Resolution dependency of sinking Lagrangian particles in ocean general circulation models

Abstract

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.

A global mean sea surface temperature dataset for the Last Interglacial (129–116 ka) and contribution of thermal expansion to sea level change

“…However, our observed polar warming is larger than in some climate model simulations, implying the latter fail to capture one or more key feedbacks (e.g. carbon, sea ice and ice sheet feedbacks) in the climate system (Bakker et al, 2013;Otto-Bliesner et al, 2013;Thomas et al, 2020;Clark et al, 2020;Fogwill et al, 2015).…”

Sinking microplastics in the water column: simulations in the Mediterranean Sea

“…The statistical properties are chosen to be similar to the ones expected for oceanic motions below the scales resolved by the numerical model (Monroy et al, 2017;Kaandorp et al, 2020). To do so, we use for D h Okubo's empirical formulation (Okubo, 1971) that parameterizes the effective horizontal eddy-diffusion below a spatial scale ℓ as D h (ℓ) = 2.055 × 10 −4 ℓ 1.55 m 2 /s.…”

“…Maritime-activity sources (Gago et al, 2018) can contribute to that. Another reason could be that their special and deformable shape results in a strongly reduced settling velocity (Bagaev et al, 2017) that allows long distance horizontal transport (Nooteboom et al, 2020). In any case, it is difficult to estimate the amount of microfibers in the oceans due to sampling issues and to their absence from statistics of mismanaged plastic waste (Carr, 2017;Barrows et al, 2018), and we will not consider them further in this paper.…”

Sinking microplastics in the water column: simulations in the Mediterranean Sea

“…(4) with and without this "noise" term modeling small scales (using β = 0.99) is around 8 % for the horizontal displacements and 5 % for the vertical ones. The figures become 12 % and 5 %, respectively, when evolving the particles for 20 d. These errors are moderate, although they may be of importance under some circumstances (Nooteboom et al, 2020). We consider these figures as a baseline to evaluate corrections to the simple Eq.…”

“…Maritime-activity sources (Gago et al, 2018) can contribute to that. Another reason could be that their special and deformable shape results in a strongly reduced settling velocity (Bagaev et al, 2017) that allows long-distance horizontal transport (Nooteboom et al, 2020). In any case, it is difficult to estimate the amount of microfibers in the oceans due to sampling issues and to their absence from statistics of mismanaged plastic waste (Carr, 2017;Barrows et al, 2018), and we will not consider them further in this paper.…”