Environmental Science and Engineering Seminar

The Earth's climate is affected by the inherently turbulent large-scale oceanic flows that continuously exchange energy with smaller-scale perturbations (eddies). Of particular interest here are the mesoscale eddies that are efficient in releasing the gravitational potential energy thus being essential for dynamics of large-scale flows. Due to computing power limitations the state-of-the-art climate models do not have sufficient resolution to represent mesoscale eddies and hence their effects are being parameterized. The most successful and widely used is the Gent-McWilliams parameterization that represents eddy buoyancy fluxes via homogenization of the isopycnal layer thicknesses. Here, we question the validity of its key assumption that the eddy fluxes are related to large-scale gradients in a Markovian way. Using an eddy-resolving ocean model, we demonstrate that the mesoscale eddy field has a finite memory of past ocean states. The presence of this memory leads to several differences in the transient evolution of ocean currents which we expose using numerical simulations and theoretical arguments. One of the most dramatic implications is the emergence of a new decadal mode of large-scale ocean variability that can be sustained via atmospheric noise forcing. This mode is essentially filtered out in current climate models that use conventional Markovian eddy parameterizations.  At last, we discuss a fundamental reason for the presence of eddy memory, speculating that it provides a way for the ocean to equilibrate in a fastest possible way.