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.