Geometry of the land-ocean distribution in the model. Left: global ocean distribution where the eigenfunctions are the SPHs. Right: bounded circular ocean distribution where the eigenfunctions are the SCHs. In the bounded case, the frame of reference ℛ_oc: (𝒪, e_x, e_y, e_z) introduced in Sect. 2.1 is such that e_z corresponds to the symmetry axis going through the centre of the ocean basin, while it is not specified in the global case due to the spherical symmetry.