Category Archives: Fortnightly Weather Review

1.1 During May 2024, much of the equatorial region experienced above-average rainfall, while much of the southern parts of the Maritime Continent and the Philippines experienced below-average rainfall (Figure 1). A mix of near- and above-average rainfall was recorded over much of Mainland Southeast Asia, although there was disagreement over Cambodia with GSMaP-NRT (Figure 1, left) recording near- to above-average rainfall, while CMORPH-Blended (Figure 1, right) recorded below-to near-average rainfall. The largest negative (drier) anomalies were recorded over southern Philippines, Java, and parts of northeastern Borneo in GSMap-NRT (Figure 1, left) and CMORPH-Blended (Figure 1, right). The larger positive (wetter) anomalies were recorded over Sumatra and central Borneo (in both GSMap-NRT and CMORPH-Blended).
1.2 The observed rainfall anomaly pattern of a mix of below- and above-average rainfall over the northern ASEAN region and above-average rainfall over the equatorial region is consistent with the predictions from the subseasonal weather outlooks for May 2024 (29 April – 12 May 2024 , 13 – 26 May 2024 and 27 May – 9 June 2024).

Figure 1: Rainfall anomalies for May 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 ASEAN region in May 2024 (Figure 2). The warmest anomalies (more than 2°C above average) were recorded over southern Thailand and northeastern Borneo. Based on ERA-5 reanalysis, Temperatures in May 2024 were among the 10% warmest values for May over southern Sumatra, southern Thailand and Myanmar, parts of Borneo and most of the Philippines.

Figure 2: Temperature anomalies for May 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 for much the second half of May 2024, based on the RMM index (Figure 3). In the first week of May, an MJO signal propagated eastwards through the Maritime Continent (Phases 4 and 5) before becoming inactive. In the second half of the May, the signal reached re-emerged over the Indian Ocean (Phases 2 and 3) and remained there until the end of the month. For May, Phases 2 to 5 tend to bring wetter conditions to parts of the Maritime Continent (the western Maritime Continent in Phase 2, much of the region for Phases 3 and 4, and the eastern Maritime Continent in Phase 5). Therefore, the MJO may have contributed to the wetter conditions in the equatorial region in May.

 

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 El Niño event has now ended, although the lingering effect of the El Niño still likely contributed to the warmer temperatures observed in Figure 2.

1.1 During April 2024, much of the northern ASEAN region experienced below-average rainfall, while much of the southern ASEAN region experienced above-average rainfall (Figure 1). Based on both GSMaP-NRT (Figure 1, left) and CMORPH-Blended (Figure 1, right) satellite-derived rainfall estimates, below-average rainfall was recorded over much of the northern ASEAN region, as well as the western Maritime Continent and parts of Borneo. The largest negative (drier) anomalies were recorded over southern Thailand, Cambodia, and parts of central Borneo. In contrast, most of the southern ASEAN region experience near- to above-average rainfall, although GSMaP-NRT shows larger positive (wetter) anomalies over Java and Sulawesi (Figure 1, left) compared to CMORPH-Blended (Figure 1, right).

1.2 The observed rainfall anomaly pattern of below-average rainfall over much of the northern ASEAN region and above-average rainfall over the southern ASEAN region is consistent with the predictions from the subseasonal weather outlooks for April 2024 (1 – 14 April 2024 and 15 – 28 April 2024).

Figure 1: Rainfall anomalies for April 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 ASEAN region in April 2024 (Figure 2). The warmest anomalies (more than 3°C above average) were recorded over eastern Thailand, parts of Lao PDR, and northern and central Viet Nam. Based on ERA-5 reanalysis, the April 2024 was one of the warmest on record, with temperatures for most of the region in the top 10% warmest values for April.

