Tipping of the double-diffusive regime in the Southern Adriatic pit in 2017 in connection with record high salinity values

Abstract. Whenever salinity decreases with depth the water column is either unstable or in a state called salt fingering (SF), which exhibits increased vertical mixing. Analysis of a high-frequency time series of thermohaline data measured at the EMSO-E2M3A Regional Facility in the Southern Adriatic Pit (SAP) from 2014 to 2019 reveal that in the south Adriatic SF is the dominant regime. The same time series shows the presence of a very saline core of the Levantine Intermediate Water that penetrated with unprecedented strength during the winter of 2016/17 at around 550 dbar and even higher salinity water above. The effect of strong heat loss at the surface during that winter allowed deep convection to transport this high salinity water from the intermediate to the deep layers within the pit. This resulted in an increase in SF throughout the water column. In the subsurface layer (350 to 550 dbar) the increase is from 27 % to 72 % of observations. Consequently, we observe an alteration of vertical stratification throughout the water column during winter 2016/17, from a stratified water column to an almost homogeneous water column down to 700 dbar, with no return in the following years.