Influences of Solar Forcing at Ultraviolet and Longer Wavelengths on Climate

Abstract

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.

The Footprint of the 11‐Year Solar Cycle in Northeastern Pacific SSTs and Its Influence on the Central Pacific El Niño

“…Past studies have found evidence that the solar cycle may induce significant climate variability in the atmosphere and oceans. 11-year signals or modulation effects, for example, have been found in the stratospheric temperature, ozone concentration, and meridional circulation (e.g., Kodera & Kuroda, 2002;Shindell et al, 2020). In the troposphere, solar cycle signals have been found in jetstream variations related to the northern and southern annular modes (e.g.…”

How much has the Sun influenced Northern Hemisphere temperature trends? An ongoing debate

“…That is, the multi-decadal trends in TSI appear to be relatively modest compared to the rising and falling over the ~11-year Solar Cycle component. As we will discuss in Section 2.6, many researchers have identified evidence for a significant ~11-year temperature variability in the climate of the mid-troposphere to stratosphere [65][66][67][76][77][78]91,[211][212][213][214][215][216], which has been linked to the more pronounced ~11-year variability in incoming solar ultraviolet irradiance [8,[217][218][219][220][221][222][223][224][225]. However, in terms of surface temperatures, the ~11-year component seems to be only of the order of 0.02-0.2°C over the course of a cycle [40,[226][227][228][229][230].…”

Solar atmosphere radiative transfer model comparison based on 3D MHD simulations

“…The results over the past decades clearly indicate that solar variability has an influence on the Earth's climate; for an overview see e.g., Matthes et al (2017) and Shindell et al (2020). However, the exact quantification of the solar influence on climate -besides other natural forcings and the anthropogenic contribution -is still debated.…”