Category Archives: Fortnightly Weather Review

CR58

May 16, 2025

Review of Regional Weather for April 2025

1. Overview

1.1 During April 2025, much of the western Maritime Continent and western Mainland Southeast Asia experienced above-average rainfall, while much of the southern and northeastern Maritime Continent experienced below-average rainfall (Figure 1). The largest positive (wetter) anomalies were recorded over the Malay Peninsula. Outside of the regions mentioned above, there was either disagreements between GSMaP-NRT and CMORPH-Blended datasets or no notable anomalies. For much of the central and eastern equatorial region CMORPH-Blended recorded below-average rainfall, while GSMaP-NRT recorded above-average rainfall. CMORH-Blended also recorded below-average rainfall for Cambodia, while GSMaP-NRT was near-average.

1.2 The observed rainfall anomaly pattern of above-average rainfall over much of western Mainland Southeast Asia and the western Maritime Continent and drier conditions over the northeastern Maritime continent is consistent with the predictions from the subseasonal weather outlooks for April 2025 (31 March – 13 April 2025, and 14 – 27 April 2025). The observed pattern is also partly consistent with the April 2025 seasonal outlook of above-normal rainfall over western Mainland Southeast Asia, although the northwestern Maritime Continent was predicted to have a higher chance of above-normal rainfall.

Figure 1: Rainfall anomalies for April 2025 based on GSMaP-NRT data (left) and CMORPH-Blended data (right). The climatological reference period is 2001-2024. Green colour denotes above-average rainfall (wetter), while orange denotes below-average rainfall (drier).

1.3 Below-average and near-average temperatures were recorded over most of Southeast Asia in April 2025, apart from over northern Myanmar and the Philippines (Figure 2). The coldest anomalies (-1°C – -2°C below average) were recorded over parts of western and southern Myanmar, associated with regions of above-average rainfall. The warmest anomalies (1°C – 2°C above average) were recorded over northern Myanmar.

Figure 2: Temperature anomalies for April 2025 based on ERA-5 reanalysis. The climatological reference period is 2001-2024. 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 for most of April based on the RMM diagram (Figure 3). During the last week of April, there were signs of an MJO signal over the Western Pacific (Phases 6 and 7) and the Western Hemisphere (Phase 8). Phases 6 and 8 tend to bring drier conditions to parts of the Maritime Continent at this time of the year, although this is not noticeable in the April rainfall anomalies, particularly for GSMaP-NRT.

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 ENSO-neutral conditions were present in April 2025. Sea surface temperatures in the Nino3.4 region (used to monitor ENSO) indicated a return to ENSO-neutral. Key atmospheric indicators of La Niña (reduced cloudiness and stronger trade winds in the central Pacific) were present earlier in March, but have since shown signs of weakening, in line with a transition to ENSO-neutral conditions.

April 28, 2025

Review of Regional Weather for March 2025

1. Overview

1.1 During March 2025, much of the Maritime Continent experienced above-average rainfall, except parts of the Philippines, the Malay Peninsula and the eastern Maritime Continent which experienced below-average rainfall (Figure 1). For Mainland Southeast Asia, below-average rainfall was recorded over parts of northeastern and southern region, with near average elsewhere. The largest positive (wetter) anomalies were recorded around the eastern coast of Sumatra to the southern tip of Malay Peninsula, while the largest negative (drier) anomalies were recorded over the northwestern Maritime Continent. There were disagreements between GSMaP-NRT and CMORPH-Blended datasets on the extent and magnitude of the dry anomalies, in particular over northern Lao PDR, northern Vietnam, parts of southern Philippines, southern Sumatra, Java and southeast Borneo, and Papua, with CMORPH-Blended showing stronger and more extensive dry anomalies as compared to GSMaP-NRT.

1.2 The observed rainfall anomaly pattern of above-average rainfall over much of the Maritime Continent is consistent with the predictions from the subseasonal weather outlooks for March 2025 (3 – 16 March 2025, and 17 – 30 March 2025). The observed pattern of above-average rainfall over the Maritime Continent is also consistent with the March 2025 seasonal outlook, although the below-normal rainfall over parts of the Maritime Continent is not well represented.

Figure 1: Rainfall anomalies for March 2025 based on GSMaP-NRT data (left) and CMORPH-Blended data (right). The climatological reference period is 2001-2024. Green colour denotes above-average rainfall (wetter), while orange denotes below-average rainfall (drier).

