El Niño/Southern Oscillation (ENSO)

Ocean-Atmosphere conditions in the Tropical Pacific have now returned to a near normal state after a rapid demise of the El Niño event, which peaked in late 2015.

Sea Surface Temperature anomalies in the central Pacific are close to normal, and slightly cooler than normal sea surface temperatures have emerged in the eastern equatorial Pacific.

The latest monthly SST anomaly in the NINO3.4 region is +0.5°C, the NINO3 region (eastern Pacific: 90°W – 150°W) is currently at +0.28°C, and the NINO4 index (in the western Pacific) is at +0.7°C. Cooler than normal sub-surface waters have continued to spread eastward from the western Pacific, and temperatures are more than 4°C below normal between 50 and 100m depth east of about 160°W. Enhanced convective activity and rainfall near and east of the Dateline in the equatorial Pacific (usually associated with El Niño events) has weakened further in May, and zonal wind in the equatorial Pacific has returned to near normal, and weak westerly wind anomalies (i.e. intensified trade winds) are present in the far western Pacific.

The Southern Oscillation Index (SOI) has recently switched to positive values (value for May 2016: +0.4). The ENSO Precipitation Index (ESPI) is also on the La Niña side of neutral with a value of -0.43 (value to the 7th of June 2016). The Madden-Julian Oscillation (MJO) was weak in the western Pacific over May as a whole. At the forecast horizon of 14 days, the Climate Prediction Center (CPC) dyamical and statistical forecasts are in disagreement, but overall there are low chances of increased intra-seasonal convective activity in the western Pacific.

International guidance indicates that neutral ENSO (El Niño - Southern Oscillation) conditions are most likely (54% chance) over the next three month period (June – August 2016), as a whole, but a transition to La Niña is also possible over the same period (43% chance). The likelihood of La Niña conditions establishing in the Pacific increases later on, with 58% chance in September-November 2016 and 61% in December 2016 – February 2017.

Surface temperature anomalies (ºC) for May 2016, data is from the NOAA OISST Version 2 dataset, available at the NOAA’s Climate Data Center (ftp://ftp.cdc.noaa.gov/Datasets/noaa.oisst.v2.highres/).