Forecasting the climate response to volcanic eruptions: prediction skill related to stratospheric aerosol forcing

The last major volcanic eruptions, the Agung in 1963, El Chichon in 1982 and Pinatubo in 1991, were each associated with a cooling of the troposphere that has been observed over large continental areas and over the Western Pacific, the Indian Ocean and the Southern Atlantic. Simultaneously, Eastern tropical Pacific temperatures increased due to prevailing El Ni?o conditions. Here we show that the pattern of these near-surface temperature anomalies is partly reproduced with decadal simulations of the EC-Earth model initialised with climate observations and forced with an estimate of the observed volcanic aerosol optical thickness. Sensitivity experiments highlight a cooling induced by the volcanic forcing, whereas El Ni?o events following the eruptions would have occurred even without volcanic eruptions. Focusing on the period 1961?2001, the main source of skill of this decadal forecast system during the first 2 years is related to the initialisation of the model. The contribution of the initialisation to the skill becomes smaller than the contribution of the volcanic forcing after two years, the latter being substantial in the Western Pacific, the Indian Ocean and the Western Atlantic. Two simple protocols for real time forecasts are investigated: using the forcing of a past volcanic eruption to simulate the forcing of a new one, and applying a two-year exponential decay to the initial stratospheric aerosol load observed at the beginning of the forecast. This second protocol applied in retrospective forecasts allows a partial reproduction of the skill attained with observed forcing.The research leading to these results has received funding from the Ministerio de Economía y Competitividad (MINECO) as part of the VOLCADEC (ref.CGL2015–70177-R) and HIATUS (ref.CGL2015–70353-R) projects, as well as the program SPECS funded by the European Commission’s Seventh Framework Research Programme under the grant agreement 308378. We thank the Met Office for providing the HadCRTU4 observational dataset. All the analysis and figures have been produced with the s2dverification R package (https://cran.r-project.org/ web/packages/s2dverification/index.html) and the simulations have been launched with the Autosubmit workflow manager (http://autosubmit.readthedocs.io/en/latest/)Peer Reviewe