The Atmospheric Hydrologic Cycle over the Arctic Basin from Reanalyses. Part II: Interannual Variability

Abstract

Previously, the atmospheric moisture budgets over the Arctic Basin as represented by reanalysis data from the National Centers for Environmental Prediction?National Center for Atmospheric Research (NCEP?NCAR) reanalysis and from the European Centre for Medium-Range Weather Forecasts reanalysis were evaluated for the overlap period of 1979?93 and found to be very similar to each other and to the available observations. Here emphasis is on the 50 yr of the NCEP?NCAR reanalysis (January 1949?May 1999) to depict the interannual variability of the atmospheric moisture fluxes across 70°N and their convergence farther north. Precipitation minus evaporation (P ? E) calculated from moisture flux convergence is compared with three large-scale circulation patterns that strongly affect the interannual variability of P ? E over the Arctic and its environs: the North Atlantic oscillation (NAO), the Arctic oscillation (AO), and the North Pacific oscillation (NPO). The impact of the NAO and the closely related AO on Arctic Basin P ? E is found to be marked, with a P ? E:NAO winter correlation of 0.49 (0.56 for the AO). On an annual basis, Arctic Basin P ? E is much more closely correlated with the NAO (0.69) than with the AO (0.49), consistent with the Atlantic Ocean domination of the northward poleward moisture flux across 70°N. Regional analysis confirms that the NAO impact on P ? E is concentrated around the periphery of the North Atlantic Ocean and extends north into the Arctic Ocean during winter. The NAO and AO differ in their P ? E modulation over the northern Eurasia sector with the AO being much more important for all seasons except summer (winter AO:P ? E correlation 0.53, NAO:P ? E correlation 0.16), consistent with its much stronger impact on the atmospheric circulation in that area. The NPO was associated with a much more modest modulation of Arctic Basin P ? E (winter correlation of 0.33 and annual value of 0.10), with its regional signal being strongest over Alaska, northwestern Canada, and areas to the north. About 40% of the interwinter variance of P ? E over the sector that includes northeastern Canada is linked with the combined influence of the NAO?AO and NPO. A region of large poleward moisture transport variability during summer was previously identified over western Siberia, east of the Urals, associated with the development of the Urals trough. Here it is shown that this is due to an opposing circulation pattern, with high (low) poleward moisture transport over the west Siberian plain during low (high) poleward moisture transport over Scandinavia. A pronounced trough?ridge pattern accompanies this circulation regime that is primarily confined to July. Because the summer moisture transport dominates the annual total for this region, these circulation patterns produce this area's large interannual poleward moisture transport variability.