An Available Potential Energy Perspective on North Atlantic Weather Regimes

The variability of the large-scale flow over the North Atlantic is well described by a set of quasi-stationary recurrent flow patterns, so-called weather regimes. Each weather regime is associated with a characteristic life-cycle and typical large-scale flow configurations, often involving the occurrence of atmospheric blocks in specific regions and particular shifts of the storm track. Due to their relevance for surface weather and, for example, wind energy production, understanding and accurately predicting the weather regime life cycles is highly important. However, the processes leading to the onset and persistence of weather regimes are not well understood. In particular, latent heat release in ascending air streams and air-sea interactions along the Gulf Stream are thought to play a fundamental role in the onset of weather regimes and their mutual transitions.

Here, we focus on a recently introduced diagnostic of available potential energy (APE), which is defined locally for each air parcel and is a measure of the energy that is available for conversion into kinetic energy (baroclinic conversion). APE directly links diabatic processes with the large-scale dynamics, which makes it ideal for the study of latent heat release and air-sea interactions in weather regime life cycles. In a case study of an episode of European blocking - a weather regime associated with an anticyclone over the British Isles and an intensified storm track in the western North Atlantic - in the period between 20 and 27 of February 2019, we study the distribution of APE and its tendencies along the paths of two intense cyclones originating in the Gulf Stream region. Thereby, we elucidate the balance of diabatic and adiabatic contributions to the APE budget of the cyclones. This particular event has been chosen due to its connection with strong air-sea interaction over the Gulf Stream and because it lead to a record-breaking warm spell in the United Kingdom, the Netherlands, and Northern France, where temperatures reached above 20°C. This case study is then complemented by a climatological analysis of the APE tendencies during the various weather regime life cycles.