Baltic Sea

BMP271 according to
〈S〉 V = 1 V
∫ z t
z b
S(z)A(z) dz , (3.7)
where horizontal homogeneity is assumed. An analogous procedure was used to obtain estimates
of the volume averaged rate of change of potential temperature.
Two questions immediately arise from this approach: (a) How representative are temporal
snapshots from a CTD prole in the presence of possible short-term uctuations, and (b) to
what extent can proles measured in the basin's centre be used to represent basin-scale volume
averages? To obtain an answer for the rst question, the temporal variability from short-term
uctuations, likely caused by near-inertial wave motions, has been investigated. Three dierent
CTD casts were compared, obtained between 17 - 18 February 1998 within a period of 13 h,
i.e. within slightly less than one inertial period of about 14 h. Resulting proles(Fig. 3.10)
demonstrate that in the lowest 20 m of the water column the temporal variability is much
smaller than the long-term spatial decay of salinity and temperature. The average of these
three proles will be identied as Prole 1 on 17 February 1998 (see Table 3.3).
Although the above mentioned assumption of spatial homogeneity in S and θ turned out to
be more demanding, it can be justied only for densest water masses below 225 m depth
where both quantities exhibit a small lateral variability, see Fig. 3.3 and Fig. 3.5. Therefore,
equation (3.7) was evaluated for a volume below z t = −225 m with a small volume correction
for proles that did not reach the deepest part of the basin. The resulting averaged salinities
and temperatures for the three dates are compiled in Table 3.3 and marked in Fig. 3.10.
Based on the proles shown in Fig. 3.10, the mean gradients ∂S/∂z ≈ −0.0075 (g/kg)/m
were estimated for salinity and ∂θ/∂z ≈ −0.0118 K/m for potential temperature between
205 and 225m. Thereafter the lower bounds for the uxes and diusivities were computed,
Table 3.4. Finite-dierencing over the whole time window of these three proles resulted in
κ S = 1.6×10 −5 m 2 /s and κ θ = 2.1×10 −5 m 2 /s. Corresponding values computed for the rst
and second half of the interval are consistently lower or higher, respectively, than these average
values. Overall, however, diusivities computed by this method agree within approximately
30 %.
40