Changing available energy for extratropical cyclones and associated convection in Northern Hemisphere summer
Author(s)Gertler, Charles G.; O’Gorman, Paul A.
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The circulation of the Northern Hemisphere extratropical troposphere has changed over recent decades, with marked decreases in extratropical cyclone activity and eddy kinetic energy (EKE) in summer and increases in the fraction of precipitation that is convective in all seasons. Decreasing EKE in summer is partly explained by a weakening meridional temperature gradient, but changes in vertical temperature gradients and increasing moisture also affect the mean available potential energy (MAPE), which is the energetic reservoir from which extratropical cyclones draw. Furthermore, the relation of changes in mean thermal structure and moisture to changes in convection associated with extratropical cyclones is poorly understood. Here we calculate trends in MAPE for the Northern extratropics in summer over the years 1979–2017, and we decompose MAPE into both convective and nonconvective components. Nonconvective MAPE decreased over this period, consistent with decreases in EKE and extratropical cyclone activity, but convective MAPE increased, implying an increase in the energy available to convection. Calculations with idealized atmospheres indicate that nonconvective and convective MAPE both increase with increasing mean surface temperature and decrease with decreasing meridional surface temperature gradient, but convective MAPE is relatively more sensitive to the increase in mean surface temperature. These results connect changes in the atmospheric mean state with changes in both large-scale and convective circulations, and they suggest that extratropical cyclones can weaken even as their associated convection becomes more energetic.
DepartmentMassachusetts Institute of Technology. Department of Earth, Atmospheric, and Planetary Sciences
Proceedings of the National Academy of Sciences of the United States of America
Proceedings of the National Academy of Sciences
Gertler, Charles G. and Paul A. O’Gorman. "Changing available energy for extratropical cyclones and associated convection in Northern Hemisphere summer." Proceedings of the National Academy of Sciences of the United States of America 116 (2019): 4105-4110 © 2019 The Author(s)
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