El Niño is no longer considered a local anomaly off the coast of Peru: it is a global climate pendulum swinging every 2–5 years. However, its behavior changes on decadal timescales, and these shifts are poorly predictable. Surrounding this work are studies of solar wind, neutrino wind, and rotating spheres, all seeking how external forces interact with internal stratification. This article stands somewhat apart: it links not just wind and thermocline, but specifically two mathematically distinct parts of the wind field to two different time scales of the ocean.
The authors took wind and sea surface temperature data from the ERA5 archive for 1960–2020, and temperature and salinity profiles from the database of the Institute of Atmospheric Physics, Chinese Academy of Sciences. They represented the tropical Pacific as a two-layer system: a warm upper layer and a cold lower layer, separated by the thermocline. They then introduced a dimensionless wind stress, the ratio of ordinary stress to the product of upper-layer density and the square of the first baroclinic mode speed. This ratio was decomposed via Helmholtz theorem into irrotational (divergent) and solenoidal (rotational) parts. To relate thermocline depth to the Niño-3.4 index, they constructed an integral relation with a transfer function found using empirical orthogonal functions.
The transfer function proved robust: its structure did not change whether calculated over the entire 1960–2020 period or only 1960–1990. It showed that even small changes in thermocline depth in the east or west of the basin strongly affect temperature in the center, while large changes in the center itself have little effect. Reconstructing the Niño-3.4 index using only the equilibrium part of the thermocline gave a correlation of 0.83 with observations but missed extreme El Niños. Adding q-waves generated by the rotational part of the wind markedly improved the fit. The q-waves themselves had a period of 25–27 years and depended weakly on initial conditions. The reduced gravity in the tropical Pacific grew linearly at about 0.66·10⁻⁴ m/s² per year, reflecting increased stratification due to uneven layer warming.
This work separates two mechanisms that were previously mixed into one concept of "wind forcing." The irrotational part of the wind drives fast, interannual oscillations, while the solenoidal part drives slow, decadal ones through interaction with changing stratification. This differs from neighboring studies, which typically consider either only zonal wind or only the thermocline, but do not distinguish the mathematical components of the wind field. Here it is shown for the first time that the rotational part of the wind, which has no intrinsic long-period variability, can generate slow ocean oscillations by multiplying with the slowly changing reduced gravity.
The method can be extended to other basins and longer data records to test how universal the separation of fast and slow modes is. It would also be interesting to include salinity as an independent factor in reduced gravity changes, since an accelerating hydrological cycle may alter regional salinity trends. For events like the 2023–2024 El Niño, when wind anomalies were weak but ocean heating strong, finer approximations than geostrophic will be needed to capture the interannual variability of q-waves.
The results are important for climate modeling and seasonal forecasts: they provide a simple way to separate the fast component of El Niño from the slow one. This could improve prediction of extreme events and understanding of the tropical Pacific response to global warming.
Conduct a systematic sensitivity test of the transfer function to the choice of period for EOF analysis. Investigate the role of salinity in reduced gravity changes and its influence on decadal Niño-3.4 variability.
An unresolved question in climatology is what processes govern decadal and interdecadal El Niño variability and why extreme events become more frequent with warming. This work proposes a concrete mechanism: slow changes in ocean stratification modulate the response to the rotational part of the wind, which may explain observed regime shifts.
This section is addressed to the author of the paper and their colleagues. Neighbouring works were found by meaning-based search across our archive, not by word overlap. We do not review: we show the field around the work.
What the work does
The authors decomposed wind stress into its irrotational and solenoidal parts and linked them to fast and slow oscillations of the thermocline. Using reanalysis data and a two-layer model, they obtained a stable transfer function and showed that adding q waves from the rotational component of the wind improves the reproduction of extreme El Niño events. Reconstructing the Niño-3.4 index with these waves achieves a correlation of 0.83 with observations.
Where its strength is
This work is the first to separate the contributions of the irrotational and solenoidal wind components to El Niño dynamics, showing that slow changes can arise from the interaction of the fast rotational component with slowly varying stratification.
Where it could go next
Worth trying
Why this may matter
If the proposed mechanism is confirmed, it will provide a physically based way to disentangle the sources of decadal El Niño variability, which could improve long-term climate forecasts and our understanding of how the tropical Pacific system responds to global warming.
Nearby works in our archive
The archive is sparse around this work: the closest review is only 0.52, against a usual 0.64. It looks like few have drilled nearby.
The first scientific mention of El Niño dates to 1892, but Peruvian fishermen knew of the warm current off their coast at Christmas since the 17th century and called it "the Christ child."
In Kim Stanley Robinson's novel "Forty Signs of Rain," climate scientists battle the consequences of abrupt changes in ocean currents reminiscent of El Niño regime shifts, which here are explained by slow modulation of stratification.