The Representation of Surface Temperature Trends in C3S Seasonal Forecast Systems

We assess near‐surface temperature and sea surface temperature trends in 8 seasonal forecast systems in the Copernicus Climate Change Service archive, over the common hindcast period (1993–2016). All but one of the systems show a faster warming of the global‐mean, relative to observations in both boreal summer and winter seasons. On average, systems warm at 0.21 K/decade and 0.22 K/decade for winter and summer, respectively, compared to 0.17 K/decade and 0.19 K/decade for ERA5. In summer, forecast systems tend to show an excessive warming of the tropical Pacific, tropical Atlantic and southern mid‐latitudes, which contributes to the difference in global warming rates compared to observations. In contrast, greater warming in the northern mid‐latitudes contributes most to trend differences for winter. The faster warming of models over this period has important implications for seasonal forecasts of future global and regional temperature and suggests further work is required to understand this bias.