Maintenance of the boreal-forest rainbelts during northern summer

It is not unreasonable to expect that boreal forests which exist along 60 o N in the Eurasian and North American continents were created and are maintained by warm seasonal rainfall. As revealed from satellite observations and various precipitation sources, zonally elongated rainbelts appear along these forests. Previous studies show that a relationship may exist between the frontal zone along the Arctic seaboard and regional patterns of high latitude precipitation. It was observed by this study that baroclinic zones associated with strong Arctic westerlies coincide with minor storm tracks and boreal-forest rainbelts only in Eastern Canada. In contrast, this coincidence does not occur in Northern Europe, East Siberia, and the Alaska-Pacific coast, because boreal-forest rainbelts in these regions are located further south of strong Arctic westerlies and ahead of high-latitude troughs over Central Eurasia, the Bering Sea, the Labrador Sea, and the Norwegian Sea. Therefore, instead of baroclinicity along strong Arctic westerlies, favorable environments for the formation of minor storm tracks are developed by positive vorticity advections ahead of these high-latitude troughs. The water vapor budget analyses performed with NCEP and GEOS-1 reanalyses show that the boreal-forest rainbelts are essentially maintained by the convergence of water vapor flux associated with transient disturbances at high latitudes.