How does El Nino affect upwelling?
During an El Niño event, upwelling of cooler oceanic water decreases along the S. American coast.
El Niño is the warm phase of the El Niño Southern Oscillation or ENSO. During El Niño, upwelling of cooler oceanic water decreases along the S. American coast.
What typically happens is that the trade winds move from S. America to Asia/Australia, and warm surface water is dragged away from the coast and colder, nutrient water rises to shallow depths.
Process of upwelling:
During El Niño, the trade winds are very weak and upwelling doesn't happen. Without this upwelling of nutrient-rich waters, the Peruvian fishing industry suffers (read more here).
Cold and warm phase of ENSO:
You can read about El Niño in more detail here.