The effects of stratosphere-troposphere coupling on the decadal predictability of the climate system.

The coupled ocean-atmosphere CMCC-CMS model is used to investigate the influence of the stratosphere on the decadal predictability. A set of decadal prediction experiments are performed for the 1960-2005 period, following the CMIP5 protocol using historical radiative forcing conditions, followed by RCP4.5 scenario settings from 2006 onward. The decadal predictions consist in 3-member ensembles of 10-year simulations starting at 5-year intervals, with the ocean initial states provided by ocean reanalyses differing by assimilation methods and assimilated data. A purpose of this work is to asses the impact of the initialization to reproduce climate variations with respect to an uninitialized climate simulation performed for the same time period of the predictions using identical forcing conditions. Further analyses were performed using the high top MPI-ESM-MR coupled model of the Max Plank Institute for Meteorology with ocean-atmosphere every year initialized state. Anomaly correlation coefficient (ACC) of sea surface temperature (SST) and zonal mean zonal wind (ZMZW) were performed to assess the likely skill for climate predictions and analyse the low-frequency variability of the stratosphere through the quasi biennial oscillation (QBO) and the polar vortex