Abstract
Recent oceanographic field measurements and high-resolution numerical modelling studies have revealed intense, transient, submesoscale motions characterised by a horizontal length scale of 100–10,000 m. This submesoscale activity increases in the fall and winter when the mixed layer (ML) depth is at its maximum. In this study, the submesoscale motions associated with a large-scale anticyclonic gyre in the central Gulf of Taranto were examined using realistic submesoscale-permitting simulations. We used realistic flow field initial conditions and multiple nesting techniques to perform realistic simulations, with very-high horizontal resolutions (> 200 m) in areas with submesoscale variability. Multiple downscaling was used to increase resolution in areas where instability was active enough to develop multi-scale interactions and produce 5-km-diameter eddies. To generate a submesoscale eddy, a 200-m resolution was required. The submesoscale eddy was formed through small-scale baroclinic instability in the rim of a large-scale anticyclonic gyre leading to large vertical velocities and rapid restratification of the ML in a time-scale of days. The submesoscale eddy was confirmed by observational data from the area and we can say that for the first time we have a proof that the model reproduces a realistic submesoscale vortex, similar in shape and location to the observed one.
Highlights
Ocean circulation is highly turbulent and occurs over a very wide range of scales, ranging from a few centimetres to thousands of kilometres
In order to test and quantify the improvement obtained by the higher resolution child model compared with the coarse resolution parent model, we evaluated the root mean square error (RMSE) between the quantities simulated by each nested model ψm and the observed quantities ψo, defined by: RMSE =
The average salinity RMSE values are approximately 0.2 PSU in the mixed layer, 0.15 PSU in the thermocline and 0.07–0.09 in the deep waters for Mediterranean Forecasting System (MFS) and the nested models. These values are comparable to those obtained by Tonani et al (2009) for the RMSE of MFS analyses and they are within the present day values of RMSE of other analysis systems (Brassington 2017)
Summary
Ocean circulation is highly turbulent and occurs over a very wide range of scales, ranging from a few centimetres to thousands of kilometres. New high-resolution oceanographic field measurements and numerical simulations have revealed intense, transient, submesoscale motions characterised by a horizontal length scale of 100–10,000 m. We used realistic flow field initial conditions and a multiple nesting approach to achieve an open ocean horizontal resolution of 200 m, which is suitable for resolving submesoscale processes in this region and we used CTD measurements to confirm our submesoscalepermitting model predictions. This innovative approach will enhance our understanding of submesoscale eddy development and their dynamics.
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