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Sunday, October 12, 2008

Fun With Sunspot Numbers


Every now and then I’ll find some phrase or comparison in a blog post or comment or a climate research paper that initiates a new search for a correlation or causal relationship between solar irradiance and ENSO. While I did not expect to find a blatantly obvious ENSO-like signal in a new way of looking at the solar cycle, it didn’t hurt to plot some curves when I had a new idea. Unfortunately, there were no magical hidden solar cycle curves that correlated with ENSO, but what I found did prove to be interesting and might turn out to be of value to someone else.

The latter graphs in this post come from a thought (It was my thought. I won’t blame anyone else.) that had to do with the possible effects of rapid changes in solar irradiance that occur during the ramp up and ramp down phases of the solar cycles. Were there significant differences in solar irradiance from solstice to solstice or from equinox to equinox, above and beyond the annual orbital and angle of incidence variations? The answer is yes. What type of effect this would have on the differences between Northern and Southern Hemisphere ocean heat content or SST, I cannot say. Also, I wouldn’t want to start to think of a process that would make this additional difference turn into a cause or initiator of ENSO, but I found the differences significant enough to post them.

There are no long-term monthly TSI data sets available online—or—I simply haven’t found them yet. However, since this is just a preliminary look, the use of sunspot numbers as a proxy for TSI should not be a problem.


The typical graph of sunspot numbers, Figure 1, illustrates the 11-year solar cycle. This graph covers the period of 1850 to 2007.

Figure 1

For a different view of the variations, in prior posts, I’ve also shown the monthly change (Example: Feb 1850 Minus Jan 1850, etc.) in sunspot number. Refer to Figure 2.
Figure 2

In Figures 3 through 5, I’ve shown the sunspot numbers for each month over the period of 1850 to 2007. Since the name of the solar phenomenon, 11-year solar cycle, reflects an average number, not the true frequency of the cycles, which vary from 9 to 14 years, there are differences in the monthly curves. The only reason I plotted these data sets was to show that there are differences and that the impacts of solar irradiance will vary from year to year (and I had the data divided into months in the spreadsheet).
Figure 3

Figure 4

Figure 5

Note: I’ve highlighted the scale of the October sunspot graph in Figure 5 to show that it was different from the other eleven graphs.

And in Figure 6, I’ve illustrated the sunspot numbers for seasons over the period of 1850 to 2007. There are seasonal variations.

Figure 6


The differences in the average of the monthly sunspot numbers between Winter and Summer and between Spring and Autumn are illustrated in Figures 7 and 8.
Figure 7

Figure 8


Figure 9 illustrates the annual difference between the December and June monthly sunspot numbers from 1850 to 2007, using the values during those months as an approximation of the values at the solstices. As noted earlier, there can be significant changes in six months, which implies another difference in the amount of solar irradiance delivered to Northern and Southern Hemispheres over the course of a year.
Figure 9

In Figure 10, the annual differences in sunspot number between equinoxes are illustrated, using the monthly sunspot numbers in March and September to approximate the values.
Figure 10


Monthly long-term sunspot data is available from the Solar Physics Group at NASA's Marshall Space Flight Center:
Specifically, this page:

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