The changes in atmospheric poleward energy transport with global warming

Under climate change, the radiative effects of changes in aerosols, water vapor, clouds, and sea ice on the atmospheric energy budget and global mean temperature have been discussed in numerous studies. It is well known that variations in climate sensitivity among global climate models (GCMs) are largely attributable to differences in these components that affect the radiation budget. Here, we investigate the latitudinal structure of these radiative effects and their influence on meridional energy transport. More specifically, we analyze the atmospheric energy budget and poleward energy transport in simulations from the WCRP CMIP3 multimodel database, and use an one-dimensional energy balance model (EBM) that diffuses moist static energy (MSE) to examine how the MSE fluxes change in response to variation in radiative fluxes. Without addressing variation of diffusivity across latitudes or in different climate states, we show that the EBM with constant diffusivity explains most of the spread in changes in energy transport among GCMs.

In this simple diffusive framework, we study how individual components affect energy transport in three latitude bands: (1) 70 degrees, where increasing poleward energy transport may cause more ice melting, (2) 40 degrees, where eddies are the strongest, and (3) the deep tropics, where GCMs do not agree with the sign of the changes in transport and where the intertropical convergence zone (ITCZ) is located. In high latitudes, positive radiative effects from melting sea ice decreases the equator-to-pole temperature gradient and prevents poleward fluxes from increasing. Models that have more ice melting tend to predict less increase in the energy transport, which could be counter-intuitive based on the argument that increasing poleward transport may lead to melting sea ice. The cooling effect of increasing low clouds over newly open ocean along the ice edge sharpens the temperature gradient and increases the energy transport in midlatitudes. Clouds and sea ice in extratropics can also influence energy transport at the equator, which may shift the ITCZ.

This work highlights that biases in clouds, surface albedo, aerosols, and ocean heat uptake will not only affect climate locally but will also influence other latitudes through energy transport. It also provides a simple framework to evaluate how climate in other latitudes may have an impact when the interest of study is focusing on a particular region.