Articles | Volume 12, issue 1
Research article
03 Feb 2016
Research article |  | 03 Feb 2016

Effects of vertical shear in modelling horizontal oceanic dispersion

A. S. Lanotte, R. Corrado, L. Palatella, C. Pizzigalli, I. Schipa, and R. Santoleri

Abstract. The effect of vertical shear on the horizontal dispersion properties of passive tracer particles on the continental shelf of the South Mediterranean is investigated by means of observation and model data. In situ current measurements reveal that vertical gradients of horizontal velocities in the upper mixing layer decorrelate quite fast ( ∼  1 day), whereas an eddy-permitting ocean model, such as the Mediterranean Forecasting System, tends to overestimate such decorrelation time because of finite resolution effects. Horizontal dispersion, simulated by the Mediterranean sea Forecasting System, is mostly affected by: (1) unresolved scale motions, and mesoscale motions that are largely smoothed out at scales close to the grid spacing; (2) poorly resolved time variability in the profiles of the horizontal velocities in the upper layer. For the case study we have analysed, we show that a suitable use of deterministic kinematic parametrizations is helpful to implement realistic statistical features of tracer dispersion in two and three dimensions. The approach here suggested provides a functional tool to control the horizontal spreading of small organisms or substance concentrations, and is thus relevant for marine biology, pollutant dispersion as well as oil spill applications.

Short summary
The paper investigates the role of vertical shear on the horizontal ocean dispersion of tracer particles or concentration fields. We compare in situ measurements to GCM numerical data of the magnitude and the typical timescale of the vertical shear. We discuss how an appropriate sub-grid-scale parameterization can account for an anomalous numerical shear dispersion due to the model poor time resolution. The analysis focuses on the mixing layer, and on the region of the south Mediterranean Sea.