The evolution of anticyclonic eddies of the Tasman Sea: vertical velocity and other properties

Eddies play a critical role in the distribution of heat and other properties in the oceans. They are most intense in regions of western boundary currents, where the kinetic energy associated with eddies is one order of magnitude greater than the kinetic energy associated with the mean flow. The East Australian Current (EAC) is the western boundary current of the South Pacific Ocean. Its flow and related mesoscale variability extend through the Tasman Sea - a region characterised by high variability associated with a complex field of eddies. Here, the path of long-lived anticyclonic eddies originating in the EAC is explored. In addition, the evolution of their properties and dynamics over time is also investigated, with a focus on their vertical velocity. These eddies are studied in fields from a free-running ocean model, from a data-assimilating ocean model, and from gridded satellite altimetry. In this thesis, I choose to examine case studies of eddies. This approach contrasts to most recent studies of eddies that tend to undertake statistical analyses of a large number of eddies. Moreover, the eddies are manually tracked - again, in contrast to a large portion of the community who have embraced datasets generated by automatic eddy-tracking algorithms. The improved accuracy and reliability of a manual approach warrants the additional time and effort to carefully track each eddy. Analysis of anticyclonic eddies that form from the EAC indicates that such eddies can “live” for over 5 years. I find that some of these eddies leave the Tasman Sea, propagate around Tasmania, and move towards the Indian Ocean. As they propagate, their amplitude is often impacted by interactions with other eddies and with the continental slope. Additionally, the speed at which eddies propagate varies considerably - with some eddies stalling in the same latitude for up to six months. As eddies propagate out of the Tasman Sea, their circulation becomes systematically more barotropic - with deeper penetrating velocities. One of the most interesting findings of this thesis is that the vertical velocity in eddies often shows alternating upward and downward cells. Such cells have been previously reported - but were not fully explained, in terms of their dynamics. Here, a dynamical explanation for these cells is provided. Specifically, the alternating cells are shown to relate to a process referred to in this thesis as “eddy distortion”. Eddy distortion refers to the change of shape of an eddy - that is, when an eddy becomes more, or less, isotropic. This distortion can be quantified by the change of sea level anomaly (SLA) associated with the eddy in time - also seen as alternating positive and negative cells. This quantity can be linked to the eddy interior dynamics and, ultimately, vertical velocity. In a region of inward distortion of an anticyclonic eddy, the SLA decreases and the water column below the permanent pycnocline vertically stretches, generating cyclonic relative vorticity and inducing upward motion. Conversely, in a region of outward distortion, the SLA increases and the water column below the permanent pycnocline vertically squeezes, generating anticyclonic relative vorticity and inducing downward motion. I find that the alternating cells are remarkably long-lived and spatially coherent. These cells typically extend from 2000 m depth to the base of the thermocline and their magnitude are often 20-50 m/d. The alternating upward and downward cells explain 30-60% of the variance of the vertical velocity within each eddy. The vertical circulation within eddies, described here, is expected to significantly impact the distribution of ocean properties, including heat, freshwater, and marine biota. Recognition of the link between eddy distortion - quantified by the temporal change of SLA - and the vertical circulation within eddies opens the door for the development of algorithms that link sea level to ocean productivity, and other oceanographic metrics.This has not been explored here - but is an exciting topic for future work.