Vertical Eddy Iron Transport in the Southern Ocean

Ryan P Abernathey1, Takaya Uchida1, Dhruv Balwada2, Galen A McKinley3, K. Shafer Smith4 and Marina Levy5, (1)Columbia University of New York, Palisades, NY, United States, (2)Lamont -Doherty Earth Observatory, Palisades, NY, United States, (3)Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY, United States, (4)New York University, Courant Institute of Mathematical Sciences, New York, NY, United States, (5)Laboratoire d'océanographie et du climat : expérimentations et approches numériques (LOCEAN), Paris, France
Abstract:
The Southern Ocean is a high-nutrient, low chlorophyll region, with primary productivity limited by light and iron availability. In the open Southern Ocean, away from strong benthic and glacial iron sources, iron is supplied to phytoplankton via vertical transport from the iron-rich interior to the euphotic zone. The strong mesoscale eddies of the Southern Ocean are known to play a first-order role in the vertical transport of heat. This study examines whether they might play a similar role in the iron budget.

We study this problem via an idealized numerical simulation of Southern Ocean physics and biogeochemistry. The model, a zonally symmetric sector meant to represent a slice across the ACC, is forced at the surface with seasonally varying wind and buoyancy inputs. We couple with an intermediate-complexity ecosystem model, with several distinct phytoplankton and zooplankton phenotypes and a full iron cycle. Nutrients, including iron, are provided via a sponge layer at the northern boundary. Because of the relatively small domain, this setup enables us to reach high spatial resolution (2 km; submesoscale permitting) while still resolving the large-scale frontal structure of the Southern Ocean. Crucially, this setup resolved the strong seasonal cycle in both biology and physics, and their interaction.

We allow the ecosystem to reach a statistical equilibrium and examine the mechanisms behind iron transport. We find that vertical eddy fluxes of iron play a major role in the iron budget; while vertical diffusion dominates within the mixed layer, eddies mediate the transport across the ML base. We use cross-spectral analysis to separate the eddy fluxes by scale, finding a larger role for submesoscale fluxes near the surface, with mesoscales dominating at depth. By varying resolution, we determine that eddy transport plays a controlling role on the ecosystem; as the eddy flux decreases moving to lower resolution, primary production declines in tandem. We also show how a properly tuned Gent-McWilliams / Redi parameterization can accurately reproduce the magnitude and seasonal cycle of the eddy iron transport in a coarse-resolution model.