Ocean model
The ocean model (usually an Ocean General Circulation Model OGCM) is used in climatology to represent the ocean circulation three-dimensionally in a climate model. Since an exclusive consideration of the atmospheric system is not in a position to adequately explain all climate phenomena, attempts are increasingly being made to couple atmospheric models ( Atmospheric Global Circulation Model , AGCM) with an OGCM. The three-dimensional approach in this case means the inclusion of the horizontal ocean currents as well as the vertical water movements.
The difficulty with an OGCM lies in the fact that the precise processes of ocean circulation have not yet been satisfactorily researched. In particular, deep-sea currents , some of which circulate very slowly, have only been researched selectively for a few decades. While surface currents (which have been well researched) have an average speed of about 0.5 - 1 km / h, the speed of oceanic deep-sea currents is significantly lower. The cycle time (the time it takes for a water particle to pass through the entire system) is between 1,000 and 10,000 years. The deep-sea currents, which have not been studied in terms of measurement technology, make it very difficult to verify the calculations of the ocean models. However, enormous progress has been made in this area in recent years.
The initial ocean models were so-called "mix ocean layer models". These are models that have only calculated the top 50 m (the mixing layer ). The processes in the lower ocean layers were parameterized here.
The most important physical quantities that are simulated in an ocean model are temperature (T) and salinity ( salinity , S). The density of a water parcel results from the temperature and salinity , which decides whether the water mass sinks or rises. This process is relevant for the global ocean circulation ( thermohaline circulation ).
Modern ocean models mostly also include biogeochemical processes such as the production and decay of organic material (mainly plankton ) and the related material cycles such as carbon (C), oxygen (O) or phosphorus (P). If an ocean model is coupled with an atmosphere model, gas exchange takes place on the ocean surface. The exchange of carbon dioxide (CO 2 ) and oxygen (O 2 ) between the atmosphere and the ocean has a significant impact on the global climate system . Around a quarter of the CO 2 emitted by humans through the combustion of fossil fuels is absorbed by the ocean, which reduces the increase in CO 2 in the atmosphere, but also leads to a change in the chemical composition of seawater ( acidification ). These processes can now be simulated well with global climate models.