Eddies and circulation: lessons from oceans, atmospheres and the GFD lab
Seminar Room 1, Newton Institute
Planetary fluids inherit the angular momentum of their planets. Concentrated and diluted, and fragmented into potential vorticity (PV), angular momentum controls the circulation under a limited budget of energy. In particular, wherever energy decays, planetary PV takes control of the dynamics. This so-called β-control is manifested in the ‘stiffness’ and scale-dependent ‘PV elasticity’ of rotating fluids which leads to many kinds of wave motion, and to limitation of turbulent mixing. The fluid finds ways, for example PV staircases and their attendant jets, of dealing with limited energy and PV control. Momentum rearrangement that follows stirring of the PV field by eddies is a key process. We see numerous zonal jets on the rapidly rotating, gas giants (strong β-control), and a weaker β-control over Earth’s subpolar atmospheric jet streams, where the kinetic energy density averages 10^6 J m^-2. The Earth’s oceans, operating at lower kinetic energy levels (10^4 J m^-2). have selected jets of much finer scale, and a dominant energy-containing eddy mode manifested as dimples on the sea surface, simply marching westward. These are strongly nonlinear baroclinic Rossby waves which do not obey the simple rules of geostrophic turbulence, namely, expansion of scale laterally and vertically toward a barotropic state, and coalescence into sparsely distributed hard-core vortices. A second mode of oceanic eddy that is widespread is the (equivalent-) barotropic mode of geostrophic flow, ‘tall’ eddies which are highly coordinated with bottom topography.
Here we describe field observations and simulations from the GFD laboratory. These demonstrate Rossby wave propagation, induction of zonal circulation and inhibition of mixing which leads to the ‘ozone hole’ in the terrestrial southern stratosphere; also transition between Rossby waves and solitary eddies which transport fluid (as in the world’s oceans), topographic production of eddies and waves, with steering by PV waveguides (formed by topography and circulation). Using a new laboratory technique known as ‘optical altimetry’ we now can see the interaction of unbalanced flows (downslope winds, gravity- and inertial waves) with the energy-containing geostrophic eddies, as seen in the upward radiation of gravity waves as storms encounter the Greenland’s icy topography.