The axial angular momentum balance of a global ocean general circulation model

In this study we examine the axial angular momentum balance of a non-eddy-resolving global ocean general circulation model, from the perspective of the geographical and seasonal variability of angular momentum and from the perspective of the torques acting on the ocean through its surfaces. Our purpose is to provide an estimate of the magnitude of the seasonal storage of angular momentum in the ocean and hence the oceanic excitation of variability in length of day, and to elucidate the role of the ocean in transferring angular momentum between the atmosphere and the Earth's crust. We provide an assessment of the reliability of the model results by examining the sensitivity of the angular momentum and torque distributions to several model parameters.Although the Southern Ocean region containing the Antarctic Circumpolar Current (ACC) makes the largest contribution to both the annual mean oceanic angular momentum and its seasonal variability, inclusion of the rest of the world ocean reduces both of these quantities to about two-thirds of the value of the Southern Ocean alone. The annual, global mean angular momentum is found to be insensitive to most model choices except for the isopycnal diffusivity. The seasonal variability, on the other hand, is insensitive to the isopycnal diffusivity, but sensitive to the smoothness of the representation of topography and moderately sensitive to horizontal and vertical friction parameterizations. The torque balance at all latitudes, including within the Antarctic circumpolar belt, is between wind stress and bottom pressure torques. Horizontal friction torques are small but non-negligible. Bottom friction and storage of angular momentum are negligible in angular momentum budgets on seasonal time scales. Two commonly used wind stress climatologies, one based on historical marine meteorological observations and the other based on operational weather analyses, differ in the sign of the globally integrated wind stress torque.