Solar forcing of winter climate variability in the Northern Hemisphere

An influence of solar irradiance variations on Earth’s surface climate has been repeatedly suggested, based on correlations between solar variability and meteorological variables1. Specifically, weaker westerly winds have been observed in winters with a less active sun, for example at the minimum phase of the 11-year sunspot cycle2–4. With some possible exceptions5,6, it has proved difficult for climate models to consistently reproduce this signal7,8. Spectral Irradiance Monitor satellite measurements indicate that variations in solar ultraviolet irradiance may be larger than previously thought9. Here we drive an ocean–atmosphere climate model with ultraviolet irradiance variations based on these observations. We find that the model responds to the solar minimum with patterns in surface pressure and temperature that resemble the negative phase of the North Atlantic or Arctic Oscillation, of similar magnitude to observations. In our model, the anomalies descend through the depth of the extratropical winter atmosphere. If the updated measurements of solar ultraviolet irradiance are correct, low solar activity, as observed during recent years, drives cold winters in northern Europe and the United States, and mild winters over southern Europe and Canada, with little direct change in globally averaged temperature. Given the quasiregularity of the 11-year solar cycle, our findings may help improve decadal climate predictions for highly populated extratropical regions. Satellite observations of solar spectral irradiance in the ultraviolet region have been subject to uncertainty; the Solar Stellar Irradiance Comparison Experiment and Spectral Irradiance Monitor (SIM) instruments aboard the Solar Radiation andClimate Experiment (SORCE) satellite mission (2004–present) are the first designed to achieve accurate long-term measurements of the solar irradiance variations over the entire ultraviolet range9. The 200–320 nm part of the ultraviolet band contributes strongly to solar heating in the middle atmosphere, largely through ozone absorption. Ozone is itself produced through the interaction between ultraviolet radiation and oxygen, giving rise to potential positive feedback10. SORCE observations made during the decline of solar cycle 23 reveal a remarkably strong decrease in mid-ultraviolet flux, some four to six times greater11 than previous spectral irradiance reconstructions12. However, before the SORCE mission, variations at these wavelengths were poorly constrained, withmeasurement uncertainty exceeding the potential solar-cycle variation13. Currently there are limited data (less than one solar cycle) so questions remain concerning accuracy and also applicability of the SIM data to other solar cycles11,14. We use the SIM observations of solar variability to estimate the change in ultraviolet radiation between the maximum and