What El Nino usually means for us:
A stormy pattern across the southern US that can turn into Nor’easters for us.
El Nino is when warm water in the tropical Pacific moves east and shifts the jet stream over North America. This happens every 3 to 7 years. The rest of that water gyre is called the Southern Oscillation. La Nina is when cool water upwelling resumes in the eastern Pacific, and the warmer water is pushed farther west.
How is El Nino measured and determined?
Sea Surface Temperatures (SST) Anomalies. Basically, where the ocean water is warmer or colder than normal. This is done through numerous remote buoys satellites. The region of above normal temperatures can expand across the tropical Pacific from Central and South America.
Video: El Nino and La Nina Explained
From NOAA’s National Ocean Service