A Comparative Study of Observed Northern Hemisphere Circulation Statistics Based on GFDL and NMC Analyses. Part II: Transient Eddy Statistics and the Energy Cycle

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Abstract
The comparison between two sets of observed circulation statistics undertaken by Lau and Oort (1981) is continued in this study by examining the temporal variance and covariance statistics in these sets. The first (GFDL) set is compiled by interpolating monthly averaged station statistics. The second set is based on twice-daily operational NMC analyses. The statistics for six winter and six summer seasons within the 1966–73 period are compared. The hemispheric fields examined include transient eddy kinetic energy at 300 mb, root-mean-squares of geopotential height and temperature at 300 and 850 mb, respectively, the horizontal transport by transient eddies of westerly momentum and geopotential height at 300 mb, and of heat at 850 mb. The patterns of horizontal eddy transports are presented in a vectorial format to delineate local relationships with the time-mean flow and the centers of eddy activity. Latitude–height distributions for zonally-averaged patterns of the above statistics are also presented. The transient eddy statistics in the two sets are in good agreement over regions with adequate data coverage. The NMC set generally gives relatively higher eddy amplitudes and stronger eddy transports over the data-sparse oceans. The maximum deviations between the two sets in these regions are about 20–30%. The two sets of analyses are further used to calculate the spatial integrals for the energy reservoirs and various energy conversion rates in the atmosphere. The transient and stationary eddies are treated separately in the formulation of the energy cycle. The largest differences are found in the transfer rate of kinetic energy from the stationary waves to the transient disturbances, and for the terms associated with the conversion of available potential energy into kinetic energy. The GFDL and NMC estimates of the other components of the energy cycle do not differ from each other by >20%. The results from both sets of analyses imply that the transient eddies are very efficient in depleting the available potential energy of the stationary waves through their ability to transport heat down the local temperature gradient. The dissipative time scale associated with this mechanism is several days.

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