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