Baltic Sea

8.8
8.6
9
9.2
9.4
9.6
7
Pot. Temperature ( o C)
6.5
6
5.5
5
9.8
4.5
8.8
9
9.2
9.4
4
11 11.5 12 12.5
Salinity (g/kg)
9.6
Figure 3.3: θ/S plots of survey M-7 (black) and M-8 (grey) on potential density surfaces (σ θ ) of 9.0
≤ σ θ < 9.1 kg/m 3 , 9.2 ≤ σ θ < 9.3 kg/m 3 , 9.4 ≤ σ θ < 9.5 kg/m 3 , 9.6 ≤ σ θ < 9.7 kg/m 3 , before
and during the terminating deep water inow; thin lateral line corresponds to 30 year averages of station
BMP271.
properties of the EGB, four 'sub-volumes' below the pycnocline were dened:
i) 9.0 ≤ σ θ < 9.1 kg/m 3 ,
ii 9.2 ≤ σ θ < 9.3 kg/m 3 ,
iii) 9.4 ≤ σ θ < 9.5 kg/m 3 ,
iv) 9.6 ≤ σ θ < 9.7 kg/m 3 .
The θ-S properties of these volumes are plotted in Fig. 3.3. Before the inow (M-7, black
circles) all densities were lower than σ θ = 9.6 kg/m 3 , and waters of category (iv) were thus
missing. More or less, the resulting θ-S clusters scattered around a single point, implying
that deep water properties were relatively homogeneous along isopycnal surfaces. After the
inow (M-8, grey circles), θ increased by about 1.5 ◦ C, while at the same time the isopycnal
variability strongly increased. Peak values occurred within the upper layer (i), where θ values
varied between 5.2 ◦ C and 6.5 ◦ C (∆θ = 1.3 ◦ C). Somewhat deeper, layer (ii) only exhibited a
thermal variability between 5.6 ◦ C and 6.5 ◦ C (∆θ = 0.9 ◦ C). This variability further decreased
to be in the range of ∆θ = 0.4 ◦ C within the layers (iii) and (iv). Consequently, the isopycnal
θ-S variability drastically decreased towards the sea bed.
These changes in thermohaline properties on four selected isopycnal surfaces conrm that all
near-bottom waters were replaced completely during that inow event. In providing a substantial
ambient density stratication, the EGB can be considered as an eective lter for
28