The use of atmospheric relative angular momentum to diagnose the Arctic oscillation

Abstract

Recent studies suggest that the North Atlantic Oscillation is a regional expression of the Northern Hemisphere Annular Mode. However, the lack of an apparent link between Atlantic and Pacific sectors questions the annular paradigm. In any case, the phenomenon is the largest and most fundamental mode of variability in the Northern Hemisphere troposphere, being responsible for much of the warming in the Northern Hemisphere surface temperature. The main aim of this study is to diagnose the Northern Hemisphere Annular Mode using an annular magnitude, such as the relative angular momentum (RAM). RAM from 500 hPa to 200 hPa is computed for the polar cap, using 60° longitude sectors as well as the whole cap, from 55 to 90°N and for the period 1958–1998, using NCEP-NCAR reanalysis data. Annual values of RAM are then correlated to temperature and geopotential height at 200, 500 and 850 hPa. The Artic region is dominated by very negative correlations for the Northern Hemisphere RAM and for the six different RAM sectors. Midlatitudes are dominated by positive correlation, with intermittent or alternating significant regions not showing a regular annular pattern. The Pacific Ocean area is only significantly correlated with the Pacific RAM sector. Since an annular pattern is characterized by its hemispheric symmetry, two symmetric latitude rings are expected, but only the polar one is detected, the rest being incomplete. Due to the proximity of the grid points near the pole, which can lead to artificial zonal symmetry, all the calculations are repeated for RAM from 45 to 65°N, finding a similar pattern with lower correlations and less symmetry. So the results do not support the annular paradigm, understood as organized along latitude sectors, but a hemispheric dimension cannot be excluded.