Sep 3, 2010
Looking to the sun
Earlier this year, UK researchers found a historical link between cold winters in Europe and low solar activity. Now some of the original team have used modern observations to test their explanation for this relationship and discover the best proxy measurement for the sun’s ultraviolet emissions.
“In the first paper, we quantified solar activity using the open solar magnetic flux – the total magnetic field leaving the top of the solar atmosphere and filling a region in space called the heliosphere, which surrounds all of the solar-system planets,” Mike Lockwood of the University of Reading and Rutherford Appleton Laboratory, UK, told environmentalresearchweb. “One question that our work prompted was ‘why was that the best indicator of solar activity to use, as opposed to, for example, the frequently used sunspot number’?”
Lockwood, the late Chris Bell and colleagues had proposed, given that ultraviolet emissions from the Sun generate ozone and heating in the stratosphere, that changes in these factors were able to modulate jet-stream behaviour, which has a profound influence on surface-weather patterns. The jet stream, which flows eastwards from North America, can readily become blocked over the Atlantic Ocean, preventing mild westerly winds from reaching Europe and enabling cold air to move down from the Arctic instead.
In their latest study Lockwood and colleagues of the University of Reading and Imperial College, London analysed proxy measurements for ultraviolet emissions from the Sun – including open solar flux, galactic cosmic ray flux and 10.7Y cm solar radio flux, which correlates closely to sunspot number. They also examined data from the Spectral Irradiance Monitor (SIM) instrument on the SORCE (Solar Radiation and Climate Experiment) satellite, and carried out a multiple-linear-regression analysis, including solar indices, on climate “re-analysis” data for 1979 to 2008 from the European Centre for Medium-Range Weather Forecasting.
“We show that the response of the lower stratosphere is stronger to open solar flux than to sunspot number or the flux of cosmic rays hitting Earth,” said Lockwood. “Importantly, in the region influenced by the northern-hemisphere jet stream we see a strong and significant solar influence all of the way to ground level, and again the response is greatest to open solar flux.”
The team found some evidence that the variations in UV irradiance are more like those in the open solar flux than those in the sunspot number. “This would explain why we obtain more clear-cut results using open solar flux,” said Lockwood. “However, making well calibrated, stable UV measurement from space is a developing art and very difficult to do. Hence there are great uncertainties in combining the most recent data – from the SORCE satellite – with older measurements. We used two methods to do this combination and the results do differ.”
That said, both methods indicated that solar UV output was lower in the recent "exceptional" solar minimum than in previous minima. “This behaviour is found in open solar flux – and is mirrored by cosmic rays – but not in sunspot number,” said Lockwood.
The finding indicates that “top-down” solar effects due to long-term changes in the amount of ultraviolet radiation emitted by the sun could be a bigger factor in tropospheric variations than previously believed. Many climate models consider only the “bottom-up” effects of the Sun’s visible and infrared emissions, which are much more stable over time.
The team believes that including this “top down” effect should help reduce uncertainty in the prediction of weather variability from year to year in Europe and western Asia, where jet-stream-blocking events are more common. “A top-down climate effect that shows long-term drift, and may also be out of phase with the bottom-up solar forcing, would mean that climate-chemistry models that have sufficient resolution in the stratosphere would become very important for making accurate regional/seasonal climate predictions,” write the researchers in their paper in Environmental Research Letters (ERL).
Lockwood said that the results also help explain persistent reports of solar influence, inferred from cosmic-ray products stored in reservoirs such as tree trunks, ice sheets and ocean sediments, on past regional climates deduced from paleoclimate ‘proxy’ data.
He is keen to stress that while the team is finding that solar variations may help explain inter-annual variability in some regional climates, this is not the same as saying that the Sun is a bigger contributor to global climate change than was thought. “When we take global averages of climate and temperature, all these regional variations are averaged out and we are not finding any larger global effect of solar variations than has already been identified in the literature, and hence in the IPCC reviews,” he explained.
Next the team will publish some results showing that the blocking phenomenon is up to 20% more common in Europe when the open solar flux is low and conversely 20% less common than average when the flux is high.
“Thus far we have concentrated on winter conditions for which these effects are most clear-cut,” said Lockwood. “A next step will be to sort out what happens at other times of year – the equinoxes and in summer. Summer, in particular, is considerably more complex and less clear-cut.”
About the author
Liz Kalaugher is editor of environmentalresearchweb.