Summary: We study a three-dimensional thermocline planetary geostrophic `horizontal' hyper-diffusion model of the gyre-scale midlatitude ocean. Let \(\Omega=M\times(-h,0)\in\mathbb{R}^3\), where \(M\) is a smooth domain in \(\mathbb{R}^2\), or \(M=(0,1)\times(0,1)\). The planetary geostrophic (PG) equations with friction and diffusion can be written as: \[ \nabla p+f\vec k\times v+{\mathcal D}=0,\qquad \partial_z+p+ T=0, \] \[ \nabla\cdot v+\partial_zw=0,\qquad\partial_tT+v\cdot\nabla T+w\partial_zT=K_vT_{zz}-\nabla\cdot q(T), \] where \({\mathcal D}\) is the friction or dissipation of momentum and \(\nabla\cdot q(T)-K_vT_{zz}\) is the heat diffusion. We show the global existence and uniqueness of the weak and strong solutions to this model. Moreover, we establish the existence of a finite-dimensional global attractor to this dissipative evolution system. Preliminary computational tests indicate that our hyper-diffusion model does not exhibit any of the non-physical instabilities near the lateral boundary which are observed numerically in other models.