ENSO Update

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Bulletin April 2022 - Summary

Issue: April 2022

The 2021–22 La Niña event continues, despite some weakening over recent weeks. Climate outlooks continue to indicate a return to neutral El Niño–Southern Oscillation (ENSO)—neither La Niña nor El Niño—during the late April to May. Even as La Niña weakens, it will continue to influence weather and climate over the country.

Atmospheric and most oceanic indicators of ENSO persist at La Niña levels. Sea surface temperatures remain cooler than average along the equator. Compared to two weeks ago, surface waters have cooled slightly in the eastern half of the equatorial Pacific. Trade winds remain stronger than average in the western Pacific. Other atmospheric indicators also remain at La Niña levels, with decreased cloudiness persisting along the Date Line and a positive Southern Oscillation Index (SOI).

The Madden–Julian Oscillation (MJO) has been weak or indiscernible for the past fortnight. Climate models indicate the MJO is likely to remain weak, having little influence on tropical weather and climate in the coming fortnight.

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Bulletin April 2022 - El Nino Oscillation

La Niña remains active in the tropical Pacific Ocean, but is weakening. Autumn (March to May) is the usual time of the year in which ENSO events decay and return to neutral.

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 April 2022 - 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 April 2022 - 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 April 2022 - Rainfall

With the La Nina-like conditions established, the country is most likely to experience above normal rainfall within the next three months, May to July 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 April 2022 - SST

5-day sub-surface temperatures:

For the five days ending 9 April 2022, sub-surface temperatures were warmer than average in the western equatorial Pacific, reaching more than 3 degrees warmer than average between 150 m and 200 m depth in a small region west of the International Date Line. Weak warm anomalies are also present just below the surface of the eastern edge of the equatorial Pacific. The slightly cool anomalies present in the central equatorial Pacific below the surface persist, although this region only has a small area cooler than 2 degrees below average.

The emergence and strengthening of warm anomalies in the sub-surface of the eastern equatorial Pacific typically foreshadow the breakdown of a La Niña event.

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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 April 2022 - 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 April 2022 - Model Outlooks

La Niña is active in the tropical Pacific Ocean, but is past its peak. Autumn (March - May) is the usual time of the year in which ENSO events decay and return to neutral.

Most of the seven international climate models surveyed by VMGD anticipate the strength of the La Niña will ease over the next three months, with a return to neutral ENSO conditions (neither El Niño nor La Niña) in mid-autumn (around April).

Five of seven models surveyed indicate sea surface temperatures in the central equatorial Pacific will meet or exceed La Niña thresholds during May. By June two models continue to exceed the threshold, and by July only one.

La Niña increases the chance of above average rainfall across much of the country. It is important to note that significant weather can still occur as La Niña comes to an end, especially as we approach the peak of the tropical cyclone season.

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Bulletin April 2022 - SPCZ

The Intertropical Convergence Zone (ITCZ) was active over the central and western equatorial Pacific, while the South Pacific Convergence Zone (SPCZ) was active shifted southwest in the western Pacific around Vanuatu, Fiji and Tonga.

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