MAGNUS CEDERLÖF
Independent climate science
Causes of temperature fluctuations in Uppsala
Created 2024-08-02
The Uppsala temperature series show huge fluctuations in temperature over time. These fluctuations must have a cause and in this article I will investigate which other weather statistics that correlates to the temperature fluctuations for different seasons.
The following weather statistics is used in the analysis:
- Uppsala wind on 10m height from ERA5 (starting 1940)
- Uppsala total cloud cover from ERA5 (starting 1940)
- Stockholm solar irradiance measurements (starting 1983)
- North Atlantic Oscillation Index from Hurrell (starting 1865)
Please note that there are dependencies between Solar irradiance and Total cloud cover. Less clouds give more solar radiation (irradiance is radiation power per unit area).
The NAO index represents the air pressure difference between Portugal and Iceland. A high NAO index means a low pressure area around Iceland. This implies in most cases an increased wind from south and west to Uppsala.
Below are tables with the statistical correlations between the Cederlöf Uppsala temperature series and the above mentioned weather statistics. Winter is defined as the months dec-jan-feb, spring as mar-apr-may, summer as jun-jul-aug and autumn as sep-okt-nov.
The P value is the null-hypothesis test. A small P value means that there is a correlation between the two data sets.
The R value is the Pearson correlation coefficient. It describes how well the two data sets correlates as a straight (linear) line.
In this analysis a P value below 0.05 together with an R value higher than 0.4 or lower than -0.4 is considered a significant correlation. These are marked green.
Correlation with Cederlöf Uppsala temperature series
Winter | P value | R value |
Wind from south | 0.00 | 0.79 |
Wind from west | 0.00 | 0.55 |
Total cloud cover | 0.54 | -0.07 |
Solar irradiance | 0.02 | -0.37 |
NAO index | 0.00 | 0.65 |
Summer | P value | R value |
Wind from south | 0.20 | 0.14 |
Wind from west | 0.03 | -0.24 |
Total cloud cover | 0.00 | -0.62 |
Solar irradiance | 0.00 | 0.83 |
NAO index | 0.00 | 0.33 |
Spring | P value | R value |
Wind from south | 0.00 | 0.48 |
Wind from west | 0.00 | 0.51 |
Total cloud cover | 0.07 | -0.20 |
Solar irradiance | 0.04 | 0.32 |
NAO index | 0.00 | 0.52 |
Autumn | P value | R value |
Wind from south | 0.00 | 0.69 |
Wind from west | 0.98 | 0.00 |
Total cloud cover | 0.37 | 0.10 |
Solar irradiance | 0.27 | 0.18 |
NAO index | 0.00 | 0.60 |
Winter
The winter season temperature has a very good correlation with the wind from south and the NAO index. A high NAO index gives strong wind from south. The correlation between NAO index and the temperature is shown below.
Basically all the winter warming that has occurred since the 1960s can be explained by low pressure areas around iceland.
Summer
The summer temperature has a very good correlation with the incoming solar radiation. See below.
The decreasing cloud cover during the summer shows also a good correlation, but not as good as with irradiance. Especially after ~1995 there is a larger discrepancy. The reason why the correlation is not as good for cloud cover compared to irradiance is not clear. Reasons might be different cloud cover during night and day or cloud cover on different altitudes gives different shielding of the sun. There might also be uncertainties in the ERA5 cloud cover data.
Spring
The situation for the spring is not as clear as for the winter and the summer. It seems to be a pretty good correlation with the wind from west and south.
There are probably many factors that affects the spring temperature, not only the wind. Since the spring is a transition from winter to summer, parts of the wind and parts of the irradiance might affect the temperature.
Autumn
For the autumn, the situation is similar to the winter. The temperature is very much dependent on the wind from south which in turn is dependent on NAO index.
Conclusions
The major driver for the winter Uppsala temperature is the wind from south/west which in turn is dependent on the position of low/high pressure areas in the Atlantic. When a low pressure area is positioned near Iceland, warm winds from south/west will increase the temperature. Also the autumn has a similar behavior as the winter.
For the summer, the incoming solar radiation drives the temperature. The correlation between solar irradiance and temperature is very strong. But for the spring the situation is not as clear. Probably many factors affect the temperature
Since most (or maybe all) of the warming seen since the 1960s can be explained by natural factors, it is hard to see that carbon dioxide emissions could have any large impact on the temperature.
All plots
All plots can be found as a zip file here: all_plots.zip
E-mail: magnus.cederlof@gmail.com