|
|
Problem >>
Strategy >>
Routes >>
Three Paths >>
Warm, Cold and Tepid Routes >>
Comparison with other GCMs >>
A17 Comparison
>>Conclusions
INTRODUCTION The thermohaline circulation of the ocean results primarily from surface warm waters that flow northward in the North Atlantic, cool and sink in the polar and subpolar regions, and then flow southward, enter the South Atlantic and eventually all the other ocean basins, where they upwell slowly into the upper oceanic layers and return to the North Atlantic within the wind-driven circulation. This circulation is known as the oceanic Conveyor Belt (CB hereafter; Broecker, 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 are the topics of permanent debates among physical oceanographers. With increased computer power and number of direct observations, different pictures for the CB have been proposed as, for instance, the Schmitz (1986) four layer CB scheme shown below.
|
|
In this framework, the major sources of debate nowadays 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 of the CB to the North Atlantic.
Historically, the upwelling process was though to take place everywhere in the ocean (Stommel and Arons, 1960). 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 proposed the Ekman suction in the Southern Ocean as a very efficient deep water converter (Döös and Coward, 1997; Toggweiler and Samuels).
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 crucial region of transformation. 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 the exception of the Bering Strait through which less than 1 Sv is 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.
For instance, classically oceanographers distinguish a
Warm Route and a Cold Route for the supply of water into the Atlantic. For the Warm Route (Gordon, 1986), the Pacific and Indian oceans are linked to the upper Atlantic via the Indonesian Throughflow with an exchange of warm water south of Africa. For the Cold Route (Broeker, 1991; Rintoul, 1991), the dominant contribution of water and heat into the Atlantic is obtained directly from the Antarctic Circumpolar Current (ACC) through the Drake Passage, south of America. The Warm Route owns its name only because of an average temperature in the Indonesia Throughflow and south of Africa warmer than the one found at the Drake Passage. While these hypothetical routes are now both well accepted, their relative magnitude and characteristics are still under debate.The aim of this poster is to recover the paths and characteristics of upper-level CB water, taking advantage of the very Lagrangian nature of the problem. The output of a complex and realistic ocean model is analyzed with newly developed Lagrangian techniques to produce a global circulation scheme that helps and completes our physical understanding of the three-dimensional ocean circulation.
|
|
|
|
|
|
|
|
|
