The Role of Upper-Ocean Mixing in Large-Scale Ocean and Climate Dynamics.

摘要

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), which are intense localized atmospheric vortices actively interacting with the ocean during their life cycle. 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 flow characteristics. The first experiment revealed that the entrainment across a sharp density interface in shear driven flows scales as the Richardson number (a non-dimensional ratio of stratification to shear) to the power of --3/2. While the second experiment, exploring the dynamics of rotating density currents, implied that the entrainment is inversely proportional to the background rotation rate of the reference frame.;Enhanced upper ocean mixing leaves a trace of a deepened mixed layer along the path of a TC and a corresponding oceanic current. However, these currents are subject to baroclinic instability that generates a series of mesoscale eddies which affect the oceanic restratification. 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 eddies 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. It is shown here, that the intensity of oceanic circulation depends of the frequency and strength of mixing events with highly intermittent mixing being less efficient compared to steady mixing. The influx of heat towards the equator creates climate conditions that resemble the past geological epochs including the Pliocene. These conditions are distinguished by their weak zonal temperature gradients at the equator, a phenomenon that modern climate models are unable to reproduce. Here, the strength of upper ocean mixing was used as a tool to explore sensitivity of the equatorial dynamics in a wide range of climates. 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 the El Nino - Southern Oscillation.;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 in a high resolution multiple-layer shallow-water model, which was devised here from scratch and explicitly resolves small scale TC forcing within a realistically large size of the ocean basin. If was found that vorticity forcing from TCs could spin up a large scale ocean circulation in the form of either a single gyre in the linear regime (weak TC) or 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.;Keywords: tropical cyclones, turbulent entrainment, frontal instabilities, large-scale ocean circulation, El Nino - Southern Oscillation.