Mapping of decadal middle Adriatic oceanographic variability and its relation to the BiOS regime

Abstract

We analyzed long‐term time series of temperature, salinity, and dissolved oxygen (DO) concentrations collected along the Palagruža Sill transect (middle Adriatic) between 1952 and 2010. The data have been mostly collected on seasonal basis, allowing for extraction of seasonal signal from the series. By applying Self‐Organizing Maps (SOM) method, a kind of unsupervised neural network method, the processes on a decadal time scale emerged as the most relevant for changes of oceanographic properties in the middle Adriatic area. Sensitivity studies revealed that oceanographic patterns obtained by SOM were not sensitive to shortening of time series, to removal of data from one station or to removal of DO from the analysis. Simultaneous SOM‐based mapping of sea surface heights in the northern Ionian Sea, with these heights serving as a proxy for the Adriatic‐Ionian Bimodal Oscillating System (BiOS), revealed asymmetry between anticyclonic and cyclonic BiOS patterns and correlated the decadal oscillations in the middle Adriatic with the reversals in the BiOS circulation regimes. These reversals are found to either rapidly change oceanographic properties in the middle Adriatic (e.g., during the Eastern Mediterranean Transient) or to change them with a time lag of 2–3 years. The mapped connections may be used for a short‐time (a few years) forecasting of the Adriatic oceanographic properties or for mapping future climate decadal oscillations as seen by ocean climate models.

Effects of the Eastern Mediterranean Sea circulation on the thermohaline properties as recorded by fixed deep-ocean observatories

“…Borzelli et al (2009), Bessières et al (2013, Gačić et al (2011Gačić et al ( , 2014, Poulain et al (2012) have confirmed that in 1997the Ionian Sea upper circulation demonstrated reversals (1997cyclonic NIG, 2006anticyclonic NIG, and again cyclonic after 2011. Additionally, both Gačić et al (2014) and Mihanović et al (2015) showed how these reversals could be rapid or slow, depending on the governing basin-scale processes that drive them.…”

Mesozooplankton spatial distribution and community structure in the South Adriatic Sea during two winters (2015, 2016)

“…BiOS is a dominant process that drives these changes on a decadal scale: the cyclonic regime brings saltier waters of Levantine origin into the Adriatic and the anti‐cyclonic pattern favours the inflow of less saline Ionian waters diluted by the Atlantic Water (Civitarese, Gačić, Lipizer, & Borzelli, ). According to the latest analyses, from 2011 onwards the Northern Ionian circulation regime shifted to a cyclonic phase, bringing saltier Aegean/Levantine waters into the Southern Adriatic, which is likely to enhance the deep‐water convection in the area (Gačić et al., ; Mihanović, Vilibić, Dunić, & Šepić, ).…”

“…According to the latest analyses, from 2011 onwards the Northern Ionian circulation regime shifted to a cyclonic phase, bringing saltier Aegean/Levantine waters into the Southern Adriatic, which is likely to enhance the deep-water convection in the area (Gačić et al, 2014;Mihanović, Vilibić, Dunić, & Šepić, 2015).…”

Unexpected Covariant Behavior of the Aegean and Ionian Seas in the Period 1987–2008 by Means of a Nondimensional Sea Surface Height Index

“…The transition between different states has been shown (through box models and idealized experiments) to be triggered by density differences between the Southern Adriatic and Aegean Seas and, in particular, has been associated with increasing (decreasing) salinity (temperature) over Aegean Sea (Amitai et al, ; Ashkenazy et al, ). Recent studies (Beuvier et al, ; Mihanović et al, ; Theocharis et al, ; Velaoras et al, ) identified three past events sharing with EMT the shift of deep waters formation from the Southern Adriatic to Aegean Sea but not the overall volume of deep waters formed: one at end of 1950s, one between 1962 and 1965 and one toward the end of 1970s.…”

“…This transient phase ended in 2013 with the establishment of a new cyclonic state. The variability observed in the circulation, and hence in SSH, can clearly affect the salinity behavior of EMED, in the Sicily Channel (Gačić et al, ) and of salt, nutrients and mixed layer depth in the Adriatic Sea (Civitarese et al, ; Gačić et al, 2010; Mihanović et al, ; Reale et al, ).…”