  The predictability of the atmosphere at short and long time scales,
associated with the coupling to the ocean, is explored in a new version of the
Modular Arbitrary-Order Ocean-Atmosphere Model (MAOOAM), based on a 2-layer
quasi-geostrophic atmosphere and a 1-layer reduced-gravity quasi-geostrophic
ocean. This version features a new ocean basin geometry with periodic boundary
conditions in the zonal direction. The analysis presented in this paper
considers a low-order version of the model with 40 dynamical variables.
  First the increase of surface friction (and the associated heat flux) with
the ocean can either induce chaos when the aspect ratio between the meridional
and zonal directions of the domain of integration is small, or suppress chaos
when it is large. This reflects the potentially counter-intuitive role that the
ocean can play in the coupled dynamics.
  Second, and perhaps more importantly, the emergence of long-term
predictability within the atmosphere for specific values of the friction
coefficient occurs through intermittent excursions in the vicinity of a
(long-period) unstable periodic solution. Once close to this solution the
system is predictable for long times, i.e. a few years. The intermittent
transition close to this orbit is, however, erratic and probably hard to
predict.
  This new route to long-term predictability contrasts with the one found in
the closed ocean-basin low-order version of MAOOAM, in which the chaotic
solution is permanently wandering in the vicinity of an unstable periodic orbit
for specific values of the friction coefficient. The model solution is thus at
any time influenced by the unstable periodic orbit and inherits from its
long-term predictability.
