La Niña—persistent abnormal cooling of sea surface temperatures (SST) in the central-eastern tropical Pacific—has profound global climate impacts. When such an event lasts more than two years (multi-year La Niña), its effects intensify. In recent decades, multi-year La Niña has become more frequent. However, how marine biogeochemical processes feed back into these events remained unclear.
A research team from the Institute of Oceanology of the Chinese Academy of Sciences (IOCAS) has now uncovered a mechanism driven by tiny phytoplankton. Their findings were published in Communications Earth & Environment on May 25.
For years, El Niño-Southern Oscillation (ENSO) studies focused mainly on ocean-atmosphere physical interactions, overlooking marine life. The researchers suspected that phytoplankton might hold the key, which contain chlorophyll and influence sunlight penetration. More chlorophyll traps solar radiation near the surface; less allows deeper warming. Using decades of observations and advanced physical-biogeochemical coupled models, they tested this heating feedback hypothesis.
The results showed that, in the western-central equatorial Pacific, multi-year La Niña strengthens east-west currents, driving higher chlorophyll levels for two consecutive years. Extra chlorophyll traps more solar radiation, initially slowing surface cooling but eventually making the mixed layer shallower. This triggers stronger meridional circulation, further cooling the region. The second-year La Niña intensifies by 8%.
In the eastern equatorial Pacific, a different pattern emerges. Influenced by a northwest Pacific anticyclone, chlorophyll levels drop sharply in the second year. Reduced chlorophyll allows sunlight to warm subsurface waters, which then rise via upwelling, weakening the second-year La Niña by a substantial 45%.
These contrasting chlorophyll patterns amplify the east-west SST gradient and reinforce air-sea coupling, challenging traditional understandings of ocean-atmosphere interactions. The researchers also found that deep chlorophyll maxima further enhance the cooling effect.
This research proves that phytoplankton-driven heating exerts critical control over multi-year La Niña on interannual timescales for the first time. The findings provide a physical basis for improving ENSO prediction models, supporting climate risk prevention under global warming.
Dr. TIAN Feng, first author of the study, noted, "Tiny phytoplankton, play an outsized role in shaping major climate events—a reminder that the ocean's smallest inhabitants hold big clues for understanding Earth's changing climate."
Sea surface chlorophyll anomaly distribution in late spring and early summer during the 1998–2000 triple La Niña event. (Image by IOCAS)
(Text by TIAN Feng)
Media Contact:
ZHANG Yiyi
Institute of Oceanology
E-mail: zhangyiyi@qdio.ac.cn
(Editor: ZHANG Yiyi)