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Hot spots of horizontal mixing and cross-stream barriers in the subpolarNorth Atlantic and their impact on the horizontal distribution of heat and the formation of mode waters

PhD student :Vasco Müller
Project supervisors :D. Kieke, M.Rhein (Germany)
P. G. Myers (Canada)
Key hypothesis
The existence of frontal barriers and hot spots of horizontal mixing dominates the spreading patterns of water mass properties in the interior subpolar North Atlantic and regulates the advective supply and removal of heat in the mode water formation regions.
The North Atlantic is a key factor in the climate system due to deepwater formation in high latitudes and simultaneously one of the regions most affected by climate change. The flow field in the subpolar North Atlantic is highly chaotic and dominated by fine filaments and eddies forming frontal barriers and hotspots of horizontal mixing. These features are believed to control the spreading patterns of water masses and their properties as well as the advective transport of heat in the mode water formation regions.
The main interest of my project is to identify these features and investigate where and how mixing takes place, if there are indeed localized hotspots of mixing and how water masses are transported across horizontal fronts (e.g. the boundary between subpolar and subtropical gyre).
To analyze this, a geometry based eddy detection and tracking algorithm is applied to over 20 years of geostrophic velocities from satellite altimetry. The velocity fields are resolved daily on a 1/4° horizontal grid. The largest numbers of eddies are found in the Irminger Sea, the eastern Labrador Sea and along the North American shelf. To estimate the heat content of individual eddies and the respective temperature fluxes, the eddy surface area and translation speed from the eddy detection and tracking algorithm are combined with a real-time global sea surface temperature (SST) analysis. It is then possible to calculate time series of eddy temperature flux across transects in different areas.
At this point, I focus mainly on the meridional eddy temperature flux across the east west transect at 47°N. This transect is located close to the gyre boundary and there are ship based measurements available for this location in order to further investigate the connection between the current system and the spacial distribution of eddies as well as the depth structure of individual eddies.
For a better understanding of the processes involved and in order to analyze the depth structure of eddies in a full 3d dataset, the analysis will be extended to a high resolution numerical model (NEMO).

Average SST in study region (1993-2014). The North Atlantic Current (NAC) and the recirculation in the Newfoundland Basin (NBR) shown in red, overflow waters and the western boundary current system (WBC) shown in white, section at 47°N shown in grey.