Speaker
Description
In a general circulation model (ECHAM6) configured to represent radiative-convective equilibrium coupled to a slab ocean, we explore a wide range of CO$_2$ concentrations. We obtain reliable statistical quantities from thousand-year-long simulations, and we characterize the horizontal scale of ascending and subsiding regions by the so-called integral length scale of the vertical velocity field at a given atmospheric level, which is based on the horizontal autocorrelation function of that field. For moderate CO$_2$ concentrations, we find weak spatial organization, which comes along with unskewed temporal variations of 1--2 K in global mean surface temperature and an almost constant climate sensitivity of 2 K. At CO$_2$ concentrations beyond four times the preindustrial value, the climate sensitivity decreases to nearly zero as a result of episodic global cooling events as large as 10 K, in association with global-scale convective self-aggregation and relying on the appearance of a low-level stratiform cloud field in the subsiding region which constitutes the global complement of the self-aggregated ascending region. We qualitatively sketch a potential description in terms of the phenomenology of spatially extended dynamical systems for the transition between the weakly organized and the self-aggregated regime.
