Oceanic rings tend to have length scales larger than the deformation r
adius and also to be long-lived. This latter characteristic, in view o
f the former, is particularly curious as many quasigeostrophic and pri
mitive equation simulations suggest such eddies are quite unstable. La
rge eddies eventually break into smaller deformation scale vortices, w
ith the attendant production of considerable variability. Here it is a
rgued that the stability characteristics of oceanic eddies and rings a
re sensitive to the presence of deep flows. In particular, eddies in w
hich the deep flow is counter to the sense of the shallow flows are of
ten more unstable than eddies with no deep flow, while eddies with cir
culations in the same sense as the shallow circulation can experience
an enhanced stability. For a given vertical shear, oceanic eddy stabil
ity can vary dramatically. (This is in contrast to quasigeostrophic th
eory, where stability properties are largely determined by vertical sh
ear.) The onset of these mechanics is quite pronounced for Gaussian oc
eanic eddies. Linear ''f''-plane stability calculations reveal a marke
d suppression of unstable growth rates for warm corotating eddies with
relatively weak deep flows. Cold eddies also experience a suppression
of instability in the corotating state, although relatively weak unst
able modes have been found. Comparisons of f- and beta-plane numerical
primitive equation experiments support these results, as well as demo
nstrate some relevant limitations. Finally, studies of dipolar eddies
and non-Gaussian circular eddies are used to examine the generality of
the results, We suggest such stability considerations may be partiall
y responsible for the observed long lives of oceanic rings. An examina
tion of the unstable normal modes from the f-plane model demonstrates
an intimate coupling between the suppression of instability and the ap
pearance of multiple critical layers. The normal-mode energetics are u
sed to demonstrate the role of upgradient momentum fluxes at the point
s of stabilization, and a heuristic argument involving critical layers
is given.