Sea surface temperatures in the eastern tropical Pacific Ocean have recently cooled to below normal and are approaching La Niña levels, while most atmospheric indicators have remained ENSO-neutral, says the World Meteorological Organization, or WMO in its latest update.
Climate models surveyed indicate that weak La Niña conditions may develop along the central to eastern tropical Pacific in the last quarter of 2017, but then weaken to ENSO-neutral in the first quarter of 2018.
La Niña conditions have a slightly higher likelihood than ENSO-neutral conditions for the coming few months, while the emergence of El Niño appears is highly unlikely. National Meteorological and Hydrological Services will continue to closely monitor changes in the state of ENSO over the coming months.
Since August, sea surface temperatures across much of the eastern equatorial Pacific have rapidly shifted from neutral to the threshold of a weak La Niña, near or just beyond one-half degree Celsius below average.
However, atmospheric patterns mainly reflect only cool-neutral conditions, with enhanced rainfall over the Maritime Continent region, somewhat suppressed rainfall near the International Dateline, and weakly enhanced easterly low-level zonal wind anomalies across parts of the Pacific.
Similarly, the pattern of sea level pressure, while leaning somewhat in the direction of La Niña, has remained in the ENSO-neutral range.
Currently, sea surface temperatures in the eastern tropical Pacific are slightly below average, with the strongest departures from average near one degree Celsius below average in the mid-eastern portion of the basin.
If La Niña conditions continue to develop as many models predict, this pattern would expand farther westward toward the Dateline, while the positive departures from average in the western Pacific would remain.
Sub-surface temperatures, from the central Pacific eastward and extending several hundred meters below the surface, have cooled markedly to become somewhat below average in the recent month; these waters often provide some indication of the coming conditions at the surface.
It is also noted that two consecutive years of La Niña following a strong El Niño, such as the one that occurred in 2015-16, is historically not uncommon.
Most dynamical models surveyed predict that sea surface temperatures in the east-central tropical Pacific Ocean will experience slight further cooling to weak La Niña conditions (0.5 to 1.0 degrees Celsius below average) over the next two months, with statistical models tending to favor maintaining only cool ENSO-neutral levels.
The forecasted cooling persists at least until the end of the year and possibly into early 2018. Based on these predictions and expert assessment, the chance of a weak La Niña developing in the final months of 2017 is about 50-55%, with the likelihood of ENSO neutral at about 45-50%. There is very little chance of El Niño developing by 2018.
It is important to note that El Niño and La Niña are not the only factors that drive global climate patterns, and that the strength of ENSO does not automatically correspond to the strength of its effects.
At the regional level, seasonal outlooks need to assess the relative effects of both the El Niño-Southern Oscillation state and other locally relevant climate drivers.
For example, sea surface temperatures in the Indian Ocean, the southeastern 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).
In summary:
- Sea surface temperatures in eastern tropical Pacific have recently cooled, approaching La Niña levels, while atmospheric patterns have largely remained ENSO-neutral;
- Models surveyed and expert opinion suggest that weak La Niña conditions may develop, with about 50-55% probability, in the final quarter of 2017;
- If La Niña conditions do develop before the end of 2017, they are likely to be weak, and would likely return to ENSO-neural in the first quarter of 2018;
- Continuation of ENSO-neutral conditions is also a plausible scenario, with 45-50% likelihood;
- Emergence of El Niño can be practically ruled out.
