The formation of North Atlantic Deep Water (NADW) involves the sinking of surface water to abyssal depths at a rate of 15 to 20 × 10⁶ m³/s. This water spreads throughout the Atlantic Ocean and is exported to the Indian and Pacific Oceans via the Antarctic Circumpolar Current and deep western boundary currents. To maintain this process, a return flow of warm water must occur within the ocean's thermocline layer, linking the main thermoclines in a thermohaline-driven global circulation cell associated with NADW formation. The return flow of warm water follows a path: Pacific to Indian flow within the Indonesian Seas, advection across the Indian Ocean in the 10°–15°S latitude belt, southward transfer in the Mozambique Channel, entry into the South Atlantic via a branch of the Agulhas Current that does not complete the retroflection pattern, northward advection within the South Atlantic subtropical gyre, and cross-equatorial flow into the western North Atlantic. The magnitude of this return flow increases as more NADW is incorporated into the upper layer of the ocean. The water mass characteristics of the return flow are gradually altered by regional ocean-atmosphere interaction and mixing processes. The cold water route, Pacific to Atlantic transport of Subantarctic water within the Drake Passage, is of secondary importance, amounting to perhaps 25% of the warm water route transport. The continuity or vigor of the warm water route is vulnerable to change as the thermohaline forcing in the northern North Atlantic and larger-scale wind-driven circulation factors vary. The interocean links within the Indonesian Seas and at the Agulhas retroflection may be particularly responsive to such variability. Changes in the warm water route continuity may influence the formation characteristics of NADW. The warm water route is proposed as the more important route for the return flow of upper layer water to the Atlantic Ocean. The NADW upwells within the world ocean, returning water to the upper layer within the Antarctic region and into the thermocline. Antarctic and thermocline upwelling may be coupled in that deep water upwelling around Antarctica contributes to the formation of Antarctic Intermediate Water, which then spreads below the thermocline and upwells into the thermocline. There are two routes by which the upper layer water can return to the Atlantic Ocean: the cold water route within the Drake Passage and the warm water route, in which Indian Ocean thermocline water is introduced to the South Atlantic south of Africa. The warm water route is proposed as the more important. The warm water route involves the upwelling of NADW returning water to the thermocline, which was lost during NADW production. The primary site of upwelling is the southern ocean, where the upwelled NADW eventually flows below the thermocline and enters the thermocThe formation of North Atlantic Deep Water (NADW) involves the sinking of surface water to abyssal depths at a rate of 15 to 20 × 10⁶ m³/s. This water spreads throughout the Atlantic Ocean and is exported to the Indian and Pacific Oceans via the Antarctic Circumpolar Current and deep western boundary currents. To maintain this process, a return flow of warm water must occur within the ocean's thermocline layer, linking the main thermoclines in a thermohaline-driven global circulation cell associated with NADW formation. The return flow of warm water follows a path: Pacific to Indian flow within the Indonesian Seas, advection across the Indian Ocean in the 10°–15°S latitude belt, southward transfer in the Mozambique Channel, entry into the South Atlantic via a branch of the Agulhas Current that does not complete the retroflection pattern, northward advection within the South Atlantic subtropical gyre, and cross-equatorial flow into the western North Atlantic. The magnitude of this return flow increases as more NADW is incorporated into the upper layer of the ocean. The water mass characteristics of the return flow are gradually altered by regional ocean-atmosphere interaction and mixing processes. The cold water route, Pacific to Atlantic transport of Subantarctic water within the Drake Passage, is of secondary importance, amounting to perhaps 25% of the warm water route transport. The continuity or vigor of the warm water route is vulnerable to change as the thermohaline forcing in the northern North Atlantic and larger-scale wind-driven circulation factors vary. The interocean links within the Indonesian Seas and at the Agulhas retroflection may be particularly responsive to such variability. Changes in the warm water route continuity may influence the formation characteristics of NADW. The warm water route is proposed as the more important route for the return flow of upper layer water to the Atlantic Ocean. The NADW upwells within the world ocean, returning water to the upper layer within the Antarctic region and into the thermocline. Antarctic and thermocline upwelling may be coupled in that deep water upwelling around Antarctica contributes to the formation of Antarctic Intermediate Water, which then spreads below the thermocline and upwells into the thermocline. There are two routes by which the upper layer water can return to the Atlantic Ocean: the cold water route within the Drake Passage and the warm water route, in which Indian Ocean thermocline water is introduced to the South Atlantic south of Africa. The warm water route is proposed as the more important. The warm water route involves the upwelling of NADW returning water to the thermocline, which was lost during NADW production. The primary site of upwelling is the southern ocean, where the upwelled NADW eventually flows below the thermocline and enters the thermoc