Variations of the Northern Hemisphere Atmospheric Energetics: 1948–2000*

Many studies have shown evidence of a major climate change in the late 1970s and early 1980s. The change comprises a reversal of the sea surface temperature anomaly pattern in the North Pacific Ocean, a lowering of the atmospheric geopotential height in the North Pacific, altered frequency and intensity of cyclones/anticyclones and severe storms in the mid-and high latitudes, along with an "abrupt" increase of the Northern Hemisphere (NH) average surface air temperature. What do these changes mean in terms of the nature of the NH atmospheric circulation in the warmer climate? Do they indicate changes of the atmospheric energy cycle and conversion/ exchange between various energy forms in the atmosphere? In which latitude regions are such changes in the energetics most significant? These are some of the questions examined in this study for the period 1948–2000 using the NCEP–NCAR reanalysis data. Major results of this study show significant increases of both the NH mean and eddy kinetic energy in boreal summer and winter in the recent two decades since 1980. The NH mean available potential energy has remained unchanged, however, even though the generation of the available potential energy has increased after 1980. The extra available potential energy generated is found to be converted to the kinetic energy by a more efficient conversion from the available potential energy to the kinetic energy in the warmer climate. The increase of the NH kinetic energy is attributed to an increase of the midlatitude kinetic energy because the kinetic energy in the tropical region has decreased slightly after 1980. Additional analysis of the NH energetics in wavenumber domain further reveals an increase of the kinetic energy for motions of planetary to regional scales. Increases of both the mean and eddy kinetic energy in the mid-and high latitudes is consistent with the reports of rising intensity of synoptic systems, cyclones and anticyclones, and severe storms in the recent two decades.