A multiplatform investigation of Istrian Front dynamics (north Adriatic Sea) in winter 2015

In the northeastern Adriatic Sea, southwest of the Istrian Peninsula, a persistent thermohaline front is formed, called here the Istrian Front (IF). A Slocum glider was operated across the IF near the entrance to the Kvarner Bay between 24 and 27 February 2015. Three Acoustic Doppler Current Profilers (ADCPs) were also deployed at the entrance of the Kvarner Bay during the same period. The glider crossed twice the IF, which was characterized by a fast response to the local wind condition, detecting strong salinity, temperature and density gradients. During the first crossing a strong northeasterly Bora wind was blowing. This resulted in a very sharp and strong thermohaline front, extended vertically in the entire water column, between saltier and warmer water to the south, and the fresher and colder water to the north. Across the front the SST changed ~ 1.2 °C within a distance of 2.4 km. On the contrary, during the second crossing, about 2 days later, under weaker wind conditions, the IF appeared to be much smoother, inclined and wider while the SST changed ~ 1.2 °C within a distance of 8 km. A strong density gradient was also reported, coincident with the thermohaline IF. From previous observations, mainly experiments in 2003, the IF was known only as a thermohaline front compensated in density. In winter 2015, the density front was strong and well defined, demonstrating a density difference of about 0.36 kg/m3 within a distance of 2.4 km. The ADCP measurements and the numerical model simulations demonstrated a circulation of cold waters exiting from the Kvarner Bay in the southern part of the entrance, while during a Bora event this outflow was taking place in the northern part.