Atmospheric Water Vapor Characteristics at 70°N

Abstract

Using an extensive rawinsonde archive, characteristics of Arctic water vapor and its transports at 70°N are examined for the period 1974–1991. Monthly-mean profiles and vertically integrated values of specific humidity and meridional vapor fluxes are computed for land stations north of 65°N for the surface up to 300 mb using once to twice daily soundings. Mean values at 70°N for these and other variables, including temperature and meridional winds, are obtained through an objective analysis of the monthly station means. The annual zonal mean specific humidity at 70°N ranges from 2.4 g kg−1 at the surface to 0.02 g kg−1 at 300 mb. Zonal-mean precipitable water ranges from 2.9 mm in February and March to 16.2 mm in July. For all months, over 95% of water vapor is found below 500 mb. Although mean winds are equatorward up to about 400 mb, the tendency for poleward winds to transport more water vapor results in a poleward annual-mean flux at all levels except at the surface, peaking at 1.5 g kg−1 m s−1 at 850 mb. Whereas over 85% of the integrated zonal-mean meridional flux is found below 500 mb for all months, a smaller percentage is found at lower levels during summer due to stronger equatorward winds. The flux convergence across 70°N is positive in all months, peaking in September at an equivalent monthly water depth of 22.1 mm averaged over the region north of 70°N. Aerological estimates of precipitation minus evaporation (P - E) for the area north of 70°N that account for changes in water storage also peak in September (26.1 mm), with the annual total of 163 mm larger than previous estimates by up to 36%. Integrated vapor transports exhibit marked longitudinal variations, with maximum annual poleward transports of 16–25 kg s−1 m−1 found over the Norwegian Sea and Baffin Bay. The Canadian Arctic archipelago is the only sector where mean integrated transports are equatorward, ranging from 1 to 10 kg s−1 m−1 depending on longitude. The September peak in P - E results from a circulation shift yielding poleward fluxes along a broad zone from near the prime meridian to 15O°E.