This is a book about the set of coupled atmosphere–ocean phenomena known collectively as ENSO (El Niño–Southern Oscillation). While it will concentrate on what is known about ENSO, its mechanism, its effects, and how predictable it is, it will also touch on what is known about the paleohistory of ENSO and what we might expect in the future as mankind puts CO2 and other radiatively active constituents into the atmosphere. The approach, while theoretical and sometimes necessarily mathematical, will concentrate on observations and on physical principles. Rigor will be acknowledged and appreciated but rarely practiced. When something in the text is stated to be known, but is not explained, the symbol ☼ (usually accompanied by a reference or footnote) will be used. This will be true of all chapters except the Preview (Chapter 1), where much will be arbitrarily stated, subsequently to be explained in the rest of the book.

Because ENSO is an intrinsically coupled ocean–atmosphere process, we will introduce the essentials of both the tropical atmosphere and ocean and explain the unique properties of each medium. Because ENSO is an intrinsically Pacific phenomenon, we will explain the unique aspects of the tropical Pacific and which of its features makes it particularly congenial for the existence of ENSO. We will describe those tropical atmospheric and oceanic mechanisms that ultimately help to explain the mechanism of ENSO. While there is no general agreement about what the ENSO mechanism is, we would expect that a similar book written a decade or so henceforth would contain much of the same material.