Figure 2: Temperature anomalies for April 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 inactive for much of April 2024, based on the RMM index (Figure 3). In the first week of April, an MJO signal propagated eastwards through the Indian Ocean (Phases 2 and 3). The signal reached the Maritime Continent (Phase 4) at the start of week 2, after which point the signal rapidly weakened and became indiscernible. It was not until the last week of April when there were signs of an MJO signal again developing over the Indian Ocean (Phase 3). For April, Phase 2 tends to bring wetter conditions for the western Maritime Continent, while Phase 3 tends to bring wetter conditions to the southern ASEAN region.

 

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 El Niño conditions over the equatorial Pacific continued weakening during April 2024, although still likely contributed to the warmer temperatures observed in Figure 2. At this time of year, warmer temperatures are typical observed when there has been a El Niño event (even if the event is weakening).

1.1 During March 2024, a mix of below-average to above-average was recorded over the southern half of the Maritime Continent, and below-average rainfall over most of the northern half of the Maritime continent (Figure 1). For Mainland Southeast Asia, below- to near-average rainfall was recorded in March over the southern region, with some pockets of above-average rainfall over the eastern and central regions. The largest positive (wetter) anomalies were recorded over central 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 northeast Borneo and southern Philippines (in both GSMaP-NRT and CMORPH-Blended).

1.2 The observed rainfall anomaly pattern of above-average rainfall over parts of the southern half of the Maritime Continent coupled with below-average rainfall over the northern half of the Maritime Continent are broadly consistent with the predictions from the subseasonal weather outlooks for March 2024 (4 – 17 March 2024 and 18 – 31 March 2024).

Figure 1: Rainfall anomalies for March 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 Maritime Continent and most of Mainland Southeast Asia (Figure 2) in March 2024. The exceptions were over parts of Myanmar and Cambodia, where below-to near-average temperature was recorded. The warmest anomalies (more than 1°C above average) were recorded over much of Thailand, parts of Lao PDR, the Malay Peninsula, central Sumatra, Borneo and Sulawesi.

Figure 2: Temperature anomalies for March 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 for much of March 2024, based on the RMM index (Figure 3). In the first and second weeks of March, the MJO signal present over the Indian Ocean (Phase 3) propagated eastwards through the Maritime Continent (Phases 4 and 5), with an increase in strength over Phase 4 at the end of Week 1. The active MJO signal continued propagating eastwards through the Western Pacific (Phases 6 and 7) in the third week of the month, before moving to Western Hemisphere (Phases 8 and 1) and weakened in strength in the last week of March. For March, Phases 4 and 5 tend to bring wetter conditions for the southern Maritime Continent, while Phases 6, 7 and 8 tend to bring drier conditions for the Maritime Continent.

 

Figure 3: The MJO phase diagram. The diagram illustrates the movement of the MJO through different phases, which correspond to different locations along the equator (denoted in the text with the first day of the month in blue and the last day of the month in red). The distance of the index from the centre of the diagram is related to the strength of the MJO. Values within the grey circle are considered weak or indiscernible (data from the Bureau of Meteorology, Australia).

 

2.2 El Niño conditions over the equatorial Pacific continued weakening during March 2024. Sea surface temperatures in the Nino3.4 region (used to monitor ENSO) show a weakening of El Niño conditions and key atmospheric indicators (cloudiness and trade winds) are consistent with weakening El Niño conditions. Weakening El Niño events tend to ease drier conditions but continue warmer-than-average conditions to much of the Maritime Continent during January – March.

1.1 During February 2024, a mix of below- to above-average was recorded over the Maritime Continent, with parts of the western half of the equatorial region receiving above-average rainfall, and below-average rainfall elsewhere (Figure 1). Over Mainland Southeast Asia, near-average rainfall was recorded in February. The largest positive (wetter) anomalies were recorded over central Sumatra 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 central parts of the Philippines (in both GSMaP-NRT and CMORPH-Blended).

1.2 The observed rainfall anomaly pattern of above-average rainfall over the western and central parts of the equatorial Maritime Continent coupled with below-average rainfall in particular over northwestern Maritime Continent, and the near-average rainfall over Mainland Southeast Asia, are broadly consistent with the predictions from the subseasonal weather outlooks for February 2024 (5 – 18 February 2024 and 19 February – 3 March 2024).

