Temperature Trends and Variability in the 20th Century reanalysis version “2c” dataset spanning 1851 to 2012

The historical reanalysis dataset generated by NOAA Earth System Research Laboratory and the University of Colorado CIRES, the Twentieth Century Reanalysis version 2 (20CRv2), is a comprehensive global atmospheric circulation dataset spanning 1871-2012, assimilating only surface pressure and using monthly Hadley Centre SST and sea ice distributions (HadISST1.1) as boundary conditions. It has been made possible through collaboration with GCOS, WCRP, and the ACRE initiative. It is chiefly motivated by a need to provide an observational validation dataset, with quantified uncertainties, for assessments of climate model simulations of the 20th century, with emphasis on the statistics of daily weather. It uses, together with an NCEP global numerical weather prediction (NWP) land/atmosphere model to provide background "first guess" fields, an Ensemble Kalman Filter (EnKF) data assimilation method. This yields a global analysis every 6 hours as the most likely state of the atmosphere, and also yields the uncertainty of that analysis.

While 20CRv2 is useful, there are opportunities for improvement. A new version (20CRv2c) released in 2015 includes an extension back to 1851 and the specification of new boundary conditions. These come from new fields of monthly COBE-SST2 sea ice concentrations and an ensemble of daily Simple Ocean Data Assimilation with Sparse Input (SODAsi.2c) sea surface temperatures generated at Texas A&M University. SODAsi.2c itself was forced with 20CR, allowing these boundary conditions to be more consistent with the atmospheric reanalysis. Millions of additional pressure observations contained in the new International Surface Pressure Databank version 3 are also included. These improvements result in 20CRv2c having comparable or better analyses, as suggested by improved 24 hour forecast skill and more realistic uncertainty in near-surface air temperature. In this presentation, the global and regional near-surface temperature trends and variability are assessed and compared climate models used in CMIP5 and to other observational estimates from the new ERA-20C surface-based reanalysis and the new HadNMAT marine air temperature reconstruction.