Remote sources of water vapor forming precipitation on the Norwegian west coast at 60 degree N - a tale of hurricanes and an atmospheric river

Precipitation amounts have increased in most areas of Norway during the last 100 years, consistent with an expected spin-up of the water cycle in a warming climate. Along with the monthly mean precipitation, heavy precipitation events have also become more frequent. In southwestern Norway where precipitation amounts are largest, the precipitation trends are strongest in fall. Climate models predict this trend to be continuing over the next few decades. In this context, we studied the most extreme precipitation event in recent years on the Norwegian southwest coast that occurred in September 2005, producing flooding and landslides. We found that this event was triggered by the transport of tropical and subtropical moisture associated with two former hurricanes, Maria and Nate, which both underwent transition into extratropical cyclones. The two former hurricanes generated a large stream of (sub)tropical air which extended over more than 40 degrees of latitude and across the North Atlantic Ocean and carried a large amount of moisture originally associated with hurricane Nate - a so-called atmospheric river or moisture conveyor belt. The mountains along the Norwegian coast caused a strong orographic enhancement of the precipitation associated with the moist air. A Lagrangian moisture tracking algorithm was employed to show that the evaporative source of the precipitation falling over Norway was distributed over large parts of the North Atlantic Ocean, and indeed included large contribution from the subtropics and smaller ones from the tropics. The moisture tracking algorithm was also applied over a 5-year period and it was found that (sub)tropical sources contributed substantially to the precipitation falling in southwestern Norway also during other events. It is, thus, likely that one reason for the strong observed and predicted positive trends of precipitation in southwestern Norway is an increased transport of (sub)tropical moisture in a warming climate. This might be particularly the case if hurricanes are becoming more frequent, as suggested by some scholars. It would also explain why the strongest trends occur in fall.