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This chapter provides an observational survey of the main elements of the tropical atmosphere and ocean needed in the sequel. In particular, the major circulation features in the atmosphere and ocean important for understanding ENSO: SST, SLP, surface winds, surface heat fluxes, the East–West overturning circulation in the Pacific, the Hadley circulation and the depth of the equatorial thermocline. Because the surface plays such a crucial role in atmosphere–ocean interactions, special emphasis will be placed on the fluxes at the surface, in particular the wind stresses, the latent heat flux and the net heat flux into the ocean. The annual cycle of the crucial quantities needed to define the climatology of the tropics: SST, SLP, precipitation, winds and thermocline depth, will be presented. “Anomalies,” including those characteristic of ENSO, can be defined relative to this climatology. The major features of ENSO and the evolution of ENSO as we now know them will be presented, with some discussion of how typical an ENSO event is likely to be. Some effects of ENSO on the globe, especially tropical temperature and precipitation, Atlantic hurricane landings and monsoon rainfall will be described. Some observations of both higher frequencies (periods less than a year) and lower frequency (especially decadal variability) will be introduced. The nature and source of climate observations relevant to ENSO It would be valuable to have an accurate picture of the Earth's atmosphere and ocean throughout the temporal evolution of climatic variability but, unfortunately, the measurement of variables important for climate has a relatively short history.

Many climatic extremes around the globe, such as severe droughts and floods, can be attributed to the periodic warming of the equatorial Pacific sea surface, termed the El Niño or Southern Oscillation (ENSO). Advances in our understanding of ENSO, in which Edward Sarachik and Mark Cane have been key participants, have led to marked improvements in our ability to predict its development months or seasons, allowing adaptation to global impacts. The book introduces basic concepts and builds to more detailed theoretical treatments. Chapters on the structure and dynamics of the tropical ocean and atmosphere place ENSO in a broader observational and theoretical context. Chapters on ENSO prediction, past and future, and impacts introduce broader implications of the phenomenon. This book provides an introduction to all aspects of this most important mode of global climate variability, for research workers and students of all levels in climate science, oceanography and related fields.

We begin by making some non-standard distinctions, solely for convenience in the following discussion. We will define “ENSO prediction” as the process of predicting the SST in the tropical Pacific a month to a year or so in advance. We will use “short-term climate prediction” to refer to the procedure of predicting the climatic conditions in the global atmosphere or over land away from the tropical Pacific a month to a year in advance. The utility of this distinction is that ENSO prediction can only be accomplished by coupled models, whereas short-term climate prediction, which depends in part on the thermal forcing due to the distribution of regions of persistent precipitation and is partly determined by the SST distribution in the tropical Pacific, can be accomplished by a global atmospheric model (with predicted tropical SST specified) but can also be accomplished by a fully coupled climate model. The distinction will become clearer in our discussion of one-tiered and two-tiered prediction below. The possibility that coupled climate models, whether simple or complex, can predict aspects of the future evolution of ENSO is not at all obvious. The atmosphere is known to be of limited predictability, basically because it is chaotic: inevitable small errors in the initial conditions grow and, depending on the growth rate, eventually limit the skill of prediction after a given time.