Baltic Sea

following these bounds will be used to determine whether diusive convection convection can
be considered to be a general consequence of such warm deep-water intrusions.
For the late-stage inow conditions (M-8), values of Tu were computed and projected on the
deep σ θ = 9.5 kg/m 3 surface (see Fig. 3.4 b). Comparing Fig. 3.4 a with Fig. 3.4 b, it becomes
evident that strong diapycnal temperature gradients, computed over a relatively large vertical
distance of around 20 - 30 m (compare Fig. 3.5), favour the local appearance of regime (i)
values of Tu. Altogether, these estimates suggest that the patterned temperature distribution
on deep isopycnals is favourable for diusive convection on selected isopycnals, in particular
near the centre of the basin (Fig. 3.4 b). In the absence of appropriate ne-scale and direct
turbulence measurements, it is, however, not possible to quantify the relative importance of
this process with respect to mixing driven by shear instabilities resulting, e.g. from internal
waves motions.
3.2.2 Vertical transects
The zonal transect at 57 ◦ 14.4 ′ N across the basin's centre during stagnant deep water conditions
(M-7) illustrates that the temperature distribution on isopycnals was quite homogeneous
(θ ≤ 5.3 ◦ C) in layers deeper than 60 m depth (see Fig. 3.5 a). No intrusions were visible
during that time. Associated values of Tu point to a slight tendency for diusive convective
mixing (see Fig. 3.6 a). The comparison of these conditions with those of the campaign M-8
(Fig. 3.6 b) clearly shows that the inow of deep warm water was associated with an upward
displacement of deep isopycnals. For instance, the density surface σ θ = 9.4 kg/m 3 was located
at a depth of about 170 m before the inow (Fig. 3.6 a). During its end (M-8, Fig. 3.5 b),
however, it was found between 140 and 150 m, thus uplifted by 37 m above the basin's centre
to occupy about 71% of the MESODYN station grid. The zonal transect of M-8 reveals that
the core of the uplift was located over the steep eastern topographic ank of the basin.
Consequently the thermohaline deep water regime dramatically changed after the inow event
(Fig. 3.5 b). The mean vertical temperature gradient had strongly increased due to the replacement
of colder bottom water by warm inow water. This points to a dominance of the
diusive convection regime (i), producing numerous local warm temperature anomalies below
the halocline down to a depth of approximately 180 m. A particularly strong intrusion was
centred around 20.07 ◦ E at a depth of approximately 170 m. In contrast, no such intrusions
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