The Contribution of Submesoscale over Mesoscale Eddy Iron Transport in the Open Southern Ocean

Uchida, Takaya; Balwada, Dhruv; Abernathey, Ryan; McKinley, Galen A.; Smith, K. Shafer; Lévy, Marina

doi:10.1029/2019ms001805

Cited by 61 publications

(86 citation statements)

References 131 publications

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“…On the seasonal scale, in addition to the "once-off" winter DFe supply to the surface by convective mixing (Tagliabue et al, 2014), this study shows that enhanced eddy activity contributes to a DFe supply of similar magnitude over the year (Figure 2c) and is thus an important supply mechanism for SO productivity (supporting 14% annual and 25% spring-summer production). Our estimate of the eddy DFe flux is likely underestimated as the horizontal resolution used (~5 km or 1/24°) is not sufficient to represent submesoscale iron fluxes (Rosso et al, 2016;Uchida et al, 2019).…”

Section: Discussionmentioning

confidence: 94%

Iron Supply Pathways Between the Surface and Subsurface Waters of the Southern Ocean: From Winter Entrainment to Summer Storms

Nicholson¹,

Lévy

Jouanno

et al. 2019

Geophysical Research Letters

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Dissolved iron (DFe) plays an immeasurable role in shaping the biogeochemical processes of the open-ocean Southern Ocean. However, due to observational constraints iron supply pathways remain poorly understood. Using an idealized eddy-resolving physical-biogeochemical model representing a turbulent sector of the Southern Ocean with seasonal buoyancy forcing and zonal winds overlaid by storms, we quantify the importance of a range of subsurface and surface iron supply mechanisms. The main physical supply pathways to the surface layer are via eddy advection and winter convective mixing in equal proportions. The associated subsurface loss of DFe is restocked via net remineralization (75%) and eddy advection (25%). Summer storms resulted in weak DFe supplies relative to the seasonal supplies (<7.6%). However, in situations of deep summer mixed layers and when interacting with underlying ocean fronts, summer storms resulted in enhanced diffusive and advective DFe supplies and raised summer primary production by 20% for several days. Plain Language SummaryThe surface of the Southern Ocean is iron depleted, which strongly limits the amount of phytoplankton growth that can occur. Every year, deep mixing in winter moves large quantities of iron from a subsurface underutilized iron reservoir to the surface alleviating the iron limitation. Once there is sufficient light in spring, phytoplankton consume this iron supply leaving the summer months iron depleted. How exactly phytoplankton are supported through the summer when iron limitations are strong remains under question. This study uses a numerical model to simulate the seasonal and intraseasonal iron supply pathways. We show that physical supplies via advection by eddies and winter mixing drive surface seasonal supplies in equal proportions. During summer, storms supply iron via intermittent mixing and thus increase primary production, particularly over regions of strong ocean fronts.

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Section: Discussionmentioning

confidence: 94%

Iron Supply Pathways Between the Surface and Subsurface Waters of the Southern Ocean: From Winter Entrainment to Summer Storms

Nicholson¹,

Lévy

Jouanno

et al. 2019

Geophysical Research Letters

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“…We use the Masachusetts Institute of Technology general circulation model 23 (MITgcm) with an embedded full ecosystem model 24,25 . The model configuration is identical to a companion paper 26 where we quantify the relative contribution of submesoscale and mesoscale dynamics on the total vertical iron transport. For completeness, details of the set up are also summarized in Supplementary Note 1.…”

Section: Submesoscale Permitting Simulation Of the Open Southernmentioning

confidence: 99%

Vertical eddy iron fluxes support primary production in the open Southern Ocean

et al. 2020

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The primary productivity of the Southern Ocean ecosystem is limited by iron availability. Away from benthic and aeolian sources, iron reaches phytoplankton primarily when iron-rich subsurface waters enter the euphotic zone. Here, eddy-resolving physical/biogeochemical simulations of a seasonally-forced, open-Southern-Ocean ecosystem reveal that mesoscale and submesoscale isopycnal stirring effects a cross-mixed-layer-base transport of iron that sustains primary productivity. The eddy-driven iron supply and consequently productivity increase with model resolution. We show the eddy flux can be represented by specific welltuned eddy parametrizations. Since eddy mixing rates are sensitive to wind forcing and largescale hydrographic changes, these findings suggest a new mechanism for modulating the Southern Ocean biological pump on climate timescales.

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“…An OGCM of �1km resolution also has a better representation of the spatial/temporal submesoscale that can be important for sinking Lagrangian particles which represent the carbon flux to the ocean bottom [64]. Although the mesoscale flow contains most of the energy that is responsible for the tracer dispersion [65], submesoscale (1-20km) dynamics have proven to be of importance for the vertical advection of iron in specific regions with strong flow-bathymetric interactions [66]. Future work could analyse the transport of sinking particles in models with higher resolutions, and with models which better represent internal tides [67] or improved interaction of the bottom-flow with topography [68].…”

Section: Plos Onementioning

confidence: 99%

Resolution dependency of sinking Lagrangian particles in ocean general circulation models

et al. 2020

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Any type of non-buoyant material in the ocean is transported horizontally by currents during its sinking journey. This lateral transport can be far from negligible for small sinking velocities. To estimate its magnitude and direction, the material is often modelled as a set of Lagrangian particles advected by current velocities that are obtained from Ocean General Circulation Models (OGCMs). State-of-the-art OGCMs are strongly eddying, similar to the real ocean, providing results with a spatial resolution on the order of 10 km on a daily frequency. While the importance of eddies in OGCMs is well-appreciated in the physical oceanographic community, other marine research communities may not. Further, many long term climate modelling simulations (e.g. in paleoclimate) rely on lower spatial resolution models that do not capture mesoscale features. To demonstrate how much the absence of mesoscale features in low-resolution models influences the Lagrangian particle transport, we simulate the transport of sinking Lagrangian particles using low-and high-resolution global OGCMs, and assess the lateral transport differences resulting from the difference in spatial and temporal model resolution. We find major differences between the transport in the non-eddying OGCM and in the eddying OGCM. Addition of stochastic noise to the particle trajectories in the non-eddying OGCM parameterises the effect of eddies well in some cases (e.g. in the North Pacific gyre). The effect of a coarser temporal resolution (once every 5 days versus monthly) is smaller compared to a coarser spatial resolution (0.1˚versus 1˚horizontally). We recommend to use sinking Lagrangian particles, representing e.g. marine snow, microplankton or sinking plastic, only with velocity fields from eddying Eulerian OGCMs, requiring high-resolution models in e.g. paleoceanographic studies. To increase the accessibility of our particle trace simulations, we launch planktondrift.science.uu.nl, an online tool to reconstruct the surface origin of sedimentary particles in a specific location.

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The Contribution of Submesoscale over Mesoscale Eddy Iron Transport in the Open Southern Ocean

Cited by 61 publications

References 131 publications

Iron Supply Pathways Between the Surface and Subsurface Waters of the Southern Ocean: From Winter Entrainment to Summer Storms

Iron Supply Pathways Between the Surface and Subsurface Waters of the Southern Ocean: From Winter Entrainment to Summer Storms

Vertical eddy iron fluxes support primary production in the open Southern Ocean

Resolution dependency of sinking Lagrangian particles in ocean general circulation models

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