Instabilities in the Agulhas Retroflection Current system: A comparative model study

The Agulhas Retroflection Current system is analyzed in two ocean general circulation models, U.K. Fine Resolution Antarctic Model (FRAM) and U.S. Parallel Ocean Program (POP). It is shown that both models have two quasizonal jets, a northerly westward flowing jet (Agulhas Current) and a more southerly eastward flowing jet (Agulhas Return Current), which act as the entrance and exits of the Retroflection region, respectively. An instability analysis has been performed on both jets, and the unstable waves are shown to be baroclinicly unstable. It is shown that this instability process is well resolved in the POP model but is poorly resolved in FRAM. An energy analysis of POP shows that both barotropic and baroclinic instability processes play a major role in the current systems, with the former process concentrated in the surface layers between the two currents, and the latter process occurring below the two currents. In FRAM a regional energy budget suggests that barotropic instability is the main mechanism by which the eddies are generated, and a meridional cross-sectional analysis shows that this occurs principally on the northern edge of the Agulhas Return Current.