The question of the mechanism for solar-variability effects on weather and climate can be separated into (1) the identification of the carrier of the solar variability and (2) the identification of the physical link between the carrier and the meteorological response. The suggestion that galactic cosmic rays (GCR), as modulated by the solar wind, are the carriers of the component of solar variability that affects weather and climate has been discussed in the literatue for 30 years, and considerable evidence for it has now accumulated. Variations of GCR occur with the 11-year solar cycle, matching the time scale of recent results for atmospheric variations, as modulated by the quasi-biennial oscillation of equatorial stratospheric winds (QBO). Variations in GCR occur on the time scale of centuries with a well-defined peak in the coldest decade of the little ice age. Here we present new evidence on the meteorological responses to variations on the time scale of a few days. The occurrence of correlations of GCR and meteorological responses on all three time scales strengthens the hypothesis of GCR as carriers of solar variability to the lower atmosphere. The responses reported here include changes in the vertical temperature profile in the troposphere and lower stratosphere and in the northern hemisphere vorticity area index, associated with Forbush decreases in GCR. The meteorological responses to Forbush decreases are in the opposite sense but otherwise are quite similar to responses that immediately follow solar flares.

This is to be expected, based on the hypothesis that particles with energy about 100--1000 MeV are the external forcing function for the tropospheric response, since large solar flares increase the particle flux and ionization and minor species production in the lower stratosphere, whereas Forbush decreases reduce them. The mechanism or mechanisms linking changes in low-energy GCR and other particles in this energy range of 100--1000 MeV to tropospheric temperature and dynamic responses have not been identified. This can be attributed to current uncertainties regarding the microphysical and electrical properties of aerosols and clouds. One possibility is the changes in clouds lead to changes in cloud radiative forcing. The height distribution of the tropospheric response and the amount of energy involved and the rapidity of the time response suggest that the release of latent heat could also be involved. These could lead to the observed tropospheric responses which are understandable in terms of changes in the intensity of cyclonic disturbances. Theoretical considerations link such changes to the observed latitudinal movement of the jet stream. ¿ American Geophysical Union 1989