Category Archives: Regional Weather Review for Year 2022

January 12, 2023

Review of Regional Weather for December 2022

1. Overview

1.1 During December 2022, a mix of below- to above-average rainfall was recorded over the Maritime Continent, while near-average rainfall was recorded over much of Mainland Southeast Asia, except for coastal parts of eastern Mainland Southeast Asia where above-average rainfall was recorded (Figure 1). The largest positive anomalies (wetter conditions) were recorded over northwestern Continent, including Malay Peninsula for both the GSMaP-NRT (Figure 1, left) and CMORPH-Blended (Figure 1, right) satellite-derived rainfall estimates. The largest negative anomalies (drier conditions) were recorded over the southern Borneo, with CMORPH-Blended data showing relatively drier anomalies as compared to GSMaP-NRT data.

1.2 The observed rainfall anomaly pattern of a mix of below- to above-average rainfall for the Maritime Continent and the above-average rainfall for eastern Mainland Southeast Asia are broadly consistent with the predictions from the subseasonal weather outlooks for December 2022 (28 November – 11 December 2022, 12 – 25 December 2022 and 26 December 2022 – 8 January 2023).

Figure 1: Rainfall anomalies for December 2022 based on GSMaP-NRT data (left) and CMORPH-Blended data (right). The climatological reference period is 2001-2021. Green colour denotes above-average rainfall (wetter), while orange denotes below-average rainfall (drier).

1.3 Warmer-than-average temperatures were recorded over northwestern Mainland Southeast Asia and parts of southeastern Maritime Continent, whereas below- to near-average temperatures were observed elsewhere over the ASEAN region. The warmest anomalies were recorded over Myanmar and the coolest over the eastern Mainland Southeast Asia.

Figure 2: Temperature anomalies for December 2022 based on ERA-5 reanalysis. The climatological reference period is 2001-2021. Red colour denotes above-average temperature (warmer), while blue denotes below-average temperature (colder).

2. Climate Drivers

2.1 An Madden-Julian Oscillation (MJO) signal emerged over the Indian Ocean (Phase 3) and the Maritime Continent (Phase 4) at the end of the first week of December, based on the RMM index (Figure 3). This signal weakened and went inside the RMM unit circle at the end of the second week of December. At the end of the third week, an MJO signal reappeared over the Maritime Continent (Phases 4 & 5), which strengthened and propagated eastwards towards the Western Pacific (Phase 6) over the last week of December. Typically, during this time of the year, Phases 4 and 5 bring wetter conditions for most of the ASEAN region whereas Phase 6 tends to bring wetter conditions for eastern Maritime Continent. Therefore, the MJO may have contributed to the wetter conditions over northern parts of the Maritime Continent region December 2022.

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 La Niña conditions were present over the tropical Pacific. Sea surface temperatures in the Nino3.4 region (used to monitor ENSO) and the atmospheric indicators over the tropical Pacific Ocean were consistent with La Niña conditions, though there are signs of weakening. La Niña events tend to bring wetter-than-average conditions to much of Southeast Asia during the September to November period.

December 15, 2022

Review of Regional Weather for November 2022

1. Overview

1.1 During November 2022, a mix of below- to above-average rainfall was recorded over the Maritime Continent, while near-average rainfall was recorded over much of Mainland Southeast Asia, except for southern and northeastern Mainland Southeast Asia where a mix of below- to above-average rainfall was recorded (Figure 1). The largest positive anomalies (wetter conditions) were recorded over southern Mainland Southeast Asia and central Borneo, for the GSMaP-NRT (Figure 1, left) and CMORPH-Blended (Figure 1, right) satellite-derived rainfall estimates datasets respectively. The largest negative anomalies (drier conditions) were recorded over central Sumatra and northern half of the Philippines, with CMORPH-Blended data showing relatively drier anomalies as compared to GSMaP-NRT data.

1.2 The observed rainfall anomaly pattern of a mix of below- to above-average rainfall for the Maritime Continent and the above-average rainfall for southern Mainland Southeast Asia are broadly consistent with the predictions from the subseasonal weather outlooks for November 2022 (31 October – 13 November, 14 – 27 November and 28 November – 11 December), although the wetter condition over central Borneo was not included in any of the outlooks.

