Since April 2018, sea surface temperatures across the east-central tropical Pacific have been at neutral levels, and the atmospheric indicators of the ENSO state have also suggested mainly neutral conditions. For example, upper level winds, cloudiness patterns and sea level pressure patterns continue to reflect neutral conditions. During the last several weeks, however, low-level winds in the west tropical Pacific Ocean have been anomalously westerly, which is an indicator of the possible coming onset of El Niño conditions.
The temperature of the waters at depth, from the central Pacific eastward and extending several hundred meters below the surface, have been moderately above average since April 2018. The waters at depth often provide an indication of the coming ENSO conditions at the surface, and suggest that the currently neutral sea surface temperatures may warm during September and into the fourth quarter of 2018, possibly reaching El Niño levels by the fourth quarter. The above-average water below the surface already extends to the surface in parts of the east-central tropical Pacific, causing warming of the sea surface, although not yet sufficiently to reach the El Niño threshold.
About three-quarters of the models surveyed predict that sea surface temperatures in the east-central tropical Pacific Ocean will warm to weak El Niño levels beginning late in the third or the fourth quarter of 2018. The time frame of this warming includes the immediate upcoming period of September-November. There is some uncertainty in these model predictions, as indicated by the forecasts ranging from a lack of an El Niño to a moderately strong El Niño event. The average prediction is for sea surface temperatures in the east-central tropical Pacific to warm to approximately 0.6 to 1.2 degrees Celsius above average during the period November 2018 through January 2019. Based on the model predictions and expert assessment, the probability for the development of El Niño is considered to be about 70% for September through the end of 2018 and into early 2019. This probability implies that El Niño development is more than twice as likely as a continuation of neutral conditions through the end of this period. If El Niño does emerge, its strength is currently uncertain but a strong event appears unlikely.
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 take into account the relative effects of both the El Niño/Southern Oscillation state and other locally relevant climate drivers. For example, sea surface temperatures of 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).