Spontaneous transition to superrotation driven by a large-amplitude MJO in very warm climate simulations

Recent paleoclimate proxy reconstructions indicate that global-mean surface temperatures in past warm climates may have been some 10-15C higher than today. Little is known about atmospheric dynamics at temperatures this warm. In particular, is it possible that the general circulation may transition to a qualitatively different state at warm enough temperatures? We study this question in a set of simulations using NCAR's Commmunity Atmosphere Model coupled to a slab ocean with a range of CO2 concentrations extending from preindustrial values (280 ppm) up to 8960 ppm (5 doublings). We find that when equatorial surface temperatures exceed ∼33°C, the model undergoes a transition to equatorial superrotation, a state with strong annual‐ and zonal‐mean westerlies on the equator. The transition is driven by zonal momentum convergence due to large‐amplitude transient eddies on the equator. These eddies have a structure similar to the observed Madden‐ Julian Oscillation (MJO). The model's MJO variability is weaker than observed when simulating the modern climate but increases sharply with temperature, coming to dominate the tropical variability and mean state of the warmest climates. We conduct sensitivity experiments to explore the mechanisms responsible for intensification of the tropical waves.