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 pe riods of a few months to many years. Its essential simplification when comp ared with a conventional full-depth model (FDM) is the specification of an abyssal climatology for material properties. Observational analyses of temp erature and salinity fluctuations demonstrate their degree of confinement t o 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 tho se 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 solu tions agree well in both the mean state and short-term climate fluctuations , even for cases for which the model parameters and forcing are modestly in consistent with the UOM's abyssal climatology. A UOM can therefore be a use ful, efficient tool for studies of coupled climate dynamics and sensitivity to forcing fields and model parameters, and for hypothesis testing about t he roles of the abyssal ocean.