Figure 1: Rainfall anomalies for February 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 Maritime Continent and much of Mainland Southeast Asia (Figure 2) in February 2024. The exceptions were over parts of Myanmar and Cambodia, where below-to near-average temperature was recorded. The warmest anomalies (more than 2°C above average) were recorded over some parts of eastern Thailand and southern Lao PDR.

 

Figure 2: Temperature anomalies for February 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 for much of February 2024, based on the RMM index (Figure 3). In the first and second weeks of February, an active MJO signal was present over the Western Pacific (Phases 6 and 7). In the third week of the month, the active MJO signal weakened in strength in Phase 7 and became e indiscernible in the last week of February. Typically for February, Phases 6 and 7 tend to bring drier conditions for the western Maritime Continent.

 

Figure 3: The MJO phase diagram. The diagram illustrates the movement of the MJO through different phases, which correspond to different locations along the equator (denoted in the text with the first day of the month in blue and the last day of the month in red). The distance of the index from the centre of the diagram is related to the strength of the MJO. Values within the grey circle are considered weak or indiscernible (data from the Bureau of Meteorology, Australia).

 

2.2 El Niño conditions over the equatorial Pacific continued weakening during February 2024. Sea surface temperatures in the Nino3.4 region (used to monitor ENSO) continue to show El Niño conditions but has likely past its peak, and key atmospheric indicators (cloudiness and trade winds) are consistent with the weakening El Niño conditions. El Niño events tend to bring drier and warmer-than-average conditions to much of the Maritime Continent during December – February.

1.1 During January 2024, a mix of below- to above-average was recorded over the Maritime Continent, with much of the central and western parts receiving above-average rainfall, and the northeastern parts receiving below-average rainfall (Figure 1). Over Mainland Southeast Asia, near-average rainfall was recorded in January. The largest positive (wetter) anomalies were recorded over southern Philippines and southern parts of the Malay Peninsula 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 central Philippines (in both GSMaP-NRT and CMORPH-Blended).

1.2 The observed rainfall anomaly pattern of above-average rainfall over most of the western half of Maritime Continent, the drier than average conditions over the northeastern parts of the Maritime Continent, and the near-average rainfall over Mainland Southeast Asia are broadly consistent with the predictions from the subseasonal weather outlooks for January 2024 (25 December 2023 – 7 January 2024, 8 – 21 January 2024 and 22 January – 4 February 2024).

Figure 1: Rainfall anomalies for January 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 Maritime Continent and much of Mainland Southeast Asia (Figure 2) in January 2024. The exceptions were over parts of Myanmar and Cambodia, where below-to near-average temperature was recorded. The warmest anomalies (more than 1°C above average) were recorded over Thailand, Lao PDR, northern Viet Nam, Sulawesi, and eastern parts of Borneo.

 

Figure 2: Temperature anomalies for January 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 throughout the entire month of January, based on the RMM index (Figure 3). In the first and second weeks of January, an active MJO signal was present over the Indian Ocean (Phases 2 and 3). In the third week of the month, the active MJO signal was propagating eastwards through the Maritime Continent (Phases 4 and 5) and remained active during the last week over the Western Pacific (Phases 6 and 7). Typically for January, Phase 3 tends to bring wetter conditions for western Maritime Continent, Phase 4 tends to bring wetter conditions for the much of the Maritime Continent, and Phases 6 and 7 tend to bring drier conditions for the western Maritime Continent. Phases 5 tends to bring a mix of wetter and drier conditions.

 

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 El Niño conditions persisted over the equatorial Pacific during January 2024. Sea surface temperatures in the Nino3.4 region (used to monitor ENSO) continue to show El Niño conditions, with key atmospheric indicators (cloudiness and trade winds) still supporting these conditions but started to weaken. The positive Indian Ocean Dipole event has likely ended. El Niño events tend to bring drier and warmer-than-average conditions to much of the Maritime Continent during December – February.

