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| Titel |
Exchanges between the open Black Sea and its North West shelf |
| VerfasserIn |
Georgy Shapiro, Fred Wobus, Feng Zhou |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250086946
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| Publikation (Nr.) |
 EGU/EGU2014-1064.pdf |
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| Zusammenfassung |
| Exchanges between the vast NW shelf and the deep basin of the Black Sea play a significant
role in maintaining the balance of nutrients, heat content and salinity of the shelf
waters. Nearly 87 % of the Black Sea is entirely anoxic below 70 to 200m and
contains high levels of hydrogen sulphide (Zaitsev et al, 2001), and this makes the
shelf waters particularly valuable for maintaining the Black Sea ecosystem in good
health. The increase in salinity of shelf waters occurs partially due to exchanges
with more saline open sea waters and represents a threat to relics and endemic
species.
The shelf-break is commonly considered the bottle-neck of the shelf-deep sea exchanges
(e.g. (Huthnance, 1995, Ivanov et al, 1997). Due to conservation of potential vorticity, the
geostrophic currents flow along the contours of constant depth. However the ageostrophic
flows (Ekman drift, mesoscale eddies, filaments, internal waves) are not subject to the
same constraints. It has been shown that during the winter well mixed cold waters
formed on the North West shelf propagate into the deep sea, providing an important
mechanism for the replenishment of the Cold Intermediate Layer ( Staneva and Stanev,
1997). However, much less is known about exchanges in the warm season. In this
study, the transports of water, heat and salt between the northwestern shelf and
the adjacent deep basin of the Black Sea are investigated using a high-resolution
three-dimensional primitive equation model, NEMO-SHELF-BLS (Shapiro et al,
2013).
It is shown that during the period from April to August, 2005, both onshore and offshore
cross-shelf break transports in the top 20 m were as high as 0.24 Sv on average, which was
equivalent to the replacement of 60% of the volume of surface shelf waters (0 – 20 m) per
month. Two main exchange mechanisms are studied: (i) Ekman transport, and (ii) transport
by mesoscale eddies and associated meanders of the Rim Current. The Ekman drift causes
nearly uniform onshore or offshore flow over a large section of the shelf break. Due to the
short duration of strong wind effects (4-7 days) the horizontal extent of cross-shelf-break
exchanges is limited to the outer shelf. The effect of Ekman drift is confined to the upper
layers. In contrast, eddies and meanders penetrate deep down to the bottom, but they are
restricted laterally. During the strong wind events of April 15 – 22 and July 1 – 4, some
0.66x1012 and 0.44x1012 m3of water were removed from the northwestern shelf
respectively. In comparison, the single long-lived Sevastopol Eddy generated a much
larger offshore transfer of 2.84x1012 m3 over the period April23to June30,
which is equivalent to 102% of the volume of northwestern shelf waters. This result
is consistent with the data obtained from satellite derived information (Shapiro
et al, 2010). The open Black Sea is generally warmer and more saline than the
northwest shelf. Hence the exchanges contribute to the increase in both salinity and
temperature of shelf waters. Over the study period, salt exchanges increased the
average density of the shelf waters by 0.67 kg m-3 and reduced the density contrast
between the shelf and deep sea, while lateral heat exchanges reduced the density of the
shelf waters by 0.16 kg m-3 and thus enhanced density contrast across the shelf
break.
This study was supported by the EU (via PERSEUS grant FP7-OCEAN-2011-287600
and MyOcean SPA.2011.1.5-01 grant 283367), Ministry of Science and Technology of China
(Grant 2011CB409803), the Natural Science Foundation of China (Grant 41276031),
Zhejiang Association for International Exchange of Personnel, and the University of
Plymouth Marine Institute Innovation Fund.
References
Huthnance, J. M., 1995. Circulation, exchange and water masses at the ocean
margin: the role of physical processes at the shelf edge, Prog Oceanogr, 35(4),
353-431,
Ivanov L.I., Besiktepe S., Ozsoy E., 1997. In: E.Ozsoy and A.Mikaelyan (eds). Sensitivity
to change: Black Sea , Baltic Sea and North Sea. NATO ASI Series, Vol. 27, Kluwer
Academic Publishers, 253-264.
Shapiro, G.I. , S.V. Stanichny, R.R. Stanychna, 2010. Anatomy of shelf–deep sea
exchanges by a mesoscale eddy in the North West Black Sea as derived from remotely
sensed data. Remote Sensing of Environment, 114 , 867–875.
Shapiro,G., Luneva,M., Pickering,J., and Storkey,D., 2013. The effect of various
vertical discretization schemes and horizontal diffusion parameterization on the
performance of a 3-D ocean model: the Black Sea case study, Ocean Science, 9,
377-390.
Staneva, J. V. and E. V. Stanev, 1997. Cold water mass formation in the Black Sea.
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to change: Black Sea, Baltic Sea and North Sea. NATO ASI Series, Vol. 27, Kluwer
Academic Publishers, 375-393.
Zaitsev Yu.P., B.G. Alexandrov, N.A. Berlinsky, A. Zenetos, 2001. Europe’s biodiversity -
biogeographical regions and seas. The Black Sea. European Environment Agency. |
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