What drives water exchanges between the Black Sea and the Sea of Azov through the Kerch Strait?

Abstract

The Kerch Strait connects the Black Sea with the shallow Sea of Azov. The latter receivessignificant continental discharges from the Don and the Kuban rivers, and the Strait is animportant pathway for exchanges of water, salt, suspended matter, nutrients, and pollutantsbetween the two seas. The mechanisms forming water circulation through the Kerch Straitare complex and combine wind forcing, barotropic and baroclinic geostrophic flows, and theadvection of extensive freshwater runoff. However, these mechanisms are not yet fullyunderstood and quantified, while in situ measurements of currents in the Strait are verysparse.We report preliminary results of a recent field experiment conducted in the Kerch Strait insummer-fall of 2023 where RDI ADCP instruments were deployed at 2 stations, one in thewestern and the other in the eastern part of the Strait and recorder vertical profiles ofcurrent velocity at 30 min temporal resolution. The total length of the series is about 3months. The observations indicated that, despite the suggested by some previouspublications, no vertical stratification of velocity was typically observed, while the currentsvaried significantly in the cross-strait direction. We identified 3 types of circulation patterns ofcirculation inside the Strait, mainly controlled by the sign and intensity of the zonal wind stresscomponent. Currents in the Strait reacted to the wind forcing with a time lag of 2 to 3hours.We also performed numerical simulations using the BSAS12 numerical model to simulatecirculation in the Black Sea and the Sea of Azov seas and study water exchange throughthe Kerch Strait. BSAS12 is a regional configuration of the ocean and sea ice generalcirculation model NEMO that covers the Black Sea and the Sea of Azov (Zavialov et al.,2020). The horizontal grid resolution of the model is 1/12◦, which is approximately 6.75 km in thestudy region. The vertical coordinate was represented by 59 vertical z levels with the finestresolution (1 m) at the upper ocean. A partial-step representation of bottom topographythat adjusts the vertical size of the model bottom level to the real depth was used. The modeldomain had an open ocean boundary at the Bosporus Strait that connects the Black Seawith the Mediterranean Sea. The model was driven by the ERA-Interim atmospheric forcing.The combined outcomes from the observations and modeling suggest that net outflow fromthe Sea of Azov to the Black Sea is induced by moderate and strong northeasterly winds,while the flow into the Sea of Azov from the Black Sea occurs mainly during wind relaxationperiods or during events of southerly winds. The direction and intensity of water exchangeexhibited wind-governed synoptic and seasonal variability, and they apparently did notdepend on the variations of river discharges to the Sea of Azov at short temporal scales.