This diagram illustrates some of the features of a coupled ocean-atmosphere model. Click on image for full size (170 Kb) Image courtesy of UCAR.

Scientists use computer models to help them understand and predict the behaviors of Earth's atmosphere and oceans. These models, which run on supercomputers, are extremely complex. Some models only cover the atmosphere, while others only deal with the oceans. A third type of model combines ocean and atmospheric models into a single integrated framework in which interactions between the oceans and atmosphere are a key feature. These combined models are referred to as "coupled models".

Let's look at an example to clarify the distinction between coupled and uncoupled models. Atmospheric models that are used to assess climate change are very much concerned with the amount of carbon dioxide (CO₂) in the air. Some processes, such as fossil fuel burning, add carbon dioxide to the atmosphere. Other processes remove carbon dioxide from the air. The world's oceans absorb large amounts of CO₂, for instance. In an uncoupled model of the atmosphere, the portion of the model that portrays the oceans always stays the same, no matter how much CO₂ the seas absorb. In a coupled model, on the other hand, the ocean model would change over time as the levels of carbon dioxide in the oceans increased. The oceans would gradually become saturated with CO₂ and would thus be able to absorb less and less from the atmosphere. This change would, in turn, alter the behavior of the atmosphere part of the model.

It probably seems obvious to you that a coupled model is more realistic. Why don't scientists always use coupled models? Coupled models are much more complex, so it takes a lot more effort to make sure they are working correctly. The greater level of complexity also means that coupled models take much longer to run, even on supercomputers. Some scenarios can be modeled pretty well with uncoupled models. Other situations demand the more complex coupled models.

This picture shows some of the "parts" of a computer model that includes both the atmosphere and the ocean. Click on image for full size (170 Kb) Image courtesy of UCAR.

Scientists use computer models to help them understand the Earth. Scientists who study the atmosphere use computer models of the atmosphere. Some scientists who study the oceans use computer models of the seas. Some scientists study both the atmosphere and the oceans. Those scientists use a special kind of model that includes both the seas and the air. These combined models are called "coupled models".

What is the difference between a coupled model and a model that isn't coupled? Let's look at an example. Computer models of the atmosphere keep track of many things, like how much carbon dioxide (CO₂) is in the air. Some parts of the model keep track of how much CO₂ is added to the air. For example, burning coal and gasoline adds CO₂ to the atmosphere. Other parts of the model keep track of how much CO₂ is taken out of the air. One way this happens is that oceans absorb CO₂ from the air. In a normal uncoupled model of the atmosphere, scientists don't keep track of how much CO₂ ends up in the oceans. The oceans don't change in that kind of model.

A coupled model is different. Changes in the atmosphere do cause changes in the ocean. Changes in the ocean part of the model can cause changes in the atmosphere part. So if lots of carbon dioxide moved from the atmosphere to the ocean, the ocean might get "full" of CO₂. It might not be able to hold any more. Or it might take in more CO₂ very slowly.

As you might guess, coupled models can be more realistic. So why don't scientists always use coupled models instead of uncoupled models? Coupled models are very, very complicated. It takes a lot of work to make sure the answers from them are right. It also takes a long time to run coupled models, even on fast supercomputers. Sometimes uncoupled models are good enough for certain types of problems. Other times scientists really need to use the more complex coupled models.

This picture shows some of the "parts" of a computer model. This model has both the atmosphere and the ocean in it. Click on image for full size (170 Kb) Image courtesy of UCAR.

Scientists use computer models to help them understand the Earth. They use them to study the atmosphere. They also use them to study the oceans. Some computer models just have the atmosphere in them. Other models just have the oceans in them. There is a third kind of model. It is more complicated than the other two kinds. It has the oceans and the atmosphere in it. Scientists call the third kind a "coupled model".

Coupled models can be more like the real world. In the real world, there are connections between the ocean and the atmosphere. When it rains over the ocean, the ocean gets more water in it. Some of the water vapor leaves the air. When the Sun heats the ocean, water evaporates from the ocean. That water goes into the air. It can form clouds.

Why don't scientists always use coupled models? The models are very, very complicated. It takes a lot of work to makes sure the answers from them are right. They also take a long time to run, even on fast supercomputers. Sometimes the models that are not coupled give answers that are good enough. Other times the problems are more complex. That's when scientists use coupled models.