Interannual variability of Black Sea's hydrodynamics and connection to atmospheric patterns

Abstract

The long term variability of the Black Sea physical processes (e.g. temperature, main circulation, cold intermediate layer, sea level) and its relation to atmospheric conditions and large scale climate patterns are investigated using an eddy-resolving tridimensional model in combination with statistical tools (e.g Empirical Orthogonal Functions, Self Organizing Maps). First, the ability of the model to represent the interannual dynamics of the system is assessed by comparing the modeled and satellite sea surface temperature (SST) and sea level anomaly (SLA) decomposed into their dominant Empirical Orthogonal Functions (EOFs). The correlation between the spatial and temporal EOFs modes derived from model and satellite is usually satisfactory and this gives some confidence in using the model as a tool to investigate not only the SST and SLA dynamics but also the dynamics of connected variables.Then, the long term variability of the Black Sea hydrodynamics is assessed by decomposing into their dominant EOFs modeled SST, SLA and selected key hydrodynamical variables associated to the main circulation and vertical structure of the water column. Significant correlations between the EOFs associated to these variables are investigated in order to link the variability of surface fields and the internal dynamics of the system.In particular, the intensity of the general cyclonic circulation (the Rim current) is shown to impact strongly on (1) the mean sea level, (2) the SST response to air temperature (AT), (3) In order to appraise the variability of atmospheric conditions over the Black Sea during 1962-2000 and their role in driving the hydrodynamics, a self-organizing maps technique is used in order to identify spatial recurrent patterns of atmospheric fields (ie. AT, wind stress and curl).The impact on these patterns of large scale climatic variability over the north Atlantic, the Eurasia and Pacific Ocean (estimated by respectively the north Atlantic oscillation (NAO), the east Atlantic/west Russia oscillation (EA/WR) and the El Niño southern oscillation (ENSO) indexes) is assessed. Distinct time scales of influence of the large scale teleconnection patterns on the AT are identified : EA/WR drives the short scale (1-5 years) variations of SST, while the long term (>5 year) trends of the NAO drive the long term SST trends.The drastic changes that have occurred in the Black Sea deep sea ecosystem at the end of the 80's are connected to an intensification of the general circulation which has promoted an export of riverine materials from the eutrophicated north-western shelf to the deep sea.Finally, in the last two decades, we find an increased duration of persistent atmospheric anomalies regime that have the potentials to drive the system away from its average state as occurred in the late 80s. If persistent in the future, such long lasting atmospheric anomalies may have a significant impact on the ecosystem functioning.

Understanding the Impact of Environmental Variability on Anchovy Overwintering Migration in the Black Sea and its Implications for the Fishing Industry

“…Temperature plays a critical role because of the special climatic feature of the Black Sea, where an exceptionally large zonal west-east temperature gradient exists with fall-winter temperatures in the southeastern region being ∼4-5 • C higher than the northwestern region (Buongiorno Nardelli et al, 2010;Capet et al, 2012). Anchovy can swim at high speeds of up to five body-lengths/s (bl/s) for long periods of time, as observed in Peruvian anchovy (Peraltilla and Bertrand, 2014), and similar swimming speeds have also been observed in the Black Sea anchovy (A.C. Gucu, pers.…”

Drivers, mechanisms and long-term variability of seasonal hypoxia on the Black Sea northwestern shelf – is there any recovery after eutrophication?

“…It resolves the currents and the physical state of the sea, which are temperature, salinity, internal mixing and surface elevation. The implementation used in the frame of these simulations is described in Capet et al (2012), and validated in detail with satellite sea surface temperature (SST) and sea level elevation data.…”

“…This indicates that climatic drivers mostly drive a shorter timescale variability while eutrophication drivers act on a longer term. Interestingly, the role of temperature introduces an oscillation in the interannual variations of hypoxia, which is related to the East Asia/West Russia atmospheric teleconnection pattern (Capet et al, 2012).…”

“…The validation of the simulated physical variables is described in Capet et al (2012) where the interannual variability of the model- simulated SST and sea surface elevation is compared with satellite observations. This validation exercise demonstrates the model ability to correctly reproduce the interannual variability of SST and associated hydrodynamical fields such as the dynamics of the mixed layer.…”

Decline of the Black Sea oxygen inventory

“…The resulting cold intermediate layer (CIL) is a major feature of the Black Sea vertical structure. Cold intermediate water formation and advection by the cyclonic basin-wide Rim Current (Öszoy and Ünlüata, 1997;Capet et al, 2012) ventilate the oxycline and thereby influence variability in the depth of the upper suboxic interface . Recently, atmospheric warming (Oguz et al, 2006) was shown to reduce the ventilation of the lower oxic layer (Tugrul et al, 2014;Pakhomova et al, 2014).…”