Figure 1: Rainfall anomalies for November 2022 based on GSMaP-NRT data (left) and CMORPH-Blended data (right). The climatological reference period is 2001-2021. Green colour denotes above-average rainfall (wetter), while orange denotes below-average rainfall (drier).

1.3 Warmer-than-average temperatures were recorded over most of Southeast Asia north of 10°N, except for some northwesternmost parts of Mainland Southeast Asia where below- to near-average temperatures are observed. In contrast, below- to near-average temperatures were recorded for most of the regions south of 10°N during November 2022 (Figure 2), except for south-eastern parts of the Maritime Continent where above-average temperatures are observed. The warmest anomalies were recorded around northern Viet Nam, with the coolest anomalies over central parts of Java islands.

Figure 2: Temperature anomalies for November 2022 based on ERA-5 reanalysis. The climatological reference period is 2001-2021. Red colour denotes above-average temperature (warmer), while blue denotes below-average temperature (colder).

2. Climate Drivers

2.1 An Madden-Julian Oscillation (MJO) signal was present over the Western Pacific and the Western Hemisphere (Phases 7 and 8) in the first week of November, however it weakened and became indiscernible by the second week, based on the RMM Index (Figure 3). At the beginning of the third week, an MJO signal emerged over the Maritime Continent (Phase 4), which strengthened and propagated eastwards towards the Western Pacific (Phase 7), before becoming weak again at the end of the month. Typically, during this time of the year, Phase 6 tends to bring wetter conditions for eastern Maritime Continent while Phases 7 and 8 tends to bring drier conditions for the western Maritime Continent. Also for November, Phases 4 and 5 bring wetter conditions to much of the Southeast Asia. Therefore, the MJO may have contributed to the wetter conditions over southern coastal parts of Mainland Southeast Asia and southeastern Maritime Continent region in November 2022.

2.2 La Niña conditions were present over the tropical Pacific. Sea surface temperatures in the Nino3.4 region (used to monitor ENSO) and the atmospheric indicators over the tropical Pacific Ocean were 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. Over the Indian Ocean, a negative Indian Ocean Dipole (IOD) was present and showing signs of weakening. Negative IOD tends to bring wetter-than-average conditions for much of Southeast Asia.

November 11, 2022

Review of Regional Weather for October 2022

1. Overview

1.1 During October 2022, much of the ASEAN region experienced above-average rainfall (Figure 1). The largest positive anomalies (wetter conditions) were recorded over northern Philippines (due to Tropical Storm Nalgae which made landfall in end-October), based on both satellite-derived rainfall estimates datasets (GSMaP-NRT and CMORPH-Blended). In contrast to the above-average rainfall, northwest Mainland Southeast Asia experienced near- to above-average rainfall, while parts of northern Viet Nam and Papua experienced below- to near-average rainfall. The two datasets agree well over most of the ASEAN region. However, there are some discrepancies over parts of the western Maritime Continent, Borneo and southern Viet Nam, where GSMaP-NRT recorded near- to above-average rainfall whereas CMORPH-Blended recorded a mix of below- to above-average rainfall.

1.2 The observed rainfall anomaly pattern of above-average rainfall for much of the Maritime Continent region is broadly consistent with the predictions from the subseasonal weather outlooks for October 2022 (3 – 16 October and 17 – 30 October). Overall, much of Mainland Southeast Asia experienced near- to above-average rainfall for October. The subseasonal outlooks predicted drier conditions over western and central Mainland Southeast Asia for middle of the October. The observed anomalies also supported this with lower rainfall anomalies compared to the rest of the region.

Figure 1: Rainfall anomalies for October 2022 based on GSMaP-NRT data (left) and CMORPH-Blended data (right). The climatological reference period is 2001-2021. Green colour denotes above-average rainfall (wetter), while orange denotes below-average rainfall (drier).

