Observation and modeling of eddies and dipoles in the Southern Indian Ocean

Abstract

Analysis of the along track satellite data near the southern tip of Madagascar shows that the separation in this region occurs in the form of counter rotating pairs – dipoles. The integral of relative vorticity across the dipole's pathway is almost zero which supports the hypothesis that the western boundary current (WBC) is a source for the dipole formation, assuming the validity of the Stewart constraint for the WBC: the net relative vorticity across the WBC is zero.

For the numerical experiments three separation regions are chosen: the southern and northern tips of Madagascar as well as South Africa. It is shown that with the use of simplified linear and piecewise linear wind profiles, which are applied only for a 20-degree eastern sector of the domain, it is possible to generate at least three types of separation: shedding eddies from the southern tip of Madagascar, retroflection of the Agulhas Current and a free jet from the northern tip of Madagascar.

Furthermore, slip and no-slip boundary conditions as they apply in Hallberg Isopycnal Model (HIM) are discussed here. It is shown that only under no-slip boundary conditions, isolated dipoles might be generated at the separation. Analysis of the integrals of relative vorticity across the western boundary layer and across the dipole’s pathway in southern Madagascar illustrates that the source of dipoles at the separation is the WBC, which is in correspondence with observations.