Trends and Pathways of the Contributions of Southeastern Subtropical Pacific Water to the Equatorial Undercurrent

Abstract Substantial interannual-to-decadal variations in the sources and pathways of the Equatorial Undercurrent (EUC) water have been found in observational and modeling studies, yet long-term trends remain underexplored due to limited record lengths and insufficient evaluation of climate model skill in simulating EUC water sources and pathways. As a necessary first step for model skill evaluation, here we investigate EUC water sources and pathways by implementing an offline particle tracer scheme applied to two high-resolution ocean reanalysis datasets, Global Ocean Reanalysis and Simulation (GLORYS) and Ocean Reanalysis System, version 5 (ORAS5). In agreement with observational studies and prior model results, we find that the primary off-equatorial source of the EUC is the southeastern subtropical Pacific (SESP) (15% of the total from the tropical Pacific). During 1979–2023, we identify an increasing trend in the contribution of this source to the western-central Pacific portion of the EUC, alongside a decreasing trend in its contribution to the eastern Pacific portion. Furthermore, we find a deepening trend in the water pathway from SESP into the EUC during 1979–2023, along with a westward shift in the pathway toward the eastern Pacific portion of the EUC during 2003–22. We attribute the trends in the SESP source to increased mixed layer depth and vertical downward pumping. The westward shift trend in the pathway is due to accelerated southwestward currents in SESP, driven by increased southwest trades over the region. The identified trends may contribute to the strengthening of the equatorial Pacific zonal sea surface temperature gradient by allowing for enhanced water mixing along the preferred pathway toward the EUC and cooling of the EUC core.