Dipoles of the South-east Madagascar Current

Abstract

From an analysis using almost 20 years of satellite altimetry data and in situ observations, we have shown thatat the southern tip of Madagascar the SEMC breaks up in a series of nearly symmetric dipolar vortex pairs of approximately zero net relative vorticity. They appear at a frequency of about 4–6 per year. Most vortex pairs break up shortly after their formation and interact with previously formed cyclones and anticyclones. The velocity shear across the SEMC is so large that the dynamics of the evolution of the dipole is dominantly nonlinear. A surprising consequence is that there is no retroflection of the SEMC nor does it continue as an unstable westward jet. Instead the lobes develop symmetrically and the dynamics appear more similar to that of modon formation [Flierl et al., 1983]. The EKE south of Madagascar was shown to correlate with the strength of the SEMC prior to separation. Such a relationship could not be found by Palastanga et al. [2006] probably because they used a more upstream satellite transect to determine the surface velocity of the SEMC. Within the SEMC, there is a short decorrelation length scale (also observed in the Gulf Stream [Sturges and Hong, 2001]), and the velocity at our transect does not significantly correlate with the velocity at locations further than 450 km upstream. The very early retroflection of the AC in 2001 was an exceptional one. Only one more appeared in the 18 year period (in 2008). In both cases, it needed a concerted action of cyclonic meanders of the AC and the ARC to shortcut the retroflection loop with an active role of the Madagascar dipoles in generating the Agulhas cyclones. Interannual variability in the separation region appeared related with the ENSO cycle, but not significantly with the IOD. Significant correlations were found between the NINO3.4 index and both the strength of the SEMC and the EKE southwest of Madagascar. Positive and negative lags are between 250 and 500 days. Preceding a positive (negative) ENSO phase, stronger (weaker) velocities in the SEMC were observed. Following a positive (negative) phase, weaker (stronger) velocities in the SEMC were observed. We are presently investigating the possible explanations of these relations and their impact.