1.3 A mix of normal and below-average temperatures were recorded over eastern half of Mainland Southeast Asia whereas above-average temperatures were recorded over the western half of Mainland Southeast Asia in March 2025 (Figure 2). Elsewhere, above-average temperature were recorded over much of the Maritime Continent, except over the northwestern region with below- to near-average temperature. The coldest anomalies (-1°C – -2°C below average) were recorded over parts of central Vietnam and northeastern Thailand, while the warmest anomalies (1°C – 2°C above average) were recorded over western Myanmar.

Figure 2: Temperature anomalies for March 2025 based on ERA-5 reanalysis. The climatological reference period is 2001-2024. Red colour denotes above-average temperature (warmer), while blue denotes below-average temperature (colder).

2. Climate Drivers

2.1 In the first week of March, an active Madden-Julian Oscillation (MJO) signal was observed over the Western Hemisphere (Phase 1). In the second week of the month, the signal propagated eastwards through the Indian Ocean (Phases 2 and 3) and continued its eastwards propagation over the Maritime Continent (Phase 4) in the third week. The signal weakened towards the end of third week of March over the Maritime Continent and became indiscernible in the last week of the month. During this time of the year, Phase 1 typically bring drier conditions to the Maritime Continent. Phase 2 and 3 tend to bring wetter conditions over the western Maritime Continent, while Phases 4 and 5 typically bring wetter conditions over much of the Maritime Continent. This observed MJO signal during March is in line with the above-normal rainfall anomalies in Figure 1.

2.2 In March 2025, La Niña conditions were present. Sea surface temperatures in the Nino3.4 region (used to monitor ENSO) indicated La Niña conditions and atmospheric indicators (cloudiness and trade winds) also showed signs of La Niña. La Niña events tend to bring wetter-than-average conditions to much of the Maritime Continent during this time of the year.

March 11, 2025

Review of Regional Weather for February 2025

1. Overview

1.1 During February 2025, the Maritime Continent experienced a mix of below- to above-average rainfall, with generally below-average rainfall over parts of the central and southern regions, and above-average rainfall for the northeastern region (Figure 1). Over Mainland Southeast Asia, there were no notable anomalies, typical of the dry season for this region. The largest positive (wetter) anomalies were recorded over the central Philippines, while the largest negative (drier) anomalies were recorded over Java. There were disagreements between GSMaP-NRT and CMORPH-Blended datasets, with GSMaP-NRT showing stronger and more extensive dry anomalies as compared to CMORPH-Blended. More disagreements between the datasets are also evident across the Maritime Continent, including over the Malay Peninsula, Sumatra, southern Borneo, and Papua, with CMORPH-Blended tending to show drier conditions over these regions as compared to GSMaP-NRT.

1.2 The observed rainfall anomaly pattern of above-average rainfall over the northeastern Maritime Continent and below-average rainfall over the central and southern Maritime Continent isare consistent with the predictions from the subseasonal weather outlooks for February 2025 (3 – 16 February 2025, and 17 February – 2 March 2025). The observed pattern of above-average rainfall over northeastern Maritime is also consistent with the February 2025 seasonal outlook, although the below-normal rainfall over parts of the Maritime Continent is not well represented.

Figure 1: Rainfall anomalies for February 2025 based on GSMaP-NRT data (left) and CMORPH-Blended data (right). The climatological reference period is 2001-2023. Green colour denotes above-average rainfall (wetter), while orange denotes below-average rainfall (drier).

1.3 Below- to near-average temperatures were recorded over central and eastern Mainland Southeast Asia and parts of the western Maritime Continent in February 2025 (Figure 2). Elsewhere, above-average temperatures were recorded over western Mainland Southeast Asia and the central and eastern Maritime Continent. The coldest anomalies (-1°C – -2°C below average) were recorded over northern Viet Nam, while the warmest anomalies (1°C – 2°C above average) were recorded over central Myanmar.

Figure 2: Temperature anomalies for February 2025 based on ERA-5 reanalysis. The climatological reference period is 2001-2023. Red colour denotes above-average temperature (warmer), while blue denotes below-average temperature (colder).

