Abstract
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.
Highlights
The role of small-scale motion in geophysical flows is receiving renewed attention (Kantha and Clayson, 2000), concerning the hydrodynamical modelling, as well as in relation to the biological consequences of specific phenomena
We demonstrate that (i) small-scale motions enabling tracer pairs to explore the whole mixing layer do not modify the Mediterranean Forecasting System (MFS) horizontal dispersion properties, due to the anomalous persistence of vertical gradients of the horizontal velocities in the MFS model, which overrides the small-scale fluctuations; (ii) differently, the horizontal relative separation resulting from the introduction of the 2-D Kinematic Lagrangian Model (KLM) is fast enough to become dominant with respect to the anomalous shear effect produced by the MFS solution
We have discussed the effect of vertical shear onto the horizontal pair dispersion of tracer particles in a www.ocean-sci.net/12/207/2016/
Summary
The role of small-scale motion in geophysical flows is receiving renewed attention (Kantha and Clayson, 2000), concerning the hydrodynamical modelling, as well as in relation to the biological consequences of specific phenomena (see e.g. Durham and et al, 2013). The Kinematic Lagrangian Model (KLM) here adopted can be 2-D, to better account for the horizontal dispersion due to mescoscale eddies, or 3-D, to simulate vertical turbulent-like motions in the ocean mixing layer. Both dynamics are often underestimated in GCMs. our primary interest is in the former situation, we will discuss both. Since the comparison is performed in one location only, its results might not be of general validity Rather, they point to differences that can arise between model and data, which in turn motivate the sensitivity study presented, where we discuss the relative dispersion properties of neutrally buoyant tracers by means of numerical simulations.
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