Long-Term Trends of the Atmospheric Circulation and Moist Static Energy Budget in the JRA-55 Reanalysis 

Abstract

The atmospheric circulation response to global warming is an important problem that is theoretically still not well understood. This is a particular issue since climate model simulations provide uncertain, and at times contradic- tory, projections of future climate. In particular, it is still unclear how a warmer and moister atmosphere will affect midlati- tude eddies and their associated poleward transport of heat and moisture. Here we perform a trend analysis of three main components of the global circulation}the zonal-mean state, eddies, and the net energy input into the atmosphere}and examine how they relate in terms of a moist static energy budget for the JRA-55 reanalysis data. A particular emphasis is made on understanding the contribution of moisture to circulation trends. The observed trends are very different between the hemispheres. In the Southern Hemisphere there is an overall strengthening and during boreal summer, also a poleward shifting, of the jet stream, the eddies, and the meridional diabatic heating gradients. Correspondingly, we find an overall strengthening of the meridional gradients of the net atmospheric energy input. In the Northern Hemisphere, the trend pat- terns are more complex, with the dominant signal being a clear boreal winter Arctic amplification of positive trends in lower-tropospheric temperature and moisture, as well as a significant weakening of both bandpass and low-pass eddy heat and moisture fluxes. Consistently, surface latent and sensible heat fluxes, upward and downward longwave radiation, and longwave cloud radiative fluxes at high latitudes show significant trends. However, radiative fluxes and eddy fluxes are in- consistent, suggesting data assimilation procedures need to be improved.