Le Mouel et al. (2010) explain the correlations in Figure 1 as being due to a plausible physical link of the 11-year solar activity cycle to a systematic modulation of tropospheric zonal wind (since winds above 30 km contribute less than 20% of Earth’s angular momentum, as proxied by A). They also make the important point that although the IPCC and others usually rule out the role of solar irradiance impact on terrestrial climate because of the small interannual changes in the solar irradiance, such an argument does not apply to the plausible link of the large seasonal incoming solar radiation in modulating the semiannual oscillations in the length-of-day amplitude. Consequently, Le Mouel et al. (2010a) say their paper «shows that the Sun can (directly or indirectly) influence tropospheric zonal mean-winds over decadal to multidecadal time scales.» And noting that «zonal mean-winds constitute an important element of global atmospheric circulation,» they go on to suggest that «if the solar cycle can influence zonal mean-winds, then it may affect other features of global climate as well, including oscillations such as the NAO and MJO, of which zonal winds are an ingredient [Wheeler and Hendon 2004].» Therefore, «the cause of this forcing,» as they describe it, «likely involves some combination of solar wind, galactic cosmic rays, ionosphere-Earth currents and cloud microphysics.»
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