2. Climate Drivers

2.1 In the first week of the February, a Madden-Julian Oscillation (MJO) signal was observed over the Maritime Continent (Phase 5). The MJO propagated eastwards through the Western Pacific (Phases 6 and 7) in the first two weeks of February, reaching the Western Hemisphere (Phase 8) at the start of the third week. The MJO signal weakened during the third week, becoming inactive shortly before emerging at the end of the month in the Western Hemisphere (Phase 1). For February, Phase 5 typically brings wetter conditions over much of the Maritime Continent and Phase 6 tends to bring wetter conditions over the eastern Maritime Continent. Phase 7 tends to bring drier conditions for the western Maritime Continent, and Phases 8 and 1 typically bring drier conditions to much of the Maritime Continent. This observed MJO signal is in line with the mix of below- to above-normal rainfall anomalies in Figure 1.

2.2 In February 2025, La Niña conditions were present. Sea surface temperatures in the Nino3.4 region (used to monitor ENSO) indicated La Niña conditions and atmospheric indicators (cloudiness and trade winds) also showed signs of La Niña. La Niña events tend to bring wetter-than-average conditions to much of the Maritime Continent during this time of the year.

February 18, 2025

Review of Regional Weather for January 2025

1. Overview

1.1 During January 2025, much of the Maritime Continent experienced above-average rainfall, with only parts of southern Philippines and the Malay Peninsula experiencing below-average rainfall (Figure 1). Over Mainland Southeast Asia, there were no notable anomalies, typical of the dry season for this region. The largest positive (wetter) anomalies were recorded over the southern tip of Malay Peninsula, eastern coast of Sumatra, and northwest Borneo, while the largest negative (drier) anomalies were recorded over southern Philippines. There were disagreements between GSMaP-NRT and CMORPH-Blended datasets on the extent and magnitude of the dry anomalies over southern Philippines, with GSMaP-NRT showing stronger and more extensive dry anomalies as compared to CMORPH-Blended. More disagreements between the datasets are also evident over Sumatra and western Papua, with CMORPH-Blended tending to show drier conditions over these regions as compared to GSMaP-NRT.

1.2 The observed rainfall anomaly pattern of above-average rainfall over much of the Maritime Continent is consistent with the predictions from the subseasonal weather outlooks for January 2025 (6 – 19 January 2025, and 20 January – 2 February 2025). The observed pattern is also consistent with the January 2025 seasonal outlook of above-normal rainfall over parts of the Maritime Continent.

Figure 1: Rainfall anomalies for January 2025 based on GSMaP-NRT data (left) and CMORPH-Blended data (right). The climatological reference period is 2001-2023. Green colour denotes above-average rainfall (wetter), while orange denotes below-average rainfall (drier).

1.3 Below-average temperatures were recorded over much of Mainland Southeast Asia in January 2025 (Figure 2). Elsewhere, positive temperature anomalies were recorded over parts of Myanmar and the eastern and northeastern Maritime Continent. The coldest anomalies (-2°C – -3°C below average) were recorded over parts of northeastern Thailand and northern Cambodia, while the warmest anomalies (1°C – 2°C above average) were recorded over northwestern Myanmar.

Figure 2: Temperature anomalies for January 2025 based on ERA-5 reanalysis. The climatological reference period is 2001-2023. Red colour denotes above-average temperature (warmer), while blue denotes below-average temperature (colder).

2. Climate Drivers

2.1 In the first week of the January, no active Madden-Julian Oscillation (MJO) signal was observed. During the second week of the month, an active MJO signal emerged, and was present over the Western Hemisphere (Phases 8 and 1). Then, the signal propagated eastwards through the Indian Ocean (Phases 2 and 3) in the third week of January and continued its eastwards propagation over the Maritime Continent (Phases 4 and 5) in the last week of the month. During this time of the year, Phases 8 and 1 typically bring drier conditions to the Maritime Continent. Phase 2 and 3 and tend to induce wetter conditions over the western Maritime Continent, while Phases 4 and 5 typically bring wetter conditions over much of the Maritime Continent. This observed MJO signal during January is in line with the above-normal rainfall anomalies in Figure 1.

2.2 In January 2025, La Niña conditions were present. Sea surface temperatures in the Nino3.4 region (used to monitor ENSO) indicated La Niña conditions and atmospheric indicators (cloudiness and trade winds) also showed signs of La Niña. La Niña events tend to bring wetter-than-average conditions to much of the Maritime Continent during this time of the year.

