Multi-year La Niña events — so-called "double-dip" or even "triple-dip" La Niñas — are becoming more common. But why do these events persist for multiple years in the first place?
Researchers from the Nanjing University of Information Science and Technology and the University of Hawaii discovered two distinct pathways that can lead to long-lasting La Niña conditions and highlighted an important mechanism that has been largely overlooked.
The study is published in Advances in Atmospheric Sciences on May 15.
El Niño and La Niña are the warm and cool phases of a recurring climate pattern in the tropical Pacific Ocean that influences weather worldwide. El Niño is characterized by unusually warm sea surface temperatures in the central and eastern Pacific, while La Niña brings cooler-than-normal conditions to the same region.
Whilst it is rather uncommon for El Niño to last more than a year, it is no longer rare to see La Niña events persist for two years, a phenomenon often referred to as a "double-dip" La Niña. These prolonged events could result in extended climate extremes and devastating weather events that take a toll on community resilience, the tourism industry and agriculture.
"Currently, a widely accepted hypothesis is that multi-year La Niña events are triggered by preceding extreme El Niño events, but this mechanism explains only about 30% of the total multi-year La Niñas observed over the past century," said Tim Li, the corresponding author of the study.
So, what accounts for the remaining 70%?
The answer, the team found, may lie in a pattern of anomalous sea surface temperatures south of the equator, known as the South Pacific Meridional Mode (SPMM).
When cooling extends farther into the South Pacific in spring, it alters atmospheric circulation by strengthening easterly winds along the equator. These winds enhance the upwelling of cold water from the deep ocean and push warm surface waters away, reinforcing the cooling at the ocean's surface.
Using atmospheric model experiments, the researchers confirmed that such a wind response can further sustain La Niña by slowing its natural decay. As a result, the cooling persists through the summer and can re-intensify in the following autumn when ocean-atmosphere feedbacks become stronger. This process, known as a season-dependent coupled ocean-atmosphere instability, is a positive, or self-reinforcing, feedback between the ocean and atmosphere that becomes particularly strong at certain times of year.
The findings suggest that when determining whether La Niña will persist into the following year, how it evolves after it peaks is just as important as how it begins. Multi-year La Niña events can develop through two key routes.
"The first route is driven by strong upper-ocean heat discharge associated with a preceding super El Niño, which induces thermocline anomalies that slow La Niña decay via Bjerknes feedback," Li said.
The Bjerknes feedback is a special type of ocean-atmosphere interaction where changes in sea surface temperatures affect atmospheric conditions, which in turn influence sub-surface ocean temperatures, creating a self-reinforcing cycle.
"The second route involves the influence of meridionally extended sea surface temperature anomalies, which strengthen equatorial easterlies, enhance upwelling, and delay the decay of La Niña," Li said.
In both cases, the cold anomalies can persist through summer and re-develop in autumn, leading to multi-year events.
By uncovering this dual mechanism, the researchers provide a more complete framework for understanding one of the most influential drivers of global climate variability. Their findings could help improve predictions of prolonged La Niña events, which have been linked to extended droughts, flooding and other extreme weather impacts worldwide.
The researchers aim to test how well current climate models capture these two distinctive routes and to explore how longer-term climate conditions may influence their behavior.
"Our ultimate goal is to improve forecasts of multi-year La Niña events and their far-reaching impacts, an advance that could strengthen climate preparedness and resilience on seasonal to decadal timescales," Li said.
