Interdecadal modes of variability through a hierarchy of ocean models

The fundamental mechanisms of interdecadal variability of the thermohaline circulation are investigated through a hierarchy of ocean models, from quasigeostrophic (QG), shallow water (SW), to planetary geostrophic (PG) and primitive equations (PE). In idealized geometry, this variability occurs sponta-neously under constant (thermal or haline) forcing, and hence does not rely on so-called mixed boundary conditions. Linear stability analyses show it appears through an unstable or weakly damped linear mode. This mode is tracked through the hierarchy of ocean models, from the classical Rossby basin modes in QG models, to the deformed and recirculating baroclinic modes when a mean gyre circulation is included, to the unstable modes when large-scale baroclinic instability is enabled (SW, PG). Here the results from a 2.5-layer shallow-water model are presented, and show the emergence of large-scale low-frequency basin modes, the most weakly damped being trapped in the closed geostrophic contour pool when the mean circulation has a realistic amplitude. 1