A New Climatology of Air–Sea Density Fluxes and Surface Water Mass Transformation Rates Constrained by WOCE

Abstract

Global air?sea heat and freshwater flux data, constrained by World Ocean Circulation Experiment (WOCE) hydrographic section transports, is used to construct a new global density flux climatology. Global transformations calculated using this density flux dataset show two regimes: surface waters with density less than ?1023.3 kg m?3 are transformed to lighter density classes with a maximum rate of 130 Sv (1 Sv ≡ 106 m3 s?1) at σ ? 1021.6 kg m?3, and surface waters with density greater than 1023.3 kg m?3 are transformed to denser density classes with a maximum rate of 100 Sv at σ = 1025.4 kg m?3. At higher density (σ = 1027 kg m?3) the net transformation rates vanish, reflecting heat loss in the Northern Hemisphere balanced by Southern Hemisphere freshening. This results in a kink in the global transformation rate, which is attributed to the presence of Drake Passage. Further analysis of the control run of the third Hadley Centre global climate model, HadCM3, suggests this feature to be robust and to reflect the ?channel? geometry of the Southern Ocean and the ?basin? geometry of the Northern Hemisphere.