A possible mechanism of solar modulation of the spatial structure of the North Atlantic Oscillation

The spatial structure of the North Atlantic Oscillation (NAO) in winter differs significantly according to the solar cycle. The NAO is confined to the Atlantic sector during low solar activity (LS), whereas it has a hemispherical structure during high solar activity (HS). The present study focuses on identifying the involved processes. The results of analyses suggest that the solar activity influence originates in the stratopause region, where the mode of interannual variation in the zonal-mean zonal winds changes according to solar activity. In the early winter of HS, the leading zonal wind mode has a meridional dipole-type structure that extends downward from the stratosphere to the troposphere through interaction with planetary waves. The dipole-type anomaly in the zonal winds produces a hemispherical scale seesaw pattern in the surface pressure between the polar region and surrounding regions. Thus the NAO during HS has a hemispherical pattern. During LS, the downward extension of zonal-mean zonal wind anomalies is weak, so the regional-scale variation becomes dominant in the troposphere.