The impact on the ENSO (if it existed at the time) is, as far as I know, unknown but since South America was still here to limit it eastward I don't think it ever reached Africa.
References:
Haug, G. H. & R. Tiedemann. 1998. Effect of the formation of the Isthmus of Panama on Atlantic Ocean thermohaline circulation. Nature, 393: 673-676.
Kennett, J. P. & L. D. Stott. 1990. Proteus and Proto-oceanus: ancestral Paleogene oceans as revealed from Antarctic stable isotopic results; ODP Leg 113. In P. F. Barker et al. (Eds), Proceedings of the Ocean Drilling Program, Scientific Results, College Station, TX (Ocean Drilling Program), 113: 865-880.
Lunt, D. J., P. J. Valdes, A. Haywood & I. C. Rutt. 2008. Closure of the Panama Seaway during the Pliocene: implications for climate and Northern Hemisphere glaciation. Climate Dynamics, 30: 1-18.
Lyle, M., Gibbs, S., Moore T. C., Rea, D. K., 2007. Late Oligocene initiation of the Antarctic Circumpolar Current: evidence from the South Pacific. Geology, 35(8): 691-694.
Mudelsee, M. & M. E. Raymo. 2005. Slow dynamics of the Northern Hemisphere glaciation. Paleoceanography, 20: PA4022.