An upper-ocean model for short-term climate variability

The authors propose and assess principles for the design of an upper-ocean model (UOM) suitable for studies of large-scale oceanic variability over periods of a few months to many years. Its essential simplification when compared with a conventional full-depth model (FDM) is the specification of an abyssal climatology for material properties. Observational analyses of temperature and salinity fluctuations demonstrate their degree of confinement to the upper ocean. Two idealized models for diffusive penetration of tracer fluctuations and for wind-driven currents show that the UOM approximations are usually accurate for the phenomena of interest. A UOM for the oceanic general circulation is constructed, and its solutions are compared with those of an equilibrium FDM. From a stratified resting state, the UOM spins up to an equilibrium state over a period of about 30 yr. The UOM and FDM solutions agree well in both the mean state and short-term climate fluctuations, even for cases for which the model parameters and forcing are modestly inconsistent with the UOM's abyssal climatology. A UOM can therefore be a useful, efficient tool for studies of coupled climate dynamics and sensitivity to forcing fields and model parameters, and for hypothesis testing about the roles of the abyssal ocean.