“[Abstract]: One of the mysteries regarding Earth’s climate system response to variations in solar output is how the relatively small fluctuations of the 11-year solar cycle can produce the magnitude of the observed climate signals in the tropical Pacific associated with such solar variability. Two mechanisms, the top-down stratospheric response of ozone to fluctuations of shortwave solar forcing and the bottom-up coupled ocean-atmosphere surface response, are included in versions of three global climate models, with either mechanism acting alone or both acting together. We show that the two mechanisms act together to enhance the climatological off-equatorial tropical precipitation maxima in the Pacific, lower the eastern equatorial Pacific sea surface temperatures during peaks in the 11-year solar cycle, and reduce low-latitude clouds to amplify the solar forcing at the surface.
[Text]: It has long been noted that the 11-year cycle of solar forcing is associated with variousphenomena in Earth’s climate system, in both the troposphere and stratosphere (1–9). Because the amplitude of the solar cycle (solar maximum to solar minimum) is relatively small, about 0.2 W m−2 globally averaged (10), and the observed global sea surface temperature (SST) response of about 0.1°C would require more than 0.5Wm−2 (11), there has always been a question regarding how this small solar signal could be amplified to produce a measurable response.
Postulated mechanisms that could amplify the relatively small solar forcing signal to produce such responses in the troposphere include changes in clouds in the troposphere caused by galactic cosmic rays, or associated global atmospheric electric circuit variations, though neither has been plausibly simulated in a climate model. However, there are two other plausible mechanisms, though each has not yet produced a modeled response of the magnitude seen in the observations.
The first involves a “top down” response of stratospheric ozone to the ultraviolet (UV) part of the solar spectrum that varies by a few percent. Peaks in solar forcing cause the enhanced UV radiation, which stimulates additional stratospheric ozone production and UV absorption, thus warming that layer differentially with respect to latitude. The anomalous temperature gradients provide a positive feedback through wave motions to amplify the original solar forcing. The changes in the stratosphere modify tropical tropospheric circulation and thus contribute to an enhancement and poleward expansion of the tropical precipitation maxima (5, 12–16). The first demonstration of the top-down mechanism in a modeling study showed a broadening of the Hadley cells in response to enhanced UV that increased as the solar-induced ozone change was included (17).
A second “bottom up” mechanism that can magnify the response to an initially small solar forcing involves air-sea coupling and interaction with incoming solar radiation at the surface in the relatively cloud-free areas of the subtropics. Thus, peaks in solar forcing produce greater energy input to the ocean surface in these areas, evaporating more moisture, and that moisture is carried by the trade winds to the convergence zones where more precipitation occurs. This intensified precipitation strengthens the Hadley and Walker circulations in the troposphere, with an associated increase in trade wind strength that produces greater equatorial ocean upwelling and lower equatorial SSTs in the eastern Pacific, a signal that was first discovered in observational data (1, 2). The enhanced subsidence produces fewer clouds in the equatorial eastern Pacific and the expanded subtropical regions that allow even more solar radiation to reach the surface to produce a positive feedback (18, 19). Dynamical air-sea coupling produces a transition to higher eastern equatorial SSTs a couple of years later (20, 21). There is observational evidence for a strengthened Hadley circulation in peak solar forcing years associated with intensified tropical precipitation maxima, a stronger descending branch in the subtropics, and a stronger ascending branch in the lower latitudes (3); a poleward expansion of the Hadley circulation in peak solar years, with stronger ascending motions at the edge of the rising branch, as well as a stronger Walker circulation with enhanced upward motions in the tropical western Pacific connected to stronger descending motions in the tropical eastern Pacific (7); and enhanced summer season off-equatorial climatological monsoon precipitation over India (6, 22). This cold event– like response to peak solar forcing is different from cold events (also known as La Niña events) in the Southern Oscillation in that, among other things, zonal wind anomalies in the stratosphere are opposite in sign (23).” “Amplifying the Pacific Climate System Response to a Small 11-Year Solar Cycle Forcing” Article in Space.com. Article in ScienceDaily. Article in Christian Science Monitor. Editorial at SEPP. h/t IceCap