1.3 The northwestern and eastern parts of the ASEAN region experienced above-average temperatures during October 2022, with below- to near-average temperatures elsewhere (Figure 2). The warmest anomalies were recorded around Papua, with the coolest anomalies over parts of southern Sumatra, southern Borneo and eastern Java. Most of the cooler anomalies occurred for areas that also experienced above-average rainfall.

Figure 2: Temperature anomalies for October 2022 based on ERA-5 reanalysis. The climatological reference period is 2001-2021. Red colour denotes above-average temperature (warmer), while blue denotes below-average temperature (colder).

2. Climate Drivers

2.1 There was no discernible Madden-Julian Oscillation (MJO) signal in the first week of October, based on the RMM Index (Figure 3). An MJO signal emerged over the Western Pacific (Phase 6) and propagated eastward to Phase 7 in the second week of October. This signal then maintained its strength and remained in Phase 7 for the rest of the October. Typically, during this time of the year, Phase 6 tends to bring wetter conditions for eastern Maritime Continent while Phase 7 tends to bring drier conditions for the western Maritime Continent.

2.2 Over the tropical Pacific Ocean, La Niña conditions were present. Sea surface temperatures in the Nino3.4 region (used to monitor ENSO) and the atmospheric indicators over the tropical Pacific Ocean remain consistent with La Niña conditions. La Niña events tend to bring wetter-than-average conditions to much of the Maritime Continent during the September to November period. Over the Indian Ocean, a negative Indian Ocean Dipole (IOD) was present. Negative IOD tends to bring wetter-than-average conditions for much of Southeast Asia.

October 14, 2022

Review of Regional Weather for September 2022

1. Overview

1.1 During September 2022, much of the ASEAN region experienced above-average rainfall (Figure 1). The largest positive anomalies (wetter conditions) were recorded over parts Viet Nam and southern Borneo based on both satellite-derived rainfall estimates datasets (GSMaP-NRT and CMORPH-Blended). In contrast to the above-average rainfall, much of Peninsular Malaysia, the Philippines as well as northern Myanmar, northern Laos PDR, northern Sumatra, and northern Papua experienced below- to near-average rainfall. The two datasets agree well over most of the ASEAN region. However, there are some discrepancies over the central Myanmar and northern Laos PDR where GSMaP-NRT recorded mix of below- to above-average rainfall whereas CMORPH-Blended recorded near-average rainfall.

1.2 The observed rainfall anomaly pattern of above-average rainfall for most of the Maritime Continent region are broadly consistent with the predictions from the subseasonal weather outlooks for September 2022 (5 – 18 September and 19 September – 2 October). For Mainland Southeast Asia, the southern parts overall experienced above average rainfall in September, where the subseasonal outlooks also predicted wetter conditions in the first half of the September. While the northern parts of Mainland Southeast Asia experienced a mix of below- to -above average rainfall for September, the subseasonal outlooks predicted drier conditions towards the end of the month.

Figure 1: Rainfall anomalies for September 2022 based on GSMaP-NRT data (left) and CMORPH-Blended data (right). The climatological reference period is 2001-2021. Green colour denotes above-average rainfall (wetter), while orange denotes below-average rainfall (drier).

1.3 The southeastern parts of the Maritime Continent and northwestern Myanmar experienced above-average temperatures during September 2022, with below- to near-average temperatures elsewhere (Figure 2). The warmest anomalies were recorded over Papua, with the coolest anomalies over parts of Thailand, Cambodia, southern Borneo, and southern Sumatra. Most of the cooler anomalies occurred for areas that also experienced above-average rainfall.

Figure 2: Temperature anomalies for September 2022 based on ERA-5 reanalysis. The climatological reference period is 2001-2021. Red colour denotes above-average temperature (warmer), while blue denotes below-average temperature (colder).

2. Climate Drivers

2.1 The Madden-Julian Oscillation (MJO) signal was weak or indiscernible for most of September 2022, based on the RMM Index below (Figure 3). As there was no significant MJO activity in the entire September, it would have little contribution to rainfall conditions over the ASEAN region.

