Arctic cyclones and their representation in climate models

Synoptic-scale Arctic cyclones are atmospheric perturbations of thousands of kilometers size associated with a low pressure that form in the Arctic or move towards this region. They live between a few days and more than a month and are characterized by strong surface winds and precipitation. The specificity of Arctic cyclones, in comparison with mid-latitude ones, is that some of them, long-lived ones, have a cold-core vertical structure. Others have a warm-core mid-latitude-like structure. Arctic cyclones form a major hazard if human activities are to be continued, even intensified, in the Arctic region. As they transport heat and humidity, they also represent a key feature to understand Arctic climate, especially in a context of global warming. In order to simulate Arctic faster warming i.e. polar amplification in a realistic way, it is crucial to better understand Arctic cyclones life cycles and to improve their representation in the models. The aim of our study is to evaluate this representation in the IPSL climate model by analysing HighResMIP simulations run at four different spatial resolutions (Low (LR), Middle (MR), High (HR) and Very High (VHR)) and comparing with ECMWF-ERA5 reanalysis. To do so, Arctic cyclones are detected and tracked by applying the algorithm TempestExtremes to the mean sea level pressure (MSLP) field. Different cyclones characteristics are analysed: their frequency, their lifetime and their intensity. Maps of genesis, lysis and track densities are also analysed. Preliminary results show that the higher the resolution, the higher the number of tracks. The model represents Arctic cyclones quite well in summer but is less skillful in winter with an overrepresentation of the most intense ones, especially above the continents and sea ice. This bias increases with the resolution. The ability of the model to represent interactions of Arctic cyclones with tropopause polar vortices and the transitions from baroclinic structure to axisymmetric cold-core structure is also investigated.