Climate models of varying complexity are used to study and predict climate. The so-called General Circulation Models (GCMs, sometimes equivalently referred to as Global Climate Models) of the atmosphere try to represent all the important process that contribute to atmospheric circulation at all locations and timescales. They will be considered further later. It is sometimes useful to simplify a problem in order to understand it better. If we are interested in a particular region and timescale, it may be possible to isolate a few of the key driving processes, from the complex of ones operating in the full climate system (Figure 1.1).

The following is an example of a simplified model of the atmosphere designed to highlight one effect of the SST on the tropical atmosphere. It turns out that this effect is a key one for determining the local atmospheric response to a change in SST on the seasonal to interannual timescale in the tropics. The model only applies to the tropics, because the assumptions it makes are only valid in the tropics. A more detailed discussion of the model can be found in Lindzen and Nigam (1987). In the analysis, they asked the question "for a given seasonal mean SST pattern, what is the seasonal mean wind pattern to expect?".

The assumptions made and the equations used can be studied (click here, optional unit, the Lindzen-Nigam model), but a subjective understanding can be gained without consideration of the equations. It is known that over tropical oceans, there is usually a well-mixed low layer of the atmosphere, usually about the lowest 3km of the atmosphere. This is called the boundary layer. Because this layer is well-mixed, the temperature of the layer reflects the temperature of the ocean surface. Thus, over warm SST, there will exist an approximately 3km layer of atmosphere that reflects the warm SST. Warm air is lighter than cold air. Thus, if we sit on a ship in a region of warm SST, immediately above us is 3km of warm air which weighs on our heads less than if we were sitting on a ship in a region of cold SST, with 3km of cold atmosphere above us. In other words, there is a difference in surface atmospheric pressure. Assume the difference in SST is about 2C across 2,000km of ocean. The difference in surface atmospheric pressure that results from the SST difference over the 2,000km is sufficient to drive substantial winds at the surface - winds blow from the high surface pressure to the low surface pressure. This is schematically shown in Fig. 1.4. It is seen that winds will tend to come together (converge) over warm water, with no option but for air to be forced to rise (it cannot go down, into the ocean!). Such rising air often gives rise to increased precipitation in the region if conditions through the depth of the atmosphere are favourable, but this will be returned to later.