ENSO Update

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Bulletin April 2019 - Summary

ENSO Outlook has been downgraded to El Nino WATCH, meaning the chance of El Nino developing in 2019 is approximately 50%; which is still double the normal likelihood.

A moderate to strong pulse of Madden-Julian Oscillation has tracked eastwards across the Maritime Continent and the Western Pacific during the last fortnight. The MJO currently lies on the other side of the planet, hence has no direct influence on rainfall in Vanuatu at this time.

Warm pool convection was displaced to the east from its normal position in April while the ITCZ was stronger than normal. The SPCZ was displaced to the southwest, influencing rainfall in parts of Vanuatu in April.

Sea Level was higher than normal in the equatorial Pacific and most of the south Pacific. Vanuatu experienced near normal sea level in April.

Rainfall for April 2019 was normal to above normal across Vanuatu. Sola, Lamap and Aneityum recorded above normal rainfall, with Lamap and Aneityum being their fourth and third-wettest April on record respectively.
The remaining stations throughout Vanuatu recorded normal rainfall.

Eight tropical cyclones (Liua, Owen, Penny, Mona, Neil, Oma, Pola and Travor) formed in the 2018-19 south Pacific cyclone season, including late-season cyclone activities such as TC Ann. The long-term average for the period 1969-70 to 2017-18 is nine cyclones. Although the official cyclone season ended in 30 April 2019, historical data has shown cyclones can form outside the normal cyclone season, especially if an El Nino event exist.



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Bulletin April 2019 - El Nino Oscillation

Although indicators have been close to El Nino thresholds over the past several months, recent signs have emerged of a weakening of these patterns. As a result, the Australian Bureau of Meteorology has downgraded the ENSO Outlook to El Nino WATCH. This means the chance of El Nino developing in 2019 is approximately 50%, which is still double the normal likelihood.

While sea surface temperatures in the tropical Pacific Ocean remain close to El Nino levels, water below the surface of the ocean has been gradually cooling. A cooling of water at depth can lead to a cooling of the ocean surface, which can reduce the length of an El Nino event if one develops. If an El Nino does develop, it is likely to be weak and short-lived as most models indicate that the ocean will likely cool as we approach winter season.Atmospheric indicators such as the Southern Oscillation Index (SOI) and cloudiness near the Date Line have generally remained in the neutral range.



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Bulletin April 2019 - MJO

The Madden-Julian Oscillation (MJO) is an eastward moving "pulse" of cloud and rainfall near the equator that typically recurs every 30 to 60 days. It affects countries in the Indian Ocean and the western equatorial Pacific (including Vanuatu) in terms of wind variations, cloudiness and rainfall. If an active phase of MJO tracks over the Maritime Continent , it will enhance rainfall in countries along the western equatorial Pacific including Vanuatu.

A pulse of the MJO currently lies on the other side of the planet, in the African latitudes. At this time, Vanuatu is not influenced directly by rainfall associated with the MJO. However, last week the MJO was tracking across the Pacific and late-season tropical cyclone activity formed in the western equatorial Pacific, mostly affecting Australia and nearby countries.

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Bulletin April 2019 - Cloud

Displayed below are global maps of outgoing long-wave radiation (ORL) highlighting regions experiencing more or less cloudiness. In the bottom panel, negative values (purple shading) represent above normal cloudiness while positive values (brown shading) represent below normal cloudiness.

(Note the displayed images are out of date)
The latest 30 day ORL ending in 21 May 2019 indicate normal to above normal cloudiness over Vanuatu, which coincided with normal and above normal rainfall experienced in the previous month.

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Bulletin April 2019 - Rainfall

Rainfall for April was normal to above normal across Vanuatu. Sola, Lamap and Aneityum recorded above normal rainfall, with Lamap and Aneityum being their fourth and third-wettest April on record respectively. The remaining stations through out Vanuatu recorded normal rainfall.

Average rainfall for Sola on a 30 year timescale (1971-02) is approximately 450.0mm. In April, Sola station recorded above normal rainfall at 534.4mm.

Average rainfall for Pekoa on a 30 year timescale (1971-02) is approximately 250.0mm. The station recorded normal rainfall at 213.5mm.

Average rainfall for Lamap on a 30 year timescale (1971-02) is approximately 220.0mm. Rainfall for April was 411.0mm, which was well above normal.

Average rainfall for Bauerfield on a 30 year timescale (1971-02) is approximately 210.0mm. In the previous month, the station recorded normal rainfall at 254.3mm.

Average rainfall for Port Vila on a 30 year timescale (1971-02) is approximately 210.0mm. In April, the station recorded normal rainfall at 225.0mm.

Average rainfall for Whitegrass on a 30 year timescale (1971-02) is approximately 110.0mm. Whitegrass station recorded normal rainfall at 71.3mm in April.

Average rainfall for Aneityum on a 30 year timescale (1971-02) is approximately 220.0mm. Rainfall recorded for April was 664.2mm, which was well above normal.

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Bulletin April 2019 - SST

The sub surface temperature anomalies displayed on the top image shows a clear cooling of water beneath the equatorial Pacific in April at a 100m depth in the western Pacific. Compared to March, the cooling further extends east from the central pacific, and slightly intensified in the western Pacific. Vanuatu is situated at latitudes of 160°E - 170°E.

Sea surface temperatures (SSTs) for April remained warmer than average across most of the tropical Pacific Ocean. SST anomalies for the week ending 12 May were warmer than average across nearly the entire tropical Pacific Ocean. Compared to 2 weeks ago from 14 May, the surface of the ocean has warmed slightly in the west and cooled slightly in the east - a weakening of the typical El Nino pattern.

The Nino3.4 value for April and for the week ending 12 May are +0.7 °C respectively, typically indicating ENSO neutral phase.

SSTs over Vanuatu are half to one degrees above average. No alert is issued for Coral Reefs as the outlook for the next 4 weeks remains at No Stress through out Vanuatu.

oceanportal.spc.int
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Bulletin April 2019 - MSLP

The April mean sea level pressure (MSLP) anomaly map shows negative anomalies less than -1 hPa to the north and east of the southern Cook Islands. Positive anomalies greater than +1 hPa were present in the western Pacific especially over eastern Federated State of Micronesia in the north Pacific and over the Coral Sea region to Australia in the south Pacific.

MSLP anomalies for April indicate normal pressure over Vanuatu waters. No Low or high pressure systems have tracked within the Vanuatu Area of Responsibility (AoR) in April to fluctuate MSLP.

(Note the displayed image is out of date)



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Bulletin April 2019 - Model Outlooks

While most climate models forecast El Niño-like warmth to persist the early austral winter, the ocean is forecast to cool over late winter and spring.

While seven of eight surveyed climate models anticipate El Niño thresholds will be met for June, for August and September two models remain in El Niño territory and two are at borderline values, with the remaining four models indicating neutral conditions are most likely.

Due to the natural cycle of ENSO, Model accuracy through autumn months (March, April, and May) is lower than at other times of the year. Forecast accuracy improves for outlooks issued in June.

SCOPIC and POAMA models favor below normal rainfall for Vanuatu within the next three months (June-Aug). Confidence is low to moderate.


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Bulletin April 2019 - SPCZ

ITCZ evident just north of the equator across the Pacific, extending westward to a mass of convection west of the Date Line. Extending from this at an unusually sharp angle was a diffuse looking SPCZ, principally affecting Tuvalu and Fiji. This convective mass was displaced to the east from its normal location. There was also a line of convection just south of the equator from the central Pacific towards the South American coast.

Vanuatu is directly influenced by rainfall associated with the SPCZ in April.


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