1.1 During December 2023, a mix of below- to above-average rainfall was recorded over the Maritime Continent (Figure 1). Near- to above-average rainfall was recorded over much of the western Maritime Continent and Borneo, with below- to near-average over much of the rest of the region. Over Mainland Southeast Asia, near-average rainfall was recorded for most of the region. The largest positive (wetter) anomalies for December were recorded over the Malay Peninsula and northern Sumatra 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 central Philippines (in both GSMaP-NRT and CMORPH-Blended), as well as Java and parts of Papua (CMORPH-blended only). The other notable difference between the two datasets was over central Viet Nam where CMORPH-Blended recorded above-average rainfall, while GSMAP-NRT recorded near average.

1.2 The observed rainfall anomaly pattern of above-average rainfall over the western Maritime Continent, below- to near- average rainfall for the rest of the Maritime Continent, and near-average over Mainland Southeast Asia, is broadly consistent with the predictions from the subseasonal weather outlooks for December 2023 (27 November – 10 December 2023, 11 – 24 December 2023 and 25 December 2023 – 7 January 2024).

Figure 1: Rainfall anomalies for December 2023 based on GSMaP-NRT data (left) and CMORPH-Blended data (right). The climatological reference period is 2001-2022. Green colour denotes above-average rainfall (wetter), while orange denotes below-average rainfall (drier).

1.3 Above-average temperatures were recorded over Southeast Asia in December 2023 (Figure 2). The warmest anomalies (more than 2°C above average) were recorded over parts of Mainland Southeast Asia, including parts of Myanmar and northwestern Thailand.

 

Figure 2: Temperature anomalies for December 2023 based on ERA-5 reanalysis. The climatological reference period is 2001-2022. 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 over the Indian Ocean (Phase 3) at the start of December 2023 (Figure 3). This signal propagated eastwards during December, weakening temporarily as it moved through the Western Pacific (Phases 6 and 7) and Western Hemisphere (Phase 8) towards the middle of the month. By the end of December, the MJO signal had strengthened again and returned to the Indian Ocean (Phase 2). Considering the phases with the stronger MJO signal (based on the RMM index), typically for this time of year, Phases 4 and 5 bring wetter conditions for most of the ASEAN region whereas Phases 1 and 2 bring drier conditions to the eastern Maritime Continent, with Phase 2 also bringing wetter conditions to the western Maritime Continent.

 

Figure 3: The MJO phase diagram. The diagram illustrates the movement of the MJO through different phases, which correspond to different locations along the equator (denoted in the text with the first day of the month in blue and the last day of the month in red). The distance of the index from the centre of the diagram is related to the strength of the MJO. Values within the grey circle are considered weak or indiscernible (data from the Bureau of Meteorology, Australia).

 

2.2 El Niño conditions persisted over the equatorial Pacific during December 2023. Sea surface temperatures in the Nino3.4 region (used to monitor ENSO) continue to show El Niño conditions, with key atmospheric indicators (cloudiness and trade winds) also supporting these conditions. The positive Indian Ocean Dipole started to weaken in December. El Niño events tend to bring drier and warmer-than-average conditions to much of the Maritime Continent during December – February. Positive Indian Ocean Dipole events also tend to bring drier conditions to most of the southern half of the Maritime Continent during this period.

1.1 During November 2023, a mix of below- to above-average rainfall was recorded over Southeast Asia (Figure 1). Near- to above-average rainfall was recorded over much of Mainland Southeast Asia, and a mix of below- and above-average rainfall was recorded over much of the Maritime Continent. The largest positive (wetter) anomalies were recorded over central Philippines and eastern coast of the Malay Peninsula 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 southern Java and Papua (in both GSMaP-NRT and CMORPH-Blended).

1.2 The observed rainfall anomaly pattern of above-average rainfall over parts of Mainland Southeast Asia and the mix below- and above-average rainfall over the Maritime Continent are broadly consistent with the predictions from the subseasonal weather outlooks for November 2023 (30 October – 12 November 2023 and 13 – 26 November 2023).

