Hysteresis of European Summer Mean and Extreme Precipitation in a CO2 Removal Experiment

This study investigates the changes in the mean and extreme summer precipitation over Europe in a CO2-removal experiment, in which CO2 concentration is quadrupled and reduced back to the current climate level symmetrically over a 280-year period. Due to the pronounced summertime drying caused by greenhouse gas-induced warming, the mean and extreme precipitation decrease and bounce back during the CO2 ramp-up (RU) and ramp-down (RD) period, respectively. Interestingly, the degree of reduction in the mean precipitation is two times that of extreme precipitation. Also, the mean precipitation shows hysteresis with respect to CO2 forcing, which is absent in extreme precipitation. We show that the changes in the mean precipitation, in which convective precipitation is the dominant component, are tightly associated with those of the mean moist convective instability (MCI). Compared to the RU period, the mean MCI is lower during the RD period with a drier lower troposphere which is due to lower soil moisture and weaker surface evaporation. In contrast, the changes in extreme precipitation, whose major component is large-scale precipitation, closely follow those of the mean column relative humidity. Our results suggest that the partitioning of total precipitation into convective and large-scale components in models needs to be considered when analyzing model-simulated future projections of regional precipitation changes.