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


 
 

Water path structure for each of the three different origins

The Lagrangian diagnostics demonstrate an advantage with respect to the classical Eulerian ones: its capability to materialise the path followed by water masses during their journey in the global ocean. In fact, the method is able to recover the particular 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 in 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 Routes first hypothezised by Rintoul (1991) with the variant that the structure of the route given by the model enhances the Indian-Atlantic connection advanced by Gordon et al. (1992) for the cold waters coming from DRAKE.

ITFL Path: The Indonesian Throughflow water that reaches the equatorial Atlantic travels westward across the entire Indian Basin 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 classicaly known as the Warm Water Route of Gordon (1986). The transport, 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 penetrate 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. 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 negligible.

 

The Pacific contribution to the return flow that enters the Indo-Atlantic domain from South of Tasmania has never been considered in the classical pictures of the global thermohaline circulation (Gordon, 1986; Broeker, 1991; Rintoul, 1991; Gordon et al., 1992; Schmitz, 1995, 1996a, 1996b). Nevertheless, the inversions performed by Metzl et al. (1990) in the Indian Ocean always produced a westward flow in one or more of the subsurface layers.

Evidence of a westward flow of Intermediate Water south of Australia (WOCE SR3 section) is documented by Rintoul and Bullister (1999) and from Ganachaud (2000) inversions. This flow is present all the year long even if its intensity varies seasonally (Rintoul, personal communication). Circulation schemes at Intermediate Water levels show westward flow immediately south of Australia, returning 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. The Lagrangian diagnostic applied to the ORCA dynamical fields confirms this hypothesis and goes further as it permits to link the TAS leakage to the North Atlantic circulation.

 

Sabrina Speich: Sabrina.Speich@univ-brest.fr