Intensification of Pacific tropical instability waves over the recent three decades

Tropical instability waves (TIWs), one of the most prominent mesoscale oceanic phenomena in the tropical Pacific, play important roles in climate and ecosystem. Due to limited observations and the difficulty in estimating equatorial current velocity, long-term changes in TIWs remain unknown.

 

Rather than the geostrophic equilibrium (Bonjean and Lagerloef, 2002), the TIW currents exhibit a momentum balance between inertial forces (local accelerations and advections), the Coriolis force and the pressure gradient force. Using a shallow water diagnostic model that retains the inertial forces, we have produced the TIW surface currents since 1993 based on the satellite altimetry sea surface height observations. The results have been well validated with moored observations of ocean velocities in TAO array (Wang et al., 2020, https://doi.org/10.1175/JPO-D-20-0063.1).

 

The satellite altimetry-derived TIW currents (1993-2021) have shown that TIWs have strengthened during this period, with their eddy kinetic energy (EKE) increasing by 12% per decade (~10 J m-3 per decade). The trend has been corroborated by other three independent datasets: satellite-observed sea surface temperature (1982-2021), moored currents from TAO (1980s-2020), and a global eddy-resolving ocean circulation model data (OFES2, 1958-2021). They consistently imply that the intensification is concentrated on the equatorial Yanai-mode TIWs. EKE budget based on OFES2 model data suggests that the increased EKE is attributed to the increased barotropic (primary) and baroclinic (secondary) instabilities. The former is due to the strengthened south equatorial currents (SEC), and the latter is due to the decreased mixed layer stratification and increased equatorial buoyancy fronts. The underlying mechanism is an enhanced cross-equatorial asymmetric warming in the eastern tropical Pacific since the 1990s that forces the changes in the equatorial multiscale ocean dynamics. As a feedback effect on the heat budget of cold tongue SST, the intensified TIWs lead to increased eddy dynamic heating effects of ∼70% since the 1990s near the equator, with implications for predicting and projecting tropical Pacific climate changes. (https://doi.org/10.1038/s41558-023-01915-x)