Mozambique Channel eddies in GCMs: A question of resolution and slippage
Hydrographic observations in the 21st century have shown that the flow within the Mozambique Channel is best described by a series of large poleward-propagating anticyclonic eddies, rather than, as previously thought, a continuous intense western boundary current. The portrayal of this region in various runs of the NEMO 75-level model is found to vary between those two descriptions depending upon the resolution used and the implementation of the model’s lateral boundary conditions. In a comparison of 1/4° resolution runs, the change of these conditions from free-slip to no-slip leads to the mean southward flow moving further offshore, with greater variability in the zonal and meridional velocities as the flow organises itself into eddies, and a reduction in total transport. If a realization of a model is unable to get these aspects of the physical flow correct, then this will significantly reduce its ability to show a realistic biological signal or long-term response to climate change. Further south, beyond Durban, the application of no-slip conditions similarly causes the mean Agulhas Current to lie further offshore, making it much more able to simulate Natal Pulses. âº Choice of lateral boundary conditions in ocean GCM affects flow in Mozambique Channel. âº No-slip condition forces flow offshore, making it more unstable, leading to eddies. âº Changing resolution has little impact on total transport. âº Natal Pulses are more easily generated in a model with no-slip boundary conditions.