ENSO Update

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Bulletin October 2020 - Summary

Issued: November 2020

"La Niña is likely to continue through December 2020 to February 2021"

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

Central and eastern tropical Pacific Ocean sea surface temperatures remain at La Niña levels, as do most atmospheric indicators including trade winds and cloudiness. The Southern Oscillation Index (SOI) has moved back into neutral values, most likely due to the influence of a passing MJO event. The SOI is expected to return to La Niña levels in the coming weeks.

La Niña typically brings above normal rainfall over much of Vanuatu during its driest months (August, September, October). Current climate outlooks indicate rainfall during November 2020 to January 2021 is likely to be above average for most of the country.

Most models suggest La Niña will peak in December, with around half the models anticipating a strong event, hence the strength of the La Niña is likely to be moderate to strong. While there is some possibility that the peak strength could reach levels similar to the 2010-12 La Niña, there are some differences. The 2010-12 La Niña became established earlier in 2010 compared to the late development of the 2020 La Niña. Impacts of the 2010-12 La Niña were enhanced by a negative Indian Ocean Dipole (IOD) and warm sea surface temperatures in the western Pacific. In contrast, the IOD is currently neutral and is likely to remain neutral for the rest of 2020.

The Madden-Julian Oscillation (MJO) is in the western Pacific Ocean, and is expected to remain at moderate strength as it moves across the Pacific Ocean. At this stage, it has no influence over rainfall in Vanuatu.

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Bulletin October 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 tropical Pacific Ocean sea surface temperatures remain at La Niña levels, as do most atmospheric indicators including trade winds and cloudiness. The Southern Oscillation Index (SOI) has moved back into neutral values, most likely due to the influence of a passing MJO event. The SOI is expected to return to La Niña levels in the coming weeks.

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.

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Bulletin October 2020 - MJO

The Madden-Julian Oscillation (MJO) is in the western Pacific Ocean, and is expected to remain at moderate strength as it moves across the Pacific Ocean. At this stage, it has no influence over rainfall in 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 October 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 October 2020 - Rainfall

Rainfall has been normal to above-normal across much of the country for the past three months although the country is going through its Dry Season. The La Niña condition across the region has significantly influenced recent rainfall patterns over the country.
Vanuatu is expected to experience above-normal rainfall over the next three months, from November 2020 to January 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 October 2020 - SST

Top Chart:
The four-month sequence of equatorial Pacific sub-surface temperature anomalies (to September) shows cooler than average water extending across the top 200 m of the sub-surface of the equatorial Pacific around and east of the Date Line. The strength and extend of cooler than average water has increased month-on-month compared to both August and July.

Weak warm anomalies persist across large parts of the column depth in the western equatorial Pacific.

Bottom Chart:
For the five days ending 11 October, sub-surface temperatures were cooler than average in parts of the central to eastern equatorial Pacific, reaching more than 3 degrees cooler than average in a small region around 140°W between 100 and 150 m depth. These cool anomalies have weakened slightly compared to two weeks ago. Temperatures are close to average in most of the sub-surface of the equatorial Pacific Ocean.

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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 October 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 October 2020 - Model Outlooks

All of the international climate models surveyed by VMGD indicate the current La Niña will persist until at least January 2021. Most models reach their peak in December, and all but one indicate thresholds will still be met in February.

La Niña increases the 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 wet Season.

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Bulletin October 2020 - SPCZ

The South Pacific Convergence Zone (SPCZ) extended from the PNG Islands eastwards across the Solomon Islands, northern Vanuatu and Tuvalu to Samoa. The SPCZ looked rather weak east of Samoa.

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