Changing available energy for extratropical cyclones and associated convection in Northern Hemisphere summer

Abstract

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.

Keywords: available energy; climate change; convection; extratropical cyclones.

Fig. 1.

Observed changes in summer (JJA) temperature and moisture of the Northern extratropics. (A) Median JJA temperature trend in 10° latitude bands from the IUKv2 radiosonde dataset (3) (1979–2015), and (B) median JJA specific humidity trend in 10° latitude bands from the homoRS92 radiosonde dataset (4) (1979–2010). See Methods for datasets and calculation details.

Fig. 2.

Visualization of MAPE calculations. Parcel rearrangements in the calculation of (A) moist MAPE and (B) nonconvective MAPE based on climatological JJA zonal-mean temperatures and relative humidities from the ERA-Interim reanalysis (1979–2017). Black contours (contour interval 100 hPa) show the pressure of a given air parcel in the minimum-enthalpy state, referred to as the reference pressure. Arrows schematically indicate vertical motion of parcels. The red dashed lines show where the reference pressure is equal to the pressure. Blue shading in A indicates a region of lower-tropospheric air bounded by a discontinuity in the reference pressure distribution, whose ascent to the upper troposphere corresponds to the release of convective instability.

Fig. 3.

Time series and trends of energetic reservoirs for summer in the Northern extratropics. (A) Percent anomaly from climatological (1979–2017) mean for nonconvective MAPE (blue line) and EKE (purple line). (B) Convective MAPE, which is defined as the difference between moist MAPE and nonconvective MAPE. All results shown are for JJA over 20–80N based on ERA-Interim reanalysis. Trends and associated 90% confidence intervals are given in each panel. The dashed black lines show the linear best-fit trends for (A) nonconvective MAPE and (B) convective MAPE.

Fig. 4.

Energetic reservoirs of idealized atmospheres. (A) Nonconvective MAPE and (B) convective MAPE in idealized atmospheres over the latitude band 20–80°N as a function of mean surface temperature and mean surface meridional temperature gradient in that latitude band. The idealized atmospheres are representative of Northern Hemisphere summer (see Methods for details). Contour intervals are 100 J kg⁻¹ in A and 1.25 J kg⁻¹ in B. Red arrows indicate changes in the JJA atmosphere based on ERA-Interim trends over 1979–2017 (see SI Appendix, Fig. S7 and Methods for details), and white dashed line extends these changes along the same slope for clarity.