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. ⺠Choice of lateral boundary conditions in ocean GCM affects flow in Mozambique Channel. ⺠No-slip condition forces flow offshore, making it more unstable, leading to eddies. ⺠Changing resolution has little impact on total transport. ⺠Natal Pulses are more easily generated in a model with no-slip boundary conditions.