La Niña, a climate event marked by persistent abnormal cooling of sea surface temperatures (SST) in the central-eastern tropical Pacific, exerts a profound and lasting impact on global climate and ecosystems. When a La Niña event lingers for more than two years—known as a multi-year La Niña—its effects become even more far-reaching. In recent decades, such prolonged La Niña events have occurred with increasing frequency, and climate models predict they may become more common by the end of this century. Yet, one critical question remained unanswered: how do marine biogeochemical processes respond to and feed back into multi-year La Niña events? Our research set out to solve this puzzle—and uncovered a surprising, game-changing mechanism driven by tiny marine organisms: phytoplankton.
As a research group focused on ocean mesoscale dynamics at the Institute of Oceanology, Chinese Academy of Sciences, we have long been fascinated by the complex interactions between ocean physics and marine ecology. For years, studies of El Niño-Southern Oscillation (ENSO)—the climate cycle encompassing both El Niño (warming) and La Niña (cooling)—largely focused on ocean-atmosphere physical interactions. The role of marine life, especially microscopic phytoplankton, was often overlooked. But we suspected these tiny organisms, which contain chlorophyll and color the ocean green, might hold a key to understanding multi-year La Niña.
Phytoplankton are the base of the marine food web, but they also influence how sunlight interacts with the ocean. Chlorophyll in phytoplankton absorbs shortwave solar radiation. More chlorophyll means more sunlight is trapped near the ocean surface; less chlorophyll allows sunlight to penetrate deeper into the ocean. We hypothesized that this simple biological process could create a heating feedback loop that alters the development of multi-year La Niña. To test this, we combined decades of historical observational data with state-of-the-art ocean physical-biogeochemical coupled models, running detailed simulations to track how chlorophyll variability shapes La Niña’s evolution.
What we found was striking: phytoplankton-driven heating feedback is a key physical-ecological coupling mechanism that fundamentally reshapes multi-year La Niña—and its effects differ dramatically across the tropical Pacific.
In the western-central equatorial Pacific, multi-year La Niña brings stronger zonal (east-west) ocean currents. These currents drive persistently higher chlorophyll levels (positive anomalies) for two consecutive years. The extra chlorophyll traps more solar radiation in the ocean’s mixed layer (the warm, upper layer of the ocean). Initially, this slows surface cooling during the early stages of La Niña. But over time, the trapped heat warms the mixed layer, making it shallower. This triggers stronger north-south (meridional) ocean circulation, which cools the western-central Pacific further. The result? The second year of La Niña becomes 8% more intense.
In stark contrast, the eastern equatorial Pacific tells a different story. Influenced by a northwest Pacific anticyclone and the North Pacific Meridional Mode, chlorophyll levels drop sharply (negative anomalies) in the second year of La Niña. With less chlorophyll, more sunlight penetrates deep into the ocean, warming subsurface waters. This warm subsurface water then rises to the surface via background upwelling currents, weakening the second year of La Niña by a substantial 45%.
Adding to the complexity, the contrasting chlorophyll patterns between the east and west amplify the SST gradient across the equatorial Pacific. This, in turn, reinforces air-sea coupling feedbacks, making the cold anomaly in the western-central Pacific even stronger. We also discovered that deep chlorophyll maxima (high phytoplankton concentrations in deeper waters) further amplify this cooling effect.
This discovery challenges long-standing assumptions about ocean-atmosphere interactions. For the first time, we have proven that phytoplankton-driven heating feedback exerts critical control over multi-year La Niña on interannual timescales. Beyond advancing basic climate science, our findings have tangible real-world value.
ENSO is one of the most influential climate cycles on Earth, and accurate ENSO prediction is vital for mitigating climate risks—from droughts and floods to extreme weather events that impact agriculture, water resources, and communities worldwide. By identifying this new biological feedback mechanism, we provide a key physical basis for improving ENSO prediction models. Our work also deepens our understanding of how extreme climate events evolve under global warming, offering scientific support for climate risk prevention and adaptation.
This research was a true collaborative effort, bringing together scientists from the Institute of Oceanology (Chinese Academy of Sciences), Nanjing University of Information Science and Technology, and Beijing Normal University. Cross-disciplinary work always comes with challenges: integrating physical oceanography and marine biogeochemistry required close communication and shared expertise. Running the complex coupled model simulations also demanded significant computational resources and careful data validation. But overcoming these hurdles made our breakthrough all the more rewarding.
Looking ahead, there is much more to explore. We plan to investigate how phytoplankton feedbacks interact with other climate drivers, and whether similar biological mechanisms influence El Niño events. We also hope our findings will encourage the broader climate community to fully integrate marine ecological processes into future climate models.
Tiny as they are, phytoplankton play an outsized role in regulating Earth’s climate. Our study reveals their hidden power to reshape major climate events like multi-year La Niña—a reminder that the ocean’s smallest inhabitants hold big clues to our planet’s most pressing climate questions.