Relation of Tropical–Extra-tropical Ventilation to Equatorial Pacific Ocean Heat Content Variability

Equatorial Pacific ocean heat content (OHC) variability is a key component of tropical climate dynamics, especially in the evolution of the El Niño–Southern Oscillation (ENSO). Recent work has shown that the strong 2023–2024 El Niño was driven primarily by oceanic processes, independent of the classical positive Bjerknes feedback. Climate models project that such events will become more frequent under continued warming, highlighting the need to better understand the mechanisms regulating equatorial OHC variability. While local air–sea interactions dominate variability on sub-annual timescales, the contribution of ventilated waters from extra-tropical source regions on annual to decadal timescales remains less clearly defined.

This study focuses on the following gaps in our understanding of tropical–extra-tropical ventilation: First, which physical mechanisms regulate the magnitude and variability of subduction fluxes in the extra-tropical source regions? Second, to what extent is the commonly assumed adiabatic transport along isopycnal pathways valid, and how strongly do diapycnal mixing and nonlinear temperature–salinity effects modify water-mass properties during advection? Third, how do these processes together control the contribution of remote ventilation to equatorial Pacific OHC variability?

To address these questions, we employ a Lagrangian framework based on ocean reanalysis data, tracking subducted water parcels from their source regions to the equatorial Pacific. By analyzing the evolution of temperature and salinity along particle trajectories, we assess the relative roles of subduction variability, interior isopycnal transport, and diapycnal mixing in shaping equatorial OHC variability on interannual to decadal timescales.