Category Archives: Fortnightly Weather Review

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).

 

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.

 

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.

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).

 

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.

 

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.

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).

 

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.

 

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.

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).

 

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.

 

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.

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.

1.1 During August 2023, a mix of below- to above-average rainfall was recorded over Southeast Asia. Over Mainland Southeast Asia, below-average rainfall was recorded over central and southern parts of the region, while a mix of below- to above-average rainfall was recorded over the rest of the region. However, there was some discrepancy over Cambodia, with GSMaP-NRT indicating predominantly near- to above-average rainfall, while CMORPH-Blended indicating well below-average rainfall. Over the Maritime Continent, below-average rainfall was recorded over much of the southern half of the region, with below- to above average rainfall over much of the northern half. For Southeast Asia, the largest positive (wetter) anomalies were recorded over parts of western Myanmar for GSMaP-NRT (Figure 1, left) and CMORPH-Blended (Figure 1, right) satellite-derived rainfall estimates. The largest negative (drier) anomalies were recorded over parts of Cambodia (CMORPH-Blended) and southern Sumatra, parts of the Maluku Islands and southern Papua (both GSMaP-NRT and CMORPH-Blended).

1.2 The observed rainfall anomaly pattern of below-average rainfall over the southern half of the Maritime Continent and a mix of below- to near-average rainfall elsewhere is broadly consistent with the predictions from the subseasonal weather outlooks for August 2023 (7 – 20 August 2023 and 21 August – 3 September 2023).

 

1.3 Above-average temperatures were recorded over most of Southeast Asia, apart from over northern Mainland Southeast Asia and the southern Maritime Continent where a mix of below- to above-average temperature was recorded. The warmest anomalies (more than 1°C above average) were recorded over parts of Thailand, Cambodia, central and southern Viet Nam, as well as the central equatorial region.

 

2. Climate Drivers

2.1 For much of August, there was no coherent Madden-Julian Oscillation (MJO) signal. While there were some indications of an MJO signal based on the RMM index, there was no eastward propagation until the last week of August. During the last week of August, based on the RMM Index, an MJO signal propagated from the Western Hemisphere (Phase 1) to the Indian Ocean (Phases 2 and 3). 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.

2.2 El Niño conditions were present during August 2023. Sea surface temperatures in the Nino3.4 region (used to monitor ENSO) were consistent with El Niño conditions, with signs of El Niño-like response in key atmospheric indicators. El Niño events tend to bring drier-than-average conditions to much of the Maritime Continent during the June to August period.

1.1 During July 2023, a mix of below- to above-average rainfall was recorded over Southeast Asia. Over Mainland Southeast Asia, below-average rainfall was recorded over the northeastern parts of the region, while a mix of below- to above-average rainfall was recorded over the rest of the region. However, there was some discrepancy over Cambodia, with GSMaP-NRT indicating predominantly above-average rainfall, while CMORPH-Blended indicating well below-average rainfall. Over the Maritime Continent, above-average rainfall was recorded over much of the western half of the region, with below-average rainfall over much of the eastern half. The largest positive (wetter) anomalies were recorded over parts of southern Myanmar, southern Viet Nam, and northern Philippines for GSMaP-NRT (Figure 1, left) and CMORPH-Blended (Figure 1, right) satellite-derived rainfall estimates. The largest negative (drier) anomalies were recorded over parts of Papua (CMORPH-Blended) and northern Viet Nam and coastal Myanmar (both GSMaP-NRT and CMORPH-Blended).

1.2 The observed rainfall anomaly pattern of a mix of below- to near-average rainfall over the ASEAN region is broadly consistent with the predictions from the subseasonal weather outlooks for July 2023 (26 June – 9 July 2023, 10 – 23 July 2023 and 24 July – 6 August 2023).

 

1.3 The observed rainfall anomaly pattern of a mix of below- to near-average rainfall over the ASEAN region is broadly consistent with the predictions from the subseasonal weather outlooks for July 2023 (26 June – 9 July 2023, 10 – 23 July 2023, and 24 July – 6 August).

 

2. Climate Drivers

2.1 At the start of July, the Madden-Julian Oscillation (MJO) signal was present over the Indian Ocean (Phase 2). This signal decayed during the first week of July, with no significant MJO activity for the rest of the month. Typically for June, Phase 2 tends to bring drier conditions in northern Southeast Asia and bring wetter conditions to the western Maritime Continent (somewhat discernible in Figure 1).

2.2 El Niño conditions were likely present during July 2023. Sea surface temperatures in the Nino3.4 region (used to monitor ENSO) were consistent with El Niño conditions, with some signs of El Niño-like response in some of the key atmospheric indicators (cloudiness). El Niño events tend to bring drier-than-average conditions to much of the Maritime Continent during the June to August period.

