Impact of eddy-induced transport on the Lagrangian structure of the Upper Branch of the thermohaline circulation

Abstract

The effect of the eddy induced transport (EIT) on the Lagrangian structure of the upper branch of the thermohaline circulation is investigated. The Lagrangian pathways, transport, and flow characteristics such as the large-scale chaotic mixing are examined in the OCCAM global, eddy-permitting ocean general circulation model. The motions of water masses are traced employing Lagrangian trajectories. These are computed using both the time-averaged Eulerian velocity and a velocity field that contains the EIT. In all aspects of the flow investigated the neglect of the EIT leads to severely biased results. Below the mixed layer divergences of eddy mass fluxes nearly cancel those of the mean flow. As a result, diapycnal motion is reduced by the EIT. In the surface layer, the EIT counteracts the Ekman flow. This compensation is found to hold both locally and nearly everywhere in the basin. Typically, the surface layer EIT reduces the Ekman transport by 50%. Both reduced diapycnal motion and compensation of the Ekman flow prolong the circulation in wind-driven gyres and counteract dispersion of particles into the interior. Subsequently, the distribution of Lagrangian transport times becomes more peaked at shorter timescales and the transport times between sections decrease. At longer timescales the functional time dependence of the distribution is significantly changed. The spreading of particles and water masses without the EIT is governed by the "wrong" physics. The fact that the EIT makes the flow more aligned along isopycnals, and subsequently more quasi two-dimensional, implies reduced chaotic mixing.