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The general circulation of the atmosphere
By S. Tibaldi* and R
Mureau
* Current address: University of Bologna,
Department of Physics, Via Imerio 46, 40126 Bologna, Italy
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4 The energy and momentum budgets: The role of the eddies
We will now discuss the energy cycle of the AGC by means of the so-called Lorenz box diagram. Fig. 6 shows an example of the diagram. The four main boxes represent available potential energy (PE) and kinetic energy (KE), while upper boxes contain energy terms of the zonal mean flow and lower boxes energy terms due to the eddy departures from the zonal mean. Arrows between boxes represent conversion terms, and therefore processes responsible for them. Conversions between PE and KE are due to baroclinic processes, while conversions between zonal and eddy KE are due to barotropic processes. For exact derivations of the energy and conversion terms in a quasi-geostrophic approximation, the student is referred to Holton's (1979) textbook.
Figure 6 . The observed energy cycle for the global atmosphere. Energy amounts inside each box are given in 105 J m-2, and rates of generation, conversion and dissipation in W m-2. Terms not directly measured are shown in parentheses.
Figure 7 Schematic picture of the dominant mechanism of northward transport of momentum by eddies in mid- latitudes of the northern hemisphere.
The solar heating provides input in the PM and PE boxes (to understand why in the PE box as well, think of the longitudinally inhomogeneous cloud cover), while dissipative processes provide output from KM and KE boxes (dissipative processes require motion, and therefore kinetic energy to take place). The mean meridional circulations are responsible for CZ conversions (but they are small); such symmetric zonal circulations are, however, baroclinically unstable. This instability, together with the presence of large-scale mountains and land-sea contrasts, generate eddies that are responsible for both CA (available zonal to available eddy) and CE (available eddy to kinetic eddy) energy conversions, and therefore to the main baroclinic energy cycle, whose main task is to transport heat in the north-south direction in a more efficient way than a symmetric Hadley-type circulation could do. Such eddies, however, due to the N-S tilt of their axes, also transport momentum in the N-S direction (see Fig. 7 ). This property is essential because it is the convergence of the latitudinal flux of momentum due to the eddies (together with the convergence of vertical eddy momentum flux), see Fig. 8 , that maintains those large-scale, meandering `tubes'of concentrated westerly momentum characteristic of the mid-latitudinal atmosphere, called jet streams. The position and intensity of such jets is therefore determined by subtle energy and momentum balance requirements.
This can also be inferred by looking at the precise geographical relationships between the meanders of the mean 500 hPa NH flow (see Fig. 9 (a)) and the time variance due to all transient eddies of short periods (Fig. 9 (b)) and to eddies of longer periods (Fig. 9 (c)). It is evident how high-frequency eddies tend to be generated more in the jet exit regions, while low-frequency eddies have their highest activity in correspondence of the highest flow diffluence.
We can conclude this section by underlining how important the interactions between the eddies and mean flow are. The eddies exist because the mean flow is both unstable and subjected to non-axisymmetric boundary conditions, but the essential mean flow characteristics are, in turn, heavily dependent upon the eddies themselves. Any GCM that aims at representing satisfactorily the mean state of the atmosphere and its variability around this mean state should therefore correctly model these eddy/mean-flow interaction.
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07.06.2002 |
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