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.