Observation, Preconditioning and Recurrence of Exceptionally High Salinities in the Adriatic Sea

Abstract

The paper aims to describe the preconditioning and observations of exceptionally high salinity values that were observed in summer and autumn of 2017 in the Adriatic. The observations encompassed CTD measurements carried out along the well-surveyed climatological transect in the Middle Adriatic (the Palagruža Sill, 1961–2020), Argo profiling floats and several glider missions, accompanied with satellite altimetry and operational ocean numerical model (Mediterranean Forecasting System) products. Typically, subsurface salinity maximum, with values lower than 39.0, is observed in the Southern Adriatic (usually between 200 and 400 m), related to ingressions of saltier and warmer waters originating in the eastern Mediterranean (Levantine Intermediate Water—LIW). However, seasonally strong inflow of warm and high salinity waters (S > 38.8) has been observed much closer to the surface since spring 2015. The main LIW core deepened at the same time (to 400–700 m). Such double-maxima vertical pattern was eventually disturbed by winter convection at the beginning of 2017, increasing salinities throughout the water column. A new episode of very strong inflow of high salinity waters from the Northern Ionian was observed in late winter and spring of 2017, this time restricted almost to the surface. As most of 2017 was characterized by extremely dry conditions, low riverine inputs and warmer than usual summer over the Adriatic and Northern Ionian, salinity values above the sharp and shallow (15–40 m) thermocline significantly increased. The maximum recorded salinity was 39.26, as measured by the Argo float in the Southern Adriatic. Surface salinity maximum events, but with much lower intensity, have been documented in the past. Both past events and the 2017 event were characterized by (i) concurrence with overall high salinity conditions and cyclonic or transitional phase of the Adriatic-Ionian Bimodal Oscillating System, (ii) very low river discharges preconditioning the events for a year or more, (iii) higher-than-average heat fluxes during most of the summer and early autumn periods, forming a stable warm layer above the thermocline, and (iv) higher-than-average E-P (evaporation minus precipitation) acting on this warm surface layer. Importantly, the 2017 event was also preceded by strong near-surface inflow of very saline waters from the Northern Ionian in early 2017.

Characterization of the Atlantic Water and Levantine Intermediate Water in the Mediterranean Sea using 20 years of Argo data

“…2g, dropping from about 100 to 650 m (maximum whisker values). The distributions tend to be symmetric in most of the Mediterranean Sea, except for the Adriatic Sea, where a strong LIW bimodality in the depth domain is found, with two peaks located at ∼ 190 and ∼ 500 m (here not shown), in agreement with Kokkini et al (2019) and Mihanović et al (2021). In this respect, Mihanović et al (2021), analysing observed data from CTD measurements, Argo floats, several glider missions, satellite observations, and operational ocean numerical model products, provide a possible explanation for this double-maxima vertical pattern, suggesting that these two peaks may be explained by two concurrent events: the winter convection at the beginning of 2017, which leads higher salinities in the water column, and a very strong inflow of high-salinity waters from the North Ionian Gyre in late winter and spring of 2017, which this time is almost restricted to the surface.…”

“…-Abrupt shifts in the LIW depth are found in the Adriatic sub-basin from ∼ 200 to ∼ 500-600 m at different time steps (trend 2.609 ± 115.404 m yr −1 ), highlighting a bimodal behaviour of the LIW depth and an intense dense water production activity, as also shown by Kokkini et al (2019) and Mihanović et al (2021).…”

“…Non-significant trends are found in the other regions. Abrupt shifts are found in the Adriatic subbasin from ∼ 200 to ∼ 500-600 m at different time steps (trend ∼ 2.609 ± 115.404 m yr −1 ), highlighting a bimodal behaviour of the LIW depth and an intense dense water production activity as also shown by Kokkini et al (2019) and Mihanović et al (2021). Previous studies attribute dramatic shifts in the Adriatic hydrological properties to the BiOS and the Eastern Mediterranean Transient (e.g.…”

“…The SAP is an important deep-water convection site in the Mediterranean Sea (e.g. Kokkini et al, 2019;Mauri et al, 2020;Mihanović et al, 2021;Azzaro et al, 2012;Bensi et al, 2013), and therefore it is also considered a crucial area from a climatic perspective. The temporal distribution of the float data is different in the various sub-basins: the longest time series are available in the Ionian, Cretan, and Levantine regions, with data from 2001 to 2019; followed by the Algerian, Ligurian, and Tyrrhenian sub-basins where data are available after 2003; and then by the Adriatic Sea with data available only after 2009.…”

On the Circulation and Thermohaline Properties of the Eastern Mediterranean Sea

“…This current is northward oriented, whatever the circulation mode of the NIG (cyclonic or anticyclonic; see Figure 2 of Menna et al, 2019b andNotarstefano et al, 2019). The LSW transported by the coastal northward current along the western Greek coast could be responsible for the near-surface salinity maximum observed by Kokkini et al (2018Kokkini et al ( , 2020 in the South Adriatic Pit (SAP) during 2015-2016, by Kassis and Korres (2020) in 2005-2010 and by Mihanovic et al (2021) in 2017. These authors describe an anomalous salinity pattern in the SAP, where, the maximum is located in the near-surface layer (around 100 m depth) instead of in the intermediate layer occupied by the LIW.…”