Investigating the impact of tides in the North Aegean on the deep water formation events (DWFe) during Eastern Mediterranean Transient (EMT).

The North Aegean Sea is one of the most interesting seas of the Mediterranean, being under the dominant impact of the Black Sea waters though the so-called Turkish Strait System (TSS – including the Dardanelles and Bosphorus Straits and the Marmara Sea). Moreover, it constitutes a potentially deep water formation site of the Eastern Mediterranean Sea along with the Adriatic Sea. Previous studies for the region focused – rightly – on the crucial role of low salinity Black Sea waters in controlling the overall thermohaline function and dynamics of the North Aegean. None of the previous modeling approaches studied the impact of tides in the mixing processes and the production of extremely dense water, especially during 1987, 1992 and 1993 when major deep water formation events took place in the region. In this work we examine the tidal impact via several long term simulations using a high resolution (1.0 km) ocean model covering the period from 1985 to 2013.

The Regional Ocean Model System (ROMS) was used for two 28-year-long hindcasts. A computational grid of approximately 1.0 km horizontal resolution in both directions and 31 vertical sigma (σ) levels was develop to cover the region of interest that extends from 22.5ºE to 27.25º E in longitude and 38.35º N to 41.2º N in latitude. Atmospheric forcing fields from ECMWF ERA5 reanalysis dataset with a spatial resolution of 0.25 degrees and hourly time step were used. The inflow from the Black sea to the North Aegean was treated as an open (east) boundary condition and data from Vladimir Maderich work was used as input. The tidal forcing, in total eight (8) harmonics – four diurnal and four semidiurnal – came from Oregon State University (OSU) inverse global tidal model. Two identical simulation – in terms of model setup and input – were conducted: the first without and the second with tidal forcing.

After extensive validation of the model’s results, using all available in situ data from different platforms, a comprehensive analysis was conducted and our findings reveal that model results employing tidal forcing exhibit a closer proximity to observations than non-tidal results, thus validating the necessity to incorporate tidal forcing. Furthermore, the use of barotropic tidal forcing enchanced meridional exchanges of heat, salt and buoyancy between the North and South Aegean, thus increasing the stratification and buoyancy content of the upper water column prior to winter mixing. The most significant and surprising result however is that the dense-water volumes produced using tidal forcing were much higher than the ones without tides, a fact signifying the complexity of processes involved prior to and during dense-water formation.

This work was partly covered by the project “Coastal Environment Observatory and Risk Management in Island Regions AEGIS+” (MIS 5047038), implemented within the Operational Program “Competitiveness, Entrepreneurship and Innovation” (NSRF 2014-2020), co-financed by the Hellenic Government (Ministry of Development and Investments) and the European Union (European Regional Development Fund).