ENSO Update

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Bulletin February 2021 - Summary

Issued: February 2021

"La Niña has passed peak, but influence expected to continue"

The 2020–21 La Niña has passed its peak, but its influence is expected to continue. All El Niño Southern Oscillation (ENSO) indicators still remain within the La Niña threshold. Climate model outlooks indicate the ENSO will return to neutral during Autumn (March to May 2021). The wetter influence from La Niña is likely to continue for the shorter term, with three-month climate outlooks indicating above normal rainfall is still likely for some parts of the country.

The Madden-Julian Oscillation (MJO) is currently located over the central Pacific Ocean and is expected to move eastwards during the coming fortnight towards the tropical Americas. Heavy rain associated with MJO is less likely for Vanuatu in the coming weeks.

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Bulletin February 2021 - El Nino Oscillation

The 2020–21 La Niña is likely to have peaked with respect to atmospheric and oceanic patterns in the tropical Pacific.

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. Normal climate conditions are experienced over Vanuatu during Neutral ENSO phase.

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Bulletin February 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 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 February 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 February 2021 - Rainfall

Vanuatu experienced extreme rainfall in January 2021. Reports flooding over low lying areas, bridges/rivers, and landslides were seen in some parts of the country. Whitegrass has experienced its wettest January in 50 years of record. All other stations also recorded above normal rainfall in January 2021. Rainfall outlook for the next three months (March - May 2021) shows the Vanuatu group is still likely to experience above normal rainfall.

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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 February 2021 - SST

Top Chart:
The four-month sequence of equatorial Pacific sub-surface temperature anomalies (to 31 January) shows cooler than average water extending across the top 200 m of the sub-surface of the equatorial Pacific east of around 160°E. The strength and spatial extent of cooler than average water has remained fairly consistent since September, with some strengthening in January.

Warm anomalies persist across large parts of the column depth west of the Date Line, with warm anomalies underlying the shallower cool anomalies between the Date Line and 160°E. These warm anomalies have strengthened month-on-month since October.


Bottom Chart:
For the five days ending 14 February, sub-surface temperatures were above average between around 100 and 250 m depth west of the Date Line, reaching more than three degrees above average around 150°E and 150 m depth. In the eastern half of the equatorial Pacific, cool anomalies persist across the top 150 m, reaching four degrees cooler than average in a small region around 140°W and 100 m depth. Anomalies in both the west and east have strengthened compared 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 February 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 February 2021 - Model Outlooks

The 2020-21 La Niña has passed its peak, with most of the Climate Models surveyed by VMGD anticipating sea surface temperatures in the central and eastern Pacific will return to neutral values by mid-autumn (March - May). It is typical for La Niña to continue to influence Vanuatu climate, even as the La Niña weakens.

La Niña increases the likelihood of above-average rainfall across much of the northern and eastern parts of the country. It also increases the chance of tropical cyclones and earlier first rains of the wet season, as we saw this season.

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Bulletin February 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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