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-e ddy-resolving global ocean general circulation model, from the perspec tive of the geographical and seasonal variability of angular momentum and from the perspective of the torques acting on the ocean through it s surfaces. Our purpose is to provide an estimate of the magnitude of the seasonal storage of angular momentum in the ocean and hence the oc eanic excitation of variability in length of day, and to elucidate the role of the ocean in transferring angular momentum between the atmosp here and the Earth's crust. We provide an assessment of the reliabilit y of the model results by examining the sensitivity of the angular mom entum and torque distributions to several model parameters. Although t he Southern Ocean region containing the Antarctic Circumpolar Current (ACC) makes the largest contribution to both the annual mean oceanic a ngular 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 angu lar momentum is found to be insensitive to most model choices except f or the isopycnal diffusivity. The seasonal variability, on the other h and, is insensitive to the isopycnal diffusivity, but sensitive to the smoothness of the representation of topography and moderately sensiti ve to horizontal and vertical friction parameterizations. The torque b alance at all latitudes, including within the Antarctic circumpolar be lt, is between wind stress and bottom pressure torques. Horizontal fri ction torques are small but non-negligible. Bottom friction and storag e of angular momentum are negligible in angular momentum budgets on se asonal time scales. Two commonly used wind stress climatologies, one b ased on historical marine meteorological observations and the other ba sed on operational weather analyses, differ in the sign of the globall y integrated wind stress torque.