ENSO Update

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Bulletin June 2020 - Summary

Issued: 8 July 2020. Next Issue: 22 July 2020.

The El Niño–Southern Oscillation (ENSO) remains neutral. The ENSO Outlook is at La Niña WATCH, indicating the chance of La Niña forming in 2020 is around 50%—roughly double the average likelihood.

Other ENSO indicators, such as the Southern Oscillation Index (SOI), trade winds, and cloudiness near the Date Line, are remain consistent with a neutral ENSO state.

The Madden–Julian Oscillation (MJO) is not currently having any widespread influence on rainfall patterns across Vanuatu.

Cloudiness was above normal over the country in June.

Vanuatu to experience above normal to normal rainfall from August - October.

The surface and sub-surface of the tropical Pacific Ocean have cooled over recent months. While the cooling trend has eased compared to two weeks ago, over half of the surveyed international climate models anticipate this cooling will approach or exceed the threshold for La Niña during spring.

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Bulletin June 2020 - El Nino Oscillation

The El Niño Southern Oscillation (ENSO) Outlook is at La Niña WATCH, indicating the chance of La Niña forming in 2020 is around 50%—roughly double the average likelihood.

El Niño Southern Oscillation (ENSO) is a single climate feature that has three states: El Niño, La Niña, and Neutral.

During an El Niño, temperatures in the tropical Pacific Ocean, both at the surface and below 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 (Transition Phase from Wet Season to Dry Season), and starts to decay in Summer (December - February). In Vanuatu, below normal rainfall with warmer temperatures are associated with El Niño.

During a La Niña, temperatures in the tropical Pacific Ocean, both at the surface and below 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 (Transition Phase from Wet Season to Dry Season), 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, are neutral.

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Bulletin June 2020 - MJO

A moderately strong pulse of the Madden-Julian Oscillation (MJO) is currently located around the east coast of Africa. It is forecast to track eastwards and weaken in strength over the next fortnight. The MJO is not currently having any widespread influence on rainfall patterns across Vanuatu.

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 June 2020 - 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.

Cloudiness near the Date Line was below average over the past fortnight and has generally been below average since early to mid-March. Overall, cloud patterns are consistent with neutral ENSO 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 June 2020 - Rainfall

Rainfall has been above normal over the northern islands to below normal over the southern islands in June. Vanuatu to expect above normal to normal rainfall over the next three months (August - October 2020).

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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 June 2020 - SST

The four-month sequence of equatorial Pacific sub-surface temperature anomalies (to June) shows cooler than average waters extend in a band across the majority of the equatorial Pacific, between about 100 and 200 m in the central equatorial Pacific and rising to the top 100 m depth at the eastern edge of the equatorial Pacific. 

Compared to May, the cooler than average waters have contracted towards the surface and into the eastern half of the equatorial Pacific while the very weak warm anomalies in the west have spread slightly in extent. 

Since January, the pattern of cooler anomalies at depth has persisted, providing conditions favourable for potential La Niña development. 

Top Image - Monthly Sub-surface Temperatures:
This diagram shows temperatures below the sea surface along the equator. During an El Nino, extreme warming of sub-surface temperatures will be evident in the eastern Pacific (RED color will shift and intensify towards the right side of the diagram). While during a La Nina, extreme cooling of sub-surface temperatures will be evident in the eastern Pacific (BLUE color will shift and intensify towards the right side of the diagram). On the graph, Vanuatu is positioned between 166E and 170E.

Bottom Image - Weekly Sub-surface Temperatures:
This diagram shows sub-surface temperatures for the latest 5 days.

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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 June 2020 - MSLP

The June mean sea level pressure (MSLP) anomaly map shows weak positive anomalies over Vanuatu, with the northern region experiencing +1 anomalies and the southern region experiencing +2 anomalies.

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 June 2020 - Model Outlooks

Most international climate models surveyed by VMGD indicate central tropical Pacific sea surface temperatures in the NINO3.4 region will cool in the coming months. Two of the eight surveyed models reach the La Niña threshold during August, with three more models approaching thresholds in September and November. The other three models remain more clearly at neutral levels. 

ENSO events—El Niño or La Niña—typically begin to develop during the southern hemisphere autumn to winter, before strengthening in winter to spring. 

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Bulletin June 2020 - SPCZ

A convective region was centred just north of the equator, with the strongest intensity near 140°E (A), a little west of its April location. The ITCZ extended eastwards out from this convective region across the northern tropical Pacific (B), while the SPCZ was very weak and fragmented, being mostly contained west of the Date Line (C). An anomalous region of cloud and rain was situated between 20°S and 40°S from 160°E to 150°W.

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