Abstract/Summary

Polar lows (PLs) are intense mesoscale cyclones that form at high latitudes during winter. Their high wind speeds and heavy precipitation can substantially impact offshore infrastructures and coastal communities over regions such as Scandinavia, Russia and Japan. However, large uncertainties regarding their climatology, interaction with the ocean and response to climate change still remain. Using an automatic tracking method and specific identification criteria, a reliable long-term climatology of PLs and their environment is derived from two atmospheric reanalyses. The mean number of PLs differs significantly between reanalyses, however the inter-annual variability of PL numbers is highly correlated between both datasets. PLs activity from these reanalyses is found consistent with observations and literature. The large-scale environment of PLs is found to play a role in the inter-annual variability of PL numbers. The possible impact of PLs on the ocean circulation over the Nordic Seas is investigated using high resolution simulations from a coupled global climate model. As seen in previous studies based on an ocean model with parametrized PLs, this thesis shows, in high resolution climate model simulations, a clear positive link between the ocean surface heat fluxes and PL occurrences. However, in this study, no evidence is found that PLs influence on the ocean density is sufficient to destabilize the water column and trigger deep ocean convection over the Nordic Seas. Finally, for the first time, the representation of PLs and their environment are assessed in a high resolution atmosphere-only global climate model, for both present climate conditions and a future climate scenario. Furthermore, the impact of the resolution of the model on the representation of PLs is assessed using simulations from three different horizontal resolutions for both climate conditions. Overall the PL numbers are expected to decrease in the future, mainly due to an increase in static stability. However, regional differences appear and new areas for PL occurrence emerge over the Arctic Ocean. The horizontal resolution of the climate model is found to affect the mean numbers of PLs but not their activity.