Recent paleoclimate proxy reconstructions show that tropical surface temperatures may have been as high as 35-40C in the Early Cenozoic. Here, we study the tropical atmospheric circulation's response to temperatures in this range using a full-complexity atmospheric general circulation model (AGCM). We find that when equatorial surface temperatures exceed ～33C, 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.
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