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| Titel |
Inter-annual variability of vertical distribution of temperature and salinity in the Gulf of Finland (Baltic Sea) in summer. |
| VerfasserIn |
T. Liblik, U. Lips |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250023007
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| Zusammenfassung |
| The Gulf of Finland is a 400-km long and 80-120-km wide elongated sub-basin of the Baltic
Sea. It has no sill at the entrance area separating the Gulf from the open Baltic Sea and its
maximum cross-section depth decreases from > 100 m at the entrance to < 30 m in the
eastern part. The salinity distribution in the surface layer is characterized by an increase from
1-3 in the east to 6 (on the practical salinity scale) in the west and by a slight decrease across
the Gulf from south to north. The water column in the deeper areas of the Gulf reveals a
three-layer vertical structure in summer – the upper mixed layer, the cold intermediate
layer and a saltier and slightly warmer near bottom layer can be distinguished.
These layers are separated by two pycnoclines – by seasonal thermocline usually
situated at the depths of 10-20 m and by permanent halocline at the depths of 60-70
m.
According to the monitoring data a decrease of salinity in the deep layers of the Gulf
observed from late seventies was replaced by a salinity increase in nineties. During the same
period an average surface layer temperature increase has been observed. Main aim of the
present study was to describe the vertical structure of temperature and salinity fields and its
inter-annual variations in the Gulf of Finland in summer (June-August). We have analysed
the data collected in 1987-2008 to show whether the suggested trends in salinity
and temperature can be confirmed and what kind of changes did the pycnoclines
experience.
CTD measurements were carried out onboard research vessels by Neil Brown Mark III
and Seabird SBE-19 CTD profilers. Altogether data of 2145 CTD casts were used in the
present study. Higher number of CTD casts was available in the western and southern part of
the Gulf. The CTD data have been processed and stored as vertical profiles with a resolution
of 0.5 m. Wind data for the corresponding period were obtained from the Kalbådagrund
meteorological station (Finnish Meteorological Institute) located in the central part of the
Gulf.
Upper mixed layer thickness as well as thermocline and halocline location, their thickness
and steepness were estimated. Mean values on the basis of each profile, and monthly and
seasonal mean values of temperature and salinity in the upper mixed layer and in the
near-bottom layer were estimated.
On the basis of data from 1987-2008, the mean water temperature in the 5-m surface layer
of the central Gulf of Finland in June was 12.3 Ë C and the mean salinity – 4.5. Same
parameters in July and August were, respectively, 16.8 Ë C, 5.0 and 17.1 Ë C, 4.9. A
relatively high inter-annual variability of the surface layer and the near-bottom layer
temperature and salinity, and the salinity gradient between the surface and the near-bottom
layer was revealed. The highest salinity and temperature values were recorded at the 70-meter
depth in the central Gulf in 1997 and 2006. These extreme T,S-values and strong vertical
gradient of salinity were related to the exceptional wind pattern prevailed in the area –
easterly winds were prevailing more than usually and a quite low average wind speed was
measured.
The upper mixed layer thickness varied mostly between 4 and 18 m and its variability was
the highest in August. A rough analysis was made to find a link between the average
wind speed and the estimated upper mixed layer thickness. The results showed a
significant correlation (r = 0.39) between the 3-day (a day when CTD cast was
performed and 2 days before it) mean wind speed and the calculated mixed layer
thickness.
The deeper border of thermocline was most probably found in a range 20-30 m. Similarly
to the upper mixed layer thickness, the variability of the thermocline deeper border had the
highest variability in August. The estimated thickness of thermocline in July-August varied
mostly between 6 and 21 m. Mean vertical temperature gradient in the thermocline was 0.95 Ë
C m-1 and salinity gradient 0.09 m-1.
The observed changes of the seasonal temperature and salinity values as well as the
vertical structure of these fields was related to the general atmospheric circulation
patterns and to the occurred salt water inflows from the North Sea to the deep layers
of the central Baltic Sea. The possible influence of the variations in the vertical
structure of the water column to the ecosystem of the Gulf of Finland is discussed. |
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