Extreme winter 2012 in the adriatic: An example of climatic effect on the biOS rhythm

Adriatic and Ionian seas are Mediterranean subbasinslinked through the Bimodal Oscillating System mechanismresponsible for decadal reversals of the Ionian basinwidecirculation. Altimetric maps showed that the last cyclonicmode started in 2011 but unexpectedly in 2012 reversedto anticyclonic. We related this “premature” inversionto the extremely strong winter in 2012, which causedthe formation of very dense Adriatic waters, flooding Ionianflanks in May and inverting the bottom pressure gradient. UsingLagrangian float measurements, the linear regression betweenthe sea surface height and three isopycnal depths suggeststhat the southward deep-layer flow coincided with thesurface northward geostrophic current and the anticycloniccirculation regime. Density variations at depth in the northwesternIonian revealed the arrival of Adriatic dense watersin May and maximum density in September. Comparison betweenthe sea level height in the northwestern Ionian and inthe basin centre showed that in coincidence with the arrivalof the newly formed Adriatic dense waters the sea level waslowered in the northwestern flank, inverting the surface pressuregradient. Toward the end of 2012, the density gradientbetween the basin flanks and its centre went to zero, coincidingwith the weakening of the anticyclonic circulation andeventually with its return to the cyclonic pattern. Thus, thepremature and transient reversal of Ionian surface circulationoriginated from the extremely harsh winter in the Adriatic,resulting in the formation and spreading of highly dense bottomwaters. The present study highlights the remarkable sensitivenessof the Adriatic–Ionian BiOS to climatic forcing.

Adriatic and Ionian seas are Mediterranean sub-basins linked through the Bimodal Oscillating System mechanism responsible for decadal reversals of the Ionian basin-wide circulation. Altimetric maps showed that the last cyclonic mode started in 2011 but unexpectedly in 2012 reversed to anticyclonic. We related this "premature" inversion to the extremely strong winter in 2012, which caused the formation of very dense Adriatic waters, flooding Ionian flanks in May and inverting the bottom pressure gradient. Using Lagrangian float measurements, the linear regression between the sea surface height and three isopycnal depths suggests that the southward deep-layer flow coincided with the surface northward geostrophic current and the anticyclonic circulation regime. Density variations at depth in the northwestern Ionian revealed the arrival of Adriatic dense waters in May and maximum density in September. Comparison between the sea level height in the northwestern Ionian and in the basin centre showed that in coincidence with the arrival of the newly formed Adriatic dense waters the sea level was lowered in the northwestern flank, inverting the surface pressure gradient. Toward the end of 2012, the density gradient between the basin flanks and its centre went to zero, coinciding with the weakening of the anticyclonic circulation and eventually with its return to the cyclonic pattern. Thus, the premature and transient reversal of Ionian surface circulation originated from the extremely harsh winter in the Adriatic, resulting in the formation and spreading of highly dense bottom waters. The present study highlights the remarkable sensitiveness of the Adriatic-Ionian BiOS to climatic forcing. OI Notarstefano, Giulio/0000-0002-8532-2097; , Laura/0000-0003-1714-1451; Bensi, Manuel/0000-0002-0548-7351; CIVITARESE, Giuseppe/0000-0002-4572-6177; Menna, Milena/0000-0002-0149-0502; cosoli, simone/0000-0002-4417-9538