On correlations between the North Atlantic Oscillation, geopotential heights, and geomagnetic activity

Abstract

We investigate correlations between geomagnetic activity indices, the North Atlantic Oscillation (NAO), and stratospheric geopotential heights. It is shown that the correlation between the geomagnetic index Ap and the NAO index is high and significant since about 1970, that it is significant during winter only, that it was not significant before about 1970, and that the correlations are dominated by quasi-decadal scales of variability. Analysis of the spatial pattern of correlations, restricted to the Northern Hemisphere and wintertime, shows that significant correlations between Ap and sea-level pressures and between Ap and stratospheric geopotential heights are found for the period 1973-2000. However, for the period 1949-1972 no significant correlations are found at the surface while significant correlations are still found in the stratosphere. This might indicate that a solar forcing, primarily acting in the stratosphere, is propagating its influence downward in the later period but not in the earlier.