Geometric remapping of particle distributions in the Discrete Element Model for Sea Ice (DEMSI v0.0)

A new sea ice dynamical core, the Discrete Element Model for Sea Ice (DEMSI), is under development for use in coupled Earth system models. DEMSI is based on the discrete element method, which models collections of ice floes as interacting Lagrangian particles. In basin-scale sea ice simulations the Lagrangian motion results in significant convergence and ridging, which requires periodic remapping of sea ice variables from a deformed particle configuration back to an undeformed initial distribution. At the resolution required for Earth system models we cannot resolve individual sea ice floes, so we adopt the sub-gridscale thickness distribution used in continuum sea ice models. This choice leads to a series of hierarchical tracers depending on ice fractional area or concentration that must be remapped consistently. The circular discrete elements employed in DEMSI help improve the computational efficiency at the cost of increased complexity in the effective element area definitions for sea ice cover that are required for the accurate enforcement of conservation. An additional challenge is the accurate remapping of element values along the ice edge, the location of which varies due to the Lagrangian motion of the particles. In this paper we describe a particle-to-particle remapping approach based on well-established geometric remapping ideas that enforces conservation, bounds-preservation, and compatibility between associated tracer quantities, while also robustly managing remapping at the ice edge. One element of the remapping algorithm is a novel optimization-based flux correction that enforces concentration bounds in the case of non-uniform motion. We demonstrate the accuracy and utility of the algorithm in a series of numerical test cases.

Wind and wave effects on surface currents in the Mediterranean Sea

<p>The knowledge of Lagrangian motion is of a great importance due to their impact on the properties of transported material like the Essential Ocean Variables (phytoplankton, temperature, pCO2, etc), or other material like plastics debris, oil spill pollution, etc. In this study we analyze the influence of the wind and waves in the transport and mixing properties at the upper layers of the Mediterranean Sea. In this context, we propose a new approach for current velocity where we take into account the wind-wave interaction and the variability that it inserts into the current velocity through Ekman and Stokes components.</p><p><br>Surface currents, Ekman, Stokes, Lyapunov exponent</p>

On Material Transport by Shelfbreak Eddies

The Lagrangian motion in the eddy field produced from an unstable retrograde jet along the shelf break is studied from idealized numerical experiments with a primitive equation model. The jet is initially in thermal wind balance with a cross-isobath density gradient and is not subjected to any atmospheric forcing. Over the course of the model integration, the jet becomes unstable and produces a quasi-stationary eddy field over a 2-month period. During this period, the cross-slope flow at the shelf break is characterized by along-slope correlation scales of O(10) km and temporal correlation scales of a few days. The relative dispersion of parcels across isobaths is found to increase with time as tb, where 1 < b < 2. This mixed diffusive–ballistic regime appears to reflect the combined effects of (i) the short length scales of velocity correlation at the shelf break and (ii) the seaward excursion of monopolar and dipolar vortices. Cross-slope dispersion is greater offshore of the front than inshore of the front, as offshore parcels are both subducted onshore below density surfaces and translated offshore with eddies. Nonetheless, the exchange of parcels across the jet remains very limited on the monthly time scale. Particles originating from the bottom experience upward displacements of a few tens of meters and seaward displacements of O(100) km, suggesting that the eddy activity engendered by an unstable along-slope jet provides another mechanism for bottom boundary layer detachment near the shelf edge.