1.1 During June 2023, a mix of below- to above-average
rainfall was recorded over the ASEAN region. Over Mainland Southeast Asia,
above-average rainfall was recorded in the eastern parts of the region, while
a mix of below- to above-average rainfall was recorded over the rest of the
region. Over the Maritime Continent, above-average rainfall was recorded over
the western Maritime Continent, with a mix of below- to above-average rainfall
elsewhere. The largest positive anomalies were over northeastern Borneo for
both the GSMaP-NRT (Figure 1, left) and CMORPH-Blended (Figure 1, right)
satellite-derived rainfall estimates.1.2 The observed rainfall anomaly pattern of below- to
near-average rainfall over the ASEAN region is broadly consistent with the
predictions from the subseasonal weather outlooks for June 2023 (29 May – 11 June 2023</a >,
12 – 25 June 2023</a >
and
26 June – 9 July 2023</a >).

 

1.3 Above-average temperatures were recorded over most of
Mainland Southeast Asia, with near- to above-average temperature over most of
the Maritime Continent. Near- to below-average temperatures were recorded over
the southern Maritime Continent. The warmest anomalies (more than 2 °C
above-average) were recorded over southern parts of the Malay Peninsula, Lao
PDR and northeast Thailand.

 

2. Climate Drivers

2.1 A Madden-Julian Oscillation (MJO) was active at the start
of June 2023. An MJO signal propagated eastwards from the Western Hemisphere
and Africa (Phase 1) and to the Indian Ocean (Phases 2 and 3) in the first
half of June. In Week 3, the MJO signal weakened and became insignificant. An
MJO signal re-emerged in Week 4 over the Western Hemisphere and Africa (Phase
1) and propagated eastwards to the Indian Ocean (Phase 2). Typically for June,
Phases 1 and 2 tends to bring drier conditions in northern Southeast Asia
while Phases 2 and 3 tend to bring wetter conditions to the western Maritime
Continent.

2.2 Over the past month, there have been signs of El
Niño-like conditions. El Niño sea surfaces temperatures indices are near or
exceeding El Niño thresholds, whereas key atmospheric indicators (cloudiness
and wind anomalies) displayed neutral conditions for most of June.

1.1 During May 2023, below- to near-average rainfall was
recorded over much of Mainland Southeast Asia, with a mix of below- to
above-average rainfall over the Maritime Continent. Over Mainland Southeast
Asia, below-average rainfall was recorded over most of the central and
southern part of the region with the largest negative anomalies (drier
conditions) over southern Myanmar and southern Thailand. Elsewhere in Mainland
Southeast Asia, the rainfall was below- to near-average. Over the Maritime
Continent, the largest positive anomalies (wetter conditions) were recorded
over eastern Borneo with the largest negative anomalies over parts of western
Borneo for both the GSMaP-NRT (Figure 1, left) and CMORPH-Blended (Figure 1,
right) satellite-derived rainfall estimates.

1.2 The observed rainfall anomaly pattern of below- to
near-average rainfall over Mainland Southeast Asia, and no widespread regional
anomalies over the Maritime Continent, is broadly consistent with the
predictions from the subseasonal weather outlooks for May 2023 (1 – 14 May 2023
and
15 – 28 May 2023).

 

1.3 Above-average temperatures were recorded over most of
Mainland Southeast Asia, with near- to above-average temperature over most of
the Maritime Continent. Near- to below-average temperatures were recorded over
the southern Maritime Continent. The warmest anomalies (more than 2 °C
above-average) were recorded over southern parts of the Malay Peninsula, Lao
PDR and northeast Thailand.

 

2. Climate Drivers

2.1 The Madden-Julian Oscillation (MJO) was active during May
2023. An MJO signal propagated eastwards from the Maritime Continent (Phase
4), reaching the Western Pacific (Phases 6 and 7) during third week of May,
and then the Western Hemisphere (Phase 8) at the end of the month. Typically
for May, Phase 4 brings wetter conditions to much of the Maritime Continent,
and Phase 5 tends to bring wetter conditions to the eastern Maritime
Continent. Phase 6 tends to bring drier conditions to the western half of the
Maritime Continent. Phases 7 and 8 bring drier conditions to much of Southeast
Asia.

2.2 The tropical Pacific was likely in an ENSO neutral state
during May. The sea surface temperatures in the Nino3.4 region (used to
monitor ENSO) continued to warm, although on average were still within the
neutral range. Atmospheric indicators over the tropical Pacific Ocean
indicated neutral conditions.

