The ocean, like the atmosphere, is a fluid component of the climate system and must be represented in climate models. Heat and water are passed between the ocean and atmosphere, and these processes must be represented as accurately as possible. Also, the wind speed at the surface affects the way that the top of the ocean is mixed and so how rapidly it responds to changing atmospheric temperatures.
Ocean “weather systems” or eddies, which play an important role in the climate system, tend to be much smaller (up to about 100km) than atmospheric weather systems, so the ocean components of climate models tend to have a finer resolution than the atmosphere components. Oceans take much longer to react to changes in the balance between incoming and outgoing radiation than the atmosphere. This means that ocean models need to run for many decades if they are to be included in climate predictions. These factors mean that they require significantly more computing power than atmosphere models. This is sometimes avoided by using a simplified model called a “slab ocean”, which effectively just represents the top 50m of the ocean, with none of the deep sea currents which can transport a huge amount of heat, albeit extremely slowly. The effects of the currents therefore need to be parameterized.
Both a slab ocean model and a ‘complete’ ocean model will be ‘coupled’ to the atmosphere model in the climateprediction.net experiment. The complete ocean model used by the climateprediction.net experiment in fact has the same horizontal resolution (2.5° in latitude by 3.75° in longitude) as the atmosphere, and 20 vertical levels, with finer vertical resolution near the surface.
The coupled model runs asynchronously, which means that the atmosphere model runs first for some time then the ocean model runs for some time, taking turns. In the case of the model used in the climateprediction.net experiment, the individual components run for one day at a time.
Fluxes of heat, wind, and freshwater are passed between the ocean model and the atmosphere model at the ocean-atmosphere interface.