Global changes in mesoscale currents and coherent eddies from satellite altimetry
Creators: name: Martinez Moreno, Josue
Publisher: name: The Australian National University
Year: 2021

description: Over recent decades, changes in the climate system have fundamentally modified properties of the ocean. These adjustments include alterations of the sea level, the sea surface temperature, and the ocean circulation from regional to global scales. The ocean surface responds to changes in the climate system through the exchange of heat, carbon, and momentum. Numerous studies have examined the response of sea level and ocean heat content to climate change; however, it remains unknown how the ocean surface currents have adjusted to the climate system over the past decades. Ocean currents have a wide range of spatial and temporal scales, from submesoscales (100m to 10 km), to mesoscale (10 to 100 km), to large-scale overturning. Dynamically, mesoscale flows are crucial in the transport and mixing of tracers such as heat, salt, and nutrients, but they also constitute one of the major reservoirs of the time-varying kinetic energy, namely eddy kinetic energy (EKE). Mesoscale flows consist of three distinct processes: coherent eddies, jets, and waves. The temporal evolution of the ocean currents and the mesoscale field is yet to be quantified. In this thesis, we use satellite observations and a coherent eddy tracking algorithm (TrackEddy) to investigate the temporal evolution of the surface mesoscale, coherent eddies, and jets over the last 27 years. We find a multi-decadal readjustment of the eddy and mesoscale field to changes in the climate system from two independent satellite products. Eddy kinetic energy and sea surface temperature gradients show a significant strengthening of the eddy field at rates of 1.2%-1.8% per decade. Furthermore, regions dominated by mesoscale processes such as the Antarctic Circumpolar Current (ACC) and ocean boundary currents have undergone a greater change over the observational record at rates of 5% and 2.5% per decade, respectively. This global and regional readjustment of the surface currents has crucial implications in the exchange of heat and carbon between the ocean and atmosphere. The Southern Ocean is responsible for capturing a considerable proportion of anthropogenic carbon and heat into the ocean. Eddy-rich regions in the Southern Ocean are further investigated through the decomposition of the flow field into coherent eddies and non-coherent features (jets). The energy budget analysis performed in coherent eddies shows a robust increasing trend over the last two decades. This increase is correlated with larger coherent eddy amplitudes and the strengthening of wind stress. Furthermore, a strong seasonal cycle and the dominance of coherent eddies over jets in certain regions of the ocean were observed. With these results in mind, we then investigate the global climatology of the eddy field, coherent eddies, and jets, as well as regions dominated by each process. The eddy field constitutes a major proportion of the ocean kinetic energy budget. Using a dynamical decomposition of the eddy field, we estimated that coherent eddies and jets constitute around 40%-60% of the energy of the eddy field. This result highlights that coherent eddies and jets can dominate regional energy budgets in the ocean. The climatology of the coherent eddy kinetic energy and eddy properties suggest the seasonal cycle of coherent eddies is driven by wintertime generation of numerous coherent eddies, interaction with the eddy flow, and coalescing over a timescale of several months to become larger and more energetic. The results presented in this thesis advance our understanding of the temporal evolution and climatology of the eddy field. Although the dynamical mechanisms that generated an intensification of the coherent eddy field remain unknown, the results show clear changes in the distribution, intensity, and evolution of the eddy field, with which numerical models can be improved to better resolve the eddy field.