Solar cycle modulation of the Southern Annular Mode -A simulation with a chemistry climate model -

Abstract:

Previous studies of the observation show that the Southern Annular Mode (SAM) tends to be modulated according to the 11-year solar cycle. In fact, late-winter SAM signal in the lower troposphere tends to be extended to the upper stratosphere and persist until the following summer in the solar-max years, whereas it is mostly restricted in the troposphere and disappears quickly in the solar-minyears.

To examine the source of such solar cycle modulation of the SAM, three sets of 42-year runs of our chemistry-climate model had been performed. The only difference of these three runs is a small realistic change of the ultra-violet (UV) radiation.

Our model satisfactorily simulates the solar cycle modulation of the SAM signal; the stratosphere-troposphere coupling tends to be stronger with the increase of the UV. The result suggests that the source of observed solar cycle modulation comes from UV change.

To examine how the solar cycle modulation occurs with small change of the UV, lagged regression analysis is performed based on the dominant SVD mode of zonal-mean wind and vertical component of E-P flux on October for each run. We found that although signals of zonal wind and E-P flux are very similar in October, they develop to very different states according to the UV strength. In fact, clear downward propagation of stratospheric SAM signal to the troposphere is found on December in stronger UV run, but such downward signal is very unclear for weaker UV run. Analysis of wave propagation/forcing shows that such difference of downward propagation of stratospheric signal originates mostly from the wave propagation in the troposphere. In fact, wave propagation tends to more enhanced toward the tropical troposphere in stronger UV run, but vertical propagation is more enhanced for weaker UV run. Possible scenario of solar cycle modulation of SAM signal is discussed.