Large‐scale mass transports, water mass formation, and diffusivities estimated from World Ocean Circulation Experiment (WOCE) hydrographic data

Abstract

A global inversion is used to estimate the oceanic mass transports and vertical diffusivities based on a subset of hydrographic data from the World Ocean Circulation Experiment (WOCE) and Java Australia Dynamic Experiment. The analysis is based on the linear inverse “box” model of Ganachaud and Wunsch [2000] that consistently combines the transoceanic sections. A globally consistent solution is obtained for a depth‐independent adjustment to the thermal wind field, the freshwater flux divergences, the Ekman transport, and the advective and diffusive dianeutral fluxes between layers. A detailed error budget permits calculation of statistical uncertainties, taking into account both the nonresolved part of the solution and the systematic errors due to the temporal oceanic variability. The horizontal structure of the transports for different density classes is analyzed regionally. During the WOCE period (1985–1996), net meridional transports are estimated, with 16 ± 2 Sv (10 6 m 3/s) of North Atlantic Deep Water (NADW) being produced in the northern North Atlantic, moving southward, entraining Antarctic Bottom Water (AABW), and Antarctic Intermediate Water increasing in volume transport to 23 ± 3 Sv at 30°S. In the Southern Ocean, 21 ± 6 Sv of bottom water was formed from lower Circumpolar Deep Water, which corresponds approximately to the lower NADW density range. Bottom water inflows (NADW + AABW mixture) to the Atlantic, Indian, and Pacific Oceans are estimated at 6 ± 1.3, 11 ± 4, and 7 ± 2 Sv, respectively. In the Indian and Pacific Oceans this water returns southward at deep and intermediate levels. Property anomaly conservation constraints permit estimation of dianeutral diffusivities in deep layers, with a global average of 3.7 ± 0.7 cm 2 s−1 north of 30°S.