Nutrient Streams and the Subduction process: their effect on global nutrient distributions

Lead Research Organisation: University of Liverpool
Department Name: Earth Surface Dynamics


Biological production leads to an export of organic matter from the surface, sunlit ocean, which is important in transferring carbon within the ocean and in providing a biological drawdown of carbon dioxide from the atmosphere. This export can only be sustained over several years if there is a compensating supply of new nutrients to the upper ocean. Traditionally, this nutrient supply has been explained in terms of a vertical advective or diffusive transfer. In this proposal, we instead explore how the horizontal transfer of nutrients acts to maintain the surface nutrient concentrates and, thus, sustain export production. We propose to examine the following scientific questions: 1. How do boundary currents (through nutrient streams) and the subduction process (the exchange of fluid between the mixed layer and thermocline) control surface nutrient distributions over ocean basins? 2. How does interannual variability in mode water formation and spreading affect the nutrient reservoir of the upper thermocline for subtropical gyres? 3. How do fine-scale eddies sustain nutrient concentrations within the upper thermocline? These questions are addressed through a combination of data analyses and coupled isopycnic circulation and nitrogen model experiments.
Description The biological productivity of the North Atlantic is often understood in terms of local physical processes, involving the vertical upwelling or vertical mixing of nutrients from the deep to the surface ocean.

This grant demonstrated how the horizontal redistribution of nutrients by transport processes, involving the western boundary currents, gyre circulations and mesoscale eddies, helps to determine the nutrient supply to the surface ocean.
In turn, this supply of nutrients then sustains the biological productivity.

In particular, the transport of nutrients ultimately originating from the Southern Ocean are of central importance in sustaining productivity of the North Atlantic.
Exploitation Route This work provides a nutrient context for follow up work examining the distribution of trace metals, such as examined in the international GEOTRACERS programme.
Sectors Environment