Multiple oscillatory modes of the global ocean circulation

Abstract

For the first time, the (linear) stability of the global ocean circulation has been determined explicitly. In a low-resolution general circulation model, a steady state is computed directly by solving the elliptic boundary value problem. The stability of this solution is determined by solving the generalized eigenvalue problem. Although the steady global circulation is (linearly) stable, there are two interesting oscillatory modes among the least stable ones, with periods of about 3800 and 2300 yr. These modes are characterized by buoyancy anomalies that propagate through the ocean basins as they are advected by the global overturning circulation. The millennial timescale is set by the time it takes for anomalies to travel, at depth, from the North Atlantic to the North Pacific. Further analyses confirm that the advective feedback between the steady flow and buoyancy anomalies is an essential process in the propagation mechanism. The growth rate of the millennial modes is controlled by vertical mixing. It is argued that these internal ocean modes may be a relevant mechanism for global climate variability on millennial timescales.