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

Abstract

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.

Wintertime dynamics in the coastal northeastern Adriatic Sea: the NAdEx 2015 experiment

“…For example, it seems that the dynamics of the thermohaline front that stretches from Kvarner Bay towards the open Adriatic (Lee et al, 2005;Kuzmić et al, 2006;Poulain et al, 2011) are highly variable over time, resembling near-diurnal wave-like oscillations. The question remains of whether these oscillations are driven by diurnal tides (Cushman-Roisin and Naimie, 2002), Adriatic seiches (Cerovečki et al, 1997), inertial oscillations (Orlić, 1987) or some other phenomena that may induce significant oscillatory currents in the region (Kokkini et al, 2017). In addition, as seen from glider measurements, the front may completely change and even vanish over a daily timescale and may have a strong impact on the thermohaline and dense-water dynam-ics in the region.…”

“…However, the front weakened the day after, when the glider returned over approximately half of the same track (turnover occurred on 26 February around 02:00 CET). Kokkini et al (2017) ascribed the variability in the front to wind forcing, where strong bora wind favours a sharp front. This front has also been observed and investigated during previous wintertime campaigns (Lee et al, 2005;Poulain et al, 2011).…”

Dense water formation in the coastal northeastern Adriatic Sea: the NAdEx 2015 experiment

“…For example, it seems that dynamics of the thermohaline front that stretches from Kvarner Bay towards the open Adriatic (Lee et al, 2005;10 Kuzmić et al, 2006;Poulain et al, 2011) is highly variable in time, resembling a near-diurnal wave-like oscillations. A question is if these oscillations are driven by diurnal tides (Cushman-Roisin and Naimie, 2002), Adriatic seiches (Cerovečki et al, 1997), inertial oscillations (Orlić, 1987) or some other phenomena that may induce significant oscillatory currents in the region (Kokkini et al, 2017). In addition, as seen from glider measurements, the front may completely change and even vanish over a daily timescale, and may have a strong impact on the thermohaline and dense water dynamics in the region.…”

North Adriatic Dense Water: lessons learned since the pioneering work of Mira Zore-Armanda 60 years ago

“…However, these contributions have been estimated by numerical models, which -particularly inside the Kvarner Bayare sensitive to both forcing from the atmosphere and to freshwater load (e.g., Vilibić et al, 2016, Vodopivec et al, 2022, see also the discussion in Section 5). Already during the generation, the dense waters are pushed out by the bora wind from the Kvarner Bay through numerous channels, generating strong thermohaline fronts between dense waters and warmer and saltier waters being advected from the southeast with the Eastern Adriatic Current (Lee et al, 2005;Carniel et al, 2016a;Kokkini et al, 2017).…”

“…Consequently, veins of dense waters are created off the Kvarner Bay reaching rapidly the major NAddW bottom current coming from the northern Adriatic shelf (Mihanović et al, 2013;Pranić et al, 2023). This flow also creates strong thermohaline fronts off the Kvarner Bay (Lee et al, 2005), where enhanced mesoscale activity occurs after the wind relaxation (Kokkini et al, 2017). Finally, this flow rapidly decreases after the end of the bora episode, as the DWF area in the Kvarner Bay is deeper than the outer open Adriatic shelf, so the dense waters are kept in the bottom layer inside the bay (Janeković et al, 2014;Pranić et al, 2023).…”