Vorticity balance for mid-ocean mesoscale eddies at an abyssal depth

Abstract

Current fluctuations in mid-ocean are dominated by mesoscale eddies, which have typical time scales of weeks to months and horizontal scales of tens to hundreds of kilometres. Such mid-ocean eddies may have an important part in the general ocean circulation, and intensive efforts have been made to observe current and density fluctuations in the world's oceans1–4. Data sets describing eddy field in a selected region such as the Mode1 and Polymode3 regions have been obtained in the western North Atlantic. The local dynamics of eddy motions, however, have seldom been discussed on the basis of field measurement data. This report examines the vorticity balance for mesoscale eddies as observed from current meter data. Currents at an abyssal depth were measured at five mooring stations in a cross pattern in the mid-ocean of the western North Pacific. Relative vorticity is estimated from horizontal derivatives of 10-day mean velocities. Its local time change is accounted for by the advection of planetary vorticity within the estimated error. The vorticity balance is primarily linear and barotropic for mesoscale eddy motions at this depth.