ENSO Update

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Bulletin August 2020 - Summary

Issued: 2 September.

Recent cooling of the surface of the tropical Pacific Ocean, changes in weather patterns, and continued ocean cooling forecast by climate models suggest La Niña could become established in spring 2020.

The ENSO outlook remains at La Niña ALERT. This means the chance of La Niña forming in 2020 is around 70%.

While most key indicators remain within ENSO-neutral range, there have been further signs of La Niña development in the past fortnight. The central tropical Pacific Ocean has continued to cool and trade winds remain stronger than average, while the Southern Oscillation Index (SOI) has exceeded La Niña thresholds in recent days. Equatorial cloudiness near the Date Line also remains below average.
All of the surveyed international climate models anticipate further cooling of the tropical Pacific Ocean.

La Niña typically increase the chance of above normal rainfall across much of Vanuatu.

The Madden-Julian Oscillation (MJO) is currentlyin the Indian Ocean, and is expected to become weak as it approaches the Pacific.

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Bulletin August 2020 - El Nino Oscillation

The El Niño Southern Oscillation (ENSO) remains at Niña ALERT. The chance of La Niña forming in 2020 has increased to 70%. Most climate models indicate the La Niña threshold could be met by October 2020, and is likely to persist into December.

El Niño Southern Oscillation (ENSO) is a single climate feature that has three states: El Niño, La Niña, and Neutral.

During an El Niño, sea surface temperatures in the Tropical Pacific Ocean, both at the surface and the sub-surface tends to WARM. Surface pressure changes across the Pacific; higher in the west, and lower in the east. Trade winds weaken and sometimes reverse. Cloudiness increases near the Date Line. El Niño events tend to develop in Autumn to Winter (March - August), and starts to decay in Summer (December - February). In Vanuatu, below normal rainfall with warmer daytime temperatures are associated with El Niño.

During a La Niña, sea temperatures in the tropical Pacific Ocean, both at the surface and the sub-surface tends to COOL. Surface pressure changes across the Pacific; higher in the east, and lower in the west. Trade winds are much stronger than normal. Cloudiness decreases near the Date Line. La Niña events also tend to develop in Autumn to Winter (March - August), and finishes the following Autumn (March - May). In Vanuatu, above average rainfall with cooler temperatures are associated with La Niña.

During a Neutral phase, all ENSO indicators (Sea surface temperatures, Southern Oscillation Index (SOI), trade winds, and cloudiness near the Date Line) in the tropical Pacific Ocean, remain within the neutral range.

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Bulletin August 2020 - MJO

The Madden-Julian Oscillation (MJO) is currently over the Indian Ocean, and is expected to become weak or indiscernible in the coming weeks.

What is an MJO (Madden-Julian Oscillation)?
An MJO is a group of cloud and rain that circulates the globe near the equator, and usually occurs every 30-60 days. When an MJO passes over an area, it brings heavy rain with varying wind speed. During cyclone season, an MJO can help develop tropical cyclones by fueling any overlying low pressure systems.

The diagram below shows the track of the MJO for the past 40 days (coloured lines). [Click to enlarge the diagram]. When the lines are within the circle, MJO is weak and has no influence on rainfall. Outside the circle, the MJO is active and will usually move in an anti-clockwise direction. Vanuatu will most likely experience related rainfall from an MJO event when it moves over the Maritime Continent on the diagram.

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Note: Due to frequent images update from source provider, the quoted Sources might not give the same images as shown

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Bulletin August 2020 - Cloud

Cloudiness near the Date Line was below average over the past fortnight and has generally been below average since March.

The above maps show regions experiencing more or less cloudiness. The top map is the total outgoing long-wave radiation (OLR), and the bottom map is the cloud anomaly. Outgoing longwave radiation (OLR) is often used as a way to identify tall, thick, convective rain clouds. This means, a region which experiences lower outgoing radiation is an indication of more cloudiness over the area.

The purple shading indicates higher than normal, active or enhanced tropical weather (above normal cloudiness), while brown shading indicates lower than normal cloud or suppressed conditions.

Equatorial cloudiness near the Date Line typically increases during El Niño (negative OLR anomalies) and decreases during La Niña (positive OLR anomalies). 

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Note: Due to frequent images update from source provider, the quoted Sources might not give the same images as shown

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Bulletin August 2020 - Rainfall

Rainfall forecast for September to November 2020:
Above normal rainfall is forecasted for Vanuatu in the next three months.

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Note: Due to frequent images update from source provider, the quoted Sources might not give the same images as shown

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Bulletin August 2020 - SST

The four-month sequence of equatorial Pacific sub-surface temperature anomalies (to 27 August) shows cooler than average water extending across the top 150 m of the sub-surface of most of the equatorial Pacific, extending eastward from around 160°E. Compared to July, the strength and extend of cooler than average water has increased.

Weak warm anomalies persist across large parts of the column depth west of the Date Line.

Since January, the pattern of cooler anomalies at depth has persisted, providing conditions favorable for potential La Niña development. Renewed cooling in August has reinforced these patterns.

For the five days ending 30 August, sub-surface temperatures were cooler than average in the central to eastern equatorial Pacific between 50 and 150 m. The strength of cool anomalies in the sub-surface has decreased slightly compared to two weeks ago, reaching up to 3 degrees cooler than average around 150°W. Elsewhere in the sub-surface of the equatorial Pacific Ocean temperatures were close to average.

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Note: Due to frequent images update from source provider, the quoted Sources might not give the same images as shown

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Bulletin August 2020 - MSLP

The mean sea level pressure (MSLP) over Vanuatu has slightly dropped by -1 over Vanuatu in the past month.

Regions of HIGH pressure anomalies (brown shades) are associated with lower rainfall, while higher rainfall are associated with regions of LOW pressure anomalies (purple shades).

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Note: Due to frequent images update from source provider, the quoted Sources might not give the same images as shown

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Bulletin August 2020 - Model Outlooks

All the international climate model indicate central tropical Pacific sea surface temperatures will cool further in the coming months. Three of eight models exceed the La Niña threshold during September, with two more models surpassing the threshold in October. For November and December, six out of eight models indicate La Niña is most likely.

ENSO events—El Niño or La Niña—typically begin to develop during the southern hemisphere autumn to winter, before strengthening in winter to spring. 

La Niña typically bring above normal rainfall to most of Vanuatu.

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Bulletin August 2020 - SPCZ

The inter-tropical Convergence Zone (ITCZ) was suppressed in the western Pacific and displaced northwards in the central and eastern Pacific. The South Pacific Convergence Zone (SPCZ) was also suppressed and displaced south over the Solomon Islands.

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