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Solar Forcing of Winter Climate Variability in the Northern Hemisphere

Reference
Ineson, S., Scaife, A.A., Knight, J.R., Manners, J.C., Dunstone, N.J., Gray, L.J. and Haigh, J.D. 2011. Solar forcing of winter climate variability in the Northern Hemisphere. Nature Geoscience 4: 753-757.
In 2009, nearly 31 years after direct satellite observations of solar irradiance from space, the science team from the Spectral Irradiance Monitor (SIM) on board the Solar Radiation and Climate Experiment (SORCE) satellite led by Jerald Harder and Juan Fontenla (Harder et al. 2009) published a bombshell result that offers a new view on how large the solar UV (in the wavelength region of 200-400 nm or so) has varied between April 2004 and February 2008. Figure 1 shows that the previous extrapolation of solar UV irradiance by Lean (2000) significantly underestimated the direct SIM observations by a factor of ten!


Figure 1. Comparison of the direct solar UV irradiance change by SIM versus the old model result from Lean (2000) revealed that the old estimate was smaller by about a factor of 10 . Adapted from Harder et al. (2009).

Naturally, ever since the publication of the Harder et al. paper, many other researchers have attempted to quantify the plausible impacts of such a large UV irradiance change. In the present study, Ineson et al. (2011) modeled the nature of stratospheric and tropospheric responses, focusing on the solar UV irradiance change in the narrow 200 to 320 nm wavelength passband. This they did by using the UK Met Office's general circulation model that has an upper boundary at 85 km height so that they could study how this solar UV irradiance change may travel from its initial impact zones in the mesosphere and stratosphere down through the atmosphere to potentially modulate tropospheric and near-surface climate, focusing on the Northern Hemisphere during winter. So what did they find?

Figure 2 shows the modeled zonal mean temperature difference throughout the whole column of atmosphere from 85 km down to the surface for solar minima minus solar maxima conditions. In general, very large temperature cooling of about 1 to 2°C in the mesosphere and stratosphere were seen in response to the large solar UV irradiance difference between solar activity minima and maxima, as suggested by Harder et al.'s SIM data. However, there are also regions of significant warming in the middle to high latitude regions of the surface-troposphere in the Northern hemisphere, which is probably a result of dynamical adjustments (rather than any strict radiation effects imposed from the top and throughout the whole atmospheric column). This finding contradicts the argument that the whole atmospheric column will warm or cool homogenously, which argument has been used by climate alarmists to dismiss any significant role of solar irradiance forcing on climate.


Figure 2. Large temperature changes modeled for solar minimum minus solar maximum in the mesosphere and stratosphere which in turn able to nudge atmospheric circulation towards weaker westerly winds during winters with a less active Sun . Adapted from Ineson et al. (2011).

After incorporating the realistic amplitude for solar UV irradiance change in their model, Ineson et al. were able to find a consistent pattern of surface pressure and temperature responses that corresponded to negative Arctic Oscillation or North Atlantic Oscillation-like patterns during solar minima. Given such findings, the authors suggest that under such atmospheric circulation patterns, i.e., decreased tropospheric westerly flow in the Atlantic sectors, the solar UV irradiance forcing may help explain the cold winters in northern Europe and the United States observed in temperature patterns of recent years (see Figure 3 focusing especially on the cool region over Northern Europe). Although the authors also modeled mild winters over southern Europe and Canada, the direct comparison with the observational data is less favorable (again, see Figure 3).


Figure 3. Modeled (left) and observed (right) winter surface temperature difference for solar minimum minus solar maximum . Adapted from Ineson et al. (2011).

It is clear that more work remains before we fully embrace the top-to-bottom physical mechanism scenario for how solar forcing can trigger responses on the Earth climate system, as described in Ineson et al. (2011). The effect of solar UV radiation on ozone chemistry as well as the role of solar irradiance changes in visible and near-infrared portion of the wavelength are two key deficiencies in this paper that must be fully incorporated into the climate models in a consistent way.

Additional References
Harder, J.W., Fontenla, J.M., Pilewskie, P., Richard, E.C. and Woods, T.N. 2009. Trends in solar spectral irradiance variability in the visible and infrared. Geophysical Research Letters 36: 2008GL036797.

Lean, J. 2000. Evolution of the Sun's spectral irradiance since the Maunder Minimum. Geophysical Research Letters 27: 2000GL000043.

Archived 2 November 2011