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 the Climate Model Intercomparison Project Phase 6 (CMIP6) under global warming. We find that while the intensity of precipitation within the Intertropical Convergence Zone (ITCZ) increases, the ITCZ also shifts towards 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 gradient along the equator. Ultimately, we argue that it is reduced vertical friction theory due to increased ocean stratification and hence larger Richardson number that explains the faster EUC. These intricate balances control future changes in equatorial currents, and the uncertainties of projected changes need to be further examined.