Strong El Niño conditions were observed over the winter of 2023-24, leading to long lasting periods of split flow in the jet stream aimed at Western North America. This is a typical El Niño pattern, where the northern split persistently aims storms at southeast Alaska and the southern branch aims at California. Despite this, Oregon has still come away with near to above average precipitation. Washington remained dry compared to average.
El Niño is already starting to fade thanks to upwelling west of the Galápagos Islands bringing cool water up from below to the surface. Cooling sea surface temperatures are likely to spread across the equatorial Pacific through the spring leading to neutral conditions by the start of summer. Modeling indicates that we won’t spend long there, instead continuing our dive to La Niña conditions by the end of summer.
In their latest forecast, the Climate Prediction Center gave a 79% chance of ENSO neutral conditions for the April-June period and a 55% chance of La Niña conditions for the June-August period. For the Pacific Northwest, above average temperatures and below average precipitation are favored during the summer.
The El Niño-Southern Oscillation, abbreviated ENSO, refers to sea surface temperatures in the equatorial Pacific Ocean. During El Niño, sea surface temperatures in this region are above average while La Niña occurs during below average conditions. This sort of long-term climate anomaly with continent- or global-scale impacts is called a teleconnection. It is not uncommon for strong El Niños to rapidly evolve into La Niña conditions. This same thing occurred in 1973, 1983, 1988, 1998, 2010, and 2016.
La Niña summers can bring increased hurricane activity in the North Atlantic thanks in part to changes in the upper level wind pattern over North America and the Atlantic. Heading into winter, average La Niña conditions feature strong high pressure in the Gulf of Alaska commonly aiming the jet stream at the Pacific Northwest for above average precipitation and below average temperatures.
Even though the storm track may be aimed at the Pacific Northwest, evidence shows that on average strong regional storm events are more common during ENSO neutral years. The Columbus Day Storm of 1962 occurred during an ENSO-neutral year but the 2006 Hanukkah Eve Storm was part of a weak La Niña.
These are, of course, just average conditions. Meteorologists have a fairly small dataset to work with with regard to ENSO conditions with reliable observations only going back to the 1950s. On top of this, there are several other teleconnections and factors at play to determine what conditions are observed.
Forecasting this far in advance can be fairly unreliable but there is some utility in getting an idea of what conditions are favored. Average regional conditions can indicate what sort of resources will be available for water supply, commodities, and energy. Forecasts for the Atlantic hurricane season will come out over the next couple months which governments and aid agencies will use to prepare.
The featured image is sea surface temperature anomalies for the Pacific on March 5 via NOAA Coral Reef Watch.
