Category Archives: Fortnightly Weather Review

CR58

May 17, 2022

Review of Regional Weather for April 2022

1. Overview

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

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

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

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

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

2. Climate Drivers

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

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

April 19, 2022

Review of Regional Weather for March 2022

1. Overview

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

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

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

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

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

2. Climate Drivers

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

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

March 14, 2022

Review of Regional Weather for February 2022

1. Overview

1.1 During February 2022, the Maritime Continent experienced a mix of below- to above-average rainfall. Mainland Southeast Asia experienced a mix of near- and above-average rainfall relative to its typical dry season, with above-average rainfall over much of Thailand, Cambodia, northern Lao PDR and northern Viet Nam (Figure 1). Both satellite-derived rainfall estimates datasets (GSMaP-NRT and CMORPH-Blended) show large positive anomalies (wetter conditions) over the Malay Peninsula and northern Sumatra, and large negative anomalies (drier conditions) over Java Island. There are some discrepancies between the two datasets, as CMORPH-Blended (Figure 1b) recorded below-average rainfall for much of Borneo and southern Sumatra, while GSMaP-NRT (Figure 1a) recorded near- to above-average rainfall with isolated areas of below-average rainfall over the mentioned region.

1.2 The above-average rainfall for the western Maritime Continent and mixed below- to above-average rainfall for much of the rest of the Maritime Continent is in line with much of the subseasonal weather outlooks for February 2022, which overall predicted above-average rainfall over the region around the Malay Peninsula and southern Thailand (24 January – 6 February, 7 – 20 February, and 21 February – 6 March). The exception is over the Philippines, where above-average rainfall was predicted during February, whereas Figure 1 shows the country experienced a mix of below- to above-average rainfall.

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

1.3 A mix of below- to above-average temperature was recorded in Southeast Asia in February 2022, with below- to near-average temperature recorded over much of Mainland Southeast Asia and the Malay Peninsula, above-average temperature over much of the Philippines, southern Viet Nam, and southern Sumatra, and near-average temperature recorded elsewhere (Figure 2). The largest negative anomalies (more than 2°C cooler) were over northwestern and northeastern parts of Mainland Southeast Asia.

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

2. Climate Drivers

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

February 17, 2022

Review of Regional Weather for January 2022

1. Overview

1.1 During January 2022, the Maritime Continent experienced a mix of below- and above-average rainfall, with Mainland Southeast Asia experiencing its typical dry season, apart from northern Lao PDR and Viet Nam that experienced above-average rainfall (Figure 1). For the Maritime Continent, both satellite-derived rainfall estimates datasets (GSMaP-NRT and CMORPH-Blended) show large negative anomalies (drier conditions) over the Malay Peninsula and the southern half of the Philippines. However, there are some discrepancies between the two datasets elsewhere. CMORPH-Blended (Figure 1b) recorded below-average rainfall for much of the region, apart from the northern Philippines, and isolated parts of Sumatra and Borneo. GSMaP-NRT (Figure 1a) however, recorded more extensive areas of above-average rainfall with isolated areas of below-average rainfall primarily over parts of Borneo and Java.

1.2 The mix of below- and above-average rainfall for the Maritime Continent with no clear regional pattern is in line with the subseasonal weather outlooks for January 2022, which predicted a mix of below- to above-average rainfall depending on the outlook (27 December – 9 January, 10 – 23 January, and 24 January – 6 February), although the wetter conditions over northern Lao PDR and Viet Nam were not included in any of the outlooks.

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

1.3 Near- to above-average temperature was recorded in Southeast Asia in January 2022, apart from northern Myanmar where below- to near-average temperature was recorded (Figure 2). The largest positive anomalies (more than 1°C warmer) were over central and eastern parts of Mainland Southeast Asia. For the Maritime Continent, the largest positive anomalies were over the Malay Peninsula (0.25°C – 1°C warmer), with the rest of the area near- or slightly above-average.

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

2. Climate Drivers

2.1 A Madden-Julian Oscillation (MJO) signal was present over the Western Pacific (Phase 7) at the start of January 2022, based on the MJO phase diagram below (Figure 3). In the second week of January, the signal entered the Western Hemisphere (Phase 8) and then weakened rapidly. In the second half of January, there was no MJO signal. Phase 7 tends to bring drier conditions to the western Maritime Continent and expanding in Phase 8 to most of the Maritime Continent at this time of the year.

