This dataset contains initialised seasonal hindcast experiments for the 20th century, produced using a version of the ECMWF's Intergrated Forecasting System (IFS), and initialised from ECMWF reanalysis (ERA-20C/CERA-20C). Two types of hindcast experiment are included in this dataset: CSF-20C (Coupled Seasonal Forecasts): This is a hindcast performed with the model fully coupled to the Nucleus for European Modelling of the Ocean (NEMO) ocean model, initialised from CERA-20C reanalysis data. ASF-20C (Atmospheric Seasonal Forecasts): This is a hindcast performed with prescribed Sea Surface Temperature (SST) and sea-ice boundary conditions at the surface, initialised form ERA-20C reanalysis data. Hindcasts were started every year between 1901-2009 (inclusive) on the 1st February, 1st May, 1st August and 1st November and are run out for four months from the start date. For each start date, there are either 51 or 25 ensemble members (depending on the season/hindcast). Monthly mean values for the following variables have been archived here on CEDA (in alphabetical order): LHF (latent heat-flux) OLR (outgoing long-wave radiation) PREC (precipitation) SHF (sensible heat-flux) SIC (sea-ice concentration) SLP (sea-level pressure) SST (sea surface temperature) T2 (2m temperature) U250 (zonal wind at 250 hPa level) U850 (zonal wind at 850 hPa level) Us (10m zonal wind speed) V250 (meridional wind at 250 hPa level) V850 (meridional wind at 850 hPa level) Vs (10m meridional wind speed) Z500 (geopotential at 500 hPa level) Files for each ensemble member are archived separately and the filetree is in the following format: [EXPname]/[VARname]monthly_[EXPname]_[INITmonth]START_ENSmems/[VARname]monthly_[YEAR]_M[ENSnum].nc Where: [EXPname] =CSF-20C or ASF-20C [VARname] = Z500, Vs, V850, ... etc. [INITmonth] = Feb, May, Aug or Nov [YEAR] =1901, 1902, 1903, ... etc. [ENSnum] =0, 1, 2, 3, ... etc. More details of these hindcasts and some previous analysis of these datasets have appeared in the following papers: Weisheimer, A., Schaller, N., O'Reilly, C., MacLeod, D. A., & Palmer, T. (2017). Atmospheric seasonal forecasts of the twentieth century: multi‐decadal variability in predictive skill of the winter North Atlantic Oscillation (NAO) and their potential value for extreme event attribution. Quarterly Journal of the Royal Meteorological Society, 143(703), 917-926. O'Reilly, C. H., Heatley, J., MacLeod, D., Weisheimer, A., Palmer, T. N., Schaller, N., & Woollings, T. (2017). Variability in seasonal forecast skill of Northern Hemisphere winters over the twentieth century. Geophysical Research Letters, 44(11), 5729-5738. MacLeod, D., O'Reilly, C., Palmer, T., & Weisheimer, A. (2018). Flow dependent ensemble spread in seasonal forecasts of the boreal winter extratropics. Atmospheric Science Letters, 19(5), e815. Parker, T., Woollings, T., Weisheimer, A., O'Reilly, C., Baker, L., & Shaffrey, L. (2019). Seasonal predictability of the winter North Atlantic Oscillation from a jet stream perspective. Geophysical Research Letters, 46(16), 10159-10167. Weisheimer, A., Decremer, D., MacLeod, D., O'Reilly, C., Stockdale, T. N., Johnson, S., & Palmer, T. N. (2019). How confident are predictability estimates of the winter North Atlantic Oscillation?. Quarterly Journal of the Royal Meteorological Society, 145, 140-159. Weisheimer, A., Befort, D. J., MacLeod, D., Palmer, T., O’Reilly, C., & Strømmen, K. (2020). Seasonal forecasts of the 20th Century. Bulletin of the American Meteorological Society, (2020).

In support of the COAPEC Thematic Programme the BADC has extracted seasonal forecast ensemble data from the ECMWF MARS (Meteorological Archive and Retrieval System) archive. These data are also known as "Hindcasts" as they are forecasts run retrospectively. Since the data is part of the ECMWF Operational system BADC users must successfully apply for access to this dataset before they can obtain the data. The ECMWF produced two sets of runs, System 1 and System 2. The data archived at the BADC are the System 2 runs which use the atmospheric component Cy23r4 of the IFS (Integrated Forecasting System) with a horizontal resolution of TL95 at 40 levels in the vertical. This is the same cycle of the IFS used for the ERA-40 re-analysis. A detailed description of the ECMWF Seasonal Forecasting system can be found on the ECMWF web site. Products: The BADC has extracted monthly means, maxima, minima and standard deviations for the available surface variables from 1987 to 6 months before the present date. Atmospheric variables are only currently available as monthly means. The data is held as part of the main BADC Operational ECMWF archive. For each month there are six forecast months archived, with 5 ensemble members for 10 months of the year, and 40 ensemble members in May and November of each year from 1987-2001. From 2002 onwards there are 40 ensemble members per month. There are 33 parameters held on surface or single levels and 6 parameters available on pressure levels. The data is held on a regular 1.875 x 1.875 degree grid in GRIB format.