Monthly Archives: June 2023

June 21, 2023

Review of Regional Weather for May 2023

1. Overview

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).

Figure 1: Rainfall anomalies for May 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
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.

Figure 2: Temperature anomalies for May 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 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.

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 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.

June 20, 2023

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June 9, 2023

Subseasonal Weather Outlook (12 – 25 June 2023)

Issued: 09 June 2023
First forecast week: 12 – 18 June 2023
Second forecast week: 19 – 25 June 2023

Figure 1: Rainfall Outlook

Figure 2: Temperature Outlook

Wetter conditions are predicted over parts of northeast Mainland Southeast Asia in Week 1 (12 – 18 June) and over northwest Mainland Southeast Asia in Week 2 (19 – 25 June).

Drier conditions are predicted over most of the Maritime Continent and parts of southern Mainland Southeast Asia in Week 2 (19 – 25 June).

Warmer than usual temperature is expected over much of the Maritime Continent in Week 1 (12 – 18 June). These warmer conditions are predicted to persist and extend northwards over southern Mainland Southeast Asia in Week 2 (19 – 25 June).

A Madden-Julian Oscillation (MJO) signal was present over the Western Hemisphere and Africa (Phases 8 and 1) in the first week of June based on the RMM Index. Most models predict this MJO signal to continue propagating eastward through the Indian Ocean (Phases 2 and 3) and decay over the Maritime Continent (Phases 4 and 5) during the forecast period.

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.

Review of Regional Haze Situation for May 2023

1.1 Inter-monsoon conditions prevailed over the ASEAN region in May 2023 and the wind conditions were light and variable in direction on most days (Figure 1). Two tropical cyclones affected parts of the region in May 2023, namely Tropical Cyclone Mocha over Bay of Bengal (Figure 2) and Super Typhoon Mawar over the western Pacific Ocean (Figure 3). The storms brought brief periods of widespread rainfall and strong winds over Myanmar and the Philippines, respectively. According to the Joint Typhoon Warning Centre (JTWC), Super Typhoon Mawar was the strongest Northern Hemisphere tropical cyclone ever recorded in the month of May. By the end of May 2023, the inter-monsoon period had gradually transitioned into the Southwest Monsoon for most parts of the ASEAN region.

1.2 Over the northern ASEAN region, dry conditions continued over much of the Mekong sub-region in early May 2023. Elevated hotspot activity persisted in many parts of the sub-region, particularly in Lao PDR and Myanmar (Figure 4). On many days, isolated to scattered hotspots were detected in northern and western parts of the sub-region, where moderate to dense smoke haze, occasionally transboundary, was observed (Figure 6 – 9). Following the return of showers from the second week of May 2023, hotspot activity over the Mekong sub-region had steadily decreased. The ASMC Alert Level for the sub-region was downgraded to Alert Level 1 on 9 May 2023, and subsequently the end of dry season for the sub-region was declared on 24 May 2023.

1.3 For the southern ASEAN region, the weather was rainy and cloudy for most of May 2023. However, periods of drier conditions began to set in over some parts of the region from last week of the month, as monsoon rain band shifted further north from the Equator. Few to isolated hotspots, some with localised smoke plumes, were observed in the fire-prone areas of Sumatra, Kalimantan, and Peninsular Malaysia. A slight increase in hotspot activity was also detected, following the gradual onset of Southwest Monsoon conditions (Figure 5). On 29 May 2023, ASMC activated Alert Level 1 for the southern ASEAN region, signaling the start of the traditional dry season.

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

Figure 2: Tropical Cyclone Mocha over the Bay of Bengal and made landfall over Myanmar on 15 May 2023. (Source: Himawari-9 satellite surveillance)

Figure 3: Super Typhoon Mawar over the western Pacific Ocean brought widespread rainfall and strong winds over the Philippines during the last week of May 2023. (Source: Himawari-9 satellite surveillance)

Figure 4: Hotspot counts for the northern ASEAN region for May (2019-2023) based on NOAA-20 satellite surveillance. Hotspot counts were generally comparable to the counts in previous years, with majority of the hotspots detected in Lao PDR and Myanmar.

Figure 5: Hotspot counts for the southern ASEAN region for May (2019-2023) based on NOAA-20 satellite surveillance. Hotspot counts were slightly higher than the counts in previous years due to the gradual onset of the traditional dry season towards the end of May 2023.

Figure 6: Distribution of hotspots detected in May 2023 based on NOAA-20 satellite surveillance. Scattered hotspots were persistent over northern parts of Thailand and Lao PDR, as well as in parts of Myanmar and Viet Nam. Elsewhere in the ASEAN region, there were isolated and short-lived hotspots over the fire-prone areas in southern Sumatra and western Kalimantan.

Figure 7: Maximum fire intensity in May 2023 based on NOAA-20 satellite surveillance. The overall fire intensity was moderate over much of the Mekong sub-region, but particularly high over northern Lao PDR where clusters of hotspots persisted.

Figure 8: Number of days with moderate to dense smoke haze observed in May 2023 based on satellite imagery, ground observations, and air quality reports. Hazy conditions persisted mainly over western and northern parts of the Mekong sub-region.

