
To answer these questions we must look at how fluids behave on a rotating sphere and how that influences the thermocline.
Sources:
The El Niño-Southern Oscillation Phenomenon - Sarachik, E. & Cane, M.
El Niño and Southern Oscillation (ENSO): A Review:
Mechanism of Kelvin and Rossby waves during ENSO events:
Charging El Niño with off‑equatorial westerly wind events:
Higher frequency of Central Pacific El Niño events in recent decades relative to past centuries: