Dynamical aspects of coherent eddies in the North Atlantic Ocean : Insights from Satellite Observations

Sammanfattning: Oceanic mesoscale eddies, often referred to as the “weather of the ocean”, have horizontal scales of O(10) − O(102) kilometers and timescales spanning days to months. These structures comprise a complex system of coherent eddies (meaning they retain their shape and structure over time and space), filaments, and spirals that influence the transport of heat, salt, and nutrients in the ocean. In this thesis, we focus on studying coherent eddies along the pathways of the North Atlantic Meridional Overturning Circulation using satellite observations, ocean reanalyses, and in-situ data. We apply an automatic algorithm to detect eddies and a Lagrangian trajectory model, based on satellite-observed current velocities, to understand the formation and dynamics of the eddies. The thesis begins by studying the low-frequency variability of anticyclonic eddies in the Gulf Stream region, which are generally warmer than surrounding waters. Our results show an increase in their formation in the late 2000s, linked to a northward shift in the Gulf Stream and a decrease in water advected from the Labrador Sea. These results contradict previous studies that reported an abrupt regime change in 2000 in the number of warm anticyclones based on sea surface temperature data. The second study focuses on eddies in the Iceland Basin. Our analysis shows that the interannual variability in the number of anticyclonic eddies follows a decadal pattern consistent with the ocean heat content variability of the eastern subpolar North Atlantic. In addition, our Lagrangian model shows that the amount of subtropical versus subpolar water advected into the Iceland Basin affects the generation of anticyclonic eddies through baroclinic instability. In the third study of the thesis, we use both Eulerian and Lagrangian methods to calculate the kinetic energy of mesoscale eddies in the Nordic Seas. Our study reveals that coherent eddies contribute significantly to the total eddy energy compared to other mesoscale fluctuations, and there are geographic variations in the distribution of eddy energy for both anticyclonic and cyclonic eddies. It is noteworthy that the low-frequency variability of anticyclonic ed- dies tends to follow the local ocean heat content variability even in these high-latitude oceans. The final study focuses on the structure and variability of the recently discovered deep jet in the Faroe-Shetland Channel, which transports cold and dense overflow water southward. Our study finds a strong correlation between the interannual variability of the number of anticyclonic eddies in the channel and the transport of the deep water. Further investigation is necessary to comprehend how eddies modulate the overflow transport. The results in this thesis indicate that changes in ocean heat content play a key role not only in our climate but also in the number of eddies on low-frequency time scales at subtropical, subpolar, and even higher latitudes. This can have consequences for the marine ecosystem.