Drivers of the projected changes to the Pacific Ocean equatorial circulation

Abstract

Climate models participating in the third Coupled Model Inter Comparison Project (CMIP3) suggest a significant increase in the transport of the New Guinea Coastal Undercurrent (NGCU) and the Equatorial Undercurrent (EUC, in the central and western Pacific) and a decrease in the Mindanao current and the Indonesian Throughflow. Most models also project a reduction in the strength of the equatorial Trade winds. Typically, on ENSO time scales, a weakening of the equatorial easterlies would lead to a reduction in EUC strength in the central Pacific. The strengthening of the EUC projected for longer timescales, can be explained by a robust projected intensification of the south‐easterly trade winds and an associated off‐equatorial wind‐stress curl change in the Southern Hemisphere. This drives the intensification of the NGCU and greater water input to the EUC in the west. A 1½‐layer shallow water model, driven by projected wind stress trends from the CMIP3 models demonstrates that the projected circulation changes are consistent with a purely wind driven response. While the equatorial winds weaken for both El Niño events and in the projections, the ocean response and the mechanisms driving the projected wind changes are distinct from those operating on interannual timescales.