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Influences of Solar Forcing at Ultraviolet and Longer Wavelengths on Climate

Solar forcing has contributed minimally to modern global warming, but its role in decadal and regional climate change and the mechanisms underlying those impacts remain incompletely understood. Analyses of modern observations show inconsistent surface climate responses to the solar cycle, though a clear signal is found aloft. A “top‐down” mechanism connecting high altitudes to the surface has been documented, as has a “bottom‐up” mechanism mediated by the ocean, but their relative importance remains unclear. To investigate these issues, we performed simulations using the GISS E2‐R climate model exploring both cyclic and constant solar forcing. Simulations were driven by irradiance variations across the spectrum, and at only short (<310 nm) wavelengths, which trigger the “top‐down” mechanism, and long (≥310 nm) wavelengths, which initiate the “bottom‐up” mechanism. We find weak surface temperature response to cyclic solar forcing across all wavelengths despite a clear stratospheric response. In contrast, persistent solar forcing induces clear impacts in both the stratosphere and troposphere, including at the surface. For persistent forcing, tropical areas warm, which is almost entirely attributable to long wavelength forcing, whereas boreal winter extratropical responses include areas of warming and cooling with comparable magnitude impacts for short and long wavelength forcings. Both appear to excite similar annular mode responses, so that there is not a clear separation between the “top‐down” and “bottom‐up” mechanisms. It seems clear, however, that longwave forcing and the “bottom‐up” mechanism dominate the tropical response to solar forcing, whereas both wavelengths/mechanisms can be important at middle to high latitudes, especially during the boreal winter.