Can we trust CMIP5/6 future projections of European winter precipitation?

Abstract

IPCC models project a likely increase in winter precipitation over northern Europe under a high-emission scenario. These projections, however, typically rely on relatively coarse ∼100 km resolution models that can misrepresent important processes driving precipitation, such as extratropical cyclone activity, and ocean eddies. Here, we show that a pioneering 50 km atmosphere–1/12° ocean global coupled model projects a substantially larger increase in winter precipitation over northwestern Europe by mid-century than lower-resolution configurations. For this increase, both the highest ocean and atmosphere model resolutions are essential: only the eddy-rich (1/12°) ocean projects a progressive northward shift of the Gulf Stream. This leads to a strong regional ocean surface warming that intensifies air–sea heat fluxes and baroclinicity. For this then to translate into a strengthening of North Atlantic extratropical cyclone activity, the 50 km atmosphere is essential, as it enables enhanced diabatic heating from water vapor condensation and an acceleration of the upper-level mean flow, which weaken vertical stability. Our results suggest that all recent IPCC climate projections using traditional ∼100 km resolution models could be underestimating the precipitation increase over Europe in winter and, consequently, the related potential risks.

Increased wintertime European atmospheric blocking frequencies in General Circulation Models with a coupled eddy-permitting ocean

“…The time-mean SST biases of NE and EP GCM simulations with respect to ERA5 reanalysis data are shown in Figures 3a and 3b, respectively. Both the GS and central North Atlantic areas have large SST biases in the two GCM ensembles, which is consistent with previous studies 14,17 . However, we find significantly lower mean SST biases in GCMs with EP ocean resolution in most areas, with a few exceptions (e.g., Labrador Sea, Fig.…”

“…We also show that EP GCMs outperform NE GCMs in reproducing the wintertime NAC position and underlying mean SST distribution, as well as better resolving patterns of variability in the high troposphere geopotential heights. Therefore, based on earlier assumptions 14,16,17,30 and our 22 GCM ensemble, we hypothesize these GCM bias reductions induced by the presence of mesoscale eddies as a strong source of improvement for WEAB frequencies. This result implies that greater confidence can be attributed to paleoclimate simulations or future projections in terms of WEAB activity and associated climate extremes when considering GCMs with an EP ocean resolution.…”

“…This implies that, in the ensemble used here, setting an EP resolution for the ocean model grids appears to be a necessary condition for the increase in atmospheric resolution to be useful. Indeed, if the presence of mesoscale eddies in the oceans tends to increase the realism of the simulated mean climate 17,27,36,37 , their presence is thus required to satisfactorily see any improvement in resolving more specific climate features, such as extratropical storms 38 , storm tracks 39 , or wintertime blocking formation (Fig. 2).…”

“…3). Indeed, both NAC and GS were also shown to influence wintertime weather patterns and storms over Europe 17,40 , notably by forcing Rossby waves 41 .…”

“…Another major outlook from this research is the potential benefit of employing GCMs with an eddy-rich ocean resolution, which can be obtained for nominal resolutions as low as 0.1°x0.1°2 7,28 . Unlike EP ocean grids, which only include mesoscale eddies with broader horizontal extents (100 km), eddy-rich ocean grids incorporate smaller-scale eddies (20 to 100 km), resulting in a significantly higher number of these mesoscale eddies and a better represented mean climate 17,27,28,29,37 . The use of an eddy-rich grid was notably shown to further improve most benefits and bias reductions seen in EP GCMs, notably in terms of SST mean biases 27 .…”