Seasonal Variation of Stratified Flow Behavior in A Sea Strait: A Field Study

Straits connecting two large water bodies present a highly strong, complex, and stratified flow structure. The barotropic (related to water level) and baroclinic (related to density) structure of the neighboring seas and the morphology of the strait are decisive on the exchange flow properties through the strait.

As it is a typical example of hydrodynamically complex straits, in this paper, the annual flow structure of the Bosphorus is analyzed. A long-term (one year) current profiles (at three locations), water levels (close to both entrances), CTD measurements at some measurement stations (both at the surface and on the bottom), meteorological (wind speed, wind direction, and atmospheric pressure variation at both entrances) measurements and discharges of the Danube River, which controls the water level of the strait during the late spring, were analyzed.

The Bosphorus is one of the most strategic and busiest waterways in the world connecting the Blacksea to the Mediterranean with the Dardanelles. It presents a two-layer flow structure and the upper layer flow is incomparably much dynamic than the lower one. The results of the study may be highlighted as follows:

1) The water level difference (Δη) between both entrances of the Bosphorus, which is the driving forcing for the southward upper layer flow, shows notable fluctuations throughout the year.

2) The meteorological set-up (wind speed, and atmospheric pressure) is much severe and decisive over Δη during the autumn and the winter, which causes large fluctuations in order of 40 cm (O(40 cm)) over a few day scales. During this period of the year, the typical two-layer flow structure of the strait frequently disappears, and one layer, either southward or in the opposite direction depending on the wind directions, dominates the water column at the measurement locations.

3) The freshwater intrusion to the Blacksea from the major rivers (especially the Danube River) reaches the Bosphorus with around one-month phase (time) lag and controls the water level difference and, so, the current structure of the strait for around 40-45 days from late Spring to early Summer. This period of the year and the rest of the summer is meteorologically calm and, as a result, the water level difference and the current structure is much stable during this time compared to the rest of the year.

4) The seasonal salinity and temperature variations are higher at the surface compared to the bottom. The notable fluctuations are observed both in salinity (> 5 PSU) and in temperature (~ 5 °C) over a short-term period (from a few days to a week) due to severe meteorological conditions which are evident during the fall and winter.

5) The salinity of both layers show temporal variation. The salinity of the top layer was around 17 PSU at the Blacksea entrance of the strait. Due to the mixing, this value increased to 22 PSU at the Sea of Marmara entrance. The salinities of the bottom layer starts from 38 PSU at the south entrance and drops to 32-35 PSU at the north one.

Keywords: stratified flows, measurement, The Bosphorus, hydrodynamics, the Danube River.