Studying subtropical thermocline ventilation and circulation using tritium and 3 He

Abstract

A 13‐year time series of tritium and 3 He in the Sargasso Sea near Bermuda shows the downward penetration of the bomb tritium and excess 3 He maxima into the main thermocline at a rate of about 17 m yr −1 . A simple box‐mixing model effectively mimics both the evolution of the tracer maxima and their thermocline inventories. The thermocline inventory of ζ (the sum of tritium and 3 He) near Bermuda has halved over duration of the time series. This can be used to estimate a regional average nitrate efflux from the main thermocline of 0.7±0.2 mol (N) m −2 yr −1 . In the eastern subtropical North Atlantic I combine 3 H‐ 3 He age with salinity, oxygen, and geostrophy to compute isopycnal diffusivities, oxygen consumption rates, and absolute velocities. The reference level velocities are determined almost solely by the 3 H‐ 3 He age equations and to an accuracy of about 1 mms −1 . A classic absolute velocity spiral is seen. The depth variation of computed vertical velocities is consistent with vorticity conservation and extrapolates to within errors of surface Ekman pumping and local subduction rates. Isopycnal diffusivities are largely constrained by the salinity equations and are ∼1200 m 2 s −l at a depth of 300 m, decreasing gradually downward. Diffusivities apparently decrease toward the surface, likely an artifact of unsteadiness in the salinity equations. The oxygen utilization rates (OURs) determined by the oxygen equations decrease exponentially with depth. Vertical integration of OUR yields a net water column oxygen demand of 4.1±0.8 mol m −2 yr −1 , which corresponds to an export production of 2.5±0.5 mol m −2 yr −1 carbon. I present a simple scheme to show how to relate the vertical profiles of 3 H‐ 3 He age to the subduction rate as a function of depth projected back to the surface outcrop. The shallowest horizon has a subduction rate indistinguishable from local climatological Ekman pumping rates but gradually exceeds the projected rates with depth. The difference may be attributed to the increasing importance of buoyancy‐forced subduction at higher latitudes.