Ekman-driven buoyancy flux in quasi-geostrophic flow

In this investigation, the effect of Ekman pumping on a quasi-geostrophic (QG) system is explored via the vertical buoyancy flux. The vertical buoyancy flux is the quantity in QG flows that is responsible for the adiabatic transfer between kinetic energy (KE) and available potential energy (APE), as well as the slow-time evolution of the mean buoyancy. Ekman pumping (or suction) is a phenomenon that arises through conservation of mass at no-slip boundaries of rotating fluid systems. Three-dimensional QG numerical simulations are run with and without Ekman pumping at the bottom boundary, as well as with and without a realistic stratification profile. Through theory and numerical experiment, it is shown that Ekman pumping drives a conversion of energy from APE to KE at small scales, and from KE to APE at large scales, even in the absence of a mean isopycnal slope. It is also shown that Ekman pumping affects the mean buoyancy by slightly weakening the stratification near the bottom boundary.