Severe Autumn storms in future Western Europe with a warmer Atlantic Ocean

Post date: Oct 24, 2014 8:27:34 PM

2014/10/10. In a previous news-item (More hurricanes to hit Western Europe due to global warming) I have reported on results of a series of high-resolution future climate simulations (25km resolution) carried out at KNMI in a project named "Future Weather". Resolution is sufficiently high to admit hurricanes to be properly resolved. It was found that in a future climate these are more likely to travel in the direction of Western Europe. Higher ocean temperatures reduce their decay rates as they leave the tropical areas making it more likely to be picked up by the jet-stream, undergo extratropical transition and re-intensify. In a paper that has just appeared in Climate Dynamics, we have looked at this dataset again, and found some very interesting results. A figure from one of the most intense storms of the dataset, is pasted below, along with the abstract of the paper itself.

Fig.1. Horizontal (a–c) and vertical (d–f) fields for Amy at 3 differ- ent stages; TC (left), extratropical transition (middle) and re-intensification (right). In the upper panels, shading represents 850 hPa EPT (K), black contours are mean sea level pressure (hPa), cyan indicates 250 hPa wind speed starting at 40 m/s with 10 m/s intervals and the yellow and white contour depict 33 and 50 m/s wind speed at 850 hPa. The lower panels contain PV (PVU) in shading and contours of horizontal wind speed (thin lines indicate 20, 30 and 40 m/s, thick lines 50 and 60 m/s). At the bottom left, the location of the storm centre at the respective times.

Abstract: "Simulations with a very high resolution (~25 km) global climate model indicate that more severe Autumn storms will impact Europe in a warmer future climate. The observed increase is mainly attributed to storms with a tropical origin, especially in the later part of the twentyfirst century. As their genesis region expands, tropical cyclones become more intense and their chances of reaching Europe increase. This paper investigates the properties and evolution of such storms and clarifies the future changes. The studied tropical cyclones feature a typical evolution of tropical development, extratropical transition and a re-intensification. A reduction of the transit area between regions of tropical and extratropical cyclogenesis increases the probability of re-intensification. Many of the modelled storms exhibit hybrid properties in a considerable part of their life cycle during which they exhibit the hazards of both tropical and extratropical systems. In addition to tropical cyclones, other systems such as cold core extratropical storms mainly originating over the Gulf Stream region also increasingly impact Western Europe. Despite their different history, all of the studied storms have one striking similarity: they form a warm seclusion. The structure, intensity and frequency of storms in the present climate are compared to observations using the MERRA and IBTrACS datasets. Damaging winds associated with the occurrence of a sting jet are observed in a large fraction of the cyclones during their final stage. Baroclinic instability is of great importance for the (re-)intensification of the storms. Furthermore, so-called atmospheric rivers providing tropical air prove to be vital for the intensification through diabatic heating and will increase considerably in strength in the future, as will the associated flooding risks."