Changes in Available Potential Energy in Northern Hemisphere Summer in Response to Climate Change

The satellite era has seen significant changes in the northern hemisphere midlatitude storm tracks in summer, including a robust downward trend in eddy kinetic energy (Coumou et al., 2015). There are also indications that convection is becoming more important, with an upward trend in the convective component of Eurasian precipitation (Ye et al., 2017). The midlatitude storm tracks draw energy from the mean available potential energy (MAPE), defined as the amount of kinetic energy that can be released by reversible parcel rearrangements in the atmosphere. Here, we examine the dry MAPE, the moist MAPE, and the convective component of moist MAPE in reanalysis products over the satellite era. We find robust downward trends in dry MAPE, consistent with the decreases in eddy kinetic energy. However, we also find robust upward trends in the convective component of moist MAPE, possibly consistent with the observed increase in the convective fraction of precipitation. By considering idealized atmospheric states, we explore how moist MAPE and its convective component respond to changes in the mean surface temperature and surface meridional temperature gradient that are characteristic of climate change. Finally, a simple one-column approach is used to explore the basic mechanisms underpinning the increases in the convective component of moist MAPE.