Northern Hemisphere climate trends in reanalysis and forecast model predictions: The 500 hPa annual means

The lead time dependent climates of the ECMWF weather prediction model, initialized with ERA‐40 reanalysis, are analysed using 44 years of day‐1 to day‐10 forecasts of the northern hemispheric 500‐hPa geopotential height fields. The study addresses the question whether short‐term tendencies have an impact on long‐term trends. Comparing climate trends of ERA‐40 with those of the forecasts, it seems that the forecast model rapidly loses the memory of initial conditions creating its own climate. All forecast trends show a high degree of consistency. Comparison results suggest that: (i) Only centers characterized by an upward trend are statistical significant when increasing the lead time. (ii) In midilatitudes an upward trend larger than the one observed in the reanalysis characterizes the forecasts, while in the tropics there is a good agreement. (iii) The downward trend in reanalysis at high latitudes characterizes also the day‐1 forecast which, however, increasing lead time approaches zero.