January 16, 2025

Review of Regional Weather for December 2024

1. Overview

1.1 During December 2024, the western Maritime Continent experienced below-average rainfall, while rest of the Maritime Continent as well as parts of southeastern Mainland Southeast Asia experienced predominately above-average rainfall (Figure 1). Elsewhere in Mainland Southeast Asia, there were no notable anomalies, typical of the dry season for this region. The largest positive (wetter) anomalies were recorded over southern Thailand, southern Viet Nam, and central Borneo, while the largest negative (drier) anomalies were recorded over Sumatra and the southern Malay Peninsula in both the GSMaP-NRT and CMORPH-Blended datasets.

1.2 The observed rainfall anomaly pattern of a below-average rainfall over parts of the western Maritime Continent, and above-average rainfall over much of the rest of the Maritime Continent and southeastern Mainland Southeast Asia is consistent with the predictions from the subseasonal weather outlooks for December 2024 (25 November – 8 December 2024, 9 – 22 December 2024), and 23 December 2024 – 5 January 2025). The observed pattern is also consistent with the December 2024 seasonal outlook of above-normal rainfall over most of the Maritime Continent except over parts of Borneo and Sumatra.

Figure 1: Rainfall anomalies for December 2024 based on GSMaP-NRT data (left) and CMORPH-Blended data (right). The climatological reference period is 2001-2023. Green colour denotes above-average rainfall (wetter), while orange denotes below-average rainfall (drier).

1.3 Above-average temperatures were recorded over much of the Maritime Continent and northwest Mainland Southeast Asia in December 2024 (Figure 2), with below-average temperatures recorded over southeast Mainland Southeast Asia. The warmest anomalies (1°C – 2°C above average) were recorded over parts of Myanmar and northwest Thailand and Lao PDR as well as parts of Sumatra. The coolest anomalies (-1°C below average) were recorded over parts of Cambodia, Lao PDR, and Viet Nam.

Figure 2: Temperature anomalies for December 2024 based on ERA-5 reanalysis. The climatological reference period is 2001-2023. 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 slowly propagated from Phase 4 to Phase 7 during December (Figure 3). In the first half of the month, the signal propagated through the Maritime Continent (Phases 4 and 5), reaching the Western Pacific (Phases 6 and 7) during the second half of the month. Phases 4 and 5 typically bring wetter conditions for much of Southeast Asia, while Phase 6 typically bring wetter conditions to the eastern Maritime Continent and Phase 7 typically brings drier conditions to the western Maritime Continent. This pattern of below-average rainfall in the western Maritime Continent and above-average rainfall for much of the rest of the region is in line with the anomalies in Figure 1.

2.2 In December 2024, La Niña or La Niña-like conditions were present. Sea surface temperatures in the Nino3.4 region (used to monitor ENSO) indicated La Niña conditions and atmospheric indicators (cloudiness and trade winds) also showed signs of La Niña. However, there is some uncertainty as to whether the conditions will persist long enough to declare a La Niña event. La Niña events tend to bring wetter-than-average conditions to much of the Maritime Continent during the June to August period.

December 16, 2024

Review of Regional Weather for November 2024

1. Overview

1.1 During November 2024, the ASEAN region experienced a mix of below- and above-average rainfall. For the Maritime Continent, much of the western half experienced above-average rainfall while the eastern half experienced a mix of below- to above-average rainfall (Figure 1). For Mainland Southeast Asia, above-average rainfall was recorded over parts of central and northern Myanmar and southern Thailand, with below-average rainfall over parts of eastern Mainland Southeast Asia and near-average rainfall elsewhere, although there is disagreement over central Viet Nam with GSMaP-NRT (Figure 1, left) recording below-average rainfall, while CMORPH-Blended (Figure 1, right) recorded above-average rainfall. The largest positive (wetter) anomalies were recorded over the Malay Peninsula, while the largest negative (drier) anomalies were recorded over southeastern Viet Nam and central Philippines in both GSMaP-NRT and CMORPH-Blended.

1.2 The observed rainfall anomaly pattern of a mix of below- and above-average rainfall in the ASEAN region, with above-average rainfall over the western Maritime Continent is consistent with the predictions from the subseasonal weather outlooks for November 2024 (28 October – 10 November 2024, 11 – 24 November 2024), and 25 November – 8 December 2024).

