Representation of tropical and extratropical trends in ECMWF seasonal hindcasts

Climate trends represent one source of predictability for climate forecasts. Hence, it is important for seasonal prediction systems to reproduce observed trends in the climate system. This contribution presents an assessment of trends in seasonal hindcasts from the European Centre for Medium-Range Weather Forecasts (ECMWF). We investigate trends in the tropics and extratropics, as well as potential links between those.

In the tropics, the focus is on processes related to El Nino – Southern Oscillation (ENSO). The hindcasts exhibit a spurious sea surface warming trend in the equatorial Pacific (i.e., a tendency towards El Nino in more recent years), which is mainly related to an underestimation of the atmospheric circulation response to the observed strengthening of the equatorial Pacific zonal sea surface temperature gradient. The trend errors are most pronounced for boreal summer and autumn (independent of start date). Furthermore, the trend errors are similar to those found in free coupled climate model simulations and suggest a biased response of the atmospheric model to changes in the anthropogenic forcing.

In the extratropics, we focus on summer-time upper tropospheric circulation and surface temperature. The ensemble mean completely misses the clear observed circulation trends (resembling a wave-5 pattern) and the associated centers of enhanced surface warming. Possible implications and causes of the missed trends will be discussed.