Oceanic nutrient and oxygen transports and bounds on export production during the World Ocean Circulation Experiment

Abstract

Large‐scale oceanic transports of nutrient and oxygen are estimated from selected hydrographic sections from the World Ocean Circulation Experiment spanning the world ocean. A so‐called geostrophic inverse box model is used to calculate the velocity field across the transoceanic sections. The circulation is required, a priori, to conserve mass, salt, top‐to‐bottom silicate, and subsurface heat and PO( = 170P + O2). The resulting estimate of the time mean circulation is used to compute horizontal and vertical nutrient transports and their residual sources and sinks associated with biogeochemical processes. Locally, the remineralization rate is, in general, below our uncertainty level, with magnitudes consistent with in situ measurements (0 ± 0.1mol N m−2 yr−1 to 0.7 ± 0.25 mol N m−2 yr−1 below about 100 m). Because of correlations between errors, the export production becomes significant when integrated globally, with 390 ± 240 kmol Si s−1 and 57 ± 40 kmol N s−1(420 ± 290 kmol C s−1) between 47°N and 30°S and at 2000 m. Examination of N* provides estimates for nitrate fixation and denitrification, either consistent (North Atlantic) or contradictory (Indian) with previous findings. Similarily, oxygen utilization rates and net air‐sea exchanges are provided. A net uptake of oxygen at high latitudes and an outgassing at low latitudes are found. Examination of the pseudotracer O* suggests an important role of dissolved organic matter in the oxygen budget. Oceanic oxygen and O* are found in global balance within uncertainties.