1. How do meteorologists forecast El Nino and La Nina?
2. Effects of El Nino and La Nina with regards to ocean currents and air currents.
Meteorologists have not been very successful at forecasting them so far; it is mainly a question of monitoring it to see of they is there.
La Nina conditions often follow the year after El Nino.
To learn about the effects, read
http://www.nationalgeographic.com/elnino/mainpage.html
El Nino- surface temps warm
La Nina- surface temps cool