Category Archives: Fortnightly Weather Review

CR58

September 14, 2021

Review of Regional Weather for August 2021

1. Overview

1.1 During August 2021, there was above-average rainfall recorded over much of the ASEAN region, while below-average rainfall was recorded over the northeastern ASEAN region (Figure 1). The largest positive anomalies (wetter conditions) were found across the equatorial region for both satellite-derived rainfall estimates datasets (GSMaP-NRT and CMORPH-Blended). The two datasets were also in agreement that the largest negative anomalies over land (drier conditions) were observed over parts of northern Viet Nam. However, there were some discrepancies over the southern Myanmar, where CMORPH-Blended recorded wetter conditions while GSMaP-NRT recorded drier conditions.

1.2 The observed rainfall anomaly pattern of above-average rainfall for much of the ASEAN region is broadly consistent with the predictions from the subseasonal weather outlooks for August 2021 (26 July – 8 August, 9 – 22 August and 23 August – 5 September).

Figure 1: Rainfall anomalies for the month of August 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 the northern ASEAN region and southern Maritime Continent experienced above-average temperatures during August 2021 (Figure 2). The warmest anomalies (above-average temperature) were recorded over northern Viet Nam, where the drier conditions were observed. For the equatorial region and central and northern Myanmar, near- to below-average temperature was recorded.

Figure 2: Temperature anomalies for the month of August 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 August 2021, a Madden-Julian Oscillation (MJO) signal was present in the Western Hemisphere (Phase 8, Figure 3), which weakened by the middle of the first week based on the RMM plot. The signal emerged over the Indian Ocean (Phase 2) in the second week of August, with a rapid strengthening of the signal in the RMM plot and slow eastward propagation. This MJO signal weakened in the second half of August while it was still over the Indian Ocean. The mix of below- to above-average rainfall for the ASEAN region is indicative of the MJO signal: Phase 2 tends to bring drier conditions over the northeastern ASEAN region and wetter conditions for the western Maritime Continent at this time of the 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 The equatorial Pacific Ocean was in the ‘ENSO-neutral’ state while a negative Indian Ocean Dipole (IOD) was present in August. At the seasonal timescale, negative IOD events tend to bring wetter conditions to the southern Maritime Continent.

August 13, 2021

Review of Regional Weather for July 2021

1. Overview

1.1 During July 2021, there was a mix of below- to above-average rainfall over the ASEAN region (Figure 1). The largest positive anomalies (wetter conditions) were over the coastal parts of southern Myanmar, southern Cambodia, northwestern Philippines, as well as the Maluku Islands for both satellite-derived rainfall estimates datasets (GSMaP-NRT and CMORPH-Blended). The two datasets were also in agreement that the largest negative anomalies (drier conditions) were over the central Philippines. However, there were some discrepancies over the western and central Maritime Continent, where CMORPH-Blended recorded drier conditions than GSMaP-NRT.

1.2 The observed rainfall anomaly pattern (i.e. mix of below- to above-average rainfall) is broadly consistent with the predictions from the subseasonal weather outlooks for July 2021 (28 June – 11 July and 12 – 25 July). The only portion of the ASEAN region with a consistent outlook for July were the wetter conditions in the southeastern Maritime Continent, which agrees with the observed anomaly pattern.

Figure 1: Rainfall anomalies for the month of July 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 the ASEAN region experienced above-average temperatures during July 2021 (Figure 2). The warmest anomalies (above-average temperature) were recorded over Lao PDR and the southern Maritime Continent, followed by central and northern Myanmar, Viet Nam, and central and southern parts of the Philippines. Elsewhere, near- to above-average temperature were recorded.

