Properties of low-level wind speed jets over the Arctic Ocean

The Arctic is warming about 4 times the global average rate. Models still struggle to reproduce correctly the observed rate of change of the Arctic climate system. One model key weakness is the correct representation of the vertical structure of the lowest part of the atmosphere, known as the boundary layer. Low-level jets (LLJ) play an important role in the boundary layer because they can modify the turbulence structure, by generating turbulent mixing aloft. The boundary layer and the turbulence aloft generated by the LLJs are poorly represented in large-scale models. In this thesis, we analyse the annual cycle of LLJs during the MOSAiC expedition, using MOSAiC datasets and ERA5. Our results show that LLJs are common during the entire year, with a seasonal and geographical variability of more LLJs during the winter months and close to the sea ice edge. The observed LLJs have a typical speed between 6 and 14 m s-1, being fastest in winter and during the transition period between winter and the start of the summer. The peak in observed jet height distributions was below 250 m throughout the year, lower in winter and higher in summer. ERA5 has an annual mean bias of about -0.6 m s-1 and 90 m for the LLJ speed, and height, respectively. The most common LLJ forcing mechanism is baroclinicity, about 64% show a baroclinic forcing, while only 8% rely purely on inertial forcings. Clear baroclinic LLJs and purely inertial LLJs tend to have a similar speed and height during the year around 10 m s-1 and 240 m, with only small seasonal variations: during winter clear baroclinic LLJs slightly faster (9 m s-1) than purely possible LLJs (8 m s-1), while during summer clear baroclinic LLJs slightly slower (12 m s-1) than purely possible LLJs (13 m s-1). Additionally, we found that LLJs tend to be faster and higher for weaker stable BLs in comparison to stronger stable BLs. We also analyse the turbulence kinematic (TKE) energy profiles and found that LLJs are associated with local maximum in the TKE values around 50 m above and below the LLJ core, and for shallow jets than 300 m, this local maximum interacts more strongly to the turbulence generated due to the surface friction. The results from this thesis shed light into the important role of LLJs in the Arctic Climate System.