The record rainfalls in July 2012 that triggered a devastating flash flood in the Russian town of Krymsk would not have been possible without the previous 30 years of rising sea surface temperatures in the Black Sea, suggests a paper published online in Nature Geoscience. The authors found that this warming trend triggered changes in atmospheric dynamics that favour extreme rainfall events.
Extreme weather events are difficult to attribute to incremental climate warming because natural variability can rarely be excluded; yet, such information is particularly sought after. Edmund Meredith and colleagues ran a suite of simulations with a very-high-resolution atmospheric model to investigate the effects of warming sea surface temperatures on the generation of convective storms, which are often associated with extreme rainfall.
Studying the Krymsk region, they found that regional rainfall levels are 300% higher on average in simulations with observed Black Sea surface temperatures, compared to simulations where the warming trend over the past 30 years was removed. Specifically, the simulations showed that without past surface ocean warming, deep convection (the heat-driven transport of warm, moist air to the upper atmosphere, active in thunderstorms, which often results in heavy precipitation) does not develop in the region. The findings thereby reveal a physical mechanism that directly links a rapid increase in extreme rain events to a gradual rise in sea surface temperatures.
In an accompanying News & Views article, Friederike Otto writes that the work “represents a crucial step towards combining the understanding of an event with an assessment of its likelihood of occurrence.”