ENSO Update

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Bulletin April 2021 - Summary

Issue: May 2021

The El Niño–Southern Oscillation (ENSO) remains neutral. Climate model outlooks indicate this neutral phase will last at least until September. With little sign of El Niño or La Niña developing, the ENSO Outlook status is INACTIVE.

Oceanic indicators of ENSO persist at neutral levels, with Pacific sea surface temperatures close to average across most of the equatorial region. Beneath the surface, temperatures are also near-average. The Southern Oscillation Index (SOI), an atmospheric indicator of ENSO, has now seen both the 30-day and 90-day SOI return to neutral levels. Similarly, winds and cloudiness near the Date Line persist at levels consistent with a neutral ENSO state.

After stalling in the Pacific over the past week, the Madden–Julian Oscillation (MJO) is currently over tropical America. It is forecast to move eastwards towards Africa during the coming fortnight. At this time of the year, the MJO has no influence over Vanuatu.

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Bulletin April 2021 - El Nino Oscillation

The 2020–21 La Niña is now INACTIVE. ENSO has returned 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 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 April 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 April 2021 - Rainfall

The northern islands still experienced much rainfall in March. Sola, Pekoa and Lamap recorded above normal rainfall. On the other hand, the southern region of Vanuatu experienced below normal to normal rainfall. Some parts of the northern region are still likely to experience above normal rainfall in the coming 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 April 2021 - SST

Top Chart:
The four-month sequence of equatorial Pacific sub-surface temperature anomalies (to 22 April) shows weak cool anomalies remain across the top 50 to 100 m of the central and eastern equatorial Pacific. The strength of these cool anomalies has decreased significantly compared to March, in line with the return to a neutral ENSO state.

Warm anomalies persist across parts of the column depth west of the Date Line. These warm anomalies have decreased in strength and spatial extent compared to March.

Bottom Chart:
For the five days ending 25 April, sub-surface temperatures were close to average across most of the equatorial Pacific.

Warm anomalies persist in a small region between around 100 and 200 m depth in the western equatorial Pacific, reaching more than three degrees above average near the western edge of the basin. These warm anomalies are stronger than they were two weeks ago. On the whole, the sub-surface is consistent with an ENSO neutral state.

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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 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 April 2021 - Model Outlooks

The El Niño–Southern Oscillation (ENSO) is currently neutral. All of the international climate models surveyed by the Bureau anticipate NINO3.4 will remain neutral until at least September.

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Bulletin April 2021 - SPCZ

A large convective region associated with the monsoon and Western Pacific Warm Pool, was centred over the Maritime Continent (A). The ITCZ extended eastwards out from this convective region across the northern tropical Pacific (B), while the SPCZ extended from the PNG Islands eastwards to the area north of Fiji, then southeast across Tonga to a region well south of the Southern Cook Islands (C).

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