Science Briefs
Do Variations in the Solar Cycle Affect Our Climate System?
There have been many arguments as to whether or not the eleven-year sunspot cycle affects our weather and climate. With our increased ability to monitor the sun, we are now aware that there is a small change in the total solar irradiance accompanying shifts from solar maximum conditions (with many sunspots) to solar minimum (with, basically, none). There is also a more substantial change in the ultraviolet (UV) portion of the solar spectrum, with direct impacts primarily in the stratosphere (above ~10km).
Figure 1. Total solar irradiance over the past three solar cycles, since 1975, varying between 1365 and 1367 W/m2. (Click for large JPEG or PDF.)
The effect of these changes on our temperature record has been noted by some researchers, and, like the change in solar irradiance, it too appears to be small. But there is little agreement on just how that change arises. Furthermore, there are claims that the sunspot cycle is associated with changes in storm tracks and rainfall. How could this happen with so little change in total energy?
To understand the processes involved, we recently completed an extensive series of climate model experiments, involving 1600 simulated years with varying UV and total solar irradiance (TSI). Our experiments show that the solar cycle influences tropospheric rainfall patterns in a manner consistent with some observations, with increased solar activity favoring precipitation north of the equator (for example, the South Asian monsoon) and decreased precipitation both near the equator and at northern mid-latitudes. The word "favoring" is used advisedly; in the experiments it is a "weighting of the dice", an increase in the likelihood of these effects while accounting for less than one standard deviation of the variability (a result found in observations as well). Locally it can account for 15-20% of rainfall totals. The influence also seems to have been modified by global warming, and so its effectiveness may change with time. The impact of the solar cycle on precipitation in the model experiments arises from two different mechanisms, the first involving UV changes, the second total solar irradiance.
The increase of incident solar UV during solar maximum conditions leads to increased generation of stratospheric ozone in the mid-to-upper stratosphere, which ultimately results in greater ozone in the tropical lower stratosphere. This helps warm that region via both short- and long-wave absorption. In response to this more stable vertical profile for tropical tropospheric processes, tropical convection preferentially shifts off the equator, favoring monsoonal effects during Northern Hemisphere summer and on the annual average.
Figure 2, at right. Results show the percentage of the 1600 years of experiments during which solar maximum conditions produced increased (green) or decreased (brown) precipitation at different latitudes on the annual average. The top panel shows the experiments which used climatological (unchanging) SSTs; here the influence comes primarily from the solar UV variations affecting the stratosphere. The bottom panel is for the experiments with historically-varying SSTs, in which TSI changes have influenced the surface. Precipitation decreases occur greater than 50% of the time south of the equator in both figures, but decreases in mid-latitudes result primarily from the UV changes (top figure). Recent variations in SSTs due to other sources (such as greenhouse gases) appear to have minimized the mid-latitude response. (Click for large GIF or PDF.)
In addition, the solar-plus-ozone change leads to increased tropical stratospheric warming in the mid-to-upper stratosphere during solar maximum conditions. Higher latitudes during Southern Hemisphere winter receive no such augmentation, and the increased latitudinal temperature gradient results in stronger stratospheric west winds. Via the interaction of these wind changes and planetary waves propagating up from the troposphere, the circulation in the stratosphere weakens, a response characterized by greater relative upwelling in the Southern Hemisphere extratropics, and more downwelling in the northern extratropics. This downwelling has a tendency to extend into the troposphere, limiting convection and rainfall during Northern Hemisphere summer at these latitudes, producing drier conditions. This effect is seen in some paleoclimate records and has been attributed to solar influence.
Total solar irradiance changes, though of small magnitude, do appear to affect sea surface temperatures (SSTs), most obviously at latitudes where cloud cover is small and irradiance is abundant, such as the Northern Hemisphere subtropics during summer. The increased SSTs then help intensify circulations spiraling away from the subtropics, again favoring reduced rainfall near the equator and to the south, as well as northern mid-latitudes. Hence, both the UV and TSI forcings produce similar effects, with the latter helping to sharpen the response.
SSTs however have been influenced by other forcings, such as greenhouse gases, over the last few decades, and these transient changes will obviously affect the solar cycle influence. Similarly, increased carbon dioxide in the stratosphere has led to gradual cooling conditions, which affects the UV influence on the stratospheric circulation. So while the solar influence may have produced a broadly similar hydrologic response for many centuries, it now competes with potentially stronger perturbations. Its effect may well decrease with time.
Reference
Rind, D., J. L. Lean, J. Lerner, P. Lonergan, and A. Leboissetier, 2008: Exploring the stratospheric/tropospheric response to solar forcing. J. Geophys. Res., 113, D24103, doi:10.1029/2008JD010114.
Note: PDF documents require the free Adobe Reader or compatible viewing software to be viewed.
Contact
Please address all inquiries about this research to Dr. David Rind.
