Dissertation Abstract
The role of upper-ocean mixing in large-scale ocean and climate dynamics
Manucharyan, Georgy E 2014 www.whoi.edu/people/gmanucharyan
Department of Geology and Geophysics, Yale University (United States), 251 pp.
The objective of this research is to understand the effects of small scale processes on large scale dynamics in fluid flows and to assess its implications for climate. An example of small scale processes that are central to this study are tropical cyclones (TC). Despite the great strength of an individual cyclone and its serious economical impacts on coastal population, the cumulative effects of such rare events on large scale oceanic circulation and climate remain largely unexplored.
The study begins with an investigation of small-scale turbulent entrainment processes driven by shear instabilities of the wind-generated ocean currents that arise in the growing oceanic mixed layer during the passage of a TC. The mixed layer growth depends on the turbulent entrainment coefficient which despite its common use in geophysical applications remains poorly constrained by observations. Two sets of laboratory experiments performed here identified the dependence of the entrainment coefficient on the key parameters: the interfacial Richardson number and the background rotation rate.
Enhanced upper ocean mixing leaves a trace of a deepened mixed layer along the path of a TC and corresponding baroclinicly unstable oceanic currents. Here, the instabilities of upper ocean fronts were analyzed with the aim of a high resolution primitive equation model of fluid flow. Theory-based analysis of the data showed that most unstable modes are self-propagating dipoles that detach and have a probability to escape the influence of the meandering front. Shallow fronts that separate mixed layers of approximately equal depth were found to have the highest probability of dipole escape. The general conclusions of the study found immediate application in the Arctic Ocean dynamics explaining persistent observations of sub mixed-layer anticyclones far from their formation sites.
The long-term implications for the ocean circulation are explored in a context of two processes: upper ocean mixing and the vorticity forcing from the cyclonic core of the TC. Upper ocean mixing by TCs results in cold sea surface temperature anomalies and an increased atmospheric heat flux into the ocean. Cumulative effects result in an oceanic circulation that transports heat polewards and equatorwards creating climate conditions that resemble permanent El Nino conditions of the Pliocene (epoch ~3 million years ago). It is shown that despite dramatic changes in mean state there are corresponding changes in the driving mechanisms that explain a persistent interannual variability dominated by ENSO.
The cyclonic winds in the core of a TC leave a scar of negative potential vorticity anomaly along its track, that manifests itself in the lifted thermocline. These anomalies eventually split into series of eddies that move towards the western boundary while interacting with other eddies and currents. Such a convoluted dynamics is explored here and it was found that vorticity forcing from TCs could spin up a large-scale ocean circulation in the form of a double gyre in a nonlinear regime (strong TC).
The study demonstrates that fluid flows have a strong memory of past forcing events and that a series of localized small scale perturbations could aggregate to form large-scale features.
The study begins with an investigation of small-scale turbulent entrainment processes driven by shear instabilities of the wind-generated ocean currents that arise in the growing oceanic mixed layer during the passage of a TC. The mixed layer growth depends on the turbulent entrainment coefficient which despite its common use in geophysical applications remains poorly constrained by observations. Two sets of laboratory experiments performed here identified the dependence of the entrainment coefficient on the key parameters: the interfacial Richardson number and the background rotation rate.
Enhanced upper ocean mixing leaves a trace of a deepened mixed layer along the path of a TC and corresponding baroclinicly unstable oceanic currents. Here, the instabilities of upper ocean fronts were analyzed with the aim of a high resolution primitive equation model of fluid flow. Theory-based analysis of the data showed that most unstable modes are self-propagating dipoles that detach and have a probability to escape the influence of the meandering front. Shallow fronts that separate mixed layers of approximately equal depth were found to have the highest probability of dipole escape. The general conclusions of the study found immediate application in the Arctic Ocean dynamics explaining persistent observations of sub mixed-layer anticyclones far from their formation sites.
The long-term implications for the ocean circulation are explored in a context of two processes: upper ocean mixing and the vorticity forcing from the cyclonic core of the TC. Upper ocean mixing by TCs results in cold sea surface temperature anomalies and an increased atmospheric heat flux into the ocean. Cumulative effects result in an oceanic circulation that transports heat polewards and equatorwards creating climate conditions that resemble permanent El Nino conditions of the Pliocene (epoch ~3 million years ago). It is shown that despite dramatic changes in mean state there are corresponding changes in the driving mechanisms that explain a persistent interannual variability dominated by ENSO.
The cyclonic winds in the core of a TC leave a scar of negative potential vorticity anomaly along its track, that manifests itself in the lifted thermocline. These anomalies eventually split into series of eddies that move towards the western boundary while interacting with other eddies and currents. Such a convoluted dynamics is explored here and it was found that vorticity forcing from TCs could spin up a large-scale ocean circulation in the form of a double gyre in a nonlinear regime (strong TC).
The study demonstrates that fluid flows have a strong memory of past forcing events and that a series of localized small scale perturbations could aggregate to form large-scale features.