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

Substantial interannual-to-decadal variations in the sources and pathways of the Equatorial Undercurrent (EUC) water have been found in observational and modeling studies. However, identifying and estimating long-term trends in response to external climate forcings remains challenging due to the short length of the records and the lack of a systematic investigation of coupled climate model skill in simulating the EUC water sources and their pathways. A necessary first step in this direction is a comprehensive investigation of EUC water sources and pathways in high-resolution ocean reanalysis datasets, which would enable detailed model bias assessments and consequent constraints of projected EUC changes. This study undertakes this step, by implementing an offline particle tracer scheme using a high-resolution ocean reanalysis dataset (GLORYS). In agreement with observational studies and prior model results, we find that the majority of the off-equatorial EUC water originates from the southeastern subtropical Pacific (SESP) (14% of total). We identify a westward and increasing trend in this SESP source in the western-central Pacific portion of the EUC, and a southwestward and deepening trend in the water pathway from the SESP into the eastern Pacific portion of the EUC. We attribute these trends to the increased mixed layer depth and accelerated northwestward currents in SESP resulting from increased southwest trade winds over the region. The identified trends in the SESP water sources and pathways may contribute to the strengthening of the equatorial Pacific zonal SST gradient by allowing for enhanced water mixing along the preferred pathway towards the EUC and cooling of the EUC core. Thus, this study provides insights into the non-local ocean contributions to the tropical Pacific SST response to greenhouse gas forcing, which is characterized by significant disagreements between climate model simulations and observations in the past decades, casting doubt into the projected decrease of the zonal SST gradient under strong anthropogenic forcing in the future.