In order to understand what drives climate variability and to better disentangle anthropogenic from natural variability, one mostly has to rely on climate model simulations. This is because observational data are sparse and not evenly distributed over time and geographically. Reanalysis provides a continuous three-dimensional reconstruction of climate variability by making best use of observational data to constrain model dynamics, using a mathematical method called data assimilation.

The project CoRea will produce a coupled reanalysis of high importance to the climate research community. The new reanalysis will come as a form of ensemble and provide a probabilistic estimate of the climate from 1850 to the present. Data assimilation will only update the ocean component of an Earth system model, and as a consequence the reanalysis will be of great importance to understand the role of the ocean in the rest of the climate system (e.g., the atmosphere and sea ice). The reanalysis will be the first one to use an advanced data assimilation method (Ensemble Kalman Smoother) that allows for model state updates varying with the regime of the climate and ensures the smoothness of the climate reconstruction. A theoretical methodology will be revisited to achieve the updates on several characteristic time scales of the climate. The new method will maximise the influence of the rich contemporary observations backwards in time for several decades. The project CoRea will be carried out by three early career researchers, which will foster the development of a new generation of climate researchers. CoRea will help to establish national interdisciplinary cooperation between NERSC, NORCE and UiB within the Bjerknes Centre for Climate Research, and to further build an internationally competitive group that operates at the forefront of climate research.