September 15, 2022

Review of Regional Weather for August 2022

1. Overview

1.1 During August 2022, much of the ASEAN region experienced above-average rainfall (Figure 1). The largest positive anomalies (wetter conditions) were recorded over the southern parts of Myanmar, western Borneo, and the Maluku Islands based on both satellite-derived rainfall estimates datasets (GSMaP-NRT and CMORPH-Blended). In contrast to the above-average rainfall, parts of northern Myanmar, the Malay Peninsula, northern Borneo, the Philippines, and Papua experienced below- to near-average rainfall, or a mix of between above- and below-average rainfall. The two datasets agree well over most of the ASEAN region. However, there are some discrepancies over the Philippines and west Papua where GSMaP-NRT recorded near- to above-average rainfall whereas CMORPH-Blended recorded below- to near-average rainfall.

1.2 The observed rainfall anomaly pattern of above-average rainfall for much of the Maritime Continent region is broadly consistent with the predictions from the subseasonal weather outlooks for August 2022 (25 July – 7 August, 8 – 21 August and 22 August – 4 September). While overall for August much of Mainland Southeast Asia experienced above-average rainfall as well, the subseasonal outlooks predicted wetter conditions in the second week, and drier conditions towards the end of the month.

Figure 1: Rainfall anomalies for August 2022 based on GSMaP-NRT data (left) and CMORPH-Blended data (right). The climatological reference period is 2001-2021. Green colour denotes above-average rainfall (wetter), while orange denotes below-average rainfall (drier).

1.3 The northernmost and southernmost parts of the ASEAN region experienced above-average temperatures during August 2022, with below- to near-average temperatures elsewhere (Figure 2). The warmest anomalies were recorded over northern Myanmar and Papua, with the coolest anomalies over parts of Sumatra and Cambodia. Most of the cooler anomalies occurred for areas that also experienced above-average rainfall.

Figure 2: Temperature anomalies for August 2022 based on ERA-5 reanalysis. The climatological reference period is 2001-2021. Red colour denotes above-average temperature (warmer), while blue denotes below-average temperature (colder).

2. Climate Drivers

2.1 There were some signs of a Madden-Julian Oscillation (MJO) signal over the Maritime Continent (Phases 4 and 5) in the first week of August, however there was no discernible signal by the second week, based on the RMM Index (Figure 3). An MJO signal emerged over the Indian Ocean (Phase 2) towards the end of August, its strength briefly amplifying at the start of the last week in August, before becoming weak again at the end of the month. Typically, Phase 2 tends to bring drier conditions over the northeastern ASEAN region and wetter conditions for the western Maritime continent at this time of year, while Phases 4 and 5 bring wetter conditions to northern parts of the Maritime Continent. Therefore, while there was little MJO activity for most of August, the MJO signals near the start and end of the month may have contributed to the wetter-than-average conditions.

2.2 Over the tropical Pacific Ocean, La Niña conditions were present. Sea surface temperatures in the Nino3.4 region (used to monitor ENSO) and the atmospheric indicators over the tropical Pacific Ocean remain consistent with La Niña conditions. La Niña events tend to bring wetter-than-average conditions to much of the Maritime Continent during the June to August period. Over the Indian Ocean, a negative Indian Ocean Dipole (IOD) was present. Negative IOD tends to bring wetter-than-average conditions for much of Southeast Asia.

August 12, 2022

Review of Regional Weather for July 2022

1. Overview

1.1 During July 2022, most of the southeastern Maritime Continent region and central/eastern parts of Mainland Southeast Asia experienced above-average rainfall (Figure 1). The largest positive anomalies (wetter conditions) were recorded over the southern coastal parts of Mainland Southeast Asia, eastern Sulawesi, western Papua, and the Maluku Islands based on both satellite-derived rainfall estimates datasets (GSMaP-NRT and CMORPH-Blended). In contrast, most of the northern/central 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. The two datasets agree well over most of the ASEAN region. However, there are some discrepancies over the western parts of Peninsular Malaysia where GSMaP-NRT recorded near-average rainfall whereas CMORPH-Blended recorded below-average rainfall, and over southern Sumatra where GSMaP-NRT recorded near- to above-average rainfall whereas CMORPH-Blended recorded near-average rainfall.

