Yale professor Ron Smith explains the El Niño/Southern Oscillation (ENSO), the dominant pattern of variability in the equatorial Pacific Ocean, in this lecture from his course The Atmosphere, the Ocean and Environmental Change. He walks through the two extreme states, El Niño and La Niña, and how they show up in sea surface temperature, sea level pressure, thermocline depth, and trade wind strength. The lecture covers terminology, symptoms of El Niño, ENSO indices, and current ocean data, then turns to how weakened trade winds reduce coastal upwelling off South America and cut into fish and agricultural productivity. A closing section shifts to ice in the climate system and the physical properties of ice. Recorded at Yale in fall 2011, the talk is data heavy, referencing real measurement records to ground the physical mechanisms being described.