January 13, 2022

Review of Regional Weather for December 2021

1. Overview

1.1 During December 2021, coastal central parts of Viet Nam, Peninsular Malaysia and southern half of the Philippines experienced above-average rainfall (Figure 1). The largest positive anomalies (wetter conditions) were recorded over the central Philippines and the Peninsular Malaysia (due to Super Typhoon Rai and Tropical Depression 29W respectively, which made landfall in mid-December), for both satellite-derived rainfall estimates datasets (GSMaP-NRT and CMORPH-Blended). For the rest of the Mainland Southeast Asia, rainfall anomalies were negligible, as expected at the start of dry season for the northern ASEAN region. A mix of above- and below-average rainfall was experienced for most regions in the equatorial region. There is some discrepancy between the two satellite datasets for the below-average rainfall for central parts of Borneo, with more extensive drier conditions based on CMORPH-Blended (Figure 1, right) compared to near- to above-average rainfall for GSMaP-NRT (Figure 1, left). Elsewhere in the southern Maritime Continent, wetter-than-average rainfall conditions were recorded over much of the land masses.

1.2 The observed large-scale rainfall anomaly pattern (i.e. above-average rainfall in regions linked to tropical storm and cyclone activity and below-average rainfall over parts of equatorial region) is broadly consistent with the predictions from the subseasonal weather outlooks for December 2021 (29 November – 12 December, 13 – 26 December, and 27 December – 9 January), apart from the positive anomalies (wetter conditions) for Peninsular Malaysia not highlighted in the outlooks.

Figure 1: Rainfall anomalies for the month of December 2021 based on GSMaP-NRT data (left) and CMORPH-Blended data (right). The climatological reference period is 2001-2020. Green colour denotes above-average rainfall (wetter), while orange denotes below-average rainfall (drier).

1.3 Below-average temperatures were recorded over central and eastern regions of Mainland Southeast Asia in December 2021 (Figure 2). In contrast, above-average temperatures (more than 1°C warmer) were recorded over parts of Myanmar. Elsewhere, near- to slightly above-average temperature were recorded.

Figure 2: Temperature anomalies for the month of December 2021 based on ERA-5 reanalysis. The climatological reference period is 2001-2020. Red colour denotes above-average temperature (warmer), while blue denotes below-average temperature (colder).

2. Climate Drivers

2.1 A Madden-Julian Oscillation (MJO) was present over the Western Pacific (Phase 6) at the start of December 2021, based on the MJO phase diagram below (Figure 3). From second week of December onwards, this signal was not very coherent (very slow eastwards propagation towards Phase 7) and likely influenced by the background La Niña conditions. Phase 6 tends to bring wetter conditions to eastern Maritime Continent, while Phase 7 tends to bring drier conditions to the western Maritime Continent at this time of the year.

December 14, 2021

Review of Regional Weather for November 2021

1. Overview

1.1 During November 2021, a mix of below- to above-average rainfall was recorded over the Maritime Continent, while near-average rainfall was recorded over much of Mainland Southeast Asia, except for above-average rainfall recorded over southern Mainland Southeast Asia (Figure 1). The largest positive anomalies (wetter conditions) were recorded over southern Mainland Southeast Asia for both satellite-derived rainfall estimates datasets (GSMaP-NRT and CMORPH-Blended). The largest negative anomalies (drier conditions) were recorded over central Sumatra and the Philippines, with larger negative anomalies based on CMORPH-Blended data compared to GSMaP-NRT.

1.2 The observed rainfall anomaly pattern of a mix of below- to above-average rainfall for the Maritime Continent and the above-average rainfall for southern Mainland Southeast Asia are broadly consistent with the predictions from the subseasonal weather outlooks for November 2021 (1 – 14 November and 15 – 28 November).

Figure 1: Rainfall anomalies for the month of November 2021 based on GSMaP-NRT data (left) and CMORPH-Blended data (right). The climatological reference period is 2001-2020. Green colour denotes above-average rainfall (wetter), while orange denotes below-average rainfall (drier).