Figure 9: Moderate smoke haze was observed in eastern Myanmar, northern Thailand and northern Lao PDR on 6 May 2023. Blown by the prevailing winds, transboundary smoke haze was observed to drift eastwards across northern Thailand, northern Lao PDR, and parts of Viet Nam. (Source: NOAA-20 satellite surveillance)

June 1, 2023

20th ASEAN COF (May 2023, Online)

Twentieth Session of the ASEAN Climate Outlook Forum (ASEANCOF-20)

25, 26, 30 May 2023, ASMC

Consensus Bulletin for June-July-August (JJA) 2023 Season

The ASEAN Climate Outlook Forum (ASEANCOF) is an avenue to collaboratively develop consensus-based seasonal climate outlooks and related information on a regional scale. The forum’s outlook and its activities contribute significantly to one of the key roles of the ASEAN Specialised Meteorological Centre (ASMC), which is to conduct climate and seasonal prediction for the Association of Southeast Asian Nations (ASEAN) region through pooling the expertise of ASEAN National Meteorological and Hydrological Services (NMHSs). In 2021, the ASEANCOF Working Group was established with the goal to guide and support the long-term development of ASEANCOF, in particular with regard to the implementation of objective outlooks.

The Twentieth session of ASEANCOF (ASEANCOF-20) was organised by ASMC and the ASEANCOF Working Group. Participants from the NMHSs of ASEAN Member States created a consensus forecast for the boreal summer monsoon 2023 in the ASEAN region. The consensus for the June-July-August (JJA) 2023 outlook was achieved through an online session, which included presentations from different NMHSs, questionnaires, and discussions regarding the current climate conditions and predictions for Southeast Asia. In particular, ASEANCOF considered the influence of the El Niño Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD) on the climate system over Southeast Asia. The theme of ASEANCOF-20 was ‘Advances in climate services on subseasonal to seasonal timescales’. On the last day of ASEANCOF-20, a sharing session was held on the impact of El Niño on Southeast Asia, involving NMHSs and users of ASEANCOF information.

Conditions and Outlook

Recent analysis of sea surface temperature (SST) anomalies over the equatorial Pacific shows average to slightly above-average SSTs across most of the equatorial Pacific Ocean, and along with atmospheric indicators such as trade wind strength and cloudiness, indicate ENSO-neutral conditions. In the Indian Ocean, the Indian Ocean Dipole (IOD) is also neutral.

An El Niño is highly likely to become established during JJA 2023. After JJA 2023, most global climate models predict the El Niño conditions to continue until the end of the year. The strength of the upcoming El Niño is uncertain, although some models are predicting that it could be moderate to strong.

A positive IOD is predicted to become establish during JJA 2023. Most models predict the IOD to return to neutral before the end of 2023.

The onset of the Southwest monsoon season has been or is expected to be later than average for much of Mainland Southeast Asia, while near average for the rest of the region. Over western parts of the region, the strength of the Southwest monsoon is expected to be stronger than average, with weaker than average or near average strength for the rest of the region, based on model predictions and supported by the predicted El Niño conditions.

During JJA 2023, tropical cyclone frequency is predicted to be below to near average around the South China Sea and the Philippine Sea and near average around the Bay of Bengal.

Taking into consideration the national level forecasts, the present state of the climate, and the forecasts available from the GPCs, ASEANCOF-20 agreed on the following consensus-based outlook for JJA 2023 over the ASEAN region:

RAINFALL

Over most of the southern ASEAN region, below- to near-normal rainfall is predicted. The exceptions include northeastern Borneo and eastern Maritime Continent where near- to above-normal rainfall is predicted.

Over much of the northern ASEAN region, a mix of near- to above-normal rainfall is predicted. Above-normal rainfall is predicted over parts of southwestern and southeastern Mainland Southeast Asia, while near- to above-normal rainfall is predicted elsewhere over this region. Above-normal rainfall is predicted for much of the western half of the Philippines, with near- to above-normal rainfall elsewhere¹.

TEMPERATURE

Near- to above-normal temperature is predicted over Southeast Asia¹. Near-normal temperature is predicted over parts of the southern Maritime Continent, including southern Sumatra and Java. Near- to above-normal temperature is predicted over the southwestern and southeastern parts of Mainland Southeast Asia, Malaysia, and the Philippines. Elsewhere in Southeast Asia, above-normal temperature is predicted.

¹This is based on the climatology period 1991-2020. However, at the national level, Myanmar is using the 1981-2010 climatology as their base period.

Refer to Annex A for reference on what is meant by “above, near, or below normal” in the outlook. For more information on the boreal (Northern Hemisphere) summer monsoon outlook and further updates on the national scale, the relevant NMHSs should be consulted (see Annex B).

Consensus Maps for JJA 2023 Season

The following maps provide the probabilistic outlooks for JJA 2023 season in terms of tercile categories of “Above Normal” (AN: upper tercile), “Near Normal (NN: middle tercile) and “Below Normal” (BN: lower tercile).

PROBABILISTIC RAINFALL OUTLOOK

PROBABILISTIC TEMPERATURE OUTLOOK

Meeting Documents

ASEANCOF-20 Outlook Bulletin

Acknowledgements

ASEANCOF would like to convey its appreciation to the NMHSs of the ASEAN Member States for sharing their national level forecasts, the GPCs, the Southeast Asia Regional Climate Centre – Network, RIMES, UN ESCAP, and other partners of ASEANCOF for sharing their products and expertise, and the World Meteorological Organization Regional Office in Asia and the Southwest Pacific (WMO-RAP) for their continued support of ASEANCOF.