ENSO Update

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Bulletin September 2021 - Summary

Issue: October 2021

The El Nino Southern Oscillation (ENSO) is at La Niña ALERT. This is due to continued cooling in the tropical Pacific Ocean and climate models showing sustained La Niña conditions over December - February. La Niña events increase the chances of above-average rainfall for Vanuatu.

La Niña Indicators:
1) Sea Surface Temperatures in the central tropical Pacific Ocean have cooled over the past three months, edging closer to La Niña levels. These cooler waters are supported by cooler than average eaters beneath the surface.
2) Southern Oscillation Index (SOI) is approaching La Niña levels.
3) Cloudiness near the Date Line is also approaching La Niña levels.
4) Trade Winds were slightly stronger than average across the far western tropical Pacific, and close to average in the central and eastern tropical Pacific.

Most international Climate models indicate weak La Niña conditions are likely for the coming months.

The Madden-Julian Oscillation (MJO) is currently weak or indiscernible. Most models indicate the MJO is likely to remain weak over the coming week to fortnight. When the MJO is weak or indiscernible, it has little influence on tropical climate.

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Bulletin September 2021 - El Nino Oscillation

 ENSO Outlook is at La Niña ALERT. This means there is a 70% chance for La Niña to develop in the coming months.

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, extreme below normal rainfall which results in drier conditions 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 normal rainfall which results in extreme wetter conditions 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. Normal climate conditions are experienced over Vanuatu during Neutral ENSO phase.

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Bulletin September 2021 - MJO

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 rainfall related to 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 September 2021 - Cloud

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 September 2021 - Rainfall

With the La Nina-like conditions, the country is most likely to experience above normal rainfall within the next three months, November to January 2022.

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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 September 2021 - SST

5-day sub-surface temperatures:

For the five days ending 24 October 2021, sub-surface temperatures were close to average across most of the equatorial Pacific. A volume of cooler than average water is present between 50 and 150 m depth in the eastern equatorial Pacific, reaching more than four degrees cooler than average between 135°W to 115°W. The strength of these cool anomalies has increased compared to two weeks ago.

Overall, sub-surface temperatures remain consistent with an ENSO-neutral state. However, if this volume of cooler than average water continues to shoal (rise towards the surface) and progress eastward, it is likely to lead to further cooling of surface waters in the eastern equatorial Pacific Ocean, and may support further development of La Niña.

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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 September 2021 - MSLP

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 September 2021 - Model Outlooks

All of the seven international climate models surveyed by the Bureau anticipate further cooling of tropical Pacific sea surface temperatures. All models surveyed indicate La Niña thresholds may be met during November. Five of the seven models indicate this cooling will be sustained at La Niña levels until at least January 2022—long enough to meet minimum La Niña event criteria (i.e. at least three months). Four models continue the event into February, but by March only one model continues to meet the threshold.

La Niña typically enhances spring rainfall in northern and eastern parts of Vanuatu, and the shift towards cooler forecast values of NINO3.4 may be contributing to the wetter than average rainfall outlooks for parts of the country even if a full-fledged event does not eventuate.

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Bulletin September 2021 - SPCZ

The Intertropical Convergence Zone (ITCZ) was active around the western and central Pacific, while the South Pacific Convergence Zone (SPCZ) was generally suppressed except for some activity between eastern PNG and western Solomon Islands.

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