The thermohaline circulation of the ocean results primarily from warm surface waters that flow northward in the North Atlantic, cool and sink in the subpolar and polar regions, and then flow southward, enter the South Atlantic and eventually all the ocean basins, where they upwell slowly into the upper oceanic layers and return to the North Atlantic with the wind-driven circulation.

This circulation is known as the oceanic Conveyor Belt (CB hereafter; Broeker, 1987). Its flux is estimated to lie between 14 and 17 Sv (Schmitz, 1995).

For the past fifteen years, the nature and the very existence of the CB as a coherent ocean circulation on the global scale have been the topics of permanent debate among physical oceanographers. While the observations of the ocean and compute power increase, various pictures for the CB have been proposed, such as the four layer CB scheme of Schmitz (1986) here shown below.

In this framework, the major sources of debate consist in the definition of the upwelling regions of the deep and intermediate layers and the characteristics and paths of warm limb waters composing the return flow to the North Atlantic of the CB.

Historically, the upwelling process was thought to take place everywhere in the ocean (Stommel and Arons, 19??). Nevertheless, observations of turbulence and dye diffusion indicate a background diapycnal mixing too low to be consistent with this hypothesis (Munk, 1966; Munk and Wunsh, 1998). More recently, various modelling studies have advanced the Ekman suction in the Southern Ocean as a very efficient deep water transformer (Toggeweiler and Samuels, 1998; Döös and Coward, 1997).

Therefore, the Southern Ocean appears to be not only the channel through which the North Atlantic Deep Water (NADW) is able to spread over the global ocean but also a very important region of transformation of the latter. Moreover, the Southern Ocean is the unique link to the Atlantic Ocean for Pacific and Indian waters forming the upper branch of the CB (with exception of the Bering Strait through which flows less than 1 Sv of water linked to the CB). Yet, the exact kind and amount of water masses that form the return flow of the CB in the North Atlantic as well as their paths and origins are still uncertain and they feed various controversial hypotheses.

The aim of this poster is to recover the paths and characteristics of upper-level CB taking advantage of the very Lagrangian nature of the problem. The output of a complex and realistic ocean model is analysed with newly developed Lagrangian techniques to produce a global circulation scheme that helps and completes our physical understanding of the three-dimensional ocean circulation.