wind anomalies) remaining consistent with the weakening La Niña conditions. La Niña events tend to bring wetter-than-average conditions to parts of Southeast Asia during the March to May period.

1.1 During March 2021, much of the land masses of the Maritime Continent experienced above-average rainfall (Figure 1). The largest positive anomalies (wetter conditions) were recorded over western coastal parts of Sumatra, eastern coastal parts of Peninsular Malaysia and western Borneo based on both satellite-derived rainfall estimates datasets (GSMaP-NRT and CMORPH-Blended). For Mainland Southeast Asia, rainfall anomalies were largely negligible for most of the region, as expected during the dry season for the northern ASEAN region, except for the southern tips of Thailand and Myanmar where the dry conditions are more pronounced.
 

1.2 The observed large-scale rainfall anomaly pattern (i.e. above-average rainfall over land masses of the Maritime Continent is broadly consistent with the predictions from the subseasonal weather outlooks for March 2021 (22 Feb – 7 Mar, 8 – 21 Mar, 22 Mar – 4 Apr), apart from the anomalies for some parts of southern Sumatra and central Java not highlighted in the outlooks.

Figure 1: Rainfall anomalies for the month of March 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 Most parts of Southeast Asia south of 10°N experienced near-average temperatures during March 2021 (Figure 2). In contrast, warmer-than-average temperatures were recorded over most of Southeast Asia north of 10°N, except for some south-eastern parts of Mainland Southeast Asia where below- to near-average temperatures are observed.

 

Figure 2: Temperature anomalies for the month of March 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 During the first half of March 2021, the Madden-Julian Oscillation (MJO) signal was weak (Figure 3). In the second half of March, it strengthened in the Western Hemisphere (Phases 8 and 1) and propagated eastward through the Indian Ocean (Phases 2 and 3) and western Maritime Continent (Phase 4). Phase 2 and Phase 3 tend to bring wetter conditions to western Maritime Continent and whole of Maritime Continent respectively.

 

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

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.

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.

1.1 During December 2020, coastal south-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 and central Philippines, and southern Thailand based on both satellite-derived rainfall estimates datasets (GSMaP-NRT and CMORPH-Blended), partly linked to the tropical storm Krovanh in Week 3. For the rest of Mainland Southeast Asia, rainfall anomalies were negligible, expected at the start of dry season for the northern ASEAN region. A mix of above- and below-average rainfall was experienced for most regions in the equatorial region. There is some discrepancy between the two satellite datasets for the below-average rainfall for southern and central parts of Borneo, with more extensive drier conditions based on CMORPH-Blended (Figure 1, right) compared to GSMaP-NRT (Figure 1, left). Elsewhere in the southern Maritime Continent, wetter-than-average rainfall conditions were recorded over much of the land masses.

1.2 The observed large-scale rainfall anomaly pattern (i.e. above-average rainfall in regions linked to tropical storm and cyclone activity and below-average rainfall over parts of equatorial region) is broadly consistent with the predictions from the subseasonal weather outlooks for December 2020 (30 Nov – 13 Dec, 14 – 27 Dec).

Figure 1: Rainfall anomalies for the month of December 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 December 2020 (Figure 2). In contrast, warmer-than-average temperatures (more than 1°C warmer) were recorded over parts of Myanmar and northern Philippines. Warmer-than-average temperatures were also recorded over much of the equatorial region (3°S to 2°N). Elsewhere, the temperature for December was closer to average.

 

Figure 2: Temperature anomalies for the month of December 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 weak and slow-moving Madden-Julian Oscillation (MJO) signal was present in the Maritime Continent (Figure 3, Phase 5) at the start of December 2020, momentarily strengthened in Week 2 and then weakened as it reached Western Pacific (Phase 6) in the second half of December 2020. Phase 5 typically brings wetter conditions to eastern half of the Southeast Asia (observed in the rainfall anomalies for December 2020, 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) 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 December period.

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).

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.

 

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.

 

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.

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).

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.

 

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.

 

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.

1.1 During September 2020, most of the equatorial region and southern parts of the Mainland Southeast Asia experienced above-average rainfall (Figure 1). The largest positive anomalies (wetter conditions) were recorded in the coastal equatorial regions, southern Thailand, southern Cambodia and southern Viet Nam based on both satellite-derived rainfall estimates datasets (GSMaP-NRT and CMORPH-Blended). In contrast, most of the Philippines recorded below-average rainfall. The rest of regions either experienced near-average rainfall for this time of year, or a mix between above- and below-average.

1.2 The observed large-scale rainfall anomaly pattern (i.e. above-average rainfall in the equatorial regions and below-average rainfall in the Philippines) is broadly consistent with the predictions from the subseasonal weather outlooks for September 2020 (7 – 20 Sep, 21 Sep – 4 Oct). The only notable anomalies forecasted for Mainland Southeast Asia were the wetter conditions over its southern and eastern coastal regions (linked to the MJO signal in phases 4 and 5).

Figure 1: Rainfall anomalies for the month of September 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 Most parts of equatorial Southeast Asia experienced near-average temperature during September 2020 (Figure 2), coinciding with areas that experienced wetter conditions during the same period. Warmer anomalies (≥ 0.5°C) are concentrated over Mainland Southeast Asia, northern parts of the Philippines and eastern Maritime Continent with the warmest anomalies in September (≥ 2.0°C) in northern Myanmar.

 

Figure 2: Temperature anomalies for the month of September 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 weak and very slow-moving Madden-Julian Oscillation (MJO) signal was present in the Maritime Continent (Figure 3) during the month of September, momentarily strengthened in the middle of September for a few days and also towards the end of the month. At this time of year, phases 4 and 5 typically bring above-average rainfall for coastal regions in Southeast Asia between 5°N and 20°N.

 

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).