the global three-dimensional ocean circulation: Lagrangian diagnostics based on general circulation model results

We point here a general question: where do water masses come from and where are they going. Pathways and mechanisms for oceanic heat and fresh water transports are critical issues in the understanding of the present climate and its stability. Indeed, the ocean circulation transfers heat and fresh water between different climate regimes and between different ocean basins. This transport is coupled to convective overturning which links the full ocean volume to the climate at decade-to-century time-scales. Inter-ocean circulation is generally implied except for sinking regions, which are compensated by local upwellings. But what is its form and how does it feed back into convective processes and their associated climate phenomena. Transports and pathways are often inferred by combining distinct (mostly Eulerian) sources of data and matching available pieces of knowledge on a basin or global scale. Yet, the most natural approach to estimate flows' origins and pathways is to follow the movement of water masses and their transformation. In this study we derive new quantitative and qualitative information about the description of the ocean mean seasonal state. This is obtained from combining quantitative Lagrangian diagnostics with observational climatologies and the dynamics computed by high performance ocean models. Numerical Lagrangian trajectories suggest the existence of a new route that brings relatively cold waters from the Pacific Ocean to the North Atlantic via the rather deep Tasman outflow. This "Tasman leakage" constitutes a sizeable component of the upper branch of the global ThermoHaline Circulation (hereafter THC) and represents an extension to the prevailing views, which hitherto emphasized the routes via the Drake Passage (the "cold route") and the Indonesian Throughflow (the "warm route"). Moreover, model pathways stress the role of the wind in influencing the return flow to the North Atlantic. The three southern subtropical gyres result intimately linked. Indeed, the wind field structure of the Southern Hemisphere and the limited southward extension of the African and Australian continents (compared to South America) allow a subtropical "supergyre" to exist. This wind-driven circulation is fairly deep, and reaches the lower thermocline. It affects strongly the modelled Southern Hemisphere Subantarctic mode and Antarctic intermediate water circulations. This would imply that the wind forcing in the Southern Hemisphere is strongly related to the freshwater transport into the South Atlantic and to the dynamical regime and stability of the THC. Additional examples of Lagrangian diagnostic applications will focus on specific circulation patterns of the Indian Ocean.