Presenter: Michael Follows (Massachusetts Institute of Technology)

Description:
The delivery of essential nutrients and the long-term maintenance of biological productivity in the ocean's subtropical gyres has presented a challenge for several decades. Despite efficient recycling of nutrients within the euphotic zone, gravitational sinking and subduction leads to a sustained loss of essential elements from the surface layers. Physical transport provides the majority of the nutrient resupply. We quantified the contributions of isopycnal, diapycnal, time-mean and eddy fluxes in maintaining biological productivity in an eddy-permitting, global simulation of ocean physics, biogeochemistry and ecology (MITgcm-Darwin). Here we focus on the south-eastern sector of the North Pacific subtropical gyre. There, diapycnal mixing and rectified adiabatic heaving resupply essential macronutrients into the euphotic layer, depleting the thermocline. Within the thermocline, the southwards, isopycnal, mean flow pushes nutrients upgradient and thus acts as a net sink of nutrients from the gyre. Thermocline nutrient concentrations are ultimately maintained by a combination of the remineralization of organic detritus and down-gradient, isopycnal eddy-stirring; the latter is enhanced at the edge of the gyre, fuelled by sharp nutrient gradients. On the long term, this spatially inhomogeneous, eddy-driven supply of nutrients to the gyre contributes to balance the slow trickle of sinking organic particles through the thermocline and into the abyss.