Upper Branch Routes of the Thermohaline Conveyor Belt
  Abstract >> Problem >> Strategy >> Routes >> Three Paths >> Warm, Cold and Tepid Routes >> Comparison with other GCMs >> A17 Comparison >>Conclusions


 
 

Water path structure for each of the three different origins

Lagrangian diagnostics show here a first advantage with respect to the classical Eulerian ones: their ability to materialize the path followed by water masses along their journey in the global ocean. In fact, the method is able to recover the specific structure of each of the three different routes.

 

DRAKE Path: 5.8 Sv of water coming from DRAKE reach the equatorial Atlantic section partly by circulating around (and recirculating within) the South Atlantic subtropical gyre, and partly by entering the Indian Ocean before coming back to the Atlantic basin via the Agulhas Current System (ACS). This is the materialization of the classical Cold Water Route first hypothesized by Rintoul (1991) with the variant that the structure of the route given by the model enhances the Indian-Atlantic connection proposed by Gordon et al. (1992) for the cold waters coming from DRAKE.

ITFL Path: The Indonesian Throughflow water that does reaches the equatorial Atlantic crosses the entire Indian Basin westward before turning south and flowing essentially in the Mozambique Channel. Then, captured by the Indian subtropical gyre and the ACS the water is able to enter the Atlantic basin. Before flowing southward in the Mozambique Channel, part of the ITFL water takes a long journey in the Northern Indian Ocean. This path is clasically known as the Warm Water Route of Gordon (1986). Its intensity, 5.8 Sv, is of the same order of that of DRAKE.

TAS Path: Water flowing westward just south of Tasmania and north of the ACC is able to cross the Indian Ocean, trapped in the northern boundary of the Indian subtropical gyre, and penetrates the Atlantic basin via the ACS. This origin has never been considered as a possible way for the ocean circulation to balance the thermohaline flow in the North Atlantic (Gordon, 1986; Broeker, 1991; Rintoul, 1991; Gordon et al., 1992; Schmitz, 1995, 1996a, 1996b). So this path represents a new route for the Upper Branch of the Conveyor Belt. With a contribution of 3.2 Sv, its input is not negligeable.

 

The inversions performed by Metzl et al. (1990) in the Indian Ocean always produced a westward flow in one or more subsurface layers.

Evidence of a westward flow of Intermediate Water south of Australia (WOCE SR3 section) is documented by Rintoul and Bullister (1999) and by Ganachaud (2000) inversions. This flow is present all around the year long even if its intensity varies seasonally (Rintoul, personal communication). Circulation schemes at Intermediate Water levels show a westward flow immediately south of Australia, transmitting water to the Indian Ocean (Reid, 1965; Taft, 1963; Johnson, 1973). Indication of a possible penetration of this intermediate layer water into the south-east Indian basin can be found in Reid (1986) and Fine (1993). More specifically, Fine (1993) evokes a possible South Pacific-Indian Ocean link via South Australia in order to explain the measured CFCs zonal gradient of the South Indian Ocean Intermediate Waters. Lagrangian diagnostic applied to the ORCA dynamical fields confirm this hypothesis and permit to link the TAS leakage to the North Atlantic circulation.

 

Sabrina Speich