Monthly Archives: October 2023

October 27, 2023

Subseasonal Weather Outlook (30 October – 12 November 2023)

Issued: 27 October 2023
First forecast week: 30 October – 5 November 2023
Second forecast week: 6 November – 12 November 2023

Figure 1: Rainfall Outlook

Figure 2: Temperature Outlook

Drier conditions are predicted over much of the southwestern and central Maritime Continent in Week 1 (30 October – 5 November). These drier conditions are predicted to persist in Week 2 (6 – 12 November), particularly over much of the southwestern and southern Maritime Continent.

Wetter conditions are predicted over central Mainland Southeast Asia in Week 1 (30 October – 5 November).

Warmer than usual temperature is predicted over much of Mainland Southeast Asia in Week 1 (30 October – 5 November). These warmer conditions are predicted to gradually ease in Week 2 (6 – 12 November). Over much of the Maritime Continent, warmer than usual temperature is predicted in the next fortnight (30 October – 12 November).

During the second half of October, no clear Madden-Julian Oscillation (MJO) signal was present. Most models predict an MJO signal over the Western Hemisphere and Africa (Phases 8 and 1) during the forecast period based on the RMM Index.

The outlook is assessed for the region in general, where conditions are relative to the average conditions for the corresponding time of year. For specific updates on the national scale, the relevant ASEAN National Meteorological and Hydrological Services should be consulted.

October 24, 2023

Alert20231024 – Downgrade of Alert Level 2 to Alert Level 1 for the Southern ASEAN region

Downgrade of Alert Level 2 to Alert Level 1 for the southern ASEAN region

In recent days, increased rainfall has helped to improve the overall hotspot and smoke haze situation over the southern ASEAN region. While this has reduced the risk of transboundary haze occurrence, isolated hotspots and localised smoke plumes may still develop in fire-prone areas during brief periods of dry weather.

Based on surveillance by the NOAA-20 satellite, a total of 57 and 47 hotspots were detected in Sumatra, and a total of 5 and 8 hotspots were detected in Kalimantan on 22 and 23 October 2023, respectively. The hotspot and haze situation over the southern ASEAN region is expected to further improve as more rainfall is forecast in the coming weeks.

October 23, 2023

Review of Regional Weather for September 2023

1. Overview

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.

October 13, 2023

Subseasonal Weather Outlook (16 – 29 October 2023)

Issued: 13 October 2023
First forecast week: 16 October – 22 October 2023
Second forecast week: 23 October – 29 October 2023

Figure 1: Rainfall Outlook

Figure 2: Temperature Outlook

Drier conditions are predicted over most of the southwestern Maritime Continent in Week 1 (16 – 22 October). These drier conditions are predicted to extend eastwards to cover much of the Maritime Continent in Week 2 (23 – 29 October).

Wetter conditions are predicted over most of Mainland Southeast Asia in Week 1 (16 – 22 October).

For temperature, warmer than usual temperature is predicted over most of the Maritime Continent in the next fortnight (16 – 29 October).

In the second week of October, a Madden-Julian Oscillation (MJO) signal was present over the Western Hemisphere and Africa (Phase 1) based on the RMM index. Most models predict the MJO signal to decay with no significant activity during the forecast period.

October 5, 2023

Review of Regional Haze Situation for September 2023

1.1 Southwest monsoon conditions persisted over the ASEAN region in September 2023. The prevailing low-level winds blew mainly from the southeast over areas south of the Equator and turned to blow from the southwest or west over areas north of the Equator (Figure 1). There were two notable tropical cyclones during the review period, namely Typhoon Saola and Typhoon Haikui, which developed one after another from late August to early September 2023 (Figure 2). Heavy rains and strong winds brought by the storms had affected many areas in northern parts of the ASEAN region, particularly the Philippines, where significant events of floods and landslides were reported.

1.2 The southern ASEAN region remained under its traditional dry season throughout September 2023. Dry conditions prevailed over many parts of the region while there were isolated shower activities over parts of northern Sumatra, Malaysia, and northeastern Kalimantan (Figure 1). Over the fire-prone areas in Sumatra and Kalimantan, hotspot activity gradually escalated from middle of the month and the overall hotspot count for the month of September 2023 was the highest since 2019 (Figure 3). Most of the hotspots with high fire intensity were detected in southern Sumatra and southern Kalimantan while isolated hotspots were also detected in central Sumatra and western Kalimantan on some days (Figure 4 and 5). During the first three weeks of September 2023, slight to moderate smoke plumes were observed mainly in the Indonesian provinces of South Sumatra, South Kalimantan, Central Kalimantan, and West Kalimantan. On some days, transboundary smoke haze was observed to drift northwards from West Kalimantan into western Sarawak (Figure 8). Towards the last week of the month, the smoke haze emanating from persistent hotspot clusters in southern Sumatra and southern Kalimantan became denser and more widespread and was observed to drift towards the northwest and north (Figure 6, 7, and 8). On 29 Sep 2023, ASMC activated Alert Level 2 for Sumatra, indicating a risk of transboundary haze for the region.

1.3 The weather was generally wet over the northern ASEAN region in September 2023 as its traditional wet season prevailed. Hotspot activity was subdued by widespread rains and the overall hotspot count in September 2023 was comparable to the count in previous years (Figure 9). While there were isolated hotspots detected in parts of central Thailand and Viet Nam, these hotspots were mostly short-lived and with low fire intensity (Figure 4 and 5). No significant smoke haze was observed over the region during the review period (Figure 6).

Figure 1: Average Daily Rainfall and Mean Winds for September 2023. (Source: JAXA GsMaP and GFS, respectively)

Figure 2: Typhoon Saola and Typhoon Haikui which affected many parts of the northern ASEAN region during late August and early September 2023. (Source: Himawari-9 satellite surveillance)

Figure 3: Hotspot counts for the southern ASEAN region for September (2019-2023) based on NOAA-20 satellite surveillance.

Figure 4: Distribution of hotspots detected in September 2023 based on NOAA-20 satellite surveillance.

Figure 5: Maximum fire intensity in September 2023 based on NOAA-20 satellite surveillance.

Figure 6: Number of days with moderate to dense smoke haze observed in September 2023 based on satellite imagery, ground observations, and air quality reports.

Figure 7: Haze situation over Sumatra in September 2023. (Source: Himawari-9 satellite surveillance)

Figure 8: Haze situation over Kalimantan in September 2023. (Source: Himawari-9 satellite surveillance)

Figure 9: Hotspot counts for the northern ASEAN region for September (2019-2023) based on NOAA-20 satellite surveillance.