Figure 2: Temperature anomalies for the month of July 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 July 2021, a Madden-Julian Oscillation (MJO) signal was present in the Indian Ocean (Phases 2 and 3, Figure 3). In the second week of July the signal propagated eastwards over the Maritime Continent (Phases 4 and 5). The signal then weakened slightly as it moved over the Western Pacific (Phases 6 and 7), although it strengthened again entering the Western Hemisphere (Phase 8) at the end of July. The mix of below- to above-average rainfall for the ASEAN region is indicative of the near complete circumnavigation of the MJO signal: Phases 3 to 4 tend to bring wetter conditions to much of the region, while Phases 7 to 8 tend to bring drier conditions, and Phase 2 and Phase 5 bring a mix of drier and wetter conditions at this time of the year.

2.2 The equatorial Pacific Ocean is currently in an ‘ENSO-neutral’ state. A negative Indian Ocean Dipole (IOD) has developed in the Indian Ocean, with warmer sea surface temperatures southwest of Java during July. At the seasonal timescale, negative IOD events tend to bring wetter conditions to the southern Maritime Continent.

July 14, 2021

Review of Regional Weather for June 2021

1. Overview

1.1 During June 2021, there was a mix of below- to above-average rainfall over the ASEAN region (Figure 1). Much of the western and central Maritime Continent (except for parts of eastern Borneo), and the north-eastern parts of Mainland Southeast Asia experienced above-average rainfall. The largest positive anomalies (wetter conditions) were recorded over central Java regions and northern Viet Nam (due to Tropical Storm Koguma made landfall in mid-June) for both satellite-derived rainfall estimates datasets (GSMaP-NRT and CMORPH-Blended). A band of below-average rainfall between 8°N and 14°N was recorded, in particular parts of southern Mainland Southeast Asia. Elsewhere, the land masses of the eastern Maritime Continent predominately experienced below-average rainfall, except for central parts of the Philippines which experienced above-average rainfall due to Tropical Storm Choi-Wan in early June.

1.2 The observed rainfall anomaly pattern (i.e. below-average rainfall over southern and northern parts of the Philippines , above-normal rainfall in the Java region) is broadly consistent with the predictions from the subseasonal weather outlooks for June 2021 (31 May – 13 June, 14 – 27 June, 28 June – 11 July), apart from the positive anomalies (wetter conditions) for central Sumatra and the north-eastern parts of Mainland Southeast Asia not highlighted in the outlooks.

Figure 1: Rainfall anomalies for the month of June 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 the Mainland Southeast Asia experienced near- to above-average temperatures during June 2021, in particular warmer temperature for northern Myanmar, central Thailand, northern Cambodia and northern Viet Nam (Figure 2). Above-average temperatures were also experienced over northern Philippines and southern Maritime Continent. Elsewhere in the Maritime Continent, near-average temperatures were experienced in most of the region, with below-average temperatures for northern Sumatra and parts of Peninsular Malaysia.

Figure 2: Temperature anomalies for the month of June 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 The Madden-Julian Oscillation (MJO, Figure 3) signal was incoherent for most of the first three weeks of June 2021 due to interference with other tropical waves. In the last week of June, an MJO signal emerged over the Western Hemisphere and Africa (Phase 1) and propagated eastwards to over the Indian Ocean (Phase 2). Phase 2 tends to bring wetter conditions to western Maritime Continent.

2.2 The equatorial Pacific Ocean is currently in an ‘ENSO neutral’ state. There are some signs of a negative Indian Ocean Dipole (IOD) developing in the Indian Ocean, with warmer sea surface temperatures south-west of Java. At the seasonal timescale, negative IOD events tend to bring wetter conditions to the southern Maritime Continent.

June 14, 2021

Review of Regional Weather for May 2021

1. Overview

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.

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.

May 14, 2021

Review of Regional Weather for April 2021

1. Overview

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.

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.

April 12, 2021

Review of Regional Weather for March 2021

1. Overview

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.

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.

March 17, 2021

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).

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.

February 18, 2021

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.

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.

January 12, 2021

Review of Regional Weather for December 2020

1. Overview

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).

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.

December 15, 2020

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).

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.

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.