On the origin of the solar cycle modulation of the winter North Atlantic
Oscillation
Abstract
Previous studies show that the variability associated with the
winter-mean North Atlantic Oscillation (NAO) tends to be largely
modulated according to the solar cycle so that the anomalous zonal wind
signal extends to the upper stratosphere when solar activity is high,
but the vertical extension is limited when solar activity is low. To
examine the physical mechanism of the solar cycle modulation of the
winter-mean NAO, momentum and wave-energy budget analyses have been
performed using the Japanese Reanalysis (JRA-55) dataset from 1958 to
2021. Momentum analysis shows that the enhancement of zonal wind in High
Solar (HS) years is due to direct enhancement of wave forcing.
Wave-energy analysis shows that baroclinic energy conversion from
zonal-mean flow in the region from the surface to the middle
stratosphere and barotropic energy conversion to the zonal-mean flow
around the middle stratosphere tend to be enhanced in HS winters. Both
enhancements are related to the enhancement of the zonal wind. Our
analysis suggests that the climatological basic states tend to become
more unstable during HS winters.