1.3 Above average temperature was recorded over western Mainland Southeast Asia, the western Maritime Continent, as well as the northern Philippines and eastern Indonesia. Below-average temperature was recorded over northeastern Mainland southeast Asia and southern Indonesia (Figure 2). Elsewhere, near-average temperature was recorded. The largest positive anomalies (warmer temperatures) were over western Myanmar, and largest negative anomalies (cooler temperatures) were over northern Viet Nam and eastern Java.

Figure 2: Temperature anomalies for the month of November 2021 based on ERA-5 reanalysis. The climatological reference period is 2001-2020. Red colour denotes above-average temperature (warmer), while blue denotes below-average temperature (colder).

2. Climate Drivers

2.1 There was no Madden-Julian Oscillation (MJO, Figure 3) present at the start of November 2021, based on the MJO phase diagram below. From second week of November onwards, there were signs of a MJO signal over the Maritime Continent (Phases 4 and 5). However, this signal was not very coherent and likely influenced by the background La Niña conditions. Phases 4 and 5 tend to bring wetter conditions to parts of the ASEAN region around 10°N, while Phase 5 tends to bring drier conditions to the western Maritime Continent.

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

November 16, 2021

Review of Regional Weather for October 2021

1. Overview

1.1 During October 2021, above-average rainfall was recorded over much of the northern ASEAN region, while a mix of below- to above-average rainfall was recorded over the southern ASEAN region (Figure 1). For the northern ASEAN region, above-average rainfall was recorded apart from over central and northern Myanmar and parts of the Philippines. The largest positive anomalies (wetter conditions) were recorded over eastern Mainland Southeast Asia for both satellite-derived rainfall estimates datasets (GSMaP-NRT and CMORPH-Blended), which was associated with Severe Tropical Storm Kompasu. A second tropical storm, Tropical Storm Lionrock, crossed the central Philippines at the start of October, contributing to the large positive anomalies in the ocean region around the Philippines (GSMaP-NRT). For the southern ASEAN region, negative anomalies (drier conditions) were recorded over western parts as well as northern Borneo, although the negative anomalies were larger based on CMORPH-Blended data compared to GSMaP-NRT. Elsewhere in the southern ASEAN region, rainfall tended to be near-to above-average during October 2021.

1.2 The observed rainfall anomaly pattern of above-average rainfall for much of the northern ASEAN region and a mix of below- to above-average rainfall for the southern ASEAN region is broadly consistent with the predictions from the subseasonal weather outlooks for October 2021 (4 – 17 October and 18 – 31 October).

Figure 1: Rainfall anomalies for the month of October 2021 based on GSMaP-NRT data (left) and CMORPH-Blended data (right). The climatological reference period is 2001-2020. Green colour denotes above-average rainfall (wetter), while orange denotes below-average rainfall (drier).

1.3 Most of the ASEAN region experienced near to above-average temperatures during October 2021 (Figure 2). The exception was over eastern parts of Mainland Southeast Asia, which experienced below- to near-average temperatures. The largest positive anomalies (warmer temperatures) were over northern Myanmar and the western Maritime Continent where negative rainfall anomalies (drier conditions) were also recorded.

Figure 2: Temperature anomalies for the month of October 2021 based on ERA-5 reanalysis. The climatological reference period is 2001-2020. Red colour denotes above-average temperature (warmer), while blue denotes below-average temperature (colder).

2. Climate Drivers

2.1 The Madden-Julian Oscillation (MJO, Figure 3) signal was present over the Maritime Continent at the start of October 2021 (Phases 4 and 5). However, this signal weakened rapidly as it approached the Western Pacific (Phase 6) likely influenced by the background La Niña conditions. For the rest of October, there was no discernible MJO signal, based on the MJO phase diagram below. Phases 4 and 5 tend to bring wetter conditions to parts of the ASEAN region around 10°N, while Phase 5 tends to bring drier conditions to the western Maritime Continent.

