Projected North Equatorial Current/Countercurrent toward an El Niño–Like Condition in the Western Pacific under Moderate and Worst-Case CO2 Emission Scenarios

Abstract Validating the mean state and historical trends of the North Equatorial Current (NEC) and North Equatorial Counter Current (NECC) in 36 phase 5 of the Coupled Model Intercomparison Project (CMIP) models and 32 phase 6 CMIP models with those in ocean reanalysis products, this study extracts the most qualified 17 model outputs to explore the projected changes in NEC and NECC in the western Pacific Ocean under scenarios of moderate- and worst-case CO 2 emissions. NEC and NECC are robustly predicted to have an El Niño–like change, in which the NEC (NECC) tends to intensify with a poleward (equatorward) movement, during the twenty-first century under both scenarios. Meanwhile, the Kuroshio and Mindanao Current are projected to become weaker. A change in the meridional gradient of basin-scale wind stress curl in the tropical Pacific Ocean is the major contributor to the centennial changes in NEC and NECC; however, anomalous wind stress curl dipole on the two sides of NEC off the eastern Philippines is the major factor in controlling the western boundary currents. This study not only represents a step forward in understanding the impact of global warming on the western tropical Pacific surface circulation but also serves as a reference for downscaled modeling, which can better resolve regional circulation and western boundary currents, for selecting suitable inputs from a large number of CMIP models. Significance Statement The North Equatorial Current and Countercurrent are important surface currents transporting massive heat and mass zonally across the tropical Pacific Ocean. With verified modeling outputs of phases 5 and 6 of the Coupled Model Intercomparison Project (CMIP), this study aims at understanding the 100-yr tendencies of the two currents in the western Pacific Ocean during the twenty-first century. The outcome shows that the North Equatorial Current (Countercurrent) tends to intensify with a poleward (equatorward) movement associated with the changes in the meridional gradient of wind stress curl across the entire tropical Pacific Ocean. The 100-yr tendencies are similar to those during an El Niño event. Meanwhile, Kuroshio and Mindanao Current are projected to be weaker during the twenty-first century due to the anomalous wind stress curl dipole off the Philippines in the western tropical Pacific Ocean. This study provides an understanding about the impact of global warming on the western tropical Pacific surface circulation and helps to select suitable initial conditions, boundary conditions, and forcing from a large number of CMIP models when conducting a downscaling modeling for the western Pacific area.