1.1 During April 2023, below- to near-average rainfall was recorded over Mainland Southeast Asia, with a mix of below- to above-average rainfall over the Maritime Continent. Over Mainland Southeast Asia, below-average rainfall was recorded over most of the central and southern part of the region with the largest negative anomalies (drier conditions) over southern Thailand. Elsewhere in Mainland Southeast Asia, the rainfall was near-average. Over the Maritime Continent, the largest positive anomalies (wetter conditions) were recorded over southern Sumatra with the largest negative anomalies over parts of Borneo for both the GSMaP-NRT (Figure 1, left) and CMORPH-Blended (Figure 1, right) satellite-derived rainfall estimates.

1.2 The observed rainfall anomaly pattern of below- to near-average rainfall over Mainland Southeast Asia, below-average rainfall over central Maritime Continent , and no widespread regional anomalies elsewhere over the Maritime Continent, is broadly consistent with the predictions from the subseasonal weather outlooks for April 2023 (3 – 16 April 2023 and 17 – 30 April 2023).

 

1.3 Above-average temperatures were recorded over most of Mainland Southeast Asia, with near- to above-average temperature over most of the Maritime Continent. The warmest anomalies (more than 2 °C above-average) were recorded over parts of Lao PDR and northern Thailand.

 

2. Climate Drivers

2.1 The Madden-Julian Oscillation (MJO) was active during April 2023. An MJO signal propagated eastwards from the Maritime Continent (Phase 5), reaching the Western Hemisphere (Phases 8 and 1) during third week of April, and then just entering again the Maritime Continent (Phase 4) at the end of the month. Typically for April, Phases 7, 8, and 1 bring drier conditions to the Southeast Asia, while Phases 3 to 5 bring wetter conditions, particularly in southern Southeast Asia.

2.2 The tropical Pacific was in an ENSO neutral state during April. Overall, sea surface temperatures in the Nino3.4 region (used to monitor ENSO) and the atmospheric indicators over the tropical Pacific Ocean indicated neutral conditions.

1.1 During March 2023, a mix of below- to above-average rainfall was recorded over much of the Maritime Continent. Parts of the northern Maritime Continent, including Philippines and northeastern Borneo, received below- to near-average rainfall, while parts of the equatorial region received above-average rainfall. Over Mainland Southeast Asia, near-average rainfall was recorded over most of northwestern and eastern region, and below-normal elsewhere. The largest positive anomalies (wetter conditions) were recorded over the central parts of Borneo 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 parts of southern Sumatra and Sulawesi in both CMORPH-Blended data and GSMaP-NRT data.

1.2 The observed rainfall anomaly pattern of a mix of below- to above-average rainfall for the Maritime Continent and below- to near-average rainfall for Mainland Southeast Asia are broadly consistent with the predictions from the subseasonal weather outlooks for March 2023 (6 – 19 March 2023 and 20 March – 2 April 2023).

 

1.3 Below- to near-average temperatures were recorded over much of the western and central Maritime Continent, with above-average temperatures over parts of the eastern Maritime Continent. A mix of below- to above-average temperatures were recorded over the northern and central Mainland Southeast Asia and predominantly below-average temperatures were observed over southeastern Mainland Southeast Asia. The warmest anomalies were recorded over northern Viet Nam, and the coolest over the southeastern Mainland Southeast Asia.

 

2. Climate Drivers

2.1 A Madden-Julian Oscillation (MJO) signal was present over the Western Pacific (Phase 6) at the start of March. This MJO signal propagated eastwards and strengthened as it passed through the Western Pacific (Phase 7) and Western Hemisphere (Phases 8 and 1) in the first half of March. In the second half of March, this signal relatively weakened and continued propagating eastwards through the Western Pacific (Phase 1) and Indian Ocean (Phases 2 and 3). The MJO signal weakened at the end of month and became indiscernible over the Maritime Continent (Phase 4), based on the RMM Index. Typically, during this time of the year, Phase 6 tends to bring wetter conditions to eastern Maritime Continent, while 7 and 8 tends to bring drier conditions to much of the Maritime Continent. Phases 1 and 2 tends to bring drier conditions over the eastern Maritime Continent, whereas Phase 3 tends to bring wetter conditions to the western Maritime Continent. Phase 4 usually brings wetter conditions to much of the Maritime Continent.

2.2 La Niña conditions have now ended, and the tropical Pacific is now in an ENSO neutral state. Overall, sea surface temperatures in the Nino3.4 region (used to monitor ENSO) and the atmospheric and oceanic indicators over the tropical Pacific Ocean, indicate neutral conditions.