ENSO Update

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Bulletin November 2021 - Summary

Issue: December 2021

La Niña has become established in the Pacific. The El Niño Southern Oscillation (ENSO) outlook has been raised to La Niña. Climate models suggest this La Niña will be short-lived, persisting until early 2022. La Niña events increase chances of above average rainfall across much of the country.

Several indicators of ENSO now show clear La Niña patterns. Sea surface temperatures in the tropical Pacific are close to La Niña thresholds ,with climate model outlooks expecting them to cool further. In the atmosphere, cloud and wind patterns are typical of La Niña, indicating the atmosphere is now responding to, and reinforcing, the changes observed in the ocean.

Model outlooks indicate that this La Niña will be short-lived and of weak to moderate strength, possibly ending in January – March 2022. Regardless whether this event will be short-lived, the presence of La Niña-like patterns increase the chances of above average rainfall in Vanuatu.

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Bulletin November 2021 - El Nino Oscillation

ENSO Outlook is at La Niña.

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 November 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 November 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 November 2021 - Rainfall

With the La Nina-like established, the country is most likely to experience above normal rainfall within the next three months, December 2021 to February 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 November 2021 - SST

5-day sub-surface temperatures:

For the five days ending 21 November 2021, sub-surface temperatures were close to average across most of the equatorial Pacific. Due to the sparsity of observations in the eastern half of the equatorial Pacific (point observations are indicated by Xs on the sub-surface temperature plot), it is hard to be confident as to what has occurred with the volume of cooler than average water present below the surface. However, cooling of surface waters would indicate it is likely this relatively cool sub-surface water has risen closer to the surface, with shoaling in the eastern equatorial Pacific contributing to surface level changes.

In the west a volume of slightly warmer than average water persists, reaching more than two degrees warmer than average between 100 and 150 m depth between the Date Line and the west of the basin. The strength of these warm anomalies remains similar to two weeks ago.

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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 November 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 November 2021 - Model Outlooks

La Niña is active in the tropical Pacific Ocean.

Over recent weeks coupling has been seen between the ocean and the atmosphere, meaning cloud and wind patterns in the atmosphere are responding to and reinforcing changes in ocean temperatures below. Once this positive feedback loop is established, it is what sustains these changes in ocean and atmosphere and helps maintain La Niña conditions for an extended period.

All seven of the international climate models surveyed by VMGD anticipate further cooling of tropical Pacific sea surface temperatures. All models surveyed indicate La Niña thresholds are likely to be met during December and January. Four models continue the event into February, but by March only one model continues to meet the threshold.

In general, the models anticipate that this will be short-lived, and of weak to moderate strength though there is some variation in predicted peak strength between the various models. For 2021–22 to be considered a La Niña year, the event will have to be sustained for at least three months.

Regardless of whether La Niña thresholds are sustained for three months or for a shorter period, the presence of La Niña-like patterns in the Pacific increases the chances of above-average rainfall for northern and eastern Australia during spring and the coming summer.

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Bulletin November 2021 - SPCZ

In July there was significantly enhanced convection over the Philippines, Indonesia, Palau, western Federated States of Micronesia, northern Papua New Guinea and northern Solomon Islands. The Intertropical Convergence Zone (ITCZ) further east was close to its average July position but marginally suppressed near the Date Line. In the south Pacific, the South Pacific Convergence Zone (SPCZ) extended further east than normal and was enhanced over northern Papua New Guinea and northern Solomon Islands. Convection was suppressed in the vicinity of New Caledonia, Vanuatu, Fiji, Samoa and the southern Cook Islands.

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