How is the Increase in the Variability of ENSO Events over the Last 6000 Years Linked to the Mean State Change of the Tropical Pacific Ocean ?

El Niño events are the dominant mode of tropical interannual climate variability. This phenomenon, coupled with changes in atmospheric pressure related to the Southern Oscillation, modifies the distribution of surface water temperatures and weather conditions via atmospheric teleconnections. To better understand the linkages between changes in ENSO characteristics and changes in the Pacific ocean mean state, we use two transient simulations of the last 6000 years performed with the IPSL model that differ in resolution and presence (or not) of dynamical vegetation. The objective is to test several hypothesis raised in the literature on the role of the thermocline and the different factors constraining its changes with time. This study will put an emphasis on the role of ocean dynamics. Several modelling studies indicate that an insolation-forced reduced equatorial upwelling feedback during the Mid-Holocene may be responsible for the less frequent ENSO events compared to modern. A few hypotheses have been made to explain this reduction in ENSO variability and equatorial upwelling feedback in the Mid-Holocene compared with today : subduction of warmer-than-normal South Pacific mode waters into the equatorial subsurface and tilt of the thermocline in the Warm Pool. Using specific diagnoses, we discuss the relative strength of different processes and highlight the differences between the processes explaining the long-term trend in variability and those characterising multidecadal to centennial variability.