Island Climate Update 211 - April 2018
Weak La Niña conditions continued in the tropical Pacific during March 2018, but trends in low-level winds and in sub-surface ocean temperatures during the month indicate that the event is coming to an end.
The Southern Oscillation Index (SOI) is currently positive at about +1.1 for March 2018. Sea surface temperatures (SSTs) remained colder than average in the eastern equatorial Pacific Ocean but have returned to near normal around the Dateline. However, the layer of colder than normal SSTs is now very shallow (75 metres or less, except close to the South American coast), with warmer than normal sub-surface water spreading eastwards during March as far as 120°W.
The trade winds have been very variable over the past few months: the easterlies were stronger than normal from July 2017 through to mid-January 2018, but since that time low-level (850 hPa) westerlies have encroached almost to the International Dateline. March 2018 rainfall and convection anomalies in the tropical Pacific remain consistent with La Niña conditions, the pattern being reinvigorated since February: the Intertropical Convergence Zone remains shifted north of its climatological position in the central Pacific, whereas the South Pacific Convergence Zone was extremely weak during March.
In summary, while weak La Niña conditions remain present in the Pacific Ocean, the expansion and intensification of warmer than normal subsurface ocean waters 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 transition to an ENSO-neutral state over the next 3 month period (75% chance over April – June 2018). ENSO-neutral remains the most likely outcome over the late-winter season (July – September 2018). The forecast models predict about an equal chance of the Pacific remaining neutral or transitioning towards El Niño over the spring (September – November 2018).