419
--- - ------------------------------------- ~-------~
--- - --------- ------------------------ ------- --------Figure 4: Sketch of the two-layer model. There is a density jump PI - P2 across the
interface of the layers giving rise to a reduced gravity g' = g{P2 - PI)/ P2.
where h = HI! H2 is the ratio of the mean layer heights. The coefficient
f-L describes the interfacial friction in the momentum balance of the two
layers.
We will focus on scales L of motion which are large compared to the
internal Rossby radius ~nt = (g' HIHd f6(Hl +H2))~. With L2 » g'Hd f6
the relative vorticity \7 2 Wi is much smaller than the stretching termproportional to TJ and, because of R2 » RTnt' the upper surface may be replaced
by a rigid lid. Hence, we may further simplify the dynamics by neglecting
the relative vorticity and the surface stretching term, which leads to the
large scale QPV s
(18)
This approximation implies an infinite phase velocity of barotropic Rossby
waves, hence the barotropic part of the flow reacts instantaneously to