When talking about heatwaves most people mean surface-heatwaves. Which makes sense, since most of us live on the surface, agriculture is happening (mostly) on the surface and the infrastructure which is potentially affected is on the surface. Fig. 1 shows the maximum temperature anomalies during a heatwave in Scandinavia in late spring 2013. But how does such a heatwave look in the free atmosphere above the planetary boundary layer?

Fig. 1: Daily maximum temperature anomalies for May 18th 2013 with respect to the 1979-2014 mean. Based on data from the E-OBS data set.

In my PhD I work a lot with data from so called GPS radio occultation (RO) measurements. RO is a remote sensing technique which uses the radio signals emitted by GPS satellites to retrieve information about the free atmosphere, e.g., the temperature. Through the setup of the measurement geometry the RO system is able to provide vertically very highly resolved profiles of atmospheric temperatures (as opposed to, e.g., surface temperatures or layer temperatures).

Fig. 2 shows a time series of temperature anomalies for the same heatwave as in Fig. 1, not for the surface, but for the free atmosphere. Temperatures in the troposphere correlate with the surface temperatures and show positive anomalies.But the feature most people immediately ask about is the blue region at about 250hPa (about 10km). The temperature anomaly suddenly changes signs and it is colder than normal in this region. How does that happen?

Fig. 2: Timeseries of daily temperature anomalies with respect to the 2006-2016 mean. The temperatures are averaged over 10°E-30°E and 60°N-70°N.

I find it hard to explain this feature without a sheet of paper at hand. In fact, there are several things going on. To explain some of them Fig. 3 shows temperature profiles throughout the troposphere and parts of the stratosphere (red is May 18th 2013 and grey is the 2006-2016 mean of all 18ths of May). Up to about 300 hPa the temperatures on May 18th 2013 are higher than normal. The dashed grey vertical line at the hight where the red lines cross the gray ones marks the climatological tropopause (the upper end of the troposphere) according to a  standard definition. This region is white-ish in Fig. 2 since at that height there is hardly any temperature anomaly.

Fig. 3: Mean temperature profiles in the region of 10°E-30°E and 60°N-70°N for (red) May 18, 2013 and (grey) the 2006-2016 mean. Left are absolute temperatures, right are anomalies with respect to the mean. The x-signs mark the tropopause heights.

During heatwave conditions several things happen: the mean tropopause for May 18th 2013 is about 1 km higher than the climatological mean (the reason for that is a topic for another time). Since the temperature continues to decrease with height until it reaches this higher tropopause, the tropopause is also about 5°C cooler than the climatological one. Hence the blue region in Fig. 2. The darkest region (i.e., the region with the strongest negative anomalies) correspondingly marks the height of the actual tropopause. In the stratosphere the influence of the surface heatwave finally starts to dissolve.

So, there are regions in the upper troposphere/lower stratosphere where surface-heatwaves lead to colder-than-normal conditions through an increase in tropopause height.  To sum up:

  • surface heatwaves are connected to warmer-than-normal conditions in the troposphere.
  • at the height of the climatological tropopause the anomalies switch from positive to negative
  • at the actual tropopause the negative anomalies are strongest
  • in the stratosphere the effect of surface heatwaves dissolves as other factors (such as ozone) become more important.

* The climatological tropopause it is the mean of all tropopause-heights in the region over 10 years and not the tropopause of the climatological profile to be precise.

I acknowledge the Wegener Center for Climate and Global Change (WEGC), University of Graz, Austria for providing their radio occultation data, on which Fig. 2 & 3 are based.

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