Meridional Heat Transport in the DeepMIP Eocene ensemble: non-CO2 and CO2 effects

The total meridional heat transport (MHT) is relatively stable across different climates. Nevertheless, the strength of individual processes contributing to the total transport are not stable. Here we investigate the MHT and its main components especially in the atmosphere, in five coupled climate model simulations from the Deep-Time Model Intercomparison Project (DeepMIP). These simulations target the Early Eocene Climatic Optimum (EECO), a geological time period with high CO₂ concentrations, analogous to the upper range of end-of-century CO₂ projections. Preindustrial and early Eocene simulations at a range of CO₂  levels (1x, 3x and 6x preindustrial values) are used to quantify the MHT changes in response to both CO₂ and non-CO₂ related forcings. We found that atmospheric poleward heat transport increases with CO₂, while the effect of non-CO₂ boundary conditions (e.g., paleogeography, land ice, vegetation) is causing more poleward atmospheric heat transport on the Northern and less on the Southern Hemisphere. The changes in paleogeography increase the heat transport via transient eddies at the mid-latitudes in the Eocene. The Hadley cells have an asymmetric response to both the CO₂ and non-CO₂ constraints. The poleward latent heat transport of monsoon systems increases with rising CO₂ concentrations, but this effect is offset by the Eocene topography. Our results show that the changes in the monsoon systems’ latent heat transport is a robust feature of CO₂ warming, which is in line with the currently observed precipitation increase of present day monsoon systems.