Black Sea thermohaline properties: Long‐term trends and variations

Abstract

The current knowledge about spatial and temporal dynamics of the Black Sea's thermohaline structure is incomplete because of missing data and sparse distribution of existing measurements in space and time. This study presents 56 year continuous simulations of the Black Sea's hydrodynamics using the 3D General Estuarine Transport Model (GETM), without incorporating any relaxation toward climatological or observational data fields. This property of the model allows us to estimate independent temporal trends, in addition to resolving the spatial structure. The simulations suggest that the intermediate layer temperature is characterized by a weak positive trend (warming), whereas the surface temperature does not show a clear linear trend. Different salinity trends have been established at the surface (negative), upper (weaker negative) and main halocline (positive). Three distinct dynamic periods are identified (1960–1970, 1970–1995, 1995–2015), which exhibit pronounced changes in the Black Sea's thermohaline properties and basin circulation. Strengthening of the main cyclonic circulation, accompanied by intensification of the mesoscale anticyclonic eddy formation is found. Both events strongly affect the sea surface salinity but contribute in opposing directions. Specifically, strong composite large‐scale circulation leads to an increase in sea surface salinity, while enhanced formation of mesoscale anticyclones decreases it. Salinity evolution with time is thus the result of the competition of these two opposing yet interdependent processes.

Sea Surface Temperature in the Mediterranean: Trends and Spatial Patterns (1982–2016)

“…SST warming rates are not only consistent but have accelerated throughout the study period, in a context of accelerated global warming observed during this century. It is also noticeable the difference from the closer Black Sea, where Miladinova et al (2017) did not find any SST significant trend for the 1960-2015 period while Shapiro et al (2010) found a warming in the 1985-2000 period embedded in an overall downward trend for the twentieth century. Shaltout and Omstedt (2014) found a 0.51°C/decade increase for the Black Sea from 1982 to 2012 that is higher than the global 0.36°C/decade we have found in the Mediterranean for 1982-2016 but lower than the decadal trends from 1993 to 2016.…”

Response to Referee 1

“…We are aware of the various references pointed out and indeed we have tried to test the effects of the modeling improvements on CIL, which equivocally shows reduced CIL over the recent years (in parallel with some recent results, e.g. Miladinova et al, 2017Miladinova et al, , 2018Stanev et al, 2019). However, we only had to compare model results either on selected domain averages or at selected stations, producing the overall climatic trends captured in the references.…”

“…Even the quality of the figure is very low I can see that the CIL is almost absent. Many recent studies based on ARGO float data (Stanev et al, 2013(Stanev et al, , 2014(Stanev et al, , 2017 and numerical modelling (Miladinova et al, 2017 and clearly show the presence of CIL in the period 2009-2015. CIL is eroded but exists in winterspring.…”

“…While a general overview of historical development of Black Sea modelling is not in order within the present context, we only give a short review of Black Sea modeling that indirectly has accounted for Bosphorus fluxes in the previous models. To the best of authors' knowledge, modelling of the Black Sea circulation and hydrography allowing for a system of open boundary conditions applied at the Bosphorus has not been attempted in the available literature so far, although a number of stand-alone Black Sea models have attempted indirectly accounting for fluxes at the Bosphorus (Stanev et al, 2003(Stanev et al, , 2004(Stanev et al, , 2005Miladinova et al, 2017Miladinova et al, , 2018). Our incremental model strategy, on the other hand, could be compared to the addition of an "Atlantic Box" preferred in the early phases of Mediterranean Forecasting System, MFS (Oddo et al, 2009), which only recently have been updated to involve further refinements of coupled systems (CMEMS, 2017).…”

The short timescale variability of the oxygen inventory in the NE Black Sea slope water

“…In the NE Black Sea, the maximum amplitude of short-term (7-8 min) subsurface waves on the shelf may be as high as 16 m (Bondur et al, 2018). In the NW Black Sea, intensive subsurface waves in the lower part of the pycnocline in the halocline were primarily observed in the continental slope area in autumn, winter, and summer (Morozov et al, 2017). During these measurements in the main pycnocline in December 2012, vertical current profiles of the large internal waves were observed with vertical lengths of 50-60 m.…”