Time-Periodic Quasigeostrophic Motion under Dissipation and Forcing
  The quasigeostrophic equation is a prototypical geophysical fluid model. In this paper, we consider time-periodic motions of this model under dissipation and time-dependent wind forcing. We show that when the wind forcing is time-periodic and the spatial square-integral of the wind forcing is bounded in time, the full nonlinear quasigeostrophic model has time-periodic motions, under some conditions on $\beta$ parameter, Ekman number, viscousity and the domain size. 
Jinqiao Duan