Changes in Cyclone Characteristics over the North Atlantic in the Norwegian Earth System Model

• Main Author: Tunes, Sofie Mykkeltveit
• Format: Master Thesis
• Language: English
• Published: 2021
• Subjects: CMIP6; Extra-tropical cyclones; storm tracks; climate change; North Atlantic; Sea ice
• Online Access: http://hdl.handle.net/10852/92145; http://urn.nb.no/URN:NBN:no-94685

Motivated by the low confidence in regional storm track changes, this study investigates the winter North Atlantic extra-tropical storm track for the NorESM2-MM model in phase 6 of the Coupled Model Intercomparison Project. The state of the storm track from 1980 to 2014 is compared to that in ERA-Interim using a Lagrangian objective cyclone tracking algorithm, applied to the mean sea-level pressure. Changes in cyclone characteristics are examined by comparing the abrupt-4xCO2 and SSP5-8.5 scenario simulations to control runs. A good agreement is found between the NorESM2 and ERA-Interim. The uncertainties that still persist in the model are suggested to be a result of a too coarse horizontal resolution, and the inability of the model to resolve diabatic processes. The results in this study differ from the general consensus of a poleward shift of the genesis latitude. A tripole structure is found with more genesis in mid-latitudes (40-60 °N) and less to the north and south. This is found to be a result of the negative meridional temperature gradient anomaly in mid-latitudes which creates a locally enhanced baroclinicity due to sea-ice and SST processes. An overall reduction in the number of storms of about 10 % is found in both scenarios. The reduction is linked to the decrease in the low-level meridional temperature gradient, which reduces the available potential energy for the cyclones. In addition, a more efficient poleward heat transport is suggested due to increased water vapor in a future climate, so less storms could perform the same heat transport. The storms translate farther, both polewards and eastwards. This is linked to the strengthening of the upper jet and the increased cyclone-related precipitation. No change was found for the cyclones lifetimes, which indicate that they also travel faster. The number of weak storms are expected to increase. Even if more latent heat are expected in the future, the overall effect of moistening is to make the storms weaker by improving the efficiency of poleward heat transport and reducing the baroclinicity. Thus, we can get more precipitation, despite no increase in the intensity.