The representation of winter Northern Hemisphere atmospheric blocking in ECMWF seasonal prediction systems

Abstract

The simulation and prediction of winter Northern Hemisphere atmospheric blocking in the seasonal prediction systems from the European Centre for Medium‐Range Weather Forecasts (ECMWF) is analysed. Blocking statistics from the operational November‐initialised seasonal hindcasts are evaluated in three generations of models: System3, System4, and System5 (SEAS5). Improvements in the climatological representation of blocking are observed in the most recent model configurations, with reduced bias over North Pacific and Greenland. Minor progress is seen over the European sector, where SEAS5 still underestimates the observed blocking frequency. SEAS5 blocking interannual variability is underestimated too and is proportional to the climatological frequency, highlighting that a negative bias in the blocking frequency implies an underestimation of the interannual variance. SEAS5 predictive skill and signal‐to‐noise ratio remain low, but interesting positive results are found over Western and Central Europe. Improved forecasts with reduced ensemble spread are obtained during El Niño years, especially at low latitudes. Complementary experiments show that the statistics of blocking are improved following atmospheric and oceanic resolution increase. Conversely, they remain largely insensitive to coupled model sea‐surface temperature (SST) errors. On the other hand, the implementation of stochastic parameterisations tends to displace blocking activity equatorward. Finally, by comparing seasonal hindcasts with climate runs using the same model, we highlight that the largest contributors to the chronic underestimation of blocking are persistent errors in the atmospheric model. It is also shown that SST errors have a larger impact on blocking bias in climate runs than in seasonal runs, and that increased ocean model resolution contributes to improved blocking more effectively in climate runs. Seasonal forecasts can thus be considered a suitable test‐bed for model development targeting blocking improvement in climate models.

Large discrepancies in the representation of compound long-duration dry and hot spells over Europe in CMIP5

“…The higher persistence of MSLP above 1012 hPa in CMIP5 models is also seen above thresholds between 1008 and 1022 hPa (not shown), hence the result is insensitive to the specified MSLP threshold within this range. Notably, this result is in contrast with previous studies indicating that atmospheric blocking does not persist enough in most climate models (Antsey et al, 2013;Masato et al, 2013;Dunn-Sigouin and Son, 2013;Davini et al, 2021). However, results of studies focussing on blocking may not be directly…”

On the relationship between Scandinavian extreme precipitation days, atmospheric blocking and Red Sea coral oxygen isotopes

“…Global climate simulations using numerical models commonly show an underestimation of atmospheric blocking frequencies, and even coupled model intercomparison project models do not achieve the observed blocking frequency during winter in the European sector (Davini and D'Andrea 2020). Given the importance of atmospheric blocking in the seasonal prediction systems of the European Centre for Medium-Range Weather Forecasts (Davini et al 2021), we suggest that the winter coral δ 18 O record from the northern Red Sea extending back to the year 1751 (Felis and Mudelsee 2019a), could be used as a complementary source of information about the frequency of winter atmospheric blocking in the European sector on longer time scales during the pre-instrumental period. As some climate models project a decrease in blocking activity over Europe in a warmer future climate (e.g.…”

Predictive Skill of Teleconnection Patterns in Twentieth Century Seasonal Hindcasts and Their Relationship to Extreme Winter Temperatures in Europe

“…Further research into why the EA/WR often exhibits such different behavior compared to the other patterns is required; a potential reason could be a mismatch between the EA/WR pattern definitions in ERA-Interim and ERA-20C/ASF-20C. We also note that the EA/WR resembles the Northern European blocking pattern (Scherrer et al, 2006), which has been shown to be difficult to predict by seasonal forecasting systems (e.g., Davini et al, 2020).…”