Investigation of the responses of the general circulation at 700 mbar to solar geomagnetic disturbance

The northern hemisphere 700-mbar contour heights from 20° to 70°N for the period 1947–1970 are studied in conjunction with 272 key days, where the daily increase of the Ci index equals or exceeds 1.0. The superposed epoch method is applied from 33 days before to 66 days after the key day for a variety of zonal and meridional indices. It is shown that the 700-mbar height difference between 20° and 55°N increases significantly in winter 4 days following geomagnetic disturbance (in summer a less prominent but statistically significant increase is found 2 days earlier). The effect is most clear in winter in the quadrant 90°–175°W and corresponds to a 7% increase in the mean geostrophic westerly flow. The statistical significance of the results is established by applying Student's t test to the difference of each daily mean from the continuum. Synoptic analyses of the departures of the mean 700-mbar contour heights from seasonal climatology following geomagnetic disturbance reveal that the effect proceeds with the growth and development of large negative centers in the latitude belt 40°–60°N and smaller positive departures at lower latitudes.