Séminaire des doctorants du LOCEAN
Antoine Nasser & Kenza Himmich
Séminaire de Antoine Nasser et Kenza Himmich, doctorants du LOCEAN
Présentation 1 : Searching for better numerical solutions along piecewise-constant coastlines (Antoine Nasser)
Adcroft & Marshall (1998) studied the formulation of lateral boundary conditions in a 1°/4 shallow water model. By comparing the solution obtained on a mesh, ideally aligned with the coastline to rotated ones, it is found that all stationary solutions differ from the aligned one. They concluded that a staircase coastline exerts a form drag that slows down the recirculation. Here, we reproduce these results both in vector and flux forms and test the combination of free-slip and no-slip boundary conditions with the stress tensors’ vorticity-divergence and symmetric forms. We show that, given any angle of rotation, the equilibrium solutions remain similar provided that the system has numerically converged (while keeping the physics unchanged). The reason for this behaviour is discussed. It appears that having many spatial steps to discretise the internal radius of deformation is necessary not only to solve eddies in the open ocean but to accurately represent flows along coastlines and isobaths. Surprisingly, with the symmetric tensor, whatever the spatial resolution, applying a free-slip condition does not seem to influence the dynamics, as it acts like a no-slip condition. This suggests that the way the boundary condition is implemented is not suitable.
Présentation 2 : Drivers of Antarctic sea-ice advance (Kenza Himmich)
The period of sea-ice advance has a key influence over the Antarctic climate and ecosystem. Over the last 4 decades, sea-ice advance has been occurring later in the Weddell and Ross Seas and earlier west of the Antarctic Peninsula. However, not much is known on the drivers of the observed changes nor on the physical processes determining the date of advance in the Southern Ocean. To progress understanding, we identify dynamic and thermodynamic processes controlling sea-ice advance. We diagnose the contribution of sea ice drift to the sea ice concentration budget at the time of advance. We explore the thermodynamic interaction between lower atmosphere and upper ocean cooling before freeze-up using satellite, in-situ and reanalysis products. We find that near the winter ice-edge, ice advance can only occur by transport of ice from higher to lower latitudes. Elsewhere in the seasonal ice zone, advance is mostly due to freezing with little or no contribution of ice drift. Where thermodynamic ice growth is dominant, the date of advance is more strongly linked to the seasonal maximum of the mixed layer heat content than to the seasonal maximum sea-surface temperature. This emphasizes the importance of the mixed layer depth during the ocean cooling phase before freeze-up. We also find that the atmosphere is leading ocean cooling and freeze-up as indicated by the comparison of mixed layer and surface-air cooling timings. There is a clear relationship between the day when surface air temperature goes under the ocean freezing point and the day of advance. This day occurs on average 45 days after the atmosphere has reached the freezing point. We will investigate the possible physical mechanisms behind such a relationship. We hope the understanding of the fundamental processes driving sea-ice advance will help us to better comprehend the observed changes of Antarctic sea-ice and better constrain its future evolution.
Lieu : Salle de l’UFR TEB – SU – 4 place Jussieu – Paris 5 – T46/56 2ième étage