Air-mass Origin in the Arctic. Part II: Response to Increases in Greenhouse Gases

Abstract

Future changes in transport from Northern Hemisphere (NH) midlatitudes into the Arctic are examined using rigorously defined air-mass fractions that partition air in the Arctic according to where it last had contact with the planetary boundary layer (PBL). Boreal winter (December–February) and summer (June–August) air-mass fraction climatologies are calculated for the modeled climate of the Goddard Earth Observing System Chemistry–Climate Model (GEOSCCM) forced with the end-of-twenty-first century greenhouse gases and ozone-depleting substances. The modeled projections indicate that the fraction of air in the Arctic that last contacted the PBL over NH midlatitudes (or air of ‘‘midlatitude origin’’) will increase by about 10% in both winter and summer. The projected increases during winter are largest in the upper and middle Arctic troposphere, where they reflect an upward and poleward shift in the transient eddy meridional wind, a robust dynamical response among comprehensive climate models. The boreal winter response is dominated by (;5%–10%) increases in the air-mass fractions originating over the eastern Pacific and the Atlantic, while the response in boreal summer mainly reflects (;5%) increases in air of Asian and North American origin. The results herein suggest that future changes in transport from midlatitudes may impact the composition—and, hence, radiative budget—in the Arctic, independent of changes in emissions.