Shallow, Intermediate, and Deep Overturning Components of the Global Heat Budget

Abstract

The ocean's overturning circulation and associated heat transport are divided into contributions based on water mass ventilation from 1) shallow overturning within the wind-driven subtropical gyres to the base of the thermocline, 2) overturning into the intermediate depth layer (500?2000 m) in the North Atlantic and North Pacific, and 3) overturning into the deep layers in the North Atlantic (Nordic Seas overflows) and around Antarctica. The contribution to South Pacific and Indian heat transport from the Indonesian Throughflow is separated from that of the subtropical gyres and is small. A shallow overturning heat transport of 0.6 PW dominates the 0.8-PW total heat transport at 24°N in the North Pacific but carries only 0.1?0.4 PW of the 1.3-PW total in the North Atlantic at 24°N. Shallow overturning heat transports in the Southern Hemisphere are also poleward: ?0.2 to ?0.3 PW southward across 30°S in each of the Pacific and Indian Oceans but only ?0.1 PW in the South Atlantic. Intermediate water formation of 2 and 7 Sv (1 Sv ≡ 106 m3 s?1) carries 0.1 and 0.4 PW in the North Pacific and Atlantic, respectively, while North Atlantic Deep Water formation of 19 Sv carries 0.6 PW. Because of the small temperature differences between Northern Hemisphere deep waters that feed the colder Antarctic Bottom Water (Lower Circumpolar Deep Water), the formation of 22 Sv of dense Antarctic waters is associated with a heat transport of only ?0.14 PW across 30°S (all oceans combined). Upwelling of Circumpolar Deep Water north of 30°S in the Indian (14 Sv) and South Pacific (14 Sv) carries ?0.2 PW in each ocean.