Extreme daily precipitation in coastal western Norway and the link to atmospheric rivers

This work investigates the link between the most extreme daily precipitation events (EDP) observed since 1900 on the west coast of Norway and the large-scale moisture fluxes over the North Atlantic Ocean. Using station precipitation data, vertically integrated water vapour (IWV) from Special Sensors Microwave Imager/Sounder (SSMIS) satellite observations and the state of the art NOAA-20th Century (NOAA-20C) reanalysis, it is shown that 55 out of 58 EDPs are associated with narrow plumes of intense low level moisture defined as atmospheric rivers. Despite the high spatial correlation between IWV fields in the SSMIS and NOAA-20C datasets, the significant positive relationship between the maximum amount of observed precipitation at all stations and the IWV content hitting the coastal terrain is only observed in the SSMIS dataset. Further, the composite analyses of synoptic conditions show that the preferred circulation type consists of a mean sea level pressure (MSLP) dipole-pattern where a high pressure system over central Europe and a series of low pressure systems to the east of Iceland and over the Norwegian Sea are present. The west coast of Norway is located in the exit region of the anticyclonically curved upper tropospheric polar jet stream implying that the coupling of upper-troposphere and surface dynamics begins to weaken at the time of EDPs. It is also found that the primary synoptic-scale precursors are persistent positive 500-hPa height geopotential and MSLP anomalies over central Europe up to 10 days before the occurrence of EDP events.