Already been explained. During periods of global warming, you will tend to get stronger and more frequent El Nino's. During periods of Global Cooling you will tend to get stronger and more frequent La Nina's. That is just a tendency. There is no causation between El Nino and La Nina to Global Warming or Cooling, simply because global implies oceans and atmosphere. And the AGW acolytes have no idea what happens to the deep oceans during the La Nina and El Nino. Now there is causation between La Nina/ El Nino and atmospheric cooling and warming. But Global implies atmosphere plus oceans. The correct way to word the situation is that El Nino causes Global Atmospheric warming. And if you know anything about physics, you know that the deep oceans must cool if the atmosphere warms assuming that solar energy is a constant. Conservation of Energy, my friend.
You also need to assume that the earth is retaining a constant proportion of the energy received.
Greenhouse gases are postulated to be causing an increase in the amount of energy retained.