2.2 By the end of October 2021, La Niña conditions were present in the tropical Pacific. Sea surface temperatures in the Nino3.4 region (used to monitor ENSO) continued to cool, with some signs of La Niña conditions in the atmosphere. A weak negative Indian Ocean Dipole (IOD) was present and at the seasonal timescale, negative IOD events tend to bring wetter conditions to the southern Maritime Continent.

October 13, 2021

Review of Regional Weather for September 2021

1. Overview

1.1 During September 2021, there was above-average rainfall recorded over much of the ASEAN region (Figure 1). The exceptions were northern Sumatra and northern Philippines, and also parts of northern Myanmar, Peninsular Malaysia, Borneo, and Papua, which received a mix of below- to near-average rainfall. The largest positive anomalies (wetter conditions) were recorded over eastern Mainland Southeast Asia (due to Tropical Storm Conson and Tropical Storm Dianmu which made landfall in early-September and late-September respectively) and southern Myanmar for both satellite-derived rainfall estimates datasets (GSMaP-NRT and CMORPH-Blended), with GSMaP-NRT also recording larger anomalies over eastern Borneo, northern and eastern Sulawesi, and Maluku Islands.

1.2 The observed rainfall anomaly pattern of above-average rainfall for much of the ASEAN region is broadly consistent with the predictions from the subseasonal weather outlooks for September 2021 (23 August – 5 September, 6 – 19 September and 20 September – 3 October).

Figure 1: Rainfall anomalies for the month of September 2021 based on GSMaP-NRT data (left) and CMORPH-Blended data (right). The climatological reference period is 2001-2020. Green colour denotes above-average rainfall (wetter), while orange denotes below-average rainfall (drier).

1.3 Most of the ASEAN region experienced below to near-average temperatures during September 2021 (Figure 2), in line with the wetter conditions recorded. The exceptions were warmer-than-average temperatures recorded over the southeastern Maritime Continent and northern Myanmar.

Figure 2: Temperature anomalies for the month of September 2021 based on ERA-5 reanalysis. The climatological reference period is 2001-2020. Red colour denotes above-average temperature (warmer), while blue denotes below-average temperature (colder).

2. Climate Drivers

2.1 The Madden-Julian Oscillation (MJO, Figure 3) signal over the Indian Ocean (Phase 3) appeared incoherent for most of September 2021 due to interference with other tropical waves. In the last week of September, an MJO signal emerged over the Maritime Continent (Phase 4). Phase 3 tends to bring wetter conditions to western Maritime Continent while Phase 4 tends to bring wetter conditions to much of the region at this time of the year.

2.2 In September, there have been some signs of La Niña-like conditions developing in the tropical Pacific. However, atmospheric indicators (OLR and wind anomalies) have not shown consistent La Niña-like conditions yet. A weak negative Indian Ocean Dipole (IOD) was present and at the seasonal timescale, negative IOD events tend to bring wetter conditions to the southern Maritime Continent.

September 14, 2021

Review of Regional Weather for August 2021

1. Overview

1.1 During August 2021, there was above-average rainfall recorded over much of the ASEAN region, while below-average rainfall was recorded over the northeastern ASEAN region (Figure 1). The largest positive anomalies (wetter conditions) were found across the equatorial region for both satellite-derived rainfall estimates datasets (GSMaP-NRT and CMORPH-Blended). The two datasets were also in agreement that the largest negative anomalies over land (drier conditions) were observed over parts of northern Viet Nam. However, there were some discrepancies over the southern Myanmar, where CMORPH-Blended recorded wetter conditions while GSMaP-NRT recorded drier conditions.

1.2 The observed rainfall anomaly pattern of above-average rainfall for much of the ASEAN region is broadly consistent with the predictions from the subseasonal weather outlooks for August 2021 (26 July – 8 August, 9 – 22 August and 23 August – 5 September).

Figure 1: Rainfall anomalies for the month of August 2021 based on GSMaP-NRT data (left) and CMORPH-Blended data (right). The climatological reference period is 2001-2020. Green colour denotes above-average rainfall (wetter), while orange denotes below-average rainfall (drier).

1.3 Most of the northern ASEAN region and southern Maritime Continent experienced above-average temperatures during August 2021 (Figure 2). The warmest anomalies (above-average temperature) were recorded over northern Viet Nam, where the drier conditions were observed. For the equatorial region and central and northern Myanmar, near- to below-average temperature was recorded.

