WMO: Weak La Nina Developed

World Meteorological Organization, 14 December 2017

Sea surface temperatures in the eastern tropical Pacific Ocean have recently cooled to weak La Niña levels. Similarly, most atmospheric indicators are now consistent with the early stages of a La Niña event. Climate models indicate that weak La Niña conditions are likely to persist into the first quarter of 2018. A return to ENSO-neutral conditions before early 2018 is less likely, while the emergence of El Niño before the second quarter of 2018 appears very remote. National Meteorological and Hydrological Services will continue to closely monitor changes in the state of ENSO over the coming months.

Since October, sea surface temperatures across much of the tropical Pacific have shifted from neutral to weak La Niña levels, with temperatures of 0.5 to 1.0 degrees Celsius below average in the east-central part of the basin. Atmospheric indicators also show La Niña patterns, including enhanced precipitation over the far western Pacific and an area of reduced rainfall near the International Date Line. Additionally, enhanced trade winds are observed in the western tropical Pacific and sea level pressures are higher than normal in the eastern Pacific and lower than normal in the west – both indicative of La Niña conditions.

In central to eastern Pacific, waters at depth have remained moderately cooler than average since October and extend several hundred meters below the surface. These waters, which often provide an indication of the coming conditions at the surface, do not suggest any warming of the cooler-than-average sea surface temperatures during December and into January.

Most models surveyed predict that sea surface temperatures in the east-central tropical Pacific Ocean may experience some additional cooling over the next two months, but will remain in the weak La Niña range (0.5 to 1.0 degrees Celsius below average). A small number of models predict cooling to more than 1.0 degrees Celsius below average. Based on these predictions and expert assessment, the chance of La Niña continuing into the first quarter of 2018 is 70-80%. There is virtually no chance of El Niño developing before the second quarter of 2018.

In summary:
  • Weak La Niña conditions have recently developed in both the sea surface temperatures and the associated key atmospheric patterns;
  • La Niña conditions are 70-80% likely to continue into the first quarter of 2018.
  • La Niña is likely to be weak (0.5 to 1.0 degrees Celsius below average), with a slight chance for moderate strength (1.0 to 1.5 degrees Celsius below average).