Weak La Niña conditions continued in the tropical Pacific during February 2018, but the current state of the Ocean-Atmosphere system in the Equatorial Pacific indicates that it is now reaching its decay phase.
Below average sea surface temperatures (SSTs) remained present in the central and eastern equatorial Pacific Ocean but weakened compared to January 2018. The NINO3.4 index is negative at – 0.58°C (was – 0.68°C last month), and the far eastern Pacific (NINO indices 1 and 2) experienced rapid warming over the course of February.
The enhanced trade winds that were present in the western Pacific during January 2018 have vanished, and strong positive zonal wind anomalies (weakened trade winds) replaced them during February. February 2018 has also seen the breakdown of the rainfall and convection anomalies in the tropical Pacific which were previously consistent with La Niña conditions. The Southern Oscillation Index (SOI) has shown considerable variability since the beginning of the year, and is currently on the El Niño side of neutral, with a preliminary value of -0.85 for February 2018.
In summary, while weak La Niña conditions remain present in the Pacific Ocean, the breakdown of the atmospheric signals, along with expansion and intensification of warmer than normal subsurface ocean waters towards the central Pacific, signal that La Niña has reached its decay phase. This tendency is expected to continue over the next few months: the international consensus is for a rapid transition to an ENSO-neutral state over the next three month period (69% chance over March – May 2018). ENSO neutral
remains the most likely outcome over the winter season (June – August 2018), but the models indicate that a transition towards El Niño becomes increasingly likely thereafter (45% chance for El Niño conditions to emerge over the September – November 2018 period).
Download the PDF: Island Climate Update 209