Figure 1: Rainfall anomalies for November 2024 based on GSMaP-NRT data (left) and CMORPH-Blended data (right). The climatological reference period is 2001-2023. Green colour denotes above-average rainfall (wetter), while orange denotes below-average rainfall (drier).

1.3 Above-average temperatures were recorded over much of the ASEAN region in November 2024 (Figure 2), with below-average temperatures recorded over a few scattered points of Mainland Southeast Asia. The warmest anomalies (more than 1°C above average) were recorded mainly over northern Viet Nam based on ERA-5 reanalysis.

Figure 2: Temperature anomalies for November 2024 based on ERA-5 reanalysis. The climatological reference period is 2001-2023. Red colour denotes above-average temperature (warmer), while blue denotes below-average temperature (colder).

2. Climate Drivers

2.1 The Madden-Julian Oscillation (MJO) was active from the beginning of the first week of November 2024, based on the RMM index (Figure 3). An MJO signal was present over the Western Hemisphere (Phases 8 and 1) during the first week of November, which weakened for a short period towards the end of week 2 as it propagated eastwards. During the third week, the MJO signal emerged and remained active over the Indian Ocean (Phases 2 and 3), and propagated eastwards to the Maritime Continent (Phase 4) during the last week of the month. Phases 8 and 1 typically bring drier conditions for much of Southeast Asia during this time of the year. For November, Phases 2 and 3 tend to bring wetter conditions to the western Maritime Continent (in line with Figure 1), and Phase 4 tends to bring wetter conditions to the Maritime Continent.

2.2 In November, there were signs of La Niña-like conditions. The Nino3.4 index was approaching the La Niña threshold in October, and cool subsurface temperature anomalies in the central and eastern Pacific indicated La Niña-like conditions. However, westerly wind anomalies in the eastern tropical Pacific region during early November likely slowed the cooling process of the sea surface temperatures.

November 15, 2024

Review of Regional Weather for October 2024

1. Overview

1.1 During October 2024, the ASEAN region experienced a mix of below- and above-average rainfall. For the Maritime Continent, much of the western region experienced below-average rainfall while much of the eastern region as well as northern and central Philippines experienced above-average rainfall (Figure 1). Above-average rainfall was recorded over much of southern Mainland Southeast Asia, with below-average rainfall over parts of northwestern and eastern Mainland Southeast Asia, although there was disagreement over central Viet Nam with GSMaP-NRT (Figure 1, left) recording below-average rainfall, while CMORPH-Blended (Figure 1, right) recorded above-average rainfall. The largest negative (drier) anomalies were recorded over southern Sumatra in both GSMap-NRT and CMORPH-Blended. The largest positive (wetter) anomalies were recorded over central Philippines (in both GSMap-NRT and CMORPH-Blended), partly associated with Tropical Storm Trami and Typhoon Kong-rey in October 2024.

1.2 The observed rainfall anomaly pattern of a mix of below- and above-average rainfall in the ASEAN region, with below-average rainfall over the western Maritime Continent is consistent with the predictions from the subseasonal weather outlooks for October 2024 (30 September – 13 October 2024 and 14 – 27 October 2024).

Figure 1: Rainfall anomalies for October 2024 based on GSMaP-NRT data (left) and CMORPH-Blended data (right). The climatological reference period is 2001-2023. Green colour denotes above-average rainfall (wetter), while orange denotes below-average rainfall (drier).

1.3 Above-average temperatures were recorded over the much of ASEAN region in October 2024 (Figure 2). The warmest anomalies (more than 1°C above average) were recorded mainly over southern Sumatra based on ERA-5 reanalysis.

Figure 2: Temperature anomalies for October 2024 based on ERA-5 reanalysis. The climatological reference period is 2001-2023. Red colour denotes above-average temperature (warmer), while blue denotes below-average temperature (colder).

2. Climate Drivers

2.1 2.1 The Madden-Julian Oscillation (MJO) was active from the end of the second week of October 2024, based on the RMM index (Figure 3). An MJO signal was present over the Western Hemisphere (Phase 1) at the start of October, which quickly weakened and remained inactive for much of the first two weeks of October. At the end of the second week, the MJO signal emerged over the Maritime Continent (Phases 4 and 5), propagated eastwards through the Maritime Continent (Phases 4 and 5) in Week 3, and through the Western Pacific (Phases 6 and 7) during the last week of the month. For October, Phases 4 to 5 tend to bring wetter conditions to parts of southern Mainland Southeast Asia and the Philippines (in line with Figure 1), Phase 6 tends to bring wetter conditions over the Philippines, and Phase 7 tends to bring drier conditions to western Maritime Continent.