1.2 The observed rainfall anomaly pattern of above-average rainfall for southeastern Maritime Continent region and central/eastern parts of the Mainland Southeast Asia is broadly consistent with the predictions from the subseasonal weather outlooks for July 2022 (27 June – 10 July, 11 – 24 July and 25 July – 7 August).

Figure 1: Rainfall anomalies for July 2022 based on GSMaP-NRT data (left) and CMORPH-Blended data (right). The climatological reference period is 2001-2021. Green colour denotes above-average rainfall (wetter), while orange denotes below-average rainfall (drier).

1.3 Much of the ASEAN region experienced above-average temperatures during July 2022 (Figure 2). The warmest anomalies were recorded over western/northern Myanmar and northern Lao PDR, followed by northern Viet Nam, Peninsular Malaysia, parts of the Philippines, and southeastern parts of the Maritime Continent. Elsewhere, near-average temperatures were generally recorded.

Figure 2: Temperature anomalies for July 2022 based on ERA-5 reanalysis. The climatological reference period is 2001-2021. 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 was present mostly over eastern Indian Ocean (Phase 3) and the Maritime Continent (Phases 4 and 5) in the first week of July 2022, based on the MJO phase diagram below (Figure 3). In the second and third week of July, the MJO signal weakened and became indiscernible. This signal then re-emerged over the Western Hemisphere (Phase 1) towards the end of the third week, propagated eastwards and weakened over the western Indian Ocean (Phase 2) in the last week of July. Typically, Phases 1 and 2 bring drier conditions in northern Southeast Asia while Phases 2 and 3 tend to bring wetter conditions to the western Maritime Continent in July. Also for July, Phase 4 bring wetter conditions in northern Southeast Asia while Phase 5 bring wetter conditions to the southeastern Maritime Continent. Therefore, the MJO may have contributed to the drier conditions over northern half of the Philippines, and wetter conditions over southern coastal parts of Mainland Southeast Asia and southeastern Maritime Continent region in July.

2.2 Over the tropical Pacific Ocean, weak La Niña conditions were present for much of July 2022. Sea surface temperatures in the Nino3.4 region (used to monitor ENSO) and the atmospheric indicators over the tropical Pacific Ocean remain consistent with weakening La Niña conditions. La Niña events tend to bring wetter-than-average conditions to much of the Maritime Continent during the June to August period. Over the Indian Ocean, there were continuing signs of a developing negative Indian Ocean Dipole (IOD) in July. Negative IOD tends to bring wetter-than-average conditions for much of Southeast Asia.

July 14, 2022

Review of Regional Weather for June 2022

1. Overview

1.1 During June 2022, below-average rainfall was recorded over northeast Maritime Continent and coastal parts of southern Mainland Southeast Asia, while near- to above-average rainfall was recorded elsewhere (Figure 1). The largest positive anomalies in rainfall (wetter conditions) were recorded over northern Myanmar, while the largest negative anomalies in rainfall (drier conditions) was recorded over southern Myanmar and Papua for both satellite-derived rainfall estimates datasets (GSMaP-NRT and CMORPH-Blended). The two datasets agree well over most of the ASEAN region. However, there are some discrepancies over western coast of Myanmar where GSMaP-NRT recorded below-average rainfall whereas CMORPH-Blended recorded near- to above-average rainfall, and over Cambodia where GSMaP-NRT recorded near- to above-average rainfall whereas CMORPH-Blended recorded below- to near- average rainfall.

1.2 The observed rainfall anomaly pattern of above-average rainfall for parts of the Maritime Continent and below-average rainfall for southern Mainland Southeast Asia is broadly consistent with the predictions from the subseasonal weather outlooks for June 2022 (30 May – 12 June and 13 – 26 June).

