An Available Potential Energy Perspective on North Atlantic Weather Regimes

Abstract

<p><span>The variability of the large-scale flow over the North Atlantic is well described by a set of quasi-stationary </span><span>recurrent flow patterns, so-called weather regimes.</span> <span>Each weather regime is associated with a characteristic </span><span>life-cycle and typical large-scale flow configurations, often involving the occurrence of atmospheric blocks in </span><span>specific regions and particular shifts of the storm track.</span> <span>Due to their relevance for surface weather and, for </span><span>example, wind energy production, understanding and accurately predicting the weather regime life cycles is </span><span>highly important. However, the processes leading to the onset and persistence of weather regimes are not well </span><span>understood. In particular, latent heat release in ascending air streams and air-sea interactions along the Gulf </span><span>Stream are thought to play a fundamental role in the onset of weather regimes and their mutual transitions.</span></p><p><span>Here, we focus on a recently introduced diagnostic of available potential energy (APE), which is defined </span><span>locally for each air parcel and is a measure of the energy that is available for conversion into kinetic energy </span><span>(baroclinic conversion).</span> <span>APE directly links diabatic processes with the large-scale dynamics, which makes it </span><span>ideal for the study of latent heat release and air-sea interactions in weather regime life cycles. In a case study </span><span>of an episode of European blocking - a weather regime associated with an anticyclone over the British Isles and </span><span>an intensified storm track in the western North Atlantic - in the period between 20 and 27 of February 2019, we </span><span>study the distribution of APE and its tendencies along the paths of two intense cyclones originating in the Gulf </span><span>Stream region. Thereby, we elucidate the balance of diabatic and adiabatic contributions to the APE budget of </span><span>the cyclones. This particular event has been chosen due to its connection with strong air-sea interaction over </span><span>the Gulf Stream and because it lead to a record-breaking warm spell in the United Kingdom, the Netherlands</span><span>, and Northern France, where temperatures reached above 20</span><span>°</span><span>C. This case study is then complemented by a </span><span>climatological analysis of the APE tendencies during the various weather regime life cycles.</span></p>