5. Which models are the climate modellers using at the Alfred Wegener Institute – and what are your objectives?

Weather models mainly take into account the physical processes in the atmosphere. The processes in the ocean, such as large currents or the development of sea ice, are not taken into account in detail. But they have a significant impact on the climate. The AWI experts also use such atmospheric models - for example, the ECHAM model developed by the Max Planck Institute for Meteorology and operated at the Climate Research Centre in Hamburg.

In order to describe the processes in the sea more precisely, a special ocean model has been developed at the AWI over the past few years - FESOM. Among other things it simulates ocean currents and sea ice. AWI researchers have linked the models ECHAM and FESOM into a coupled model – the "AWI Climate Model". It allows climate simulations that do not only consider the atmosphere, but also the ocean and sea ice in detail. With the AWI Climate Model, simulations can be carried out on a global scale as well as in detail for certain regions. This is due not least to the special properties of FESOM.

Unlike many other ocean models, FESOM does not use cubes as grid boxes. Instead, it works with unstructured grids. Put simply, a flexible network of triangles is placed on the globe. The benefit: It is possible to draw the triangles larger and smaller and thus have a higher resolution for certain areas - for example, individual ocean currents. Using this method, the AWI researchers were able to simulate the climate phenomenon El Niño with higher resolution over several centuries. In another case, the researchers calculated how the currents under the ice shelf of Antarctica could change in the future. The ice shelf is the marginal part of the Antarctic glaciers which slowly moves to the sea. The ice shelf floats as a gigantic ice sheet on the water, but is still firmly connected to the glacier. With FESOM, the researchers are able to simulate the currents that will melt the ice shelf in the course of climate change. It is feared that the Antarctic glaciers will slip faster into the sea when the ice shelf disappears. This could accelerate the sea level rise.

In addition to the coupled climate model consisting of ECHAM and FESOM, AWI researchers also operate the NAOSIM model which focusses on the ocean and sea ice. This model is fed with a lot of knowledge about the physics of sea ice, taking into account the thickness and the concentration of ice floes. Both excert a strong influence on the heat exchange between the atmosphere and the ocean. While with the coupled AWI climate model global climate simulations are possible, NAOSIM only depicts the processes in the Arctic and the North Atlantic. “Altogether, we can use our models to handle the entire spectrum from small to large scales," says Helge Gößling. "We can carry out global climate simulations, but we can also zoom into certain marine areas." For example, the AWI participates with the AWI Climate Model in international climate model intercomparison projects which ultimately serve as a basis for the assessment reports of the Intergovernmental Panel on Climate Change (IPCC).