What causes major heatwaves?

The 2019/20 heat wave in Australia has resulted in days that have been called the ‘hottest on record’. There are disputes about whether this is the case, with factors such as the Urban Heat Island effect, the BoM re-calculating historic temperatures downwards and excluding all data prior to 1910 (whether the data were measured in Stevenson Screens or not).

Leaving aside the argument about whether the 2019/20 year has been record heat, there is an interesting study on the role and impact of soil moisture on heat waves. The study closely analyzes five major heat waves that occurred between 2010 and 2016 in different parts of the world: Russia, the U.S. Midwest, Australia, Europe, and South Africa. 

The study concluded:

“Considering all analyzed events, atmospheric circulation and soil moisture play similarly important roles, each contributing between 20% and 70% to the events’ anomalies. This highlights that the role of thermodynamics can be just as important as that of the dynamics for temperature extremes, a possibly underestimated feature.”

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