The centre of the Southern Adriatic is an area where vertical mixing (up welling, wintertime convection) has a rather prominent role homogenizing physical and chemical seawater properties, and controlling the primary production (up welling water is a source of nutrients in the euphotic zone which stimulates the growth of phytoplankton).
The E2-M3A Station is deployed in the southern Adriatic Sea, at about 1200 m depth, a well known area of dense, deep water formation through open sea convection. These newly formed deep water masses are then exported through the Strait of Otranto to the rest of the Eastern Mediterranean basin.

The dynamics of the area is dominated by the presence of a quasi-permanent cyclonic gyre that intensifies in the winter season creating the conditions for the production of dense and oxygenated waters with the contribution of the saltier intermediate layer dominated by the Levantine Intermediate Water (LIW).
The Adriatic Dense Water (AdDW) formed prevalently by the open-ocean vertical convection, is then exported through the Strait of Otranto to the rest of the Eastern Mediterranean basin and becomes the main component of the Eastern Mediterranean Deep Water (EMDW).

The recorded time-series at the E2-M3A station can be used for studies of long-term changes of Adriatic response to local climatic forcing. The volume of the dense water formed surrounding the station can be correlated to the winter heat losses.
The amount of the dense water formed also determines the water exchange between the Adriatic and Eastern Mediterranean.

Water masses circulation in the Adriatic Sea:

NAdDW = North Adriatic Dense Water;
AdDW = Adriatic Dense Water;
LIW = Levantine intermediate Water
AW = Atlantic Water AdDW = Adriatic Dense Water LIW = Levantine Intermediate Water

Adriatic - Ionian Bimodal Oscillating System (BiOS):

Recently, cyclical occurrences of the high salinity in the Levantine and the Eastern Mediterranean Transient (EMT) preconditioning have been explained in terms of the feedback mechanism called Adriatic-Ionian Bimodal Oscillating System (BiOS), i.e. the decadal reversals of the Ionian upper-layer basin-wide circulation from cyclonic to anticyclonic and vice versa (Gačić et al., 2011).

The Mid-Ionian Jet, which brings Atlantic Water (AW) into the Levantine basin, is reinforced or weakened by the Ionian cyclonic or anticyclonic circulation, respectively. This then results in a varying intensity of the AW advection towards the Levantine and consequently in a varying dilution of the Levantine surface waters. Considering that the LIW is formed in the Levantine, in the area of Rhodes Gyre, obviously the LIW salinity will change as a function of the intensity of the Levantine surface water dilution by the AW.

During the Ionian anticyclonic mode the AW flow is mainly deflected northeastward affecting the northern Ionian and southern Adriatic. In that situation the flow of the AW towards the Levantine is reduced, the Levantine surface waters become saltier and the same applies to the LIW. The Ionian anticyclonic circulation mode is thus the preconditioning factor for the EMT-like phenomena (Gačić et al., 2011). As far as the Adriatic Sea is concerned, it is more prone to winter convection when the Ionian circulation is cyclonic bringing there the salty waters of the Levantine origin.

In the opposite circulation pattern, under the influence of the AW, the vertical stability of the water column in the Adriatic hampers the vertical convection. In addition, the water formed is of a lower salinity due to the freshening of the upper part of the Adriatic water column. However, we have to take into considerations that the winter convection in the Adriatic represents an interplay between airsea heat fluxes and the buoyancy, meaning that the winter meteorological conditions can affect strongly the efficiency of the Adriatic as a source of the EMDW.

Acidification

The increasing CO2 concentration in the atmosphere through the exchanges with the surface ocean is determining a process called Ocean Acidification on a global scale. This process could have a negative impact on many marine organisms from different trophic levels, with potentially large consequences for the marine ecosystem as well as carbon cycling. In the global ocean the ocean acidification process has been estimated to be more pronounced in the area where deep water formation takes places, due to the influence of low seawater temperature which enhances the solubility of atmospheric CO2 and the physical pump.
The Adriatic is an important player for the circulation in the Mediterranean Sea, which on a smaller scale behaves as an ocean, and represents one of the most important sites of deep water formation. The automatic monitoring of the carbonate system set up on the E2-M3A buoy is measuring two of its main variables i.e. the partial pressure of CO2 (pCO2) and the pH. This allows a better understanding of the processes influencing the carbonate system in the South Adriatic, especially during winter cooling, when dense waters are formed. The aim of the carbonate system automatic monitoring is to understand the role of this physical pump for the transfer process of the CO2 when winter cooled waters sink forming the Adriatic Dense Water which, flowing through Otranto strait, contribute to the Eastern Mediterranean Deep Water. Therefore, following the CO2 transfer would allow to understand the acidification processes in deep waters.
The distribution of pCO2 along the Bari-Dubrovnik section in March 2014 is showed below. It is representative for the winter period. At 300-500m depth in the eastern side, the Levantine Intermediated Water (LIW) revealed an elevated pCO2 concentration. High values were also present close to the bottom, with the presence of Adriatic Dense Water (AdDW).
The pHT measured at in-situ temperature (below) showed an inverse pattern with respect to pCO2 in the same period and along the same transect. Due to the influence of biological activity and seawater temperature, at the surface the pHT was elevated and gradually decreased with the depth. Between 300 and 500 m in the eastern side of the section, low pHT values were present in association with the LIW core. However, in the deepest part of the Southern Adriatic Pit, the AdDW revealed the most acidified waters.
These findings point out the role of North Adriatic Sea in sequestering and storing a great amount of atmospheric CO2 during deep water formation, which in turns could cause the expansion of the ocean acidification process into the deeper layer of the Adriatic Sea and, consequently, of the whole Mediterranean Sea.


FixO3 is a FP7 Collaborative Project coordinated by the
National Oceanography Centre, Southampton, UK
For more information please contact: V. Cardin
Last update: March 2, 2015 -- Webmaster: R.Iungwirth