Response of the Aegean Sea surface characteristics to turbidity variations, using numerical simulations.

Abstract

<p>The thermohaline and dynamic characteristics of the upper ocean can be affected by the way the sunlight is absorbed and scattered on the surface layer. Changes in light penetration can be investigated through the turbidity of the layer, which is determined by a synthesis of terrestrial inputs (atmospheric and riverine), and the biological activity in a region, of both natural and anthropogenic origin. To examine the effects that turbidity variability has on the surface-layer characteristics of the ocean, a twin modeling sensitivity experiment was performed, using as a case study the Aegean Sea, NE Mediterranean. The Aegean Sea is an oligotrophic region with most nutrient inputs located in the northern coasts of the basin, creating a north-to-south chlorophyll-a gradient, with the highest concentrations on the North and lowest on the South. The first experiment corresponds to a very clear ocean, and the second incorporates the turbidity field, varying according to chlorophyll-a concentration.<br>The experiments were implemented using the NEMO models' ocean component (v3.6) for the region (34.05-41.16 °N, 22.29-28.98 °E) and for the period 1997-2001, discretized on a 1/36° Arakawa-C grid, with 75 partial step vertical levels. Atmospheric inputs are ERA-5 reanalysis products of the ECMWF service, whereas inputs for initial and boundary values have been derived from the Copernicus database. First, a two-band light penetration scheme was applied, using a Jerlov Type-I extinction depth at 23.0 m, representing the constant-low turbidity field. An RGB scheme was applied for the second experiment, using the multiyear monthly mean of the chlorophyll-a concentration variable derived from the ESA-CCI service. The sea surface variables' response is examined for the final year of the experiment. The results indicate that the RGB-scheme experiment estimates elevated sea surface salinity and temperature values, with the most significant difference in salinity located in the northern part of the basin, where there is a strong influence of the inflow of Black Sea Water from the Dardanelles Straits. Elevated eddy kinetic energy is observed in the gyres formed in the Cretan Sea.</p>