An additional fall in temperature was detected by the Vida buoy on 13 February 2012, after the extreme bora winds temporarily decreased in strength.  A minor fall in sea surface temperature was present, however, an extreme fall in temperature had emerged at sea bottom, a few hours after the sea surface temperature fall had been noticed. Over a few hours, the temperature at the bottom dropped from around 7.7°C to 4.9°C. This temperature equals the lowest temperature that has ever been measured in the Slovenian sea by the Marine Biology Station of the National Institute of Biology: The “RI” station in the freshwater Rizana river mouth measured the same temperature in February 2005 at a depth of 0.3 m.

The temperature of 4.9°C is approximately one degree above the lowest temperature ever measured at a few 10 m of depth in the Gulf of Trieste (in the Northern Adriatic Sea, if not in the entire Adriatic Sea):  In the hard winter of February 1929, Aristocle Vatova measured a temperature of 3.95°C at a depth of 22 m right in front of the entrance to the Gulf of Koper.

What happened after the bora had settled? On 13 February 2012, the bora's strength had been decreasing (Fig. 1) until approximately 8:00 CET, and then again began to build up. The fall in temperature at the bottom is connected to the local current turn: prior to the fall in temperature the current had a positive north component, almost 4 cm/s, which rapidly dropped into -negative values, specifically to -8 cm/s (Fig. 2). This means that the cold water mass flowed from the northern area to the buoy.

Figure 1

During the bora period, the current in the thin layer at the water surface flows out from the Gulf of Trieste in the direction of the bora. At lower depths, a compensation current is present, which preserves the water mass in the Gulf of Trieste and is the consequence of the pressure gradient force due to the water mass accumulated by the wind in the direction from Trieste towards Venice. Since the water surface near Venice is higher than the surface near Trieste, the pressure force drives the current at depth from Venice towards Trieste. When the strength of the bora decreases, the wind current is no longer present.

Figure 2

The cooling of the water mass at same wind strength in various parts of the Gulf of Trieste depends on the depth of the bottom. The mass, cooled at the surface, which is driven horizontally by the wind, is denser due to cooling and also continuously sinks and mixes with the deeper water. At the site, where the depth of the sea bottom is shallow, the cooled water mass at the surface mixes with a small quantity of water during sinking. The same applies to the sinking of the cooled surface water mass in the part that has greater depth. Therefore, at the same wind intensity at both locations, the temperature of the water column at the site, where the depth of the bottom is shallow cools more than the water column at the site, where the depth of the bottom is greater. Therefore, during the bora winds, the shallower northern Italian part of the Gulf of Trieste, the depth of which increases towards the Slovenian part of the Gulf, is cooled more than the southern Slovenian part of the Gulf, which also has a plate-like bottom with a depth, mostly greater than 20 m. During the blowing of the bora, especially of an extreme bora, horizontal differences in temperature evolve, consequently causing differences in the density of seawater, if we disregard the differences in saltiness.

When the bora wind's strength decreases, relaxation processes begin, for instance the inertial current caused by the Earth's rotation. However, the second relaxation process is more important due to the horizontal density gradient: the so called density or gravitation current emerges. The colder water mass in the northern part of the Gulf of Trieste cuneately lowers towards the deeper bottom in the northern half of the Gulf of Trieste. At the same time, the process in the opposite direction evolves above the descent of the dense water mass: at the surface, the surface dense current builds up, which transfers the warmer water mass from the southern part to the northern part.

Therefore, the oceanographic buoy did not measure the lowest temperature during the hurricane bora winds, but approximately half a day later, when the wind's strength decreased and when the colder water mass rushed from the northern part of the Gulf of Trieste at the bottom of the sea. Of course, the temperature of this mass on the northern side was even lower than the temperature of the mass, measured at the bottom right beneath the oceanographic buoy. During monitoring on 14 February 2012, the ARPA FVG regional environmental agency of Friuli-Venezia-Giulia measured a temperature in the Laguna di Grado lagoon that was lower than 1.6°C at a depth of 0.5 m at the site, where the depth of the bottom is barely 4 m (S saltiness equalled 32.6; typical saltiness in the Gulf of Trieste is normally higher than 37). On 15 February 2012, measurements performed at the sea bottom at a depth of 22.5 m on the Slovenian side (at C2 station, 13° 36,80'E, 45° 34,80'N), which is located in front of the entrance to the Gulf of Koper, showed a temperature of 4.47°C (S = 38.5), which was lower than the temperature measured on 13 February 2012 under the Vida buoy. This is just one half a degree above the record minimum temperature, ever measured at depths greater than 20 m.

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Prepared by: Assoc. Prof. Vlado Malacic, PhD, Head of the Marine Biology Station of the National Institute of Biology

	The Primorske novice newspaper published an article by Ilona Dolenc on 16 February 2012, titled “Vida Measures 4.9 Degrees Celsius at Sea”. The article can be viewed in the PDF document. The article is owned by the Primorske novice newspaper and its authors.