Weak Equatorial Superrotation During the Past 250 Million years

For modern Earth, the annual-mean equatorial atmosphere is flowing from east to west or called easterly winds. This is mainly due to the deceleration effect of the seasonal cross-equatorial flows of the Hadley cells, against the acceleration effect of equatorial Rossby and Kelvin waves excited from tropical convection and latent heating release. In this work, we examine the evolution of equatorial winds during the past 250 million years (Ma) using the global Earth system model CESM1.2.2. Three climatic factors different from the modern Earth, solar constant, atmospheric CO2 concentration, and land-sea configuration, are considered in the simulations. We find that the equatorial winds in the upper troposphere change the sign to westerly flows or called atmospheric superrotation in certain eras. The strength of the superrotation is comparable to the magnitude of the present easterly winds, several meters per second, not strong. This phenomenon occurs when the waves are relatively stronger and/or the Hadley cells are relatively weaker, which in turn are due to the changes in the three factors.