During La-Nina conditions, stronger trade winds bring up deep cold water as winds travel up the coast line of South America. The winds carry the water from Antarctica (or of similar latitude). This water creates a cold pool of the South American coast and is circulated through the E. Pacific leading to cool SSTs in the Nino regions.
Subsequently this creates a cascade affect into the Indian Ocean. The Indian Ocean can be monitored in many ways, and one of them is the Indian Ocean Dipole (IOD) which measures SST anomalies. During La-Nina phases, the IOD becomes negative, favoring warmer SSTs near the Western Australian Coast and cooler SSTs near Africa
But back to the Walker Cell…
The opposite scenario exists for El-Nino. Weaker trade winds establishes a weaker Walker Cell Circulation. The result is that the cold pool off the SA coast line isn’t able to travel up and around into the equator. In fact, due to low pressure and convection moving from Australia/Indonesia into the Central Pacific, warmer waters from those areas move into the ENSO regions. They have less trade winds to fight against, less resistance. The cold pool off SA isn’t there or cannot counterbalance the weakened Walker Circulation.