Tropical Instability Waves and Oceanic Mixing Dynamics

Tropical instability waves (TIWs) are large‐scale, meandering oscillations of sea surface temperature and currents in the equatorial Pacific that arise from instabilities of the zonal current system. As they propagate westward, TIWs enhance vertical shear and generate turbulence, driving diapycnal mixing across the thermocline. This mixing modulates heat and nutrient transport between the upper ocean and deeper layers, with implications for climate variability, the El Niño–Southern Oscillation and marine ecosystems. TIWs interact with mean currents such as the Equatorial Undercurrent, undergo barotropic and baroclinic instability, and can excite subsurface modes that peak tens of metres below the surface. The resulting turbulence patterns influence regional SST fronts, alter air–sea heat fluxes and shape the global heat budget, making TIWs a critical component of tropical ocean dynamics and climate predictability.

Research from Nature Portfolio

High‐resolution coupled atmosphere–ocean prediction experiments have demonstrated that resolving fine‐scale SST fluctuations associated with TIWs improves medium‐range forecast skill. These studies identified a two‐way feedback in which barotropic instability of equatorial currents and baroclinic instability at sharp density fronts both supply energy to anticyclonic eddies, which in turn reinforce the density gradients that sustain TIWs. Another recent investigation using global high‐resolution climate simulations revealed that submesoscale ocean eddies with horizontal scales below several hundred kilometres substantially dampen the amplitude of El Niño and La Niña events by modulating upward heat fluxes during warm and cold phases. Finally, analyses of deep‐reaching thermocline mixing in the equatorial Pacific cold tongue have shown that TIW‐induced shear, together with the Equatorial Undercurrent, lowers the local Richardson number and triggers mixing events down to the lower flank of the undercurrent, with frequency and depth varying under different ENSO conditions.

Research from all publishers

Observational studies have uncovered a subsurface mode of TIWs in the eastern Pacific in which zonal velocity oscillations peak at 70–90 m depth with periods of 5–20 days. Although less energetic than surface‐intensified waves, these subsurface TIWs contribute disproportionately to vertical shear changes and hence drive mixing at mid‐thermocline levels. Multiyear turbulence measurements in both Atlantic and Pacific cold tongues have revealed a remarkably consistent deep cycle of turbulence beneath the mixed layer, with diurnal increases in dissipation rate that scale with the product of wind stress and current shear. Foundational numerical work also established a linear relationship between interannual TIW activity and ENSO intensity, showing that stronger La Niña conditions enhance TIW amplitude via increased SST gradients, and that baroclinic instability of the northern SST front is a primary generation mechanism.

Tropical Instability Waves and Oceanic Mixing Dynamics publication trend

The graph below shows the total number of articles in tropical instability waves and oceanic mixing dynamics across all publications each year (not limited to Nature Index journals).

Technical terms

Tropical instability waves (TIWs): Large‐scale oscillations of SST and currents in the equatorial ocean driven by current and density front instabilities.

Barotropic instability: Instability arising when horizontal shear in the flow extracts kinetic energy from the mean current.

Baroclinic instability: Instability due to vertical density gradients, converting potential energy of stratification into eddy kinetic energy.

Equatorial Undercurrent (EUC): An eastward subsurface jet beneath the equator that influences vertical shear and mixing.

Richardson number: A dimensionless ratio of stratification to shear; values below a critical threshold indicate potential for turbulence.

Submesoscale eddies: Ocean eddies with horizontal scales of a few to several hundred kilometres that modulate heat flux and damping of large‐scale variability.

References

1. Effective generation mechanisms of tropical instability waves as represented by high-resolution coupled atmosphere–ocean prediction experiments. Scientific Reports (2023).
2. El Niño/Southern Oscillation inhibited by submesoscale ocean eddies. Nature Geoscience (2022).
3. Deep-reaching thermocline mixing in the equatorial pacific cold tongue. Nature Communications (2016).
4. The Subsurface Mode Tropical Instability Waves in the Equatorial Pacific Ocean and Their Impacts on Shear and Mixing. Geophysical Research Letters (2019).
5. Deep Cycle Turbulence in Atlantic and Pacific Cold Tongues. Geophysical Research Letters (2022).
6. A linear relationship between ENSO intensity and tropical instability wave activity in the eastern Pacific Ocean. Geophysical Research Letters (2003).