2.2 The tropical Pacific was in an ENSO neutral state during October.

October 15, 2024

Review of Regional Weather for September 2024

1. Overview

1.1 During September 2024, the Maritime Continent experienced a mix of below- and above-average rainfall, with much of Mainland Southeast Asia experiencing above-average rainfall . For the Maritime Continent, much of the eastern equatorial region experienced below-average rainfall while the northern Philippines and the southern Maritime Continent experienced above-average rainfall (Figure 1). Above-average rainfall was recorded over much of northern and eastern Mainland Southeast Asia, although there was disagreement over the southern parts of Mainland Southeast Asia with GSMaP-NRT (Figure 1, left) recording near- to above-average rainfall, while CMORPH-Blended (Figure 1, right) recorded a mix of below- and above-average rainfall. The largest negative (drier) anomalies were recorded over Papua, southern Philippines and northeast Borneo in GSMap-NRT (Figure 1, left) and CMORPH-Blended (Figure 1, right). The larger positive (wetter) anomalies were recorded over northern Viet Nam and northern Philippines (in both GSMap-NRT and CMORPH-Blended), partly associated with Typhoon Yagi in early September.

1.2 The observed rainfall anomaly pattern of above-average rainfall over much of the northern ASEAN region and a mix of below- and above-average rainfall is consistent with the predictions from the subseasonal weather outlooks for September 2024 (2 – 15 September 2024 and 16 – 29 September 2024).

Figure 1: Rainfall anomalies for September 2024 based on GSMaP-NRT data (left) and CMORPH-Blended data (right). The climatological reference period is 2001-2023. Green colour denotes above-average rainfall (wetter), while orange denotes below-average rainfall (drier).

1.3 Above-average temperatures were recorded over the much of ASEAN region in September 2024 (Figure 2). The warmest anomalies (more than 1°C above average) were recorded over northern Myanmar, southern Sumatra, Sulawesi and parts of Borneo, based on ERA-5 reanalysis.

Figure 2: Temperature anomalies for September 2024 based on ERA-5 reanalysis. The climatological reference period is 2001-2023. Red colour denotes above-average temperature (warmer), while blue denotes below-average temperature (colder).

2. Climate Drivers

2.1 The Madden-Julian Oscillation (MJO) was active during much of September 2024, based on the RMM index (Figure 3). For much of the first half of September, an MJO signal propagated eastwards through the Maritime Continent (Phases 4 and 5). In the third week of the month, the signal was active over the Western Pacific (Phases 6 and 7), before propagating eastwards through the Western Hemisphere (Phases 8 and 1) during the last week of the month. For September, Phases 4 to 5 tend to bring wetter conditions to parts of southern Mainland Southeast Asia and the Philippines (in line with Figure 1), while Phases 8 and 1 tend to bring drier conditions to these regions.

2.2 The tropical Pacific was in an ENSO neutral state during September.

September 13, 2024

Review of Regional Weather for August 2024

1. Overview

1.1 During August 2024, a mix of below- and above-average rainfall was recorded over the Southeast Asian region (Figure 1). Much of the equatorial region and northern and eastern Mainland Southeast Asia recorded above-average rainfall, while parts of eastern Mainland Southeast Asia, the Philippines, and the southern Maritime Continent recorded below-average rainfall. The largest positive (wetter) anomalies were recorded over the western coast of Myanmar and parts of Borneo based on GSMaP-NRT (Figure 1, left) and CMORPH-Blended (Figure 1, right) satellite-derived rainfall estimates. In contrast, the largest negative (drier) anomalies were recorded over the northern Philippines, central Viet Nam and southern Lao PDR (in both GSMaP-NRT and CMORPH-Blended) and Cambodia (CMORPH-Blended only).

1.2 The observed rainfall anomaly pattern of above-average rainfall over the equatorial region and a mix of below- and above-average rainfall elsewhere are broadly consistent with the predictions from the subseasonal weather outlooks for August 2024 (5 – 18 August 2024 and 19 August – 1 September 2024).

