The sensitivity of climate and climate change to the efficiency of atmospheric heat transport

Atmospheric heat transport (AHT) moderates spatial gradients in surface temperature, and its efficiency (hereinafter referred to as diffusivity) shapes the distribution of moist static energy and the hydrological cycle. Using a linear downgradient rule for AHT, we diagnose zonal-mean diffusivity using observational and model data. We find it varies two- to threefold with season and latitude, but is nearly invariant across different climate states. We then employ a moist energy balance model (MEBM) to explore the impacts of changing the magnitude and spatial pattern of diffusivity on the climatology and climate response to forcing. Spatial anomalies in diffusivity in the extra-tropics have a larger impact on temperature and hydrology than diffusivity anomalies in the tropics. We demonstrate that compensating dynamical adjustments in the MEBM act to mute the impact of changing diffusivity patterns on the resulting climate. We isolate the impacts of spatial patterns of forcing, ocean heat uptake, radiative feedbacks, and diffusivity on the spatial pattern of climate change; and find that the pattern of climate change is least sensitive to the detailed pattern of diffusivity. Overall, these results suggest that although diffusivity is far from spatially invariant, understanding the climatology and spatial patterns of climate change does not depend on a detailed characterization of the spatial pattern of diffusivity.