This website and the links therein may be a good primer on ocean currents. Regarding the effects of melting ice on currents, you may be thinking of the slowdown of the Atlantic Meridional Overturning Circulation (AMOC) - this NASA website gives some good information about that. The possible effect of a slowing AMOC on El Nino/La Nina is a good question and something that I don't think is settled. This recent study suggests a slowdown of the AMOC contributes to more ENSO variability, which would mean more extreme swings from La Nina to El Nino. However, more research is needed.