Tropical Pacific mean state and ENSO changes: sensitivity to freshwater flux and remnant ice sheets at 9.5 ka BP

Abstract

The early Holocene, about 9,500 years ago (9.5 ka BP), was still affected by the presence of remnant ice sheets and their melting. Using as a reference a simulation of the early Holocene, the present study explores the relative contribution of these two factors on the climate mean state and ENSO variability in the tropical Pacific with the IPSL coupled model. The melting water flux and remnant ice sheets both induce a North Atlantic cooling and a southward shift of the ITCZ. Atmospheric teleconnections and local coupled ocean–atmosphere feedbacks lead to a remote SST cooling in the eastern equatorial Pacific. Both forcing factors also weaken the SST annual cycle in the eastern equatorial Pacific, closely related to the zonal wind stress anomalies. Compared with the early Holocene reference run, the freshwater flux experiment exhibits enhanced ENSO amplitude. A feedback analysis suggests that it is due to the wind–thermocline feedback. The remnant ice sheet experiment does not show significant change in ENSO. It exhibits a slight SST variability increase at the east coast and a reduction in the middle of the basin driven by the net heat flux feedback. All experiments reproduce the classical Eastern Pacific (EP) El Nino, and the central Pacific El Nino. The freshwater flux forcing strengthens the amplitude of EP El Nino events due to the stronger wind–thermocline feedback in this experiment. These results suggest that ice sheet melting and the remnant ice-sheet have partially counteracted the insolation forcing in the early Holocene.