Figure 1: Rainfall anomalies for November 2023 based on GSMaP-NRT data (left) and CMORPH-Blended data (right). The climatological reference period is 2001-2022. Green colour denotes above-average rainfall (wetter), while orange denotes below-average rainfall (drier).

1.3 Above-average temperatures were recorded over the Maritime continent and much of Mainland Southeast Asia (Figure 2). The warmest anomalies (more than 1°C above average) were recorded over southern Myanmar, southern Sumatra, Java, Sulawesi, and Papua.

 

Figure 2: Temperature anomalies for November 2023 based on ERA-5 reanalysis. The climatological reference period is 2001-2022. Red colour denotes above-average temperature (warmer), while blue denotes below-average temperature (colder).

2. Climate Drivers

2.1 No active Madden-Julian Oscillation (MJO) was observed during the first week of November 2023, based on the RMM index (Figure 3). In the second week of November, an MJO signal emerged over the Western Pacific (Phase 6), propagated eastward and maintained its strength in Phase 7. The MJO signal continued propagating eastwards through the Western Hemisphere and Africa (Phases 8 and 1) in Week 3, before decaying over the Indian Ocean (Phase 2) in the last week of the November. Typically for November, Phase 6 tends to bring wetter conditions for eastern Maritime Continent while Phase 7 tends to bring drier conditions for the western Maritime Continent. Phases 8 and 1 typically bring drier conditions for much of Southeast Asia during this time of the year, while Phase 2 tend to bring wetter conditions to parts of the western Maritime continent and drier conditions to eastern parts.

 

Figure 3: The MJO phase diagram. The diagram illustrates the movement of the MJO through different phases, which correspond to different locations along the equator (denoted in the text with the first day of the month in blue and the last day of the month in red). The distance of the index from the centre of the diagram is related to the strength of the MJO. Values within the grey circle are considered weak or indiscernible (data from the Bureau of Meteorology, Australia).

 

2.2 El Niño conditions persisted over the equatorial Pacific during November 2023. Sea surface temperatures in the Nino3.4 region (used to monitor ENSO) continue to show El Niño conditions, with key atmospheric indicators (cloudiness and trade winds) also supporting these conditions. A positive Indian Ocean Dipole is also present. El Niño events tend to bring drier and warmer-than-average conditions to much of the Maritime Continent during September to November. Positive Indian Ocean Dipole events also tend to bring drier conditions to most of the southern half of the Maritime Continent during this period.

1.1 During October 2023, a mix of below- to above-average rainfall was recorded over Southeast Asia (Figure 1). Over much of Mainland Southeast Asia, near- to above-average rainfall was recorded. Over much of the Maritime Continent, below-average rainfall was recorded. For Southeast Asia, the largest positive (wetter) anomalies were recorded over central Viet Nam 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 Sumatra and the Philippines (in both GSMaP-NRT and CMORPH-Blended).

1.2 The observed rainfall anomaly pattern of above-average rainfall over much of Mainland Southeast Asia and below-average rainfall over the Maritime Continent are broadly consistent with the predictions from the subseasonal weather outlooks for October 2023 (2 – 15 October 2023 and 16 – 29 October 2023).

Figure 1: Rainfall anomalies for October 2023 based on GSMaP-NRT data (left) and CMORPH-Blended data (right). The climatological reference period is 2001-2022. Green colour denotes above-average rainfall (wetter), while orange denotes below-average rainfall (drier).

1.3 Above-average temperatures were recorded over the Maritime continent and much of Mainland Southeast Asia (Figure 2). The warmest anomalies (more than 2°C above average) were recorded over southern Sumatra.

 

Figure 2: Temperature anomalies for October 2023 based on ERA-5 reanalysis. The climatological reference period is 2001-2022. Red colour denotes above-average temperature (warmer), while blue denotes below-average temperature (colder).

