ENSO Update

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Bulletin July 2020 - Summary

Issued: 5 August. Next Issue: 19 August.

The El Nino Southern Oscillation (ENSO) outlook has been raised to La Nina WATCH since mid-June 2020. This means there is a 50% chance that a La Nina event could develop towards the end of the southern hemisphere spring (September - November 2020).

Sea temperatures over the tropical Pacific Ocean have continued to cool since April 2020, and are likely to cool further in the coming months.

Atmospheric indicators of ENSO, such as the latest trade winds have been stronger than average while the Southern Oscillation Index (SOI) has increased since the beginning of July. Equatorial cloudiness near the Date Line is below average. These signs are consistent with a developing La Nina.

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Bulletin July 2020 - El Nino Oscillation

The El Nino Southern Oscillation (ENSO) is at La Nina WATCH. The chance of a La Nina forming in 2020 is around 50%.

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 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 (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, 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 (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, remain within the neutral range.

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Bulletin July 2020 - MJO

The Madden-Julian Oscillation (MJO) is over the Maritime Continent but is forecast to weaken over the coming week, therefore its influence on rainfall patterns across Vanuatu is likely to be weak.

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 July 2020 - Cloud

Cloudiness near the Date Line was below average over the past fortnight and has generally been below average since March. Overall, cloud patterns are still within ENSO neutral conditions.

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 July 2020 - Rainfall

Rainfall forecast for August-October 2020:
Above normal rainfall forecasted for the northern region (Torba, Sanma, Penama and Malampa), and normal rainfall forecasted for the southern region (Shefa and Tafea).

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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 July 2020 - SST

The four-month sequence of equatorial Pacific sub-surface temperature anomalies (to July) shows areas of cooling than average water in both the central and eastern equatorial Pacific, between 100 and 200 m in the central equatorial Pacific and the top 100 m depth in the east. Compared to June, the strength and extend of cooler than average water has increased.

Since January, the pattern of cooler anomalies at depth has persisted, providing conditions favorable for potential La Nina development, although July has seen some weakening of these patterns.

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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 July 2020 - MSLP

The mean sea level pressure (MSLP) anomaly map shows normal to weak positive anomalies over Vanuatu.

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 July 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 July 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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