A Slower North Equatorial Countercurrent but Faster Equatorial Undercurrent in a Warming Climate

Abstract We analyze century-end projections for the tropical Pacific upper-ocean currents simulated within phase 6 of the Coupled Model Intercomparison Project (CMIP6) under global warming. We find that while the intensity of precipitation within the intertropical convergence zone (ITCZ) increases, the ITCZ also shifts toward the equator and broadens, which reduces wind stress curl north of the equator. Consequently, the North Equatorial Countercurrent (NECC) shifts equatorward, following the ITCZ, and weakens, despite the more intense ITCZ. The strength of the North Equatorial Current (NEC) and the South Equatorial Current (SEC) also decreases due to the weakening of the Walker circulation and the corresponding wind stress. However, despite the weaker winds, the Equatorial Undercurrent (EUC) intensifies as it shoals due to stronger vertical stratification induced by surface warming. Furthermore, we find a slightly stronger zonal pressure gradient along the core of the EUC, instead of a weaker one expected from weaker wind stress and sea surface height (SSH) gradient along the equator. Ultimately, we suggest that the reduced vertical friction, driven by enhanced ocean stratification and a higher Richardson number, is essential for the accelerated EUC. These intricate balances control future changes in equatorial currents, and the uncertainties of projected changes need to be further examined.