Abrupt Intensification of ENSO Forced By Deglacial Ice-Sheet Retreat

Abstract:

The influence of ice-sheet retreat on the El Niño-Southern Oscillation (ENSO) variability is studied using a transient simulation in NCAR-CCSM3 forced only by the variations of continental ice sheets during the last deglaciation. The most striking feature is an abrupt strengthening of ENSO (by ~25%) at 14 ka BP in response to a significant retreat (an equivalent ~25 m sea-level rise) of the Laurentide ice sheet (LIS). This abrupt intensification of ENSO is caused mainly by a sudden reduction of the equatorial annual cycle through the nonlinear mechanism of frequency entrainment, rathe­­r than by the change in the large-scale tropical coupled ocean-atmosphere feedbacks (thermodynamic and mean advection dampings, the thermocline, the zonal advection and the Ekman upwelling feekbacks). The decrease of the annual cycle is a result of the reduction of the cross-equatorial SST contrast in the eastern Pacific. The more symmetric SST about the equator can be achieved predominantly by the anomalous easterly from abrupt polarward shift of jet stream in the Northern Hemisphere (NH), which propagates to tropical northeastern Pacific Ocean surface and is then reinforced by Wind-Evaporation-SST feedback. The fast sea-ice expansion in the NH induced by ice-sheet retreat could be another factor that contributes to the reduction of the cross-equatorial SST contrast and the intensification of ENSO.