On the role of moist and dry processes in atmospheric blocking biases in the Euro-Atlantic region in CMIP6

Abstract. Synoptic- and large-scale features of atmospheric flow such as extratropical cyclones, Rossby wave packets, and atmospheric blocking modulate mid-latitude weather and climate. However, several studies have shown strong biases in the frequency and location of these features in state-of-the-art global climate models. One notable and persistent bias is an underestimation of the atmospheric blocking frequency in the Euro-Atlantic region. In this study, we validate the representation of synoptic- and large-scale features of the North Atlantic flow in eight climate models of the Coupled Model Intercomparison Project Phase 6 (CMIP6), taking the ERA5 reanalysis as a reference. Validation includes atmospheric blocking, storm tracks, eddy heat and moisture fluxes, and warm conveyor belts (WCBs). The selected CMIP6 models underestimate the atmospheric blocking frequency over the eastern North Atlantic and Europe in winter (December to February) by up to 80 %. The frequency biases result from combined biases at different spatial and temporal scales described in the following. First, we define the background flow as the most frequent value of the daily time series of the meridional gradient of geopotential height at 500 hPa. In the CMIP6 models, the strongest latitudinal geopotential gradients are shifted equatorward in the North Atlantic basin. This shift favours more zonal and stronger winds to the south of the climatological jet. The differences in the background flow affect Rossby wave breaking and blocking onset and persistence, as illustrated by analysing the eddies in the Euro-Atlantic region. We find an equatorward shift in the eddies in CMIP6 that accelerates the mean flow in the exit region of the Atlantic jet, as indicated by a reduction in the divergence of E vectors. The shift in the eddies leads to a less diffluent flow in the east Atlantic and, thus, a less favourable flow for blocking formation. Second, we find a negative bias in WCB outflow frequency in the CMIP6 models in the North Atlantic. Reduced WCB outflow indicates weaker transport of low potential vorticity (PV) from the lower to the upper troposphere by moist diabatic processes and consequently weaker downstream ridge amplification and, therefore, a less diffluent flow over the eastern North Atlantic and weaker diabatic contributions to blocking. The negative WCB outflow bias can be linked to an underestimation of the meridional moisture transport at low levels in the climatological WCB inflow area in the western North Atlantic. Thus, the misrepresentation of moist processes contributes to the negative blocking biases. Accordingly, an improved representation of the moist processes in the next generation of climate models could improve the blocking representation.