Phd Thesis - Thermohaline staircases in the Arctic Ocean: Detection, evolution, and interaction
Thermohaline staircases consist of a series of horizontal, well-mixed layers, each on the order of a meter thick, separated by thin interfaces, across which temperature and salinity make abrupt jumps. While they have been consistently observed several hundred meters below the surface of the Arctic Ocean for over fifty years, little is known about their long-term evolution. Such stratification structures affect the propagation of internal waves and, because of an effect called internal gravity wave tunnelling, interactions between internal waves and staircases can be complex. This thesis presents a novel method of detecting thermohaline staircase layers in observations, analyzes their evolution on a decadal scale, and examines their interactions with internal waves.
Inspired by the patterns made by observations of thermohaline staircases in temperature-salinity space, I develop a novel detection method. Using the Hierarchical Density-Based Spatial Clustering of Applications with Noise algorithm, I find I can detect and connect staircase layers across datasets of hydrographic profiles from the Canada Basin in the Arctic Ocean. This offers an advantage over previous detection methods which treat each profile individually as, here, the sprawling horizontal nature of the layers can be analyzed.
Using this clustering method, I identify layers in the Beaufort Gyre Region which span over 1000 km horizontally and persist for nearly two decades. In addition to reproducing many results from previous studies, I find the layers to be evolving in time. The layers are sinking at approximately the same rate as the overall downwelling in the region. I also find that layers in the upper staircase are warming while layers near the bottom are cooling.
I develop a set of numerical experiments to examine the interactions between internal waves and idealized staircase stratification structures. For structures with one layer, I find the transmission of waves decreases monotonically as the layer thickness gets larger relative to the wavelength. With multiple layers present, I find peaks in transmission for particular ratios of thickness to wavelength, the patterns of which become more complex as more layers are added. I also reproduce the results of a laboratory experiment, finding the same pattern of reflection and transmission of waves.
Schee, M.G. (2024) “Thermohaline staircases in the Arctic Ocean: Detection, evolution, and interaction,” University of Toronto Doctoral Thesis, URI: hdl.handle.net/1807/140974