Category Archives: Fortnightly Weather Review

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.

 

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 tropical Pacific was in an ENSO neutral state during August.

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.