Polar mesoscale cyclones in ERA5 and CARRA

Sammanfattning: Polar mesoscale cyclones (PMCs) are low-pressure systems that form in polar regions. Their small size and short lifespan pose challenges for coarse resolution models in capturing PMCs and their associated air-sea mass and energy transfer. To address the influence of resolution on PMCs, a comparison between a higher resolution dataset (CARRA with 2.5 km resolution) and a lower resolution reanalysis dataset (ERA5 with 30km resolution) over a 10-year period in the North Atlantic is conducted by employing an automatic algorithm. The results revealed that CARRA detected a greater number of PMC activities, highlighting the benefits of higher resolution data in reducing uncertainties during tracking. PMCs identified using CARRA exhibited smaller vortex sizes, but higher relative vorticity and faster movement speeds compared to ERA5 results. Notably, the typical vortex diameters derived from ERA5 and CARRA were 80 km and 120 km, respectively. Furthermore, the climatology of PMCs in the North Atlantic is presented. Monthly variations indicated that the majority of PMCs occurred during winter, with only a few cases developing in summer. The spatial distribution exhibits that the highest density of PMCs was observed in the Irminger Sea and the Norwegian Sea. It also shows that the presence of sea ice can influence the PMC density, as a significant number of cases formed near the sea ice edge. The comparison results indicate that the current coarse climate simulation may underestimate the influence of PMCs in the Arctic due to the misrepresentation of them, thus introducing uncertainties in the climate prediction.