Figure 2: Temperature anomalies for the month of August 2021 based on ERA-5 reanalysis. The climatological reference period is 2001-2020. Red colour denotes above-average temperature (warmer), while blue denotes below-average temperature (colder).

2. Climate Drivers

2.1 At the start of August 2021, a Madden-Julian Oscillation (MJO) signal was present in the Western Hemisphere (Phase 8, Figure 3), which weakened by the middle of the first week based on the RMM plot. The signal emerged over the Indian Ocean (Phase 2) in the second week of August, with a rapid strengthening of the signal in the RMM plot and slow eastward propagation. This MJO signal weakened in the second half of August while it was still over the Indian Ocean. The mix of below- to above-average rainfall for the ASEAN region is indicative of the MJO signal: Phase 2 tends to bring drier conditions over the northeastern ASEAN region and wetter conditions for the western Maritime Continent at this time of the year.

2.2 The equatorial Pacific Ocean was in the ‘ENSO-neutral’ state while a negative Indian Ocean Dipole (IOD) was present in August. At the seasonal timescale, negative IOD events tend to bring wetter conditions to the southern Maritime Continent.

August 13, 2021

Review of Regional Weather for July 2021

1. Overview

1.1 During July 2021, there was a mix of below- to above-average rainfall over the ASEAN region (Figure 1). The largest positive anomalies (wetter conditions) were over the coastal parts of southern Myanmar, southern Cambodia, northwestern Philippines, as well as the Maluku Islands for both satellite-derived rainfall estimates datasets (GSMaP-NRT and CMORPH-Blended). The two datasets were also in agreement that the largest negative anomalies (drier conditions) were over the central Philippines. However, there were some discrepancies over the western and central Maritime Continent, where CMORPH-Blended recorded drier conditions than GSMaP-NRT.

1.2 The observed rainfall anomaly pattern (i.e. mix of below- to above-average rainfall) is broadly consistent with the predictions from the subseasonal weather outlooks for July 2021 (28 June – 11 July and 12 – 25 July). The only portion of the ASEAN region with a consistent outlook for July were the wetter conditions in the southeastern Maritime Continent, which agrees with the observed anomaly pattern.

Figure 1: Rainfall anomalies for the month of July 2021 based on GSMaP-NRT data (left) and CMORPH-Blended data (right). The climatological reference period is 2001-2020. Green colour denotes above-average rainfall (wetter), while orange denotes below-average rainfall (drier).

1.3 Most of the ASEAN region experienced above-average temperatures during July 2021 (Figure 2). The warmest anomalies (above-average temperature) were recorded over Lao PDR and the southern Maritime Continent, followed by central and northern Myanmar, Viet Nam, and central and southern parts of the Philippines. Elsewhere, near- to above-average temperature were recorded.

Figure 2: Temperature anomalies for the month of July 2021 based on ERA-5 reanalysis. The climatological reference period is 2001-2020. Red colour denotes above-average temperature (warmer), while blue denotes below-average temperature (colder).

2. Climate Drivers

2.1 At the start of July 2021, a Madden-Julian Oscillation (MJO) signal was present in the Indian Ocean (Phases 2 and 3, Figure 3). In the second week of July the signal propagated eastwards over the Maritime Continent (Phases 4 and 5). The signal then weakened slightly as it moved over the Western Pacific (Phases 6 and 7), although it strengthened again entering the Western Hemisphere (Phase 8) at the end of July. The mix of below- to above-average rainfall for the ASEAN region is indicative of the near complete circumnavigation of the MJO signal: Phases 3 to 4 tend to bring wetter conditions to much of the region, while Phases 7 to 8 tend to bring drier conditions, and Phase 2 and Phase 5 bring a mix of drier and wetter conditions at this time of the year.

2.2 The equatorial Pacific Ocean is currently in an ‘ENSO-neutral’ state. A negative Indian Ocean Dipole (IOD) has developed in the Indian Ocean, with warmer sea surface temperatures southwest of Java during July. At the seasonal timescale, negative IOD events tend to bring wetter conditions to the southern Maritime Continent.