Relation between the meridional distribution of potential vorticity and the Lagrangian mean circulation in the troposphere

Abstract

The meridional distribution of potential vorticity (PV) in the troposphere is examined in termsof the Lagrangian transport by using an idealistic general circulation model. A zonally uniformforcing and uniform boundary conditions are applied to the model to particularly examine thePV structure in the mid-latitudes and the subtropics. Trajectories of air parcels released fromeach grid point of the model and Lagrangian changes in PV are calculated for a period of 60 days. Values of PV of each parcel are changing along the Lagrangian motions due to thediabatic effect, the frictional effect and the mixing effect which has smaller scales than thoseresolvable in the model. Both diabatic and frictional effects are dominant in the lower layers, and the mixing effect is larger in the other regions. It is found that the zonal mean PV changeshave different characteristics between the “Underworld” in which isentropes intersect the groundand the “Middleworld” in which isentropes are above the ground and intersect the tropopause.In the Underworld, the zonal mean PV changes are determined by the equatorial flow in thelower layers. In particular, the PV changes are negative in the lower layers of the low- and themid-latitudes. (The sign of PV tendency is for the northern hemisphere. The southern hemispherictendency is opposite as in the followings.) This negative tendency is due to the diabaticeffect near the surface. In the Middleworld, there remain positive and negative tendency regions, which are resulted from the isentropic mixing. In general, if a parcel moves poleward in themid-latitudes, the value of PV increases, whereas the value of PV decreases if a parcel movesequatorward. The sign of the Lagrangian mean change in PV corresponds to whether theLagrangian mean motions cross the PV contours equatorward or poleward in the meridionalplane. In particular, the contour of no change in PV has a similar shape to that of meridionaldistribution of PV in the mid-latitudes. This indicates that the directions of the Lagrangianmean motions are very close to those of the isolines of PV at the points on this contour.