El Nino weakening fast, La Nina may develop – WMO

Source: World Meteorological Organization
Country: World

The previously strong 2015-16 El Niño is now weakening rapidly, but it is still likely to influence climate patterns in some regions until mid-year, UN weather agency reported.

The previously strong 2015-16 El Niño is now weakening rapidly. However it is still likely to influence climate patterns in some regions until mid-year. Climate prediction models indicate a return to ENSO-neutral during May 2016, with odds now increasing of La Niña development in the third quarter. A resurgence of El Niño is highly unlikely in 2016. National Meteorological and Hydrological Services will closely monitor changes in the state of ENSO over the coming months.

Ocean temperatures in the central and eastern tropical Pacific Ocean exceeded +2.0 degrees Celsius above average between October 2015 and February 2016, but are now in the process of returning to ENSO-neutral levels. In early May, these areas had cooled to between +0.5 degrees Celsius and +1.0 degrees Celsius above average.

Atmospheric indicators that showed very strong El Niño patterns early in the year had weakened significantly towards ENSO-neutral values by early May. Such indicators include lower than normal atmospheric pressure across the central and eastern Pacific Ocean, weakened and on occasion reversed low-level Pacific trade winds, and above-average cloudiness and increased rainfall near and east of the International Date Line. Historically, El Niño events often persist through much of the first quarter of the year following their development – and occasionally into the second quarter – before returning to neutral. Because of the strength of this El Niño, it has persisted through early May, albeit at weak levels, but the situation will likely return to neutral levels before the end of May.

Between January and early May 2016, temperatures below the surface of the tropical Pacific, to the east of the International Date Line, transitioned from being well above average to below average, as cool waters at depth in the western and central equatorial Pacific Ocean expanded both eastward and upwards towards the surface. In the eastern quarter of the tropical Pacific, surface waters have recently become cooler than average. While the surface waters in the central and east-central Pacific currently remain warmer than average, below average sea temperatures exist at shallow depths, suggesting that the surface waters are likely to cool further in the coming months. Historically, La Niña has followed several strong El Niño events, including the 1997-98 event.

Currently, all dynamical and statistical prediction models surveyed predict that the sea surface temperatures in the east-central tropical Pacific Ocean will cool further in the coming months, with many models predicting temperatures to be in the range of -0.5 to +0.5 degrees Celsius from average during the overlapping 3-month periods May-July and June-August. Beginning in the July-September period and continuing through the remainder of 2016, more than half of the models predict east-central tropical Pacific Ocean temperatures will drop to more than 0.5 degrees Celsius below average, indicative of at least weak La Niña conditions. However, some uncertainty remains as forecasts made at this time of the year typically have less accuracy than those made during the second half of the year.

The 3-month mean sea surface temperature in the central tropical Pacific of well over 2 degrees Celsius above average during the peak of the 2015-16 El Niño indicates that it was comparable in strength to the previous very strong events of 1982-83 and 1997-98. While the peak ocean temperatures were approximately as strong as those of the 1997-98 event, other aspects of the 2015-16 El Niño were weaker, such as the sea surface temperature and subsurface temperature in the eastern one-third of the tropical Pacific, and the eastward extent of enhanced cloudiness and rainfall along the equator.

A careful watch will be maintained on the oceanic and atmospheric conditions in the tropical Pacific in the coming months to better assess the dissipation of El Niño and any possible transition to La Niña.

It is important to note that El Niño and La Niña are not the only factors that drive global climate patterns. Further, the strength of an El Niño event may not necessarily closely correspond to its climate impacts occurring in various regions of the world. At the regional level, seasonal outlooks need to assess the relative impacts of both the El Niño or La Niña state and other locally relevant climate drivers. For example, the sea surface temperature of the Indian Ocean, the south-eastern Pacific Ocean and the Tropical Atlantic Ocean are also known to influence the climate in the adjacent land areas. Regionally and locally applicable information is available via regional and national seasonal climate outlooks, such as those produced by WMO Regional Climate Centres (RCCs), Regional Climate Outlook Forums (RCOFs) and National Meteorological and Hydrological Services (NMHSs).

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