Figure 1: Rainfall anomalies for the month of June 2022 based on GSMaP-NRT data (left) and CMORPH-Blended data (right). The climatological reference period is 2001-2021. Green colour denotes above-average rainfall (wetter), while orange denotes below-average rainfall (drier).

1.3 The western Maritime Continent recorded below-average temperature, while the southeastern Maritime Continent and eastern Mainland Southeast Asia recorded above-average temperature in June 2022 (Figure 2). The largest positive anomalies (more than 1°C warmer than normal conditions for June) were over Papua and northern Viet Nam.

Figure 2: Temperature anomalies for the month of June 2022 based on ERA-5 reanalysis. The climatological reference period is 2001-2021. 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 was present mostly over the Western Hemisphere (Phases 8 and 1) and the Indian Ocean (Phases 2 and 3) in June 2022 based on the MJO phase diagram below (Figure 3). In the first week of June, the MJO signal propagated eastward over the Western Hemisphere (from Phase 8 to Phase 1). For the second and third week of May, the MJO signal weakened and remained relatively stationary over the Western Hemisphere (Phase 1), with also the influence from other tropical waves. This signal then strengthened again and propagated rapidly eastwards over the Indian Ocean (Phase 2 and 3) in the last week of June. Typically, Phases 1 and 2 bring drier conditions in northern Southeast Asia while Phases 2 and 3 tend to bring wetter conditions to the western Maritime Continent in June. Therefore, the MJO may have contributed to the drier conditions over southern Mainland Southeast Asia and wetter conditions over the western Maritime Continent in June.

2.2 Over the tropical Pacific Ocean, La Niña conditions were present but show signs of weakening. Sea surface temperatures in the Nino3.4 region (used to monitor ENSO) and the atmospheric indicators over the tropical Pacific Ocean were weakening, indicating weak La Niña conditions in June. La Niña events tend to bring wetter-than-average conditions to much of the Maritime Continent during the June to August period. Over the Indian Ocean, there were signs of negative Indian Ocean Dipole development as the Dipole Mode Index (DMI), which is used to measure the Indian Ocean Dipole (IOD) signal, became increasingly negative and had transited into the negative phase in June. Negative IOD tends to bring wetter-than-average conditions for much of Southeast Asia.

June 17, 2022

Review of Regional Weather for May 2022

1. Overview

1.1 During May 2022, near- to above-average rainfall was recorded over much of the northern ASEAN region, while a mixture of below- to above-average rainfall was recorded over the southern ASEAN region (Figure 1). For the northern ASEAN region, the largest positive anomalies in rainfall (wetter conditions) were recorded over the southern Myanmar for both satellite-derived rainfall estimates datasets (GSMaP-NRT and CMORPH-Blended). For the southern ASEAN region, above-average rainfall was recorded over central Indonesia and a mix of below- to above-average elsewhere. The two datasets agree well over most of the ASEAN region. However, there are some discrepancies over western Borneo and southern Sumatra where CMORPH recorded below- to near-average rainfall whereas GSMaP-NRT recorded near- to above-average rainfall.

1.2 The observed rainfall anomaly pattern of above-average rainfall for parts of the northern ASEAN region, particularly Myanmar, is broadly consistent with the predictions from the subseasonal weather outlooks for May 2022 (2 – 15 May and 16 – 29 May). The drier conditions predicted over the equatorial region for 16 – 29 May, can be observed over parts of the western and eastern equatorial region, although overall in May it was relatively wet over the central equatorial region.

Figure 1: Rainfall anomalies for the month of May 2022 based on GSMaP-NRT data (left) and CMORPH-Blended data (right). The climatological reference period is 2001-2021. Green colour denotes above-average rainfall (wetter), while orange denotes below-average rainfall (drier).

1.3 Most parts of northern ASEAN region recorded below-average temperature in May 2022 (Figure 2). The largest negative anomalies (2°C cooler than normal conditions for May) were over northeastern Thailand and northern Viet Nam. For most of the southern ASEAN region, normal to above-average temperature were recorded with largest warm anomalies over Peninsular Malaysia and southeastern Indonesia.

