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, a geological time period with high CO2 concentrations, analog to the upper range of end‐of‐century CO2 projections. Preindustrial and early Eocene simulations, at a range of CO2 levels 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 oceanic poleward heat transport decreases. The non‐CO2 boundary conditions cause more MHT toward the South Pole, mainly through an increase in the southward oceanic heat transport. The changes in paleogeography increase the heat transport via transient eddies at the northern mid‐latitudes in the Eocene. The Eocene Hadley cells do not transport more heat poleward, but due to the warmer atmosphere, especially the northern cell, circulate more heat in the tropics, than today. The monsoon systems' poleward latent heat transport increases with rising CO2 concentrations, but this change is counterweighted by the globally smaller Eocene monsoon area. 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.
N2 - Plain Language Summary: In the Earth's climate system both the atmosphere and the ocean are transporting heat through different processes from the tropics toward the poles. We investigate the transport of the atmosphere in several climate model set ups, which aim to simulate the very warm climate of the early Eocene (∼56–48 Myr ago). This period is relevant, because the atmospheric CO2 concentration was close to our pessimistic projection of CO2 concentration for the end of the century. In our study we separate the results into transport changes due to the different set up of the Eocene, and transport changes due to larger CO2 concentration values. We found that with rising CO2 values the atmosphere transports more heat from the tropics to the poles. The different location of the continents and seas is influencing the heat transport of the midlatitude cyclones. The Eocene tropical meridional overturning circulation's poleward heat transport does not increase, but it circulates more heat than today. The monsoon systems seem to be affecting a globally smaller area in the Eocene, but they are also more effective in transporting heat. This conclusion is in line with the observation, that current day monsoon systems' precipitation increases, as our CO2 concentration rises. N2 - Key Points:The latent heat transport of the monsoon increases through the Eocene higher CO2 concentration, but it is reduced by the Eocene topography
The poleward heat transport of midlatitude cyclones is higher in the Northern Hemisphere in the Eocene, due to the different topography
The Eocene northern Hadley cell circulates more heat, than in the present, while its net poleward heat transport is even less than today