Figure 1: Rainfall anomalies for August 2024 based on GSMaP-NRT data (left) and CMORPH-Blended data (right). The climatological reference period is 2001-2023. Green colour denotes above-average rainfall (wetter), while orange denotes below-average rainfall (drier).

1.3 Above-average temperatures were recorded over most of Southeast Asia in August 2024, apart from over the equatorial region where near- to above-average temperatures were recorded and northern and western Mainland Southeast Asia where below-to above average temperatures were recorded (Figure 2). The regions where below- and near-average temperatures were recorded are in line with those with above-average rainfall. The coolest anomalies were over northern Lao PDR (around 1°C below average), while the warmest anomalies (more than 2°C above average) were recorded over southern Sumatra.

Figure 2: Temperature anomalies for August 2024 based on ERA-5 reanalysis. The climatological reference period is 2001-2023. 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 strengthened over the Western Hemisphere (Phase 1) in the second week of August, after no discernible signal for much of the first week based on the RMM index (Figure 3). This signal propagated eastward for the rest of August, reaching the Indian Ocean (Phase 2) in the third week, and the Maritime Continent (Phase 4) in the last week. Typically for August, Phases 1 and 2 tend to bring drier conditions to northeastern Southeast Asia, while Phases 2 and 3 bring wetter conditions to the western Maritime Continent, and Phase 4 brings wetter conditions to northern parts of the Maritime Continent. MJO activity therefore likely contributed to the above-average rainfall in the equatorial region, and drier conditions over parts of eastern Mainland Southeast Asia and the Philippines.

2.2 The tropical Pacific was in an ENSO neutral state during August.

August 19, 2024

Review of Regional Weather for July 2024

1. Overview

1.1 During July 2024, below-average rainfall was recorded over parts of the southern ASEAN region, in particular over the equatorial region, while above-average rainfall was recorded over parts of the central equatorial region and near-average rainfall over the southernmost region (Figure 1). For the northern ASEAN region, above-average rainfall was recorded over eastern and southern Mainland Southeast Asia as well as over the Philippines, while a mix of below- to above-average rainfall was recorded over northern Mainland Southeast Asia. The largest positive (wetter) anomalies were recorded over southern coast of Myanmar based on GSMaP-NRT (Figure 1, left) and CMORPH-Blended (Figure 1, right) satellite-derived rainfall estimates. In contrast, the largest negative (drier) anomalies were recorded over northern Sumatra (in both GSMaP-NRT and CMORPH-Blended).

1.2 The observed rainfall anomaly pattern of above-average rainfall over parts of the southern Mainland Southeast Asia coupled with below-average rainfall over parts of the equatorial region are broadly consistent with the predictions from the subseasonal weather outlooks for July 2024 (24 June – 7 July 2024 , 8 – 21 July 2024 and 22 July – 4 August 2024).

Figure 1: Rainfall anomalies for July 2024 based on GSMaP-NRT data (left) and CMORPH-Blended data (right). The climatological reference period is 2001-2023. Green colour denotes above-average rainfall (wetter), while orange denotes below-average rainfall (drier).

1.3 Above-average temperatures were recorded over most of the Maritime Continent and northern Mainland Southeast Asia (Figure 2) in July 2024. The exceptions were over central and southeastern Mainland Southeast Asia, where below-to near-average temperature was recorded over parts of eastern Thailand, Cambodia and Vietnam. The warmest anomalies (more than 1°C above average) were recorded over Sumatra.

Figure 2: Temperature anomalies for July 2024 based on ERA-5 reanalysis. The climatological reference period is 2001-2023. Red colour denotes above-average temperature (warmer), while blue denotes below-average temperature (colder).

2. Climate Drivers

2.1 The Madden-Julian Oscillation (MJO) was present over the Indian Ocean (Phase 3) for the first week of July based on the RMM index (Figure 3). The MJO signal weakened and became indiscernible for much of the rest of July, except during mid-July when an MJO signal emerged briefly for a few days over the Maritime Continent (Phases 4 and 5). Typically for July, Phases 3 and 4 tend to bring wetter conditions to the Maritime Continent and southern Mainland Southeast Asia, and Phase5 tends to bring wetter conditions to the eastern Maritime Continent.

2.2 The tropical Pacific was in an ENSO neutral state during July, although the lingering effect from the El Niño still likely contributed to the warmer temperatures observed in Figure 2.