Figure 2: Temperature anomalies for the month of May 2022 based on ERA-5 reanalysis. The climatological reference period is 2001-2021. Red colour denotes above-average temperature (warmer), while blue denotes below-average temperature (colder).

2. Climate Drivers

2.1 There was no discernible Madden-Julian Oscillation (MJO) signal at the start of May 2022 based on the MJO phase diagram below (Figure 3). In the first week of May, an MJO signal emerged over the Indian Ocean (Phase 2). For the second week of May, the MJO signal first became relatively weaker as it moved eastwards over the Indian Ocean (Phase 3). This signal then strengthened again and continued moving rapidly eastwards over the Maritime Continent (Phase 4 and 5) and Western Pacific (Phase 6). In the third week of May, the signal moved further east over the Western Pacific (Phase 7) and Western Hemisphere (Phase 8), before weakening and becoming indiscernible. Towards the end of May, an MJO signal emerged again over the Western Pacific (Phases 6 and 7). For the central Maritime Continent, Phases 3, 4, and 5 can all bring wetter conditions, while for the western Maritime Continent, Phases 6 and 7 tend to bring drier conditions. Therefore, the MJO may have contributed to the wetter conditions over parts of the central Maritime Continent and drier conditions over parts of the western Maritime Continent during May.

2.2 Over the tropical Pacific Ocean, La Niña conditions were present. Sea surface temperatures in the Nino3.4 region (used to monitor ENSO) and the atmospheric indicators over the tropical Pacific Ocean were consistent with La Niña conditions. La Niña events tend to bring wetter-than-average conditions to much of Southeast Asia during the March to May period. Over the Indian Ocean, there were signs of a negative Indian Ocean Dipole developing in May but the Dipole Mode Index (DMI), which is used to measure the Indian Ocean Dipole (IOD) signal, remained neutral. Negative IOD tends to bring wetter-than-average conditions for much of Southeast Asia.

May 17, 2022

Review of Regional Weather for April 2022

1. Overview

1.1 During April 2022, near- to above-average rainfall was recorded over much of the northern ASEAN region, while a mix of below- to above-average rainfall was recorded over the southern ASEAN region (Figure 1). For the northern ASEAN region, the largest positive anomalies (wetter conditions) were recorded over the Philippines for both satellite-derived rainfall estimates datasets (GSMaP-NRT and CMORPH-Blended), which partly was associated with Tropical Storm Megi during the middle of April. For the southern ASEAN region, negative anomalies (drier conditions) were recorded over the eastern parts. Elsewhere in the southern ASEAN region, rainfall tended to be a mix of below- to above-average. There are some discrepancies between the two datasets, as GSMaP-NRT recorded a mix of below- to above-average rainfall for much of southern Sumatra and Borneo, while CMORPH-Blended recorded below- to near-average rainfall with isolated areas of above-average rainfall over the mentioned regions.

1.2 The observed rainfall anomaly pattern of above-average rainfall for parts of the northern ASEAN region, including the Philippines and a mix of below- to above-average rainfall for the southern ASEAN region is broadly consistent with the predictions from the subseasonal weather outlooks for April 2022 (4 – 17 April, and 18 April – 1 May).

Figure 1: Rainfall anomalies for the month of April 2022 based on GSMaP-NRT data (left) and CMORPH-Blended data (right). The climatological reference period is 2001-2021. Green colour denotes above-average rainfall (wetter), while orange denotes below-average rainfall (drier).

1.3 Most parts of northern ASEAN region recorded below-average temperature in April 2022 (Figure 2). The largest negative anomalies (less than 1°C cooler) were over northeastern Thailand, Lao PDR, and Viet Nam. Elsewhere, there was near- to above-average temperature, with most of the regions recording above-average temperature south of the equator.

Figure 2: Temperature anomalies for the month of April 2022 based on ERA-5 reanalysis. The climatological reference period is 2001-2021. Red colour denotes above-average temperature (warmer), while blue denotes below-average temperature (colder).

