How was the atmospheric/oceanic circulation different prior to the formation of the Isthmus of Panama?
Concerning the thermohaline circulation: the defining part of it, I think, is the formation of bottom water in the polar regions.

The formation of Antarctic Bottom Water (AABW) in the Southern Ocean is strongly linked to the presence of the Antarctic Circumpolar current (ACC). The ACC was possible only after the opening of the Tasmanian Gateway and the Drake Passage (between Antarctica &amp; South America) which both happened in the Late Eocene / earliest Oligocene. The ACC is thought to have been the deep, strong current it is today since the Late Oligocene at the very latest (Lyle et al. 2008), so the AABW is also thought to have existed since the Oligocene.

However as far as the North Atlantic Deep Water (NADW) formation is concerned: it is linked indeed in part to the Gulf Stream and the Northern Hemisphere glaciation. While the latter only occurred ca. 3.6Ma (e. g. Muddelsee &amp; Raymo 2005), the Gulf Stream is a direct consequence of the closure of the isthmus of Panama: the shallowing (and therefore closure to deep current) of the Panama Seaway is thought to have happenned at ca. 4.6Ma (Haug &amp; Tiedemann 1998) with a complete closure at 2.6Ma (Lunt et al. 2008).

So it is rather safe to say that the thermohaline circulation as we know it today settled probably around 3.6Ma (therefore mid-Pliocene).

Back in the Eocene (55 to 33Ma), the formation of bottom water is thought to have occurred in middle to low latitude (e. g. Kennett &amp; Stott 1990) leading to a very completely different kind of global circulation.

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. &amp; R. Tiedemann. 1998. Effect of the formation of the Isthmus of Panama on Atlantic Ocean thermohaline circulation. Nature, 393: 673-676.
Kennett, J. P. &amp; 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 &amp; 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. &amp; M. E. Raymo. 2005. Slow dynamics of the Northern Hemisphere glaciation. Paleoceanography, 20: PA4022.