Winter-Spring Southern Oscillation Index (SOI)

• Red bars indicate years when the winter-spring SOI was –10 or lower
• Recognized El Niño years have their bars labelled
  Note that years marked with an asterisk (*) are ambiguous in that, depending on definitions, some scientists class these as El Niño years while others don't
• A lack of data from the late 1800s and early 1900s makes it difficult to classify years during this period