| According to the framework of energy cycle proposed by Lorenz in1967, the atmospheric energy cycle is estimated on a monthly, quarterly and annual basis in the Northern Hemisphere using NCEP/NCAR Reanalyses for the period of1958-2011. Both the available potential and kinetic energy are decomposed into components for eddies and zonal and time mean circulations. The temporal and spatial distribution and variations of energy reservoirs and conversions have been investigated in the present paper. The major results are summarized as follow:(1) The atmospheric energy cycle exists annual variations obviously. Reservoirs and conversion rates change in the same phase, with values larger in winter, smaller in summer. The interhemispheric exchanges of zonal mean available potential energy at equator may play an important role in the Northern Hemispheric energy budget. When in winter, the energy is transported northward from the Southern Hemisphere into the Northern Hemisphere, and vicevers in summer. The annual variations of generations of the two available potential energies and dissipations of kinetic energies are roughly out of phase. A maximum in generation of the available potential energies takes place in summer or fall. While dissipations of both zonal mean and eddy kinetic energy are stronger in winter and weaker in summer.(2) Available potential energies are mainly distributed in the mid-lower troposphere. The zonal mean energy has a maximum in high latitudes and closes to zero near35°N, and the eddy energy is found largest in mid-and high-latitudes and smallest at equator. Kinetic energies are mainly reserved in the upper troposphere. The zonal mean energy is larger in mid-low latitudes and smaller in equator and high latitudes, whereas the eddy energy is larger in middle and high latitudes. Further more, the mid-latitudes are the regions where the energy converts from one kind into another actively. The maximum conversion rates between the available potential energies and between the eddy energies appear in low troposphere, whereas the other two conversions appear with maximum rates in high troposphere.(3) As compared to those estimated values obtained from the monthly mean circulations, the yearly mean values of zonal mean energies are smaller and the eddy energies are larger in the annual mean circulations, which contains more eddies of various time scales.(4) Evidently, the energy cycle of the Northern Hemispheric atmosphere varies on interannual (interdecadal) time-scale and also changes with the long term trends. It shows that the annual mean atmospheric energy cycle varies with periodicities of2-4a. But, winter mean energy cycle changes with periodicities of3-6a, and summer mean cycle with periodicities of2-5a. In the case of annual mean, the increases of energy conversion rates facilitate both the decreases of zonal mean energies and the increases of eddy energies, indicating a close relationship between the reservoirs and conversions of the energy cycle. The increasing eddy energies suggest that eddy motions in troposphere become more active, and the conversion processes of atmospheric energies have been somewhat intensified in recent years. The trends of winter mean energy cycle are similar to those of the annual mean’s. But the trends of the summer’s are different. Except the conversion rate between the eddy energies has been increased, reservoirs and other conversion rates have generally changed with decreasing trends, indicating that eddy motions in summer troposphere become weaker. These results are helpful for us to better understand the atmospheric energetics and the mechanisms of the general circulation changes in the Northern Hemisphere. |
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