2. Climate Drivers

2.1 No active Madden-Julian Oscillation (MJO) was observed for much of October 2023. An MJO was present in the Maritime Continent (Phase 5) on the first two days of November before decaying. There were signs of an MJO signal developing over the Western Pacific and Western Hemisphere (Phases 7 and 8) in the fourth week. Typically for October, Phase 7 tends to bring drier conditions to the western Maritime Continent and Phase 8 tends to bring drier conditions to much of the Maritime Continent.

 

Figure 3: The MJO phase diagram. The diagram illustrates the movement of the MJO through different phases, which correspond to different locations along the equator (denoted in the text with the first day of the month in blue and the last day of the month in red). The distance of the index from the centre of the diagram is related to the strength of the MJO. Values within the grey circle are considered weak or indiscernible (data from the Bureau of Meteorology, Australia).

 

2.2 El Niño conditions persisted over the equatorial Pacific during October 2023. Sea surface temperatures in the Nino3.4 region (used to monitor ENSO) continue to show El Niño conditions, with key atmospheric indicators (cloudiness and trade winds) also supporting these conditions. A positive Indian Ocean Dipole is also present. El Niño events tend to bring drier and warmer-than-average conditions to much of the Maritime Continent during September to November. Positive Indian Ocean Dipole events also tend to bring drier conditions to most of the southern half of the Maritime Continent during this period.

1.1 During September 2023, a mix of below- to above-average rainfall was recorded over Southeast Asia (Figure 1). Over much of Mainland Southeast Asia, above-average rainfall was recorded. Over the Maritime Continent, below-average rainfall was recorded over much of the southern of the region, with below- to above average rainfall over the northern half. For Southeast Asia, the largest positive (wetter) anomalies were recorded over coastal Myanmar, southern Viet Nam, and northern 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 parts of Cambodia (CMORPH-Blended), southern Borneo, Java, Sumatra, and southern Papua, and parts of Philippines (both GSMaP-NRT and CMORPH-Blended).

1.2 The observed rainfall anomaly pattern of above-average rainfall over most of Mainland Southeast Asia, while below-average rainfall elsewhere over the southern half of the Maritime continent are broadly consistent with the predictions from the subseasonal weather outlooks for September 2023 (4 – 17 September 2023 and 18 September – 1 October 2023).

Figure 1: Rainfall anomalies for September 2023 based on GSMaP-NRT data (left) and CMORPH-Blended data (right). The climatological reference period is 2001-2022. 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, where a mix of below- to above-average temperature was recorded (Figure 2). Near- to above-average temperatures were recorded over Mainland Southeast Asia. The warmest anomalies (more than 1°C above average) were recorded over parts of southern Sumatra and northern Myanmar.

 

Figure 2: Temperature anomalies for September 2023 based on ERA-5 reanalysis. The climatological reference period is 2001-2022. Red colour denotes above-average temperature (warmer), while blue denotes below-average temperature (colder).

2. Climate Drivers

2.1 A Madden-Julian Oscillation (MJO) signal developed over the Indian Ocean (Phase 3) during the first week of September 2023, based on the RMM Index below (Figure 3). During the second week of September, the signal weakened and appeared to stall over Phase 3 and Phase 4. However, towards the end of September the signal began propagating eastwards through the Maritime Continent (Phases 4 and 5), more typical of an MJO signal. Usually for September, Phases 4 and 5 bring wetter conditions to parts of southern Mainland Southeast Asia and the Philippines, somewhat in line with Figure 1, while Phase 3 has little impact on the region’s rainfall.

 

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 El Niño conditions persisted over the equatorial Pacific during September 2023. Sea surface temperatures in the Nino3.4 region (used to monitor ENSO) were consistent with El Niño conditions, with El Niño-like response in key atmospheric indicators. A positive Indian Ocean Dipole also developed. El Niño events tend to bring drier and warmer-than-average conditions to much of the Maritime Continent during September to November. Positive Indian Ocean Dipole events also tend to bring drier conditions to most of the southern half of the Maritime Continent during this period.