Overview

Earlier this year, there were signs that a weak El Nino in the tropical Pacific Ocean could continue through the fall and even into the upcoming winter season, but there is now substantial agreement amongst numerous computer forecast models that La Nina conditions are likely to become established over the next couple of months and current observations back this notion. La Nina is a naturally occurring oceanic cycle that produces colder-than-normal sea surface temperatures (SSTs) in the equatorial Pacific Ocean whereas El Nino is associated with warmer-than-normal SSTs. The formation of La Nina in the tropical Pacific Ocean will likely have important ramifications around the world including significant impacts on the upcoming winter season, next summer’s tropical season, and global temperatures.

Computer model forecasts support the formation of La Nina

Numerous independently-made computer forecast models depict a change from the current near-neutral conditions in the tropical Pacific Ocean to La Nina conditions by the winter of 2017-2018. The plume of model El Nino Southern Oscillation (ENSO) model forecasts from mid-September indicate a transition to La Nina conditions are quite likely to take hold by later this fall. Indeed, some models are predicting a fairly strong La Nina by the middle of the upcoming winter season with sea surface temperatures as much as 1.5°C below-normal in the “Nino 3.4” region (central tropical Pacific).

Sea surface temperatures have indeed changed dramatically in the tropical Pacific Ocean between the spring and today with a “wavy” pattern of colder-than-normal water (bottom panel, blue region) now showing up in the same area that exhibited widespread warmer-than-normal SSTs back in April 2017 (top panel, yellow/orange region).