Differences in Poleward Energy Transport between Two Versions of the Community Earth System Model

The tropics receive more energy from solar heating than they lose to longwave emission, which results in a net energy uptake at low latitudes. The opposite is true at the poles, which experience a net energy loss on average. This meridional heating gradient implies a necessary Poleward Energy Transport (PET) from the tropics to higher latitudes, which is accomplished mainly through Atmospheric Heat Transport (AHT) and Oceanic Heat Transport (OHT). A recent study found that aerosol forcing contributed to a large anomaly in PET in the second half of the 20th century in a large ensemble of historical simulations performed with the Community Earth System Model (CESM), version 2. That study also found that OHT was the primary contributor to the total PET anomaly, which they discussed in the context of recent work that has shown that CMIP6 models (including CESM2) tend to overestimate the response of the Atlantic Meridional Overturning Circulation (AMOC) to historic aerosol forcing. In this study, we compare the response of the PET to historic forcings across two generations of the CESM and find that the PET response to aerosol forcing is larger in CESM2 compared to CESM1. We then investigate which physical processes may be contributing to the dissimilarity in PET. We find that CESM2 exhibits larger PET and AMOC anomalies versus CESM1, which we attribute to an increased AMOC response to aerosol forcing in the newer model. We also find that the cloud radiative effect is larger in CESM2 versus CESM1.