The plumbing of the global biological pump: Efficiency control through leaks, pathways, and time scales

We systematically quantify the pathways and time scales that set the efficiency,E bio, of the global biological pump by applying Green‐function‐based diagnostics to a data‐assimilated phosphorus cycle embedded in a jointly assimilated ocean circulation. We consider “bio pipes” that consist of phosphorus paths that connect specified regions of last biological utilization with regions where regenerated phosphate first reemerges into the euphotic zone. The bio pipes that contribute most to E bio connect the Eastern Equatorial Pacific (EEqP) and Equatorial Atlantic to the Southern Ocean ((21 ± 3)% ofE bio), as well as the Southern Ocean to itself ((15 ± 3)% ofE bio). The bio pipes with the largest phosphorus flow rates connect the EEqP to itself and the subantarctic Southern Ocean to itself. The global mean sequestration time of the biological pump is 130 ± 70 years, while the sequestration time of the bio pipe from anywhere to the Antarctic region of the Southern Ocean is 430 ± 30 years. The distribution of phosphorus flowing within a given bio pipe is quantified by its transit‐time partitioned path density. For the largest bio pipes, ∼1/7 of their phosphorus is carried by thermocline paths with transit times less than ∼300–400 years, while ∼4/7 of their phosphorus is carried by abyssal paths with transit times exceeding ∼700 years. The path density reveals that Antarctic Intermediate Water carries about a third of the regenerated phosphate last utilized in the EEqP that is destined for the Southern Ocean euphotic zone. The Southern Ocean is where (62 ± 2)% of the regenerated inventory and (69 ± 1)% of the preformed inventory first reemerge into the euphotic zone.