Why Moist Dynamic Processes Matter for the Sub-Seasonal Prediction of Atmospheric Blocking Over Europe

In recent years, there has been growing evidence that latent heat release in midlatitude weather systems such as warm conveyor belts (WCBs) contributes significantly to the onset and maintenance of blocking anticyclones (blocked weather regimes). Still, numerical weather prediction (NWP) and climate models struggle to correctly predict and represent atmospheric blocking in particular over Europe. Here, we elucidate the representation of WCB activity in 20 years of extended winter (1997-2017) of European Centre for Medium-Range Weather Forecast's IFS reforecasts around the onset of blocking over Europe (EuBL) employing different perspectives. First, we show that the model struggles to predict EuBL onsets already at 10-14 days lead time in line with a misrepresentation of WCB activity in the ensemble mean. However, we also find cases with accurate EuBL forecasts even in pentad 4 (15-19 days). This subset of successful forecasts at extended-range lead times goes in line with accurate WCB forecasts over the North Atlantic several days prior to the blocking onset. Second, investigating the time-lagged relationship of blocking onset and WCB activity, we find that WCB activity over the North Atlantic emerges well prior to the onset of the block and that different pathways into EuBL exist in the reforecasts compared to reanalysis. Finally, we find indication of predictability associated with a Rossby wave train emerging from the North Pacific. Although our study can not disentangle the roles of intrinsic predictability limits and model deficiencies, we show that correct predictions of EuBL go along with distinct patterns of WCB activity. Warm conveyor belts (WCBs) are weather systems associated with low pressure systems which occur predominantly over the ocean regions of the midlatitudes. Several recent studies highlight the role of latent heat release due to cloud formation in WCBs for the development of long-lived high pressure systems. However, current weather prediction and climate models struggle to accurately predict these high pressure systems, particularly over Europe (EuBL). This study, based on 20 years (1997-2017) of forecast data, reveals challenges in predicting WCB activity together with the onset of EuBL, especially within a lead time of 10-14 days. Successful predictions at extended lead times (15-19 days) align with precise forecasts of WCB activity over the North Atlantic and North Pacific, indicating a connection between these regions. Examining the timing of the onset of EuBL and WCB activity, we show that WCB activity over the North Atlantic precedes EuBL. Additionally, we identify both correct and incorrect pathways leading to EuBL and suggest that predictability of EuBL may arise from specific atmospheric patterns, particularly related to Rossby waves in the North Pacific. Warm conveyor belt (WCB) activity around the onset of atmospheric blocking over Europe (EuBL) is analyzed in reanalysis and sub-seasonal reforecasts Correct WCB prediction provides a sub-seasonal window of forecast opportunity for EuBL onset Synoptic activity over the North Pacific supports the development of a teleconnection that affects EuBL onset