2. Climate Drivers

2.1 The Madden-Julian Oscillation (MJO) signal was weak or indiscernible for much of April 2022, based on the MJO phase diagram below (Figure 3). At the start of the month, the MJO was indiscernible. In the second week of April, there were signs of eastward progression of a signal, however, this signal was weak and remained inside the unit circle. Around the middle of April, the signal strengthened slightly, moving outside the unit circle in the Western Hemisphere (Phase 8) before becoming indiscernible again by the third week in April as it moved towards the Indian Ocean (Phase2). Phase 8 tends to bring drier conditions to much of the Maritime Continent, while Phase 1 tends to bring drier conditions only to parts of the eastern Maritime Continent. Therefore, the MJO may have contributed to the drier conditions in the April in the southeastern Maritime Continent.

April 19, 2022

Review of Regional Weather for March 2022

1. Overview

1.1 During March 2022, above-average rainfall was recorded over much of the northern ASEAN region, while a mix of below- to above-average rainfall was recorded over the southern ASEAN region (Figure 1). For the northern ASEAN region, above-average rainfall was recorded apart from over central and northern Myanmar. The largest positive anomalies (wetter conditions) were recorded over northeastern Mainland Southeast Asia and southern half of the Philippines for both satellite-derived rainfall estimates datasets (GSMaP-NRT and CMORPH-Blended), which was associated with tropical depression 01W that formed on the last three days of March 2022. For the southern ASEAN region, negative anomalies (drier conditions) were recorded over western parts of Sumatra as well as western and central Borneo, although the negative anomalies were larger based on CMORPH-Blended data (Figure 1b) compared to GSMaP-NRT (Figure 1a). Elsewhere in the southern ASEAN region, rainfall tended to be near- to above-average during March 2022. There are some discrepancies between the two datasets, as GSMaP-NRT recorded above-average rainfall for much of central Java and northern Sumatra, while CMORPH-Blended recorded near- to below-average rainfall with isolated areas of above-average rainfall over the mentioned regions.

1.2 The observed rainfall anomaly pattern of above-average rainfall for much of the northern ASEAN region and a mix of below- to above-average rainfall for the southern ASEAN region is broadly consistent with the predictions from the subseasonal weather outlooks for March 2022 (21 February – 6 March, 7 – 20 March, and 21 March – 3 April).

Figure 1: Rainfall anomalies for the month of March 2022 based on GSMaP-NRT data (left) and CMORPH-Blended data (right). The climatological reference period is 2001-2021. Green colour denotes above-average rainfall (wetter), while orange denotes below-average rainfall (drier).

1.3 Most parts of Southeast Asia north of 15°N experienced above-average temperatures during March 2022 (Figure 2). The largest positive anomalies (more than 1°C warmer) were over northern and eastern parts of Myanmar, northwest Thailand, northern Lao PDR and northern Viet Nam. Elsewhere, near-average temperatures were experienced in most of the region, with above-average temperatures for Papua and southern and central parts of Sumatra, and below-average temperatures for parts of the southern Mainland Southeast Asia.

Figure 2: Temperature anomalies for the month of March 2022 based on ERA-5 reanalysis. The climatological reference period is 2001-2021. Red colour denotes above-average temperature (warmer), while blue denotes below-average temperature (colder).

2. Climate Drivers

2.1 The Madden-Julian Oscillation (MJO) signal was indiscernible at the start of March 2022, based on the MJO phase diagram below (Figure 3). In the second week of March, an MJO signal developed over the Western Hemisphere (Phase 1). The MJO signal then propagate eastward over the Indian Ocean (Phase 2 to Phase 3), before weakening and becoming indiscernible in the last week of March. Phase 2 tends to bring drier conditions to the southeastern Maritime Continent while Phase 3 tends to bring wetter conditions to the western Maritime Continent.

2.2 Over the tropical Pacific Ocean, La Niña conditions were present but there are signs of weakening. Sea surface temperatures in the Nino3.4 region (used to monitor ENSO) and the atmospheric indicators over the tropical Pacific Ocean were consistent with La Niña conditions. La Niña events tend to bring wetter-than-average conditions to much of the Maritime Continent during the December to March period.