Airmass Origin in the Arctic. Part I: Seasonality

The first climatology of airmass origin in the Arctic is presented in terms of rigorously defined airmass fractions that partition air according to where it last contacted the planetary boundary layer (PBL). Results from a present-day climate integration of the Goddard Earth Observing System Chemistry–Climate Model (GEOSCCM) reveal that the majority of air in the Arctic below 700mb last contacted the PBL poleward of 608N. By comparison, 62% (60.8%) of the air above 700mb originates over Northern Hemisphere midlatitudes (i.e., ‘‘midlatitude air’’). Seasonal variations in the airmass fractions above 700mb reveal that during boreal winter air from midlatitudes originates primarily over the oceans, with 26% (61.9%) last contacting the PBL over the eastern Pacific, 21% (60.87%) over the Atlantic, and 16% (61.2%) over the western Pacific. During summer, by comparison, midlatitude air originates primarily over land, overwhelmingly so over Asia [41% (61.0%)] and, to a lesser extent, over North America [24% (61.5%)]. Seasonal variations in the airmass fractions are interpreted in terms of changes in the large-scale ventilation of the midlatitude boundary layer and the midlatitude tropospheric jet.