ENSO Update

Select Issue:
Bulletin September 2021 - Summary

Issue: September 2021

The El Nino Southern Oscillation (ENSO) remains neutral with most oceanic and atmospheric indicators within the neutral range. While pressure patterns show some La Nina-like characteristics as indicated by the latest Southern Oscillation Index (SOI) 30-day value of +15.1, it is likely that some of this shift in pressure is driven from warm conditions in the eastern Indian Ocean. Most climate model outlooks indicate the central tropical Pacific is likely to cool in the coming months, with three of seven models indicating this cooling will be enough to reach La Nina thresholds by September to November 2021, with the remaining four models staying neutral.

The negative Indian Ocean Dipole (IOD) has become ACTIVE in July and continues to influence rainfall for much of the southern and eastern parts of Australia, although this may have little to no effect over the islands of Vanuatu.

The Madden-Julian Oscillation (MJO) is currently in the Western Hemisphere and has no influence on rainfall over Vanuatu at this time.

Print this part
Bulletin September 2021 - El Nino Oscillation

Although the El Nino Southern Oscillation (ENSO) remains neutral (meaning there is currently no El Nino or La Nina active), two out of seven models indicate a La Nina event could be possible around September to November, while the remaining four models indicate a neutral state.

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, extreme below normal rainfall which results in drier conditions 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 normal rainfall which results in extreme wetter conditions 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.

Print this part
Bulletin September 2021 - MJO

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 rainfall related to an MJO event when it moves over the Maritime Continent on the diagram.

Large view

Source

Large view

Source

Large view

Source

Note: Due to frequent images update from source provider, the quoted Sources might not give the same images as shown

Print this part
Bulletin September 2021 - 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.

Equatorial cloudiness near the Date Line typically increases during El Niño (negative OLR anomalies) and decreases during La Niña (positive OLR anomalies). 

Large view

Source

Large view

Source

Note: Due to frequent images update from source provider, the quoted Sources might not give the same images as shown

Print this part
Bulletin September 2021 - Rainfall

The country is most likely to experience normal rainfall in August, while above normal rainfall is favored for September and October 2021.

Large view

Source

Large view

Source

Large view

Source

Note: Due to frequent images update from source provider, the quoted Sources might not give the same images as shown

Print this part
Bulletin September 2021 - SST

5-day sub-surface temperatures:

For the five days ending 29 August, sub-surface temperatures were close to average across the equatorial Pacific.

The subsurface remains consistent with an ENSO-neutral state.

Large view

Source

Large view

Source

Note: Due to frequent images update from source provider, the quoted Sources might not give the same images as shown

Print this part
Bulletin September 2021 - 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).

Large view

Source

Note: Due to frequent images update from source provider, the quoted Sources might not give the same images as shown

Print this part
Bulletin September 2021 - Model Outlooks

The El Niño–Southern Oscillation (ENSO) is currently neutral. Most of the international climate models surveyed by VMGD anticipate tropical Pacific sea surface temperatures will cool during the coming months, but vary in how much cooling they expect.

Four of the seven surveyed models anticipate a continued ENSO-neutral state for the coming months, despite some cooling.

Two of the seven surveyed models sustain NINO3.4 at La Niña thresholds for at least three months during spring to early summer (i.e. long enough to indicate a full-fledged event), with one more anticipating La Niña thresholds are likely to be exceeded during November and December.

La Niña typically enhances spring rainfall in northern and eastern parts of Vanuatu, and the shift towards cooler forecast values of NINO3.4 may be contributing to the wetter than average rainfall outlooks for parts of the country even if a full-fledged event does not eventuate.

Print this part
Bulletin September 2021 - SPCZ

The South Pacific Convergence Zone (SPCZ) was displaced south of its climatological normal position. The Intertropical Convergence Zone (ITCZ) was close to its climatological normal position.

Print this part