This activity investigates the oceanographic and climatic characteristics of El NiÃo/La NiÃa (ENSO) events using observational data from moored ocean buoys in the tropical Pacific Ocean. Data are obtained from NOAA's Tropical Atmosphere Ocean (TAO) project website which provides a web-based interface for accessing and displaying oceanographic data. In addition to providing an introduction to ENSO, this activity is designed to give students practice interpreting real oceanographic observations by emphasizing the description and identification of patterns in large data-sets. Students first describe patterns in sea-surface and cross-sectional transects of ocean temperatures and surface winds associated with "normal", El NiÃo, and La NiÃa years and then use this as a basis for classifying observations from unknown years and interpreting connections between oceanographic and atmospheric processes occurring in the tropical Pacific.