ENSO Update

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Bulletin November 2020 - Summary

Issued: December 2020

"La Niña continues, cooling in the tropical Pacific Ocean returns"

La Niña continues in the tropical Pacific. International climate models suggest it is likely to continue at least into February 2021.

Central and eastern Pacific Ocean sea surface temperatures (SSTs) are at La Niña levels. Models suggest the current La Niña will strengthen further, peaking in December 2020 or January 2021 at moderate to strong levels.

Most oceanic and atmospheric indicators reflect a mature La Niña. Recent variability in the Southern Oscillation Index have been related to the Madden-Julian Oscillation (MJO), rather than the state of the La Niña.

Climate outlooks indicate December 2020 to February 2021 rainfall is likely to be above average for most of the country. Current La Niña conditions, though not as strong as 2010-12, warmer than average waters over Vanuatu, climate change are contributing to the increased chances of rainfall over Vanuatu.

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Bulletin November 2020 - El Nino Oscillation

La Niña continues in the tropical Pacific. International climate models suggest it is likely to continue at least into February 2021.

Central and eastern Pacific Ocean sea surface temperatures (SSTs) are at La Niña levels. Models suggest the current La Niña will strengthen further, peaking in December 2020 or January 2021 at moderate to strong levels.

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 November 2020 - MJO

The Madden-Julian Oscillation (MJO) is over the eastern Indian Ocean. It is expected to become weak or indiscernible as it moves from the Indian Ocean into the Maritime Continent. The MJO is expected to move into the Australian region during early December.

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 November 2020 - Cloud

Cloudiness near the Date Line was below average over the past fortnight and has generally been below average since early to mid-March.

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 November 2020 - Rainfall

Rainfall has been above-normal across much of the country for the past three months, that is from August to October 2020. The La Niña condition across the region has greatly influenced rainfall patterns for these past three months.
Vanuatu is expected to experience above-normal rainfall over the next three months, from December 2020 to February 2021.

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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 November 2020 - SST

Top Chart:
The four-month sequence of equatorial Pacific sub-surface temperature anomalies (to 19 November) shows cooler than average water extending across the top 200 m of the sub-surface of the equatorial Pacific from around the Date Line and eastward across the basin. The strength and extent of cooler than average water has increased month-on-month since July.

Weak warm anomalies persist across large parts of the column depth in the far western equatorial Pacific. Compared to October, these warm anomalies now extend further eastward at depth, covering the area around the Date Line below 150 m depth.

Bottom Chart:
For the five days ending 22 November, sub-surface temperatures were below average in the eastern equatorial Pacific, reaching more than 3 degrees below average in a region between 140°W and 110°W at 50 to 150 m depth. These cool anomalies cover a larger region than they did two weeks ago. In the west, weak warm anomalies, reaching more than 2 degrees above average, extend between about 100 to 150 m depth west of 170°W.

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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 November 2020 - 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).

Mean Sea Level Pressure over Vanuatu within the past month is normal.

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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 November 2020 - Model Outlooks

All international climate models surveyed by the Vanuatu Meteorology and Geo-hazards Department indicate the current La Niña will likely persist until at least February 2021. Most climate models reach their peak in December, with some peaking in January. Six of eight models indicate thresholds will still be met in March, although all six show central Pacific sea surface temperatures much declined from the summer peak.

While some models indicate that the current La Niña could possibly reach similar strength to the La Niña of 2010-21, La Niña conditions are currently weaker than at the same point in 2010. Sea surface temperatures in the central and eastern tropical Pacific are the coolest since the end of the La Niña event in 2012, but they are not as cool as during spring or early summer 2010.

La Niña increases likelihood of above average rainfall across much of Vanuatu. La Niña increases the chance of cooler than average daytime temperatures for large areas. It also increases the chance of tropical cyclones, and earlier first rains of the northern wet season.

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Bulletin November 2020 - SPCZ

The Intertropical Convergence Zone (ITCZ) was shifted to the north, while the South Pacific Convergence Zone (SPCZ) was more active east of Fiji.

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