This chapter deals with the basic atmospheric processes involved in coupled atmosphere–ocean interactions over the tropical oceans, and in particular, those processes needed for a description and analysis of ENSO.

In order to begin the discussion, we have to define some basic atmospheric quantities; in particular, the virtual temperature, the dry static energy and the moist static energy. In terms of these quantities, we examine dry adiabatic ascent, i.e. the temperature changes that would exist if a dry parcel were lifted without the addition of heat, and moist adiabatic ascent, i.e. the temperature changes that a saturated moist parcel would have if lifted with the only internal source of heat being the condensation of parcel water vapor and the subsequent rain out of the water from the parcel.

We then use a classic diagnosis of waves in the tropical Pacific ITCZ to illustrate some unusual differences between tropical and midlatitude atmospheric motions. In particular, the horizontal divergences in these tropical waves are large, in contradistinction to the midlatitudes where geostrophy constrains the horizontal divergences (and therefore the vertical velocities) to be small. The reason for these large tropical divergences is that heating of the atmosphere by deep clouds does not produce much temperature change. Rather, cloud heating by deep cumulonimbus clouds produces synoptic vertical velocities whose adiabatic cooling just balances the cloud condensation heating.