Weekly Seminar: Fall 2013
Speaker: Dr. Alberto Scotti (University of North Carolina)
Title: "Stirred, not shaken: mixing in the Ocean"
Date: Friday, December 6, 2013
Time: 11:00 a.m.
Location: Gilman Hall 50 (Marjorie M. Fisher Hall)
The current understanding of the mechanisms that sustain the oceanic Meridional Overturning Circulation (MOC), an important component of both present and past global-scale climate, emphasizes the role of mechanical forcing, either as wind-driven Ekmann pumping or tidally induced mixing below the main pycnocline, while downplaying the role of the thermodynamic forcing in the form of the meridional gradient in buoyancy applied at the surface. The latter is discounted based on Sandström's so-called "theorem", in reality an inference drawn from experiments, and on a theoretical result used to argue that a flow solely driven by a meridional buoyancy gradient, herein refereed to as Horizontal Convection (HC), cannot sustain a turbulent flow (Paparella and Young, 2002). In this talk, we summarize work that we have been doing to elucidate some of the issues related to Horizontal Convection. Numerical experiments spanning 7 orders of magnitude in the appropriate Rayleigh number show that part of the flow remains turbulent. However, as the Rayleigh number approaches a critical value the meridional circulation undergoes a transition, which suggests that Sandström's inference may be correct in the sense that at sufficiently large values of the Rayleigh number the overturning circulation becomes shallow. Somewhat paradoxically, the deep circulation is weakened by an excess of mixing near the surface, rather than by a lack of mixing, as the convetional energetics approach seem to imply.
Dr. Scotti is an Associate Professor in the Department of Marine Sciences at the University of North Carolina at Chapel Hill. He holds a Laurea in Physics from the University of Milano (1992), and an MS (1995) and PhD (1997) in Mechanical Engineering from the Johns Hopkins University. His research interests include theoretical and computational environmental fluid mechanics with an emphasis of turbulence and mixing in stratified flows and nonlinear internal waves. He is a member of the American Geophysical Union.