ENSO Update

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Bulletin October 2020 - Summary

Issued: October 2020

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

As the La Niña is currently underway in the tropical Pacific, all surveyed international climate models surveyed by VMGD indicate this La Niña will persist through the southern hemisphere summer, this includes the months of December 2020, into February 2021.

Most models suggest the La Niña will strengthen, peaking in December. Around half the models anticipate a strong event, meaning there is a possibility it could reach similar strength to the La Niña of 2010-12. However, models forecast this event will be shorter, possibly ending in the first quarter of 2021. The strength of the La Niña impacts on Vanuatu are often related to the strength of the event. La Niña typically brings above-normal rainfall over the country.

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Bulletin October 2020 - El Nino Oscillation

The El Niño Southern Oscillation (ENSO) has now reached La Niña. This La Niña event is expected to strengthen over the coming months before peaking in December. The strength of the event could possibly reach the 2010-12 La Niña event, meaning it is likely to be a moderate - strong event. This event however, is expected to be short-lived, meaning it should end within the first few months of 2021.

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 currently in the Maritime Continent and is expected to increase in strength as it moves into the Western Pacific Ocean. At this time of the year, the MJO may increase the likelihood of above average rainfall over Vanuatu as it passes through the western Pacific.

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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Source

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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Source

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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