R code for multi-decadal variation of ENSO forecast skill since the late 1800s

The enclosed R code is intended for conducting tropical sea surface temperature (SST) hindcasts using a model-analog technique. The verification dataset used in this analysis is taken from CERA-20C, covering the period from 1901 to 2009. The library dataset is taken from the CMIP6 CESM2 pre-industrial control simulation. The results have been extensively discussed in the paper by Lou et al. (2023) titled "Multi-decadal variation of ENSO forecast skill since the late 1800s," which has been accepted in principle for publication by npj Climate and Atmospheric Science. For more detailed discussions, please refer to the aforementioned paper.