Data Assimilation Through Upscaling and Nudging (DATUN) is a technique to assimilate climate proxy data (e.g. from tree rings, corals, lake sediments or ice cores) into atmosphere-ocean-cryosphere climate model simulations with the goal of reconstructing the global climate during the last millennium. This method is a combination of (1) the statistical reconstruction of past climate variability from proxy data and (2) the modeling of the forced component of climate change using estimates of past climate forcings to drive climate model integrations. It is a physically-based , spatial and temporal interpolation of paleoclimatic information by providing an estimate of the global past climate trajectory which is consistent with model physics but is "nudged" towards the observed large-scale climate state derived from proxy data.

DATUN consists of two steps. The first step is the empirical upscaling of multy-proxy networks to corresponding large-scale circulation states, e.g. the amplitudes of the Arctic or Antarctic Oscillation, acknowledging the fact that more detailed historical information is not available from proxy data. This is followed by an assimilation step in which a newly developed pattern nudging technique is used to force ("nudge") the atmospheric circulation in a climate model  to be consistent with these large-scale patterns, without affecting components of the climate state that are not constrained by the proxy data while including synoptic-scale variability.

The DATUN technique has been developed by Martin Widmann and Hans von Storch at the GKSS Research Centre. As a sub-contractor of GKSS in the nationally-funded German DEKLIM project, O.A.Sys implemented the pattern nudging method into the ECHAM4  Atmospheric General Circulation Model of the Max Planck Institute for Meteorology.

References:

Jones, J. M. and Widmann, M., 2004: Reconstructing large-scale variability from palaeoclimatic evidence by means of Data Assimilation Through Upscaling and Nudging (DATUN). In: Fischer, H., Kumke, T., Lohmann, G., Flöser, G., Miller, H., von Storch, H., and Negendank, J. F. W. (eds.), 2004: The KIHZ project: towards a synthesis of Holocene proxy data and climate models. Springer, Heidelberg, Berlin, New York. ISSN 1437-028X.

von Storch, H., Cubasch, U., González-Rouco, F., Jones, J. M., Voss, R., Widmann, M., and Zorita, E., 2000. Combining paleoclimatic evidence and GCMs by means of data assimilation through upscaling and nudging (DATUN). Proc. 11th Symposium on Global Change Studies, American Meteorological Society, Long Beach, CA.

Widmann, M., J.M. Jones and H. von Storch, 2004: Reconstructions of large-scale circulation and data assimilation in paleoclimatology. PAGES Newsletter, Vol 12, No 2, pp. 12-13.