Master Thesis Defense Elloise Jensen Fangel-Lloyd

Title: Fewer storms as a consequence of climate change. Decreasing baroclinicity observed in Lorenz energetics and
EOF analyses of climate data.

Abstract:
Recent literature on climate change shows that warming is not evenly distributed across the Earth; the poles warm  disproportionately to the Equator in a mechanism known as polar amplification. The meridional temperature gradient generates baroclinic development, which is the driving force behind extratropical cyclones. This implies that less baroclinic development and so fewer storms in the midlatitudes is a consequence of climate change; however, tracking storms is not straightforward.
This work studies this change by accessing baroclinicity directly, through two proxies. The first is by an Empirical Orthogonal Function (EOF) decomposition of geopotential height data, which is validated by several methods. Following this, the appropriate mode is selected and validated. The second is by calculation of eddy kinetic energy of the Lorenz energy cycle by use of wind fields. To study a long and uninterrupted time series, climate reanalysis data is used.

Overall, this work concludes that baroclinicity has indeed decreased over the studied time period. The differences method found a significant decrease in the size of the baroclinic mode, whereas a Kolmogorov-Smirnov analysis found a difference for the filtered case only. When applied to the energetics analysis, both the differences method and the Kolmogorov-Smirnov method found a decrease in both kinetic and eddy kinetic energy. Kinetic energy was found to decrease more than eddy kinetic energy, which has implications on some of the other terms in the Lorenz energy cycle. It was also found that both kinetic and eddy kinetic energy are Ornstein-Uhlenbeck processes with a relaxation time on the order of a week.
Confirmation of a decrease in baroclinic development can be seen as a proof of the polar amplification phenomenon and climate change-induced warming as a whole. A detailed understanding of future storm behaviour is beyond the scope of this work, but it is clear that climate change has significantly affected weather in the midlatitudes, and is expected to continue to do so.

Supervisors: Peter Ditlevsen and Eigil Kaas