Using data assimilation to study extratropical Northern Hemisphere climate over the last millennium 1Institute for Coastal Research, GKSS Research Centre, Geesthacht, Germany
2School of Geography, Earth and Environmental Science, University of Birmingham, UK
3Institut d'Astronomie et de Géophysique George Lemâitre, Université catholique de Louvain, Belgium
4Royal Netherlands Meteorological Institute (KNMI), De Bilt, The Netherlands
5O.A.SYS – Ocean Atmosphere Systems GmbH, Hamburg, Germany
6Royal Netherlands Meteorological Institute (KNMI), De Bilt, The Netherlands
Received: 10 June 2009 – Published in Clim. Past Discuss.: 07 September 2009 Abstract. Climate proxy data provide noisy, and spatially incomplete information
on some aspects of past climate states, whereas palaeosimulations with
climate models provide global, multi-variable states, which may however
differ from the true states due to unpredictable internal variability
not related to climate forcings, as well as due to model deficiencies.
Using data assimilation for combining the empirical information from
proxy data with the physical understanding of the climate system
represented by the equations in a climate model is in principle a
promising way to obtain better estimates for the climate of the past.
Revised: 31 March 2010 – Accepted: 17 May 2010 – Published: 30 September 2010
Data assimilation has been used for a long time in weather forecasting
and atmospheric analyses to control the states in atmospheric General
Circulation Models such that they are in agreement with observation
from surface, upper air, and satellite measurements.
Here we discuss the similarities and the differences between the data
assimilation problem in palaeoclimatology and in weather forecasting,
and present and conceptually compare three data assimilation methods
that have been developed in recent years for applications in palaeoclimatology.
All three methods (selection of ensemble members, Forcing Singular
Vectors, and Pattern Nudging) are illustrated by examples that are
related to climate variability over the extratropical Northern
Hemisphere during the last millennium. In particular it is shown that
all three methods suggest that the cold period over Scandinavia
during 1790–1820 is linked to anomalous northerly or easterly
atmospheric flow, which in turn is related to a pressure anomaly that
resembles a negative state of the Northern Annular Mode.
Citation: Widmann, M., Goosse, H., van der Schrier, G., Schnur, R., and Barkmeijer, J.: Using data assimilation to study extratropical Northern Hemisphere climate over the last millennium, Clim. Past, 6, 627-644, doi:10.5194/cp-6-627-2010, 2010.