A Climate Reconstruction for 1750-1850 A.D. Using Data Assimilation
Abstract:Climate reconstructions for the pre-instrumental period are either based on climate proxy data or on numerical simulations. However, both approaches are associated with substantial uncertainties. In principle, the best state estimates can be expected by employing data assimilation (DA) techniques, which systematically combine the empirical information from proxy data with the representation of the processes that govern the climate system given by climate models. Here, we reconstruct the climate for 1750-1850 A.D by employing an ensemble member selection DA method, in which the ensembles are generated sequentially for sub-periods based on the analysis of previous sub-periods. The simulated period is very interesting for testing the assimilation method, because of the strong volcanic eruptions, such as the Tambora (Indonesia, 1815 A.D.) and Cosiguina (Nicaragua, 1835 A.D.) eruptions, and their dynamical effects on the ocean and the atmosphere. The period also has several advantages. Firstly, it is directly before the instrumental period and a relatively high number of climate data, e.g. early instrumental records and historical documents, is available for this period. Additionally, a relatively high number of proxies have contributed to the PAGES 2k continental reconstructions which are used in the assimilation, thus reducing errors.
We use the Max Planck Institute for Meteorology’s model MPI-ESM. In the so called on-line DA method, a 20-member ensemble was generated for the first year of the analysis period, 1750 A.D, by introducing small perturbations in the atmospheric diffusion field. Simulations with 10-year duration were run, starting from the previous best member's initial conditions, until the year 1850 A.D. We reconstruct different variables of the climate system, such as the surface air temperature, geopotential height and sea-surface temperature, and indices such as the Meridional Overturning Circulation (MOC), North Atlantic Oscillation (NAO), and the Northern and Southern Annular Modes (NAM/SAM) and compare them with various proxy-based and early instrumental reconstructions, which are partly independent of the data that have been assimilated.