ON THE MAINTENANCE OF POTENTIAL VORTICITY IN ISENTROPIC COORDINATES

We present a diagnostic study of the maintenance of the potential vort icity (PV) on isentropic surfaces, in the troposphere to the lower str atosphere, using ten years of analyses produced by the European Centre for Medium-Range Weather Forecasts. After a brief three-dimensional d escription of the general circulation in isentropic coordinates, we ex amine the budgets of the PV evolution equation. By decomposing the flo w into its mean, high-frequency transient and low-frequency transient parts, we assess the role of these various scales of motion in the mai ntenance of the mean PV distribution. The contribution of vortical and thickness fluxes is also investigated. One interesting result is the key role of the divergent part of the transient flow along the tropopa use in the midlatitudes, which creates a large PV sink region. We give here a tentative explanation. The PV maps are also used to generate a climatology of a wave activity called pseudomomentum, that indicates four centres of wave activity over the Atlantic and Pacific. The first two are located in the northern hemisphere storm tracks and the two o thers in the upper tropospheric subtropical areas between the 310 K an d the 350 K levels.